I am soooooooo green to this industry! I was wondering if anyone out there could explain to me how the hysteresis setting on a pressure indicator affects how the pressure controller operates? Do keep in mind you're speaking to a newbie, so dumb it down as much as possible, please.

 

Badly. There is nothing good about hysteresis.

Your controller is trying to reach a measurment that will allways be somewhere within the hysteresis limit. Your Indicator will allways read a little high or a little low - so the controller will try to correct an error that is not really there. So your controller will hunt up and down trying to acheive a setpoint that it can never reach. And because its moving the valve, the process gets disturbed. Eventually your valve will start to wear out because its moving all the time. The worn valve also causes real hysteresis, because it never reaches the position your controller needs it to be at. So your process disturbance gets even worse. And so it continues.

NEVER use hysteresis in the measured value for a controller.

Rob
www(.)lymac.co.nz

 

If you're a newbie and you're using the word 'hysteresis', you must have picked it up from something involved with this controller, so I'll propose that you have an on-off pressure controller.

Every controller has a setpoint, which is the pressure value you want the system to be. The indicator shows you the instantaneous value of what you actually get, the present pressure value.

An on-off controller can only open the valve fully or close the valve fully; it cannot modulate the valve between its open and closed positions.

The result is that an on-off controller can not hold the system pressure exactly at setpoint, regardless of demand useage. An on-off controller opens the valve when the pressure gets too low and closes the valve when the pressure rises to a suitable point. If you plot the pressure deviations over time on a graph, the trend looks like a sawtooth.

The hysteresis the range over which the valve opens and closes. For example, as volume is consumed, pressure drops. When the pressure drops to the lower hysteresis pressure point, the controller opens the valve. Fluid flows, pressure increases. When the pressure gets to the upper hysteresis pressure point, the controller closes the valve.

The pressure setpoint is typically half way between the upper hysteresis point and the lower hysteresis point.

If the hysteresis band is too narrow, the valve will cycle too often and wear. The sawtooth gets flattened out and the pressure remains closer to the ideal setpoint, but at the cost of wear and tear on the regulator.

If the hysteresis band it too wide, the swings between pressure can be too far from setpoint and affect whatever it is the pressure is providing downstream. Poor performance.