6 replies to this topic

[Edwingineer]

Dear all,

 

our piping system for liquefied gas is protected against line block with a small safety valve. It is boiling liquid at -160 deg. C, so not 'just' a thermal relief valve, however the required capacity is very low. Pipe design pressure is 16 bar.

The valves relief into a header, connected back to the tank gas phase, which is operating at 1 bar, but may be at 4 bar, which is its design pressure. So the backpressure is a variable superimposed back pressure, although the main source of backpressure changes very slowly with time and could be seen as a constant backpressure of maximum 4 bar.

 

I was planning on installing conventional safety valves set at 12 bar cdtp. Even with the tank at 4 bar, the valves would still open at 16 bar. The backpressure would influence the valve's capacity but if the remaining capacity is sufficient, the piping would be sufficiently protected. Blowdown is not an important parameter.

 

First question: is this allowed?

 

The contractor has decided to use balanced safety valves because his vendor has simply informed him that when backpressure is above 10% you need to use balanced type valves. But the valves are still set at 12 bar. I believe that if the tank is at 4 bar, the balanced safety valve will still open at 12 bar and not at 16 bar, which is also safe, but a waste of money.

 

Question 2: am I correct?

 

Thank you,

Edwin

 

 

 

 

 

 

[fallah]

Edwin,

 

1- Allowed, because relieving pressure doesn't exceed the design pressure of piping (16 barg) 

 

2- Yes, especially because at such low temperature the bellows of this type of PSV is severely subject to brittle fracture damage...

[Edwingineer]

Thank you, Fallah,

 

I must say that the manufacturer (Leser) is a reputable one from Germany. They sell these safety valves for cryogenic temperature, closed bonnet, balanced type. It is a series 433, with modulated action. The bellows should be rated for the low temperatures (I will verify).

From another post I read yesterday, I learned that API 520 states that balanced type bonnet should allways be vented to atmosphere. That conflicts with the design of the selected valve. We're on a ship in Europe, and API is not a specification, but I'm still interested to know how this is possible. I will follow up with them and keep you posted.

 

Edwin

[fallah]

 

I must say that the manufacturer (Leser) is a reputable one from Germany. They sell these safety valves for cryogenic temperature, closed bonnet, balanced type. It is a series 433, with modulated action. The bellows should be rated for the low temperatures (I will verify).

From another post I read yesterday, I learned that API 520 states that balanced type bonnet should allways be vented to atmosphere. That conflicts with the design of the selected valve. We're on a ship in Europe, and API is not a specification, but I'm still interested to know how this is possible. I will follow up with them and keep you posted.

 

 

Edwin,

 

Yes, in general, the bonnet of balanced bellows type PSV should be vented to atmosphere...

 

Also, as far as i know, modulating action characteristic can be provided just by pilot operated PSV...hence please recheck and let's know the result...

 

And yes, Leser is a reputable company in the field of PSV manufacturing and i was involved in a gas refinery project for which we had many technical meetings with them to specify and purchase the required PSVs for mentioned project...but if you need to have a detailed clarifications on the vague points you did refer, you should provide the detail info of the PSV you are purchasing from Leser...

[shan]

Higher back pressure may reduce the conventional PSV relieving capacity.  It seems to me that you selected an oversized PSV to compensate the reduced PSV relieving capacity due to higher back pressure.  You may need checking if the oversized PSV will have chattering problem when the back pressure is low.

[Edwingineer]

Thank you Shan,

 

you are right. It's impossible not to oversize when the scenario is manual blocking of a volume of 2 liters of pipe; the required capacity is very low. But the modulate action valve should handle the low quantity of gas without chattering.

 

Fallah, I will set up a meeting with Leser to discuss details and keep you informed. After Xmas, I'm afraid..

[Nikolay_]

Hello Edwingineer,

 

Could you please tell me the operation pressure of your pipe? Is the pressure below 12 bar?

 

Regards,

Nikolai