9 replies to this topic

[krishnamurthy]

we generally provide minimum flow recirculation line  for centrifugal pumps (i.e  from the discharge line some minimum flow is recirculated back to the upstream vessel of the centrifugal pump through restriction orifice) .

But for positive displacement pump we do not provide the minimum flow recirculation line. what is the reason for this. please explain in detail. i have searched in net i found that centrifugal pump should always be flooded. but  i am not clear with this explanation, please explain.

[Rahimzadeh]

Dear:

Centrifugal pump the impeller rotation causes the liquid become hot in other word the kenematic energy chenges to heat and causes the liquid become vapour but in reciporecating pump due to incompressibility of liquid the pressure rises very rapidly that must be released by psv.

 

best

[tarafdar]

Hi,

 

It is not allowed to operate a positive displacement pump with a closed delivery valve.

[Art Montemayor]

A positive displacement (PD) pump operates precisely according to its title: it positively displaces liquid entering its chamber – regardless of anything else as long as its driver continues to input motive energy.  Anything done externally by the operator (with the exception of decreasing the driver’s rpm) does nothing to lessen the pump’s liquid displacement.  This simply means that the pump will continue to pump the same flow rate whether the operator throttles the discharge or not.  HOWEVER, if you throttle the pump’s discharge (or block it off) you will force the pump’s discharge pressure to increase almost exponentially due to the liquid’s relative incompressibility.  This is a hazardous situation and that is why all PD pumps require a mandatory pressure relief valve in their discharge piping upstream of any blocking device.  That is the reason you should not try to force a portion of the discharge fluid to recirculate back to the suction side of a PD pump by throttling.  With a PD pump, it is like the song,“All or nothing at all” as far as volumetric capacity is concerned – unless you can vary the pump’s speed.

 

A centrifugal pump, being a dynamic device, has a dead head with a maximum discharge pressure.  Hence, at 100% blocked discharge, a portion of its discharge can be diverted back to its source and thereby maintain at least a small, minimum flow through the pump that ensures its seal lubrication and self-cooling.  A PD pump can’t do that.  If you can’t accept its flow rate downstream during a part of its operation, you either shut it down, reduce its speed, or divert the discharge through some instrumented procedure.

[krishnamurthy]

thanks  a lot for your explanation sir.

[farid.k]

Art Montemayor; m.r.rahimzadeh:

 

What if some engineer designs the PD pump discharge equipped with a minimum flow line that uses an orifice type that is routed back to the reservoir?  Will it not work to protect against any over pressure if the discharge pipe is closed?  Just asking.

[fallah]

farid.k,

 

That is recycle line mostly for required capacity adjustment of the PD pump, and one cannot take credit on it for pump protection against over pressure due to closed discharge. Then the PRV should be used in discharge line of the PD pumps for over pressure protection...

[Ajay S. Satpute]

Farid,

 

Please refer below link to support your query.

http://www.driedger....dp/CE2_PDP.html

 

 

 

Regards.

 

Ajay S. Satpute

[farid.k]

fallah, Ajay S. Satpute

 

Thanks for the reply.

How about the set pressure? What i mean here is, set pressure normally will be specified by vendor or by purchaser? Sometime it is little bit confusing because some of the PD pump has come with internal relief valve and set pressure will be set at weakest point of the pump internal parts and the problem will occur if the set pressure given by vendor is higher than design pressure of discharge piping. Let say the pump come with internal relief valve, thus we don’t have to design the external relief valve isn’t?

[fallah]

 

How about the set pressure? What i mean here is, set pressure normally will be specified by vendor or by purchaser? Sometime it is little bit confusing because some of the PD pump has come with internal relief valve and set pressure will be set at weakest point of the pump internal parts and the problem will occur if the set pressure given by vendor is higher than design pressure of discharge piping. Let say the pump come with internal relief valve, thus we don’t have to design the external relief valve isn’t?

 

farid.k,

 

Internal relief valve with very limited capacity, is being considered by vendor just for pump protection and nothing to do other than protects the pump for short therm conditions of closed discharge. The set pressure of the internal PRV is equal to or lower than the MAWP of the pump.

You have to consider the external PRV with the set point equal to or lower than the design pressure of the pump's discharge line with the relief load equal to the nominal pump capacity which would be much higher than that of the internal PRV...