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Facts At Your Fingertips: Specification Considerations for Control Valves

| By Scott Jenkins

Properly specifying process control valves for a plant project is critical to achieving efficient and effective processes. This one-page reference discusses key considerations for specifying control valves.

Valve specification

Process engineers should take the following aspects into consideration when specifying control valves to ensure that the valves are manufactured according to requirements.

FIGURE 1. The diagram illustrates a control valve with fluid flowing through. The pressure drop is represented by ∆P

Flow coefficient and size. The size of a control valve is derived from the flow coefficient (Cv), which is a parameter defined as volumetric flowrate (in gal/min) of water through the valve at 60°F when pressure drop across the valve is 1 psi (Cv is calculated using the formula given in the standard ISA-75.01.01-2007).

Valve controllability. The valve’s controllability must be sufficient over the full range of flowrates that the valve will experience. This can be ensured by estimating the maximum Cv and minimum Cv that correspond to maximum and minimum flowrate, respectively. In general, the controllability of a control valve is acceptable if its travel at maximum flowrate does not exceed 90% of the rated travel, and if travel at minimum flowrate is in the range of 10 to 20% of the rated travel. This means the ratio of estimated maximum Cv to estimated minimum Cv should preferably not be more than 15. If the ratio far exceeds this value, travel at minimum flow may be less than 10% of the rated travel, or the travel at maximum flow may be greater than 90% — both scenarios mean poor controllability of the valve. In that case, pressure drop across the control valve should be increased so that the target ratio can be lowered, as shown in Equation (1). For incompressible fluids, the ratio of maximum Cv to minimum Cv is given by Equation (1):

(Cv)max / (Cv)min = (max flowrate / min flowrate)(∆Pmin / ∆Pmax)0.5                          (1)

Cavitation. When fluid is flowing through a control valve, the minimum pressure occurs at the vena contracta, and then pressure increases along the path of flow until the fluid reaches the outlet of the control valve. The vena contracta is the point in the flow path where the flow area is minimum, the velocity is maximum and, hence, pressure is minimum. For liquids, if the pressure at the vena contracta is less than the vapor pressure of the liquid, vapor bubbles will form. Downstream of the vena contracta, pressure recovery takes place, resulting in higher pressure at the valve outlet than at the vena contracta. If pressure at the control valve outlet exceeds the vapor pressure, the vapor condenses and bubbles collapse. Collapsing bubbles impact the valve body and create noise. This phenomenon is cavitation.

Multiple operating cases. Control valves are generally specified for three operating cases — minimum, normal and maximum flowrates, with the corresponding pressure drops. There may be more than three. In such situations, normal flowrate and corresponding pressure drop should be specified in accordance with the normal operating case, whereas other operating cases (if there are more than two) should be narrowed down to two cases.

When narrowing down the operating cases, Cv should be estimated for each case. Then, minimum and maximum flowrates (and corresponding pressure drops) should be specified in such a way that they correspond to the minimum and maximum Cv of the control valve, and the Cv corresponding to all other cases should lie between minimum and maximum Cv. As the actual Cv is not available when a control valve is specified, the estimated Cv should be used.

Valve type. Butterfly valves, which are compact and generally lower cost, are often the first choice. However, constraints may dictate otherwise. For instance, if high pressure drop across the valve is required, a globe valve may be a better choice. Because the resistance of a globe valve is higher than that of a butterfly valve, higher pressure drop can be obtained across a globe valve with reasonable size. Cavitation can often be avoided with globe valves. V-notch ball valves are preferred where high rangeability is required. Standard, round-ported ball valves are generally used for on-off applications.

Leakage class. The allowable control-valve-seat leakage is specified in terms of ANSI/FCI 70-02- 2006 leakage class. This standard recognizes six classes of allowable seat leakage (Class I is the highest allowable leakage and Class VI is the least allowable leakage). Generally, control valves for CPI applications are specified with leakage Class IV. However, when tight shutoff is required, at least Class V should be specified. Control valves discharging to a flare, or controlling fuel flow to a burner, should be specified with Class VI leakage.

Flow characteristics. The most common types of inherent flow characteristics are the following:

Linear — A valve with an ideal linear inherent flow characteristic produces a flowrate that is directly proportional to the amount of valve plug travel, throughout the travel range.

Equal percentage — Ideally, for equal increments of valve plug travel, the change in flowrate regarding travel may be expressed as a constant percentage of the flowrate at the time of the change.

Quick opening — A valve with quick-opening flow characteristic provides a maximum change in flowrate at low travel rates. A quick-opening characteristic is basically linear through the first 40% of valve plug travel (corresponding to 70% of maximum flowrate), and there is little increase in flowrate with further increase in plug travel. 

Editor’s note: This column is based on Singh, Satyendra K., Key Considerations in Specifying Control Valves, Chem. Eng., March 2017, pp. 84–87.