Flow direction of control valves has a relationship with a failure action, High Delta Pressure, Cavitation, and % opening of the valve.
Control Valves Flow Direction
There are 2 types of control valve flow direction :
- Flow to Open (FTO, or Tending to Open),
- Flow to Close (FTC, or Tending to Close).
Flow to Open Valves
Flow to Open valve means that the fluid flow go through and facing the plug directly, where Flow to Close valve means that the fluid flow go through over the plug. Flow to Open or Flow to Close flow direction of control valve have a relationship with the failure action of a valve.
This failure action means failure when the actuator is considered not present (not a failure just because of signal loss or instrument air lost). If we use a Flow to Open valve, when we lost the actuator the fluid flow that go through the valve will make the plug open in the globe valve type. If we use Flow to Close valve, when we lost the actuator, the fluid flow that go through the valve will make the plug close in the globe valve type.
Flow to Close Valves
Flow direction in relationship with high delta pressure could make a valve fail to operate properly in Flow to Close type. When we use a Flow to Close valve in a high delta pressure application, it could make a valve seat damaged due to over thrust. When we use a Flow to Open valve in a high delta pressure application, it will make a plug tend to open against the actuator thrust.
Some of the valve manufacturer used a Flow to Close valve to reduce the cavitation for liquid application. Some of them also recommend using a Flow to Open valve to expand the gas flow and reduce the power in gas fluid application. The used of the Flow to Close or Flow to Open for such application shall be consult with the respective manufacturer.
One important thing that must be considered if we use the Flow to Close valves is the % opening of the valve that must be in adequate higher than 20% opening to avoid a hydraulic hammer that can occur if the plug operates very close to the seat.
As a general practice, use flow direction Flow to Open as a default valve type, because this type have less than problem rather than flow direction Flow to Close. Also, we assume that there are no cavitating occur as a preliminary. Detail design shall be as per Vendor calculation of the flow condition in the valve (cavitation, flashing etc).
Usually vendor will recommend using Flow to Close valve when the fluid flow is cavitating, high noise etc. If the application is an angle valve (usually used in high delta pressure), it is more recommend to use a flow to close valve since it will close when any actuator component fail and also could reduce the valve cavitation and noise.