TABLE OF CONTENTS In general, large pressure differentials create high velocities through a valve and in downstream piping. This in turn creates turbulence and vibration in liquid applications and high noise levels in gas applications. The velocity is inversely related to the pressure losses and gains as the flow moves through the vena contracta (Fig. 9.24). The velocity reaches its maximum peak just slightly after the vena contracta, which is when the pressure is at its lowest point.
The following general rules apply to velocities: Liquids should generally not exceed 50 ft/s (15.2 m/s) (or 30 ft/s or 9 m/s in cavitating services). Gases should not exceed sonic speeds (Mach 1.0). And, mixtures of gases and liquids (such as flashing applications) should not exceed 500 ft/s (152 m/s). These general rules can vary, however, according to the size of the valve. For example, smaller-sized valves can normally handle higher velocities, while larger valves only handle lower velocities.
Generally, process liquids that have temperatures close to the saturation point must keep the velocity at or under 30 ft/s (9 m/s) to avoid the fluid pressure from falling below the vapor pressure and creating cavitation. The 30-ft/s rule also applies to cavitating services, where higher velocities result in greater cavitation damage to downstream piping. Lower velocities will also reduce problems associated with flashing...
