Valve Selection. Honeywell AUTOMATIC CONTROL SI Edition

VALVE SELECTION AND SIZING

Ball valves provide tight shut-off, while full port models have low flow resistance, and reduced port models can be selected for modulating applications.

Butterfly valve: A valve with a cylindrical body, a shaft, and a rotating disc (Fig. 4). The disc rotates 90 degrees from open to closed. The disc seats against a resilient body liner and may be manufactured for tight shut-off or made smaller for reduced operating torque but without tight close-off. Butterfly valves are inherently for twoway operation. For three-way applications, two butterfly valves are assembled to a pipe tee with linkage for simultaneous operation.

STEM

BODY

RESILIENT

SEAL

DISC

M12247

Fig. 4. Butterfly Valve.

Double-seated valve: A valve with two seats, plugs, and discs.

Double-seated valves are suitable for applications where fluid pressure is too high to permit a single-seated valve to close. The discs in a double-seated valve are arranged so that in the closed position there is minimal fluid pressure forcing the stem toward the open or closed position; the pressure on the discs is essentially balanced.

For a valve of given size and port area, the double-seated valve requires less force to operate than the single-seated valve so the double-seated valve can use a smaller actuator than a single-seated valve. Also, double-seated valves often have a larger port area for a given pipe size.

A limitation of double-seated valves is that they do not provide tight shut-off. Since both discs rigidly connect together and changes in fluid temperature can cause either the disc or the valve body to expand or contract, one disc may seat before the other and prevent the other disc from seating tightly.

Flanged-end connections: A valve that connects to a pipe by bolting a flange on the valve to a flange screwed onto the pipe. Flanged connections are typically used on large valves only.

Globe valve: A valve which controls flow by moving a circular disk against or away from a seat. When used in throttling control a contoured plug (throttling plug) extends from the center of circular disk through the center of the seat for precise control (Fig. 1).

Pilot-operated valve: A valve which uses the differential between upstream and downstream pressure acting on a diaphragm or piston to move the valve plug. Pilotoperated valves are suitable for two-position control only. The valve actuator exerts only the force necessary to open or close the small pilot port valve that admits fluid flow into the diaphragm or piston chamber.

Reduced-Port valve: A valve with a capacity less than the maximum for the valve body. Ball, butterfly, and smaller globe valves are available with reduced ports to allow correct sizing for good control.

Screwed-end connection: A valve with threaded pipe connections. Valve threads are usually female, but male connections are available for special applications. Some valves have an integral union for easier installation.

Single-seated valve: A valve with one seat, plug, and disc. Singleseated valves are suitable for applications requiring tight shut-off. Since a single-seated valve has nothing to balance the force of the fluid pressure exerted on the plug, it requires more closing force than a double-seated valve of the same size and therefore requires more actuator force than a double-seated valve.

Three-way valve: A valve with three ports. The internal design of a three-way valve classifies it as a mixing or diverting valve. Three-way valves control liquid in modulating or two-position applications and do not provide tight shut-off.

Two-way valve: A valve with one inlet port and one outlet port. Two-way valves control water or steam in twoposition or modulating applications and provide tight shut-off in both straight through and angle patterns.

VALVE MATERIAL AND MEDIA

Valves with bronze or cast iron bodies having brass or stainless steel trim perform satisfactorily in HVAC hydronic systems when the water is treated properly. Failure of valves in these systems may be an indication of inadequate water treatment. The untreated water may contain dissolved minerals

(e.g., calcium, magnesium, or iron compounds) or gases (e.g., carbon dioxide, oxygen, or ammonia). Inadequate treatment results in corrosion of the system. Depending on the material of the valve, the color of the corrosion may indicate the substance causing the failure (Table 1).

ENGINEERING MANUAL OF AUTOMATIC CONTROL

431

VALVE SELECTION AND SIZING

Table 1. Corrosive Elements in Hydronic Systems.

Brass or Bronze Component

Corrosive Substance

Chloride

Ammonia

Carbonates

Magnesium or Calcium

Oxides

Sulphide (Hydrogen)

Iron

Corrosion Color

Light Blue-Green

Blue or Dark Blue

Dark Blue-Green

White

Black (water)

Black (Gas)

Rust

Iron or Steel Component

Corrosive Substance Corrosion Color

Magnesium or Calcium

Iron

White

Rust

Glycol solutions may be used to prevent hydronic systems freezing. Glycol solutions should be formulated for HVAC systems. Some available glycol solutions formulated for other uses contain additives that are injurious to some system seals.

In addition, hydronic seals react differently to water and glycol such that when a new system is started up with water or glycol the seals are effective. The hydronic seals are likely to leak if the system is later restarted with media changed from to water to glycol or glycol to water. To prevent leakage part of the process of media changeover should include replacing seals such as, pump and valve packing.

VALVE SELECTION

Proper valve selection matches a valve to the control and hydronic system physical requirements. First consider the application requirements and then consider the valve characteristics necessary to meet those requirements. The following questions provide a guide to correct valve selection.

— What is the piping arrangement and size?

The piping arrangement indicates whether a two-way or three-way mixing or diverting valve is needed. The piping size gives some indication of whether the valve requires a screwed end or a flanged end connection.

— Does the application require two-position control or proportional control? Does the application require a normally open or normally closed valve? Should the actuator be direct acting or reverse acting?

In its state of rest, the valve is normally open or closed depending on the load being controlled, the fluid being controlled, and the system configuration.

For chilled water coils, it is usually preferable to close the valve on fan shutdown to prevent excessive condensation around the duct and coil, and to save pumping energy. This may be accomplished with either normally closed valves or a variety of other control schemes. Lower cost and more powerful normally open valve assemblies may be used with the close-onshutdown feature and allow, in the case of pneumatic systems, the capability to provide heating or cooling in the event of air compressor failure.

Converter control valves should be normally closed and outdoor air preheat valves should be normally open.

— Is tight shut-off necessary? What differential pressure does the valve have to close against? How much actuator close-off force is required?

Single-seated valves provide tight shut-off, while doubleseated valves do not. Double seated valves are acceptable for use in pressure bypass or in-line throttling applications.

The design and flow capacity of a valve determine who much actuator force is required for a given close-off.

Therefore, the valve must first be sized, then, the valve and actuator selected to provide the required close-off.

— What type of medium is being controlled? What are the temperature and pressure ranges of the medium?

Valves must be compatible with system media composition, maximum and minimum temperature, and maximum pressure. The temperature and pressure of the medium being controlled should not exceed the maximum temperature and pressure ratings of the valve.

For applications such as chlorinated water or brine, select valve materials to avoid corrosion.

— What is the pressure drop across the valve? Is the pressure drop high enough?

The full open pressure drop across the valve must be high enough to allow the valve to exercise control over its portion of the hydronic system. However, the full open pressure drop must not exceed the valves rating for quiet service and normal life. Closed pressure drop must not exceed valve and actuator close-off rating.

432 ENGINEERING MANUAL OF AUTOMATIC CONTROL

VALVE SELECTION AND SIZING

GLOBE VALVE

Globe valves are popular for HVAC applications. They are available in pipe sizes from 12 mm to 300 mm and in a large variety of capacities, flow characteristics, and temperature and pressure capabilities. They provide wide rangeability and tight shutoff for excellent control over a broad range of conditions.

Globe valves are made in two-way, straight or angle configurations and three-way mixing and diverting designs.

Globe valves close against the flow and have arrows on the body indicating correct flow direction. Incorrect piping can result in stem oscillations, noise, and high wear.

A two-way globe valve has one inlet port and one outlet port

(Fig. 5) in either a straight through or angle pattern. The valve can be either push-down-to-close or push-down-to-open.

Pneumatic and electric actuators with linear motion to operate globe valves are available for operation with many control signals.

BUTTERFLY VALVE

Butterfly valves (Fig. 6) control the flow of hot, chilled, or condenser water in two-position or proportional applications.

Butterfly valves are available in two-way or three-way configurations. Tight cutoff may be achieved by proper selection of actuator force and body lining. The three-way valve can be used in mixing or diverting applications with the flow in any direction. The three-way valve consists of two butterfly valves that mount on a flanged cast iron tee and are linked to an actuator which opens one valve as it closes the other. Minimum combined capacity of both valves occurs at the half-open position.

IN IN

PUSH-DOWN-TO-CLOSE PUSH-DOWN-TO-OPEN

C2328

Fig. 5. Two-Way Globe Valves.

M10403

BALL VALVE

Ball valves are available for two-position applications either manual (hand) or power operated or for modulating applications with direct coupled electric actuators. Ball valves are relatively low cost and provide tight close off and available in two-way and three-way configurations. As with all other valves, ball valves must be properly sized to provide good flow control.

When used in modulating service, ball valves must be specifically designed for modulating service as compared to two-position service. Packing must provide leak-free sealing through thousands of cycles to ensure trouble-free HVAC service. The ball and stem should be made of stainless steel or similar material that minimizes sticking to the seat.

Two-way ball valves have equal percentage flow control characteristics and flow can be in either direction

Three-way ball valves can be used in either mixing or diverting service. They have linear flow control characteristics for constant total flow.

Fig. 6. Butterfly Valve.

When butterfly valves are used for proportional control, they must be applied using conservative pressure drop criteria. If the pressure drop approaches the critical pressure drop, unbalanced forces on the disc can cause oscillations, poor control, and/or damage to the linkage and actuator, even though the critical flow point is not reached.

Butterfly valves are usually found in larger pipe sizes. For example, two butterfly valves could be piped in a mixing application to control the temperature of the water going back to the condenser. The valves proportion the amount of tower water and condenser water return that is flowing in the condenser water supply line.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

433

VALVE SELECTION AND SIZING

TWO-WAY VALVE

Two-way valves are available as globe, ball, or butterfly valves.

The combination of valve body and actuator (called valve assembly) determines the valve stem position. Two-way valves control steam or water in two-position or proportional applications

(Fig. 7). They provide tight shutoff and are available with quickopening, linear, or equal percentage flow characteristics.

SUPPLY

TWO–WAY

VALVE

LOAD

RETURN

C2329

Fig. 7. Two-Way Valve Application.

Ideally, a control system has a linear response over its entire operating range. The sensitivity of the control to a change in temperature is then constant throughout the entire control range.

For example, a small increase in temperature provides a small increase in cooling. A nonlinear system has varying sensitivity.

For example, a small increase in temperature can provide a large increase in cooling in one part of the operating range and a small increase in another part of the operating range. To achieve linear control, the combined system performance of the actuator, control valve, and load must be linear. If the system is linear, a linear control valve is appropriate (Fig. 8). If the system is not linear, a nonlinear control valve, such as an equal percentage valve, is appropriate to balance the system so that resultant performance is linear.

100%

NONLINEAR SYSTEM

RESPONSE

RESULTANT

LINEAR SYSTEM

CONTROL

0%

TEMPERATURE

EQUAL PERCENTAGE

CONTROL VALVE

100%

C2330

Fig. 8. Linear vs Nonlinear System Control.

QUICK-OPENING VALVE

A quick-opening two-way valve includes only a disc guide and a flat or quick-opening plug. This type of valve is used for two-position control of steam. The pressure drop for a quickopening two-way valve should be 10 to 20 percent of the piping system pressure differential, leaving the other 80 to 90 percent for the load and piping connections. Figure 9 shows the relationship of flow versus stem travel for a quick-opening valve. To achieve 90 percent flow, the stem must open only 20 percent. Linear or equal percentage valves can be used in lieu of quick-opening valves in two-position control applications as the only significant positions are full open and full closed.

100%

90%

QUICK-OPENING

CONTROL VALVE

0% 20%

STEM TRAVEL

100%

C2331

Fig. 9. Flow vs Stem Travel Characteristic of a Quick-Opening Valve.

LINEAR VALVE

A linear valve may include a V-port plug or a contoured plug. This type of valve is used for proportional control of steam or chilled water, or in applications that do not have wide load variations. Typically in steam or chilled water applications, changes in flow through the load (e.g., heat exchanger, coil) cause proportional changes in heat output. For example,

Figure 10 shows the relationships between heat output, flow, and stem travel given a steam heat exchanger and a linear valve as follows:

100%

90%

100%

90%

100%

90%

20% 20% 20%

0% 20%

FLOW

GRAPH A

90% 100% 0% 20%

STEM TRAVEL

GRAPH B

90% 100% 0% 20%

STEM TRAVEL

90% 100%

GRAPH C

C2332

Fig. 10. Heat Output, Flow, and Stem Travel Characteristics of a Linear Valve.

434 ENGINEERING MANUAL OF AUTOMATIC CONTROL

VALVE SELECTION AND SIZING

— Graph A shows the linear relationship between heat output and flow for the steam heat exchanger. Changes in heat output vary directly with changes in the fluid flow.

— Graph B shows the linear relationship between flow and stem travel for the linear control valve. Changes in stem travel vary directly with changes in the fluid flow.

NOTE: As a linear valve just starts to open, a minimum flow occurs due to clearances required to prevent sticking of the valve. Some valves have a modified linear characteristic to reduce this minimum controllable flow. This modified characteristic is similar to an equal percentage valve characteristic for the first 5 to

10 percent of stem lift and then follows a linear valve characteristic for the remainder of the stem travel.

— Graph C shows the linear relationship between heat output and stem travel for the combined heat exchanger and linear valve. Changes in heat output are directly proportional to changes in the stem travel.

Thus a linear valve is used in linear applications to provide linear control.

EQUAL PERCENTAGE VALVE

An equal percentage valve includes a contoured plug or contoured V-port shaped so that similar movements in stem travel at any point in the flow range change the existing flow an equal percentage, regardless of flow rate.

EXAMPLE:

When a valve with the stem at 30 percent of its total lift and existing flow of 0.25 L/s (Table 2) opens an additional

10 percent of its full travel, the flow measures 0.40 L/s or increases 60 percent. If the valve opens an additional 10 percent so the stem is at 50 percent of its full travel, the flow increases another 60 percent and is 0.64 L/s.

100%

90%

100%

Table 2. Stem Position Vs Flow for

Equal Percentage Valve.

Stem

Change Position

— 30% open

10% increase 40% open

10% increase 50% open

Rate

0.25 L/s

0.40 L/s

0.64 L/s

Flow

Change

—

60% increase

60% increase

An equal percentage valve is used for proportional control in hot water applications and is useful in control applications where wide load variations can occur. Typically in hot water applications, large reductions in flow through the load (e.g., coil) cause small reductions in heat output. An equal percentage valve is used in these applications to achieve linear control.

For example, Figure 11 shows the heat output, flow, and stem travel relationships for a hot water coil, with 94

°

C entering water and 10

°

C entering air and an equal percentage valve, as follows:

— Graph A shows the nonlinear relationship between heat output and flow for the hot water coil. A 50 percent reduction in flow causes a 10 percent reduction in heat output. To reduce the heat output by 50 percent, the flow must decrease 90 percent.

— Graph B shows the nonlinear relationship between flow and stem travel for the equal percentage control valve.

To reduce the flow 50 percent, the stem must close

10 percent. If the stem closes 50 percent, the flow reduces

90 percent.

— Graph C shows the relationship between heat output and stem travel for the combined coil and equal percentage valve. The combined relationship is close to linear. A 10 percent reduction in heat output requires the stem to close

10 percent, a 50 percent reduction in heat output requires the stem to close 50 percent, and a 90 percent reduction in heat output requires the stem to close 90 percent.

The equal percentage valve compensates for the characteristics of a hot water application to provide a control that is close to linear.

100%

90%

50% 50% 50%

0% 10% 50%

FLOW

GRAPH A

100%

10%

0% 50%

STEM TRAVEL

GRAPH B

90% 100%

10%

0% 10% 50%

STEM TRAVEL

GRAPH C

90% 100%

C2333

Fig. 11. Heat Output, Flow, and Stem Travel Characteristics of an Equal Percentage Valve.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

435