I am trying to estimate new PSV for old drum. Basically we are using old drum in our plant, relocating it to new location and using it with totally different service.

New PSV inlet size matches with old drum PSV nozzle, but inlet and outlet piping pressure drop is coming 5.70% and 16.51% respectively.

I am using safety factor of 1 to calculate del. P /100 ft. I want to know what is standard safety factor is used in industry.

Do we at all need to apply safety factor for PSV inlet and outlet piping pressure drop?

As far as i know we apply safety factor to cover pressure drop due to unknown pipe design (one more elbow or lil. extra pipe in piping design state). But in my case i made sure that pipe designers are using exact joints and pipe length that i used for piping pressure drop.

Your input on this will be highly appriciated.

Thanks,

amit Joshi

We don't know if your service needs to comply to API STD 521 for your company's SAFETY policy. We also don't know what type of PSV e.g. conventional type, etc and what is blowdown for selected PSV.

If API STD 521 shall be complied, one of the requirement in this standard is the pop action pilot operated and conventional spring loaded type PSV inlet and outlet shall be sized based on rated flow whilst modulating type pilot-operated may consider required flow.

You mentioned that "...inlet and outlet piping pressure drop is coming 5.70% and 16.51% respectively.". Can you please detail the 5.7% and 16.51% ? Does it refer to 5.7% of set pressure, relieving pressure or others parameter ? What type of PSV ?

Personally i do not worry much on the safety factor used but i worry your system experiencing chattering and under capacity PSV...

If you provides required informations, may be we can provides some useful advice.

I believe JoeWong covered the technical aspects of your query very well, so I'll offer a bit more subjective analysis.

I would discard safety factors very reluctantly. True, safety factors often cover uncertainties and unforeseen details that tend to diminish as the design proceeds, falling to very low values as the system is constructed. Ideally, recommended safety factors would follow a similar trend and diminish as uncertainties disappear. In fairness, it's pretty hard though to come up with a set of rules that implement a practice like this.

Consider, however, that the accumulation of rust, scale, poorly installed gaskets, and fugitive unspecified items can act to decrease system's capacity below expectations. We cannot anticipate, nor can we overcome all such events. I give past designers credit for having developed reasonable safety factors based on much experience yielding a near ideal compromise between "having one's head stuck in the sand" (clueless) and being wildly paranoid. Perhaps I'm overly optimistic.
Doug

Thank Joe and Doug,

PSV i am selecting is conventional PSV and it's blowdown pressure is 181.5 Psig.

inlet and outlet piping pressure drops are based on set pressure.

I believe Doug provided the answer that i was looking for. Since I know almost every detail of my preocess. I do not need to add any safety factro, unless i am trying to add it for improper installation or rust etc.

Thanks for your help.

Amit