When sizing and selecting control valves, it’s important to consider more than just flow capacity
(Cv). Valves must also be able to withstand the pressures they will encounter during operation.
These requirements are typically divided into two main categories:
- Body Pressure Rating
- Close-Off Pressure Rating
Body Pressure Rating
The Body Pressure Rating (sometimes called the body and stem or shell rating) defines the
maximum static pressure that a valve body and stem can safely contain without leaking.
For chilled and hot water systems, this rating is based on the total pressure acting on the valve
— including the water column, fill pressure, and pump pressure.
Manufacturers express this rating in different ways, depending on the valve type and standard
used.
Common Body Pressure Rating Terms
WOG (Water, Oil, Gas):
Indicates a non-shock pressure rating at ambient temperature.
Water refers to standard liquid water.
Oil refers to a free-flowing liquid with higher viscosity and lubricating properties.
Gas refers to vaporized liquids (e.g., steam), not combustible gases.
Expressed in psi: for example, 600 WOG = 600 psi.
CWP (Cold Working Pressure):
Defines the maximum allowable working pressure for temperatures between –20°F and 100°F.
Expressed in psi: for example, 200 CWP = 200 psi.
PN (Pressure Nominal):
Expressed in bar, a metric pressure unit.
Example: PN25 = 25 bar (1 bar ≈ 14.5 psi).
Some manufacturers list both bar and psi ratings.
ANSI / ASME Flange Classes:
Flanged or lug-style valves are categorized according to ANSI classes under ASME B16
standards:
Class 125 and 250 — for cast iron (ASME B16.1)
Class 150 and 300 — for ductile iron, steel, and stainless steel (ASME B16.42 / B16.5)
Each class corresponds to a temperature–pressure chart that defines allowable limits. Some
valves may have body pressure ratings that differ from their flange class ratings.
- Close-Off Pressure Rating
The Close-Off Pressure Rating defines the maximum differential pressure (ΔP) that a valve can
hold across its ball or disc while maintaining its rated seat leakage.
This rating accounts for the entire valve assembly — including the actuator and linkage — and
must exceed the maximum differential pressure expected in the system.
For a two-way valve, this value is typically based on the maximum pump differential
pressure.
⚙️ Example:
A Control Characterized Valve (CCV) may have a 200 psi close-off rating, but a maximum
ΔP rating of only 30 psi due to trim or design limitations.
Summary
Body Pressure Rating → Structural limit of the valve body and stem under static
pressure.
Close-Off Pressure Rating → Maximum pressure differential the valve can seal without
leakage.
Both ratings are critical for ensuring safety, performance, and longevity in HVAC valve
applications.