4 replies to this topic

[Joko Susanto]

Dear Experts,

 

Have a good day.

Kindly please advise me for below matter.

 

When we perform design Pressure safety valve for Gear Pump and Reciprocating pump, should we put set pressure smaller than shut off pressure of pumps.? Or can we keep set pressure same as shut off pressure. ?

Is there any reference / standard&code for this matter?

 

For your explaination would be highly appreciated.

 

Thank you,

Joko

[samayaraj]

Hi Joko,

 

Gear pump & reciprocating pumps are types of positive displacement (PD) pumps which displaces a constant volume of liquid per unit rotation of shaft. Here we cannot use the term "shutoff head". Shutoff head is applicable for dynamic pumps (eg. Centrifugal pump) which is the maximum pressure the pump can create when the discharge valve is completely closed. In PD pumps, if you close the discharge valve, pressure will rise abruptly and the weakest part (either pump internals/ discharge piping etc.) will fail. Hence to avoid such failure, a safety valve will be provided in the discharge side. In many of the PD pump, safety valve is fixed internally to safeguard the pump if started running with discharge valve in closed condition.

 

For safety valve set pressure, you need to keep below the weakest part in your system. It will be generally the "discharge piping".

[Joko Susanto]

Hi Mr. Samarayaj,.

 

Thank you for your quick response.

 

Well noted.

If we couldn't use term "shut off head", so then How to decide the design pressure of such PD pump? Could we apply 1.1xdischarge pressure? Is there any rule of thumb of it?

 

FYI. Some of safety valves in our project are designed with set pressure similar with the design pressure of the pump system.

 

Your further advised is appreciated.

 

Kind regards,

Joko

[fallah]

 

Well noted.

If we couldn't use term "shut off head", so then How to decide the design pressure of such PD pump? Could we apply 1.1xdischarge pressure? Is there any rule of thumb of it?

 

FYI. Some of safety valves in our project are designed with set pressure similar with the design pressure of the pump system.

 

 

Joko,

 

Design pressure of the PD pump will be specified by the pump manufacturer. The discharge line's PSV set pressure will be the lower value of PD pump's design pressure and downstream piping/system's design pressure....

[Art Montemayor]

Samayaraj is precisely correct.  The term "shut off" head is reserved only for dynamic (centrifuga)l pumps.  A positive displacement pump theoretically can continue to build up its discharge pressure until one three things happens:

• The driver burns or trips out;
• The protected system ruptures;
• The pump casing ruptures.

With such tremendous potential for doing harm, the set pressure on a positive displacement device is always done in accordance with the protected system's needed set pressure - just as Fallah has stated.  It is very important for engineers to fully understand all pressure terms used in designing pressure relief valves - as well as fully understanding the fundamental principles of the type of pump(s) employed.