Honeywell MS4103 manual Valve Sizing, Mixing valve application has a maximum pressure, 253

Appendix A: Valve Selection and Sizing

		THREE-WAY
		MIXING VALVE
HOT	LOAD		HOT
WATER			WATER
SUPPLY	BYPASS		RETURN

A. LOAD BYPASS IN MIXING VALVE APPLICATION

THREE-WAY

DIVERTING VALVE

SUPPLY LOAD RETURN

BYPASS

B. LOAD BYPASS IN DIVERTING VALVE APPLICATION

C2335A

Fig. 13. Three-Way Valve Applications.

MIXING VALVE

A mixing valve provides two inlet ports and one common outlet port. The valve receives liquids to be mixed from the inlet ports and discharges the liquid through the outlet port (Fig. 12). The position of the valve disc determines the mixing proportions of the liquids from the inlet ports.

The close-off pressure in a mixing valve equals the maximum value of the greater inlet pressure minus the minimum value of the downstream pressure.

EXAMPLE:

A mixing valve application has a maximum pressure of

25 psi on one inlet port, maximum pressure of 20 psi on the other inlet port, and minimum downstream pressure of

10 psi on the outlet port. The close-off pressure is 25 psi – 10 psi = 15 psi. The application requires a mixing valve with at least a 15 psi close-off rating. The actuator selected must have a high enough force to operate satisfactorily.

In globe mixing valve applications, the force exerted on the valve disc due to unbalanced pressure at the inlets usually remains in the same direction. In cases where there is a reversal of force, the force changes direction and holds the valve disc off the seat, cushioning it as it closes. If the pressure difference for the system is greater than the pressure ratings of available globe mixing valves, use a ball mixing valve or two butterfly valves in a tee configuration.

Globe mixing valves are not suitable for modulating diverting valve applications. If a mixing valve is piped for modulating diverting service, the inlet pressure slams the disc against the seat when it nears the closed position. This results in loss of control, oscillations, and excessive valve wear and noise. Mixing valves are acceptable using about 80 percent of the close-off rating, but not recommended, in two-position diverting valve applications.

DIVERTING VALVE

A globe diverting valve provides one common inlet port and two outlet ports. The diverting valve uses two V-port plugs which seat in opposite directions and against the common inlet flow. The valve receives a liquid from one inlet port and discharges the liquids through the outlet ports (Fig. 12) depending on the position of the valve disc. If the valve disc is against the bottom seat (stem up), all the liquid discharges through the side outlet port. If the valve disc is against the top seat (stem down), all the liquid discharges through the bottom outlet port.

The close-off pressure in a diverting valve equals the maximum value of the inlet pressure minus the minimum value of the downstream pressure.

Globe diverting valves must not be used for mixing service. As with mixing valves used for diverting service, media pressure drop across the valve can cause it to slam shut with resulting loss of control.

EXAMPLE:

A diverting valve application has 20 psi maximum on the inlet port, one outlet port discharging to the atmosphere, and the other outlet port connecting to a tank under 10 psi constant pressure. The pressure difference between the inlet and the first outlet port is 20 psi and between the inlet and second outlet port is 10 psi. The application requires a diverting valve with at least 20 psi close-off rating.

Valve Sizing

Every valve has a capacity index or flow coefficient (Cv). Typically determined for the globe and ball valves at full open and about 60 degrees open for butterfly valves. Cv is the quantity of water in gpm at 60F that flows through a valve with a pressure differential of 1 psi. Sizing a valve requires knowing the medium (liquid or gas) and the required pressure differential to calculate the required Cv. When the required Cv is not available in a standard valve, select the next closest and calculate the resulting valve pressure differential at the required flow to verify to verify acceptable performance.

After determination of the valve Cv, calculation of the flow of any medium through that valve can be found if the characteristics of the medium and the pressure drop across the valve are known.

APPENDIX

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