9 replies to this topic

[curious_cat]

Is the possibility of reverse flow into a vessel via a relief valve a credible scenario or does a relief valve itself act as a check against reverse flow? If the relief line is connected to a common header which has other vessels which may vent flammable / toxics into it how does one normally design for this situation?

 

I think a downstream NRV is a strict no-no due to the possibility of a restriction. I guess a downstream rupture disk to ensure positive isolation is one option? What other options are there?

 

Could a pilot operated valve or a soft seating relief valve be counted upon to  ensure sufficient sealing against backflow?

[fallah]

Hi,

 

Back flow through a conventional or balanced bellows PSV could be a credible scenario if there would be the possibility of unsuccessful reseating after PSV opening and at the same time back pressure would be higher than vessel pressure; then because rupture disk, if to be installed upstream or downstream of the PSV, has already been ruptured cannot help avoiding probable back flow...

 

Therefore, if the excessive back pressure will lead to possibility of back flow through a PSV; it might a pilot operated PSV equipped with back flow prevention facility be the best option to avoid, provided that the fluid would be clean enough and the relieving temperature would be tolerable by PSV in material standpoint... 

[S.AHMAD]

In HAZOPS we normally do not consider backflow through PSV a credible scenario since it is considered double jeorpardy for it to happen namely the vessel that PSV is protecting must be at lower pressure (e.g. shutdown) than the discharge pressure and the PSV is damaged.

Edited by S.AHMAD, 30 November 2013 - 09:42 PM.

[curious_cat]

Therefore, if the excessive back pressure will lead to possibility of back flow through a PSV; it might a pilot operated PSV equipped with back flow prevention facility be the best option

 

Thanks!

 

Do you have a link to a PSV with back flow prevention? I didn't know that's an option. Is it a design modification? 

[fallah]

  

Thanks!

 

Do you have a link to a PSV with back flow prevention? I didn't know that's an option. Is it a design modification? 

 

 

Hi,

 

Please find attached from API 520 showing a POSV equipped with back flow prevention....

Attached Files

•  Pages from API 520.pdf   43.95KB   71 downloads

[fallah]

In HAZOPS we normally do not consider backflow through PSV a credible scenario since it is considered double jeorpardy for it to happen namely the vessel that PSV is protecting must be at lower pressure (e.g. shutdown) than the discharge pressure and the PSV is damaged.

 

AHMAD,

 

Ok, although back flow through a PSV wouldn't normally be considered as a credible scenario; but as per API 520 when a POSV is to be installed on, let say, a vessel then any of the below situation shall be investigated and if would be applied then the back flow would be credible and back flow preventer should be specified:

 

- Vessel can be depressured and isolated , let say for maintenance, while routed to an active flare header;

- A vacuum might be created at the inlet connection due to a process upset;

- There would be a possibility of more pressure in PSV downstream than in its upstream;

- There would be a combination of relieving from multiple PSVs into a common header can create superimposed back pressure in excess of the current pressure at PSV upstream.

[Malek]

I have a learnt lesson about that in the next door plant.

while carrying out partial plant shutdown activities which included (Re-installing a re-calibrated POSV which is not equipped with a back flow preventer accessory), a fault gas detection caused a total plant blow down which allowed back flow from the flare header to the vessel which was already opened for maintenance and then caused fire, fortunately no one injured.

It was a bad day and lost millions of dollars to get the plant online again. 

[Bobby Strain]

You might want to look for another location if the plant next door will do such dumb things.

 

Bobby

[AnandS]

I have a learnt lesson about that in the next door plant.

while carrying out partial plant shutdown activities which included (Re-installing a re-calibrated POSV which is not equipped with a back flow preventer accessory), a fault gas detection caused a total plant blow down which allowed back flow from the flare header to the vessel which was already opened for maintenance and then caused fire, fortunately no one injured.

It was a bad day and lost millions of dollars to get the plant online again. 

Malek,

As per my understanding, main cause of your next door plant incident is no proper operation shutdown isolation philosophy. If you are taking the vessel in maintenance, it should be positively isolated that means your PSV should be also positively isolated. you can believe the flow back flow preventer as positive isolation during maintenance.

[S.AHMAD]

Agreed with AnandS, The PSV should be properly isolated using blinds as necessary.