8 replies to this topic

[Sherif Morsi]

Hi,

 

A relief valve installed downstream a positive displacement pump has two routing options:

1- Go back to the supply vessel/tank

2- Go back to the pump suction line taking into consideration a minimum distance away from the suction nozzle to avoid flow discontinuities (5D - 10D).

 

One opinion says, PD pumps don't need it and it's only centrifugal pumps. The other says both types need it. If so, are there any references supporting either?

 

Regards,

Sherif

[Sherif Morsi]

It should also be noted that the suction line is fed from an atmospheric tank

[senthil13]

As far as i know, there is no standard that dictates that. It all depends on the system design limitations. Doesn't matter what type of pump we use.

 

I have seen almost all PD pumps with relief valve on discharge of the pump. Not on all centrifugal pumps. there are some special cases where centrifugal pumps also needs high pressure protection.

 

Good practice is to route the discharge to the source tank if possible.

[AlertO]

Hi Sherif

API 521 (section 4.4.2.2 version Y2014) states about closed outlet in pump system. You can use this standard to have a conclusion in your team.

The recommendations by senthil13 is practical and you can follow them.

[Sherif Morsi]

Thanks guys, appreciate your feedback

[Art Montemayor]

Sherif:

 

After spending more than 50 years practicing process and project engineering as well as managing engineering projects, I can strongly state:

 

You should take the need of pressure relief valve on the discharge of a positive displacement pump AS MANDATORY.  Common sense should tell you that unless you do that, you are running the risk of either burning up the pump's driver, or causing a pipe, pump, or other mechanical rupture.

 

You should always send the discharge of a pressure relief valve to a safe environment.  This can be the original source or supply suction tank - but always with consideration for any potential over pressure or mechanical failure in the same.

 

NEVER, NEVER send the discharge of a pressure relief valve to the suction of the pump.  I consider this a potentially dangerous and very stupid thing to even think about.  Whoever dreamed of this "option" doesn't have any experience, knowledge, or process design capability.  Serious and major chemical process companies never allow this to be done.

 

Who is the "genius" that is suggesting that a positive displacement pump doesn't need a discharge pressure relief valve?  He/she have probably never attended or participated in a Hazop and should be banned from one for lacking any common sense with respect to safety and potential hazards.

 

I have always tried to design my centrifugal pumps so that their maximum attainable discharge pressure never reaches the MAWP of any piping or vessels it is connected to on the discharge side.  This obviously allows me to eliminate the need for any pressure relief device on the discharge of the centrifugal pump.  Depending on the dead head pressure this may or may not be applicable.

 

There are no references required to prove what I have stated.  Common sense and experience among any capable process design engineer deems it so.  The countless Hazops I have attended and led clearly demonstrate these facts.  Our forums on this website have expounded these basic and well-known principles for many years.  All you have to do is read through them and learn these basic process truths from other engineers.

 

I am very concerned that no one on this thread has yet to mention that ALL centrifugal pumps exhibit what is called a maximum dead head pressure.  Knowing this - and the experienced knowledge that if your down-stream piping and equipment have a MAWP above that dead head pressure you logically cannot justify the need for a pressure relief valve.  This leads me to believe that today's engineers know little if anything about pumps - especially centrifugal pumps.  Shame.

[sunny143]

Sherif,

 

As per API and other standards guidelines, relief valve is required at every pump discharge piping.

 

But, if you increase the discharge piping Design Pressure to Pump shutoff pressure then no Relief valve is required (Centrifugal Only).

 

If you see the PD pump Flow/Head graph, discharge head will keep on increasing on every stroke/cycle of pump. So, Relief valve is necessary in PD pump case.

 

 

Also, Relief valve discharge causes more flow disruption and 5D-10D length requirement is for pump suction pipe length and flow meter requirement only.

[Sherif Morsi]

All,

 

Sorry for this confusion my original post has created. Of course the pumps have a discharge PSV on each discharge line. This wasn't my question my inquiry was about returning the PSV outlet line to the tank or to the suction line at minimum distance of 5D to 10D to allow for uniform flow regime upstream of the pump.

 

One mechanical engineer here said that minimum distance won't matter with PD pumps due to the sucking effect by the piston hence the flow regime is not uniform in the suction side of the pump.

 

I hope it is clearer now and sorry for the inconvenience. Your feedback is always appreciated

 

Sherif

[fallah]

Sherif,

As Art well mentioned, it is highly recommended not to conduct PSV discharge to relevant PD pump suction line.