6 replies to this topic

[ziba]

Hi:

 

I need to rerate a vessel with operating pressure of 60 Psig and 35 F and design p=150 Psig to design P=200 Psig.

 

The PSV which is set at 150 Psig popps up at summer time due to high ambient and we have NH3 relief .

 

The issue is that I think the PSV will have higher capacity once we change set point from 150 to 200 Psig but my co worker thinks the capacity will stay same.

 

the over pressure is 10% on PSV and capacity as of now is 37 #/hr with size 3/4 x1.

Another problem is that I think the header size  for ammonia relief should be reviewed once we change the  set point pressure, this is for two PSV 's , there are over 20 PSV connected to a 3" header, plant was designed for lower production but later product type changed  and no one really looked at PSV header sizing again.

 

The relief header size is 3 inch now I think it should be 6" as the biggest PSV can relief 110 #/min NH3  , I used biggest PSV in system and considered back pressure of sparge tank to be 5 Psig, hysys simulation shows pressure drop on 3" relief header to be quite high around 14 Psig,,,total drop=14+5=19 Psig).

 

 19 Psig  is 10% of set point of lowest PSV set point in system.we have PSV;s set at 250, 300 and 200 Psig all connected to same header, pipe length is over 80 ft .sch 40

 

How can I convince my manager to consider re sizing header?

Does vessel re rating and change to PSV set point cause PSV capacity increase, can I change spring size 

what are the resources available.api 520 or articles?tx

Edited by ziba, 03 December 2016 - 10:14 PM.

[Bobby Strain]

All of these things are readily calculated. You should not try to out-think each other. Get to work on the calculations instead.

 

Bobby

[fallah]

Hi Ziba,

This is really a revamp conditions rather than the simple changes...
I think you are mostly not allowed to increase the PSV set pressure to such higher value due to setting range limitation also you have to ensure having MAWP higher than 200 psig for vessel.
You have also to know about possibility of simultaneous relieving of PSV's to evaluate the capability of old header...
Then please take care about your mentioned modifications...

Good Luck

[ziba]

Hi Mr Fallah:

 

Our mechanical group have re rated the vessel to 200 Psig, the reason was the vessel had enough thickness to be rerated.

 

My question is if we change the set pressure, the capacity of PSV will go higher, and the header would be small, I am doing hysys simulation and it shows the pressure drop will be higher than 10%.

 

where is the resources available here in this website? and what do you think regarding two PSV's with same set point, do I consider added capacity of both?? thanks

Edited by ziba, 04 December 2016 - 09:54 PM.

[fallah]

 

My question is if we change the set pressure, the capacity of PSV will go higher, and the header would be small, I am doing hysys simulation and it shows the pressure drop will be higher than 10%.

 

where is the resources available here in this website? and what do you think regarding two PSV's with same set point, do I consider added capacity of both?? thanks

 

Hi Ziba,

 

In general depending on the tail pipe size and length, superimposed back pressure and also increment value, following to PSV set pressure increment the relieving load or PSV capacity might or might not be increased due to higher build up back pressure....

 

On the other hand the main point is: according to ASME UG-126 ©, "The set pressure of a PSV shall not be adjusted outside the range of set pressure specified by the relevant manufacturer.";  hence you have to evaluate the possibility of such high set pressure increment by referring to the PSV's documents or contact vendor if is available..

 

Considering two operating PSV's with the same set pressure isn't allowed and they should be set in staggered arrangement...
 

[fallah]

Hi Ziba,

It's not reasonable to keep such small size PSV for new conditions especially if in addition to mentioned reason it might the rating of its inlet/outlet nozzles don't match with the new set pressure. Then it would be better to replace the PSV with new one...

[Bobby Strain]

You have told us little about this system. If you provide some information, we might be of help. For instance, if the only cause for overpressure is fire, you may have no problem at all with your 3" header. Based on the size of the PSV, this vessel must be quite small. So, tell us what you know.

 

Bobby