Is PSV required here?
Is PSV required here?
(OP)
Dear members,
I have a question regarding the need for a PSV. We're currently designing a nitrogen system for sweep through purging of some atmospheric equipment. The maximum nitrogen flow needed for purging is 200 kg/h at atmospheric pressure and 15°C.
The nitrogen supply pressure is 7 barg and a PCV is installed to reduce the pressure to 0,5 barg in order to deliver the required flow rate to the downstream equipment. The design pressure of the piping downstream of the PCV is 10 barg and the downstream equipment is atmospheric (sufficient large opening on equipment so that pressure build-up is not possible, blocking of this opening is also not possible). Shutoff valves between the PCV and equipment shall be foreseen. They will completely shut the nitrogen supply in case of failure of the PCV (a flow transmitter shall detect the higher flow rates and gives a signal to close the shutoff valve) or failure of the nitrogen extraction (interlock).
Do you think a PSV is recommended here? In my opinion, we don't need one but I could use some feedback here (I have read quite some topics on this forum and there are a lot of experienced members here). Please let me know if you need any more information.
FYI: We still have to do a HAZOP and LOPA study but I already want to encounter as much as possible to reduce the amount of actions coming out of the HAZOP.
BR
I have a question regarding the need for a PSV. We're currently designing a nitrogen system for sweep through purging of some atmospheric equipment. The maximum nitrogen flow needed for purging is 200 kg/h at atmospheric pressure and 15°C.
The nitrogen supply pressure is 7 barg and a PCV is installed to reduce the pressure to 0,5 barg in order to deliver the required flow rate to the downstream equipment. The design pressure of the piping downstream of the PCV is 10 barg and the downstream equipment is atmospheric (sufficient large opening on equipment so that pressure build-up is not possible, blocking of this opening is also not possible). Shutoff valves between the PCV and equipment shall be foreseen. They will completely shut the nitrogen supply in case of failure of the PCV (a flow transmitter shall detect the higher flow rates and gives a signal to close the shutoff valve) or failure of the nitrogen extraction (interlock).
Do you think a PSV is recommended here? In my opinion, we don't need one but I could use some feedback here (I have read quite some topics on this forum and there are a lot of experienced members here). Please let me know if you need any more information.
FYI: We still have to do a HAZOP and LOPA study but I already want to encounter as much as possible to reduce the amount of actions coming out of the HAZOP.
BR





RE: Is PSV required here?
That is "If anything can go wrong, it will"
- What would the cost of a small PSV (plus piping) be?
Vs
- What would the cost of a catastrophic failure (with injuries) be?
Your choice.
Sometimes its possible to do all the right things and still get bad results
RE: Is PSV required here?
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Is PSV required here?
Best regards, morten
RE: Is PSV required here?
A FT / FAHH is more expensive than a PT / PAHH, so I'd go for a field installed PT at the downstream component to isolate the N2 PCV in case of pressure control failure.
RE: Is PSV required here?
If all assumptions of OP with supportive documented calculation being provided; no PSV for over pressure protection is required...
RE: Is PSV required here?
RE: Is PSV required here?
So if I understand it correctly, I should calculate the pressure drop across the equipment openings at max flow rate (PCV failure) to confirm my statement of 'a too large opening for pressure build-up'?
A detail I forgot to mention is that this equipment is located indoor. The main risk here is the escape of nitrogen in this area when the extraction system fails. In that case, the shutoff valve should close. A failure of the PCV can indeed be seen by a pressure transmitter with PAHH.
@MortenA: the maximum nitrogen pressure is specified and not higher than the piping design pressure.
RE: Is PSV required here?
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Is PSV required here?
RE: Is PSV required here?
RE: Is PSV required here?
As georgeverghese mentioned, please make sure to do a backpressure backwards calculation with CV failure flowrate. Do not account for the closing of the on-off valve. All the best!
RE: Is PSV required here?
RE: Is PSV required here?
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Is PSV required here?
RE: Is PSV required here?
I don't think you got what georgeverghese was saying. My guess is georgeverghese (georgeverghese correct me if I'm wrong please) assumed choked flow then did the rough math in his head and said, "approx 3barg". He did not say just "3 barg" or "exactly 3 barg". His intent was to convey the thought/fact that he believed flow to be choked and sonic. Therefore, it would be up to you to calculate accurately the downstream pressure at the PCV outlet.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Is PSV required here?
Yes, for a first iteration, you can assume choked flow and compute the flow based on Pd being 3barg or lower (based on the Wiki link, any Pd value less than 1.895 x 8bar abs will give the same flow when in choked flow). When you complete the built up backpressure profile calc, you will most likely get an actual backpressure at the PCV which is probably much lower than 3barg.
RE: Is PSV required here?
If you are going to rely on an open vent on the downstream system, then as a worst case you should calculate the max flow through the PCV - nitrogen system at design pressure and PCV fully open. consider also if there is a bypass on the PCV, this should be included.
Also you have to confident the vent will always be available - lock open any valves on the vent line, are there any filters or equipment that could get blocked in between the equipment being protected and the vent?
This requires detailed analysis and you cannot rely upon messages on a forum for this answer. You can't at present say in HAZOP that the system is adequately protected for overpressure unless you completed a suitable overpressure analysis.
RE: Is PSV required here?
Then your system - "atmospheric" is actually quite vague and needs to be defined. Any sort of flow in that system will introduce some sort of pressure in order to generate flow. Without knowing the precise definition of "atmospheric" it's rather hard to say. API 650 for "atmospheric" storage tanks actually defines this as a max of 18 kPa or 2.5 psi or weight of the roof, but often only inches of water guage.
From an engineering design view you really need to test the assertion that " blocking of this opening is also not possible". It is quite difficult sometimes to show that this is possible to check and confirm before you start the operation. Relying on procedures to implement safety critical activities is sometimes not allowed in a HAZOP scenario, but practicality can be introduced. Each situation is different.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Is PSV required here?