Pressure valve design opening

[picclock]

Hi
I am in the process of 'designing' a system with a pressure valve which needs to be opened periodically with lowish force, so direct acting is out of the question. The pressure on the opposite sides of the valve will vary so offsetting it with a spring is not going to do it. I have considered using a very small valve to equalise the pressure before opening the main valve, but the time taken to equalise the pressure will be a problem. A butterfly valve will give very low opening force but tend to be difficult to make and they leak.

I have seen quite complex solutions overcoming this problem with a diaphragm but want to know if there is a simpler approach. Any design ideas or pointers to web resources from you expert engineers most welcome.

Many thanks in advance

Best Regards

picclock

 

[Ken I]

Picclock, more info - substance (steam, air, uranium hexaflouride) - flow, pressure, range, permissible leak rate (thou shalt not measure zero).

Offhand how about a cross drilled taper plug (petcock) type - these can be lapped in to give good fit - turning torque pretty much unaffected by the pressure.

2c Ken

 

[picclock]

Ok More info pressure differential maybe up to 120psi max. Min probably about 10 psi. Main valve aperture is 17mm diameter. Uranium hexaflouride is solid at room temperature so I think that would cause a few difficulties, so I guess I'll stick to compressed air. Temperature range is just above ambient always less than 40C. Leakage? what leakage - as low as possible.

Problem is I would like to operate the valve to equalise the pressures with a small force, hence my needing to know about valve type mechanisms. Saw a picture of 1 on the internet that used a diaphragm as a slave to open the main valve path with just a tiny hole. I really don't know very much about these 'power assisted' type valve mechanisms hence my seeking solutions which will fit into small spaces and do the same thing. Considered a moving piston instead of a diaphragm where a spring holds it closed when pressure on either side of piston is equal, but when the pressure is reduced on one side the piston will move allowing large flow to commence.

If that sounds like garbage can do a sketch - but I'm really just looking for all the ideas out there at the moment.

Best Regards

picclock

 

[Ken I]

Picclock, the diaphragm idea is commonly used in many valve applications basically the pressure self closes the diaphragm and a small orifice / bleed apperture is all that is opened to get the diaphragm off its seat - they also seal pretty well.

See weblink - the diaphragm is much larger than the seat - pressure on the underside of the seat (inlet side) goes through the orifice in the middle of the diaphragm - pressurising the larger area to close the seal against the seat - it will stay shut until a small solenoid (or whatever) opens another slightly larger orifice to the outlet - pressure to the diaphragm is lost and the supply line backs it up - it will stay open until the solenoid closes and the pressure can once again close the diaphragm.

Often used in irrigation systems.

Tends to "slam" shut which can be a problem with water hammer in liquids (I blew the arse of a 2" diaphragm valve at the end of a long water line - you can't just suddenly stop half a ton of moving liquid).

Don't handle contaminants well. Plugging of the diaphragm orifice or interference with the slave valve seat will cause problems. They also work in one direction only.

If you can't use an off the shelf unit - perhaps you can scavenge the internals for a custom application.

Ken


http://www.google.com/imgres?imgurl=http://www.spudtech.com/images/etc/valve3.gif&imgrefurl=http://www.spudtech.com/content.asp%3Fid%3D21&usg=__rIRWDptFK5yXsl558Tb_AOdqLEs=&h=325&w=400&sz=5&hl=en&start=18&zoom=1&tbnid=rzJF7m-EowcnXM:&tbnh=101&tbnw=124&ei=k7TnTu73C4iIhQeK8O3sCg&prev=/search%3Fq%3Ddiaphragm%2Bvalve%2Bsolenoid%2Boperated%2Bhow%2Bstuff%2Bworks%26um%3D1%26hl%3Den%26sa%3DN%26rls%3DRNWE,RNWE:2008-17,RNWE:en%26tbm%3Disch%26prmd%3Divns&um=1&itbs=1

 

[Ken I]

Picclock, as an aside - I build leak testing machines and "leakproof" valves are hard to come by.

I built a tester for a manufacturer of gate valves and plumbing fittings - I asked them for a leak rate spec. they said "zero" - I said "there ain't no such thing and you can't measure it".
They were horrified when my machine showed them that everything they made - leaked.

 

[picclock]

@ Ken I
That's a really good explanation of what's happening. The real trick is that the area above the diaphragm is larger than that below, so that with equal pressure more force is exerted on the top closing the valve. A neat trick, but there's no reason that the diaphragm can't be replaced by a piston with the appropriate seal(s). This seems a very crafty way of doing things with the operating speed determined by the predefined leak and the 'turn on' opening. So if I understand correctly opening time is determined by volume above the piston (diaphragm) and the speed with which the pressure can be depleted (size of turn on opening). Closing speed is determined by size of predefined leak.

Many Thanks

Best Regards

picclock

 

[Ken I]

Picclock, as I was reading your post I was thinking a piston type might be more appropriate for our needs - then you said exactly that - so yes.

The slave valve orifice needs to be bigger than the "piston" orifice as a rule of thumb - depending on the ratio of piston to seat diameter. A needle and seat arrangement will work for the slave valve as well.

The piston orifice is often an anulus formed between the hole and a guide pin - the pin is often stepped down so that the effective area of the anulus increases as the piston raises - not to speed it up but to clear debris.
Its also easier to make a larger hole with a pin though it than a very small hole of the same cross section - additionally you can futz with the orifice by adjusting the pin diameter.

There is normally a spring behind the diaphragm (piston) to bias it.

Hope this helps.

P.S. I didn't know Uranium Hexaflouride was solid at room temperature - I was simply using something incredebly difficult to handle as an exaggeration - can you say Methylcyclopentadienyl Manganese Tricarbonyl.

Ken

 

[picclock]

@Ken I

>> can you say Methylcyclopentadienyl Manganese Tricarbonyl.

YES ! ;D

I thought that instead of having a guide pin (its a cylinder moving in a bore so why ?) I would just pop a small hole in it, say 2mm, and then use a 3mm hole for the activated one. This will give low force operation (around 1% of pressure applied) with reasonable speed, proportional to the differential pressure.

Many thanks

Picclock

 

[Ken I]

Picclock, No problem - the pin is not required - holes are typically very small, the orifice being 0.8-1.0mm and the slave valve 1.2-1.5mm - obviously smaller means less actuating force, bigger = faster valve response.

The pin does serve some functions other than guiding (its often referred to as a guide pin) as I mentioned it can help with clearing debris (particulate matter that might otherwise clog the orifice) - in a piston design it can serve as a needle valve throttle to cushion the moving piston so that it doesn't hammer into its housing. I believe in diaphragm types it also helps prevent "trumpeting" - as the diaphragm seats it can bellow like a bull elephant in must - under some circumstances.

There is also normally some redundant volume above the diaphragm (piston) to slow the response, which in fluid applications can be very violent (water hammer) otherwise.

P.S. most modern toilet cistern valves (as opposed to the old ballcock type) use a diaphragm - a much smaller float can close the slave valve hole.

A further thought - these valves are also available with remote piloting - servo type - using compressed air to close the diaphragm - the advantage is this will work both ways (you did say "equalising" which implies the pressure difference might be the "wrong" way around).

Ken

 

[Maryak]

can you say Methylcyclopentadienyl Manganese Tricarbonyl.

Yep................Methylcyclopentadienyl Manganese Tricarbonyl.

Best Regards
Bob

 

[Noitoen]

Easy http://tts.imtranslator.net/Ivd4

 

[Noitoen]

Just joking. I've seen this device working and functions well.
http://news.thomasnet.com/fullstory/Deluge-Valve-features-single-moving-piece-25488