Can a Failed Weir Diaphragm Valve in Vacuum Service be opened with more Vacuum?

[Bambie]

The diaphragm has separated from the stem inside a weir type diaphragm valve in vacuum service.

The process vacuum is holding the diaphragm in the closed position, which appears to be intact because it is not passing.

If the bonnet was drilled and tapped for a fitting and attached to a vacuum pump, it should open if the vacuum imposed is greater than the process vacuum.

Will lifting the diaphragm by removing air pressure from the bonnet impose more or less stress on the diaphragm than 'normal' operation of the stem?

If it does, should the diaphragm deflection be controlled by 'feeling' when the stem is contacted by the rising diaphragm?

 

[Compositepro]

The diaphragm should see less stress, I imagine.

 

[LittleInch]

Diagram? / Drawing

mechanical drawing?

connections from the diaphragm

How do you it's "separated from the stem"?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Bambie]

LittleInch,

I haven't been able to find a drawing of this 1960's era cast iron Saunders model 2431-S-T diaphragm valve that would provide the connection details for stem/compressor/diaphragm.

Compositepro,

I agree, bonnet pressure equalization would seem to be a good idea whenever diaphragm valves are used in vacuum service.

 

[Bambie]

PS,

Would anyone know how to approach a stress analysis of the neoprene diaphragm?

I was thinking a very conservative approach would be applying circular flat plate theory using Roark & Young with corrections applied for large deflection diaphragm stresses.

I could approximate what the reduction in stresses would be using superposition with and without bonnet pressure.

I could approximate internal dimensions from external measurements of lift, diameters and thickness, however I would be guessing at Young's Modulus and poisson's ratio for the neoprene diaphragm.

 

[Compositepro]

I do not see any point in doing the stress analysis you are talking about. What are you try to accomplish? One thing you do need to consider is that in some designs, like air operated double diaphragm pumps, the diaphragm is designed to operate with pressure always in one direction. They do not like to see pressure on the inlet. That reverse the curvature of the diaphragm and leads to fatigue failure.

 

[Bambie]

Compositepro,

The analysis would help convince Management that drilling and tapping the bonnet and body for a valve and tube connection to deliver a vacuum is a good idea.

I would like to modify all similar valves in the system to avoid more stem/compressor/diaphragm fatigue failures.

 

[Bambie]

LittleInch,

We are arranging for radiography to determine failure mode however the field hands say that they feel no resistance in the hand wheel when opening or closing the valve.

 

[hacksaw]

Sounds like you are dealing with a blockage in the pipe, not a valve failure.

What are your service conditions, and the nominal fluid properties.

 

[Bambie]

hacksaw,

Air vacuum service at ambient temperature.

 

[hacksaw]

"Saunders Patent Valves:

1. Used on applications handling chemicals and slurries
2. Not suited for high pressure
3. Provides tight shutoff
4. On-Off service only
5. Temperature limited to diaphragm material"


They are used in municiple waste water plants and not suited for fluids under vacuum. It will not work in your application.

For processes under vacuum, I've encountered elastomer sealed butterfly valves.

Your suggested use of vaccum will not likely work as the Saunders valve relies on positive process fluid to force it open.

 

[Bambie]

hacksaw,

Thanks for the advice, however these diaphragm valves have been working on vacuum air service for the past 50 years.

I suspect aging and fatigue of the neoprene elastomer has caused the diaphragm to detach from the stem/compressor.

Applying a higher vacuum to the bonnet will produce the positive process fluid load you speak of and force the diaphragm open.

 

[hacksaw]

The Sanders Patent design relies on a pneumatically powered ram to force the elastomer closed.

You seem to be referring to the purely pneumatic design that relies on external pressure to collapse elastomer sleeve. That is not the Saunders design.

What line size are you dealing with?

 

[Bambie]

hacksaw,

I am referring to 1960's era cast iron, manually actuated, Saunders model 2431-S-T diaphragm valves of various sizes.

 

[hacksaw]

Yes, and that model uses a manual (or pneumatically driven) ram to pinch the elastomer sleeve closed.

What size valve? Remember, that valve design is not suited to negative gage pressure process conditions.

 

[Bambie]

hacksaw,

Do you see any problem with equalizing pressure across the diaphragms by applying a vacuum to the bonnets during normal operation?

Yoke sleeve grease could enter the system and pressure pulses may not be dampened as effectively without air filled bonnets, which could accelerate fatigue.

On the other hand if the bonnets are air tight, then opening the valves causes the trapped air to compress and impose even larger bending moments on the diaphragm connections.

Perhaps drilling, tapping and installing pressure gauges in the bonnets would provide some insight?

 

[hacksaw]

Bambie,

you've indicated the valve is in "vacuum service," so it is unclear how "venting the bonnet" will help.

Remember you are not dealing with a conventional valve. The Sanders patent valve is only used in rare cases if at all these days, and only in positice pressure services, not vacuum.

Valve actuation that is independent of the line pressure is what you need to use. The valve you've describe squeezes the flow path closed, mechanically, and relies on positive fluid pressure to open, when the squeeze mechanism is released.

You really need to consider an alternitve valve design, once you sort out the line size.

 

[Bambie]

hacksaw,

I appreciate your interest in re-designing the entire vacuum system, however I must be content with providing a way to open a Saunders diaphragm valve that has failed closed and preventing this degradation from happening to the others.

I have attached a few pages from an archived Saunders catalog that indicate a fairly typical design for all sizes of valves.

I don't understand why you add "patent" to the Saunders name brand.

I also can't help you if you cannot grasp the concept of reducing diaphragm stress by preventing bonnet air compression through venting or, better still, applying a vacuum to the bonnet to equalize pressure differential.

 

[Bambie]

hacksaw,

Page 26 of that Saunders Catalog (attached) contains a note for applications on vacuum service.

 

[hacksaw]

Sanders Patent Valve was the name commonly used in 1965.

It is only used for one off service.

It is not suited for vacuum or negative gage pressure services, as your blockage problem proves.

You vaccum idea won't work as you have claimed a vacuum in your piping pulling the elastomer sleeve closed.

Two vacuums (one in the pipe, one in your bonnet) won't provide adequate forcing, it is a matter of a net force.

Replace the valve.