7 replies to this topic

[ahyong]

Dear Sir,
I have some questions regarding PSV and Flare System. I have been searching online for the difference between superimposed back pressure and Built-up Back Pressure. Although there are some answers online, but I am still quite confuse with it. As I know, superimposed back pressure is a constant and "already existed" in the valve while built-up back pressure "is produced". Is it correct? And also how is the chattering of the valve related to the back pressure?
Thank you for your help

[fallah]

ahyong,

Superimposed back pressure could be either constant or variable. Build up back pressure would be produced after PSV opening due to fluid flow in PSV tail pipe.

High build up back pressure due to either excessive length of PSV tial pipe or undersized PSV tail pipe can be among major causes of PSV chattering.

Fallah

[flarenuf]

Thi swas posted back in August
the minimum superimposed back pressure will in most cases be 0 barg. if the flare is not flowing any relief flows then the PSV will see only atmospheric ( plus small purge gas pressure) on the exit flange.

the maximum superimposed takes two forms as Fallah says .

Constant - pressure on back of the valve from other relief/vent flows passing into the header system. Th evalve vendor may want to adjust his spring setting to compensate for this .
Variable - this is the backpressure on the valve when it is open and venting into the header system. The bp will normally be calculated using Flarenet or similar hydraulic software.

normally conventional pSV's are designed for a maximum BP of 10% of set pressure, balanced valves 40-50%.

hope this helps
flarenuf

[ahyong]

Dear Fallah,
I have read your post regarding oversized PSV dated 19 January 2012. In the post, you described how the chattering will occur. I have 2 questions regarding your explanation on chattering.
1. "......: if flow is going to be lower than 25% of the rated capacity then the spring force ovecomes to kinetic forces and the valve would be closed" (the flow that you mentioned here is the inlet flow? rated capacity is the result that calculated with reference to tail pipe size?)
2. "On the orther hand, because at the same time the upstream pressure of the PSV is still equal to (or higher than) the set pressure, then the PSV would be opened again and this cyclic operation will lead to chattering. (as I know the purpose of the PSV is to release any overpressure of the system, if the valve finally closed meaning that the system doesnt face with any overpressure problem. Am I wrong? If that's the case then what does the upstrea pressure of PSV refers to?, why is it still possible to be higher than set pressure)

My understanding of the concept could be/most probably be wrong. Hope you can solve my confusion.
Thank you very much

[fallah]

ahyong,

1-Rated capacity of a PSV is to be calculated based on actual orifice area of the selected.PSV. The 25% of the rated capacity is an approximate limitation for a cause of PSV chattering due to reduced flow. It has been concluded based on the incoming flow to the PSV via inlet line with the size of minimum equal to PSV inlet nozzle size. Obviously, it can be applied for the higher sizes of the inlet line due to more reduction in kinetic forces.

2-When the PSV is operated in much reduced capacity (due to minor process upsets or considerable difference between the scenarios with the highest and lowest relief load), i.e. the PSV is ovesized for that relief load; the overpressure is released more quickly and the PSV would be closed temporarily, then if the cause of the overpressure is still remained again the PSV would be opened and this cycle can be repeated for several times till the cause of the overpressure be removed.

Hope above can help to solve your confusion...

Fallah

[ahyong]

Thank you Mr. Fallah for your reply. I appreciate it a lot and flarenuf too!

[alkimi]

Dear Fallah,

Could you show me the related reference which describe that 25% of the rated capacity is an approximate limitation for a cause of PSV chattering due to reduced flow? Or it is just common rule of thumbs?
Thank You for your help.


Regards,
Alkimi

[fallah]

alkini,

API 521 FIFTH Ed. section 6.3.2.2 page No. 67 included the point (25 % of the valve's rated capacity) as a limit below which spring closing force overcomes the opening force causes the PSV would be closed without complete overpressure relieving, then the valve will be opened again due to consequent pressure build up that by repeating will lead to valve chattering.

Fallah