6 replies to this topic

[shekhar dhuri]

Hi,

 

I need your suggestions on type of ball valve/s (reduced bore or full bore) required to be used for PSV isolation.  

 

I am used to see full bore isolation valve in all my past projects. However, API does not insist the upstream isolation valve has to be 'Full bore'. As long as we meet 3% pressure drop criteria for PSV inlet line sizing, can we also accept reduced bore valve ??

 

Reduced bore valves are cost effective over full bore valves. Especially, for the plant where there are many PSVs, it might be signficant cost saving.

 

Please suggest.

 

Regards,

Shekhar Dhuri

[katmar]

Do your local regulations allow you to install a valve upstream of a PSV?  Usually all valves are totally forbidden.

Edited by katmar, 03 June 2014 - 01:04 AM.

[fallah]

 

I need your suggestions on type of ball valve/s (reduced bore or full bore) required to be used for PSV isolation.  

 

I am used to see full bore isolation valve in all my past projects. However, API does not insist the upstream isolation valve has to be 'Full bore'. As long as we meet 3% pressure drop criteria for PSV inlet line sizing, can we also accept reduced bore valve ??

 

Reduced bore valves are cost effective over full bore valves. Especially, for the plant where there are many PSVs, it might be signficant cost saving.

 

Please suggest.

 

 

Hi,

 

Yes, API does not insist the upstream isolation valve has to be 'Full bore', then it can be "Reduced bore" provided that the inlet line loss doesn't exceed 3% of the PSV set pressure and as per attached from API 520 part II the minimum flow area in the isolation valve be equal to or greater than the inlet area of the PSV.

Attached Files

•  Page from API 520 Part II.docx   387.34KB   92 downloads

[Marian1]

same requirement for outlet system!

[smalawi]

Hi

 

typically gate valves are utilized for relief valve inlet isolation for ASME sec8 process vessels. they are not allowed for steam boiler service which has a different code. there is a code case under the boiler code that puts the option of a flow diverter valve certified for this use

 

the 3% rule is not really a rule but more of a guideline, it is related to chatter potential under no flow/flow conditions, you can design systems with higher pressure drop if you account precisely for the systems pressure drop and RV characteristics with the help of the vendor (provided they know what they do).  

 

i have not seen many ball valves being used expect for small bore pipes. it might not be well known but ball valves are not very safe, you have two metal parts that form a capacitor design, thus creating an electrical potential & sparks. this is more applicable for flowing systems but less for stagnant ones

 

cheers,

sm

[fallah]

sm,

 

Gate valves have so technical/operational/safety limitations (higher leakage rate and opening/closing problem in larger size) to be used as upstream isolation valves of PSVs in all cases. Then normally they are to be used for:

 

-Relevant line rating of maximum 150#, and

 

-Diameter lower than or equal to, say, 8", and

 

-Non sour/non toxic services

Edited by fallah, 03 June 2014 - 12:14 PM.

[jrtailor09]

Yes , I have seen gate valves was used earlier in PSV inlet /outlet line for the isolation purpose . it has been replaced most by the ball valve because of the above problem as fallah explained.

 

Generally client has considered Full bore valve during FEED study then he will not allow to change to reduced bore. As you had quoted on the basis of full bore valve even reduced bore ball valve suffice your requirement.

 

Thanks for highlighting this requirement.

 

Regards,