105786398-Flare-System

September 13, 2012 1

Training Program on

Basic Process Engineering Practices

By

Module – 11B Flare System


Flaring is a combustion control process in which waste gases are piped to a remote, usually elevated location and burned in an open

flame in the open air.

A specially designed burner tip, auxiliary fuel, and steam or air are

used to promote mixing for nearly complete combustion (>98 %).

The flaring process can produce undesirable by-products, including

noise, smoke, heat radiation, light, SOx, NOx, CO, and an undesired

source of ignition. However, proper design can minimize these.

What is Flaring?

Flare K.O Drum Water

seal

Flare Tip

Flare Header

Process Equipment

Process Equipment


Process plant can be subjected to excessive overpressure or

under-pressure due to process upset conditions.

Safety Valves or Rupture Discs prevent the equipment from

reaching overpressure condition i.e. protects it from exceeding

design pressure by releasing the excess gases.

The gases released in a process plant is generally hazardous.

Primary purpose of flare system is to safely take the released

gases to a flare stack and burn it.

Flare system is also used for burning gases due to emergency

venting. Example of emergency venting- Gas flaring when a

consumer shuts down.

Purpose of Flare System


Causes of Over-pressure

External fire

Blocked Valve

Process abnormality or mal-operation

Equipment or service / utility failure

Changes in ambient conditions

Runaway chemical reaction

Flare system is used to destroy flammable, toxic or corrosive

vapors, from relief valves or emergency venting.


Flare System Design Factors

Key design factors to ensure flare safety and performance

include:

Smokeless operation

Flame stability

Flare size, capacity, stack diameter

Thermal radiation

Noise level

Reliable pilot and ignition system

Flashback protection


Flare Network Components

Water

seal

Flare Tip

Flare K.O Drum

Incinerator

Mol Seal

Pilot

Burner

Flare

Ignition

System

Fuel Gas (2”) Pump

PC

Unit Flare HDR Fuel Gas

Process (Unit – 1)

Unit Flare HDR Fuel Gas

Process (Unit – 2)

Main

Flare

HDR Flare

Stack

Air


• Flares are generally categorized in two ways:

1) by the height of the flare tip (i.e., ground or elevated) and

2) by the method of enhancing mixing at the flare tip (i.e., steam-

assisted, air-assisted, pressure-assisted, or non- assisted).

• Elevating the flare can prevent potentially dangerous

conditions of high radiation at ground level or operating area of

a process unit. The distance and height of the flare stack is set

by radiation calculations (API RP 521)

• Further, the products of combustion can be dispersed above

working areas to reduce the effects of noise, heat, smoke, and

objectionable odors. Dispersion and ground level concentration

of pollutants from flare also may set the height of the flare

stack.

Flare Types


• Cracking can occur with the formation of small hot particles of

carbon that give the flame its characteristic luminosity. If there is an

oxygen deficiency and if the carbon particles are cooled to below

their ignition temperature, smoking occurs.Non-assisted flares are

more prone to smoking.

Flare Types, Contd...

• The non-assisted flare is just a flare tip without an auxiliary

provision for enhancing the mixing of air into its flame. Its use is

generally limited to gas streams that burn readily without

producing smoke.

Non- assisted flares

Smoke problem


Assisted flares

• In assisted flares, induction of air for combustion and mixing are

enhanced by various means described below.

• Steam is injected into the combustion zone to promote turbulence

for mixing and to induce air into the flame.

Steam assisted flares

Air assisted flares

• Some flares use forced air to provide the combustion air and the

mixing required for smokeless operation.

Pressure assisted flares

• Gas pressure is kept high at the battery limit of the flare to promote

mixing at the burner tip.

Flare Types, Contd...


Flare Hardware Components

Safety Relief and

Flare Header

Steam

Assisted

Flare


Steps on Designing Flare System

Determine

Controlling Load

For Each Relief

Estimate

Worst Scenario

For the Plant

Identify Cases

For Over-pressure

Identify Systems

For Relief

Protection

Select Set

Pressures

Line Sizing &

P&ID

For Flare System

Select Stack

Height, Diameter

And Distance

Select Type Of

Flare Tip,

Seals

Piping Layout Equipment

Specification


The first step is to analyze the causes of overpressure in

various equipment and systems and calculate the loads due

to safety valve popping.

External fire

Process abnormality or mal-operation

Equipment or service / utility failure

Changes in ambient conditions

Runaway chemical reaction

Once the loads are calculated, they are systematically

tabulated under above heads.

The chances of simultaneously occurring failures dictate the

flare load

Determining Flare Load in a Plant


Examples of Safety Valve Sizing Cases

Fire Case- required to be estimated for vessels 25 feet from

ground. Heat flux due to fire is taken as 21 or 34.5 MBtu/Hr/Sq. ft.

Surface up to 25 ft x heat flux x absorption factor x insulation

factor.

API RP-521 (1993) gives the equation-

Q= 21,000 x F x A 0.82 Where - Q= Heat absorption in wetted area.

A= Wetted area in sq. ft.

F= Environment Factor

(F=1 for bare surface, 0.15-0.3 for

insulated surface)

NFPA –

Q= 21,000 x F x A 0.84 Where -

(F=0.3 for bare water sprayed,

buried or insulated surface)



Blocked Flow- inadvertently closed block valve, failed-

shut control valve, power failure, pump failure with

upstream vessel level affected.

Tube rupture-differential pressure between shell side and

tube side to be evaluated.

Control valve failure- due to air failure or other causes.

Power failure – resulting pump failure, instrument air

failure, failure of agitator in vessel etc.


Steam related failure- can cause excessive steam

pressure due to fail open valve, stoppage of steam supply

with low vaporization and rising levels, high vapor load

due to excess steam.

Reflux failure- causes vapor overload. Since column is at

ground level fire case usually controls.

Thermal relief- Blocked liquid line with heat load like

steam tracing or solar radiation.

Runaway chemical reaction- should be specially

evaluated from licensor information. Usually this case or

fire case controls the PSV sizing.



Over-pressure: Blocked Discharge Case

Block discharge from well head

Oil manifold

This can happen when there is a sudden closure of valve in

any flowing pipeline. In this case, the safety valves provided

on pipeline or equipment need to be designed on full flow

rate


Heat Exchanger Tube Failure

Tube Side Shell Side

When there is a wide difference in design pressure between

the two exchanger sides and the low pressure side is

designed at a pressure less than two-third of design

pressure of high pressure side, a relief valve is required at

the low pressure side


Utility Failure Example – Cooling Water Failure

Feed

Top product

Reboiler

Condenser

Bottom product

Distillation Column

When there is a sudden failure

of cooling water in overhead

condensers of distillation

column, the column pressure

starts increasing due to loss of

reflux after 5-10 minutes.

To overcome this, a relief valve

is required that can vent the

additional quantity of vapor

generated to flare.


Control Equipment Failure- Oversupply of Heat

Feed

Top product

Reboiler

Condenser

Bottom product

Distillation Column

When the control of fuel supply

or steam supply to reboiler

fails, there could be excessive

heating resulting in rise in

column temperature and over-

pressurization.

To overcome this, a relief valve

is required that can vent the

additional quantity of vapor

generated to flare.


Selecting the Set Pressure

Depending on temperature rating of the equipment and

material of construction, design pressure or maximum

allowable working pressure (MAWP) is decided.

The set pressure of safety valve is to be equal or lower

than design pressure. It is guided by codes like API 520.


Relief Line Sizing- Guidelines

No PSV inlet line pressure drop should be greater than 3%

of the set pressure.

PSV discharge side should be at least one size higher

than the inlet side.

PSV discharge side pressure drop should not be more

than 10% of the set pressure.

Back pressure on safety valve should not exceed 10% of

set pressure. For bellows type safety valve it can be

higher.

There should be no restriction on relief lines – full bore LO

valves, no Restriction orifice, no flame arrestor etc.

Be aware of limitations of sonic flow. Sonic flow limits

maximum possible flow in a line. Do not exceed 50% of

sonic velocity.


Flare System Hardware and Network Design

After completing the design of process systems, a final flare

and relief analysis of process system should be done.

A comparative study of flare and relief loads should be

determined and the worst scenario foreseen.

Based on the worst conditions, flare load is designed.

Based on the controlling flare load, the flare equipment and

system hardware are designed-

Network of relief lines from numerous equipment with

main flare header

Flare k o drum

Liquid transfer pumps

Flare stack are designed.


Flare stacks are of three types:

Self Supported Derrick Supported Guy Supported

Flare Stacks


The height and distance of a flare is determined by the ground

level limitations of:

• thermal radiation intensity,

• luminosity,

• noise,

• height of surrounding structures, and

• the dispersion of the exhaust gases.

API RP 521 sets the guidelines for radiation and dispersion

calculations.

Stack Height


Solar Radiation

• API RP 521 provides guidelines for radiation limits for

estimating stack height.

• An industrial flare is normally sized for a maximum heat

intensity of 1,500-2,000 Btu/hr-sq ft when flaring at its maximum

design rate. At this heat intensity level, workers can remain in

the area of the flare for a limited period only.

• If, however, operating personnel are required to remain in the

unit area, the recommended design flare radiation level

excluding solar radiation is 500 Btu/hr-sq ft.

• The solar radiation is in the range of 250-330 Btu/hr-sq ft.)

Stack Height Contd..


• Flare height may also be determined by the need to safely disperse

the vent gas in case of flameout.

• The height in these cases would be based on dispersion modeling

for the particular installation conditions.

• The minimum flare height normally used is 30 feet.

Stack Height Contd..


Cold Vent

In cases where the safety relief valves are small in number and

venting possibilities are minimal, cold venting of natural gas

can be carried out in stead of flaring.

The gas should be mainly methane (much lighter than air) so

that it goes up and disperses in the air much above operating

level.

Cold venting is also done for atmospheric storage tanks or

where adequate back pressure for flare system is not available.


Atmospheric storage tank

Vent stack

• Atmospheric Storage Tank designed as per API 650 can

not tolerate back pressure of flare system. They need to

be vented.

• Atmospheric Storage Tanks for refrigerated liquids

designed as per API 620 (500 mm water) can be

connected to flare.

Flaring from Atmospheric Tanks

Refrigerated

Atmospheric

Storage tank

Flare stack


Codes and Guidelines

API RP 520 Sizing, selection, and installation of pressure

relieving devices in refineries

Part I – Sizing and Selection, 1993.

Part II – Installation, 1994.

API RP 521 Guide for pressure-relieving and depressuring

systems, 1997.

API RP 526 Flanged Steel Safety Relief Valves, Fourth

Edition, 1995.

API RP 527 Seat Tightness of Pressure Relief Valves, Third

Edition, 1991.

API Std 2000 Venting atmospheric and low pressure storage

tanks: Non-refrigerated and refrigerated, 1998.

API RP 2521 Use of pressure-vacuum vent valves for

atmospheric Loss, First Edition, 1966.

Documents

105786398-Flare-System