Flow control valves are used in constant-flow (fixed displacement pump) circuits to control actuator speed. The simplest type of flow control valve is a needle valve. Another name for a needle valve is a non-pressure-compensated flow control valve. Turning the manual adjustment on a needle valve causes the needle to move down into the orifice, thus reducing the orifice area. Pressure drop across the valve (?Pfc) is increased by continuously restricting the orifice until enough pressure is produced to cause the relief valve to crack open. At this point, extra turns will further reduce the orifice, increase the ?Pfc, increase the pressure at the relief valve, dump more fluid to the reservoir, and thus slow the actuator. The sequence of events is exactly the same as using a DCV to control actuator speed. If load pressure changes over a narrow range, the needle valve will give fairly good flow control. Again, a reminder is given. Flow across the relief valve represents an energy loss. A needle valve is inexpensive, but the operating cost is high because of the energy loss.
A pressure-compensated flow control valve has a provision for changing the ?Pfc as the load pressure changes. Total pressure at the relief valve,
Pr = ?Pfc + ?PL
is maintained nearly constant. As ?PL increases, ?Pfc decreases, and vice versa. A constant Pr means a constant load on the pump (leakage is constant) and a constant flow across the relief valve.
A partial schematic of a flow control valve is shown in Fig. 4.31a, and the full schematic in Fig. 4.31b. It is instructive to first do a force balance on the spool of the valve shown in Fig. 4.31a. Suppose the spring is a 100 psi spring, meaning that it produces a force equivalent to a 100 psi pressure. The force balance on the spool is
(PcAc – PcAr + PcAr – 100Ac)= 0
where Ac = area of spool cap end (in2)
Ar = area of spool rod end (in2)
Pc = pressure in the cavity between the two spool ends
The pressure Pc must equal 100 psi for the spool to be in force balance. The spool finds the position that maintains 100 psi in the center cavity. If the inlet pressure is 500 psi, this means that the pressure drop across the orifice shown in Fig. 4.31a is ?Po = 400 psi. This pressure drop represents an energy loss that is characteristic of this type of valve. The pressure drop between the center cavity and the outlet to the valve is 100 psi. This pressure drop sets the flow through the orifice created by the position of the handwheel adjustment.
The complete schematic for the flow control valve is shown in Fig. 4.31b. Here, the spring cavity opens to the downstream pressure. Now, the downstream pressure adds to the spring pressure to give the total pressure in the center cavity.