Pressure Vessel Air Release Point

[jph90]

Hi guys,

With reference to the attached drawing from suppliers I have a vessel that is designed to retain oil and acid under pressure. The operating pressure is 6bar. The tank is bottom fed (supply line not shown) and discharged at height. On this discharge line you will see a safety valve, and an air release and vent line (blue). My thought is that the air release valve should be at the high point of the system, and there is a chance of an air lock in the riser to the SV?

What are your thoughts? The pipework has already been installed like this by the contractors and I observed it today but am thinking it may need to be altered.

Let me know if you need any further information?

 

[LittleInch]

No, I think that's Ok, if a little odd - you would normally expect the nozzles to be reversed, i.e. vent on the tee and the relief valve on the horizontal bit, but in reality doesn't make much difference

Presumably the air release valve is good to work at 6 barg to relieve air or is this only for commissioning / first fill?

If this vessel is operating at 6 bar then I'm a bit puzzled why you need an air release valve there at all, but I guess that's a different question...

Anyway, pressure is pressure and it doesn't matter for the relief valve whether that pressure comes from liquid or air.

I would be more concerned about operating at 6 bar (+/-?) with a relief valve set pressure of 7 bar. That looks way too close to me for a simple spring relief valve.

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

 

[jph90]

Thanks LittleInch. Something I didn't point out but you may have noticed is there is also an actuated valve in place to introduce air to the system, depressurize it and drain residual oil though a drain line on the supply side of the tank. This is during the 'Emptying' process. The air release I would then function to remove air prior to the next cycle of production (e.g. we run the emptying/drain program at the end of a particular product run, prior to changing oil type).

Would air not collect in this deadleg and then impact the flow? What are your reasons for thinking it might be OK?

FYI i would not expect the vessel to really operate under 6 bar. Realistically it is 4-5 bar, however we are told op pressure of 6 bar, design of 8 bar

Appreciate the help

 

[LittleInch]

Well in essence the air release is at the top of the system.

Sure it won't remove every last molecule of air in that horizontal section where it is, but I can only assume your downstream system can accept a bit of air entrained given you are emptying and refilling it.

Once you introduce pressure into a system to drive the fluid then air just gets swept along, providing there is enough velocity and differential pressure. 6 bar sounds good to me.

If you do run it at 6 bar and the relief is actually set at 7 bar, not 8, then you could get some leakage or "chattering".

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

 

[bimr]

PRV valves should be installed on top of the pressure tank or on piping as close to the tank as possible. Valves are not allowed between the PRV and the pressure tank.

You do not show the distance from the PRV to the discharge. Because of the air, the headloss through the piping may be two phase flow and the piping should be sized accordingly.

 

[wsu1975]

All,
I agree that the nozzles should be reversed, but the current arrangement has little effect on operation.
As far as “Valves are not allowed between the PRV and the pressure tank” is a good practice, and may not be allowed in particular regulatory jurisdictions, ASME VIII, UG-126(g) (2) provides an exception clause and requirements, leading to UG-135 (d) and Non-Mandatory Appendix M. (ASME 2013 Edition).
I noticed the sketch uses the term “Safety Valve”, and there is discussion as to “Relief Valve” and “PRV” (I assume this is Pressure Relief Valve). A note of caution that we need to be aware that “Safety Valve” and “Relief Valve” have specific definitions and applications. “Pressure Relief Valve” is generic and includes both, as well as others – see ASME VIII UG-126. Just being picky!

WSU1975

 

[georgeverghese]

It does sound like the vapor space in the drum is more or less air anyway all the time.

In actual practice, the PSV will leak a little ( especially after a few months in service), and the entire feed line to the PSV would be approximately be the same composition as the vapor space in the drum after a few minutes.

 

[curtis2004]

jph90,

If your system design is 8 bar, I don't understand setting Relief Valve at 7 bar if operating @ 6 bar. As LittleInch mentioned it could be too close. I think 15 psi is minimum recommended by some of valve manufacturers. I would set 8 bar though. You have more margin that way.

Regards,
Curtis