2 replies to this topic

[mykid]

Hi all,
Need some professional advise on the following.
We have an existing separator designed with 16 barg. The existing PSVs(2x100%) are 4P6 and set at 6 barg while its operating pressure is 1.5 barg. I believed the low setpressure is to protect the downstream equipment which comprises a pump n cooler both are designed for 16barg.

We have checked the PSV size for the new fluid condition. Fire case is governing but only requires an L orifice. As the plant is currently in operation, and the existing PSVs sizes are more than adequate. Intention is to leave the PSV there.

However for inlet line sizing, if we were to use rated flow through a P orifice, the existing 4inch pipe has to change to 8inch to comply with API520. The PSVs are sitting directly on top of the separator, with less than 2m pipe length. The branch off tee is the cause of the exceedance of pressure drop.

Here is my question
1. If we retain the existing PSV P orifice and 4" inlet pipe, what is the consequence?

If its only designed for fire case, PSV chattering is not too much a concern.. firstly the vessel is designed for 16 barg, therefore overpressure of the vessel is not possible even if 3% line loss is exceeded. Secondly at high high pressure trip, the downstream pump will be tripped, therefore overpressure of downstream equipment is also not possible.

2x100% PSVs are provided for fire case.

Question: can i leave the inlet line size as it is?

If not, can i increase the PSV setpressure to match the design pressure, ie 16barg? This will give 0.48 bar inlet line loss rather than 0.18 bar.Use the high high pressure trip to protect the downstream equipment? Need 1 out of 2 voting for the high high pressure transmitter?

All input is really appreciated.

[fallah]

mykid,

 

If you can upload a simple sketch it's better to understanding the issue...

 

Anyway, seems the downstream equipment are designed for the pressure lower than 16 barg not for 16 barg as you mentioned...

 

The type and blowdown value of the PSVs aren't specified...

 

If the PSV is only for fire case the chattering due to high inlet pressure loss cannot be a major issue...

[mohit3040]

mykid,

 

Use the high high pressure trip to protect the downstream equipment? Need 1 out of 2 voting for the high high pressure transmitter?

 

Please note that as per API 521 " Fail-safe devices, automatic start-up equipment and other conventional instrumentation should not be a substitute for properly sized pressure-relieving devices as protection against single-jeopardy over pressure scenarios. There can be circumstances, however, where the use of pressure-relief devices is impractical and reliance on instrumented safeguards is needed. Where this is the case, if permitted by local regulations, a pressure-relieving device might not be required" 

 

Therefore, relying on instrument for the protection of downstream equipment is not a preferable option.

 

If its only designed for fire case, PSV chattering is not too much a concern.. firstly the vessel is designed for 16 barg, therefore overpressure of the vessel is not possible even if 3% line loss is exceeded.

 

This 3%  limit of total non recoverable pressure loss between the protected equipment and the PRV is not because of overpressure. It is because the excessive pressure loss at the inlet of PSV can cause rapid opening and closing of the valve or chattering. Which will result in lowered capacity and damage to the seating surface of PSV.

 

If there is a vapor relief through PSV, overpressure will never be an issue for the vessel even if it is designed for 6 barg pressure. Overpressure will remain same as considered for calculating the PSV relief load. But if there is a liquid relief through this PSV higher design pressure of the vessel will help during the scenario when vessel seeing the higher pressure than PSV set pressure due to static head.

 

As per my understanding the preferable solutions is either to increase the PSV inlet line size or to retain the same PSV with increased set pressure to 16 barg ( assumed that same PSV will work adequately with decreasing set pressure) and provide one more PSV at 6 barg to protect the downstream equipments.

Edited by mohit3040, 08 April 2014 - 12:58 PM.