30 replies to this topic

[Qalander (Chem)]

Reading all the posts above I was hoping someone could confirm where you have a centrifugal pump with a minimum recycle the recycle flow will be throttled back to the suction line/vessel pressure, correct? Also, can anyone provide any possible examples of what might cause back flow, and thus the need to design the suction line for the discharge pressure. Thanks.

Dear

Presumably Any accidental closure at discharge pipeline end(s);human error, mechanical malfunction e.g A 'pig' stuck-up blockage in midway or some block valve's gate dropping/stuck etc.

With one of the pump stopped,

other operating and short segment(near the other pump) may be subjected to shock of high back pressure(s)& Non Return Valve's (If provided) are not considered 100% sealing

hope seems logical to you!

I fail to realize the reasons for negative 1 can someone who gave this make me understand!

Edited by Qalander (Chem), 29 March 2010 - 08:00 AM.

[sheiko]

Reading all the posts above I was hoping someone could confirm where you have a centrifugal pump with a minimum recycle the recycle flow will be throttled back to the suction line/vessel pressure, correct? Also, can anyone provide any possible examples of what might cause back flow, and thus the need to design the suction line for the discharge pressure. Thanks.

Dear

Presumably Any accidental closure at discharge pipeline end(s);human error, mechanical malfunction e.g A 'pig' stuck-up blockage in midway or some block valve's gate dropping/stuck etc.

With one of the pump stopped,

other operating and short segment(near the other pump) may be subjected to shock of high back pressure(s)& Non Return Valve's (If provided) are not considered 100% sealing

hope seems logical to you!

I fail to realize the reasons for negative 1 can someone who gave this make me understand!

Sorry i mistake.

[kkala]

Having read all interesting views, please note that some additional explanations have been provided in the thread "When does pipeline experience pump shut-off pressure", by "go-fish" (Industrial Professionals), having lasted from 10-2-10 to 30-4-10 (at least so far).
The subject of suction design pressure is discussed in the last posts, decisions look not to be "black or white".
You can read the thread at http://www.cheresour...t-off-pressure/

[Padmakar Katre]

Dear,

Does it mean the check valve will not hold the liquid going to the stand by pump suction via discharge line? If it supports that the check valve will not hold the pressure and failure will cause the pump suction to experience the discharge pressure of the running pump. If this is is accepted, then all the pump systems with the interlocks like PAHH and PALL i.e. pressure signal from the common discharge header which will stop the running standby pump or start the standby pump, should have suction rating same as the discharge. Somewhat new thing for me as I have seen most of the cases the different rating in suction and discharge of the pumps. Please make me understand in this context.

Edited by Art Montemayor, 13 September 2010 - 12:08 PM.

[djack77494]

Padmakar, I am sorry but I'm having trouble understanding your query but will attempt an answer. The point of rating the suction piping of a system of centrifugal pumps to the same design pressure as the discharge piping is fairly common. I think it is quite conservative, but I have no problem with the approach because the reasoning is sound. Basically, if you have multiple centrifugal pumps operating in parallel and one or more fo those pumps might be switched off, then it is possible that a flow path is established from the discharge header backwards through a pump that is not running all the way to the suction isolation valve. This requires that we imagine that the idle pump's discharge isolation valve has been left open (or it leaks) and that the check valve or NRV does not fully prevent backflow. The idle pump's suction isolation valve is presumed to be closed. Even a very small leakage rate would bring the idle pump's suction piping up to the discharge header pressure since there is no outflow to limit the pressure. Since any single pump might be out of service, all suction piping back to the most upstream isolation valve for each pump would be rated at the higher (discharge) pressure. As I said earlier, I personally accept the rationale for this approach and propose carrying the higher pressure rating to and including the suction valves. If this is not clear to you, kindly explain your doubts more fully and I'll attempt to address them.

Edited by Art Montemayor, 13 September 2010 - 12:08 PM.

[Padmakar Katre]

Dear,
It's true, just slipped from mind. Anyways thanks for your reply and realizing me the actual practice. I have done the same in the past i.e. suction and discharge pipe rating same. Thanks once again.