Bellows Seal Gate or Globe

[engineer2385]

Hello Everyone,
I am new to the Valve industry and learning different designs and standards. Just came across Bellows Seal Gate and Globe. I read SP-117 and a few drawings and see that Bellows undergo Compression and Extension cycle under pressure. Recently, I observed leakage through Bellows Seal with fluid leaking through the bushing. Can someone explain more on Bellows seal from its Design point of view and how they handle fluid pressure?

Thank you

 

[venividivici]

What kind of fluid are you dealing with and at what temperature? Mechanical seals such as these are rated for pressure based on a specific gravity and temperature of the fluid. The lower the SG or the higher the temperature, the lower the pressure rating will be.

 

[pennpiper]

"I observed leakage through Bellows Seal with fluid leaking through the bushing."
This should not happen unless there is a rupture of the bellows.
Look here:

http://www.google.com/search?q=bell...ed=0CDMQyjc&ei=J1ZaVZyGE8TJsAXp4IGgBg#imgrc=_

Sometimes its possible to do all the right things and still get bad results

 

[engineer2385]

Apparently, the bushing came loose due to pressure. There was no fluid leakage through the bushing(it was wrong I said that in the first post). To me, it looks like the fluid load being applied to bellows transferred the load up until bushing making it loose. This was surprising since this being a Gate valve wherein the axial load is lesser(as compared to globe). That's why I'm trying to understand the way Bellows seal handle fluid pressure. There was no rupture of the Bellows here and this happened during the hydro test.

 

[gerhardl]

It is still a bit unclear how the different parts are located and being mecanically moved by spindle movement and fluid pressure relative to each other.

If the bellows are non-leaking apart form inlet and outlet openings for spindle and bushings, there could possibly be an operation - caused 'wrong' movement of the spindle.

The spindle could be forcing the bellows to stretch or compress (or skew) more than allowed for by the construction. An unprecise operation or forced operation against poorly machined or placed/dimensioned end-stops, sealings or bushings could give the result you are describing.

 

[bellowsmfg]

There could be several issues here so I'll hit on a few and you can look further into it.
A. Pressure Trust - When a bellow is put under pressure it produces a pressure thrust (P x Ae), where P is the Pressure and Ae is the Effective Area of the bellows. If the connecting ends are not property designed to deal with this, issues will arise.
B. Displacement - The bellow is designed to displace, ensure the displacements have been calculated correctly and compare to the design displacements of the bellows.
C. Cycles - Bellows are designed for a certain number of cycles. For your industry it is usual to try and design for 100k cycles. Other industries go a low as 1000 cycles. Determine how long it takes for your valve to reach 100k cycles (or your design number of cycles) so you know how frequently the bellow needs to be changed out. Understand each induced stress in a bellow is considered a cycle (be it pressure, thermal or displacement).
D. Vibration - Observed if there is vibration in the system. The natural frequencies of a bellow can be calculated or provided by the manufacturer. Compare these to frequencies produced in the system to ensure you do not have resonance.

That will keep you moving in the right direction for now.

All my best

Conor

Bellows Manufacturing and Research, Inc.

https://bellowsmfg.com

 

[engineer2385]

gerhardl and bellowsmfg,
Thanks for your reply. It gives me insight into design of bellows and different ways bellows can fail.