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

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926 CHAPTER 18

reciprocating pumps is pulsating. Reciprocating pumps fall into two general cate-

gories. These are the simplex type and the duplex type. In the case of the simplex

pump there is only one cylinder which draws in the ﬂuid to be pumped on the back

stroke and discharges it on the forward stroke. External valves open and close to

enable the pumping action to proceed in the manner described. The duplex pump has

a similar pumping action to the simplex pump. In this case however there are two

parallel cylinders which operate on alternate stroke to one another. That is, when the

ﬁrst cylinder is on the suction stroke the second is on the discharge stroke.

Reciprocating pumps may have direct acting drives or may be driven through a

crankcase and gear box. In the case of the direct acting drive the pump piston is con-

nected to a steam drive piston by a common piston rod. The pump piston therefore is

actuated by the steam piston directly. Reciprocating pumps driven by electric motors,

turbines, etc are connected to the prime mover through a gearbox and crankcase.

Other positive displacement pumps

There are other positive displacement pumps commonly used in the process industry

for special services. Some of these are:

Metering or proportioning pumps. These are small reciprocating plunger type pumps

with an adjustable stroke. These are used to inject ﬁxed amount of ﬂuids into a larger

stream or vessel.

Diaphragm pumps. These pumps are used for handling thick pulps, sludge, acid or

alkaline solutions, and ﬂuids containing gritty solid suspensions. They are partic-

ularly suited to these kind of service because the working parts are associated with

moving the diaphragm back and forth to cause the pumping action. The working parts

therefore do not come into contact with this type of ﬂuid which would be harmful to

them.

Characteristic curves

Pump action and the performance of a pump are deﬁned in terms of their Charac-

teristic Curves. These curves correlate the capacity of the pump in unit volume per

unit time versus discharge or differential pressures. Typical curves are shown in Fig-

ures 18.13–15. Figure 18.13 is a characteristic curve for a reciprocating simplex pump

which is direct driven. Included also is this reciprocating pump on a power drive.

Figure 18.14 gives typical curves for a rotary pump. Here the capacity of the pump

is plotted against discharge pressure for two levels of pump speed. The curves also

show the plot of brake horsepower versus discharge pressure for the two pump speed

levels.

PROCESS EQUIPMENT IN PETROLEUM REFINING 927

Figure 18.13. Characteristic curves for a reciprocating pumps.

Figure 18.14. Characteristic curves for a rotary pump.

928 CHAPTER 18

Figure 18.15. Characteristic curves for a centrifugal type pump.

Figure 18.15 is a typical characteristic curve for a centrifugal pump. This curve usually

shows four pump relationships in four plots. These are:

A plot of capacity versus differential head. The differential head is the difference

in pressure between the suction and discharge

The pump efﬁciency as a percentage versus capacity

The brake horsepower of the pump versus capacity

The net positive head (NPSH) required by the pump versus Capacity. The required

NPSH for the pump is a characteristic determined by the manufacturer

Pump selection

Most industrial pumping applications favor the use of centrifugal pumps. The promi-

nence of this type of pump stems from its ability to handle a very wide spectrum of

ﬂuids at a large range of pumping conditions. It is ﬁtting that in considering pump

selection the ﬁrst choice has to be the centrifugal pump and all others become a se-

lection by exception. Centrifugal pumps are generally the simplest in construction,

lowest initial cost and simplest to operate and to maintain. This item therefore begins

with the selection characteristics of the centrifugal type pump.

The centrifugal pump

Before looking at the selection of the centrifugal pump it is necessary to deﬁne the

following terminology associated with pumps in general. These are:

PROCESS EQUIPMENT IN PETROLEUM REFINING 929

Capacity

Differential Head

Available NPSH

Required NPSH

Capacity. This can be deﬁned as the amount of ﬂuid the pump can handle per unit

time and at a differential pressure or head. This is usually expressed as gallons per

minute at a differential head of so many pounds per square inch or so many feet.

Differential head. This is the difference in pressure between the suction of the pump

and the discharge. It is usually expressed as PSI and FEET in specifying a pump. The

following formula is the conversion from PSI to Feet:

H =

2.31 × P

where

H = the differential head in feet of ﬂuid being pumped.

P = the differential pressure of the ﬂuid across the pump measured in pounds

per square inch.

SG = the speciﬁc gravity of the ﬂuid at the pumping temperature.

Available NPSH. The available NPSH is the static head available (in feet or meters)

above the vapor pressure of the ﬂuid at the pumping temperature. This is a feature of

the design of the system which includes the pump.

Required NPSH. Is the static head above the vapor pressure of the ﬂuid required by

the pump design to function properly. The required NPSH must always be less than

the available NPSH.

Selection characteristics

Selection of any pump must depend on its ability to handle a particular ﬂuid effectively,

and the efﬁciency of the pump under normal operating conditions. The second of these

primary requirements can be determined by the pump’s characteristic curves. These

have already been described earlier in this part of the chapter, and a further discussion

on these now follows:

Capacity range

Normal

Figures 18.16 and 18.17 show the normal capacity range for various types of cen-

trifugal pumps in two different speed ranges, 3,550 rpm, 2,950 rpm.

930 CHAPTER 18

Figure 18.16. Centrifugal pumps at 3,550 rpm.

These values correspond to motor full load speeds available with current at 60 and

50 cycles, respectively. Most process applications call for these speed ranges. Lower

speeds are for low or medium head and high capacity requirements, and for spe-

cial abrasive slurries or corrosive liquids. Low capacity centrifugal pump applica-

tions may require special recirculation provisions in the process design to maintain

a minimum ﬂow through the pump. Because of practical consideration in impeller

construction, the smallest available process type centrifugal pumps are rated at about

50 gpm.

PROCESS EQUIPMENT IN PETROLEUM REFINING 931

Figure 18.17. Centrifugal pumps at 2,950 rpm.

High and low capacity ranges

Pumps above the limits shown in Figure 18.16 and 18.17 will normally require large

horsepower drivers. Special investigation of efﬁciency, speed, NPSH requirements,

etc., will normally be justiﬁed. As an example, heads at or above the limits shown

for multistage pumps at standard motor speed may be obtained by speed increasing

gears (motor drive) or turbines to give pump operating speeds above maximum motor

speeds, (NPSH requirements increase with speed).

In general, centrifugal pumps should not be operated continuously at ﬂows less than

approximately 20% of the normal rating of the pump. The normal rating for the

pump is the capacity corresponding to the maximum efﬁciency point. The following

932 CHAPTER 18

table lists minimum desirable ﬂow rates which should be maintained by continuous

recirculation, if the required process ﬂow conditions are of lower magnitude:

Minimum

continuous capacity Normal rating of

Head range feet Pump type rating, gpm pump, gpm

60 Cycle speed (3,550 rpm)

To 100 1 stg 10 60

100–350 1 stg 15 75–100

350–650 2 stg 30 150

650–1,100 2 stg 40 160

400–1,200 Multistg 15 50

1,200–5,500 Multistg 40 100–120

50 Cycle Speed (2,950 rpm)

To 75 1 stg 10 50

75–250 1 stg 15 60–80

250–450 2 stg 25 120

450–775 2 stg 30 130

250–850 Multistg 10 40

850–3,800 Multistg 30 80–100

Care must be exercised in the design of any recirculation system to insure that the re-

circulated ﬂow does not increase the temperature of pump suction and cause increased

vapor pressure and reduction of available NPSH.

For low head pumps that can operate at 1,750 or 1,450 rpm, the above normal and

minimum continuous capacities are reduced by 50%.

Effect of liquid viscosity

When suitably designed, centrifugal pumps can satisfactorily handle liquids contain-

ing solids, dirt, grit and corrosive compounds. Though ﬂuids with viscosities up to

20,000 SSU (440 centistokes) can be handled, 3,000 SSU (650 centistokes) is usually

the practical limitation from an economical operating standpoint.

Effect of suction head

An important requirement is that there be sufﬁcient NPSH at the eye of the ﬁrst stage

impeller. This is static pressure above the vapor pressure of the ﬂuid handled to prevent

vaporization at the impeller eye. Flashing of the ﬂuid produces a shock or cavitation

effect at the impeller which results in metal loss, noise, lowered capacity and discharge

pressure, and rapid damage to the pump. NPSH requirements for various centrifugal

pumps will normally vary from 6 to over 20 (in feet of ﬂuid) depending on type, size

PROCESS EQUIPMENT IN PETROLEUM REFINING 933

and speed. Vertical pumps can be built for practically no NPSH at all at the nozzle.

These will have extended barrels in order to provide the required NPSH at the eye of

the ﬁrst stage impeller.

Efﬁciency

The efﬁciency of centrifugal pumps varies from about 20% for low capacity (< 20

gpm) pumps to a range of 70–80% for high capacity > 500 gpm pumps. Extremely

large capacity pumps (several thousand gpm) may have efﬁciencies up to nearly

90%.

The rotary pumps

Rotary pumps deliver constant capacity against variable discharge pressure. This is a

feature for all positive displacement pumps. Rotary pumps are available for process

application over a range of 1 to 5,000 gpm capacity and a differential pressure up

to about 700 psi. Displacement of the pump varies directly as the speed except in

the case where the capacity may be inﬂuenced by a the viscosity of the ﬂuid. In

this case thick viscous liquids may limit capacity because they cannot ﬂow into the

cylinder fast enough to keep it completely ﬁlled. Rotary pumps are used mostly

in low capacity service where the efﬁciency of a centrifugal pump would be very

low.

Reciprocating pumps

The liquid discharge from reciprocating pumps is pulsating. The degree of pulsation

is higher for SIMPLEX pumps than for the DUPLEX type. The pulsation is also

higher for direct driven pumps than for those driven by motor or turbine through a

gear box.

Pulsation is generally not a problem in the case of small low speed pumps of this

type. It affects only the associated instrumentation which can be compensated for

by local dampeners. However as the pump speed is increased the pulsation effect

becomes more serious affecting the piping design of the system. Under these condi-

tions of high pulsation instantaneous piping pressures may often exceed the design

pressure of the piping. The piping must then be designed to meet this higher pressure

requirement which invariably results in a higher cost. As an alternative, discharge

dampeners are often considered but these too add to the cost of the pump installat-

ion.

Reciprocating pumps are used mostly in situations where their low piston speeds will

withstand corrosive and abrasive conditions. They are ideal also for pumping at low

934 CHAPTER 18

capacity against high differential head and where it is necessary to maintain a constant

ﬂow rate against a gradually increasing discharge pressure.

Evaluating pump performance

Many process engineers are involved with the day to day operation of process plants.

Much of their duties in this respect is concerned with maintaining plant efﬁciency,

locating trouble areas and solving operational problems. This Item is directed to these

engineers and presents the calculation methods to check pump performance in terms

of the pump horsepower and the available NPSH for a pump.

Brake horsepower efﬁciency

Actual running efﬁciency can be calculated from plant data. This may be compared

with a typical expected efﬁciency to evaluate the pump performance. The steps below

are followed to arrive at this efﬁciency ﬁgure.

Step 1. Obtain ﬂow rate from plant readings. Also read discharge pressure and if

available suction pressure. If this later reading is not available calculate it from

source pressure, height of liquid above pump suction and frictional loss.

Step 2. Read stream temperature and obtain Sg of stream from lab data. Calculate Sg

at ﬂow conditions.

Step 3. Calculate differential head which is discharge pressure PSIA—suction pres-

sure PSIA. Convert to differential head in feet by

P × 144

62.2 × Sg (at ﬂow cond)

Step 4. Convert feed rate to pounds per hour. Then calculate hydraulic horsepower

from the expression

Head (ft) × rate (lbs/hr)

60 (mins) × 33,000 (ft-lbs/min)

Step 5. From motor data sheet obtain pump motor running efﬁciency from plant data

read power usage in kW.

Step 6. Convert pump power to HP by dividing kW by 0.746. Multiply this by pump

efﬁciency expressed as a fraction. This is the brake horsepower.

Step 7. Divide hydraulic horsepower by brake horsepower and multiply by 100 to

give efﬁciency as a percentage.

Step 8. Check against Figure 18.16 or 18.17 to evaluate pump performance.

That is, the calculated efﬁciency within a reasonable agreement with the expected

efﬁciency given by Figure 18.16 or 18.17. Should there be a large discrepancy then

the appropriate mechanical or maintenance engineers should be informed.

PROCESS EQUIPMENT IN PETROLEUM REFINING 935

Checking available NPSH

This needs to be done if the pump is showing signs of vibration and losing suction

under normal operations.

Step 1. Obtain details of the ﬂuid being pumped (temperature, Sg, ﬂow rate, source

pressure, and vapor pressure of ﬂuid).

Step 2. Calculate the frictional pressure drop in the suction line.

Step 3. Calculate the suction pressure of the pump by taking source pressure and

adding in the static head. For this calculation take static head as being from the

bottom of the vessel (not from the liquid level).

Step 4. From the suction pressure calculated in Step 3 take out loss through friction.

Step 5. Calculate NPSH available as being net suction pressure less vapor pressure.

This is usually quoted in feet (or meters) so convert using:

PSI × 144

62.2 × SG

Step 6. Check against manufacturer’s data sheet for required NPSH. If the available

NPSH is less than required the pump will continue to cavitate. Fill tank to a level

that the vibration stops and maintain it at that level.

If the problem is really troublesome and maintaining a liquid level as suggested in

step 6 above is not practical contact the pump manufacturer. Very often he is able to

make some minor changes to the pump design that will solve the problem.

Example calculation

Example calculation No. 1—Pump brake HP efﬁciency.

Flow capacity of pump at ﬂow conditions = 200 gpm

(from plant data) = 82,632 lbs/hr

Suction pressure (plant data) = 120 psig

Discharge pressure = 450 psig

Pressure = 330 psi

Sg at ﬂow conditions = 0.827

Differential head ft =

330 × 144

62.2 × 0.827

= 924 ft

Hydraulic horsepower =

82, 632 × 924

60 × 33,000

= 38.56

From motor rating, motor efﬁciency is 92%

Plant readings show motor power usage = 48.1 kW