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

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Figure 13.34. An ion exchange unit hook up.

584 CHAPTER 13

Figure 13.35. A deaerator drum.

counter current to the water. This action removes the dissolved gasses from the water.

These gasses then leave with the steam from the top of the degassing section to be

vented to atmosphere. The gas free water free ﬂows into the main retaining section

of the deaerator.

The treated water feed is introduced to the degassing section of the deaerator through

atomizing spray/distributor. This reduces the water stream to ﬁne droplets prior to

entering the packed section. This enhances the removal of the gasses in the water.

deaerators operate at about 2–3 psig and at this pressure all of the CO

contained in

the water and most of the oxygen is removed.

The BFW pumps draw suction directly from the deaerator. To ensure that there is

available sufﬁcient NPSH for the pumps to operate properly deaerator drums are

installed on a structure at least 15 ft above the center line of the pump suction.

Most large pumps (as BFW pumps are) usually require a relatively high NPSH when

handling hot water.

The retention section of the deaerator should have as a minimum 30 minutes of surge

between HLL and LLL. The water feed to the deaerator is normally on retention

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 585

section level control. The boiler feed from the BFW pumps will be on ﬂow control

with steam drum level reset. Very often boiler feed ﬂow has low ﬂow alarm and at

very low ﬂow has automatic boiler shut down device.

Compressed air system

All oil reﬁning plants require a supply of compressed air to operate the plant and for

plant maintenance. There are usually two separate systems and these are:

Plant air system

Instrument air system

Plant air is generally supplied by a simple compressor with an after cooler. Very often

when plant air is required only for maintenance this is furnished by a mobile compres-

sor connected to a distribution piping system. Air for catalyst regeneration and the like

is normally supplied by the regular process gas compressor on the unit. Instrument air

should always be a separate supply system. Compressed air for instrument operation

must be free of oil and dry for the proper function of the instruments it supplies. This

is a requirement which is not necessary for most plant air usage. A reliable source

of clean dry instrument air is an essential requirement for plant operation. Failure of

this system means a complete shut down of the plant.

Figure 13.36 shows a typical instrument air supply system.

Atmospheric air is introduced into the suction of one of two compressors via an

air ﬁlter. The compressors are usually reciprocating or screw type non-lubricating.

Centrifugal type compressors have been used for this service when the demand for

instrument air is very high. The air compressors discharge the air at the required

pressure (usually above 45 psig) into an air cooler before the air enters one of two

dryers. One of the compressors is in operation while the other is on standby. The

operating compressor is usually motor driven with a discharge pressure operated

on/off start up switch. The standby compressor is turbine (or diesel engine) driven

with an automatic start up on low discharge pressure switch.

The cooled compressed air leaves the cooler to enter the dryers. There are two dryer

vessels each containing a bed of desiccant material. This material is either silica

gel (the most common), alumina, or in special cases zeolite (molecular sieve). One

of the two dryers is in operation with the compressed air ﬂowing through it to be

dried and to enter the instrument air receiver. The desiccant in the other dryer is

being simultaneously regenerated. Regeneration of the desiccant bed is effected by

passing through the bed a stream of heated air and venting the stream to atmo-

sphere. This heated stream removes the water from the desiccant to restore its hy-

groscopic properties. At the end of this heating cycle cooled air is reintroduced to

cool down the bed to its operating temperature. When cool the unit is ready to be

Figure 13.36. An instrument air supply system.

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 587

switched into operation for the ﬁrst dryer to start its regeneration cycle. The various

operating and regeneration phases are automatically obtained by a series of solenoid

valves operated by a sequence timer switch control. These dryers (often including the

compressor and receiver items) are packaged units supplied, skid mounted, and ready

for operation.

The instrument air receiver vessel is a pressure vessel containing a crinkled wire mesh

screen (CWMS) before the outlet nozzle. It is high-pressure protected by a pressure

control valve venting to atmosphere, and of course is also protected by a pressure

safety relief valve (not shown in diagram). The air leaves the top of this vessel to

enter the instrument air distribution system servicing all the plants in the complex.

13.3 Safety systems

A major requirement in the design and engineering of a plant or system is to ensure its

safe operation. Much of the effort in this respect is directed to determining the pressure

limits of equipment and to protect that equipment from dangerous over pressuring.

Pressure relief valves are normally used for this protection service, although under

certain conditions bursting discs may be used. This section of Chapter 13 covers the

various types of relief valves, and the procedure for calculating the correct oriﬁce size

of the valve and the valve selection.

Determination of risk

The cost of providing facilities to relieve all possible emergencies simultaneously is

prohibitive. Every emergency arises from a speciﬁc cause, the simultaneous occur-

rence of two or more emergencies or contingencies is unlikely. Hence, an emergency

which can arise from two or more contingencies (e.g., the simultaneous failure of

a control valve and cooling water) is not considered when sizing safety equipment.

Likewise, simultaneous but separate emergencies are not considered.

Every unit or piece of equipment must be studied individually and every contingency

must be evaluated. The safety equipment for an individual unit is sized to handle the

largest load resulting from any possible single contingency. If a certain emergency

would involve more than one unit, then all must be considered as an entity. The

equipment judged to be involved in any one emergency is termed “single risk”. The

single risk which results in the largest load on the safety facilities in any system is

termed “largest single risk”and forms the design basis for the equipment.

Note: The emergency which results in the largest single risk on the overall basis may

be different from the emergency(ies) which form the basis for individual pieces of

equipment.

588 CHAPTER 13

Contingencies generally fall into one of two categories—ﬁre (external) or operating

failure. Operating failure covers such contingencies as utility failure, mechanical

failure, or mal-operation.

Deﬁnitions

The terms used and the descriptions given in this item are based on data given in two

API publications. These are: API RP520 and 521. References are also made to: Part 1

ANSI Proposed Standard, “Terminology for Pressure Relief Devices”and to ASME

PTC 25.2. These publications are the safety standards commonly in current use. The

following deﬁnition of terms used in the design of safety systems helps to understand

the design and criteria of safety systems.

Accumulation

Accumulation is the pressure increase over the maximum allowable working pressure

of the vessel during discharge through the pressure relief valve expressed as a percent

of that pressure, or in PSI.

Atmospheric discharge

Atmospheric discharge is the release of vapors and gases from pressure relief and

de-pressurizing devices to the atmosphere.

Back pressure

Back pressure is the pressure existing at the outlet of the pressure relief device due to

pressure in the discharge system.

Balanced safety relief valves

A balanced safety relief valve incorporates means for minimizing the effect of back

pressure on the performance characteristics-opening pressure, closing pressure, lift,

and relieving capacity.

Blow-down

Blow-down is the difference between the set pressure and the resealing pressure of a

pressure relief valve, expressed as a percent of the set pressure, or in PSI.

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 589

Burst pressure

Burst pressure is the value of inlet static pressure at which a rupture disk device

functions.

Conventional safety relief valve

A conventional safety relief valve is a closed bonnet pressure relief valve that has the

bonnet vented to the discharge side of the valve. The performance characteristics-

opening pressure, closing pressure, lift, and relieving capacity are directly affected

by changes of the back pressure on the valve.

Design pressure

Design pressure is the pressure used in the design of a vessel to determine the minimum

permissible thickness or physical characteristics of the different parts of a vessel.

Flare

A ﬂare is a means for safe disposal of waste gases by combustion. With an elevated

ﬂare the combustion is carried out at the top of a pipe or stack where the burner and

igniter are located. A ground ﬂare is similarly equipped except that combustion is

carried out at or near ground level. A burn pit differs from a ﬂare in that it is normally

designed to handle both liquids and vapors. Flare systems are described and discussed

more fully in “Offsite Systems”of this chapter.

Lift

Lift is the actual travel of the disk away from closed position when the valve is

relieving.

Overpressure

Overpressure is the pressure increase over the set pressure of the primary relieving

device; it would be termed accumulation when the relieving device is set at the

maximum allowable working pressure of the vessel.

Note: When the set pressure of the ﬁrst (primary) pressure relief valve to open is less

than the maximum allowable working pressure of the vessel, the overpressure may be

greater than 10% of the set pressure of the valve.

590 CHAPTER 13

Pilot-operated pressure relief valve

A pilot-operated pressure-relief valve is one that has the major ﬂow device combined

with and controlled by a self-actuated auxiliary pressure relief valve. This type valve

does not utilize an external source of energy.

Pressure relief valve

Pressure relief valve is a generic term applied to relief valves, safety valves, or safety

relief valves.

Relieving conditions

Relieving conditions pertain to pressure relief device inlet pressure and temperature

at a speciﬁc overpressure. The relieving pressure is equal to the valve set pressure

(or rupture disk burst pressure) plus the overpressure. The temperature of the ﬂowing

ﬂuid at relieving conditions may be higher or lower than the operating temperature.

Set pressure

Set pressure in psig is the inlet pressure at which the pressure relief valve is adjusted

to open under service conditions. In a safety or safety relief valve in gas, vapor, or

steam service, the set pressure is the inlet pressure at which the valve pops under

service conditions. In a relief or safety relief valve in liquid service, the set pressure

is the inlet pressure at which the valve starts to discharge under service conditions.

Superimposed back pressure

Superimposed back pressure is static pressure existing at the outlet of a pressure relief

device at the time the device is required to operate. It is the result of pressure in the

discharge system from other sources. This type of back pressure may be constant or

variable; it may govern whether a conventional or balanced-type pressure relief valve

should be used in speciﬁc applications.

Vapor depressing system

A vapor depressing system is a protective arrangement of valves and piping intended

to provide for rapid reduction of pressure in equipment by release of vapors. Actuation

of the system may be automatic or manual.

Vent stack

A vent stack is the elevated vertical termination of a disposal system which discharges

vapors into the atmosphere without combustion or conversion of the relieved ﬂuid.

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 591

Types of pressure relief valves

The following is a list of those types of relief valves commonly used in industry. These

have been approved according to ASME V111 “Boiler and Pressure Vessel”code.

Conventional safety relief valves

In a conventional safety relief valve the inlet pressure to the valve is directly opposed

by a spring closing the valve, the back pressure on the outlet of the valve changes the

inlet pressure at which the valve will open. A diagram of a conventional relief valve

is shown below as Figure 13.37.

Balanced safety relief valves

Balanced safety valves are those in which the back pressure has very little or no

inﬂuence on the set pressure. The most widely used means of balancing a safety relief

Figure 13.37. A diagram of a conventional safety relief valve.

592 CHAPTER 13

Figure 13.38. A diagram of a balanced safety relief valve.

valve is through the use of a bellows. In the balanced bellows valve, the effective area

of the bellows is the same as the nozzle seat area and back pressure is prevented from

acting on the top side of the disk. Thus the valve opens at the same inlet pressure

even though the back pressure may vary. A diagram of a balanced safety relief valve

is shown as Figure 13.38.

Pilot operated safety relief valves

A pilot-operated safety relief valve is a device consisting of two principal parts,

a main valve and a pilot. Inlet pressure is directed to the top of the main valve

piston, and with more area exposed to pressure on the top of the piston than on the

bottom; pressure, not a spring, holds the main valve closed. At the set pressure the