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Pd Pump Discharge Pressure.


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#1 farid.k

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Posted 28 November 2014 - 06:39 PM

I saw a PD pump datasheet. Discharge pressure needed is only 1 barg. It goes to injection point at open cooling tower basin which is ATM. For me,that stated discharge pressure is quite low.plus the suction pressure is also around 1 bar which also means that the differential pressure is almost zero.

Question: letsay actual situation need more than 1 barg ( i.e. 2barg due to high pressure drop ), does the pump capable to pump it? I think it should be ok since it is PD pump and will force the liquid to go out no matter how big the pressure drop.

#2 fallah

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Posted 29 November 2014 - 02:01 AM

farid,

 

PD pump is capable to pump a specified flow rate against any pressure lower than the relevant maximum allowable discharge pressure...



#3 Supriyo Mukherjee

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Posted 29 November 2014 - 05:07 AM

If the pump is selected for 2 bar differential pressure and a flow rate of say 500kg/Hr, when you increase the diffrential pressure or effectively the pressure ratio the flow rate of the pump will decrease. So what it means is the pump will operate at a lower capacity than desired when the differential pressure is higher. So I would suggest to consider the requirement of the flow also in your system in case of higher differential pressure.



#4 ankur2061

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Posted 29 November 2014 - 06:20 AM

If the pump is selected for 2 bar differential pressure and a flow rate of say 500kg/Hr, when you increase the diffrential pressure or effectively the pressure ratio the flow rate of the pump will decrease. So what it means is the pump will operate at a lower capacity than desired when the differential pressure is higher. So I would suggest to consider the requirement of the flow also in your system in case of higher differential pressure.

Supriyo,

 

PD pumps do not operate in the manner which you suggest. PD pumps are constant volume flow pumps for a given speed within limiits of the NPSH and fluid viscosity and will deliver that much volume at the pump discharge flange. If you block the flow (in other words increase the resistance to the constant volume flow), the pressure on the discharge side will keep on building until the pump stalls or a mechanical failure of the pump or discharge piping occurs.

 

You will normally never find PD pumps being operated by any throttling of valves on the discharge side of the pump. It is simply not done. Proper flow control (increasing / decreasing volume flow) for PD pumps is done by either varying the speed or adjusting the stroke length (reciprocating pumps) of the pump.

 

Regards,

Ankur.

A



#5 farid.k

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Posted 29 November 2014 - 07:50 PM


If the pump is selected for 2 bar differential pressure and a flow rate of say 500kg/Hr, when you increase the diffrential pressure or effectively the pressure ratio the flow rate of the pump will decrease. So what it means is the pump will operate at a lower capacity than desired when the differential pressure is higher. So I would suggest to consider the requirement of the flow also in your system in case of higher differential pressure.

Supriyo,

PD pumps do not operate in the manner which you suggest. PD pumps are constant volume flow pumps for a given speed within limiits of the NPSH and fluid viscosity and will deliver that much volume at the pump discharge flange. If you block the flow (in other words increase the resistance to the constant volume flow), the pressure on the discharge side will keep on building until the pump stalls or a mechanical failure of the pump or discharge piping occurs.

You will normally never find PD pumps being operated by any throttling of valves on the discharge side of the pump. It is simply not done. Proper flow control (increasing / decreasing volume flow) for PD pumps is done by either varying the speed or adjusting the stroke length (reciprocating pumps) of the pump.

Regards,
Ankur.
A

Thanks ankur for the explaination.
I think the explaination by supriyo is applicable to centrifugal right?

I have an experience also that the pd pump vendor supply pd pump which has lower discharge pressure as stated in datasheet.the reason is, the discharge pressure is basically based on back pressure. For example, if there is no back pressure at pump dischare, the discharge pressure will be as per stated in the datasheet. But letsay the the discharge pressure has been increased (that caused by any means ) the discharge pressure of the pump will be increased..

Am i correct?

#6 farid.k

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Posted 29 November 2014 - 07:55 PM

farid,

PD pump is capable to pump a specified flow rate against any pressure lower than the relevant maximum allowable discharge pressure...

Fallah,
Max allowable discharge for pd pump pressure is based on what? How they determine that
I am afraid that, if i state the discharge pressure is 1 bar, vendor will come max allowable is 10% only from the discharge pressure.

#7 fallah

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Posted 30 November 2014 - 02:24 AM

farid.k,

 

It's specified by relevant vendor and is higher enough than the normal operating pressure has already submitted by customer. Anyway the PRV at pump discharge line will be set at such allowable working pressure to protect the pump against blockage of discharge line. Obviously if the design pressure of discharge line is lower than allowable working pressure of the pump, mentioned PRV would be set at design pressure of the pump discharge line to protect both pump and discharge line...



#8 samayaraj

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Posted 30 November 2014 - 05:41 AM

@ Fadri.K

 

Ya, for PD pump flow rate is independent of system resistance. Its applicable to dynamic pumps...

 

 

#Samayaraj



#9 oscarsender

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Posted 30 November 2014 - 11:30 AM

This is a chemical injection to the cooling water basin. I think this is connected to an eductor which is an injection that uses a Venturi effect to entrain the motive fluid which is the chemical. With this you need not to have a high discharge pressure at the PD pump and let the eductor do its job.


Edited by oscarsender, 30 November 2014 - 09:05 PM.





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