Selection of valves

The following service conditions should be considered before
specifying or selecting a valve:

1. Type of liquid, vapor, or gas
• Is it a true fluid or does it contain solids?
• Does it remain a liquid throughout its flow or does it vaporize?
• Is it corrosive or erosive?

2. Pressure and temperature
• Will these vary in the system?
• Should worst case (maximum or minimum values) be considered in selecting correct valve materials?

3. Flow considerations
• Is pressure drop critical?
• Should valve design be chosen for maximum wear?
• Is the valve to be used for simple shutoff or for throttling flow?
• Is the valve needed to prevent backflow?
• Is the valve to be used for directing (mixing or diverting) flow?

4. Frequency of operation
• Will the valve be operated frequently?
• Will valve normally be open with infrequent operation?
• Will operation be manual or automatic?

Body Ratings

The rating of valves defines the pressure-temperature relationship within which the valve may be operated. The valve manufacturer is responsible for determining the valve rating. ASME
Standard B16.34, Valves—Flanged, Threaded, and Welding End,
should be consulted, and a valve pressure class should be identified.
Inlet pressure ratings are generally expressed in terms of the
ANSI/ASME class ratings and range from ANSI Class 150 through
2500, depending on the style, size, and materials of construction,
including seat materials. Automatic control valves are usually either

Materials

ASME Standard B16.34 addresses requirements for valves made
from forgings, castings, plate, bar stock and shapes, and tubular
products. This standard identifies acceptable materials from which
valves can be constructed. In selecting proper valve materials, the
valve body-bonnet material should be selected first and then the
valve plug and seat trim.

Other factors that govern the basic materials selection include

• Pressure-temperature ratings
• Corrosion-resistance requirements
• Thermal shock
• Piping stress
• Fire hazard
Types of materials typically available include

• Carbon steel
• Ductile iron
• Cast iron
• Stainless steels
•Brass
•Bronze
• Polyvinyl chloride (PVC) plastic

Bodies. Body materials for small valves are usually brass,
bronze, or forged steel and for larger valves, cast iron, cast ductile
iron, or cast steel as required for the pressure and service.

Seats. Valve seats can be machined integrally of the body material, press-fitted, or threaded (removable). Seats of different materials can be selected to suit difficult application requirements. The valve seat and the valve plug or disk are sometimes referred to as the valve trim and are usually constructed of the same material selected to meet the service requirements. The trim, however, is usually of a different material than the valve body. Replaceable composition disks are used in conjunction with the plug in some designs in order to provide adequate close-off.

Maximum permissible leakage ratings for control valve seats are defined in Fluid Controls Institute (FCI) Standard 70-2.

Stems. Valve stem material should be selected to meet service conditions. Stainless steel is commonly used for most HVAC applications, and bronze is commonly used in ball valve construction.

Stem Packings and Gaskets. Valve stem packings undergo constant wear due to the movement of the valve stem; and both the
packings and body gaskets are exposed to pressure and pressure variations of the control fluid. Manufacturers can supply recommendations regarding materials and lubricants for specific fluid
temperatures and pressures.

Flow Coefficient and Pressure Drop

Flow through any device results in some loss of pressure. Some
of the factors affecting pressure loss in valves include changes in the cross section and shape of the flow path, obstructions in the flow
path, and changes in direction of the flow path. For most applications, the pressure drop varies as the square of the flow when operating in the turbulent flow range. For check valves, this relationship is true only if the flow holds the valve in the full-open position.

For convenience in selecting valves, particularly control valves, manufacturers express valve capacity as a function of a flow coefficient Cv. By definition in the United States, Cv is the flow of water in gallons per minute (at 60°F) that causes a pressure drop of 1 psi across a fully open valve. Manufacturers may also furnish valve coefficients at other pressure drops. Flow coefficients apply only to water. When selecting a valve to control other fluids, be sure to account for differences in viscosity.

Valve Cavitation & Water Hammer

Online Selection Of Control Valves (REGIN )