Self activating relief valve installed on a gate valve set pressure

[Lozzi70]

Dear all,

a gate valves user decided to approach the body cavity ovrepressure issue by means of a self activating relief valve installed on the valve body cavity (and not by means of equalization external line or hole in the valve disc). This situation involves a series of valve, with different ratio between max working pressure and design pressure (from 60% to 95%). The question is: which is the correct value to set the relief pressure in the relief valve? Is this a function of the max working pressure or of the design pressure or what?

Thanks to everybody for the support!

 

[valvesarefun]

Lozzi70,

I'm not sure I completely understand your question, but I believe you are trying to prevent over pressure of the body cavity in a gate valve, by setting up some sort of relief valve system.

My suggestion would be to set the relief valves based on the maximum expected system pressure, because ideally you want your body cavity pressure to be equal to your line pressure.

Best Regards,

VRF

 

[Lozzi70]

Thanks, valvesarefun.

You got the point, the valve user is trying to prevent over pressure in the body cavity of a gate valve by setting up a self activating relief valve.
The problem now is that the valve user doesn't know how to choose the value of the relief pressure, and he's asking the gate valve manufacturer to provide such information. The only data available are the design pressure and the working pressure.
I'm asking the engineers of this forum if there is an answer to this problem, given the design pressure and the working pressure data, or not.

Thanks again.

 

[valvesarefun]

Lozzi,

Can you give a description of the valve? Ie body, trim, seat, pressure class, and design (6D or 6A). Also state the design and working pressure.

Best regards,

VRF

 

[Lozzi70]

Thanks VRF,

one example: gate valve DN 100, pressure class: 1500, flexible wedge, pressure seal, design according EN 12516-1, body material: GP240GH, seat material: P245GH hardfaced, disc material: GP240GH hardfaced, stem material: X12Cr13.
Working pressure: 139.4 bar
Design pressure: 148.3 bar

 

[valvesarefun]

Lozzi,

First I would suggest selecting a relief valve that has a manual override feature. This way, if the relief valve is not triggered automatically, but the gate valve is still difficult to open, you can manually relieve the pressure.

Second, I would suggest that you set your relief pressure above your maximum allowable working pressure (i.e. the highest pressure the valve is expected to experience). You would have to determine an appropriate "safety factor" like say (maximum anticipated working pressure+10%). That should keep the relief valve from going off for no reason, and also allow the cavity to vent when pressure begins to build above the line pressure.

Honestly I think the first two options you mentioned (equalization external line or hole in the valve disc) are way better options, but the relief valve can be made to work if there the other options are excluded for some reason.

Best Regards,

VRF

 

[Lozzi70]

Many thanks for your help, VRF!

 

[gerhardl]

???????

As the manufacturor obviously is design responsible for this (uncommon?) solution, he is also the one responsible for the valves performance in the system.

As the pressure setting and the operation of the cavity system does not (true?) have an impact on the other parts of the pipeline system, the setting is only influencing the strength and performance of the valves.

Depending of fluid and construction of the relief valves a too low setting (excessive opening of relief) could influence the relief valves ability to close properly and operate over time.

A too high setting could influence the valve operation and not be within given strength and operational limits for the valves.

Conclusion: the only valid setting parameters for valid valve guarantee must come from the valve manyfacturor.

(Additional question: do this special type of solution have any references for longtime use you could draw on?)

 

[valvesarefun]

gerhardl,

"As the manufacturor obviously is design responsible for this (uncommon?) solution, he is also the one responsible for the valves performance in the system."

The manufacturer just designs the valve, and the customer must specify the particular way in which pressure equilization is achieved. Lozzi70, is saying that they have chosen to use the relief valve method, which is the least desireable of the 3 generally accepted methods for pressure equilization. It is least desireable because first your vented fluid usually has to be piped to a safe location(increased cost), and second because its difficult to accurately choose an appropriate pressure setting for the relief valve (particularly if system pressure fluctuates).

"As the pressure setting and the operation of the cavity system does not (true?) have an impact on the other parts of the pipeline system, the setting is only influencing the strength and performance of the valves."

The cavity pressure relief valve setting can influence the pipeline system, because if the gate is fully open then the body cavity essentially becomes a part of the pipeline. If the relief valve pressure is set too low, the relief valve will be triggered and cause system pressure drop when the gate is fully open.

"Depending of fluid and construction of the relief valves a too low setting (excessive opening of relief) could influence the relief valves ability to close properly and operate over time."

I agree. Regular maintenance and functional checks should be preformed to ensure proper function of relief valve. Relief valve material selection should also be appropriate for the intened service.

"A too high setting could influence the valve operation and not be within given strength and operational limits for the valves."

I agree, the relief valve setting should not exceed the rated working pressure of the valve. The maximum pressure/temperature ratings for most the body/shell of flanged valves can be determined by referencing ASME B16.34A, or consulting with the valve manufacturer. The relief valve's purpose must be two fold: 1. Equilize pressure between the system and the body cavity to maintain easy valve operation, 2. protect the valve itself from being damaged by over pressure.

"Conclusion: the only valid setting parameters for valid valve guarantee must come from the valve manyfacturor."

I disagree. Only the system engineer can accurately determine an appropriate relief pressure. The valve manufacturer can advise a maximum setting, but the maximum may not be appropriate in all circumstances.

"(Additional question: do this special type of solution have any references for longtime use you could draw on?)"

Unfortunately I do not. The relief valve service life will depend highly on how frequently it is triggered. The relief valve manufacturer should be able to predict a life expectancy if you can provide them with a cycle frequency.

Best Regards,

VRF

 

[gerhardl]

valvesarefun: noted, and thank you for your comments! Generally I think we agree, at least I do not disagree with your coments ;-)