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Thalstan

Thalstan

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Thalstan
My wife is a process engineer that designs safety systems for plants.

I am a root cause analysis guy with a background in system operations, EMS/SCADA, and Relays/protection systems. The relay/protection system work I deal with is really not that much different than flow and pressure in piping systems except we call it amps and voltage in wiring, otherwise, the concepts are really very similar.
 
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NaffNaffBobFace

NaffNaffBobFace

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The valve on our gas central heating boiler appears to fail closed. We found this out the day we moved in and the electrics for the boiler fried themselves that evening. Smelt as if a car battery had exploded but there was not a trace of gas escaping.
 
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Thalstan

Thalstan

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Thalstan
Yes, a fuel valve you would want to fail closed. There are instances you want a valve to fail closed. The problem was the blanket statement before of you want it to always fail closed, that is not the case. You want them to fail into a safe mode. For example, a water heater has a safety valve. That valve is supposed to be maintained and replaced every so often to ensure it works. You want that valve to fail to an open position...because if it fails closed and then you have an overheat/overpressure situation, you get waterheater rocket. See demonstration of this here.


Have you replaced/maintained your water heater safety valve recently?
View: https://youtu.be/9bU-I2ZiML0
 
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NaffNaffBobFace

NaffNaffBobFace

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Thalstan said:
Yes, a fuel valve you would want to fail closed. There are instances you want a valve to fail closed. The problem was the blanket statement before of you want it to always fail closed, that is not the case. You want them to fail into a safe mode. For example, a water heater has a safety valve. That valve is supposed to be maintained and replaced every so often to ensure it works. You want that valve to fail to an open position...because if it fails closed and then you have an overheat/overpressure situation, you get waterheater rocket. See demonstration of this here.


Have you replaced/maintained your water heater safety valve recently?
View: https://youtu.be/9bU-I2ZiML0
Click to expand...
I have not maintained it, however I have ensured it points at the house on the street no one likes so if disaster strikes at least there is a silver lining.
 
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Bambooza

Bambooza

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Thalstan said:
Yes, a fuel valve you would want to fail closed. There are instances you want a valve to fail closed. The problem was the blanket statement before of you want it to always fail closed, that is not the case. You want them to fail into a safe mode. For example, a water heater has a safety valve. That valve is supposed to be maintained and replaced every so often to ensure it works. You want that valve to fail to an open position...because if it fails closed and then you have an overheat/overpressure situation, you get waterheater rocket. See demonstration of this here.


Have you replaced/maintained your water heater safety valve recently?
View: https://youtu.be/9bU-I2ZiML0
Click to expand...

Um I think you proved the opposite point. I think all water heaters should fail close and thus remove themselves from your house in glorious fashion when they fail.
 
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Frad in'Ryth

Frad in'Ryth

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I just want to say I love this group.

I asked my buddy about his auto bar tender. He went with fail open so any built up pressure etc would release and back flow into the liquor bottles when the system was powered down.
He also designed it so all the booze was in the cabinet underneath the dispenser and touch screen, to clean/sanitize just required pulling all the heads from the bottles tossing them in a bucket of cleaner and hitting the cleaner cycle he programed for it.
 
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Sirus7264

Sirus7264

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Thalstan said:
Umm, it depends really. In certain situations, you might want valves to fail open it depends on the nature of the chemical, process, and positions in the piping. For instance, you do not want a Valve that is placed down flow of a pump to fail closed.

Case in point, a check valve operates in a manner that flow only goes one way. When installed correctly on a sump pump, it allows flow from sump pit to the discharge point, but does not allow flow back into the sump pit. Yet, if this valve fails, you want it to fail open so that when the pump turns on, water can still flow to the discharge point. You don’t want your pipes bursting or coming apart because the valve failed closed.



I also asked my wife if there are instances where you would want a valve to fail open instead of closed. Since she designs safety systems for chem plant piping and is a P.E., I feel she is a valid source.

Her response...plenty (of times)
Click to expand...
For things such as reciprocating pumps(positive displacement) you should always have a relief valve of some sort. A centrifugal pump will not increase in preasure after it dead heads. This is more of a design issue.

Now back to the main subject yes there are times that a valve needs to fail in the open position yes there are times that valves need to fail in the open position my comment was only directed for this situation as the valves on the bar tending position if they fail in the open position when they lose power the alcohol will continue pouring and wont stop until you take the alcohol out or it runs out.

A good example of a solenoid valve that would need to fail in the open position would be a electrically operated purge safety valve where you are trying to control pressure tests in a system where pressure drop has no negative impact. In general though its usually safety valves that would ever need to be failed open really and you shouldnt be using solenoide valves for safety valves.
 
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Sirus7264

Sirus7264

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Frad in'Ryth said:
I just want to say I love this group.

I asked my buddy about his auto bar tender. He went with fail open so any built up pressure etc would release and back flow into the liquor bottles when the system was powered down.
He also designed it so all the booze was in the cabinet underneath the dispenser and touch screen, to clean/sanitize just required pulling all the heads from the bottles tossing them in a bucket of cleaner and hitting the cleaner cycle he programed for it.
Click to expand...
From the sound of it he used a recirc line after the pump to relive back to the bottles with a solenoide operated releif valve i'm guessing the bottles are open to atmosphere then it makes sense. What did he use for the actual dispensing valves thought he was going to use gravity with solenoid operated valves thats why i said fail shut didnt expect him to pressurize the liquid to dispense it. it would have to be a positive displacement pump in this case with a pressure sensing line which would turn the pump on and off once pressure reaches its setpoint. good setup. He should think about using gravity over the pump for energy conservation and total cost(Wouldnt need as much piping and the pump). Also repair costs would be lower. HIs advantage though with the pump is he can have the alcohol anywhere really and switching bottles is very simplistic and he can really control the ammount of flow with choosing different pressures(Higher pressures=higher flow etc. which means faster drinks gravity would be constant.).
 
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