9 replies to this topic

[manchester]

Dear All,

Please take a look on the attached sketch I've provided. I understand from Fluid Mechanic books that static head required for this pump at its discharge is "b", instead of "a", due to siphon effect.

However, one of my senior engineer in my office told me to provide pump with static head "a". He told me that siphon effect is only happen if the pipe is already full of liquid. His concern is during initial start up, where there will be no liquid inside the pipe, then no siphon effect will happen during the start up. He added, if I calculate the static head required as "b", the pump will not be able to deliver liquid to the highest point, during start up.

Then , I confused with his argument, since it doesn't match with what I've learnt from my college's fluid mechanic book.

Please enlighten me, if his argument is correct or not?

Thanks in advance

Attached Files

•  Pump sketch.xls   32.5KB   80 downloads

[Vignesh S]

Manchester:

I hope your Sr. Engineer's statement is correct. The below link may help you.

http://www.chemicalp...s/2007/060.html

Regards,
Vignesh.

[breizh]

Let us know how you intend to start the pump at the initial stage .

Breizh

[manchester]

Let us know how you intend to start the pump at the initial stage .

Breizh

After the piping system is flushed during pre-commissioning, the liquid inside the pipe will be drained. After all of the system is ready to start up, then I will start up my pump.

1.My pump is centrifugal. So, at the beginning of start up, closed discharge valve and start the pump.
2. I monitor the pressure at pump outlet. When the pressure rise to certain amount, then I will gently open the discharge valve until full open.

breizh, If that is the case, what should be my required static head at discharge point?

[manchester]

Vignesh,

Thank you for your answer and the link you given to me.
Here I copy one of the paragrah in your link:
However, once the piping in that system is full of liquid, a siphoning effect will kick in, and the system will only require a static head of 1.3 m. This means that the entire system curve will fall, and intersect the pump curve at a higher flow rate than it will when the static head is 5.3 m. Consequently, it is essential that the pump be selected so that it can operate satisfactorily under both conditions.

I just want to confirm, is that means:
1. if I start up with empty discharge pipe, my pump curve has to able to handle static head covering the highest point?
2. if I don't start up my pump with empty discharge line (i.e. the flushing liquid / hydrostatic liquid from the precommissioning supplied by other pump to my pump discharge piping is not drained away), i just have to consider my destination height, instead of the highest point?

Thank you very much

[fallah]

manchester,

Your required static discharge head is "b" in normal operation and is "a" in start-up condition. Then, pump's working point would be correspond with static discharge head of "b", but the pump shall be able to supply static discharge head of "a" at its start-up. In fact, the selected pump shall handle both conditions are different in static discharge head value standpoint.

Fallah

[manchester]

Fallah,

Thank you very much.

So it is very important to select the pump with pump curve which covers both "a" and "b".
If the different of "a" and "b" is huge, so it is forbidden for me to select pump with flat curve

Is my statement correct?

Edited by manchester, 29 August 2012 - 01:56 AM.

[fallah]

manchester,

You are absolutely right.

Fallah

[breizh]

My question was about the discharge line , are you going to fill it with liquid prior to start the pump ?
If not the pump should be able to cope with the worst scenario (a) .

Breizh

[manchester]

breizh,
As I wrote previously,

After the piping system is flushed during pre-commissioning, the liquid inside the pipe will be drained. After all of the system is ready to start up, then I will start up my pump.

breizh, If that is the case, what should be my required static head at discharge point?

Therefore, I will start with empty line. Thanks very much for your advise and answer.