5 replies to this topic

[abcd63b]

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). 

[latexman]

 

 

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.

[Bobby Strain]

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

 

Bobby

[fallah]

 

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.

[usaidsaif]

i am trying to post this as a new topic but its not working.sorry to include here

 

 

Hello.i have a question.in a ptoject  some part of flare header is of mixed material.cs and ss
In  ss tie ins are bascially from hcdp unit wheres tempersture ranges are below -50f
First 600 m is of ss.now client want me to tie in ss and tell him that till what length temperture shall be positive so that it doesnt effect it cs portion.
How i can check that till what length my negative temp -60f which i have tie in ss will be in positive temp so that it doesnt effect cs portion

[abhi_agrawa]

Usaidsaif,

 

What you really need to do is calculate the length of bare SS piping required such that from ambient heating the temperature of the fluid increases, such that it is acceptable for CS piping. Further, since the relief can happen when the ambient temperature is low, for your calculation you should assume that the ambient temperature is the minimum ambient temperature.

 

Hope this helps,

Abhishek

 

 

 

i am trying to post this as a new topic but its not working.sorry to include here

 

 

Hello.i have a question.in a ptoject  some part of flare header is of mixed material.cs and ss
In  ss tie ins are bascially from hcdp unit wheres tempersture ranges are below -50f
First 600 m is of ss.now client want me to tie in ss and tell him that till what length temperture shall be positive so that it doesnt effect it cs portion.
How i can check that till what length my negative temp -60f which i have tie in ss will be in positive temp so that it doesnt effect cs portion