7 replies to this topic

[Cold]

Hello I got a problem here...

My pump discharge has a shut off pressure at 20barg. Downstream equipments are designed at 20barg with PSV setting at 20barg and it starts to chatter at 90% to it's set point which is at 18barg.

In this case how do i prevent the activation of my PSV in the case of a pump shut off pressure scenario?

Thanks

[Technical Bard]

You can't. If the pump reaches it's shutoff head, your PSV is going to lift. You could change the pump impeller to reduce the shutoff head, or install a PCV to limit the discharge pressure by sending liquid elsewhere.

[S.AHMAD]

1. The PSV is installed for protection against shut-off pressure or other credible scenarios?
2. The shut off pressure is 20 barg and the equipment is designed at 20 barg so the equipment is inherently designed against shut-off pressure. Under this environment, PSV is not required.
3. If the PSV is based on other scenarios, then before deciding on smaller impeller (one option), I suggest you check and calibrate the PSV since it starts to chatter at 90% of set pressure. Obviously, PSV is not correctly assembled, spring pressure may not be correct.
3. Another option is to determine the Maximum Allowable Working Pressure (MAWP) of the downstream equipment. In most cases, design pressure is lower than MAWP since the design pressure is normally determined at the design stage while MAWP is determined after fabrication. Under this situation, you can upgrade the equipment design pressure and reset PSV accordingly.
4.Another option that you can study is to take advantage of the maximum allowable overpressure (e.g. 10% or 21% for fire case). Normally the actual orifice size of the PSV installed is bigger than required. Therefore, using the actual size, back calculate to determine the relieving pressure. If for example the back-calculated pressure is 21 barg then you may consider setting the PSV at 20.5 barg.
5. My point is, before deciding on what to do in resolving your problems, study several options and present your analysis and recommendation for management decision.

Edited by S.AHMAD, 01 April 2012 - 10:14 PM.

[fallah]

Cold,

You fail to say about the operating conditions of the pump, point of measuring the pressure (90% of set pressure) while PSV is chattering and the PSV blowdown value.

Anyway, if the PSV is properly selected and assembled, you are faced with PSV chattering due to one of following reasons:

1) Psv is oversized
2) PSV inlet (or outlet) piping excessive lenght
3) PSV inlet (or outlet) piping undersized

Fallah

[Cold]

Hi all really thank u very much for your inputs. I will consider adding a PCV or upgrade my downstream equipment based on MAWP.

A question if i want to upgrade my equipments base on increasing the design pressure how do i go about to estimate it?

[S.AHMAD]

1. What type of equipment that you have?
2. Normally for any equipment, there is a stamping on the equipment that gives information on the design pressure/MAWP. or you can recalculate based on existing information such as thickness, diameter,temperature etc.
3. Normally inspection section can do such calculation and you probably required to obtain approval from your local authority that provide the certificate of fitness of pressure vessel.

[kkala]

After estimating MAWP according to previous post, you have to verify selected design pressure through an official hydrostatic test. This procedure may be complicated (see relevant posts of [url] http://www.cheresour...-for-pipelines/ [url], post No 7 and on, same general concepts apply for equipment or piping), but effective for the case of pump.
Seeing that vessel design pressure equals (max) pump shutoff pressure, PSV is assumed to cover some case of flow rate higher than what the pump delivers at a discharge pressure of 18-20 barg, as previous posts indicate. I would first try to find PSV data sheet and verify this assumption, in combination with pump curve. And "correct" PSV set pressure to 20 barg.

Edited by kkala, 03 April 2012 - 03:15 AM.

[Zubair Exclaim]

Could you attach your pump curve and psv data sheet. that would surely help a lot in defining a root cause. Till now its assumptions, you still don’t know the root cause