1

Surface Production OperationsENPE 505

Lecture Notes #8Pumps and Compressors

Hassan Hassanzadeh

EN B204Mhhassanz@ucalgary.ca

Pumps and Compressors

Learning Objectives

• choose appropriate pump and compressor for transportation of crude oil and natural gas streams

3

Pumps

Pressure

Flo

w ra

te

Reciprocating

Rotary

Centrifugal

4

Pumps and compressorsCrude oil and natural gas are transmitted over short and long distances mainly through

pipelines. Pumps and compressors are used for providing pressures required for the

transportation.

Double-action stroke in a duplex pump

Single-action stroke

in a triplex pump

The double-action stroke is used for duplex (two pistons) pumps. The single-action stroke

is used for pumps with three or more pistons (e.g., triplex pump). Normally, duplex pumps

can handle higher flow rate and triplex pumps can provide higher pressure.

PumpsReciprocating piston pumps are widely used for transporting crude oil through pipelines.

There are two types of piston strokes: the singlesingle--action piston strokeaction piston stroke and the double-

action piston stroke.

5

Triplex PumpsThe work per stroke for a single piston is expressed as

The work per one rotation of crank is

where

P = pressure, lb/ft2

L = stroke length, ft

D = piston diameter, ft.

Thus, for a triplex pump, the theoretical power is

where N is pumping speed in strokes per minute.

The theoretical horsepower is or

The input horsepower needed from the prime mover is

where em is the mechanical efficiency of the mechanical

system transferring power from the prime mover to the fluid

in the pump. Usually em is taken to be about 0.85.

1Hp=550 ft.lbf/sec

6

Triplex Pumps (cont.)The theoretical volume output from a triplex pump per revolution is

The theoretical output in bbl/day is thus

If we use inches (i.e., d [in.] and l [in.]), for D and L, then

The real output of the pump is dependent on how efficiently the pump can fill the

chambers of the pistons. Using the volumetric efficiency ev gives:

or

where ev is usually taken to be 0.88–0.98.

As the above volumetric equation can be written in d and l, then the horsepower

equation can be written in d, l, and p (psi).

7

Duplex Pumps

The work per stroke cycle is expressed as

The work per one rotation of crank is

Thus, for a duplex pump, the theoretical power is

Power = 2x

The theoretical horsepower is

The input horsepower needed from the prime mover is

8

Duplex Pumps (cont.)The theoretical volume output from the double-acting duplex pump per revolution is

The theoretical output in gals/min is thus

If we use inches (i.e., d [in.] and l [in.]), for D and L, then

The real output of the pump is

or that is,

As in the volumetric output, the horsepower equation can also be reduced to a

form with p, d1, d2, and l

9

Duplex Pumps (cont.)

we have

and

The usual form of this equation is in p (psi) and q (gal/min):

That is:

The other form of this equation is in p (psi) and qo (bbl/day) for oil transportation:

10

CompressorsWhen natural gas does not have sufficient energy to flow, a compressor station is

needed. Five types of compressor stations are generally used in the natural gas

production industry:

1. Field gas-gathering stations to gather gas from wells in which pressure is

insufficient to produce at a desired rate of flow into a transmission or distribution

system. These stations generally handle suction pressures from below

atmospheric pressure to 750 psig and volumes from a few thousand to many

million cubic feet per day.

2. Relay or main-line stations to boost pressure in transmission lines compress

generally large volumes of gas at a pressure range between 200 and 1,300

psig.

3. Re-pressuring or recycling stations to provide gas pressures as high as 6,000

psig for processing or secondary oil recovery projects.

4. Storage field stations to compress trunk line gas for injection into storage wells

at pressures up to 4,000 psig.

5. Distribution plant stations to pump gas from holder supply to medium- or high-

pressure distribution lines at about 20–100 psig, or pump gas into bottle storage

up to 2,500 psig.

11

Types of CompressorsThe compressors used in today’s natural gas production industry fall into two

distinct types:

Reciprocating (most commonly used)1. They are built for practically all pressures and volumetric capacities

2. Reciprocating compressors have more moving parts and, therefore, lower

mechanical efficiencies than rotary compressors.

3. A typical reciprocating compressor can deliver a volumetric gas flow rate

up to 30,000 cubic feet per minute (cfm) at a discharge pressure up to

10,000 psig.

Rotary compressors

Centrifugal compressor Centrifugal compressor

1. Centrifugal compressors have few moving parts because only the impeller

and shaft rotate. Thus, its efficiency is high and lubrication oil consumption

and maintenance costs are low.

2. high-capacity, low-pressure ratio machines that adapt easily to series

arrangements within a station.

3. Typically, the volume is more than 100,000 cfm and discharge pressure is

up to 100 psig.

12

Types of Compressors (cont.)Rotary compressors

Rotary blowerRotary blower

primarily used in distribution systems where the pressure differential between

suction and discharge is less than 15 psi15 psi. They are also used for

refrigeration and closed regeneration of adsorption plants.

The rotary blower has several advantages: large quantities of low-pressure

gas can be handled at comparatively low horsepower, it has small initial

cost and low maintenance cost, it is simple to install and easy to operate,

it requires minimum floor space for the quantity of gas removed, and it has

almost pulsation-less flow.

As its disadvantages, Rotary blower cannot withstand high pressures, it has

noisy operation because of gear noise and clattering impellers, it

improperly seals the clearance between the impellers and the casing, and

it overheats if operated above safe pressures.

Typically, rotary blowers deliver a volumetric gas flow rate of up to 17,000 cfm

and have a maximum intake pressure of 10 psig and a differential

pressure of 10 psi.

13

Approximate ranges of application for compressors.

14

Reciprocating CompressorsHorsepower for a single stage compression

The compression ratio in each stage should be less than six to increase

compression efficiency. The equation to calculate stage-compression ratio

iswhere Pdis, Pin, and ns are final discharge pressure, inlet

pressure, and number of stages, respectively.

For a two-stage compression, the

compression ratio for each stage

should be

For greater number of stages

k=cp/cv, T is oR, p in psi

Reciprocating Compressors

in practice, although this equation results in minimum power, the net work or

energy required varies only by a fraction of a percent for relatively large

variations in the compression ratio. This is an important fact, often used for

flexibility in design for economic and technical reasons.

Example: A gas is being compressed from 100 psia and 150 oF to 2500 psia.

Determine the compression ration with and without cooling:

s

ss

n

nn

s

do

p

pr

/1

/1/1

25100

2500=

=

=

without cooling acceptablenot andhigh too25,1 →== rnfor s

acceptable 65,2 →<== rnfor s

s

ss

n

nn

s

do

p

pr

/1

/1/1

773.2597.0

1

100

2500

97.0

1=

=

=

with cooling

acceptable 6077.5,2 →<== rnfor s

16

Reciprocating Compressors (cont.)Reciprocating compressors have a clearance at the end of the piston. This

clearance produces a volumetric efficiency ev. The relation is given by

where є is the clearance ratio defined as the clearance volume at the end of the

piston stroke divided by the entire volume of the chamber (volume contacted by

the gas in the cylinder).

In addition, there is a mechanical efficiency em of the compressor and its prime

mover.

The required minimum input prime mover motor to practically operate the

compressor (either reciprocating or rotary) is

where ev = 0,80 - 0.99 and em = 0.80 to 0.95 for reciprocating compressors, and

ev = 1.0 and em = 0.70 to 0.75 for rotary compressors.

17

Fuel consumption of prime movers versus load

The usable prime mover power ratio is

Fuel consumption of prime

movers using three types of

fuel.

Fuel consumption of prime movers

using natural gas as fuel.

18

Effect of elevation on prime mover power

19

Centrifugal Compressors

The procedure of preliminary calculations for selection of centrifugal compressors is

summarized as follows:

1. Calculate compression ratio based on the inlet and discharge pressures:

2. Based on the required gas flow rate under standard condition (q), estimate the

gas capacity at inlet condition (q1) by ideal gas law:

3. Find a value for the polytropic efficiency Ep from the manufacturer’s

manual based on q1 or where q1 is the gas capacity

at the inlet condition, cfm.

4. Calculate polytropic ratio (n-1)/n using

5. Calculate discharge temperature by

20

Centrifugal Compressors (cont.)6. Estimate gas compressibility factor values at inlet and discharge conditions.

7. Calculate gas capacity at the inlet condition (q1) by real gas law:

8. Repeat Steps 2–7 until the value of q1 converges within an acceptable

deviation.

9. Calculate gas horsepower by

Some manufacturers present compressor specifications using polytropic head

in lbf –ft/lbm defined

where R is the gas constant given by 1544/MWa in psia-ft3/lbm-R. The

polytropic head relates to the gas horsepower by

where mt is mass flow rate in lbm/min.

10. Calculate gas horsepower by:

where ∆Hpm = 50, is mechanical power losses, which is usually taken as 20 horsepower for bearing and 30 horsepower for seals.