CONTENTS 1.0
Introduction
2.0
Causes Of Flare Load
3.0
Design Guidelines
3.1 Radiation 3.2 Noise 3.3 Glc Of Toxic Combustion Products 4.0
Types Of Flares
4.1 Elevated Flares 4.2
Ground Flares
5.0
Flare System Components And Concepts
6.0
Design Of Flare Systems
6.1 Flare Load 6.2 Flare Header 6.3 Flare Stack 6.4 Knock Out Drum 6.5 Blow Down Pump 6.6 Water Seal Drum 6.7 Purge Gas Requirement 6.8 Smokeless Operation Of Flare
7.0
Inhouse Software Available For Design Of Flare System
8.0
Sample Calculations
9.0
Comparative Study Of Flare Systems Of Different Refineries
10.0
Flare Load Reduction Based On Interlocks
11.0
Flare Gas Recovery Systems
11.1
Introduction
11.2
System Description
11.3
Comp. Selection & Suction Control
11.4
Operating Feedback
11.5 Attachments 12.0
Umoe’s Concept Of Flare Gas Recovery
1.0
INTRODUCTION Primary function of Flare is to convert flammable, toxic or corrosive vapours to less objectionable components by combustion. Disposal of combustible gases, vapours and liquids by burning is accomplished in flare. This serve as an emergency disposal system to eliminate excess hydrocarbons coming to it due to: a) Release from safety valves because of an equipment failure or because of major plant emergency such as a power failure, cooling water failure or a plant fire. b) Leakages from safety valves or control valves. c) Disposal of flammable liquid-vapour products or by products which can not be marketed. Process flares are used primarily in the oil and petrochemical industries from initial production through transportation, storage, refining and processing. Flares are also used for other process applications where hydrocarbon emissions must be controlled. A few examples include: Sewage digestors, coal gasification and liquefaction, rocket engine testing and heavy water plants. The main objective of the flare has been to discharge flammable and waste gases at a safe location and burn them in order to protect the environment from pollution and hazards. Flaring, being a critical operation in many plants its design must be governed on strict safety principles. The various problems associated with flare are: a) Glare: The flare generates a considerable amount of glare particularly during emergencies. This causes inconvenience to the surrounding population and also gives the impression of unnecessary waste of resources. b) Smoke: Smoke from flares causes a considerable amount of environmental pollution. The smoke does occur in most of the flare system due to incomplete combustion of gases. c) Noise: High noise levels are created by flare system during plant upset conditions. This again causes inconvenience to the surrounding population. Prolonged exposure to excessive noise may cause mental irritation, fatigue and even deafness. Now sophisticated design of flare tips have greatly reduced the noise pollution. d) Radiation: During peak flare, the radiation level from the flame could be high. This affects the surrounding vegetation and habitation. e) Ground level concentration (GLC) High GLC of combustion products or unburnt hydrocarbons causes damage to human health, deterioration of materials and structures or creation of an area where plant/animal species cannot survive. With the growing consciousness of people in every country regarding environmental protection, there is a lot of pressure to come up with better flare systems to eliminate the problems mentioned above. ATMOSPHERIC VENTING: Another widely used disposal method is venting or direct discharge to atmosphere. There is some difference of opinion on the question of what may and what may not be vented to atmosphere. It is usually considered safe to vent hydrocarbon vapours or gases lighter than air, except hydrocarbon vapours or gases lighter than air, except hydrogen. Some designers recommend venting of vapours even as heavy as propane, provided it is vented from an elevated vent stack at a high velocity. The vent stack height is selected so that the
concentration of vapour at grade is well below the lower flammable limit of the vapours. The reliability for estimation of ground level concentrations is questionable and therefore flaring is the only satisfactory method for safe disposal of hydrocarbons heavier than air. 2..0
o
o
o o
CAUSES OF FLARE LOAD: Pressure Vessels, Heat Exchanges, operating equipments and piping are designed to contain the system pressure. The design is based on the normal operating pressure at operating temperature, the effect of any combination of mechanical loadings that is likely to occur, and the differential between the operating and set pressures of the pressure relieving device. Overpressure results because of an unbalance or disruption in normal flows of material and energy that causes accumulation or loss in certain part of the system. The process system designer must define the relief required to prevent the pressure in any piece of equipment from exceeding the maximum allowable accumulated pressure. The causes of over pressures are listed below: Closed outlet on Vessels The inadvertent closure of a block valve on the outlet of a Pressure Vessel while the plant is on stream may expose the vessel to a pressure that exceeds the maximum permissible working pressure. If closure of valve results in Overpressure, a Pressure Relief valve system is required. Inadvertent Valve Opening: In advertent opening of any valve from high pressure system to low pressure system may cause an Overpressure in connecting lines or receiving system. A pressure relief valve is required to protect the system. Check valve malfunction: The failure of check valve to close the reverse flow must be considered where potential of back flow of high pressure fluid exists. Utility Failure: Loss of any utility service, whether unitwide or local may cause overpressure. A potential failure may also lead an equipment to overpressure. In most of the case the utility failure governs the maximum failure load. Possible utility failure & equipment affected are listed below: Utility Failure Electric Cooling Water Inst. Air Steam Fuel (Oil, Gas) Inert Gas
o
Partial Failure:
Equipment Affected Pumps for circulating cooling water, boiler feed water, quench or reflux. Fans for air cooled exchanges, Cooling towers or combustion air. Condensers for Process or Utilities Service coolers for Process fluids, lubricating oil or seal oil. Transmitters and Controllers, Process regulating valves, Alarm and shutdown system Turbine drivers for Rotating Equipment, Reboilers, Reciprocating pumps, Ejectors Boilers for Process steam, Reboilers, Gas Turbines, Compressors Seal, Catalytic reactors
o o o
o o o
o
3.0
be 3.1
Partial preotectice credit can be taken for normally operating parallel equipments or stand by services that have two unrelated source of energy to the drivers. Standby services cannot be considered totally reliable hence it does not suffice insurance against overpressure. Electrical or Mechanical Failure: The failure of electrical or mechanical equipment that provides cooling or condensation in process streams can cause overpressure in process vessels. Reflux Failure: Reflux failure causes condensers flooding or loss of coolant resulting in an overpressure. Heat exchanger tube failure: When tube side pressure of an exchanger is much lower than the shell side, rupture of tube will overpressure the tube side. Thermal shock, vibration, corrosion may also cause tube failure. This must be protected with a pressure relief valve., Chemical reaction: Over pressure may occur because of unbalance reaction in a reactor. Sophisticate safety valves are used in these cases. Hydraulic Expansion: Thermal relief valves are provided to overcome overpressure because of rise in temperature. Plant Fires: Any process equipment in an operating plant that handles or processes fluid may get exposed to fire. Continuous vapour generation will lead to over pressure in system. Pressure relief valve is required to protect the vessels releasing the generated vapor. Local Power Failure: There might be local power failures in units/equipments which will cause disturbances to local equipments. This may result in over pressure in system. DESIGN GUIDELINES The permissible limits on pollution, radiation and other effects on environment have been laid down considering various aspects. Although there is no definite quantitative limit on flare and smoke emission from flare, most authorities demand that these should minimum. For thermal radiation, noise and ground level concentrations following guidelines should be followed: Radiation: Maximum permissible level is a function of the length of exposure, factors involving reaction time and human mobility. Following limits of heat intensity radiations should be observed: Permissible Design Level (K) Conditions KW/m2 Btu/Hr- ft2 1.58 500 Continuous exposure with appropriate clothing 4.73 1500 Emergency action lasting for several minutes for personnel with proper clothing but without shielding. 6.31 2000 Emergency action lasting upto one minute without shielding but appropriate clothing. 9.46 3000 Radiation level at any location where people have access.
For example at grade below the flare or a service platforms nearby tower) exposure should be limited to a few seconds sufficient for escape only. 15.77 5000 Radiation level on structure and in areas where operators are not likely to be performing duties and where shelter from radiating heat is available (for example behind equipment). Note: On towers or other elevated structures where rapid escape is not possible, ladders must be provided on side away from flare when K is greater than 6.31 KW/m2.(Reference API RP - 521, Fifth edition). The heat radiation level for personnel are with account taken of reaction time and human mobility time of between 8 and 15 seconds from the onset of heat radiation. In emergency release, a reaction time 3 - 5 seconds may be assumed and 5 - 10 seconds would elapsed before individual could seek cover or deport from the area, which would result exposure time 8 - 15 seconds. Exposure time necessary to reach the Pain threshold. Radiation intensity Time to pain threshold BTU/hr-ft2 KW/m2 Seconds 550 1.74 60 740 2.33 40 920 2.90 30 1500 4.73 16 2200 6.94 9 3000 9.46 6 4700 11.67 4 6300 19.87 2 The radiation levels take into account the influence of clothing, but excludes the contribution of wind chill factor, ability of personnel to face away from the radiation source and lack of large open areas on offshore platforms. The heat radiation figures are exclusive of an allowance for solar radiation. In our country solar radiation would account for about 350 Btu/Hr ft2 heat and it is recommended that for continuous exposure solar radiation should be taken into consideration. In that case allowable heat radiation by combustion should be taken as 500 - 350 = 150 Btu/Hr - ft2. For other conditions for calculations solar radiation should not be subtracted. 3.2
Noise: The exposure 1 minutes for noise prescribed in some of the well accepted standards are given below: Daily exposureOSHA Noise Regulations NIOSH Noise Regulations Hrs dBa dBa 8 90 85 4 95 90 2 100 95 1 105 100 0.5 110 105 0.25 115 110
3.3
GLC of toxic combustion products: Threshold limits of GLC for certain toxic substances (gases and vapors) are as follows: Gas or Vapor PPM Gas or Vapor PPM Acetaldehyde 200 Acetic acid 10 Acetic anhydride 5 Acetone 1,000 Acrolein 0.5 Acrylonitire 20 Ammonia 100 Amyl acetate 200 Amyl alcohol 100 Aniline 5 Arsinic 0.005 Benzene 35 Benzyl chloride 1 Bromide 1 Butadiene 1000 Butyl alcohol 100 Butylamine 5 Carbon dioxide 5000 Carbon disulfide 20 Carbon monoxide 100 Carbon tetrachloride 25 Chlorine 1 Chlorobenzene 75 Chloroform 100 Cresol (all isomers) 5 Cyclohexane 400 Cyclohexanol 100 Cyclohexanone 100 Gas or Vapor Cyclohexene Diacetone alcohol 1,1 Dichloroethane Diisobutyl ketone Dimethylsulfate Ethyl acetate Ethylamine Ethyl bromide Ethyl ether Ethylenedramine Ethylene dichloride Fluorine Gasoline Hydrogen selenide Isodine Isopropylamine Methyl acetate Methyl alcohol 200 2-Methyloxyethanol
PPM 400 50 100 50 1 400 25 200 400 10 100 0.1 500 0.05 0.1 5 200 25
Gas or Vapor Cyclopropane O-Dichlorobenzene Diethylamine Dimethylaniline Diethylene dioxide Ethyl alcohole (ethanol) Ethylbenzene Eltyl chloride Ethylene chlorohydrin Ethylene dibromide Ethylene oxide 100 Formaldehyde Hydrazine Hydrogen sulfide Isophorene Mesityl oxide Methyl acetylene Methyl bromide 20 Methyl chloride
PPM 400 50 25 5 100 1000 200 1000 5 25
Methylyclohexane Methylcyclohexanone Methyl amyl alcohol
500 100 25
Methylcyclohexanol Methyl formate Methylene chloride
100 100 500
Naphtha (coal or tar) pNitroaniline Nitrobenzene Nitrogen dioxide
200 1 1 5
Naphtha (petroleum) Nickel carbonyl Nitroethane Nitromethane
500 0.001 100 100
5 1 20 25 50 1000 100
Nitrotoluene Ozone Gas or Vapor Propyl ketone Phenylhydrazine Phosphine Propyl acetate Propyl ether Pyridine Stbine Sulfur dioxide Sulfur monochloride 1,1, 2,2-Tetrachloroethane Toluene (toluol) Trichloroethylene 4.0
4.1
5 0.1 PPM 200 5 0.05 200 500 10 0.1 10 1 5 200 200
Octane Pentane Gas or Vapor Phenol Phosgene (carbonyl chloride) Phorphorous trichloride Propyl alcohol Propylene dichloride Quinone Styrene Sulfur hexafluoride Sulfur pentafluoride Tetranitromethane o-Toluidine
500 1000 PPM 5 1 0.5 400 75 0.1 200 1000 0.025 1 5
TYPE OF FLARES The ideal flare is a device that burns hydrocarbons completely. The combustion should be smokeless, least noisy and with minimum inconvenience to community in terms of radiation and luminosity aspects of flare. Based on specific requirements various flaring concepts have emerged and are being practised. Various types of flare are: Elevated flares: Conventional pipe flare: Conventional flare consists of a pipe stack with a burner on top. Pilot burners remain lit near the burner, which ensure burning of hydrocarbons from the flare stack during upset conditions. Some of the common features of conventional pipe flares are problem like smoke, flame lift off and high purge requirements. This type of flare systems are simple, proven and suitable for varying loads. The noise level is also very less due to low exit velocities. Coanda Effect Flares: Skin-adhesion effect, discovered by Henri Coanda, is used in this type of flares. This effect introduces both air and turbulence in the flame area, which ensures complete combustion. High pressure gas ejected from a narrow slot follows a profile of curved surface. The Coanda profile aids in formation of a hollow cylinder of a gas and entrains air upto twenty times the gas volume. The main feature of this type of flare systems are shorter flame length, stable, flame, smokeless combustion and gives lower heat emissivity. The other advantage is that these can handle small quantities of liquid droplets. However, this requires high pressure gas, having low turndown capabilities, gives higher noise level and requires higher maintenance. Jet Mix Flares: These are available in various configurations. Multiple nozzles with several holes, wings with slotted openings or double angle drilled nozzles produce swirling action to enhance air entrainment. Instead of air going to the gas, the flared gas is spread out into ambient
air at high velocity, promoting rapid, turbulent mixing with air and thus ensuring smokeless combustion. The advantages of this type of flares are, giving short, stable, smokeless flame and low emissivity. These are provided with wind shielding and hence are less affected with wind velocity and direction. However, the problems associated are of having complicated configuration, having limited turndown capabilities, noisily combustion and higher pressure drops. Air-Assisted Flares: These flares incorporating special mixing heads that increase gas to air interface. Air is blown into the central duct allowing unrestricted passage for the low pressure relief gas flowing up the annular duct. The air is blown with the help of a axial fan. The only utility required are power for the fan and fuel gas for the pilot burner. Air assisted flares can handle heavier gases, gives low radiation and glare and having longer tip life. The disadvantages of this type of flare are, it requires utility source, having high operating cost and complex controls are required for efficient operation.Inclusion of a rotating equipment to supply combustion air reduces the system reliability. Steam Assisted Flares: These flares achieve their smokeless capabilities by mixing steam with the flare gases. The steam is introduced at one or several points to induce secondary air and turbulence to improve mixing. In addition to rendering the flame smokeless, steam reduces the flame temperature, thus reducing thermal radiation. Steam assisted flares also can handle heavier gases and have longer tip life. The steam requirement would depend on flare gas molecular weight. The operating costs are higher due to steam consumption. Multi-burner flares: The gas stream is split and burnt in multiple burners installed on common stack. This reduces flame length and thickness while increasing the flame surface and helps air or steam to penetrate the flame easily, ensuring efficient and improved flame characteristics. 4.2
Ground Flares: In ground flares, combustion occurs at grade, typically inside refractory lined steel enclosures of circular or rectangular section. Flaring load is staged and each stage comprises of multiple jets. Due to high cost and capacity limitations, these are often used in conjunction with elevated flares. There are essentially designed to dispose off small continuous loads with smokeless, invisible and less noisy combustion. Ground flare is basically installed for: Concealing combustion of continuous load Small load from cost considerations preferred. The use of ground flare requires a clear distance of about 150 M from any processing unit or storage facility. The flow rates which can be handled by a ground flare are generally constructed of fire brick or of carbon steel lines with refractory material.
5.0
FLARE SYSTEM COMPONENTS AND CONCEPTS: The essential elements of any flare system are: o Flare header o K.O. Drum o Water seal drum
o Flare gas seal o Flare tip o Ignitor The basic scheme with components is shown in sketch 5.1. The flare header collects the material relieved by safety valves for safe discuarge to the flare.The maximum allowable back pressure would be: 10% of the set pressure for conventional safety valves 50% of the set pressure for balance bellow type safety valves 70% of the set pressure for pilot operated safety valves (Maximum allowable back pressure and set pressure are expressed as gauge pressures). The number of flare headers required is decided based on temperature, allowable pressure drop, properties of hydrocarbon, moisture content etc. If there is a possibility of liquid discharge into the flare system a K.O. Drum is provided immediately adjacent to process unit. For process plant handling light hydrocarbons like C1, C2, C3, C4 the liquid blow down is also sent to Flare I.S.B.L. knock out drum. A suitable Vaporizer Armstong type or indirect methanol is used for vaporizing the liquid hydrocarbon. In case, very low temperature gases from a particular unit is put to a common flare header in offsites due to economic reasons, a flare gas superheater on the low temperature header may be required to heat the gases. In addition to this a K.O. Drum is needed near the flare stack to trap any liquid condensed en route to the flare. This is provided to prevent hazards associated with burning liquid droplets escaping from the flare stack. Considerable quantities of liquids can accumulate in flare systems because of the dew point characteristics of the complex mixture of vapours discharged at high temperatures. “As the high temperatures vapours move through the system, there is drop in temperature due to heat loss to system and surroundings. When these vapours cools to their dew point, liquid droplets are formed, K.O. Pot is designed for removal of liquid and also adequate space is provided for removed liquid. A water seal drum is provided at or near the base of the flare stack as a safeguard against ingress of air in the flare system. Air upon entering the flare system mixes with the combustible gases within the system and mixture may be with in the explosive range resulting in some serious explosion. There are numerous sources of entry of air to the flare system. When the flare system is fairly static and flow is either very small or non existent and flares stack is filled with either low molecular weight gases or gases at high temperature the draft would exist at the base of the stack. With this state of less than atmospheric pressure with the flare system it is obvious that air will leak into the system if there is opportunity for it to do so. The water seal prevents damage to the system upstream of the water seal drum. For protection of flare system downstream of the water seal, i.e. flare stack itself, entry of air from the flare outlet is prevented by: Continuous purge with oxygen free gas and Use of gas seal. To avoid a static condition of flow through the flare stack, continuous purge is maintained. Purge gases may be any oxygen free gas which would not reach its dew point under any condition of flare operation. Natural gas, inert gases and nitrogen are commonly used, but steam is not at all satisfactory as a purge gas. Calculated volumes of purge gas should enter the flare system to ensure sweeping of the system. There is divergent opinion as to the volume of purge gas which may be required. The use of gas seals installed
immediately below the flare tip are capable to restrict entry of the air into the flare system. Various types of seals are available as discussed below: The fluidic seal consists of a series of fixed baffles shaped like open ended cones (Ref. Fig - 5.2) within a flare tip. Each succeeding baffle encountered by waste or purge gas traveling aperture than below it. The resultant effect is similar to that of venturi with little drop in gas pressure. To force the air into the flare stack about 4 to 5 times the pressure of purge gas is required and hence atmospheric air can not enter the stack through the fluidic seal. The molecular seal is shown in fig. 5.3. The purge gas is forced to make two 180 o bends as it flows through the seal. The air thus encounters a trap caused by the difference in molecular weight between air and the purge gas. If the purge gas is lighter than air, the trap forms in the upper portion of the inverted cylinder. If the purge gas is heavier than air, the trap is formed in the lower portion of the outer cylinder. The seal is effective even when nitrogen is used as a purge gas. The molecular seal is heavier and large as compared to the fluidic seal because of the complexity of its internal passage. This necessitates a stronger and costlier support structure to carry the additional weight and wind resistance. Molecular seal operates at reduced efficiency if its baffles are partially filled with water or pieces of refractory from the flare tip. A drain is used for water removal, but the refractory removal could require costly maintenance. Fluidic seal due to its simplicity of construction, light weight and lower initial cost, is preferred. However for very large diameter flare stacks (48" and higher) there is a belief that molecular seal reduces oxygen ingress in flare stack. Flare tip a proprietary item, made of heat resistant alloy steel is specially designed to provide a stable and quiet flame of the desired smokeless capacity.
6.0
DESIGN OF FLARE SYSTEM
6.1
Flare Load: There are various reasons due to which hydrocarbons and/or other materials will be relieved to the flare system. During each such emergency, its quantity and is not necessarily the largest number of Kg/Hr., but it is the flow which imposes the greatest head loss while flowing thru the system. Pressure drop through the header is proportional to the square of the mass flow rate and inversely proportional to the density. It is also possible that during various emergency conditions different safety valves discharge and hence the back pressure allowable may be different. Flare load from one plant: Flare load from one plant is estimated considering: Flare load Temperature and molecular weight of gases Permissible back pressure The load is estimated for various emergencies and to arrive at design flare load, one emergency is considered at a time in one plant. Flare Load for a Complex: To define the system load, the simultaneous occurrence of two or more unrelated contingencies need not be assumed. For example, it is generally not necessary to assume
blocked outlets on process systems under fire conditions. However, under some arrangements of process equipment, a fire could possibly result in a failure of local wiring or instrument air piping, leading to the closure of valves that block off the process system. Each individual contingency should therefore be reviewed for possible resultant effects. Particular study is required for cases of failure of major utilities, such as power or cooling water. Complete failure of electrical power, cooling water, or steam to an entire plant should be considered. Where utility sources are believed to be unreliable or are not backed up by a spare unit, the effect of complete failure should be studied. This type of study, with reference to electrical power failure, commonly results in a design based on the failure of one bus, although loss of an entire distribution center or of the incoming line is occasionally used as a basis for design. The capability of steam system to pick up standby turbine loads should be reviewed in conjunction with the overall installed boiler capacity and the normal standby capacity immediately available. The most common basis for analyzing water or steam failure is the failure of one lateral rather than the entire supply. Instrument air failure is commonly considered to be a plant wide failure unless automatic makeup from an uninterrupted source is provided. Failure of the power supply to electronic or electrical instruments may also be considered plant wide unless proper standby power supplies are provided. To define the combined relieving loads under fire exposure, the probable maximum extent of a fire should be estimated. This may be done on the basis of the actual layout of facilities, considering the location of sources of combustibles, the provision of drainage, and the effects of natural barriers. Facilities that handle only gaseous fluids may be assumed to generate more localized fires than those that handle liquid combustibles. In the absence of any other governing factors, consideration of a fire incident is frequently limited to 230 square meters to 460 square meters.
6.2
Flare Load Reduction in a Complex: Flare load in a plant can be considerably reduced based on following concepts: Cooling water segregation in plant so that simaltaneous cooling water failure in different units can be avoided. Segregation of Power Distribution through various Sub-Stations.( Refer design basis of HPCL Mmbai.) Flare load reduction based on INTERLOCKs. ( Refer section 10.0 ) ( ex. When Cooling water failure occurs in a column , steam supply to col. Bottom reboiler will stop immediately.) Flare Header: The flare header must be sized so that the built-up back pressure at the outlet of relief valves does not exceed the maximum permissible value. In some cases if allowable back pressure is governed by less load and for major flare load higher back pressure is allowable then two parallel headers one LP and other HP are run if it is economical. Flare piping system 1) Individual discharge lines from PSVs. 2) Subheaders in each area connecting to discharge lines. 3) Main flare header leading to KOD. 4) Final header connecting the vapour line to flare stack. Sizing Flare Headers 1) Back pressure i.e. 10% of set pressure in psig for conventional type and 40% for balance type valve.
2) To avoid noise problem, velocity is limited. General practice (.2 - .5) Mach number is selected. Sonic velocity Vc = 223 / KT/M Vc in ft/sec K = Cp/Cv normally T = oR M = Mol. Wt. When dia of KOD exceeds 12 ft, split flow arrangement is economical For estimating back pressure at safety valves the following losses are to be added: Stack exit loss Gas seal loss (Take p = three 90 o bends of diameter equivalent to flare stack) Static head due to stack heights Entry loss to stack Entry/exit loss to water sealdrum Seal height in water seal drum Entry/exit loss to K.O. Drum Flare header drop The emergency discharge is normally at high temperatures and flare header runs many hundred meters. This results in drop in flare gas temp due to heat loss to metal and surroundings. In order to estimate pressure loss through flare headers, the total header length is divided into a number of sections and pressure drop should be calculated for each section separately for each section. A check should be made to ensure that the maximum velocity in the flare header is well below sonic velocity. Generally maximum velocity is (0.4 - 0.5) of sonic velocity is acceptable. Sonic velocity is estimated as follows: Sonic velocity = 39.3 (gkt/M) ½ ft/sec Where,g= Acceleration due to gravity 32.2 ft/sec2 K = cp/cv for the flare gases T = Temperature of gases oR M = Avg. Molecular weight of flare gases. R = 1.8 K R = 460+ F In some cases corrosive gases like H2S or explosive gases like acetylene are to be handled which should not be connected to the common header. For such gases a separate header is run to the flare stack area. A separate water seal drum is provided with separate stack and is joined with the main stack below the gas seal. 6.3
Flare Stack : Flare stack is located on downwind pile and at a remote place from operating zones.The sizing of the flare stack requires the determination of stack diameter, height of stack and flare length. Hence one simple approach for flare sizing is described: Height of flare stack depends upon following: 1) Heat released by the flare gas in KW. 2) Characteristics of flame and flame length 3) Emissivity of flame 4) Reduction intensity of flare 5) GLC of toxic gases
Radiation Intensity Emissivity valve
for peak load .1 Normal operation .3 2 2 2 D =x +H x = 20 te 20 ft/s = velocity of escape te = time of escape to a safe location For a high temp system, a separate subheader may be required upto the point, where the temp drops down to the allowable limit. A heat loss of 10 BTU/hr/ft2 may be taken as thumbrule for heat loss calculations. For low temp system, flare subheaders may similarly combined into a single low temp flare headers and piped all the way to flare stack. Here, a heat gain calculation is done to find out, whether subheader can be connected to main header or not. SIMPLE APPROACH FOR FLARE STACK SIZING Basic data Material flowing is HC vapour W = Flow rate (kg/hr) MW = Avg molecular weight T = Temp. Of fluid flowing (K) Z = Compressibility factor (Generally z = 1.0 is taken) P2 = Pressure at the tip d = Flare diameter (m) (inside) K = Ratio of specific heat H = Heat of combustion (Kj per kg) Calculation of Diameter Mach No. = 3.23 * 10-5 ( W / ( P2 * d2 )) (Z *T )/ ( K*MW)) 0.5 Flare stack dia is generally sized on velocity basis , although pressure drop should be checked. Velocity upto 0.5 Mach may be permitted for peak load and short term. 0.2 Mach is maintained for more normal and possibly more frequent conditions for low pressure flares, which depends on following criteria: a)Volume ratio of maximum conceivable flare flow to anticipated average flare flow. b) The probable timing , frequency, and duration of those flows. c) The design criteria adopted for the project to stabilize flare burning. However , for high pressure flares, a higher Mach no. can be taken. Calculation of flare length The heat liberated, Q is calculated as follows: Q = Flow rate x heat of combustion = (W/3600)* ( H) From Fig. A flare length L (m) can be found out. Calculation of Required Flare Stack: Basis: See Fig. C for dimensional reference. Sizing of flare stack is based upon the effects of radiation. The following equation by Hazen and Ludwig can be used to determine min distance from a flare to an object whose exposure to thermal radiation must be limited:
D
=
( *F*Q / (4* *K))
D
=
min distance from the mid point of flame to object being considered (in meters) = fraction of heat intensity transmitted F = fraction of heat radiated Q = heat release (lower heating value) (KW) K = Allowable radiation (KW/m2) For initial calculation may be assumed 1.0 According to Brzustowbsui & Summer, use of the fraction of heat intensity transmitted, to correct the radiation impact. = 0.79 ( 100 / r ) 1/16 (30.5 / D) 1/16 = fraction of heat intensity transmitted through the atmosphere. r = relative humidity, percent D = Distance from flares to illuminated area (m) ( see fig. C) Above equation satisfy most of flare gases except H2 & H2S which burn with little or no luminous radiation. The F factor allows for the fact that not all the heat released in a flame can be transferred by radiation. Measurement of radiation from frames indicate that the fraction of heat radiation (radiant energy per total heat of combustion) increases towards a limit, similar to the increase in the burning rate with increasing flame diameter. For peak flare load F = 0.1 normal flare load F = 0.3 Estimate flame length from Fig A indicates flame radiation center being at the flame mid point. A flame under influence of wind will tilt in the direction of wind is blowing. The lateral wind effect can be estimated from Fig.B which relates horizontal and vertical displacement of flame center to the ratio of lateral wind velocity to stack velocity. Flame length varies with emission velocity and heat release. Flow in (m3/s) Can be estimated based on ideal gas law: U = Wind velocity Uj Flare tip velocity Uj = Flow (m3/s) (( *d 2 )/4) Another method to calculate the tip exit velocity sonic velocity = 91.2 ( k* Tj / M j ) 0 . 5 ; U j = jet Mach no. * sonic velocity From Fig. B Y/ L &
Y/L
can be estimated
Where L is flame length From Hazen and Ludwig equation , D is calculated. Now corresponding to desired Mach No. stack height, H , is calculated as follows: H’
=
H+½
Y
R’
=
R-½
x
D2
=
(R’) 2 + (H’) 2
R is estimated as per design basis. This corresponds to maximum allowable radiation K at particular distance from flare stack. From above equations H is calculated. 6.4
Sizing a Knockout Drum Sizing a knock out drum is generally a trial and error process. The first step is to determine the drum size required for liquid entrainment separation. Liquid particles will separate (a) when the residence time of the vapor or gas is equal to or greater than the time required to travel the available vertical height at the dropout velocity of the liquid particles, and (b) when the vertical gas velocity is sufficiently low to permit the liquid dropout to fall. This vertical height is usually taken as the distance from the liquid surface. The vertical velocity of the vapor and gas should be low enough to prevent large slugs of liquid from entering the flare. Since the flare can handle small liquid droplets, the allowable vertical velocity in the drum may be based on that necessary to separate droplets from 300 micrometers to 600 micrometers in diameter. The dropout velocity (9) of a particle in a stream is calculated using Equation 30 as follows: Uc
=
Uc g D p1
= = = =
1.15 {g D( l -
v
)}/ ( v *C)
dropout velocity, (meters per second) acceleration due to gravity, (9.8 meters per second per second). Particle diameter, (in meters) density of the liquid at operating conditions, (kilograms per cubic meter) Pv = density of the vapor at operating conditions, (kilograms per cubic meter) C = drag coefficient (see Figure 20). The basic equation is widely accepted for all forms of entrainment separation. The second step in sizing a knock out drum is to consider the effect any liquid contained int he drum may have on reducing the volume available for vapor/liquid disengagement. This liquid may result from (a) condensate that separates during a vapor release, or (b) liquid streams that accompany a vapor release. The volume occupied by the liquid should be based on a release that lasts 20 to 30 minutes. Any accumulation of liquid retained from a prior release (from pressure relief valves or other sources) should be added to the liquid indicated in items () and (b) to determine the available vapor disengaging space. However, it would usually not be necessary to consider the following volumes relative to vapor disengaging in the following situation that in which the knock out drum is used to contain large liquid dumps from pressure relief valves from other sources where there is no significant flashing and the liquid can be removed promptly.
Economics of vessel design in selecting a drum size, which may influence the choice between a horizontal and a vertical drum. When large liquid storage is desired and the vapor flow is high, a horizontal drum is often more economical. Although horizontal and vertical knockout drums are available in many designs, the differences are mainly in how the path of vapor is directed. The various designs include the flowing: a. A horizontal drum with the vapor entering one end of the vessel and exiting at the top of the opposite end (no internal baffling). b. A vertical drum with the vapor inlet nozzle on a diameter of the vessel and the outlet nozzle at the top of the vessel’s vertical axis. The inlet stream should be baffled to direct the flow downward. c. A vertical vessel with a tangential nozzle. d. A horizontal drum with the vapor entering at each end on the horizontal axis and a center outlet. e. A horizontal drum with vapor entering in the center and exiting at each end on the horizontal axis. f. A combination of a vertical drum in the base of the flare stack and a horizontal drum upstream to remove the bulk of the liquid entrained in the vapor. This combination permits the use of large values for the numerical constant in the velocity equation. The following sample calculations have been limited to the simplest of the design. Items a and b. The calculations for items d and e would be similar, with one-half the flow rate determining one half the vessel length. The normal calculations would be used for item c and d will not be duplicated here. The following formula can be used for sizing horizontal drums for separation of 400 particle: W = 360* D2 [ (eL - eG)* M*P/ T ]½ where, W = lbs/hr of vapour eL = liquid density lbs/cu ft eG = gas density lbs/cuft M = mol. Wt. Of the vapor T = temperature of the vap in oR. P = psia, D = drum diameter in fit Similar expressions are available for vertical knock out drums. A practical formula for the vapor velocity is: V = 0.4 * (eL - eG) /eG ft/sec. 6.5
BLOWDOWN PUMP A blowdown pump is provided to transfer liquid from K.O. Drum. The pump capacity is put equal to the maximum possible condensation rate during peak flaring. However, the minimum pump capacity should be 5 m3/hr. The pump should have provision to start and stop automatically through level switches on K.O.D. The blowdown pumps selected should have minimum NPSH requirement and KOD should be elevated suitably to meet the NPSH requirement. The condensate in the KOD should be considered as subcooled for the NPSH (available) calculations. Typical NPSH (available) for the condensate pump is 2.0m and the KOD shall be elevated at .5m (min) from the grade. Higher elevation of
the KOD results in increased elevation of the flare header, thus increasing the system costs. 6.6
6.7
Water Seal Drum Seal is provided at the base of flat stack to prevent any flash back. In absence of seal a continuous entry of gas may be bled to flare. Seals are two types Liquid seal Gas Seal Liquid seals are further classified a Seal Drum & Seal pipe. In seal drum liquid seal is located. Pipe is an integral part of the stack. The purpose of seal drum is to maintain a seal of several inches on inlet flare header, not exceeding six inches otherwise, it may cause back pressure on KOPD. Water is normally used on sealing liquid in cold climate some extra provision is needed either water is heated or water is replaced by alcohol, kero etc. For designing water seal drum, Shell method is used. Refer attached fig. PURGE GAS REQUIREMENT It is essential to prevent air ingress into the flare system. The most acceptable safety measure is to ensure a positive flow thru flare system. It is normal practice in industries to supply gases to the flare system constantly to avoid a static condition of flow. These gases are referred to as purge gases. The quoted values of the minimum required purge gas velocity vary from as low as 0.06 to 3 ft/sec. However, there is general agreement that a commercially available gas seal installed immediately below the flare at the top of the stack establishes perfect safety to the flare system when the purge volume admitted is capable of maintaining stack velocity from 0.05 to 0.10 ft/sec. To be safer side it is recommended to use purge gas to have 0.1 ft/sec, stack velocity. Purge gas requirement in the flare header: With seal 0.1 ft/sec Without seal 1 ft/sec
6.8
SMOKELESS OPERATION OF FLARE Because of increasing stringent air pollution laws, smokeless flare operation is process plants has been preferred in the interest of good public relations. In the flaring of gaseous hydrocarbons, the tendency for smoke production as the gases burn is governed by the weight ratio of hydrogen to carbon in the gases but is not directly proportional to the H/C ratio by weight. There have been numerous approaches to the problem of smokeless burning of waste process gases. Since the principle means for suppression of smoke involves the various chemistries associated with reaction of water vapours with the components of the flared gas stream, there have been various schemes for delivery of the water to the burning zone as either steam or as water spray in one form or another. There has been certain limited success with water injection for smoke suppression due to the great nuisance of a spray of unvaporized water to grade in normal operation of the flare. Due to this reason steam is almost universally used for suppression of smoke in flare operation. Use of steam increases the burning rate by the creation of turbulence in the
reaction gases and the inspiration of air, thereby reducing the formation of soot. Soot formation is also reduced by the water gas reaction C = H2O = CO + H2, promoted by the addition of steam. The amount of steam for smokeless operation can be computed from: W steam = W HC (0.68 - 10.8/M) Where W steam = Steam rate lbs/hr W HC = Hydrocabron rate, lb/hr M = Molecular weight of hydrocarbon It is normally not possible to attain smokeless operation of flare at peak flaring loads, as the steam rates would be prohibitively high. The normal practice is to design the flare system for smokeless operation for any of the following reasons. 1. Normal flaring loads 2. Start-up or shutdown of a process plant, or 3. For any other situation which involves prolonged flaring. Flares for suppression of smoke are commercially available of proprietary designs., The manufacturer should be consulted on the minimum necessary steam rate. The stem for smokeless consumption can be put through a manual remote control valve or through an automatic smoke detector, located at the ground level, sending signal to the steam control valve. Total flare load is sum of governing loads from different units based on governing factor of respective units. 7.0 INHOUSE SOFTWARE AVAILABLE FOR FLARE SYSTEM SIZING Following programmes are available in LOTUS-123 in dir D:\FLR_MAN for variouscalculations. Name of file Function stak_cal.wk4 kod_cal.wk4
Stack dia calculation , press drop from flare tip to main KOD Horizontal KOD sizing
p_drop.wk4
Pressure profile calculation
T_drop.wk4
Temp. Profile calculation
Apart from above programs, Designer may refer to various packages available on PRODES. In a large chemical plant or Refineries having a very reliable power supply. Cooling water failure mostly governs the maximum flare load. In the following example, system has been segregated into two parts. Cooling Tower one supply CW to U-01, U-02, U-03, U-04 and CT-2 supplies to U-05, U-06, U-08. Cooling water failure is the governing case in each unit. Maximum fire load comes from CT-1 failure. So far design of flare header & stack maximum load corresponding to CT-1 is the governing load. According to governing load, flare stack dia and KOD have been sized. Individual unit location is specified based on plot plan. Accordingly, unit back pressure is calculated moving from KOD to individual units. Various headers & sub-headers sizes are optimized meeting the unit back pressure requirement. 8.0
Sample Calculation seets are attached herewith.
9.0
COMPARATIVE STUDY OF FLARE SYSTEMS OF DIFFERENT REFINERIES
HPCL (M)
IOCL (Panipat)
Power substation Cooling water failure is the MSS-5 failure is the governing governing case for flare case for Flare system design. header sizing. Only single largest local power failure is considered for checking the Governing load = 449 TPH flare header sizing. Governing load = 457 TPH Maximum allowable back pressure at unit B/L = 0.61 Max allowable back pressure kg/cm2g = 1.5 kg/cm2g at unit B/L Max. Allowable back press at PSV outlet = 0.71 kg/cm2g Flare header 48" (after WSD 54") Stack height = 100m Smokeless flaring capacity = 2 Tons/hr
MRPL-II For main flare header sizing and flare stack design, general electric power failure is considered. Governing load = 750 TPH. Max allowable back press. At unit B/L = 1.0 kg/cm2g
Max allowable back pressure = 1.7 kg/cm2g at PSV outlet
Max allowable back press at PSV outlet=1.2 kg/cm2g
Flare Header 54"
Flare header 54"
Stack height = 100m
Stack height = 100m
Length of Flare header= 3m Smokeless flaring capacity:
Smokeless flaring capacity = 110 TPH
15% of maximum load Water seal drum is an integral part of flare stack.
Water seal drum is an integral part of flare stack.
Horizontal water seal drum separate from flare stack.
Plant
Total governing load
Temp o C
Stack Dia/Ht (m/m)
WSD L/Dia. (M/m)
Main KOD L/Dia (m/m) 23/5.6
MW
Remarks
MRPL-II
750,000
133
1.7/100
8.4/2.8
43
Ref. attached DB for details
MRPL-I
284733
12.5/4.5
15.6
449100
50-100 1.37/ 8/4.5 100 112 1.37/100 7.2/4.8
HPCL(M)
10.8/3.6
63.2
-do-
457323
80
1 37/100 NA/4.5
13.5/4.5
34
-do-
43300
50
0.61/100 7.2/2.4
7.8/3.6
4.0
BPCL(M) IOCLPanipat CRL (DHDS)
DESIGN BASIS MRPL - II FLARE SYSTEM
10.
FLARE SYSTEM
10.1
INTRODUCTION Flare system shall be installed to burn the flammable, toxic or corrosive vapors safely which is discharged during startup, shutdown, normal operation and emergency cases. Dedicated flare system for Phase-2 is provided.
10.2
DESIGN CONSIDERATION
10.2.1 System Configuration Two flare collecting systems are provided. One is main (LP) flare header and the other is sour flare header. Hydrocarbon and sour flares released from the following units are discharged to the main (LP) flare header. (Note):
CDU/VDU/NSU Visbreaker unit Merox units Hydrocracker unit Hydrogen unit GO-HDS unit CCR/NHT unit (future) HP flare from Hydrocracker unit which is discharged independently in phase I shall be integrated with main (LP) flare header in phase 2.
Sour flares released from the following units are discharged to the sour flare header. -
Amine treating unit Sour water stripping unit Sulfur recovery unit
These two flare headers are connected at main flare knockout drum which is located near the flare stack. 10.2.2 Design Flare Load For flare system design, following cases are studied. Flare load for the various cases is summarized in the attached sheet. -
Cooling water failure
-
General electric failure
-
Fire case Blocked outlet
-
Reflux failure
-
Emergency Depressure
For the main flare header sizing and flare stack design, general electric power failure case is considered. Individual flare outlet from each units are sized as listed below. CDU/VDU
-
As per EIL Package. (General electric failure)
VBU
-
As per Phase-I (Reflux failure)
CCR + NHDT
-
General electric failure
GO+HDS
-
Fire case + Emergency depressure
MEROX
-
Fire case
HCU
-
As per Phase-I (Fire case + Emergency depressure)
ATU
-
Fire case
SWS
-
General electric failure
SRU
-
As per phase-1 (Fire case)
Hydrogen
-
As per phase-1 (PSA trip case)
Pressure profile and velocity for the flare header design shall be as follows. -
0.3 kg/cm2g at main flare KO Drum located near flare stack. 1.0 kg/cm2g at each unit BL 1.2 kg/cm2g at each SV back pressure
-
Velocity is less than Mach No. 0.4.
Pressure profile for the main (LP) flare header at general power failure case and for the sour flare header at ATU fire case is shown in the attached sheets. 10.2.3 Knock Out Drum Five (5) Nos. Of Flare Knock out drums are provided in phase-2 as per following groupings: 1)
Knock out drum in VBU area is receiving flare gas from -
VBU VBU/BLENDER MEROX
-
GO-HDS NHDT CCR
2)
Knockout drum in CDU/VDU/NSU area
3)
Knockout drum for HCU
4)
Knockout drum in SWS area is receiving flare gas form -
5)
ATU SWS SRU
Main knock out drum is receiving flare gas from all of above knock out drums.
Flare knock out drums are sized as per API RP 521. Design load for each KO drum is summarized as below: Knock out drum
Mol. Wt.
Temp. oC
Case
In VBU Area
Flow rate kg/hr 189684
37.54
175.4
General power failure
For HCU
108180
8.8
303
In SWS area
86600
50
123
Fire + Emergency depressure Fire case in ATU
Main knock out drum
741274
43
133
General power failure
10.2.4 Flare Stack Flare stack shall be designed based on the general power failure case of 750 t/h. Diameter and height of the flare stack shall be determined considering the ground level heat intensity in accordance with API RP 521. Maximum heat intensity at ground level shall be less than 4,000 kcal/m2.hr excluding solar radiation. Flare tip shall be designed considering smokeless capacity of 110 t/h which is corresponding to 15% of maximum flare load. 10.3
MATERIAL OF CONSTRUCTION - Sour flare header - Main (LP) flare header - All the knockout drum - Water seal drum epoxy cement coating - Flare stack
: : : :
Stress relieved killed carbon steel with 6mm CA Stress relieved carbon steel with 3mm CA Stress relieved killed carbon steel with 6mm CA Stress relieved killed carbon steel with 6mm CA and
:
Stress relieved carbon steel with 3mm CA
FLARE LOAD SUMMARY (PHASE-II REFINERY) Unit Name
Crude Distillation Unit (#41000) Vacuum Dist. Unit (#411000) Visbreaker Unit (#41200) Visbreaker Unit (#51200) Sulfur Recovery Unit (#41400) Sulfur Recovery Unit (#51400) LPG Merox (#41500) Kerosene/Jet Merox (#41600) Hydrocracker (Rx Sect) (#42000)
Cooli ng Wate r Pum ps Rate M W Kg/h 207, 50. 280 6
Gener al Electr ic Failur e Tem Rate M p W o C kg/h 81 283,6 69. 30 3
Tem Rate p o C kg/h 122 248,0 02
MW Tem Rate p o C kg/h 72 129 23,0 00
10,6 03 10,6 03
117
1,680
18
156
132
212
117
1,680
18
156
12,43 5 12,43 5 12,00 0
132
212
-
145/ 133
12,00 0
-
145/ 133
23,89 4 140,0 28
50
99
Capacity s
162
344
Capacity s
60,01 0
205
427
27, 4 27. 4
17,40 8
5
139
M W 68
Tem Rate p o C kg/h 207
19,21 4 19,21 4
8,31 6
2.1
146
M W
Tem Rate p o C kg/h
M W
Tem p o C
Eject steam kg/hr 100 oC, M VDU & N CDU According 007) 61
284
61
284
48,1 70
4
149
Hydrocracker (Frac.Sec) (#42100) Poly Arom Adsor ) #42200) Amine Treating (#42200) Sour Water Stripper (#42600) Hydrogen plant (#42300) Gas Oil HDS (#45500)
17,4 04
19,4 50
49
18
71
222,7 71
73. 3
14,9 49
45. 5
106
150 27,78 1
8,00 0
73
150
17,60 7
18
25. 1
157
119
Naphtha Merox (#45700) Naphtha Hydro-treater (#41700) CCR Platformer (#42400) 25,9 23
26. 1
71
59,46 6
10 0
227
109,2 51
30. 6
157
87,85 4
142
317
47,73 4
180
427
86,60 0
50
123
2,102
18
140
48,10 0
26.2
32,59 8
13.6 190
20,00 0 52,08 3 39,37 4
3308 7
57. 7
118
42,4 00
32
117
45415 77. 9
102
Based on P
Based on P
PSA trip c 41,500 kg 40,0 00
4.6
60
Pre estima Depress.
Pre estima 97.5 236
Pre estima for future
42.8 179
Preestimat for future External
Fire Total
937,249 kg/h General Power failure 741.274 kg/hr MW
43.0 133 oC CW failure total 314,212 Local power failure of HYC 288.349 kg/h
Temp.
DESIGN BASIS FOR IOCL - PANIPAT FLARE SYSTEM
FLARE SYSTEM 9.1
INTRODUCTION The flare system will be provided for safe disposal of combustible, toxic gases which are relieved from process plants and offsites during start-up, shutdown, normal operation or in case of an emergency such as: o o o o o o o
9.2
Cooling water failure Power failure Combined cooling water and power failure Instrument air failure External fire Blocked outlet/an open inlet Any other emergency
UNIT CAPACITY The flare loads from the following plants have been considered for the design of flare system: Process Unit Capacity, MMTPA 1.
CDU 6.0
2.
VDU 3.3 3.
OHCU 1.7
4.
FCCU
5.
H2 UNIT
6.
CRU
0.7 0.038 0.5 7.
VBU
8.
BBU
0.4 0.5 9. 10. 11. 12. 13.
Kerosine Merox SR LPG Merox FCC LPG Merox FCC Gasoline Merox Sulphur Block
9.3
FLARE LOADS
9.3.1 CDU/VDU For this Unit the flare loads are given below: Cooling Water System
Load kg/hr 207280
Local Power Failure
Mol wt. 65.04
Temp o C 81.09
Load kg/hr 501600
Mol wt. Temp o C 85.9 196
These loads have been taken from the package. 9.3.2 OHCU The flare loads for this unit is given below:
Fire Case (Atm col+ Stab + MTO strip Ref Drum) Load kg/hr 112447
Mol wt. 67.08
Temp o C 141
Coolin g Water Failure Load kg/hr 69872
Mol Tem. o wt C 55.9 98.4
Genera l Electri c Failure Load Mol kg/hr wt 33414 56.25 7
Tem o C 240
Emerg ency Depres surizati on Load Mol kg/hr wt 12661 3.62 2
Fire Case (Separator + Flash Drum)
Tem Load o C kg/hr 55 110585
Mol wt Temp o C 186.46 427
The loads given above are taken from safety valves relief summary for the OHCU given by UOP.
9.3.3 FCCU Cooling Water System Load Mol kg/hr wt. 40000 40.0
Temp o C 110
Local Power Failure Load kg/hr 90000
Fire Case Mol wt. Temp o C 50 210
Load kg/hr 40000
Mol wt. 376
Temp o C 427
The flare loads are taken from the preliminary estimates available form the licensor. 9.3.4 H2 UNIT The loads given below are taken from hydrogen unit flare load summary for GHP and the flare loads are prorated. Cooling Water System Load Mol kg/hr wt. 61231 13.65
Temp o C 50
Local Power Failure Load kg/hr
Fire Case Mol wt. Temp o C
Load kg/hr
Mol wt.
Temp o C
The flare load for different contingencies shall be updated when data from the hydrogen unit licensor M/s., HTAS is available. 9.3.5 CRU The data for the following contingencies is taken from the package: Cooling Water System Load Mol kg/hr wt. 70000 23.5
Temp o C 74
Local Power Failure Load kg/hr 105000
Fire Case Mol wt. Temp o C 83.8 133
Load kg/hr 51710
Mol wt. 40
Temp o C 105
9.3.6 VBU The following are the flare loads given for Visbreaker Unit and are taken from the process package. Cooling Water
Local Power
Fire Case
System Load kg/hr 8940
Failure Mol wt. 44
Temp o C 79.5
Load kg/hr 17100
Mol wt. Temp o C 58 199
Load kg/hr 23960
Mol wt. 64
Temp o C 125.8
9.3.7 BBU The offgases from this unit shall be incinerated in the incinerator provided near the unit. This unit shall not be connected to the flare system. 9.3.8 MEROX These unit are not considered to add-on to the flare load in the contingencies considered above. The flare load for these units will be furnished when data from the respective licensors are available. Cooling Water System Load Mol kg/hr wt. Hold
Temp o C
Local Power Failure Load kg/hr Hold
Fire Case Mol wt. Temp o C
Load kg/hr Hold
Mol wt.
Temp o C
9.3.9 SULPHUR BLOCK The flare loads given below for this block for different contingencies are as per the estimates available for this block (consisting of ARU, SWS unit and sulphur unit). The flare gases from the block shall be routed to the sour flare. The estimated flare loads are as follows: ARU. Cooling Water System Load Mol kg/hr wt. 25000 20.0
SWS Unit
Temp o C 150
Local Power Failure Load kg/hr Hold
Fire Case Mol wt. Temp o C
Load kg/hr Hold
Mol wt.
Temp o C
Cooling Water System Load Mol kg/hr wt. 1284.2 24.23
Temp o C 86.85
Local Power Failure Load kg/hr 13250
Fire Case Mol wt. Temp o C 18.01 160
Load kg/hr 5685
Mol wt. 46.2
Temp o C 140
Sulphur Unit The unit shall have its own incinerator and shall not be connected to the flare header.
9.4 S.N o
FLARE LOAD SUMMARY
Tem p oC 81.0 9 98.4
Load kg/hr 50160 0 15787 0
Mol wt. 85.9
Temp o C 196
Load kg/hr 112447
Mol wt 67.08
Tem p oC 141
Mol wt
Tem p oC
2.
Mol wt 65.0 4 55.9
Load kg/hr
CDU/VD U OHCU
Cooli ng Water Failur e Load kg/hr 20728 0 69872
128.4
234
110585
186.4 6
427
12661 2
3.62
55
3.
FCCU
40000 40.0
110
90000
30
210
40000
376
427
4.
H2 Unit
50
5.
CRU
61231 13.6 5 70000 23.5
83.8
133
51710
40
105
6.
VBU
8940
79.5
10500 0 17100
58
199
23960
64
125. 8
50
55
46.2
140
1.
Unit
Local Power Failure
44
74
Others
Contingency
Fire
MEROXE S 8. OFFSITE 62400 S (Note-1) 9. ARU 25000 20 150 HOLD HOLD SWS 13250 18.01 160 5685 Sulphur Unit 10. TOTAL(2) 45732 -34 -80 3 Figures in this block represents governing flare load from the respective units.
Emergency Depressurizati on
7.
LPG Sphere 1284.2
24.2 3
86.8 5
SRU down
Note 1:
In case of fire at MT bullets or Hydrogen bullets they shall also discharge to flare.
9.5
FLARE PHILOSOPHY
9.5.1 The cooling water failure becomes the governing load for the flare header sizing. 9.5.2 Only single largest Local Power failure shall be considered for checking the flare header sizing. 9.5.3 For different units the General electric failure shall not be considered as governing load as the feed to the furnace/feed pumps shall also cease leading to a low flare load. The only exceptions are OHCU and CRU, releasing high load. However this load is not governing. 9.5.4 There shall be two separate flare headers. One carrying the discharges from equipments handling high H2S contents hydrocarbons i.e. Sour flare header (carrying reliefs from the sulphur block) and the second one carrying rest of the hydrocarbon reliefs i.e. Main flare. 9.5.5 A maximum allowable back pressure of 1.5 kg/cm2g at unit battery limit shall be considered for sizing the flar header. However a maximum allowable back pressure of 1.7 kg/cm2g shall be considered at PSV outlet. 9.5.6 For sour flare a maximum allowable back pressure of 0.5 kg/cm2g shall be considered at Sulphur Block battery limit for SRU down case, however for Local power failure case and fire case a back pressure of 1.5 kg/cm2g shall be considered at sulphur block battery limit. 9.5.7 EIL had suggested routing all H2S bearing streams to the sour flare but IOCL decided to route only the Sulphur block reliefs to the sour flare header. 9.5.8 Smokeless Capacity: The flare shall be designed for a smokeless capacity equivalent to 15% of maximum anticipated fuel gas production (i.e. 15% of 12.3 TPH mol wt. 8.95) i.e. - 2 TPH. 9.5.9 Flare header shall be purged by all units maintaining a purge velocity of 0.1 fps in respective unit headers with fuel gas. 9.5.10 For monitoring flare losses, metering instrument will be provided in each unit block area and also on the main flare header. Type of instruments will be decided during P&ID stage. 9.6
THE SYSTEM The main flare header shall collect the emergency hydrocarbon reliefs from all the equipments (except equipment discharging to the sour flare header) and shall route it to main flare stack. The size of this flare header works out to be 54". To take care of the short term H2S rich reliefs the flare header shall be post weld heat treated and hardness of 200 BHN maintained. The flare stack diameter shall be 54" and height 100m. This Flare system shall have its own dedicated set of flame front generators and flame pilots. The length of this flare header is ~3 KM. To keep the height of flare header within reasonable limits and to reduce the cost of associated flare header structure, it is proposed to have one knock out drum enroute flare stack. The knock out drum shall be located
near BBU. It shall have second knock out drum near flare stack. A vertical water seal drum shall be provided at the bottom of the stack. The water seal drum is an integral part of the flare stack. It is not possible to take water seal drum under maintenance without shutdown of entire complex. Thus to avoid any such situation all the internals of water seal drum shall be of SS-304. The sour flare header shall have a knock out drum in the Sulphur block. The gases will then be routed to the flare area. The gases shall be burnt alongwith the main flare gases through a separate header near the tip of the main flare. Sour flare header shall be post weld heat treated and shall have minimum 200 BHN hardness. This flare header shall have a KOD near flare stack. This will not have any water seal drum. A separate fluidic seal, separate Flame Pilots shall be provided for this flare. The flame front generators shall be common for two flares. A schematic for flare gas collection header is given under Annexure 1.
DESIGN BASIS FOR HPCL, MUMBAI FLARE SYSTEM
8.0
FLARE SYSTEM
8.1
INTRODUCTION At present several safety valves from existing units are venting to atmosphere. The existing flare system is incapable of handling this additional load. A new flare system is envisaged to handle safety valve reliefs from the existing units presently connected to flare as well as venting to atmosphere and DHDS block.
8.2
FLARE LOADS Safety valve discharges for cooling water failure case from various units are given below: Units FRE APS
220
70
150
-
FRE VPS
7.8
20.3
150
-
HMU
36.5
70
100
-
T-1701
27.3
70
150
-
FR-APS
272
70
150
-
FR-VPS
7.8
20.3
150
-
FCCU
139
70
100
-
Propane Unit
35 50 32.2
50 50 77.7
-
T-801 (Deeth)
5.5 11.6 15.2
T-802 (Debut)
85
50.5
68.1
-
MEA Regeneration*
4.5
22.5
125
-
T-701 (PG VPLG)
52
70
150
-
DHDS
2.7
37
75
-
Hydrogen
19.2
12.32
35
-
3.5
31.5
65
Lean oil failure
SWS* ARU* SRU* *
Sour gases being routed through a separate acid header. Notes: 1. For Blocked outlet/fire case flare load details refer flare study report (dated Sept 1996) submitted to HPCL - Mumbai.
8.3
GOVERNING FLARE LOAD Power is distributed in different units in the fuel refinery & FR cooling towers through various substations viz. MSS-1, MSS-4 & MSS-5. After MS maximisation, total flare loads envisaged for MSS-1, MSS-4 & MSS-5 failure are as follows: MSS-1 FAILURE MSS-1 supplies power to FR APS, FR VPS, T-701 (PG VLPG) units & 3 pumps of FR cooling tower. Therefore, MSS-1 failure will result the power failure in the above said units & three FR cooling water pump trip. Since there is only one common CWE supply header from FR cooling tower, due to trip of three CW pumps the pressure in the header will be so low that the other pumps will also trip leading to FR cooling water system failure. UNITS (oC)
LOAD, (TPH) MW Temp. Remarks
FRE CT Not affected FRE APS affected FRE VPS -
Not
Not
affected HMU affected T-701 (BH VLPG) affected FR CT
Not
Not
FR CWS will trip FR APS Both CW & unit power failure FR VPS -doFCCU 139 43.7 failure PROPANE UNIT 5.5 35 50 11.6 50 50 T-801 (DEETH) 15.2 32.2 oil failure T-802 (DEBUT) 85 50.5 MEA REGEN.
failure 4.5 125 T-701 (PG VLPG) 51.5 70
93 CW
77.7 Lean
68.1 CW 22.5
150 Both CW & Unit power failure*.
Total (for HC flare)
307.8
MSS-4 FAILURE MSS-4 supplies power to FR APS, FR VPS, HMU, T-1701 (BH VLPG). Propane units, one pump in FR cooling tower. Therefore, MSS-1 failure will result the power failure in the above said units & all pumps in FR cooling tower. Therefore MSS-4 failure will result in power failure in the above said units & FRE cooling water system failure. Since only one pump in FR cooling water system will trip it will not affect the FR cooling water system. UNITS (oC)
LOAD, (TPH) MW Temp. Remarks
FRE CT FRE CWS will trip FRE APS Both CW & Unit power failure FRE VPS -doHMU 100 70 100 -doT-701 (BH VLPG) 27.3 70 150 -doFR CT Not affected FR APS affected
Not
FR VPS -
Not
affected FCCU affected PROPANE UNIT T-801 (DEETH) affected T-802 (DEBUT) affected MEA REGEN.
Not
Not
Not -
Not affected T-701 (PG VLPG) Not affected Total (for HC flare)
127.3
MSS-5 FAILURE MSS-5 supplies power to FCCU, T-801 (Deethaniser), T-802 (Debutaniser), MEA Regenerator & Two pumps of FR cooling water system. Therefore, MSS-5 failure will result the power failure in the above said units & two FR cooling water pump trip. Since there is only one common CW supply header from FR cooling tower, due to trip of two CW pumps the pressure in the header will be so low that the other pumps will also trip leading to FR cooling water system failure. UNITS (oC)
LOAD, (TPH) MW Temp. Remarks
FRE CT Not affected FRE APS affected FRE VPS -
Not
Not
affected HMU affected T-701 (BH VLPG) -
Not
Not
affected FR CT FR CWS will trip FR APS 272
70
150 CW
failure FR VPS 7.8 20.3
150 CW
failure FCCU -
Both CW & Unit power failure PROPANE UNIT 5.5 35 50 11.6 50 50 T-801 (DEETH) 15.2 32.2 77.7 Lean oil failure T-802 (DEBUT) 85 50.5 68.1 Both CW & Unit power failure* MEA REGEN. 4.5 22.5 12.5 T-701 (PG VLPG) 52 70
150 CW
failure Total (for HC flare)
449.1
*Since steam will be on in the reboiler, over pressurisation will be there. Therefore load should be considered. It is evident from the above load distributions that MSS-5 failure is the governing case for flare design.
8.4
PROPOSED SYSTEM
8.4.1 Hydrocarbon & sour flare headers will be segregated. Sour flare header will be steam traced. 8.4.2 Separate H/C flare KODs will be provided for FRE Unit (88-D-01); Combination Unit (88-D-02), LEU (88-D-03), DHDS Unit (By licensor) and Hydrogen Unit (By licensor). 8.4.3 A main H/C flare KOD (88-D-07) will be provided at flare stack area. 8.4.4 A common sour flare KOD (88-D-08) will be provided at the flare stack area. 8.4.5 A vertical water seal drum (88-D-09) for H/C flare will be provided. 8.4.6 No water seal drum is provided for sour flare to avoid corrosion problem caused by moist H2S/NH3. 8.4.7 Flare stack will be mounted on water seal drum. 8.4.8 Hydrocarbon flare stack will have a molecular seal and sour flare riser will have a fluidic seal. 8.4.9 A 36" H/C flare header is taken from FRE Unit. This is joined by a 30" H/C flare header from LEU. A 42" H/C flare header is taken from combination unit which expands to 48" just before the joining of 36" H/C flare header from FRE Unit to it. A 24" H/C flare header from DHDS Block is joined to the 48" main header running upto main flare KOD. It exits the KOD as 54" header before it enters the water seal drum. A 10" sour flare header will be provided from LEU, which will be connected to the 10" sour flare header from DHDS unit. The flare header runs upto sour flare KOD. After KOD the line runs upto the top of the flare stack through fluidic seal. Refer flare system P&IDs for detail.
UPPC - AURAIYA CASE STUDY
Main Flare System The flare load for various units is enclosed as Annexure-1. The flare load for the controlling case i.e. cooling water & power failure is 777 T/hr. An attempt has been made to reduce the load. The detailed analysis for the controlling case has been done with respect to instrumentation provided and rationalization of flare loads in event of simultaneous release from several columns in a unit. The details are as under: GCU As per SWEC flare load during electrical power/cooling water failure is as follows: Flare load, kg/Hr MW Temp. C Hot/Wet flare header 315390 33.75 2. Intermediate Flare Header 68855 42.08 3. Low temperature flare header 86503 28.86 TOTAL LOAD 470748 33.67 Being a complex unit with inbuilt margins for instrumented trips, total flare load has been considered for the design of flare system. CSU/C2-C3 Uunit: Control scheme has provided a number of shutdown valves at different locations to close during power/cooling water failure. The details are as follows: Two quick shutdown valves (one redundant) on inlet feed gas to UPPC. Actuation is due to high high/low low pressure and power failure. The gas will be diverted to lean gas despatch header. Shutdown valve upstream of GSU absorber to close during power failure due to tripping of lean amine pump. During power failure, GSU shutdown valve shall close and this in turn will close the feed gas shutdown valve due to upstream line pressurization. Shutdown valve in C2/C3 plant on suction line of lean gas compressor. The shutdown valve will actuate during power failure due to low low pressure in the suction side/high high pressure in the discharge/high high temp in the discharge. There is a remote possibility of failure of all shutdown valves to actuate during power failure situation. Still the possibility of pressurization upto 14 kg/cm2a has been considered. For lean gas suction line normal operating pressure is 12 kg/cm2a and PSV is set at 15 kg/cm2a. The estimated flare load due to pressurization is 2 T/Hr. For flare design, conservative figure of 5 T/hr has been considered. o
1.
o o o
LLD/HDPE Flare load summary by licensor proposal is as under Kg/Hr 1. Comonomer Column 2. High boils column
159849 149690
3. Low boils columns 64242 4. FE column 4850 TOTAL 378631 The flare load from various columns is for the conditions when unit power has failed but steam/BFW is continuous. During complex power failure, steam generation/.BFW shall also trip. Supply of steam to unit reboilers shall be limited to the system hold up/surge available. With the limited steam holdup the simultaneous release of hydrocarbons from various columns is not envisaged. For the flare design the single largest PSV release from CM Column and 10% of balance PSV release has been considered as follows: Flare load, kg/Hr MW Temp. o C 1. CM Column 159849 51.9 89.6 2. HB Column 14969 108.4 241.0 3. LB Column 6424 58.1 146.0 4. FE Column 485 58.1 138.1 Total 181727 Avg. 54.46
GUJARAT REFINERY FLARE SYSTEM The existing flare system of Gujarat Refinery is integrating GR, GREP and GRSPF in a single flare system due to separate dedicated cooling water systems. GHP is having its dedicated flare system. GR flare is connected to GHP for flexibility of using GHP flare system when GR flare is under maintenance. Recently IOCL has given a study to connect atmospheric vents of various crude units, incorporating new units viz AU-V, DHDS, H2 Unit, Sulphur block , MTBE unit and Butene1. Several alternatives were studied. The least cost option has emerged by considering the following basis: Stagger GR cooling water system with dedicated substations so that cooling water failure of AU-V, DHDS, H2 Unit and sulphur block can be considered separate from existing AUI, AU-II, CRU, PDF and Food Grade Hexane unit. Use GHP flare system for GR atmospheric vents etc by following the philosophy of single cooling water failure at a time. Provision of GHP to GR side flexibility to take GHP flare system under maintenance. Use of shell philosophy by considering tripping of source of heat whenever pressure of cooling water supply header is very low.
11.0 11.1 11.2
11.3
12.0
FLARE GAS RECOVERY SYSTEM
INTRODUCTION Excess hydrocarbons being wasted to flare system can be recovered by using a simple but effective flare gas recovery system. SYSTEM DESCRIPTION Flare gas recovery system in its most basic form is simply a compressor to compress the flared gases for its subsequent use at fuel gas in the refinery/petrochemical complex. The objective of the system is to recover excess hydrocarbons being flared and also reduction/containment of radiation, light and glare, smoke and noise in the adjoining areas. The critical aspect of the system is selection of appropriate capacity for compressor and to set its operating specifications so that it can safely and efficiently handle as wide a range of possible compositions and rates as practical. Apart from this, designed system should maintain the integrity of existing flare lines/flare stack system. To achieve this, flare system must be left as it had been. No valves automatic or manual should be place din the flare lines to divert gas from flare stack to compressor. Such mechanical devices can fail to operate when required during emergency. Experience shows that they will fail eventually then we will have no operable emergency system. Recovery system should be controlled so that it does not lower flare system pressure to that level at which air could ingress into the system. For this, suction pressure of the compressor is maintained with compressor capacity control backed up by discharge recycle control. DESCRIPTION OF RECOVERY SYSTEM Typical recovery system is shown in figure-1. Part of flared gases from main flare header downstream of stack area flare gas knock out drum are diverted to compressor through a compressor suction knock out drum to prevent any liquid which may condense in the intake line or entrained liquid from being pulled into the compressor. The knock out drum is provided with mist eliminator to allow only liquid free gases to compressor suction. For noise suppression, silencers are installed at both the suction and discharge ends of the compressor. As the gases get heated up in the compressor, these are cooled in the compressor after cooler before being led to the fuel gas header. The liquid condensate if any is removed in the knock out vessel. Design of suction and discharge knock out drum and also of after cooler are simple and will not pose any problem due to fluctuation in flared gas composition and temperature. Critical area of the flare recovery system is the selection of compressor and the scheme to maintain suction pressure.
Umoe’s concept for Flare Gas Recovery
The new concept of flare gas recovery eliminates the need of continuous flaring of gas. The total hydrocarbon gas is recovered from the flare system and routed to the fuel gas header after compression. The flare line is closed during recovery of the flare gas by means of a on/off valve installed downstream of the flare knock out drum. For safety reasons a bursting disc is installed in parallel with the isolation valve. The compression facility is designed to handle normal gas leakage rates, with a spare capacity to manage smaller releases form blowdown valves/PSVs. During large releases the valve in the flare line will open (typically, the opening time for the valve is less than 2 secs) and the recovering equipment will be isolated. In case of malfunctioning of the valve in flare line, the rupture disc will open and the gas will be flared. The flare gas ignition system will ignite the flare gas. The flare gas ignition system consists of a compressed nitrogen driven launcher contained in a stainless steel cabinet at the deck level. In addition a target plate is located below the flare tips to ignite the specially designed ignition pellet being automatically launched from the launching unit. Standard launching range 125m, Launching medium N2, Launching pressure 150 - 220 bar, Muzzle velocity 430 m/sec (free) / 320 m/sec (guided), ignition pellet dia 20 mm/wt - 43 gram). When hitting the target plate, the pellet will explode and generate a shower of sparks, each igniting along its entire path. The system launches two pellets, the first 15 seconds and the second 30 seconds after the opening of the valve in the flare line. Experience shows the gas is ignited by 9 out of 10 pellets. System Failure Scenario The following two main safety scenarios need to be considered: 1) Both the flare valve and the rupture disc fail to open. 2) The flare gas is not ignited. Scenario (1) has an extremely low probability of occurrence, sufficiently low to be ignored.Two rupture disc in parallel can be used to lower its probability further. Scenario (2) is not much different from a continuously burning flare where a sudden emergency release or strong winds can extinguish the flame. However, the safety of aviation traffic should be considered particularly and procedures in this context, should be made. This new concept of flare gas recovery reduces the emission of Nox & CO2 to a large extent. This technology has been installed in several projects in Norway and patented by M/s. UMOE Oil & Gas.
