From Control Valves

This chapter consists of an introduction to the ANSI/ISA S75.01 standard, "Flow Equations for Sizing Control Valves," followed by the latest version of this standard. Few changes are expected in later versions. However, refinements for calculating flow coefficients for very small flows and for viscous fluids are expected. Moreover, according to Dr. Hans Baumann [Ref. 1], changes are probable for the valve-style modifier Fd . Chapter 22 of this book addresses sizing by a computer program.

Required Calculations

There are several calculations used in sizing. Among them are the following:

  1. For required flow coefficient
  2. For choked possibility, cavitation, and noise
  3. For actuator
  4. For pipe size vs. valve size
  5. For acceptable velocity

It takes an energy drop of the fluid through the valve for the control valve to "control." A control valve should have available at least 10% of the system pressure drop for acceptable control and up to 30% of the system drop for good control. The "loss" of energy through the valve is loss of available energy to the " fluid, but it is accounted for by the heat generated, the noise production, the material removal, vibration, and sometimes by the increase in kinetic energy.

Control valves are...


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© 1998 Instrument Society of America

Products & Services
Hydraulic Valves
Hydraulic valves contain and transfer the flow and pressure of hydraulic fluid in hydraulic power systems. They range from simple shutoff valves to precision control valves.
Valve Actuators
Valve actuators mount on valves and, in response to a signal, move a valve to a desired position using an outside power source.
Manual Valve Actuators
Manual valve actuators do not require an outside power source. They use a handwheel or lever to drive a series of gears whose ratio results in a higher output torque compared to the input (manual) torque.
Pneumatic Valve Actuators
Pneumatic valve actuators adjust valve position by converting air pressure into linear or rotary motion.

Topics of Interest

To avoid dangerous and costly problems in their process systems industrial valve users should understand the basics of the ways in which cavitation and flashing affect control valves. Cavitation...

A control valve is a variable restriction, which is capable of being modulated, in a conduit that contains a flowing fluid. ISA S75.05 offers a more formal definition: "A control valve is a power...

Most people recognize the importance of selecting the right size control valve for a particular application. Obviously, if the control valve is too small, it will not be able to pass the required...

This chapter is offered in two parts. This first part addresses the terms useful to those involved with control valves but not deemed appropriate for the ISA Standard, ANSI/ISA S75.05, "Control Valve...

OVERVIEW Control Valve Sizing Formulas Liquid Formulas (Noncompressible Fluids) Volume basis: Weight basis: Equation 1 and Equation 2 exclude control valve calculations where flashing,...