Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing

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SUPPORT SYSTEMS COMMON TO MOST REFINERIES 553

ﬂare and slop handling system is therefore essential to the plant operation. This section

describes and discusses typical disposal systems currently in use in the oil reﬁning

industry where the hydrocarbon is immiscible with water. Where the chemical is

miscible in water special separation systems must be used.

Figure 13.17 shows a completely integrated waste disposal system for the light end

section of an oil reﬁnery. The system shown here consists of three separate collection

systems being integrated to a ﬂare and a slops rerun system. A fourth system is for the

disposal of the oily water drainage with a connection to the ﬂare and a separate connec-

tion for any oil laden skimming. This later connection would be to route the skimmed

oil to the reﬁnery slop tanks. In the three integrated systems, the ﬁrst collects all the

vapor efﬂuent streams from the relief headers. The contents of this stream will be ma-

terial normally vapor at ambient conditions. It would be the collection of the vapors

from the relief valve and the vapor venting on plant shut down or upset conditions. The

second of the three systems is the liquid hydrocarbon drainage. The material in this

system is liquid under normal ambient conditions and is collected from drain headers

used to evacuate vessels during shutdown or upset conditions. Both the ﬁrst and the

second collection systems are routed to the ﬂare knockout drum. The second (liquid

system) may also be routed to the light ends slop storage drum. The liquid phase from

the ﬂare knockout drum is also routed to the slops storage drum. The third system

is the light ends feed diversion. This allows the light ends unit to be bypassed tem-

porarily by sending the feed to the slop drum for rerunning later.

Further description and discussion of these disposal systems is given in the following

sections:

Blow-down and slop disposal

Flares

Blow-down and slop

This system generally consists of the following drums:

Non-condensable blow-down drum

Condensable blow-down drum

Water disengaging drum

A typical non-condensable blow-down drum is shown in Figure 13.18. These types of

drums are provided for handling material normally in the vapor state and high volatility

liquids. These drums receive and disengage liquid from safety valve headers, and drain

headers. These drums are often referred to as ﬂare knockout drums as the disengaged

vapor is routed directly to a ﬂare. The drum is basically a surge drum and therefore

should be sized as one using the following criteria:

1.0 Normal liquid surge is based on the daily liquid draw-off to drain per operating

day of 24 hr. This includes spillage, sample point draining, etc.

Figure 13.17. An integrated water disposal system.

Figure 13.18. A non-condensable blow down drum.

556 CHAPTER 13

2.0 The surge capacity between the HLSO (High Level Shut Off) for normal drainage

and the HLSO for feed diversion should be such as to contain the total feed to

a unit routed to this drum for a period sufﬁcient to shutdown the unit producing

the feed stream. Should there be more than one unit routed to the drum then this

surge capacity should be for the largest of a single feed streams.

3.0 The capacity between the highest HLSO and the high level alarm (HLA) should

be sized to handle the largest liquid volume that can be discharged in 30 minutes

by the relief valves constituting any single risk.

4.0 The drum must be sized for a vapor velocity above the HLA at a maximum of

100% of the critical ﬁgure calculated by:

V = 0.157

√

(ρ

− ρ

)/ρ

where

V = critical velocity in ft/sec

=liquid density in lbs/cuft

=vapor density in lbs/cuft at drum conditions of pressure and temperature.

Drum pressure. The maximum operating pressure for this drum will be about 0.5

psig or that of the water disengaging drum tied to the same ﬂare header.

Condensable blow-down drum and system is used for collection and containment

of heavier hydrocarbons with low volatility. For example this would account for the

middle and waxy distillates (kerosene, gas oils, and the like).

Figure 13.19 shows a typical blow-down drum and quench. The material entering

this system is generally above ambient temperatures. Very often hot streams directly

from operating units ﬁnd their way into this system. To handle these materials the

condensable blow-down drum is designed as a direct contact quench drum. The blow-

down material leaves the unit in a drain collection system to enter the bottom section

of the drum. Cooling water is introduced at the top of the drum and passes over a

bafﬂed tray section to contact the hot blow-down stream at the drum base. Any hot

vapors rising from the blow-down stream are condensed in the bafﬂe section of the

drum and carried down to the bottom of the drum. Uncondensed material leaving the

top of the vessel is routed to the ﬂare. The aqueous mixture containing the condensed

blow-down leaves the bottom of the drum through a seal system to enter the chemical

or oily water sewer for separation and treatment.

The following criteria is used to size this vessel:

The vapor load on the drum is based on the safety valve(s) constituting the largest

single risk

The maximum operating pressure for the drum is usually 1–2 psig

The stack may vent to atmosphere rather than the ﬂare if desired. However if vented

to atmosphere the stack should vent at least 10 ft above the highest adjacent structure.

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 557

Figure 13.19. A typical blow down drum with quench.

In any case the vent should not release to atmosphere below 50 ft above grade.

Snufﬁng steam should also be provided.

The cooled efﬂuent leaving the drum should be at 150

◦

F or colder. The cold water

supply should be controlled either by efﬂuent temperature or inlet blow-down stream

ﬂow. There should however be a bypass ﬂow of water entering the drum at all times.

The drain system from the unit(s) to the drum should be free draining into the drum.

The drum therefore should be located at a minimum height to grade. Where very

waxy materials are likely to be handled, steam tracing of lines and a steam coil in

the drum should be considered.

558 CHAPTER 13

Figure 13.20. A water separation drum.

The water separation drum arrangement is shown in Figure 13.20. The purpose of this

drum is to remove any volatile and combustible material from certain water efﬂuent

streams before they enter the sewer system.

Thus:

All water from distillate drums which have been in direct contact with ﬂammable

material such as light hydrocarbons are sent to the disengaging drum before disposal

to the sewer. The exception is where a sour water stripper is included in the plant;

then these streams are sent to this stripper.

Cooling water drainage from coolers and condensers which may have been contam-

inated with ﬂammable high volatility material are sent to the disengaging drum. So

too are steam condensate streams which fall in the same category.

The drum is located at a minimum height above grade. It operates at about 0.5 psig

and vents into the ﬂare system. The pressure and the liquid level in the drum are

maintained by the free draining of the efﬂuent through a suitable seal.

Design criteria used for the sizing of this drum are as follows:

The vapor load on the drum will be the result of high volatile material ﬂashing to

equilibrium conditions at the drum pressure. This design load is based on the largest

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 559

amount of vapor arising from a single contingency. For exchangers this contingency

will be due to a fractured tube. For liquid from a distillate drum this will be due to

a failed open control valve on the water outlet.

The liquid seal must be such as to eliminate air from the sewer system and to allow

free drainage from the drum.

An oil or chemical skimming valve is located at the water NLL. This allows for the

draw off of the oil phase from time to time. A high interface level alarm is often

included.

The ﬂare

Vapors collected in a closed safety system are disposed of by burning at a safe location.

The facilities used for this burning are called ﬂares. The most common of these ﬂares

used in industry today are:

The elevated ﬂare

The multi jet ground ﬂare

The elevated ﬂare is used where some degree of smoke abatement is required. The

ﬂare itself operates from the top of a stack usually in excess of 150 ft high. Steam is

injected into the gas stream to be burnt to complete combustion and thereby reduce

the smoke emission.

The multi-jet ground ﬂare is selected where luminosity is a problem. For example at

locations near housing sites. In this type of ﬂare the vapors are burned within the ﬂare

stack thus considerably reducing the luminosity. Steam is again used in this type of

ﬂare to reduce the smoke emission.

Figure 13.21 shows a typical arrangement of an elevated ﬂare and Figures 13.22 and

13.23 shows that for a multi jet ground ﬂare.

The elevated ﬂare

This type of ﬂare is the normal choice in the larger process industries such as the

petroleum reﬁning industry. It consists of a ﬂare stack over 150 feet in height that

contains an ignitor system, a pilot ﬂame, and the ﬂare pipe itself. The ﬂare header

enters the stack through a water seal at the base of the stack immediately above an

anchor of concrete plinth. The water seal maintains a back pressure of around 0.5

psig on the ﬂare header. The waste gas to be ﬂared moves up the stack to exit at the

top. At the stack top there is an assembly of ignitor and pilot gas which ensures the

safe burning of the waste ﬂare gasses. This assembly is shown in Figure 13.24. It

consists of three tubes all external to the stack itself and each supplied with the plant

fuel gas. The ﬁrst and largest of these tubes is the ignitor. Here the fuel gas supply

560 CHAPTER 13

Figure 13.21. An elevated ﬂare.

is mixed with air (plant or instrument air supply) before passing upwards through a

venturi tube to an igniter chamber. A spark is induced in the ignitor chamber by an

electric current of 15 Amp. The chamber and the venturi tube are located near grade

and a sight glass on the ignitor chamber enables the operator to check on the ignitor’s

operation. The ﬂame front from the ignitor travels up the ignitor tube to contact the

waste gasses that are to be ﬂared as they exit the top of the ﬂare stack. The same ﬂame

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 561

Figure 13.22. A multi jet ground ﬂare and stack.

front ignites the “On and Off ”pilot burner which is the center tube of the three and

initially ignites the permanent pilot burner at the stack top. The outlets of these three

tubes are located at the stack top such that the prevailing wind ensures that the ﬂame

from them is blown across the stack exit.

Steam is often injected into the stack at some point near the top to complete combustion

and eliminate or at least reduce smoke emission. The amount of steam normally

used for this purpose depends on the character or composition of the waste gasses.

Aromatics and oleﬁns when burnt produce a smokey ﬂame: steam injection allows

the free carbon which makes up the smoke to convert into CO and CO

which of

course are invisible gasses. An estimate of the amount of steam required for smoke

abatement is given in Figure 13.25.

A clear space around an elevated ﬂare is required to allow for the effect of heat

radiation from the ﬂare to the ground. Flares which have a heat release of 300 million

562 CHAPTER 13

Figure 13.23. Multi jet ﬂare plan with seal details.

to 1 billion btu/hr should be located at least 200 ft from the plant property line, or any

pond, separator, tankage, or any equipment that could be ignited by a falling spark.

The stack also must have a spacing of at least 500 ft from any structure or plant whose

elevation is within 125 ft of the ﬂare tip.

The elevated ﬂare stack is designed to maintain a gas velocity of between 100 and

160 ft/sec during a major blow-down to ﬂare. This rate is based on the maximum

single emergency plus any steam added to improve the burning characteristics. Above

a velocity of 160 ft/sec noise becomes a problem and the maintenance of ignition also

is dubious unless multiple ignition tubes are used. Some proprietary ﬂare tip designs

however do claim ability to handle satisfactorily velocities up to 400 ft/sec.

The multi-jet ground ﬂare

The multi-jet ﬂare provides a completely noiseless, non-luminous ﬂaring at a reason-

able cost. At normal loads the ﬂare is also essentially smokeless and is particularly

useful where continuous ﬂaring is required. Figures 13.22 and 13.23 show the el-

evation of the ﬂare stack and the plan arrangement of a two stage multi-jet ﬂare,

respectively.

The two stage arrangement shown here shows the ﬂare header being directed to one

of two seal drums or to both. The ﬁst stage seal drum operates at a back pressure of

20 ins of water at the ﬁrst stage burners at its design capacity. The second stage burners