2 replies to this topic

[krishnamurthy]

why centrifugal pumps do not require psv? please explain me in detail,in google i found the pump cant develp pressure more  than the shut off head,i cant understand,pls explain.

[samayaraj]

Hi krishnamoorthy,

centrifugal pumps are one type of DYNAMIC pump, which converts kinetic energy into pressure energy. The pumping fluid gets kinetic energy from the rotating impeller by the action of centrifugal force. As the fluid exerts out from the tip of impeller, it has more kinetic energy (has more velocity). As the impeller is placed eccentrically, the velocity of fluid reduces gradually and pressure starts increasing in the volute casing. As you close the discharge valve of pump, all the kinetic energy from the fluid is converted in to pressure( since velocity and pressure are inversely proportional). Hence, the pressure generated max will be known and further operation of centrifugal pump at shutoff head can be allowed for little time, no relief valve is required. Since, the pump casing and the discharge piping is designed for shutoff head, no relief valve is required.

In positive displacement pumps, eg piston pump, it gives a constant flow of fluid per unit rotation of shaft. Hence, the fluid goes in, should come out. If the discharge valve is closed, pressure will rise abruptly and cause failure to pump. Hence, for positive displacement pump, there is a need for safety valve before isolation valve.

Hope this will be clear.

Regards,
Samayaraj

Edited by samayaraj, 27 February 2014 - 09:27 AM.

[ColinR33]

We have to be careful about making a general statement that centrifugal pumps do not need a PSV.  In order for this statement to be true you have to make sure that the downstream piping and equipment is designed to handle the pump dead head pressure.  In a completely new installation this is the responsibility of the mechanical and process engineers to confirm this (although I have seen instances where they did not check into this carefully enough and a PSV had to be added!).  On existing systems that are being retrofitted this can become more of an issue.  Sometimes plant debottleneck projects result in a new pump being installed into an existing piping system as part of a capacity increase.  The characteristics of the new pump can be such that the deadhead pressure is above that of the piping maximum design.  In this case a PSV will be required if the existing piping cannot be re-rated (often an ugly process if it is old piping).

Cheers,

Colin