Estimate the discharge pressure of PD pump?

PD Pumps give constant flow rate at different discharge heads. The head increases as the back pressure (or obstruction to flow). Better thing is to check the head at a point where your motor is not overloading and set your relief valve accordingly or just check at what head you are getting the fluid at user end.

Regards,

We cannot opearte the pump without knowing the expected disch pressure.This if our safety rules to ensure the downstream equipment will not be impacted. How can i do it if i only know the flow????
upm

The rotary pump will work against any back pressure, provided you have the horse power or kilowatts to drive the pump,therefor this means that you will need a pressure relief valve in the discharge system or built into the pump casing.
The flow is independent of pressure. You can change the flow without upsetting the pump's efficiency.
The pump can handle high viscosity fluids efficiently.
The pump is self priming
You get a smooth pulse free flow of the liquid into the system.
You can get the desirable high head low flow combination that is need in many high pressure applications.
They give you a non-shearing action that will not degrade sensitive petrochemicals and polymers

The following table will give you a feel for the capabilities of some of the above configurations:

ROTARY PUMP GPM PSI
Gear
1200 gpm
500psi

Lobe
1200 gpm
500 psi

Progressive cavity
1000 gpm
1000 psi

Three screw
1000 gpm
500 psi

Two screw
9000 gpm
1500 psi

Vane
1000 gpm
150 psi

UMP
Positive displacement pumps are a bit different than centrifugal pumps. Theoretically the "operation curve" of a PD pump looks like a line: same flow at all pressures (at the same speed) ;-).

The flow depends more from the motor speed or stroke speed.

You can estimate the discharge pressure by evaluating the system pressure drop at the pump nominal flow. Then check with your supplier
a)if the power consumption (system pressure drop @ pump nominal flow) is lower than the available motor power
b)if the discharge pressure is lower than the design pressure of the pump (MAWP).
c) check if the discharge pressure is lower than the downstream system design pressure.

usually you would find a pressure relief valve at the discharge of the pump to avoid critical situations.
RGS

Perhaps can help:

PD pumps aren't pressure- or head-developing units as centrifugals are. They are volumetric movers, in fact, many are used as meters for this reason.

The discharge pressure would be that generated by the pipe friction of the volumetric flowrate provided by the pump plus elevations and any static pressurizing that the system may have.

Shall i assume that the discharge pressure will be the same at a the given flow??? we will adjust the pump stroke to meet the desired flow but at what pressure.
Does this mean that i need to work out the discharge piping friction at the given flow and add it to the ssyetm back pressure???
upm

To upm, yes.

As you guessed, the pump discharge pressure will be whatever is necessary to get the fluid from the pump through your equipment to its final destination. Your PSV will then be set, I would suggest, based on the lowest allowable MAWP pressure(s) in your system (equipment, piping etc). Make sure you have enough of a difference between the two to allow for variation in operational parameters.

You should also do a check at the pump's design capacity and calculated required discharge pressure that you have enough driver capacity.

If there are any existing control valves in your system, you need to make sure their control loops are compatible with your proposed stroke adjustment on the pump.

if the old pump has a motor with it... look at the nameplate data of the motor (if available) otherwise a good electrician can tell what the KW are for the motor very closely... or you can test the motor as described below...
then
KW = QP/ep
where
Q is the flow
P is the pressure
ep is the pump efficiency (assume 0.7 for low viscosity fluids and 0.85 for high viscosity fluids)
This is just ballpark figure.

If the motor nameplate is not available...
you could run the motor un-coupled get the rpm's and the amps running idle... the Full Load Amps (FLA) are usually between 5 and 6 times the No Load Amps (NLA) then...
Full load KW = 1.732*Volts*(5.5*NLA)*PF*Eff
Assume an efficiency of 0.85

The shaft size is also a good indication of the power the pump was designed for...
P = T x N

Run the motor un-coupled and you will have the rpm
Calculate the full load KW as described above

Calculate the Torque (T)

Find in the mech engineering manuals what is the required diameter for a given torque... or given the shaft diameter what torque can you carry (it's been a while but i as far as i remember you will also need the distance between bearings).

When you know what kind of power can your pump deliver knowing your required flow you can calculate what was the design pressure of the pump originally.

Another good indication of the design pressure of the pumps are the seals... have the pump overhauled and identify the seals... the seals have a design pressure that if exceeded will result in leaks.

Not very precise... but with the physical dimensions and values of the motor-pump set it is possible to have a fair idea of what are the original design parameters...

also... with positive displacement pumps it is possible to have a rough estimate of how much liquid they can hold inside, with proper deduction for nozzles and other dead spaces... the volumetric efficiency of the pump can be estimated as 85 to 90 % for well built pumps (gear, higher for piston)... with the rpm's of the motor and the actual pump displacement (theoretical x volumetric eff) you can calculate the design Q of the pump
with the kw of the motor you can verify the design discharge press of the pump.

HTH

saludos.
a.

upm (Mechanical) Jan 25, 2004 asked:

S"hall i assume that the discharge pressure will be the same at a the given flow??? we will adjust the pump stroke to meet the desired flow but at what pressure."

The outlet pressure is whatever is required to push the flow through the downstream piping (in theory at least. Another post above mentions the most common limits). This makes PD's a little dangerous (especially when used temporarely e.g. with hoses). The flow rate is controlled by rpom. If you insert a flow control valve and try to control the flow using this it will not work (very well or not at all)

"Does this mean that i need to work out the discharge piping friction at the given flow and add it to the ssyetm back pressure???"

Yes - if you want to establish if your piping will hold. As TD2K mentined above a PD pump should allways have a full flow relief valve.

Best regards

Morten