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Fail-safe control valves

January 22, 2013

I have been disappointed that there hasn’t been a groundswell from you peasants, I mean, loyal followers, propelling me to the position of Dictator of PB worldwide. Therefore, I will have to instigate floggings and they will continue until moral improves and I am propelled to glory.

Those of you without Alzheimer’s will recall that last month I wrote about barometric loops and how they can be employed to prevent your line from draining when the pump turns off. I also mentioned another method and that is to put an actuated valve on the end of the line. The way this works is that when the pump wants to stop, a signal is sent to the terminal valve and it shuts, preventing the line from draining. When the pump starts again, the valve is opened and normal service is resumed. The timing of the valve opening and closing and also the time over which the valve closes are critical and a transient pressure model is usually required to ensure the control is correct. If it is not done right, then over-pressure from rapid valve closure or line draining from closing too slowly can occur. Having a VSD on the pump does help considerably as you can slow the flow right down before closing the valve. An emergency pressure relief system just upstream of the valve is also a good idea.

One big issue with a controlled valve is what happens when something goes wrong. If it fails to close, then the line drains and this may not be too big a problem if it is only once in a while. But if it fails to open, the system doesn’t function at all and your sewage pumping station will overflow or your water supply tank will run dry. So you have to make it “fail-safe”. Contrary to popular misconception, this does not mean something that will not fail. In accordance with Murphy’s Law, everything man-made has the potential to fail. What it means is that you accept that it can and will fail, but when it does, it fails in a safe manner. For a terminal valve, this usually means that it should fail open. A common way is to have a pneumatic actuator with air pressure to close and spring return to open. So if the site loses power or air pressure or communications with the pumping station, the valve will automatically open. Hence it has failed into a safe position.

As an aside, there actually was a Murphy and his law is often misquoted as “If anything can go wrong, it will”. He was a Captain Murphy in the US Air Force. After WW2, they were developing the first ejection seats and it was some poor sod’s job to be strapped into a rocket powered sled, accelerated along a rail track and slammed into a wall. This was to determine what acceleration a human body could withstand (this was in the days before crash test dummies and computer simulations). On the sled were 19 accelerometers to give a reading of the G forces. One day, something went wrong and all the rockets on the sled fired sending the unlucky enlisted man hurtling down the rails and into the wall at full speed. The guy was pulled out of the wreckage bloody, broken and barely alive. But on the upside, they thought that when the get the readings from the accelerometers, at least they will know the absolute upper limit a body can survive

But alas, there were two ways the meters could be mounted and all 19 had been mounted incorrectly and gave no reading. This is when Cpt Murphy coined his law which actually states “When there are two or more ways of doing something and one way will end in disaster, then that is the way it will be done.” So the way to defeat Murphy’s Law is just to make sure there is only one way of doing it. That is, make it fool-proof or fail-safe.

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