While the ANSI Class rating and bolting pressure limits describe the pressure retaining ability of a control valve, the internal components (trim) typically determine the pressure drop rating of a specific valve construction. Some control valves have pressure drop ratings to the full ANSI Class pressure rating, while other control valves have limited pressure drop capabilities because of the strength limitations of the valve’s internal parts.
Flowing Versus Shutoff Pressure Drops
Specifiers need to be aware of two different pressure drop ratings (refer to Figure 8):
• The flowing pressure drop refers to the difference between the upstream pressure (P1) and the downstream pressure (P2) while the valve is throttling.
• The shutoff pressure drop is the difference between upstream pressure and downstream pressure when the valve is fully closed.
The shutoff pressure drop is generally higher than the flowing pressure drop; therefore, the pressure drop rating that is listed for most valves refers to the shutoff pressure drop, unless otherwise specified.