Control valve terminology.

article courtesy of Honeywell



AUTOMATIC CONTROL VALVE
An automatic control valve consists of a valve body and an actuator. The actuator is controlled by a pneumatic or electric signal from a remote point changing the rate of medium flow passing through the valve.

VALVE COMPONENTS
Actuator - That part of an automatic control valve which causes the valve stem to move.

Body - That part of an automatic control valve in which the flowing medium is contained.

Plug - The moveable part of the valve that makes contact with the valve seat when the valve is closed, and which varies the area controlling the flow. The three most common types of control valve plugs are contoured, V-port, and quick opening. A contoured plug controls flow by a shaped end. and is usually end-guided at the top or bottom (or both) of the valve body. A V-port plug has a cylinder (called a skirt) which rides up and down in the scat ring. The skirt guides the plug, and by virtue of shaped openings in the skirt, varies the flow area. Quick-opening plugs are machined to provide maximum flow quickly when the plug lifts from its seat. These plugs can be either end-guided or guided by wings riding in the seat ring.

Disc - That part of the plug that contacts the scat and seals off the fluid flow. Valve plugs are sometimes built so that the part of the plug contacting the seat is replaceable. This type of plug is known as the Renewable Disc type. A common example is an ordinary hot water faucet in the kitchen sink. The renewable discs are usually made of a composition material softer than metal. Many valves having all metal or nonrenewable discs have to be "ground in" to restore a damaged seating surface. The term "disc" is sometimes used to refer to the combination of plug and disc.

Guide - That part of the valve plug that keeps the plug aligned with the valve scat. Top or bottom guides on a valve plug are usually located so they do not influence flow but merely accomplish the centering function. Valve guides often have the added function of determining the valve flow characteristic. These are known as skirt guides and usually have notches or Vs cut into them to characterize flow.

Port - This term refers to the flow-controlling opening between the seat and the disc when the valve is wide open. It does not refer to body size or end connection size. Standard valve ports are the sizes normally used in the valves. Valves with reduced ports have flow contequal to a smaller valve with a standard port.

Example:

A one-inch valve with 3/4-inch reduced port size usually has the same flow-controlling area as a 3/4 inch valve with standard port size.

Trim - Trim consists of all parts of a valve that are in contact with the flowing medium but are not part of the valve shell or casting. Thus, plugs, seats, discs, stems, packing rings, etc. are all trim components. The term "trim" is usually used in connection with trim materials.

VALVE FLOW CHARACTERISTICS

Valve Flow Characteristic - The relationship between the stem travel, expressed in percent of travel, and the flow of the fluid through the valve, expressed in percent of full flow.

Quick-Opening Characteristic - Maximum possible flow as soon as the stem starts to move from a closed position. This characteristic is usually selected for two-position rather than modulating valves.

Linear Characteristic - This flow-lift relationship, if plotted on rectilinear coordinates, approximates a straight line, giving equal volume changes for equal lift changes, regardless of percent of valve opening.

Equal Percentage Characteristic - In a valve having an equal percentage characteristic, like movements of the valve stem at any point of the flow range changes the existing flow an equal percentage regardless of the existing flow.

Example:

Suppose a valve stem has been lifted 30 percent of its total lift and the flow at this time is 3.9 gal/min. Now assume that the valve opens an additional 10 percent of its full travel and that the flow increases to 6.2 gal/ nun or a 60 percent increase. Next, suppose that the valve stem moves an additional 10 percent so that it is now 50 percent open. The flow now will be 10 gal/min or another 60 percent increase in flow.

Equal percentage discs are especially useful for control jobs where occasional wide variations in loads occur.

OTHER VALVE FLOW TERMS

Travel Coefficient - The ratio between the flow at a given valve stem position and the flow through the valve at its wide open position, usually expressed as a decimal fraction. Example:

If a valve having a lift of one inch passes 100 gal/min when fully open and passes 66 gal/min at a valve lift of .5-inch, this valve is said to have a .66 valve travel coefficient at a .5-inch lift. Valve travel coefficients can be read directly from any plot showing valve flow characteristics.

Rangeability - The ratio of the maximum controllable flow to the minimum controllable flow. For instance, a valve with a rangeability of 50 to 1 and having a total flow capacity of 100 gal/min, fully open, will control flow accurately down as low as 2 gal/min. The valve may or may not have tight shutoff. However, for this particular valve you could not depend upon getting steady flow of, for instance, 1.5 gal/min if this were necessary to maintain stable load conditions. Generally, rangeabilities in the range of 50 to 1 or 40 to 1 are considered excellent for extreme precision of control. Valves with high rangeability are very expensive to manufacture since very close tolerances are involved between the disc and the seat. Competitively priced control valves ordinarily have rangeability of less than 30 to 1. In many cases wide rangeability compensates automatically for deviations from anticipated conditions without loss of control.

Turndown - The ratio between maximum usable flow and the minimum controllable flow; usually less than the rangeability. For instance, as stated above, after the 100 gal/min valve has been applied at a job, it might turn out that the most flow you would ever need through the valve is 68 gal/min. Since the minimum controllable flow is 2 gal/min, the turndown for this valve is 34 to 1. In comparing rangeability and turndown, we may say that rangeability is a measure of the predicted stability of the control valve, and turndown is a measure of the actual stability of the valve.

Tight Shut-Off - A valve having tight shutoff will have virtually no flow or leakage in its closed position. Generally speaking, only single-seated valves have tight shutoff. Double-seated valves may be expected to have a leakage of two to five percent while in closed position.

VALVE RATINGS

Capacity Index - The quantity of water in gal/min at 60 F (16 C) that flows through a given wide open valve with a pressure drop of 1 lb/in2. It is sometimes called flow coefficient. The symbol for capacity index or flow coefficient is Cv. Once the Cv of the valve has been determined, the flow of any fluid through the same valve can be calculated provided the characteristics of the fluid and the pressure drop through the valve are known.

Close-Off - The close-off rating of a valve is the maximum allowable pressure drop to which the valve may be subjected while fully closed. This rating is usually a function of the power available from the valve actuator for holding the valve closed against pressure drop, but structural parts such as the stem sometimes are the limiting factor. The close-off rating is independent of the actual valve body rating.

Example:

A valve having a close-off rating of 10 lb/in2 could be operated with an upstream pressure of 40 lb/in2 and a downstream pressure of 30 lb/in2.

Three-Way Diverting Valves - A valve which has three openings (one inlet and two outlets). Fluid entering, the inlet point can he diverted to either of the two outlet ports, in any proportion desired, by moving the valve stem. Valves designed for diverting service can usually be used in mixing applications.

Butterfly Valves - A valve consisting of a cylindrical body with a rotating vane to control flow. Tight Shutoff is obtained by an elastomer body liner into which the vane seats. Advantages of butterfly valves are low flow resistance in open position, compact overall size, and relatively low cost due to simple design. Butterfly valves are a subject unto themselves and are not further discussed in this article.

Others - There are many other types of valves that are not normally used in heating and air conditioning systems and are not covered in this article.

VALVE MATERIALS

Commercial valves with bronze or cast iron bodies having brass or stainless steel trim perform satisfactorily in heating, ventilating and air conditioning hydronic systems when proper system water treatment is used. Failure of valves in such systems is usually a sign of inadequate water treatment, air or dissolved gases entrained in the water, or solids such as silt or rust being circulated.
Some hydronic systems utilize a glycol solution to prevent freezing. Some commercially available glycol solutions contain additives that are injurious to certain elastomers sometimes used for valve packing or discs. Where glycoI is used, it should be a solution specially formulated for heating and air conditioning systems.
For other applications, such as chlorinated water or brine, select other valve materials to avoid serious corrosion.

THREE-WAY VALVE APPLICATION

Three-way valves are normally used as mixing or diverting valves to control the flow of liquids. The difference between the two applications is in the piping layout and the internal design of the valve.

Figure 1 shows a three-way mixing valve. Two pipe lines bearing liquids to be mixed connect to the valve one pipe to port "A" and the other to port "B". Connection "AB" is the outlet for the mixed liquids. The mixing proportion of the liquid depends on the position of the valve disc. Thus, a mixing valve receives liquids through two inlets, mixes them, and discharges them through one outlet.

Figure 2 shows a three way diverting valve. When the valve disc is against the top seat, the liquid enters through port "AB" and leaves through port "B". When the disc is against the bottom seat, "AB" is closed off from "B" and the flow is from "AB" to "A". The diverting valve has one inlet from which flow is directed to either of two outlets.

In a given mixing valve application, the force exerted on the valve disc due to the unbalanced pressure at the two inlets usually remains in the same direction.
The force tends to push the valve disc upward. In cases where there is a reversal of force, the force changes direction and tends to hold the valve disc off the seat, cushioning the valve disc as it closes.
If the pressure difference for the installation is greater than the pressure ratings of all mixing valves, use two globe valves or two butterfly valves in a tee configuration.

DIVERTING SERVICE

Referring to Figure 2, assume that there is a pressure of 20 lb/in2 at "AB". "B" discharges to the atmosphere, and "A" is connected to a tank under constant 10 lb/in2 pressure.
The pressure difference between "AB" and "A" is 10 lb/in2, and between "AB" and "B" is 20 lb/in2.
Therefore, a diverting valve with at least a 20 lb/in2 close-off rating is required.

MIXING VALVE SERVICE

"Close-Off" ratings are listed for three-way valves used as mixing valves only. The close-off pressure in a mixing valve installation is equal to the maximum value of the greater inlet pressure minus the minimum value of the downstream pressure,

Example:

The valve in figure 1 has a maximum of 25 lb/in2 pressure on inlet A and a maximum of 20 lb/in2 pressure on inlet "B".
The minimum downstream pressure is 10 lb/in2.
Close-Off Pressure: 25 lb/in2 minus 10 lb/in2 equals 15 lb/in2.
Select a valve with at least 15 lb/in2 pressure close-off rating.

Question:
Assume that a 2-inch valve has the required capacity. Does it have the required close-off ratings?

Answer:
The close-off rating for the 2-inch valve with an electric motorized operator is given in a pressure rating table as 67 lb/in2. The valve therefore has a high enough close-off rating to operate satisfactorily.

Mixing valves should not be used in diverting application due to the differences in internal design.
Suppose that the valve disc area of a mixing valve is two square inches. When the disc is against the lower seat, there is a 20 lb/in2 differential pressure or a total force of 40 lb. holding the valve disc down against the seat. When the disc is against the upper seat, the differential pressure is 10 lb/in2 and a force of 20 lb. holds the valve disc up against the seat. As the valve disc travels between these two extremes, there is a point at which the direction of force is reversed. The reversal is quite rapid and in some cases is almost instantaneous. To further complicate matters, the reversal is in such a direction that it tends to slam the valve closed as the disc approaches the valve seat. This rapid reversal of force is often enough to overcome the strain release mechanism of an electric operator or the diaphragm and spring of a pneumatic operator. The valve disc then bounces against the seat causing "water hammer".
Because of this inherent condition, do not use threeway valves for diverting service except when nearly the same pressures can be maintained on all three outlets at all valve disc positions. Such conditions rarely exist. For diverting valve applications, diverting valves or two globe valves will give more satisfactory service.
If the diverting action is required only occasionally, such as for heat-cool changeover, and if a fail-safe scheme can be devised to assure that the pump is not operating when the valve disc is changing position, a mixing valve may be used for diverting service.

Three applications in which three-way valves are used in mixing service are shown in Figures 3, 4 and 5. Figure 3 shows a typical piping diagram for a three-way mixing valve installed in a single zone application. This valve provides a constant volume of variable temperature water to heating loads and a variable volume of water through the boiler.

Figure 4 shows several three-way mixing valves installed in a multizone system. These valves provide a constant volume of variable temperature water to the heating or cooling loads and a variable volume of water through the boiler or chiller.

Figure 5 shows several three-way mixing valves installed in a multizone application. These valves provide a constant volume of water through the boiler or chiller and a variable volume of constant temperature water to heating or cooling loads.
It is extremely important to connect the piping properly to a three-way valve. The three-way valve is available in mixing and diverting models and is stamped with an arrow on the body to show flow direction.


Above article was compiled out of the "Pneumatic Temperature Controls Theory Manual" published by Honeywell.
Phone or fax Honeywell if you have any questions. Phone: 403-211-2222; fax: 403-253-0690




First published November 1997

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