Question #9 - Is proportional only control better than PI control for some applications?
January 9, 2004

Question:

I have seen level controllers that were proportional only, no reset. Wouldn’t control be better with reset in addition to proportional control?

Answer:

Yes and no depending upon the needs of the process.

In some cases it is important to hold the level in a vessel close to the set point. In those cases integral control is usually used.

In many cases, however, a tank or other vessel is used for surge capacity. Sometimes a tank may be installed in the process to absorb sudden changes of flow from an upstream process and reduce the rate of change, or perhaps the peak flow, to downstream units.

A typical example is a waste treatment unit that takes a constantly varying flow of effluent from a process plant, as well as storm water runoff. Their may be a surge tank between the process plant and the treatment plant. If there is a sudden increase in flow into the tank, the level will rise and the flow out of the tank will rise, albeit more slowly. As the tank level rises, the level controller will increase the flow to the treatment plant. However, the rate of increase will be far less than the rate of increase of the flow into the tank.

The control tuning can affect the ability of the surge tank to smooth sudden changes in flows. If the loop is tuned to tightly control the level, any sudden increase in flow in will result in an immediate increase in flow out. The only way to hold the level constant is to match the flow out to the flow in, defeating the purpose of the surge tank.

To increase the ability of the tank to absorb surges in flow, the gain is kept very low. The gain should be high enough to fully open the outlet valve when the tank is at its maximum level, and to close the valve when the tank is at its lowest permissible level.

Reset serves to return the level of the tank to its set point. If the reset rate is too high (reset time too short) the reset action will quickly open the outlet valve to allow more liquid to flow out than is flowing in. That is the only way to bring the level back down to its set point. Even a slow reset will have to provide for an outflow greater than the inflow if the level is to be returned to its set point. Therefore, if a high flow into the tank is expected to continue for some period of time, the treatment plant will have to be built to handle a flow larger than the maximum flow into the surge tank. With proportional only control, however, the flow out of the tank will never be higher than the maximum flow in, nor less than the minimum flow in.

Observe in Figure 1 (proportional only) that as the flow into the tank (blue) suddenly increases and decreases the level (green) rises and falls but the flow out (red) increases and decreases at a slower rate than the flow in. Except in the longest period of high flow the outflow remains well below the flow in.

In Figure 2, (proportional-integral) the flow out is very similar, but observe that the during the longest period of high flow the flow out actually increases about 20% above the flow in. This would require that the downstream treatment plant be built to handle a higher capacity than would be the case with proportional only control.

Therefore the proportional only controller may be the best choice for level control when surge control is important.

Figure 1 - Proportional only control

Figure 2 - Proportional reset control