Keep temperature control out of hot water

QUESTION FROM SEPTEMBER 2004 CHEMICAL PROCESSING

One of our processes requires water at 210°F, which is heated by a steam exchanger. Demand varies and can change stepwise. We have a small, continuous bypass flow so we can maintain temperature even when there is no flow of water. The temperature controller is tuned for this minimum-flow condition so that temperature will be stable and on set point whenever there is a demand for water.

Our problem is that temperature doesn’;t stay on set point when demand increases, especially when it changes rapidly. It seems to control below set point on rising loads and above it on falling loads. But if we tighten the PID settings during high load, the temperature cycles badly when we return to minimum flow. Is this a place for adaptive or self-tuning control? Is there another solution?

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Feed-forward will take the heat off
Think of a heat exchanger as a controller for which the gain varies inversely with flow. At minimum water flows, a small increase in steam flow will produce a certain steady-state rise in water temperature. When the water flow is doubled, the same size increase in steam rate results in only half the water temperature rise. This varying gain can be compensated for by using an equal-percentage steam valve for which the gain varies directly with its delivered flow.

The exchanger also has a time constant and dead time, which vary inversely with flow. Since optimum integral and derivative settings are related to process dead time, they can only be optimized at one flow, and the worst case (slowest dynamics) is at minimum flow. If your controller has only one value for its proportional-integral-derivative (PID) settings, optimize the integral for minimum flow and the derivative for maximum flow. Some controllers have multiple sets of PID parameters that can be invoked as a function of another variable, such as water flow or controller output — this would allow optimum tuning at two or three loads. Ideally, integral and derivative settings should be programmed to vary inversely with measured water flow.

Feed-forward control will help immensely. Measured water flow is passed through a lead-lag unit and multiplied by the output of the temperature controller to set steam flow in cascade or to position a linear steam valve. In this case, the multiplier compensates for the variable process gain. Feed-forward is fast and minimizes the need for integral and derivative action.

F. Greg Shinskey, process control consultant
North Sandwich, N.H.

 

Correct the derivative
It appears that the derivative is not set correctly and that there is some dead time in your process. Adaptive or self-tuning control may not solve this problem. An advanced controller with more than simple PID could best solve this.

Joel Heidbreder, principal engineer
Monsanto, St. Louis

Install bypass valve
A complex controller may require technical training and time to install while incurring costly downtime. I would suggest replacing the continuous bypass flow with an automated bypass/divert valve that provides full flow to and from the process depending on load. Insert a check valve at the process discharge to prevent bypassed water from backfilling.

To ensure temperature control, modify the bypass piping to achieve a flow rate that is equivalent to the flow that would pass through the process. This would also prevent water hammer when the divert valve shifts (regulating the discharge air from the solenoid also helps). The control RTD should be located at the exchanger discharge and provide feedback to the steam valve to control the water temperature.

Ron Johnson, process engineer
Alpharma Pharmaceuticals, Baltimore

Increase size of exchanger
One of the problems associated with intermittent flow is the temperature lag between low- and high-flow conditions. Adaptive control may be useful if the upsets are small and regular.

Some other ideas you might try include:

1. Increase the size of the heat exchanger so it can better handle the heat requirement during high-flow conditions. Also increase the bypass flow to minimize overshooting the temperature.
2. Create a fluid reservoir to handle the upsets. Draw from the reservoir during high-flow periods, which will tame some of the temperature variations and allow your control system to react.

Jim Stahl, engineering manager
Dekoron-Unitherm, Cape Coral, Fla.

Use a power trap
We’;ve had success with a power trap system. In this system, the inlet control valve sets the steam flow to the steam exchanger. The condensate from the exchanger drains by gravity to a reservoir drum, below which the power traps either pump or trap the condensate to the return header.

This system works well when piped correctly; the reservoir and power traps need to be equalized to the exchanger at the condensate outlet high point with air vent and check valve to the pad; properly sized check valves on the power trap inlet and outlet; and the motive steam should come from a constant steam-supply source to help pressurize the trap and discharge condensate when the inlet control valve is throttling down. However, this system requires routine maintenance about every 18 months. Otherwise, excessive water hammering will damage the exchanger, power traps and check valves.