38B5786X092 Emerson Reverse digital valve controller

Description

For example: 38B5786X092 Suppose 275 lbf. is required to close the valve calculated following the process described earlier. An air-to-open actuator with 100 square inches of diaphragm area and a bench set of 6 to 15 psig is one available option. The expected operating range is 3 to 15 psig. The precompression can be calculated as the difference between the lower end of the bench set (6 psig) and the beginning of the operating range (3 psig).

This 3 psig is used to overcome the precompression so the net precompression force must be: 3 psig X 100 sq. in. = 300 lbf. This exceeds the force required and is an adequate selection. Piston actuators with springs are sized in the same manner. The thrust from piston actuators without springs can simply be calculated as: (Piston Area)(Minimum Supply Pressure) = Available Thrust (Be careful to maintain compatibility of units) In some circumstances an actuator could supply too much force and cause the stem to buckle, to bend sufficiently to cause a leak, or to damage valve internals.

This could occur because the actuator is too large or the maximum air supply exceeds the minimum air supply available. The manufacturer normally takes responsibility for actuator sizing and should have methods documented to check for maximum stem loads.

To maintain a steady flow of liquid through the valve, the velocity must be greatest at the vena contracta, where cross-sectional area is the least. The increase in velocity (or kinetic energy) is accompanied by a substantial decrease in pressure (or potential energy) at the vena contracta. Further downstream, as the fluid stream expands into a larger area, velocity decreases and pressure increases. However, downstream pressure never recovers completely to equal the pressure that existed upstream of the valve.

The pressure differential (∆P) that exists across the valve is a measure of the amount of energy that was dissipated in the valve. Figure 5.7 provides a pressure profile explaining the differing performance of a streamlined high-recovery valve, such as a ball valve, and a valve with lower recovery capabilities due to greater internal turbulence and dissipation of energy.