How do valves control flow rate?

This post was last edited by jujiangliu on 2016-6-6 at 15:59. A valve functions as a resistance element; when the pressure difference across a system remains constant, changes in the valve’s opening degree result in changes in resistance, which in turn affects the flow rate. Nevertheless, the fluid continues to flow smoothly throughout the system. A valve changes the flow rate of a system, not just the flow rate at the valve itself.

However, according to the equation of fluid continuity, for incompressible fluids, as long as the pipe diameter remains unchanged, the flow velocity once it reaches the steady-flow region after passing through the valve will not change either; consequently, the flow rate will not change either

HLF100 series constant flow valves are automatic control valves that maintain a constant flow rate of the fluid in a pipeline, even when there are fluctuations in the flow rate of that fluid. Installing a constant flow valve on the pipeline for measuring fluids enables the flow meter to operate at a stable flow rate, improving the accuracy of oil measurement. It also prevents damage to the flow meter and other pipeline equipment caused by phenomena such as \"water hammer\" resulting from sudden changes in flow rate within the pipeline. Working principle of the HLF100 constant flow valve: When fluid enters the constant flow valve, it passes through the flow throttling section A formed by the throttle plate and the fluid channel, then through the flow passage section f formed by the valve and the outlet channel, before exiting the constant flow valve. When fluid flows through section A, the pressure acting on the front side of the throttle plate balances the force of the reverse spring, thereby enabling a constant flow rate. When the flow rate increases, the pressure on the throttle plate rises (P increases), which compresses the spring and causes the valve to move toward the outlet. The inlet area A remains unchanged, while the outlet area f decreases; this leads to an increase in the reverse pressure on the throttle plate, and the pressure difference across the throttle plate decreases (P drops), allowing the throttle plate to remain in balance at its new position. At this time, the pressure difference (P) across the throttle plate or the outlet pressure remains constant; according to the principles of Bernoulli’s equation, the flow rate can stay unchanged, thus achieving a constant-flow condition.

The valve changes the volumetric flow rate; the mass flow rate remains constant