Why are we using 3 to 15 Psig air pressure for control valve operation, Why it can't be 0 to 12 Psig?

I agree with Matt.





You can have three different types of controls: Positive pressure, Positive-Negative, or Negative. This is to say running your 3-15 or 15 to -15(positive-negative) or a vacuum depth system.





Running to zero is kind of like running to neutral. There is not a lot of useful spring stuff for 0-2psi that works well at 15 and vice versa...





..Why are we using 3 to 15 Psig air pressure for control valve operation, Why it can't be 0 to 12 Psig?
More or less agree with the first two answers. However, the 3-15psig control signal goes back to the early days of automatic control, although I have no idea why 3-15 was chosen. There was another (original) standard of 3-27psig. There's no reason why the signal could not be 0-15 (the upper limit is always 15psig), but by using 3psig at the low end you have a 'live' zero. Problem with using 0 at the low end is that it's possible to drive the signal below 0 and with most control devices a value lower than 0 cannot be seen, although the controlled device may react to that lower signal. Why are we using 3 to 15 Psig air pressure for control valve operation, Why it can't be 0 to 12 Psig?
A control valve may be normally closed or open and a spring tension keeps the valve closed or open.The usual practice is for an air pressure of 3 psi to maintain the valve at closed position. It can't be 0 psig, otherwise the valve will open.The valve is also calibrated to be 50% open at [15-3]/2 or 7.5 psi. At 15 psi, the valve opens 100%. At 12 psi, the valve is only 75% open. The reason, it is an industry standard.
Normally a low positive like this is to ensure no backwards flow or no fluid leakage to a dry side of a valve, or if the valve has a calibrated spring it is to always keep the spring under load. It would be helpful to know more about the system really as thats just a very general answer.