30 November 1998
manufacturers' literature shows Kv of 8.6, 10.4, 11.2 (Cv of 10, 12, and 13) in a 25 mm (1 inch) valve. A
valve Kv of 10.4 (Cv of 12) would be a good design choice for this application.
h. When the calculation for steam valve selection results in a Kv greater than 47.5 (Cv greater than 55),
the designer will select two valves, sized to split the flow unevenly between the valves; a good ratio is one-
third of the flow for the smaller valve and two-thirds of the flow for the larger valve, which, however, is not
always achievable from the normally available stock of valves. As an example of the calculation required, a
steam converter requires 2,722 kg/hr (6,000 lb/hr) of steam at 7 kPa (1 psig) entering the converter shell at
peak load with the steam pressure reduced from 862 kPa (125 psig) to 172 kPa (25 psig) near the
convertor. The critical pressure drop is 123 kPa (17.87 psid) as a result of using equation 2-5, and the
pressure drop across the valve is 172 - 7 = 165 kPa (25-1 = 24 psid), which is greater than the critical
pressure drop. The constant-enthalpy process of reducing the steam pressure from 862 kPa (125 psig) to
172 kPa (25 psig) results in 26.7 degrees C (48 degrees F) of superheat. The required Kv (Cv) is 77.5
(89.1) as a result of using equation 2-9. This Kv (Cv) is 3 percent larger than it would have been if the steam
were saturated at 172 kPa (25 psig) and the Kv (Cv) were calculated from equation 2.7. In this example, the
designer should show two valves in parallel and attempt to find available valves with Kvs of approximately
52 and 26 (Cvs of approximately 60 and 30). A check of manufacturers' catalogs for this example results in
the designer showing valves with Kvs of 60.5 and 19.9 (Cvs of 70 and 23) and a close-off pressure of 172
kPa (25 psig). The selection results from the following available control valve combinations, any of which
will meet the design intent:
(1) one 65 mm (2-1/2 inch) valve, Kv = 64 (Cv = 74), and one 40 mm (1-1/2 inch) valve, Kv = 15.6
(Cv = 18), or total Kv = 79.6 (Cv = 92)
(2) one 65 mm (2-1/2 inch) valve, Kv = 60.5 (Cv = 70), and one 40 mm (1-1/2 inch) valve, Kv = 19.9
(Cv = 23), or total Kv = 80.4 (Cv = 93)
(3) one 80 mm (3-inch) valve, Kv = 61.1 (Cv = 70.7), and one 40 mm (1-1/2 inch) valve, Kv = 20 (Cv
= 23.1), or total Kv = 81.1 (Cv = 93.8)
15. DETERMINING VALVE ACTUATOR CLOSE-OFF PRESSURE RATINGS.
a. The close-off rating indicates the pressure against which a valve must be able to close. Figure 2-16
is a normally open pneumatic valve. F1 is the force of the air pressure acting to close the valve. F2 is the
opposing spring pressure and F3 is the force exerted by the fluid (water pump head or steam pressure).
The value of F3 is actually the difference in pressure on the two sides of the valve plug. The valve and
actuator must be able to close the valve against fluid force F3. The expected value of F3 is part of the
hydraulic design of the piping system and depends upon the location of the valve within the system. To
provide for a safety factor, the pressure the valve must close against is normally specified at several times
the value actually expected at the valve's location. A three-way valve must operate against a similar fluid
pressure in moving from one position to another.
Figure 2-16. Typical normally-open pneumatic valve.
b. In piping circuits with two-way valves, the flow is not constant. As the valve closes, the flow
decreases. As the flow decreases, the friction losses decrease and the pump delivers more pressure. As a
valve shuts off (worst condition) the valve must take the full pump pressure at zero flow (dead head). To
provide a measure of safety and to allow for differences between design and installed conditions, close-off
pressure ratings for two-way valves should specified to be 100% to 125% of the rated pump pressure at the
design flow rate.
Figure 2-17. Close-off pressure for two-way valves.