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

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966 CHAPTER 18

rise to surge have been speciﬁed. From this the head produced by the compressor at

the surge point can be back-calculated using the head-pressure ratio relationship. The

maximum capacity point is speciﬁed to be at least 115% capacity at 85% of normal

head.

The head-capacity curve retains its characteristic shape with changes in speed. Curves

at other speeds can be obtained from the three known points on the 100% speed curve

by using the following relationships:

1. The polytropic head varies directly as the speed squared.

2. The capacity varies directly as speed.

3. The efﬁciency remains constant.

Figure 18.26 shows a typical centrifugal compressor performance curve.

Control

Speed

Speed control is the most efﬁcient type of control from an energy consideration. It

requires, however, that a variable speed driver such as a steam turbine or gas turbine,

or a variable-speed electric motor be used. The compressor is controlled by shifting

its performance curve to match the systems requirement.

Suction throttling

Adjustable inlet guide vanes. Adjustable inlet guide vanes are the most efﬁcient

method of adjusting the capacity of a constant speed compressor to match the

system characteristics. They consist of a venetian blind device that is positioned

by a rack and pinion linkage. While the guide vanes do some throttling, their main

effect is to change the velocity of the gas to that of the impeller vane by changing

the direction of ﬂow. This changes the head produced and in effect changes the

characteristic of the machine.

Suction throttle. This control consists of a control valve located in the compressor

suction which regulates the suction pressure to the compressor. The control valve

results in a greater power loss compared to adjustable inlet guide vane control since

it is a pure throttling effect. Suction throttle valves are lower in cost than adjustable

inlet guide vanes.

Discharge throttling. This control consists of a control valve located in the compres-

sor discharge. Discharge throttle valves are seldom used since they offer relatively

little power reduction at reduced capacity. The effect is simply to “push”the com-

pressor back on its curve.

PROCESS EQUIPMENT IN PETROLEUM REFINING 967

Figure 18.26. An Example of a centrifugal compressor performance curve.

Seals

Table 18.23 shows the types of seals that are commonly used in centrifugal compres-

sors. The start-up as well as the operating conditions of the compressor should be

considered in selecting a seal. Often the system is evacuated when hydrocarbons are

handled prior to its start-up. This requires that the seal be good for vacuum conditions.

968 CHAPTER 18

Table 18.23. Centrifugal compressor seals.

No Application Gas being handled Inlet pressure, psia Seal arrangement

1 Air compressor Atmospheric air Any Labyrinth

2 Gas

compressor

Non-corrosive

Non-hazardous

Non-fouling

inexpensive

Any Labyrinth

3 Gas

compressor

(note 1)

Non-corrosive or

Corrosive

Non-hazardous or

Hazardous

Non-fouling or Fouling

10–25 Labyrinth with injection

or ejection of ﬂuid

being handled.

4 Gas

compressor

Non-corrosive

Non-hazardous

Non-fouling

All pressures Oil seal combined with

Lube oil system.

5 Gas

compressor

Corrosive

Non-hazardous or

Hazardous

All pressures Oil seal with seal oil

Separate from lube oil

System.

Note 1: Where some gas loss or air induction is tolerable.

Specifying a centrifugal compressor

The process speciﬁcation must give all the information concerning the gas that is to be

handled, its inlet and outlet conditions, the utilities that are available and the service

that is required of the compressor. The process speciﬁcation sheet given here shows

the minimum that a process engineer should provide in approaching manufacturers.

An explanation of this speciﬁcation now follows covering each of the items in the

speciﬁcation.

Title block

This requires the item to be identiﬁed by item number and its title. The number of

units that the speciﬁcation refers to is also given here. For a centrifugal compressor

this will normally be just one as very seldom is a spare machine required.

Normal and rated columns

More often than not the conditions and quantities required to be handled will vary

during the operation of the machine. The two columns therefore will be completed

showing the average normal data in the ﬁrst column and the most severe conditions

and duty required by the compressor in the second. The severe conditions in column

two are for a continuous length of operation not instantaneous peaks (or troughs) that

may be encountered.

PROCESS EQUIPMENT IN PETROLEUM REFINING 969

Gas

The composition and gas stream identiﬁcation must be included as part of the process

speciﬁcation. Usually the composition of the gas is listed on a separate sheet as

shown in the example. Note in many catalytic processes that utilize a recycle gas the

composition of the gas will change as the catalyst in the process ages. Thus it will be

necessary to list the gas composition at the start of the run (SOR) and at the end of

the run (EOR).

The compressor may also be required to handle an entirely different gas stream at

some time or other. This too must be noted. For example in many petroleum reﬁning

processes a recycle compressor normally handling a light predominately hydrogen

gas is also used for handling air or nitrogen during catalyst regeneration, purging, and

start-up.

Volume ﬂow

This is the quantity of gas to be handled stated at 14.7 psia and 60 F.

Weight ﬂow

This is the weight of gas to be handled in either lbs/min or lbs/hr.

Inlet conditions

Pressure: This is the pressure of the gas at the inlet ﬂange of the compressor in psia.

Temperature: This is the temperature of the gas at the inlet ﬂange of the compressor.

Mole weight: The mole weight of the gas is calculated from the gas composition given

as part of the speciﬁcation.

: This is the ratio of speciﬁc heats of the gas again obtained from the gas mole

wt and Table 18.A.1 in the Appendix.

Compressibility factor (z): use the value at inlet conditions calculated as shown in

step 7 of the chapter ‘Calculating the Horsepower of Centrifugal Compressors’.

Inlet volume: This is the actual volume of gas at the conditions of temperature and

pressure existing at the compressor inlet. Thus:

ACFM =

SCFM × 14.7 × (inlet temp F + 460)

(60 F + 460) × inlet press psia

Discharge conditions

Pressure: This is the pressure at the compressor outlet ﬂange and is quoted in either

psia or psig.

Temperature: This is estimated using Figure 18.24.

970 CHAPTER 18

: This will be the same as inlet.

Adiabatic efﬁciency: Will be as given in Figure 18.27.

Approximate driver horsepower

This item will include the adiabatic (or gas) horsepower as calculated in the section

on calculating the horsepower of centrifugal compressors in this chapter plus the

following losses:

Leakage loss—1% of Adiabatic HP

Seal losses—35 HP for all HP ranges.

Bearing loss—5 HP for all HP ranges.

The reminder of the spec sheet contains all the essential data and requirements that may

affect the duty and performance of the compressor. Much of this is self explanatory

however there are some items that require comment. These are:

1. Most compressor installations today are under an open sided shelter with a small

overhead gantry crane assembly for maintenance.

2. Usually the lube and seal oil assemblies have their own pump and control systems.

Consequently even if the compressor itself is to be steam driven there may still be

need to give details of utilities for the ancillary equipment.

3. Details of the gas composition is essential for any development of the compressor.

This is listed on the last page of the speciﬁcation together with any notes of

importance concerning the machine and its operation.

An example calculation for a speciﬁcation sheet follows:

Example calculation

Prepare a process speciﬁcation sheet for a compressor to handle the hydrogen recycle

stream in an o-xylene isomerization plant. Details are as follows:

Fresh feed rate : 5,000 bpsd of m-xylene.

Recycle gas rate : 7,000 Scf of hydrogen per Bbl of Fresh feed.

Gas composition mole % :

Start of Run End of Run

(SOR) (EOR)

85.00 68.78

4.4 9.17

4.2 8.86

3.6 7.36

0.78 1.62

0.99 2.05

s 1.03 2.16

Figure 18.27. Adiabatic efﬁciencies for centrifugal compressors.

972 CHAPTER 18

Suction pressure : 150 psig

Reactor pressure : 500 psig.

Suction temperature : 100 F

Step 1. Calculate the mole weight of the gas.

SOR EOR

mole% MW wt Factor mole% wt Factor

85.0 2 170 68.78 138

4.4 16 70 9.17 147

4.2 30 126 8.86 266

3.6 44 158 7.36 324

0.78 58 45 1.62 94

0.99 58 57 2.05 119

s 1.03 72 74 2.16 156

Total 100.00 700 100.00 1,244

MW 7.0 12.44

Step 2. Calculate volume ﬂow of gas in SCF/min.

Total volume of H

required = 5,000 BPSD ×7,000 SCF

= 35.00 mmScf/day

= 24,306 Scf/min.

For SOR volume gas Flow =

24,306

0.85

= 28, 595 Scf/min.

For EOR volume gas ﬂow =

24,306

0.6878

= 35, 339 Scf/min.

Step 3. Calculate weight ﬂow in lbs/min.

moles/min of gas =

Scf/min

378

For SOR moles/min = 75.6

For EOR moles/min = 93.5

lbs/min for SOR = 75.6 × 7.0 = 529 lbs/min

lbs/min for EOR = 93.5 × 12.44 = 1,163 lbs/min

Step 4. Calculate ACFM at inlet conditions.

Compressor inlet pressure = 165 psia

” ” T emp = 100 F

PROCESS EQUIPMENT IN PETROLEUM REFINING 973

For SOR ACFM =

28,597 × 14.7 × 560

520 × 165

= 2,744 cft/min

For EOR ACFM =

35,339 × 14.7 × 560

520 × 165

= 3,391 cft/min

Step 5. Estimate the C

ratio.

The molal proportions will be used for this purpose. The ratio for each component

will be taken from Table 18.A.1 in the Appendix.

SOR EOR

mole% C

fact mole% C

fact

1.40 85.0 119 68.78 96.29

1.30 4.4 5.7 9.17 11.92

1.22 4.2 5.12 8.86 10.81

1.14 3.6 4.10 7.36 8.39

1.11 0.78 0.87 1.62 1.80

1.11 0.99 1.10 2.05 2.28

S 1.09 1.03 1.12 2.16 2.35

Total 100.00 131.9 100.00 133.84

Then C

for the gas is:

SOR = 1.319

EOR = 1.338

Step 6. Calculate compressibility factors.

Z =

MW × P

T × ρ

× 10.73

For SOR ﬂows Z =

7.0 × 165

560 × 0.193 × 10.73

= 0.996

wt lbs/min

ACFM

For EOR ﬂows Z =

12.46 × 165

560 × 0.343 × 10.73

= 0.998

Step 7. Calculate outlet temperature.

974 CHAPTER 18

Approx discharge temperature is read from Figure 18.24 in this chapter using the

following:

ACFM for SOR = 2,744

ACFM for EOR = 3,391

Compression ratio =

515

165

= 3.12

Inlet temp F = 100

Then

Discharge temp for SOR = 370 F

”””EOR = 340 F

Step 8. Calculate approx driver HP.

ave

× R × T

(K − 1)/K







(K −1)/K

− 1



where

= adiabatic head in ft lbs/lb

Z = compressibility factor (ave)

R = gas constant = 1,545/MW

K = adiabatic exponent C

= 1.15

T = temperature in

◦

R =

◦

F + 460

◦

= discharge pressure psia

= suction pressure psi

Then for SOR:

Had for SOR conditions = 161,063

Had for EOR conditions = 91,546

Step 9. The gas HP is obtained using the expression

Gas HP =

W × H

× 33,000

where

W = weight in lbs/min of gas

= adiabatic head in ft lbs/lb

η = adiabatic efﬁciency (0.7–0.75)

Let η

be 0.73

For SOR Gas HP = 3,536

For EOR Gas HP = 4,420

PROCESS EQUIPMENT IN PETROLEUM REFINING 975

Step 10. The driver HP is as follows:

SOR EOR

Gas HP 3,536 4,420

Leakage losses 35 44

Bearing losses 35 35

Seal losses 35 35

DRIVER HP 3,641 4,534

Calculating reciprocating compressor horsepower

Reciprocating compressors are used extensively in the process industry. They vary

in size from small units used for gas recovery (such as those on a crude distillation

overhead system) to fairly large complex machines used for recycle gas streams and

for transporting natural gas. Engineers are frequently required therefore to assess the

horsepower of these machines and their capability to handle various streams. This item

describes a method used to determine horsepower and proceeds with the following

steps:

Step 1. Obtain the capacity and the properties of the gas to be handled. Fix the ultimate

(discharge) pressure level.

Step 2. From the machine data sheet ascertain the number of stages.

Step 3. Estimate the brake horsepower from the expression:

BHP = 22 × (compression ratio/stage) × number of stages

× capacity (in cuft/day × 10

) × F

where

F = 1.0 for 1 stage

1.08 for 2 stages

1.10 for 3 stages

Ratio/Stage =

√

ratio for two stages and

√

ratio for three stages.

Step 4. Check the estimate with Figure 18.28. The use of these graphs is self explana-

tory.

Step 5. Conﬁrm actual suction conditions and compression ratio required (discharge

pressure).

Step 6. Calculate compression ratio/stage.

Step 7. Calculate 1st stage discharge pressure. This will be suction pressure times

compression ratio per stage from Step 6.

Step 8. Allow about 3% for inter-stage pressure drop then calculate second stage

discharge pressure. Check that overall compression ratio/stage is close to that cal-

culated for Step 6.