CV sizing for Steam HVAC Control valves
Steam Valve CV Sizing:
Critical Pressure Drop/Choked Flow: The steam is at a point at which more pressure drop across a valve no longer means more flow of steam. This is generally considered to be 42% of the Absolute inlet pressure which is Gauge Pressure + 14.7 Atmospheric. The valve is at a Choked condition at this point.
If it’s a 2-position valve, then you can go with whatever line size going to the coil. If you really want to check the Cv then take 10% of the design pressure inlet to the system. If the system pressure is 10 PSI, then you need a 1 psi pressure drop valve.
2 Position Valve CV Formula: Covers Low- and High-Pressure Steam:
CV= (Q/(3 (√(h x Po)) ))
Variables:
Cv = Valve Coefficient of flow which is what is the flow with a 1 psi drop across the valve.
Q = Pounds per hour of steam.
h = Pressure drop across fully open valve (2 position this needs to be 10% of the inlet gauge pressure)
Po = Outlet pressure in Psia = P1 - h
P1 = Absolute Inlet Pressure (Gauge Pressure + 14.7 Atmospheric)
Example:
Inlet Gauge Pressure = 15 PSI
Q = 1000 lb/hr.
Step 1: h= .1 x 15=1.5
Step 2: P1=(15 PSI+ 14.7)=29.7
Step 3: Po=(29.7-1.5)=28.2
Step 4: Cv= (1000/(3 (√(1.5 x 28.2)) ))
Step 5: Cv= (1000/(3 x 6.5))
Step 6: Cv= (1000/(19.51))
Step 7: Cv= 51.25
If you are in between valves for Cv then pick the size up. If it’s a 1/3 – 2/3 application, then you need a valve that is 1/3 of Cv rating and one that is 2/3 of the Cv rating.
Modulating Valve CV Low Pressure Steam under 15 PSI Steam Inlet Formula:
CV= (Q/(3 (√(h x Po)) ))
Variables:
Cv = Valve Coefficient of flow which is what is the flow with a 1 psi drop across the valve.
Q = Pounds per hour of steam.
h = Pressure drop across fully open valve (modulating under 15 PSI needs to be 80% of the inlet gauge pressure).
Po = Outlet pressure in Psia is P1 - h
P1 = Absolute Inlet Pressure (Gauge Pressure + 14.7 Atmospheric)
Example:
Inlet Gauge Pressure = 15 PSI
Q = 1000 lb/hr.
Step 1: h= .8 x 15=12
Step 2: P1=(15 PSI+ 14.7)=29.7
Step 3: Po=(29.7-12)=17.7
Step 4: Cv= (1000/(3 (√(12 x 17.7)) ))
Step 5: Cv= (1000/(3 x 14.57))
Step 6: Cv= (1000/(43.72))
Step 7: Cv= 22.87
If you are in between valves for Cv then pick the size up. If it’s a 1/3 – 2/3 application, then you need a valve that is 1/3 of Cv rating and one that is 2/3 of the Cv rating.
Example:
Modulating Valve CV High Pressure Steam over 15 PSI Steam Inlet Formula:
CV= (Q/(3 (√(h x Po)) ))
Variables:
Cv = Valve Coefficient of flow which is what is the flow with a 1 psi drop across the valve.
Q = Pounds per hour of steam.
h = Pressure drop across fully open valve (modulating valve needs to be at the critical pressure drop = absolute inlet pressure (Gauge Pressure + 14.7 Atmospheric) then multiply by .42 which is 42% of inlet absolute pressure.
Po = Outlet pressure in Psia is P1 - h
P1 = Absolute Inlet Pressure (Gauge Pressure + 14.7 Atmospheric)
Example:
Inlet Gauge Pressure = 40 PSI
Q = 1000 lb/hr.
Step 1: h= .42 x (40 PSI+14.7 PSI)=22.974
Step 2: P1=(40 PSI+ 14.7)=54.7
Step 3: Po=(54.7-22.974)=31.726
Step 4: Cv= (1000/(3 (√(22.97 x 31.73)) ))
Step 5: Cv= (1000/(3 x 26.99))
Step 6: Cv= (1000/(80.97))
Step 7: Cv= 12.35
If you are in between valves for Cv then pick the size up. If it’s a 1/3 – 2/3 application, then you need a valve that is 1/3 of Cv rating and one that is 2/3 of the Cv rating.
Example: