Safety Relief Valve Location 2

[smrodriguez4]

Hi guys,

I currently have a Nitrogen vessel that is 54"OD X 11' S/S, designed to 150psig @450F; it currently operates at 120psig @80F but it has a very difficult to get relief valve on the top of the vessel. We are thinking of relocating the valve to the side of the vessel since there is a plug on the side of that we could use. I have read that the Best Engineering practice is to keep the valve on top of the vessel but I haven't found anything that prevent's me from relocating the relief valve to the side. I don't have have a lot of experience with relief valves and pressure vessels so your feedback is greatly appreciated.

Thanks,
Sal

 

[MortenA]

I dont see any reason why you couldnt have the PSV connection on the side. Your only concerns shall be that you should not risk getting demisters and similar into the PSV or liquids draggend into the PSV either.

 

[pennpiper]

You have not given us enough information about the total situation for us to give good advice.

What is the length of the skirt or support legs?
Does this vessel have a Platform on top (if not why not)?
Does this vessel have a ladder up the side to the top?
What else is on the top of this vessel?
What is the maximum liquid level possible for this vessel?
Where is the 'side' nozzle in relation to the maximum liquid level?
Where does the PSV discharge to (a closed system or atmosphere)?



Sometimes its possible to do all the right things and still get bad results

 

[EmmanuelTop]

Installation of PSV on the vessel shell is not uncommon - particularly if vessel contains internals that can be blocked or dislodged, and end up in the relief path. Most of the cyclone separators I have seen do have PSV nozzle on the shell, probably for the same reasons. In some situations, locating PSV nozzle on the shell side is actually the best solution.

As pennpiper pointed out, you need to look at other things as well - if there is a potential for liquids to flood the PSV nozzle, or if PSV reliefs to atmosphere, then additional measures need to be taken to ensure the PSV will perform as intended.


Dejan IVANOVIC
Process Engineer, MSChE

 

[smrodriguez4]

pennpiper,

Thanks for the reply.
This vessel does not have a platform on top, I don't know why it was designed like that; It also does not have a ladder up the side to the top. We have to bring a lift every time we inspect the valve or replace it. There is nothing else in top of the vessel other then process piping that makes access to the relief valve with man lift somewhat difficult but not impossible.

As far as liquid goes, we don't expect to see any liquid in the tank plus we don't have any level sensors in that tank. However, the side nozzle that we are planning on using is at the same height as the inlet coming from the Desiccant dryer; which is about 5ft above the tank lowest level.

This PSV discharges to the atmosphere. Which may pose another concern on the safety side; If the valve happens to discharge while somebody is standing next to the vessel...

Thanks for your replies, I really appreciate your feedback.

 

[LittleInch]

You might simply be able to offset the valve by means of piping so that you could use something that only goes up the side of the vessel or even pipe it down to a more reasonable location. You are allowed a certain pressure drop on the inlet (3 or 5%, I forget which) so it might need a bigger pipe and a couple of reducers but might be an option.

I hope this vessel is outdoors otherwise Nitrogen direct to atmosphere sounds a bit risky...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[don1980]

PSVs that are designed to relieve liquid must have an inlet nozzle that is located in the liquid zone of the vessel, and PSVs designed to relieve vapor must have an inlet nozzle located in the vapor zone. From a compliance perspective, that's the determining factor in deciding where to locate the PSV nozzle, and deciding whether a PSV can be moved from one nozzle to another (Ref: API 520 Pt II).

 

[SNORGY]

The nozzle that you propose to move it to should be the same NPS or larger than the NPS of the PSV inlet.

For nitrogen at the conditions you state, I don't expect to see liquid either, unless there is something else present besides nitrogen.

The concern I would have would arise because the PSV is going to atmosphere. It might or might not be a big deal, depending on the size(s) involved, but double check the reaction forces created when the PSV lifts, and support the pipe accordingly to prevent overloading the nozzle.

 

[georgeverghese]

If this stream has no liquids in it, then you could place this PSV on the feed line to this N2 vessel also, if you find other nozzles on the vessel are not suitable. Check that you have a particulates filter downstream of the the PSA dryer / and downstream of the N2 generator also.

 

[JAlton]

If you move the PSV to the side of the vessel, it should meet the recommendations of previous commenters, but also be mounted vertically with the spindle upright. This is not a Sec VIII requirement, but most PSV are designed for vertical operation. Make sure the pressure drop through the inlet does not exceed 3% of set pressure.

JAC

 

[The Obturator]

In case you will determine a pressure drop in excess of 3 % in the inlet piping to the SRV, consider either i) Increasing the blowdown of the SRV by ring position (if fitted and also speak to the manufacturer), ii) Change the valve to a pilot operated type with remote sensing line to direct vessel pressure (modulating pilot good for all fluid phase).

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

 

[JAlton]

Adjusting Ring Position will not compensate for Pressure Drop through the inlet piping. Remote Sense Pilot is a good Option.

JAC

 

[The Obturator]

JAlton, My comment was relating to the fact that you could purposely increase the blowdown (reseat) of the SRV to avoid potential SRV eventual failure, as a result of inlet pressure losses. ie., resultant valve seat chatter. Not the cure I know, but lessening the overall scenario. Of course pilot SRV is best, but more expensive bet if less than 4" inlet (cost).

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.