Properly Size Control Valves

Oversizing afflicts many plants but is avoidable

By Michael McCarty, Emerson Process Management

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A survey of more than 500 individuals involved in process engineering, procurement, operations and maintenance at over 200 plants worldwide identified “oversizing” as the number one control valve problem. In a few instances, oversizing is intentional (e.g., to prepare for future production rate increases). However, in most cases, oversizing results from well-meaning but misguided decisions during the valve selection process. Unfortunately, an oversized valve can incur a sizable economic penalty and cause significant operating problems. So, let’s look at how to avoid oversizing.

At full operating conditions, control loops operate over a very narrow, essentially steady-state throttling range where small input signal changes to the control valve result in small valve stem or shaft movements. As you might expect, a small position change by a valve that’s oversized gives a larger-than-desired change in flow. Depending upon the accuracy of the elements in the loop, the control system then responds to correct the situation, which can result in a throttling sequence that oscillates back and forth, causing continuous variation in process conditions.

While a higher-performance digital valve controller can mask this “dithering” by the control valve to give the appearance of acceptable loop performance, oversizing problems remain. In fact, an incorrectly sized control valve can result in problems even though the plant appears to be running smoothly.

During a system startup or a turndown to 25% maximum continuous rating (MCR), operating conditions will fall outside of the oversized valve’s ability to adequately control because of the need for extremely small valve movements. Process control in this range may be near impossible depending upon the inherent flow characteristic of the valve. High gain characteristics (i.e., the amount of change divided by the amount of input) can result in instability, again causing the valve to cycle.

In addition, these off-case, low-flow conditions can lead to valve throttling occurring essentially right at the seat or seal. The resultant high-velocity flow across the narrow opening causes wear and erosion. Impingement of the accelerated process media can cut lines into sealing surfaces, an effect that reduces the control element’s ability to prevent or minimize leakage when the valve closes. Erosion to the contour of the valve’s flow-control element alters its flow characteristic and ability to control as intended. More severe cases can prompt vibration that causes additional damage and, ultimately, equipment failure.

A valve gives the best control when it’s sized to operate around 60%–80% open at maximum required flow and not less than 20% open at minimum required flow. Using a larger-than-necessary valve compromises performance, as indicated in Figure 1, which compares flow coefficient, Cv, versus travel for Nominal Pipe Size (NPS) 4 and NPS 3 valves for a service needing an NPS 3 valve.

Because of their compromised rangeability, improperly sized control valves also can cause problems during process transients. A typical control valve with an equal-percentage flow characteristic has about a 30:1 turndown ratio (i.e., the ratio of maximum Cv to minimum Cv). However, when the valve is oversized and throttling at the low end, its turndown ratio falls to 3:1 or less.

Also, installing too large a valve amplifies mechanical problems such as stiction and hysteresis, making the system difficult to control and potentially causing process upsets.

Oversizing of control valves has a domino effect. Safety relief valves must be sized to match the capacity of the control valve. Within a bypass configuration, isolation valves, bypass valves and drain valves all must be larger, which can impact the size of piping and structural pipeline supports.

Consider also that to achieve or maintain flow velocity in a loop equipped with an oversized valve requires a dramatic increase in compressor or pump horsepower (Figure 2). Sizing issues can propagate beyond erection costs into perpetual operational cost increases as pumping horsepower increases by a power of three to maintain the desired fluid velocity within the piping.

When trying to avoid — or correct — valve-sizing problems, it’s important to understand the causes of valve-sizing errors. Emerson research has identified several major contributors: multiple safety factors, selecting line-size valves, and out-of-date process data resulting from changes in process conditions or conditions that differ from the original design.

To correct systems with improperly sized equipment, it’s vital to obtain accurate process data at all expected operating conditions. Then, size the valve to perform optimally at these conditions.

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