6 replies to this topic

[m42364236k]

Dear friends,

 

For calculating the shut off pressure we have its pump curve.
The pump end-of-curve flow rate is seen in the H - Q diagram +suction pressure.

 

My questions are:
In some designs I have seen the suction vessel design pressure is considered instead of the suction pressure.
My second question
As you know, in the pump curve we have the impeller size noted.
Do we need to calculate the shut off pressure with max impeller?  Or is the normal impeller OK?

Thanks

Edited by Art Montemayor, 21 October 2014 - 08:24 AM.
Spelling, grammar, composition

[fallah]

Hi,

 

1- Design pressure of suction vessel to be considered as worst condition in safety standpoint...

2- Shut off pressure for maximum impeller size should be considered to design downstream piping and equipment...

Edited by fallah, 20 October 2014 - 12:14 PM.

[chem2013]

Dear Fallah,

 

In absence of pump curve, we generally determine downstream piping design pressure = Max suction pressure+ 1.25 * pump differential pressure at design point.

 

Now, pump differential pressure at design point will be the rated differential pressure for pump.

 

When pump curve will be available, we should validate that calculated design pressure of system as per the shut off differential pressure for rated impeller. If maximum impeller diameter is considered, this may result in revision of piping design pressure. Duty point is not defined for maximum impeller diameter and this is for future.

 

If piping is hydrotested at its full rated pressure, does it requires to specify design pressure at maximum impeller diameter? Please suggest.

 

Regards.

[Brocklesnar]

Generally, designers do not select a pump where rated condition impeller and conditions corresponding to max. impeller differ by +5%.

So,when tested pump performance curves for a trimmed impeller are available, use them to verify for downstream pipe design pressure. At a later date, if impeller size needs to be upgraded to max. for same impeller casing just verify at shut-off pressure the downstream pipe pressure is within +15% tolerance stress limit and the shut off condition is not likely to occur more than 800 hours / per year.

 

Above explanation is in line with ASME B 31.1 piping code and probably should hold good for other pipe design codes.

[chem2013]

Dear Brock,

 

Thanks for your reply.

 

What if hydrotest pressure is found to be more than maximum pump discharge pressure calculated using maximum impeller diameter at shut off condition.?

 

Regards.

[fallah]

chem12,

 

If you put a bigger impeller in the pump you will increase the maximum discharge pressure of the pump then design pressure of the discharge line. So it's good engineering practice to ensure that the discharge line is hydrotested to the maximum allowable pressure for the line class...

[Brocklesnar]

I agree with Fallah to hydro test the pipe for max. allowable pressure for pipe class selected.

However, typically for design pressures more than 200 psig, one should avoid hydro testing for max. allowable pressure because for higher design pressures in excess of 200 psig, the pipe SCH will change resulting in thick pipe and increased costs.