Intro4u2u

The piping industry has long recognized the need to control the kinetic energy levels in the transport of fluids through a pipe. The industry has created design criteria that limits the fluid velocity in the pipe to acceptable limits. For example, a normal criteria for liquids in pipes is to limit the fluid velocity to a range of 5 to 50 ft/s (1.5 to 15 m/s). Assuming normal water densities, this is equivalent to a kinetic energy of 0.16 to 16 psi (1.1 to 110 KPa). The typical criteria for gases is to keep the fluid Mach number (actual velocity divided by the fluid’s sonic velocity) below 0.15. Assuming saturated steam of 100 to 1000 psi (0.7 to 7 MPa) and a sonic velocity of 1630 ft/s (500 m/s), the kinetic energy is in the range of 1.5 to 15 psi (10 to 100 KPa).

Velocity criteria for liquids are much lower than for gases because liquid densities are much higher, resulting in higher energy levels. While the velocity limits are quite different, the kinetic energy limits are very close to the same.

Table 2 shows criteria for a valve trim’s outlet kinetic energy. The valve trim should be selected to keep the kinetic energy below these levels. The examples that follow support the values shown in the table.

For most conditions, an acceptance criteria of 70 psi (480 KPa) for the trim outlet kinetic energy will lead to a trouble free valve. In some applications, where the service is intermittent (the valve is closed more than 95% of the time) and the fluid is clean (no cavitation, flashing or entrained solids), the acceptance criteria can be increased, but should never be higher than 150 psi (1030 KPa).

In flashing service, liquid droplets are carried by their vapor at much higher velocities. To eliminate the risk of erosion, the acceptance criteria for flashing or potentially cavitating service should be lowered to 40 psi (275 KPa). The same criteria exists for liquids carrying entrained solids.

Special applications may require even more stringent kinetic energy criteria. For example, pressure letdown valves used in pump test loops must be vibration free so that proper evaluation of the pump can be made. These valves are designed with trims that reduce the kinetic energy to less than 11 psi (75 KPa). Gas or steam valves with very low noise requirements may also result in extra low trim outlet kinetic energy requirements.