Valves, Actuators & Pump Control


Factors affecting control valve performance - Part 6: Conclusion - economic results

April 2003 Valves, Actuators & Pump Control

Consideration of the factors discussed in this series of articles can have a dramatic impact on the economic results of an operating plant. More and more control valve users focus on dynamic performance parameters, such as dead band, response times and installed gain (under actual process load conditions) as a means to improve process-loop performance. Although it is possible to measure many of these dynamic performance parameters in an open-loop situation, the impact these parameters have becomes clear when closed-loop performance is measured.

The closed-loop test results shown in Figure 1 demonstrate the ability of three different valves to reduce process variability over different tuning conditions. This diagram plots process variability as a percent of the set point variable versus the closed-loop time constant, which is a measure of loop tuning. The horizontal line, labelled manual, shows how much variability is inherent in the loop when no attempt is made to control it (open-loop). The line sloping downward to the left marked minimum variability represents the calculated dynamic performance of an ideal valve assembly (one with no non-linearities). All real valve assemblies should normally fall somewhere between these two conditions.

Not all valves provide the same dynamic performance even though they all theoretically meet static performance purchase specifications and are considered to be equivalent valves. Valve A in Figure 1 does a good job of following the trend of the minimum variability line over a wide range of controller tunings. This valve shows excellent dynamic performance with minimum variability. In contrast, valves B and C designs fare less well and increase in variability as the system is tuned more aggressively for decreasing closed-loop time constants.

All three valve designs are capable of controlling the process and reducing the variability, but two designs do it less well. Consider what would happen if the poorer performing valve B was replaced with the best performing, valve A, and the system was tuned to a 2,0 second closed-loop time constant.

The test data shows this would result in a 1,4% improvement in process variability. This might not seem like much, but the results over a time can be impressive. A valve that can provide this much improvement every minute of every day can amount to significant savings over a single year. By maintaining closer adherence to the set point, it is possible to achieve a reduction in raw materials by moving the set point closer to the lower specification limit. The 1,4% improvement in this example converts to a raw material saving of 45 780 litres per day. Assuming a material cost of 50 cents per litre, the best valve would contribute an additional R22 892 per day directly to profits. This adds up to an impressive R8 361 186 per year.

The excellent performance of the better valve in this example provides strong evidence that a superior control valve assembly can have a profound economic impact. This example is only one way a control valve can increase profits through tighter control. Decreased energy costs, increased throughput, less reprocessing cost for out-of-spec product and so on are all ways a good control valve can increase economic results through tighter control. While the initial cost might be higher for the best control valve, the few extra rands spent on a well-engineered control valve can dramatically increase the return on investment. Often the extra initial cost of the valve can be paid for in a matter of days.

As a result of studies such as these, the process industries have become increasingly aware that control valve assemblies play an important role in loop/unit/plant performance. They have also realised that traditional methods of specifying a valve assembly are no longer adequate to ensure the benefits of process optimisation. While important, such static performance indicators as flow capacity, leakage, materials compatibility and bench performance data are not sufficiently adequate to deal with the dynamic characteristics of process control loops.

Summary

The control valve assembly plays an extremely important role in producing the best possible performance from the control loop. Process optimisation means optimising the entire process, not just the control algorithms used in the control room equipment. The valve is called the final control element because the control valve assembly is where process control is implemented. It makes no sense to install an elaborate process control strategy and hardware instrumentation system capable of achieving 0,5% or better process control and then to implement that control strategy with a 5% or worse control valve. Audits performed on thousands of process control loops have provided strong proof that the final control element plays a significant role in achieving true process optimisation. Profitability increases when a control valve has been properly engineered for its application.

Control valves are sophisticated, high-tech products and should not be treated as a commodity. Although traditional valve specifications play an important role, valve specifications must also address real dynamic performance characteristics if true process optimisation is to be achieved. It is imperative that these specifications include such parameters as dead band, dead time, response time, etc.

Finally, process optimisation begins and ends with optimisation of the entire loop. Parts of the loop cannot be treated individually to achieve coordinated loop performance. Likewise, performance of any part of the loop cannot be evaluated in isolation. Isolated tests under non-loaded, bench-type conditions will not provide performance information that is obtained from testing the hardware under actual process conditions.

For more information contact Alpret Control Specialists, 011 249 6700, alpret.conspec@emersonprocess.com



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Schubert & Salzer and Macsteel
September 2021, Macsteel Fluid Control , Valves, Actuators & Pump Control
The demand on valves, particularly in tyre production plants, is extreme with tyre curing presses working constantly at high temperatures and pressures.

Read more...
Chinese LNG tanker fitted with ARCA anti-surge valves
September 2021, Valve & Automation , Valves, Actuators & Pump Control
ARCA anti-surge valves avoid surging by accurately adjusting the gas circulation or blow-off flow, effectively protecting the safety of the compressor unit.

Read more...
Electrified subsea actuator for offshore production
September 2021 , Valves, Actuators & Pump Control
Bosch Rexroth has engineered the SVA R2 Subsea Valve Actuator as a disruptive innovation for electrically actuating valves in the subsea process industry. It can replace conventional hydraulic cylinders ...

Read more...
Eliminate bearing failures in TOVs
August 2021, Valve & Automation , Valves, Actuators & Pump Control
Bearing failures are among the highest root causes of failure in triple offset valves (TOV) for tight shutoff applications. This issue is directly attributed to the need to have metal bearings with very ...

Read more...
Locally manufactured knife gate valves
August 2021, Bearing Man Group t/a BMG , Valves, Actuators & Pump Control
BMG and Abrasive Flow Solutions (AFS) play a major role in the local valves sector, through the design, distribution, supply and support of a broad range of slurry related valves.

Read more...
Protect your pump and keep it longer
July 2021, Elemental Analytics , Valves, Actuators & Pump Control
Unexpectedly losing a pump is often an expensive or even a dangerous problem in a chemical plant.

Read more...
Locally valves manufactured for tailings pipeline
June 2021 , Valves, Actuators & Pump Control
The AFS manufacturing plant in Benoni is equipped with advanced equipment and is supported by specialists who offer dependable technical support to ensure the optimum safety, efficiency and extended service life of every system.

Read more...
New flange monoblock with shut-off valve
March 2021, WIKA Instruments , Valves, Actuators & Pump Control
WIKA’s new monoblock with flange design integrates a shut-off valve to separate the process from the instrument side.

Read more...
The Stafsjö HX story – greater demand calls for action
February 2021, Valve & Automation , Valves, Actuators & Pump Control
As a knife gate valve supplier, Stafsjö supplies product solutions to realise efficiency and production goals in pulp and paper applications.

Read more...
Drives for solar-powered pump systems
February 2021, Bearing Man Group t/a BMG , Valves, Actuators & Pump Control
BMG advises on how to optimise efficiency and extend the service life of solar-powered pumping systems by selecting the correct drives.

Read more...