Looking for a Report Templates? Download it for free!

Table of Contents

POSITIVE DISPLACEMENT RECIPROCATING PUMP FUNDAMENTALS—

POWER AND DIRECT ACTING TYPES

0.028 for septuplex single or double acting

acceleration head equation to approximately 20 to 25 times the

0.022 for septuplex single or double acting

suction pipe diameter for bladder types, and to approximately eight

= Acceleration of gravity = 32.2 feet/sec

(9.8 m/s

)

to 10 times the suction pipe diameter for larger volume flow

= Liquid factor of…

through types.

2.5 for highly compressible hydrocarbons

4. Increase available static head by elevating the suction vessel,

2.0 for most hydrocarbons

1.5 for water, amine, glycol

raising the fluid level in the suction vessel, or possibly lowering the

1.4 for deaerated water

pump location.

Since this equation is based on ideal conditions of a relatively

5. Cooling the fluid being pumped after it leaves the suction vessel

short, nonelastic suction line, calculated values of H

should be

to lower the vapor pressure, now making this pressure reduction a

considered as approximations only.

portion of the NPSHA.

•

Notes:

NPSHR

1. As pump speed (N) is increased, mean flow velocity (V)

Pump NPSHR for a reciprocating machine is not analogous to

also increases. Therefore, acceleration head (H

) varies as the

that of a centrifugal machine. Understanding the difference

square of pump speed.

between the two types of NPSHR is fundamental in ensuring that

the pump operates with the correct NPSH margin.

2. Acceleration head varies directly with actual suction pipe

NPSHR for a reciprocating machine is defined as the pressure at

length (L).

which the pump has experienced a loss of no more than a 3 percent

reduction in capacity to obtain satisfactory volumetric efficiency at

3. Acceleration head is a suction piping system factor that

a specific pressure and speed, and relates to pump suction valve

must be accounted for by the piping system designer. Pump

losses, pump suction manifold acceleration head, plus pump

manufacturers cannot account for this in their pump designs

suction manifold and pumping chamber frictional losses.

because of the large variety of applications and piping systems

Conversely for a centrifugal pump NPSHR relates to 3 percent

pumps are installed in.

reduction in head at a constant flow.

NPSHR for a reciprocating pump is generally determined by

4. If acceleration head is ignored or miscalculated, significant

testing on cool water, and a 3 percent loss in capacity is an indication

pump and piping systems (suction and discharge) may result.

that the pump is starting to or has entered into a cavitational state

Net Positive Suction Head

causing the reduction in capacity.

Figure 30 is a typical NPSHR curve for a 4 ¼ inch stroke single

Net positive suction head (NPSH) for positive displacement

acting triplex power pump, which has a range of plunger sizes

reciprocating pumps is normally expressed in pressure units (psi,

available with a given fluid cylinder size. As can be seen, NPSHR

kPa, Bar) since a significant portion of pump NPSHR is the

varies as a function of plunger size and pump speed within a given

pressure requirement needed to push the suction valve from its

fluid cylinder size. The key influencing factors are pump speed,

seat, to overcome friction losses and acceleration head within the

plunger size, valve spring load and spring rate, valve lift, valve

fluid cylinder, rather than an energy per unit mass (or head)

passage area, cylinder passage configuration, suction manifold

requirement.

configuration, and power end connecting rod center to center of

Expressed in pressure units it becomes independent of fluid

bearing distance to throw radius ratio.

density. NPSHR pressure units are not expressed as “gauge”

(above atmospheric pressure) nor “absolute” (above absolute zero),

but a pressure measurement above vapor pressure.

NPSH is divided into two important aspects: what is available

(NPSHA) from the suction vessel and piping, and what is required

by the pump (NPSHR).

NPSHA

Suction system NPSHA is the pressure provided above fluid

vapor pressure to ensure the pump is provided with the NPSH it

requires, plus an adequate amount of pressure to overcome piping

friction losses and acceleration head generated by the pump. To

determine the necessary minimum, total NPSHA must be equal to

or greater than pump NPSHR + pipe friction losses + acceleration

head. If suction system design cannot achieve this requirement,

NPSHA can be increased by one or more of the following remedies.

Figure 30. Typical NPSHR Curve.

1. Increase suction pipe diameter to make a larger volume of fluid

available adjacent to the pump suction connection and reduce

It has been suggested that some value above the pump

suction piping flow velocity, which will reduce acceleration head

manufacturers stated NPSHR be used by system designers to avoid

and piping friction losses.

the possibility of operating the pump in a cavitational state.

2. Shorten suction piping length by providing a more direct route

Suggested values for this margin ranges from 103 percent to 125

between the pump and suction vessel, relocate the pump closer to

percent of the NPSHR, and varies based upon the type of fluid

the suction vessel, or some combination of both. Again, reducing

being pumped. The lower the fluid specific gravity, the higher this

acceleration head and piping friction losses.

safety factor margin becomes.

Design constraints placed on the system designer can be

3. Install a suction pulsation dampener or stabilizer adjacent to the

accommodated by modifying a reciprocating pump design. The

pump suction connection. This will reduce the pulsating fluid mass

following factors can be adjusted to provide a more suitable

inertia effect by changing the suction piping length portion in the

NPSHR values.

"Positive Displacement Reciprocating Pump Fundamentals - Power And Direct Acting Types" By Herbert H. Tackett, Jr., James A. Cripe, Gary Dyson Page 11

Related Articles

Related forms

Related Categories