Guest commentary: Low-cost, high-return process control improvements, part 3
Last month we looked at two ways you can improve performance of PID loops and associated equipment. This month we consider two more.
Idea 4: Stop cycling energy loops
When energy-consuming control loops are cycling, they lose efficiency. Imagine how inefficient your car would be if you were constantly pressing and backing off the accelerator so the engine speed was constantly cycling up and down. The same thing happens in your plant. Typically as many as 40% of all control loops in process plants are cycling. A cycle in one control loop often forces cycles in other loops, so oscillation spreads throughout the plant.
To find these efficiency wasters, start by looking at key energy loops (temperatures, fuel flows, excess O 2 monitors) that are cycling. When a loop cycles, it does so at a specific period. In the loop shown in the illustration, the dominant cycle has a period of six minutes. In 80 to 90% of plant situations, cycles are not caused by the loop in question, but by some other loop. We refer to this as oscillating due to a load disturbance.
The root cause of the problem is likely another loop that is cycling at the same period, so in this case you are looking for other loops that also cycle every six minutes. The traditional method for this is slowly and methodically putting loops one-at-a-time in MANUAL, until the cycle goes away. While this is effective, it can often take weeks to find the root cause. Modern loop analysis tools allow you to get a list of all loops that cycle at a given period, and can also indicate whether the source of the cycle is oscillating-load, oscillating-tuning, or oscillating-valve. This greatly reduces the time needed to solve the problem.
Most valve-induced oscillations can be resolved by simple adjustments to the valve, positioner, or air supply.
Cost estimate: With modern tools, six hours to solve x $60/hour = $360.
Benefit: The savings for these efforts can be quite remarkable. Conservative energy cost reductions are 0.5% of your energy budget, but reductions up to 2% are common. With a $3 million energy budget, that’s a $15,000 saving annually at the low end.
Idea 5: Stop wasting pump energy
In a typical plant, 5 to 10% of control valves are oversized. Whether this happened due to design errors, changes in the process, or other reason, these valves and the pumps that supply them are major energy-wasters.
To find oversized valves, look for those that are rarely open more than about 20%. Changing the valve itself may not be practical since it could involve potentially major piping modifications, but you can save money reducing the pump’s energy consumption. Some energy-saving methods include:
• Reduce the pump speed with a VFD or similar controller;
• Replace the pump impeller with a smaller impeller; or
• Use a smaller pump motor.
In many cases, you can probably get the equipment you need from the storeroom.
Cost estimate: With modern tools, locating three valves and pumps in eight hours x $60/hour = $480 plus the cost of the parts, drive, new motor, and installation as applicable.
Benefit: Of course, the savings depends on the sizes of the pumps in question, but it can be dramatic. The example here is based on saving 20 hp with a 100 hp pump. With 100 control loops, you will typically find three to six over-sized valves. Depending on your energy costs, each horsepower costs between $600 and $1,200 per year, so if you save 20 hp, 20 x $600 = $12,000/year for each pump.
Using the five methods shown in this series, a typical plant can achieve significant savings:
• Air leaks;
• Eliminate derivative;
• Apply filters;
• Cycling energy loops; and
• Pump energy.
Of course, the full extent of the benefits depends on your starting point, and how much focus is put on control system improvement. This discussion has used very conservative numbers in estimating the value, but even a conservative estimate says you can save over $70,000 per 100 control loops, that’s over $700 per control loop in savings. When combined with other PlantTriage tools and techniques, the total value can be much larger – from 2% to 6% of total operating costs.
As you undertake these efforts, follow a simple and effective plan:
• Get started today-These are simple, easy, and effective techniques.
• Use modern tools-Consider using PlantTriage Control Loop Monitoring tools to speed up your results.
• Monitor continuously-More of these opportunities will appear over time, as plant operating conditions change.
• Document the value-Your management would like to know how you have helped the business. Don’t be shy about reporting on your efforts.
George Buckbee, P.E., is VP of marketing and product development for ExperTune . Reach him at firstname.lastname@example.org. His series Seven habits of highly successful control engineers received very high readership.
-Edited by Peter Welander, process industries editor, PWelander@cfemedia.com
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