Poor Controller Tuning Drives Up Valve Costs

How to get six-month paybacks by reducing stroking, air consumption, and wear
George Buckbee, PE
Published in Control Magazine April 02, 2002

The control valve is the workhorse of process control. In most modern PID control loops, the sensor is solid-state, as is the controller. The valve is the only component with moving parts. Therefore, it is the most subject to wear and tear, and requires frequent attention to stay in top condition. Unfortunately, it is also the most expensive component in the loop both to purchase and to repair.

Poor controller tuning contributes to wear and tear, often forcing the valve to move five or 10 times more than is actually required for good control. This article assesses the cost of poor tuning and provides some direction for the person looking to reduce valve maintenance costs.

For a PID control loop, the valve is usually the most costly component, typically representing 50% to 75% of the purchase cost of the control loop. However, it often represents closer to 90% of the ongoing maintenance costs.

These costs are well understood. Valve seals wear, packing leaks, components corrode, and linkages and springs suffer the stresses of thousands, even millions of cycles. In most industrial plants, a planned maintenance program combats the effects of this wear. It is typical for plants to spend on average between $400 and $2,000 per valve per year.

A good maintenance program will evaluate valve performance and routinely rebuild or replace valves that are out of specification. Work orders are written and the jobs are scheduled during plant shutdowns. Plant maintenance personnel are usually stretched quite thin during these shutdowns, so much of the work is completed with overtime pay or through third-party contractors, further increasing the cost.1

Wear and tear is caused by excessive operation of the valve. If the valve is constantly in motion, then the springs and linkages are being constantly stressed and fatigued. Positioner arms and other devices with moving components are also being worn in the process.




As Free as the Air We Breathe?
A well-hidden cost of control valves is compressed air. Large, expensive compressors operate with electricity 24 hours a day. If you know your plant's cost per kilowatt-hour, you can estimate the cost per year for 1 standard cubic foot per minute (SCFM) of air: 0.25 hp/SCFM x 0.745 kW/hp x 24 hr/day x 365 day/year x cost/kWh.

At a typical industrial rate of $0.06/kWh, this is roughly $98/year for each SCFM. And this doesn't count the capital, depreciation, and maintenance costs for the compressor, dryer, and distribution system. To determine the cost of air for control valve actuation, we need to know a little about the size of the valve actuator. Table I provides some typical air uses, based on actuator size and type.

Since it costs only $100 to $200 per year to tune and optimize a control loop, the savings on air alone justifies the effort with more than 200% payback.

The amount of valve movement plays a significant role in the amount of air required for the valve. For good control, of course, the valve must move. But just how much the valve must move depends on many factors, including the size of the valve, the process gain, and the size and frequency of disturbances and setpoint changes.

Top Control's experience is that most controllers, when properly tuned, require the valve to move between 1,500% and 20,000% per day; half of the time responding to noise, and half of the time to disturbances. A poorly tuned controller may move the valve two to 50 times as much.

Said another way, a properly tuned valve makes the equivalent of between 15 and 200 full strokes each day. This works out to between 1% and 14% per minute. Obviously, if we can reduce the valve movement, we can reduce the air usage. This in turn reduces valve maintenance needs, air costs, and process variability.

While we're on the topic of air losses: During a shutdown, assign someone to go hunting for air leaks. When the plant is quiet, they can quickly pinpoint air leaks (a spray bottle filled with soapy water helps). They can fix these leaks as they go, and the payback will be tremendous.

One other way to save on compressed air costs is to use an inlet guide vane to pre-spin the air entering the compressor. This reduces compressor-operating costs by approximately 10%.

How Tuning Affects Valve Costs
Of course, air costs are not the ultimate goal of the control system. A properly tuned controller will react when the process requires it, but will not over-react to process noise. In addition to tuning, a filter can be used to further reduce the noise that is seen by the controller.

Based on Top Control's experience, most control loops will see a two or three times reduction in valve movement when tuned properly. This happens through the proper use of P, I, and D parameters. Using these techniques, we have seen valve movement reduced by as much as 200 times.

Derivative (D) setting has the greatest impact on valve movement. Derivative action responds to the rate of change of the process variable (PV), or the error. Since noise represents quick changes to the PV, the derivative action responds quickly, moving the valve. In effect, derivative action will amplify noise.