7 things noncontrol people should know about control engineers
A few basic differences between control engineers and others in the plant can hinder progress toward optimization. Start a conversation to improve communications and controls. See examples and career advice. Send a link to these seven things other people should know about control engineers, so they understand.
It is no wonder "others" do not understand how to interact with control professionals. Control engineers may well be the least understood group in our society today. The U.S. government has more than 50 job codes for IT professionals but NONE for control engineers. Alaska does not offer a Professional Engineering License in control. Are control engineers part of chemical or process? Are they electrical or instrumentation? Having enjoyed over 35 years in this fulfilling arena, allow me to offer seven things "others" should know about this chosen profession.
1. Process engineering is steady-state; control engineering is real-time.
Process engineers pick setpoints and targets: 85 tons of wood chips plus 20,000 gal of chemical at a specific temperature for a certain length of time yields pulp with predictable properties. Control engineers, on the other hand, do not care about chemistry. Control cares about transfer functions and response times of the elements affecting the reaction: if the valve moves, how fast does the process change and by how much? If a different chemical is used by the process folks, odds are a least one response time will change and the loop will need to be retuned.
Process engineering is NOT control engineering. Do not assume a process engineer will succeed as a control engineer. Example: A chemical company was simultaneously setting both production and quality records on its most profitable product. A manager with a process engineering background decided a control valve was moving "too much." He ordered the valve to be locked down at its average position. He hoped to make an additional $200-500 per month on increased sales of a by-product. When the control engineer questioned the impact of the change, he was told he was being territorial, and perhaps had a personality conflict. Over the control engineer's objection the change was implemented.
Rejects increased dramatically, often changing with the ambient temperature. Within a month this once most profitable product was losing over $10,000 per day! The control engineer pointed to the change in the control strategy and the corresponding start of the problem and asked to run a simple and most basic trial with absolutely no risk: Put the original control back.
Process engineers and research scientists dismissed the idea since average temperatures, pressures, etc., remained the same (but those variables were swinging constantly). The requested trial was specifically forbidden. It was stated emphatically, "The control was NOT the source of the problem." The control engineer was excluded from the biweekly meetings on the problem. The problem continued for 6 months. Every time the product was scheduled to run, they doubled management presence around the clock (with no effect on the rejects). Finally, the plant ran for 3 days with 100% rejects. Marketing decided to discontinue sale of the product at the end of the quarter. At that point the control engineer went directly to the plant manager, pleaded his case, and then ran the trial. The problem immediately disappeared. Process engineers (even PhDs and scientists) are not necessarily control engineers.
2. Computer geeks are not control experts.
This is an easy mistake to make. The control people live in a world of human-machine interfaces (HMIs) and "block-ware," using a PC interface. Many people with process control on their business card are not control engineers. A control engineer can calculate the flow through a relief valve. A control engineer knows the effect of load swings versus source swings on the design and tuning of a control loop. A true control engineer will not spend a week trying to tune a simple flow loop while standing next to a check-valve that is mounted backward in the line.
If you know of or are a computer whiz in a control position, welcome to a fabulous field. Please start learning control. There are many sources of control training available, and this training will pay huge dividends. Every company can benefit greatly by getting this control training for employees who need it.
3. Physics rules.
While driving down the road, if the back of your car passes the front of your car, you are unstable and in trouble; the back should not go faster than the front. Likewise, the control cannot go faster than the process. Put a loop in manual and give the valve a step-change. If the process shows no change for 2 minutes and then gradually rises over the next 30 seconds until it stabilizes, you cannot use that control for disturbances that occur faster than 2 minutes. Increasing gain and decreasing poll-time will not speed up the process response. When the process response is too slow, the loop must be reengineered. Relocate sensors, find sensors with faster response, use feed-forward controls, and discover other process relationships to predict control. This is the very heart of a good control engineer. This is the type of situation where process and computer folks have problems acting like a control engineer.
4. Senior control experts know things they do not teach in school.
Good control strategy beats more expensive instrumentation. This statement may upset some, but it is another example of where a true control expert can perform where others will fail. Consider consistency, density, or temperature control. Putting a $45,000 transmitter on the main flow to directly drive the valve on the controlled flow (sweetener or dilution) is standard practice from virtually every major engineering company. It is very simple, and it is very wrong. This is the loop requiring retuning repeatedly for years and years with no success. Why? The process variable deviation may occur due to an incoming change (such as an inlet temperature change), or a header pressure change may cause a change in the controlled flow, or a production rate change may necessitate a step change in controlled flow.
Each of the three error sources requires different tuning. One size does not fit all. In fact the main control (correcting for a change in inlet value) is production rate dependent: what works at a high production rate oscillates at a low rate, and what works at a low production rate is too slow at a high rate. Control will always be poor. Instead, take a simple $600 sensor but add an orifice plate/differential pressure (DP) transmitter on the sweetener/dilution line, and things improve. Flow control on the control line eliminates flow errors due to any header pressure changes. Ratio that flow to the main product flow and production rate errors disappear, and tuning the main variable is no longer rate dependent. Now control the ratio setpoint with the consistency, density, or temperature sensor, and you have a very simple to tune, robust control that will outperform the expensive instrument. Senior control experts know this; I haven't heard of a school that teaches it.
SEE more, next page.
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