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PROCEEDINGS OF THE TWENTY-FOURTH INTERNATIONAL PUMP USERS SYMPOSIUM • 2008

1. Reduce pump speed

allows all fluid cylinder pumping chambers to become fully primed.

a. Increase the plunger or piston size within a given model and

A bypass line with a shut-off valve should be installed in discharge

stroke length.

piping between pump and check valve back to suction supply

b. For a given stroke length, increase the quantity of plungers or

source, not into the pump suction line to prevent flow discontinuity.

pistons, i.e., use a triplex pump instead of a duplex, or a quintuplex

Install a discharge check valve beyond the bypass connection to

instead of a triplex, etc.

protect pump from discharge system pressure during pump idle

c. Change to a larger (longer) stroke length pump.

periods and pump startup.

Discharges piping “dead ends” are to be avoided or provided

with dampening device. This type of feature can be responsible for

2. Change the type of suction valve used. Lightweight valves

generally require less NPSHR due to the reduction in valve

undesirable piping harmonics and can contribute to elevated levels

of vibration and noise.

cracking pressure.

For some services the natural pump pressure or flow fluctuations

may not be appropriate. In these cases it is prudent to use a

3. If reduced pump speed is possible and changing valve type is

pulsation dampener for the installation. For maximum effectiveness

not, NPSHR could potentially be reduced by using a light (less

the dampener should be mounted adjacent to the pump fluid

spring force) suction valve spring, or if pump operating is slow

enough, no suction valve spring at all.

cylinder. Recommendations for dampener size and type can be

obtained from dampener manufacturers based on details of pump

type and size, service conditions, and piping system.

Ultimately, using a booster pump to provide ample NPSH should

Install flanges or unions as close to the pump as practical to

be considered if the system constraints dictate. Such a pump

should be installed adjacent to suction supply vessel, have an

allow for fluid cylinder removal during maintenance.

Shut-off valves are required in both suction and discharge lines

NPSHR less than total suction system NPSHA, and have a

discharge head at least 20 percent greater than positive displacement

to isolate pump when maintenance is required. They should be of

reciprocating pump NPSHR + pipe friction losses + acceleration

full opening design, such as a gate valve.

head. A suction pulsation dampener or stabilizer adjacent to the

When connecting two or more pumps to a common suction

positive displacement reciprocating pump suction connection is

and/or discharge line exercise care to prevent a mutually reinforcing

also appropriate to protect the booster pump from the pulsating

pressure wave from occurring during operation. This can be

fluid mass inertia of the positive displacement reciprocating pump.

achieved by adding the capacities of all pumps that will operate

simultaneously to determine line velocities for sizing pipe and

Pump Discharge Piping System

calculating the acceleration head. The best way to avoid a mutually

reinforcing pressure wave is to install independent suction and

Listed below are the fundamental requirements for a discharge

discharge lines to each pump.

piping system.

Figure 31 gives an example of the recommendations outlined in

Piping should not be smaller than pump discharge connection,

the previous section for an appropriate pump piping system, while

and should:

Figure 32 provides an example an inappropriate pump piping

•

Be as short and direct as possible.

system configuration.

•

Be one to two sizes larger than pump discharge connection with

increasers used at pump.

•

Contain a minimum number of turns. Accomplish any necessary

turns with long radius elbows or laterals.

•

Have a fluid velocity not exceeding three times maximum

suction line fluid velocity shown in Figure 29 for pump crankshaft

operating speed.

•

Be provided with gauge and drain connections adjacent to pump.

All positive displacement reciprocating pumps deliver fluid and

build pressure until action is taken to control and stabilize the

pump’s work or a failure occurs. To protect pump, piping, and

personnel from hazards associated with operating a “positive

displacement” pump against a “dead head” a safety relief valve

Figure 31. Piping System Appropriate Design.

should always be provided between the pump and discharge valve.

The safety relief valve should be sized to pass the entire pump

capacity and the cracking pressure should be set at 10 percent over

the specified working discharge pressure and have an accumulation

pressure not exceeding 110 percent of cracking pressure.

The safety relief valve outlet connection should ideally be piped

back to the suction supply vessel. Piping back to the suction pipe

can cause discontinuities in the suction pipe flow that can result in

poor pump operation and damage. Should it become necessary

to pipe the safety relief valve back to the suction piping, the

connection into the suction pipe should be a minimum of 10

suction pipe diameters in length back toward the suction supply

vessel away from the pump suction connection. This will allow any

flow discontinuity created by the relief valve flow into the suction

pipe to be smoothed out by time and viscous effect.

A discharge bypass line from pump discharge piping back to the

suction supply vessel permits lubrication to reach critical pump and

Figure 32. Piping System Inappropriate Design.

drive parts during startup without subjecting them to high loads and

"Positive Displacement Reciprocating Pump Fundamentals - Power And Direct Acting Types" By Herbert H. Tackett, Jr., James A. Cripe, Gary Dyson Page 12

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