Issue
Can you cap the A port or B port of a mixing or diverting valve to change the valve to a two way (straight through) valve?
Product Line
Field Devices
Environment
VB-9313 series
Cause
Capping A port or B port of a mixing or diverting valve to change the valve to a two way (straight through) valve
Resolution
Schneider Electric does not recommend this modification for the VB-9313 series, because the valves are not equal percentage and therefore will not provide good control. The valve performs as a linear valve when both ports are utilized. These curves are not additive. Total flow is determined with both ports contributing. The valve will perform as a quick acting valve if the B port is plugged and a nominally linear valve if the A port is plugged.
Additional problems that can occur are:
- Insufficient force to close off. Two way valves usually require high actuator forces to provide close off.
- Valves are not designed to be tight seating. Valves are rated ANSI III therefore flow leakage through the controlled coil can be expected.
- Flow direction through the valve can not be reversed. Flow must oppose the plug when closing. Flow aiding the plug may create hammer by causing the plug to overcome the actuator force and slam against the seat.
- When converted to a two way control valve for a variable flow system, 3-way valves tend to be extremely over-sized creating poor process control.
After considering the pros and cons of making this type of modification, you may choose to proceed with this modification. The performance penalties should be less in chilled water applications and higher in hot water application.
