Getting Respect and Preventive Valve Maintenance

Touching On the Need for a larger Role and Added Recognition that Process Control Professions Deserves and Getting to the Bottom of a Mathematical Model Description for Water Treatment Plants

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A: The short answer to your question is yes, you can move the SIS valve testing to the shutdown maintenance list, but at a cost.

SIL availability calculations are performed to determine how frequently a shutdown device will be called on to act, but fail to do so. The higher the SIL rating, the lower the likelihood of failure on demand. These availability calculations are based on the dangerous undetected failure rate of the device and the frequency of testing.

You can lengthen the testing  frequency (as you suggest), but the likelihood of the valves failing to close on demand will be increased. The question you have to answer is this:  Will these valves still satisfy their SIL requirement under the new testing schedule?

People have devised ways to get around this issue with some success. One option is partial-stroke testing. This operation is performed with the use of solenoids and limit switches, which allow you to partially close the valve while in operation. This proves out the SIL control system, the wiring to the valve and the solenoid, but it doesn't really guarantee that the valve will fully close and seal upon demand. Still, it does check the functioning of a large part of the loop, and that may be enough to satisfy the SIL availability requirement.


A: The interval between testing valves depends on the performance required. You did not state the original and proposed test intervals, and so your question can't be answered qualitatively (other than saying that the "safety measures" definitely will be affected by such a change).

Was a calculation done to justify the change? What recommendations (if any) might the valve manufacturer have for test intervals? Automated partial stroking is often an effective means to extend the required manual full-test intervals. There are over a dozen manufacturers of the systems required to implement such solutions. Note that failure rates can change depending on frequency of valve movement and testing. The standards, technical reports and many books describe and show how to perform such calculations; it is nothing more than simple algebra.

Paul Gruhn, PE, CFSE,

Q: I am working in a power plant in Kosovo. As there is no possibility here of finding good literature on the control of processes I am writing to you. Can you provide me with a mathematical model description for water treatment  plants—especially for the part of a water treatment plant were I will try to implement predictive control.

Lutfi Bina

A: In process control, two general types of models are used—first-principle models and black-box models developed from the  operational data. For linear, model-predictive control, black-box, parametric (ARX, ARMA, state space) or non-parametric models (step responses) are applied. The typical procedure for building a model is applying pseudo- random excitations, collecting data and generating and validating the model
Most MPC product providers have tools for building models. You also can use Matlab Process Identification Toolbox.

You also can read more on developing models at
Also, a practical general overview on building models and model application for tuning and MPC control can be found in Instrument Engineers' Handbook (IEH), Vol. 2, Chapters 2.13 to 2.17. You can also read in the same volume in Chapter 8.39 about wastewater treatment controls.

Willy Wojsznis,

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