2000 Hook-up Book

Temperature Control Valves for Liquid Service

Water Valve Sizing Water valve capacity is directly related to the square root of the pressure drop across it, not the static system pressure. Knowing the load in GPM water or any other liquid, the minimum valve Cv required is calculated from the allowable pressure drop ( ∆ P): Cv = GPM S.G. √ ∆ P derived from... GPM(Water) = Cv √ ∆ P (Other SG Liquids) GPM = Cv ∆ P √ S.G.

If the allowable differential pres sure is unknown, the following pressure drops may be applied: • Heating and Cooling systems using low temperature hot water (below 212°F)— Size valve at 1 psi to 2-1/2 psi differential. • Heating Systems using water above 212°F— Size valve on a 2-1/2 to 5 psi differential.

• Water for process systems— Size valve for pressure drop of 10% up to 20% of the sys tem pressure. • Cooling Valves— Size for allowable differential up to full system pressure drop when discharging to atmosphere. Be sure to check maximum allowable pressure drop of the valve selected. A bellows-balanced type may be required. Only two-port valves are used on steam systems. However, when dealing with controls for water we can select either two-port or three-port valves. But we must consider the effects of both types on the overall system dynamics. A three-port valve, whether mixing or diverting, is fairly close to being a constant volume valve and tends to maintain constant pressure distribution in the sys tem, irrespective of the position of the valve. If a two-port valve were used, the flow decreases, the valve closes and the pressure or head across it would increase. This effect is inherent in the use of two-port valves and can affect the operation of other subcircuits. Furthermore, the water standing in the mains will often cool off while the valve is closed. When the valve reopens, the water entering the heat exchang er or load is cooler than expected, and it is some time before normal heating can com mence. To avoid this, a small bypass line should be installed across the supply and return mains. The bypass line should be sized to handle flow rate due to mains losses but in the absence of information, the bypass should be sized for 10% of the design flow rate. Using Two-Port and Three-Port Valves

SYSTEM DESIGN

Pump

Constantly Open Port

O

C Heating System

X Z

Three-Port Valve

Balance Valve

A

Boiler

B

Figure 43 Three-Port Mixing Valve in a Closed Circuit (Constant Volume, Variable Temperature)

Pump

Constantly Open Port

O

Z X

C Heating Plant or Process Equipment

Three-Port Valve

Balance Valve

A

Boiler

B

Figure 43A Three-Port Diverting Valve in a Closed Circuit (Constant Temperature, Variable Volume)

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