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Pressure Differential Valve for Fail Safe Operation

Cheap 5/8" poly irrigation tubing costs 5 cents/foot but degrades in the sun and unexpectably rupture creating a mess.

On solution would be to have venturis at both ends of the questionable tubing. Assuming the pressure drop across the length of tubing was low, the pressure in the throats of the venturis should be about the same if the water is flowing or if it is off.

Only when the tubing ruptures are the pressures different.

The valve could be double acting. In case of a small leak the down stream venturi would be higher and close the valve.

In case of a really large leak the pressure downstream might go to zero and the diaphram would seat the valve in the other direction.

Is there a common off the shelf diaphram or other valve that is normally open except when there's a pressure differential?

Bret Cahill

Reply to
Bret Cahill
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Dear Bret Cahill:

Teflon does not have this problem. A lot cheaper than a valve.

Which requires a second full piece of tubing, which will also degade in the Sun.

...

There is a valve used in folklifts that shuts when a hose is cut on the fork lift cylinder. Search for "velocity check". No remote sensing required... if the flow goes higher than some nominal value, the value shuts tight.

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)

We're talking thousands of feet of tubing.

They might be PVC or copper.

I was thinking about that too but what about slow leaks?

They've outlawed poly tubing solar water heaters. This is _not_ a green situation.

Now either we find a valve or the Greenland ice sheet will melt sooner than expected.

Bret Cahill

Reply to
Bret Cahill

Dear Bret Cahill:

Then you've solved your problem with the *first* piece of tubing. Make it "PVC or copper".

A pair of venturis won't catch those anyway.

A velocity check and eyeballs.

There are "sequence valves" that will do the sorts of thing you imagine, but will not work on "slow leaks".

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)

Maybe I should have said, "a few feet of PVC or copper."

A really slow leak isn't that much of a problem.

A 15% max flow rate leak would be at least few psi, more than enough for a diaphragm.

I'll reconsider the velocity check.

Bret Cahill

Reply to
Bret Cahill

Dear Bret Cahill:

I figured that out after you described solar hot water heating. Sorry. A line leaves, and returns to nearly the same spot.

The bigger it is, the more sensitive it would be. One of the venturi's probably needs to be between the hot water tank and the recirc pump.

Consider a line break. If the break is near either venturi, there will a differential pressure that can be worked with. If it is mid span, then no detection is possible.

Additionally, you will need to shut off *both* lines... not just one.

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)

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I know you are looking at mechanical solutions, but keep in mind there are electronic pressure switches available, as well as electronic flow switches. Either could be used to actuate a gate valve to stop water flow in a section affected with a leak or rupture as sensed by the switch.

I am using a flow switch in a methanol injection system that will give me an input if the demanded pump flow is not achieved at the injector.

Reply to
Mike H

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Yes, but he needs a differential flow switch... a differential pressure switch (connected to the venturis) would do that. With caveats.

David A. Smith

Reply to
dlzc

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