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

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

1

Superimposed Backpressure Greater Than Inlet Pressure


3 replies to this topic
Share this topic:
| More

#1 abcd63b

abcd63b

    Brand New Member

  • Members
  • 6 posts

Posted 28 February 2020 - 12:12 AM

Hello everyone. 

 

Section 6.4 of API 520 Part II says that if the superimposed backpressure is greater than inlet pressure, reverse flow can occur. It asks us to provide an accessory to prevent such backflow.

 

Firstly, I did not understand how such a case would be possible. Why not increase the design pressure (consequently the set pressure would be higher) of the vessel itself above the operating pressure of the flare header? 

 

I know that within Section 6.4, they have mentioned that this can happen if multiple PRVs are discharging simultaneously but would that not be considered double jeopardy? 

 

Secondly, what accessory are they talking about here? Even if they are talking about an NRV, the NRV won't open at all in my opinion of the pressure at the NRV's outlet is higher than its inlet pressure

 

Thank you for your time Sirs (and Madams if present). 



#2 latexman

latexman

    Gold Member

  • ChE Plus Subscriber
  • 1,003 posts

Posted 28 February 2020 - 07:57 AM

 

 

I know that within Section 6.4, they have mentioned that this can happen if multiple PRVs are discharging simultaneously but would that not be considered double jeopardy? 

 

Not if multiple PRVs are in the same fire area.  Not if multiple PRVs are on cryogenic storage and utilities are lost.

 

 

 

Secondly, what accessory are they talking about here? 

 

A pressure switch in the discharge that closes a pilot operated PCV would qualify, but I'm not sure that's the type of PCV you are working with.



#3 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,825 posts

Posted 28 February 2020 - 10:28 AM

Pilot operated valves will behave as described unless they are equipped with a back-flow preventer.  The condition typically occurs during a startup.

 

Bobby



#4 fallah

fallah

    Gold Member

  • ChE Plus Subscriber
  • 4,662 posts

Posted 29 February 2020 - 01:28 AM

 

Section 6.4 of API 520 Part II says that if the superimposed backpressure is greater than inlet pressure, reverse flow can occur. It asks us to provide an accessory to prevent such backflow.

 

Firstly, I did not understand how such a case would be possible. Why not increase the design pressure (consequently the set pressure would be higher) of the vessel itself above the operating pressure of the flare header? 

 

I know that within Section 6.4, they have mentioned that this can happen if multiple PRVs are discharging simultaneously but would that not be considered double jeopardy? 

 

Secondly, what accessory are they talking about here? Even if they are talking about an NRV, the NRV won't open at all in my opinion of the pressure at the NRV's outlet is higher than its inlet pressure

 

 

Hi,

 

Simultaneous discharging multiple PSVs cannot be considered as double jeopardy because they might be discharged due to a common failure such as instrument air failure.

 

If the design pressure of a vessel is lower than the flare operating pressure it should be conducted to lower pressure level of the flare, for example, from HP flare to MP flare.

 

The mentioned accessory is considering back flow preventer for pilot operated PSV to prevent back flow in the cases you described.






Similar Topics