Most control valve manufacturers can provide valves with reduced- or restricted- capacity trim parts. The reduced flow rate might be desirable for any of the following reasons:
1.Restricted capacity trim may make it possible to select a valve body large enough for increased future flow requirements, but with trim capacity properly sized for present needs .
2.Valves can be selected for adequate structural strength, yet retain reasonable
3. Large bodies with restricted capacity trim can be used to reduce inlet and outlet fluid velocities.
4. Purchase of expensive pipeline reducers can be avoided
5. Over-sizing errors can be corrected by use of restricted capacity trim parts.
Conventional globe-style valve bodies can be fitted with seat rings with smaller port size than normal and valve plugs sized to fit those smaller ports.Valves with cage-guided trim often achieve the reduced capacity effect by utilizing valve plug, cage, and seat ring parts from a smaller valve size of similar construction and adapter pieces above the cage and below the seat ring to mate those smaller parts with the valve body (figure 1-28).Because reduced capacity service is not unusual,leading manufacturers provide readily available trim part combinations to perform the required function. Many restricted capacity trim combinations are designed to furnish approximately 40% of full-size trim capacity