Theory of flow through pipes, valves and fittings

Theory of flow through valves and fittings

Chocked flow

Flow in pipes and valves theory - full content

Formation of bubbles in the valve resulting of flashing and cavitation effect reduces the flow rate through valve and limits the capacity. Limiting pressure difference for the valve is determined by experiment for each valve. Limiting pressure difference can also be calculated using:

where is:

• &Delta p_allow - maximum allowable differential pressure [psi]
• K_m - valve recovery coefficient from manufacturer literature [ - ]
• p₁ - valve inlet absolute pressure [psia]
• p_v - vapor absolute pressure of the liquid at inlet temperature [psia]
• r_c - critical pressure ratio 0,70 - 0,95

In high recovery valves, cavitation can occur on pressure drops below that produces chocked flow. Therefore cavitation index is used to determine the pressure drop at which cavitation damage will begin in high-recovery valves:

where is:

• K_c - cavitation index from manufacturer literature [ - ]
• p₁ - valve inlet absolute pressure [psia]
• p_v - vapor absolute pressure of the liquid at inlet temperature [psia]
• p_c - pressure drop that creates cavitation in high recovery valves [psi]

This equation can be used anytime outlet pressure is greater than the vapor pressure of the liquid.

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