Globe Valves
Globe valves, such as W.E. Anderson's High-Flow™ Control Valves, are a linear type valve with a rising stem and plug that seals the flow on a seat. Flow through a globe valve permits throttling control without excessive wear of the seat or plug. Since globe valves are metal seated, they have a small leak rate in the closed position. The travel distance is small in globe valves making response time quick. The pressure drop in a globe valve is typically larger than in other styles of control valves. These valves have high pressure and temperature capabilities as well as good chemical compatibility. Globe valves are typically operated with a pneumatic actuator that physically moves in response to a pneumatic control signal. Globe valves are good for very precise control of flow in smaller size diameter pipes, and are the most popular type of control valve in use.

Butterfly Valves
Butterfly valves are a rotary type valve that can be used as a control valve or a block valve. There is a disc that rotates on a shaft in butterfly valves so that when fully open the disc is parallel to the flow and when fully closed the disc is perpendicular to the flow. Elastomer liners between the disc and body achieve a tight fluid seal. Butterfly valves require a minimal amount of space and have very large flow capacities with minimal pressure drop. The valves have lower temperature capabilities due to the limit of the elastomer lining and lower pressure ratings due to the design. Butterfly valves are low maintenance, low cost, and self-cleaning. There is a large choice of actuators for butterfly valves to fit a variety of applications. Lever lock handles, gear operated hand wheels, two position electric and two position pneumatic actuators are available for on-off service. Modulating electric and pneumatic actuators are available for flow control applications.

Ball Valves
The ball valves, such as W.E. Anderson's ABV, 2BVS and BV2 Series, are rotary valves that like butterfly valves can be utilized as a control valve or a block valve. Ball valves use a rotating 90° it seals off the valve blocking the flow. Tight closure is achieved by squeezing the ball against the seat rings. Ball valves have low-pressure drop, good flow capabilities, and good temperature and pressure ratings. The valve is low cost, compact, and easy to maintain. The ball valve is best for on-off service and can be use for throttling service, although it does not give as accurate of control as a globe valve. W.E. Anderson offers ball valves with hand-levers and two position electric and pneumatic actuators for on-off service. Modulating electric and pneumatic actuators are available for flow control applications.

Terminology
Pressure Drop - The difference in upstream and downstream pressures of the fluid flowing through the valve.
Critical Flow - The flow has reached the point of being choked. At the choked condition the flow rate has hit a maximum limit and does not increase with further increase in pressure drop across the valve.
Cv or Valve Flow Coefficient - The number of U.S. gallons per minute of water at 60°F that will pass through the valve with a pressure drop of 1 psi. For example, a Hi-Flow™ valve with a maximum Cv of 10.75 has an effective port area in the full open position such that it passes 10.75 GPM of water with a pressure drop of 1 psi.
Full Port - The port diameter of the valve is the same diameter as the piping connections.
Rangeability - The ratio of maximum controllable flow in minimum controllable flow of a valve. For example, a valve with a 50 to 1 rangeability and a total flow capacity of 100 GPM at full open controls flow accurately to as low as 2 GPM.
Valve Flow Characteristic - The relationship between the stem travel or rotation of a valve, expressed in percent travel, and the fluid flow through the valve, expressed in percent of full flow.

Control Valve Sizing
The Cv method is an accepted way to size control valves. Basic equations are provided as a guide to use in sizing a control valve, and the results of the equations will only be as accurate as the information provided of the flowing conditions. The equations are broken down into the type of media - liquid, gas or steam, and whether or not the flow is critical. The critical flow equations are to be used for vapor flow when the pressure drop across the valve is greater than half of the upstream pressure. As a general guide to avoid cavitation do not size a valve for liquid service where the pressure drop is greater than 50% of the upstream pressure.

Nomenclature
Cv = Valve flow coefficient
g = Specific gravity of liquid at flowing conditions
G = Specific gravity of gas at flowing conditions
P1 = Upstream pressure, psia
P2 = Downstream pressure, psia
ΔP = Actual pressure drop (P1-P2), psi
q = Liquid volumetric flow rate, U.S. GPM
Q = Gas volumetric flow rate, SCFH
W = Steam weight (mass) flow rate, lb/hr
T = Flowing temperature,°R (460 + °F)

Once the required Cv is determined, selection of the proper size control valve can be obtained by comparing the required Cv to the Cv values for the valve. As a general rule the maximum capacity of a control valve should be 15 to 50% above the maximum process flow, and the minimum required Cv must be within the available rangeability of the valve for proper control. If only the maximum process flow rate was used to calculate Cv, then the percent travel of the valve should be checked and should fall in the range of 65 to 80% of total travel.