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FIG. 5-19: Dimensional References for Sizing a Flare Stack

FIG. 5-19: Dimensional References for Sizing a Flare Stack

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mittent and non-scheduled. The flare must be instantly available

for full emergency duty to prevent any possibility of a hazardous or environmentally offensive discharge to the atmosphere.

Wind-shields and flame-retention devices may be used to ensure

continuous piloting under the most adverse conditions. Most pilots are designed to operate at wind velocities of 160 km/h and

higher. Multiple pilots are generally provided.

•Different equation used for length flame

•Different values used for fraction of heat radiated for

flared gas by component

•The API method gives a leaner flame angle

Low Heating Value Gas Flaring

The most common flare pilot ignition system is a flame front

generator, where a flame generated by compressed air and fuel

gas is sent through a pipe at high velocity up the flare stack

to ignite the pilot gas. Spark plug type igniters are sometimes

used as well.

Low heating value gases are common in many gas plants;

for example, vent gas from a sweet gas amine system or the

feed gas to a sulfur plant. These streams can be a challenge for

a flare system. A number of tests were performed in the 1980s

to assess flare flame stability, and combustion efficiency, for a

wide range of fluids. Based on this testing it was concluded that

high heating value gases can be flared with a thermal destruction efficiency of greater than 98% over a wide range of flare

types and flare tip velocities. For low heating value gas, however, the testing found that a minimum heating value is needed,

and that flare tip velocity must be limited in order to achieve

high destruction efficiency. To flare gas streams with low heating value, the gas must be supplemented by natural gas injection in the flare header or at the flare tip, to ensure a minimum

heating value of approximately 7450 kJ/Nm3 for an unassisted

flare and 9315 to 11 180 kJ/Nm3 for an assisted flare, and the

maximum flare tip velocity must be substantially limited.

Proper flame monitoring is critical to flare operation. Typical systems consist of multiple flame detectors, or multiple

thermocouples, along with closed-circuit television.

­Flare Siting and Regulations

Flare design must comply with local, state, and federal regulations regarding pollution, noise, and location. Permits are

usually required prior to construction. Flaring of gas for the

purpose of emissions control (as opposed to relief), is regulated

in the U.S.A. by the Environmental Protection Agency (EPA),

and specific maximum flare tip velocities may apply. Standards for design of flare systems are covered by API Std 537 and

ISO23251 (API Std 521).

Smokeless Operation

Most smokeless flares utilize outside motive forces to produce efficient gas/air mixing and turbulence from the momentum transferred by the high velocities of the external motive jet

streams (steam, fuel, gas, etc.). The assist medium mass flow

requirements are low for steam and fuel gas because of their

high velocity relative to the flare gas. Flare suppliers should

be consulted, because the assist gas rate is dependent on the

flare design.

Atmospheric Vent Stacks

Atmospheric vent stacks can be used to dispose of non-toxic

hydrocarbons to the atmosphere, under the proper conditions.

In the natural gas industry, vent stacks for hydrocarbons are

typically limited to atmospheric disposal of lighter-than-air

gases. Stacks are many times used in natural gas compressor

stations to vent an individual compressor or the entire station

to the atmosphere on an emergency shutdown.

ISO 23251 (API Std 521) presents a table with suggested

injection steam rates based on the type of gas being flared. The

following fitting equation may be used for calculation of the injection steam rate for a mixture of paraffins (reference 12):

 10.8 

Wstm = Whc  0.49 –  

 MW

Before designing a vent stack system for a facility, it is

important to consider a number of factors: vent stack location

relative to plant and public facilities (permanent or temporary),

vent stack height, possibility of a combustible or toxic mixture

at grade or at an elevated platform, layers of protection in place

at upstream equipment, level controls to prevent overflow of

volatile liquids into the stack, appropriately sized knock out

drum, possibility of explosive release of energy due to detonation of a vapor cloud, radiation due to a jet fire at the vent stack

tip caused by static ignition or lightning. The decision to discharge hydrocarbons or other flammable or hazardous vapors

to the atmosphere usually requires that a dispersion analysis

be carried out to ensure that disposal can be accomplished without creating a hazard. These topics are covered extensively in

ISO 23251 (API Std 521).

Eq 5-33

For a mixture of olefins, the fitting equation becomes:

 10.8 

Wstm = Whc  0.79 –  

 MW

Eq 5-34

The water spray and air blower methods provide necessary

mixing with low velocities and greater mass flow rates. The required assist fluid injection rate is highly dependent upon the

method of injection and atomization. Wind also has a significant effect on water spray flares and may greatly reduce their




The blower assisted flare uses air to produce smokeless operation. Forced draft from a blower assists combustion and air/

gas turbulence, promoting smokeless operation. With blower

assisted flares it is common, for high capacity flares, to design

the air assist for a the portion of the maximum capacity expected during operation, and to allow a degree of smoke during

the full emergency relief. This, however, is dependant on local


The designers of relief systems should be familiar with the

following documents related to pressure relief valves in process

plants and natural-gas systems.

ASME Boiler and Pressure Vessel Code, Section I, Rules for

Construction of Power Boilers

ASME Boiler and Pressure Vessel Code, Section VIII.

­Pilots and Ignition

ASME B31.1 — Power Piping

Reliable pilot operation under all wind and weather conditions is essential. Flaring operations are for the most part inter-

ASME B31.3 — Process Piping


ASME B31.4 — Pipeline Transportation Systems for Liquid

Hydrocarbons and Other Liquids

ASME B31.8 — Gas Transmission & Distribution Systems.

API Std 520-I — Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part I – Sizing and Selection.

OSHA Publications­ — OSHA Title 29, Part 1910 — Part

1910 includes handling, storage, and safety requirements for

LPG and ammonia.

CGA (Compressed Gas Association) Publications — Series of standards covering transportation, handling, and storage of compressed gases including:

Pamphlet S-1.2 Safety Relief Device Standards

API RP 520-II — Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part II - Installation.

Part 2: Cargo and portable tanks for compressed gases.

API Std 526 — Flanged Steel Pressure Relief Valves.

Pamphlet S-1.3 Safety Relief Service Standards

API Std 527 — Seat Tightness of Pressure Relief Valves.

Part 3: Compressed Gas Storage Containers.

API Std 537 — Flare details for General Refinery and Petrochemical Service.



1.Min, T. C., Fauske, H. K., Patrick, M., Industrial Engineering

Chemical Fundamentals, (1966), pp. 50-51.


2.Brzustowski, T. A., “Flaring In The Energy Industry”, Process

Energy Combustion Science, Pergamon Press, Great Britain,

V. 2, pp. 129-144, 1976.


3.Straitz III, J. F., Nomograms “Determining Proper Flame Tip Diameter and Height”, Oil Gas and Petroleum Equipment, Tulsa,

Oklahoma, July and August, 1979.


4.“Recommendations and Guidelines — Gasoline Plants, Pamphlet 301”, Oil Insurance Association, 175 West Jackson Blvd.,

Chicago, Illinois 60604, August 1971.


5.Van Boskirk, B. A., “Sensitivity of Relief Valves to Inlet and

Outlet Line Lengths,” Chemical Engineering, August 23, 1982,

pages 77-82.


6.Schwartz, Robert E. and White, Jeff W., “Predict Radiation From

Flares,” Chemical Engineering Progress, Vol. 93, pp. 42-49, July

1997 .


7.Overa, Sverre J., Strange, Ellen and Salater, Per, “Determination of Temperatures and Flare Rates During Depressurization

and Fire,” GPA Convention, San Antonio, Texas, 15–17 March,


API Bulletin 2521 — Use of Pressure Vacuum Vent Valves

for Atmospheric Pressure Tanks to Reduce Evaporation Loss.

National Board — Pressure Relief Device Certifications NB18 (RedBook)

8.Carucci, V.M., and Mueller, R.T., “Acoustically Induced Piping

Vibration in High Capacity Pressure Reducing Systems,” ASME

Paper 82-WA/PVP-8, 1982.

9.Energy Institute, IP SAFE Hydrocarbon Leak Reduction Volume

2.00, Guidelines for the Avoidance of Vibration Induced Fatigue

in Process Pipework, ISBN 9780852934630, 2nd edition, March


API Standard 620 — Design and Construction of Large,

Welded, Low-Pressure Storage Tanks.

API Standard 650 — Welded Steel Tanks for Oil Storage.

API STD 2508 — Design and Construction of Ethane and

Ethylene Installations at Marine and Pipeline Terminals, Natural Gas Processing Plants, Refineries, Petrochemical Plants,

and Tank Farms — Covers the design, construction, and location of refrigerated (including autorefrigerated) liquefied ethane and ethylene installations, which may be associated with

one or more of the following: railroad, truck, pipeline stations,

or marine loading or unloading racks or docks.

API STD 2510 — Design and Construction of LPG Installations. Covers LPG Storage Vessels, Loading and Unloading Facilities at Marine and Pipeline Terminals, Natural Gas Processing Plants, Refineries, Petrochemical Plants, and Tank Farms.

API Specification 12F — Specification for Shop Welded

Tanks for Storage of Production Liquids.

API Specification 12D — Specification for Field Welded

Tanks for Storage of Production Liquids.

ISO 15156/NACE MR0175 Petroleum and Natural Gas Industries — Materials for Use in H2S-containing Environments

in Oil and Gas Production.

ISO 23251 (API Std 521), Pressure-Relieving and Depressuring Systems.

ISO 28300 (API Std 2000), Venting Atmospheric and LowPressure Storage Tanks (Nonrefrigerated and Refrigerated).

NFPA 30 — Flammable and Combustible Liquids Code

NFPA 58 — Liquefied Petroleum Gas Code

NFPA 59 — LP-Gas, Plant Code (Note: For Utility Plants)

NFPA 59A — Production Storage and Handling of Liquid

Natural Gas (LNG)

NFPA 68 — Standard of Explosion Prevention by Deflagration Venting

NFPA 69 — Standard of Explosion Prevention Systems

10.Ouderkirk, R., Rigorously Size Relief Valves for Supercritical

Fluids, Chemical Engineering Progress, August 2002.

11.Nezami, P.L., Distillation Column Relief Loads – Part 1, Hydrocarbon Processing, April 2008, and Part 2 – May 2008.

12.O.C. Leite, “Smokeless, Efficient, Non-toxic Flaring,” Hydrocarbon Processing, March 1991, page 77.

13.ISO 23251 API Std 521 — Pressure-relieving and Depressuring

Systems (Fifth Edition, 2007), American Petroleum Institute,

1220 L Street, NW, Washington, DC 20005.

­­­ 14.API 520-520-I — Recommended Practice for the Design of Pressure Relieving Systems in Refineries (Eighth Edition, 2008,

American Petroleum Institute, 1220 L Street, NW, Washington,

DC 20005.

15.“ASME Boiler and Pressure Vessel Code,” Section VIII, Div. 1,


16.API Std 527, “Seat Tightness for Pressure Relief Valves,” Reaffirmed 2007.



 traitz III, J. F., “Solving Flare-Noise Problems”, Inter. Noise 78, San


Francisco 8-10, May 1978, Pages 1-6.

Chiu, C. H. “Apply Depressuring Analysis to Cryogenic Plant Safety”,

Hydrocarbon Processing, November 1982, Pages 255-264.

Straitz III, J. F., “Flaring for Safety and Environmental Protection”,

Drilling-DCW, November 1977.

Kandell, Paul “Program Sizes Pipe and Flare Manifolds for Compressible Flow”, Chemical Engineering, June 29, 1981, Pages 89-93.

­Straitz III, J. F., “Make the Flare Protect the Environment”, Hydrocarbon Processing, October 1977.

Powell, W. W., and Papa, D. M., “Precision Valves for Industry”, Anderson, Greenwood Company, Houston, Texas, 1982, Pages 52-61.

­ an, S. H., “Flare Systems Design Simplified”, Hydrocarbon ProcessT

ing (Waste Treatment & Flare Stack Design Handbook) 1968, Pages





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FIG. 5-19: Dimensional References for Sizing a Flare Stack

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