October 2008

Control fundamentals

I liked "The Final Say" article by Fernando Otero in the June InTech.  

My mission in life is to try and get people to get their base layer controls working properly.  After 20 odd years in control, I specialised solely in regulatory control optimisation in 1990, and I have been working on this ever since.   I have been in many hundreds of plants worldwide, and I have trained thousands of people in what I call "practical control."

In general, I find at least 85% of all control loops operating completely inefficiently in automatic.  The whole field is an extremely grey area, with abysmal understanding on the part of the vast majority of practitioners in the field.  It is also a major problem that so many people on the process and operation side do not have a basic understanding and realization as to how much a properly optimizing base layer could help them in achieving their own targets in production, and with much less need for operator intervention.

I just wish more people would publish information like yours.  One of our biggest problems is trying to get financial figures out of companies of benefits gained after loop optimization.  We know it is huge, but there is little published giving figures.  It would be a powerful incentive for companies to spend money on getting their loops to work properly if we could only get more feedback on potential financial improvements and savings.

Mike Brown, Johannesburg, South Africa, www.controlloop.co.za

Cover story kudos

Thanks for the EXCELLENT article, "The best Ethernet may be coming" by Patrick McCurdy, Ira Sharp, and Nicholas Sheble, in the August InTech.

I learned a lot. The article was interesting and informative.  I liked the length.

I hope InTech continues to have such informative and in-depth articles.

Jim Reizner, Corporate Engineering, Procter & Gamble

Catching up on valves

Bobby Bruzina's letter (June InTech) prompted me to refresh my memory on control valves. The answer of linear (D) is correct when it says "flow coefficient" vs. travel is equal for linear valves (for a fixed Diff Pressure). However, many practicing technicians are confused, thinking the actual "low rate" will be linear with valve travel for a linear trim.

As I'm sure you know, the equal percentage valve is designed to "linearize" the actual flow, so at low openings a 1% increase in valve position gives more nearly a 1% change in flow (slower increase in flow coefficient change), and at higher flow a 1% increase in travel gives more nearly a 1% increase in flow (higher increase in opening/flow).

Unless the real flowing system is constant DP (or nearly so), the equal % will more nearly provide a linear change in actual flow for the same change in valve position (travel) at different points on the characteristic curve. The equal percentage will also provide a larger "turndown" for stable control with a properly sized valve.

It was interesting to read one company's actuator manual, which "electronically" trims the valve to equal percentage so our historical "dumb" PID can work better to control flow. Looks like it is designed for a butterfly valve, which is inherently more of a quick opening valve, not really mechanically designed to throttle. Often these will be oversized (by the line size), so that to throttle and control at the low end, you need another control valve in parallel to trim. I guess there is no linear table in the referenced manual because most rotational valves designs are not "linear trims"?

Now that we can measure flow so well, why not let a smart valve determine the actual flow characteristic curve (increasing and decreasing) and use a look-up table to position the valve for the desired flow?

Joel T. Youngblood

Refrain from 'new journalism'

I have been watching with growing aggravation the demise of the English language for some time now.  Your publication is only adding to my distress.  I fail to see how oil, grain, biodiesel, etc., pumps and flows itself into a new product, condition, or state.  The wording should still be in the passive form, i.e., the oil is pumped to storage, the biodiesel is flash dried, etc.  Unfortunately, it is not just your publication but all technical publications that are using the "new journalism."  Also, when articles are written for third graders, the technical content suffers greatly and the information and ideas are not presented in sufficient detail to convey anything new or better. In the future, please try to refrain from using the active voice when the passive "requires."

Fred A. Coon, III, P.E., Sr. Instrumentation & Control Systems Engineer