3 replies to this topic

[latexman]

helena,

You can keep the PSVs as they are (no need to change nozzle size), if you expand up (reducer turned backwards) to an outlet pipe size and main header size and knockout pot and on and on until it reaches atmosphere such that dP is not > 10% of set pressure (21% in fire case).

You may be able to add bellows to the existing PSVs pretty cheap and have more dP (if I recall correctly, dP is not > 40% of set pressure) to work with.

Or, you can swap out the PSVs for remote pilot operated PSVs and have even more dP to work with (my memory fails me on this one). I don't know if you can convert your PSVs to remote pilot or not. It's worth a phone call to the vendor.

[Nirav]

hello,

In addition to whatever latexman has said, you need to study API-RP 520, Part-1. Refer to Clause No. 3.3 Back Pressure. To be more specific, you can refer 3.3.3.1 for conventional valves.

Just to give explanation on a query : whether there is any relation between back pressure and orifice size. If you don't say anything beyond this, the simple answer is "YES". The reason....

The flow from any kind of device (orifice, control valve, etc..) depends on its area and dP across it. Flow is constant for PSV orifice because it's the "required relief capacity". You can not deviate from process requirement. So, remaining variables are "dP", "area" and "velocity".
"dP" is driving force for the flow to occur. So, if you increase back pressure, means you decrease "dP" (Set pressure is same). Which means you need to provide higher area otherwise it may affect the flowing capacity of orifice based on velocity. Now, if you decrease back pressure, you have higher dP (driving force) available and you require less area. These are theoretical aspects.

To take care of such aspects, we consider "Back pressure correction factor" Kb, for sizing the orifice area. For conventional valve, Kb=1 when outlet piping is designed in such a way that total back pressure does not go above 10% of set pressure.

For balanced bellows valve, Kb can be found from Figure 30 & 31 on API-RP 520 Part-1.
When Kb < 1, it means you are increasing orifice area to take care of higher back pressures. (The relation which you are asking).

Now, coming to your specific query,

I believe that the PSV outlet pipe size needs to be increased in order to lower the backpressure. However, my question is whether this means I need to increase the orifice size as well? Is there any relationship between the two?

I hope now you understand that if you decrease back pressure, you have higher dP available across orifice. So, you are on safer side and it does NOT require any change in orifice.
As told by latexman, you can provide "expander" in the outlet of PSV nozzle to match with the outlet line size. In no case, 97% back pressure is allowed. You MUST change line size to higher one (4" or 6" based on % of back pressure to set pressure).

Thank you,

[JoeWong]

Helena,

Some supplementary information / alerts ...

i) I wish the back pressure has been calculated base on RATED FLOW instead of RELIEF FLOW unless you are DEFINELY can demostrate that RATED FLOW is IMPPOSSIBLE. Please refer to API 521, section 5.4.1.3.

ii) SHELL DEP has indicated that back pressure limit < 65% of set pressure to some vendor (advisable limit is <50%) for Balanced bellow PSV.

iii) Supprised "Backpressure = 97% of set pressure". I hope this is a typo-mistake. If so, i wish you consult PSV vendor for confirmation and experienced people for advices...
OR do you mind to advise your application ?


ALL,
There's pressure recovery with expander downstream of PSV. Some company do not consider as in some events the pressure recovery (dP,expander,recovery) is larger than pressure drop (dP,expander,k).

Any advices ?


regards,

JoeWong