Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

3-Stage Propane Ref Performance

New Blog Entry

Steam Tracing Design- posted in Ankur's blog

0

Multiple Safety Valves

psv

5 replies to this topic
Share this topic:
| More

#1 ARM

ARM

    Veteran Member

  • Members
  • 43 posts

Posted 16 May 2017 - 12:51 AM

Hi All,

 

I have question regarding multiple safety valves, in my case two safety valves.

 

PSV A set pressure            :              10 barg

PSV B set pressure            :              10.5 barg (105% of PSV A set pressure as per API 520)

 

I have several scenarios on these two safety valves. One of the scenarios (non-fire scenario, not governing scenario) can be handled with PSV A (PSV A has adequate capacity to prevent overpressure).

 

My question: is it possible to consider only PSV A safety valve for this scenario and consider 10% pressure accumulation (instead of 16% pressure accumulation for multiple safety valves)?

 

My own opinion:

 

In reality additional safety valve (PSV A) will be partially open (if not fully open) because;

  1. Relief pressure (110% of Design Pressure) will be higher than set pressure of PSV B (105% of Design Pressure)
  2. Based on API520 Part I, Para. 4.2.1.2.4: “…Typically, the valve will suddenly surge to 50 % to 100 % lift at 2 % to 6 % overpressure…”.

If we consider that only one valve with lower set pressure (PSV A) is fully open then we may overlook potential risk of chattering since both safety valves will be open. I believe most conservative way is to consider both valves open (therefore, 16% pressure accumulation on top of design pressure as relief pressure).

 

Please let me know how you think about it.

 

Regards,

 

ARM


Edited by ARM, 16 May 2017 - 02:15 AM.


#2 fallah

fallah

    Gold Member

  • ChE Plus Subscriber
  • 4,382 posts

Posted 16 May 2017 - 01:12 AM

 

My question: is it possible to consider only PSV A safety valve for this scenario and consider 10% pressure accumulation (instead of 16% pressure accumulation for multiple safety valves)?

 

 

ARM,

 

In sizing for multiple valve applications, the total required area is to be calculated based on overpressure of 16%. Hence you should consider 16% pressure accumulation for all scenarios for which mentioned multiple PSV's are selected and installed.
 



#3 Saml

Saml

    Gold Member

  • Members
  • 206 posts

Posted 16 May 2017 - 08:21 AM

You don't say if the governing case is fire or not. If it is fire, the suplemental valve for fire only case can be set up to 110% of MAWP. ASME VIII Div 1 UG-134(b-)

 

Also, you don't need to design both with the same size. You can design, in the case you mention, the first valve to be bigger so under the non governing scenario the pressure don't reach the setpoint of the second. Or smaller you can make it smaller, so the second one won't chatter in that scenario. 


Edited by Saml, 16 May 2017 - 08:22 AM.


#4 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,256 posts

Posted 16 May 2017 - 10:56 AM

No matter what you do it is likely that the valves will "chatter" when relieving. So, I wouldn't be too concerned as to how you set them up. Just be sure they are properly sized.

 

Bobby



#5 ARM

ARM

    Veteran Member

  • Members
  • 43 posts

Posted 18 May 2017 - 02:50 AM

There is no fire scenario identified in my case. Therefore, I should keep the set pressure of additional safety valve 105% of design pressure.

 

From above replies I understand that even though capacity PSV with lower set pressure has enough capacity, I should consider both safety valves open as conservative approach to check possibility of chattering.



#6 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,256 posts

Posted 18 May 2017 - 09:24 AM

You don't need to check. Conventional valves will chatter. You worry too much about a trivial issue.

 

Bobby






Similar Topics