Positive Displacement Pump Design and Calculation


Positive Displacement Pump makes a fluid move by traping a fixed amount and forcing (displacing) that trapped volume into the discharge pipe. Some positive displacement pumps use an expanding cavity on on the suction side and a decreasing cavity on the suction side expands and the liquid flows out of the discharges side. Liquids flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The Volume is constant through each cycle operation. (Source : Wikipedia)

Positive displacement pump, unlike centrifugal or roto dynamic pump, theoritically can produce the same flow at given speed (RPM) no matter what the discharge pressure. Thus, positive displacement pumps are constant flow machines. However, a slight increase in internal leakage as the pressure increases prevents a truly constant flowrate.

A positive displacement pump must not operate againts a closed valve on the discharge side of the pump, because it has no shutoff head like centrifugal pump. A positive displacement pump operating againts a closed discharge valve continues to produce flow and the pressure in the discharge line increases until the line burst, the pump is severely damaged, or both.

A relief or safety valve on the discharge side of the positive displacement pump is therefore necessary. The relief valve can be internal or external. (Source: Wikipedia)

As my experience, discharge from positif displacement pump, can be transfered to the tank, or you can circulate back into suction pump.

How to design Positive Displacement Pump ?

API 675 Standard covers the minimum requirement for controlled volume positive displacement pumps for use in service in petroleum, chemical, and Gas industries. Both packed-plunger and diaphgram types are included. Diaphgram pump that use direct mechanical actuation are excluded.

Definiton of Terms :

The alarm point is a preset value of a parameter at which an alarm is activated to warn of a condition that requires corrective action. You can call at the field is LLL (Low-Low Level).  That  define point to shutdown pump automatically/ manual.

A controlled Volume pump is a reciprocating pump which precise volume control is provided by varying ffective strioke length. Such pumps are also known as proportioning, chemical injection, dosing or metering Pumps.

Differential pressure  is the difference between discharge pressure and suction pressure.

In a packed-plunger pump, the process fluid is in direct contact with the plunger.

In a diaphgram Pump , the process fluid is isolated from the plunger by means of hydraulically actuated flat or shaped diagram.

So, in case of project, you have to choose which is better for your process system. It will compare with system, cost, time schedule, procurement, etc.

diaphgram

Diaphgram pump (membrane pump) is a positive displacement pump that uses a combination of the reciprocating action of a rubber, thermoplastic or Teflon diaphgram and suitable valves on either side of the diaphgram (check valve, butterfly valve, etc) to pump a fluid. (Wikipedia).

Diaphgram pump need air supply from field to operate pump. It means if in your project have a compressor, or Plant air, to operate pump from Local control room, you need solenoid valve to control fluid at system.

Plunger pump is different with diaphgram pump. Plunger pump is type of positive displacement pump where the high pressuer seal is stationary and a smooth cylindrical plunger slides through the seal. This makes them different from piston pumps and allows them to be used at higher pressure. This type of pump is often used to transfer municipal and industrial sewage.

reciprocating

For plunger pump is no need air consumption. Driver use electric motor. Pump will operate from local control room with HMI.

Herewtih I inform to you format datasheet Pump as per API 675. You can prepare datasheet with several format. There are with SI Unit :

si-unit-3

si-unit-2

And from Costumary Unit :

Reference : API 675

NPSH or NPIP is indicated as either Available or required. The Net Positive Inlet Presure Available is the absolute presure Above Fluid Vapor Pressure at the pump inlet and determined as follows :

npipa2

pamax

NPIPa must bigger than NPIPr. NPIPa > NPIPr. It cause to prevent occurence of cavitation. See youtube if you want to know what effect if NPIPr > NPIPa or NPSHr > NPSHa.

Example Calculation :

Pa = 1 atm = 101,325 kpa

Pz = +0,622 mm = 6,0956 kpa  (If water level below pump, number will negative dosing-shelter

calculation-1

V = Q (flow rate) / A (area)

ɛ = koef. Losses from suction + strainer (see table)

tee

Pf = Friction Losses on Pipe line, elbow, strainer.

Pf = Hl-pipe +Hel-elbow

Pf = 0.006 kpa

Pvp = 0,0752 kg/cm2 = 7,37 kpa,

Length of pipe = 1,9 m

pamax-2

Pamax = 25,409 kpa

npip

NPIPa = 75.91 kpa = 7.74 m

NPIPr (based on manufacture pump ) = 5 m

So, NPIPa > NPIPr.

Thats it, hopefull this note can help you to design and calculate Positive Displacement Pump. Please search another books, article, calculation, etc. And you can compare one and each other.

Thanks.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s