Technology > Theory of Operation

As the pressure drop across the valve increases the flow through the valve will momentarily increase causing the pressure drop across the restriction and the piston to increase. The rise in pressure drop will cause the piston to move the pin toward the seat and hence restrict the flow. Since the piston travel is very slight the change in spring force is negligible so the pressure balance across the piston is the same after the piston reacts to the change in pressure drop across the valve. The time for the piston to react is virtually instantaneous and occurs without hunting or chatter.

For full-scale flows below two thousand gallons per day (fixed orifice design) the rate is adjusted by changing the spring tension. By moving the throttling seat away from the mating pin the spring is compressed causing the pressure drop across the restriction to increase and hence the new flow setting is increased. The fixed fluid restriction is made up of five to ten orifices in series. Cascading these orifices typically increases the minimum hole size passage by a factor of ten compared to a single orifice allowing larger debris in the fluid stream to pass.

The chance of fluid erosion in the five to ten orifices is nonexistent because the pressure drop across the orifice assembly is governed to a maximum of either 200 psid or 900 pisd full scale. The fluid passing over the near diamond hard surfaces of the throttling pin and seat dissipates most of the fluid energy.

For full-scale flow rates above two thousand gallons per day (variable orifice design) and up to fifteen gallons per minute the flow rate is adjusted by changing the size of the fluid restriction while maintaining the pressure drop across the restriction to approximately two hundred psid. The restriction is made up of a characterized globe style trim, which is controlled by a valve stem. Like the low flow version most of the fluid energy is dissipated over the throttling pin and seat eliminating fluid erosion in the globe style trim.

In SubSea Valves for flow rates below two thousand gallons per day (fixed orifice design) full scale the cascading orifice used in conjunction with a differential pressure transmitter produces a true flow rate meter. For full scale rates above two thousand gallons per day (variable orifice design) a second fixed orifice is added upstream of the globe style trim [Variable Orifice] to be used with the differential pressure transmitter for flow rate measurement. For both types of valves the seat holder or stem position can also be used to infer the flow rate to within 5% of full scale over a wide range of pressure drops across the valve.