6 replies to this topic

[J_Leo]

Hello There,

 

I am doing the calculation for a PSV located on a horizontal drum blanketed with N2.

N2 supply line design pressure: 150psig

N2 supply max operating: 105psig

PSV set pressure (which is also the design pressure of the drum): 100 psig

There is a PV on the nitrogen supply line and a PV on the vent line

 

My question:

Is it a relief scenario if PV on the supply line fails wide open or PV on the vent line fails close?

It depends on what N2 pressure to consider for the calculation. If I use the design pressure, then there is a relief. If I use the max operating pressure, then there is no relief. I know if I set the drum design pressure at 150psig, there will definitely no relief. Because some other process reasons, it is not a good practice to set the drum design pressure that high.

 

Would you please shed some light on this?

 

Thank you very much.

 

Merry Christmas!

 

Regards,

Leo

[ankur2061]

Leo,

 

I presume that what you are calling as a PV on the blanketing supply line is a pressure control valve or automatic pressure regulator which regulates the blanketing pressure in the drum. If such is the case, then in case of a 100% stuck open condition of this valve your drum can see the maximum operating pressure upstream of this PV which in your case is 105 psig.

 

Do you know the reason why the design pressure in your nitrogen supply line is 150 psig? If your nitrogen supply source pressure cannot exceed 105 psig then you don't need to consider 150 psig.

 

Many a times the piping design pressure is arbitrarily given as the design pressure corresponding to the piping class (150#. 300# etc.) at the design temperature. This is a lazy approach of trying to avoid a detailed analysis for the design pressure of the piping but still serves the purpose in a conservative manner, since the maximum design pressure for any pipe should never exceed the design pressure as per the pressure-temperature tables given in ASME B16.5 for a given material class and pound rating of the pipe.

 

One thing in your post is not clear. What is the blanket pressure in your drum and what kind of a valve or pressure regulator you are using to reduce the pressure from say 105 psig to the drum operating pressure? 

 

Hope this helps.

 

Regards,

Ankur.

[fallah]

Leo,

 

Your post has a lack of adequate info:

 

A detailed sketch...

Set points of the PVs?

Pressure of the N₂ supply source?

Is a PCV located between PV on the N₂ supply line and N₂ supply source?

Sizing case of the PVs?

[Sharma Varun]

Leo:

 

Although Ankur & Fallah have very well diagnosed your post, I feel you must also be very careful about this max operating pressure.  If possible, increase the design pressure to at least 105 psig.

[Jiten_process]

To qualify a PSV there are two approach i have seen in my experience. Specially for N2 blanketing cases. 

 

I recently involved in a project where licenser has follow the approach as "PSV is required if the design pressure of vessel (which is blanketed) is less than max. operating pressure of N2 supply upstream of PV (a pressure control valve)."

 

But this is contradicting with some EPC company where they consider design pressure in qualifying PSV requirement instead of max. operating pressure.

 

However, considering max. operating pressure is justified in my project as we know the upstream system very well. In our project LP nitrogen is used as blanketing gas. This LP N2 was supplied by reducing pressure of HP nitrogen across mechanical PRV. There is a PSV installed downstream of this PRV which is set at design pressure of LP nitrogen system.

 

Now the vessel which is blanketed by this LP nitrogen is designed at pressure of lower than design pressure of LP N2 system design pressure but a pressure same as Max. operating pressure of LP N2 system. In such cases, we did not consider the PSV for vessel for LP N2 PV fail case. This is because considering LP N2 system at design pressure where PSV provided downstream of Pressure reducing station and failing of blanketing pressure control valve is two relief contigency which normally do not considered simultaneous as per safety study practice.

 

Coming to your system, check in your nitrogen pressure system upsteam of PV. if you have other PSV in your upstream N2 system which is likes to be present then you may consider max. operating preussre in qualifying requirement of PSV for the vessel which is blanketed.

 

Hope this helps. 

Edited by Jiten_process, 24 December 2013 - 05:06 AM.

[J_Leo]

All, thank you very much for your replies.

Here are additional information for clarification
1 The blanketing nitrogen is produced from a membrane system. Feed to the system is instrument air from compression and drying system. Feed IA to the membrane system is under pressure control, set at 115 psig. The expected pressure drop through membrane is 20 psi. Since all these are vendor packages, pressure profile will have to be verified during EPC
2 The drum normal operating pressure is 40 psig.
3 PV on the supply and vent line refer to pressure control valve. NO PCV is involved

The initial thought for the drum design pressure set point is to avoid PV failure relief, if we can take max supply pressure for relief consideration.
There is a PSV on this drum for fire case anyway. I assume there should be a PSV in the membrane package which shall be set at 150 psig.

Sorry I cannot provide a sketch since I am away from office for vacation, no convenient software available for use.

Edited by J_Leo, 24 December 2013 - 01:25 PM.

[latexman]

Is contents of drum toxic, flammable, or regulated?