The Dwyer valve product line includes several types
of valves to cover a wide range of applications. Control valves are used to
continuously regulate the flow of a fluid in a system by changing the amount
of flow through the valve to control a process. The change in flow rate can
be used to control temperature, flow, pressure, level, Ph, etc. Block valves
are used to stop or block a flow in a system and are for on-off service. The
stoppage of flow from a block valve enables them to be used as a drain, bypass,
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 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.
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.
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
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.
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
= 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
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.