Page 223 - Root Cause Failure Analysis

Control Valves 21 1

One Way

One-way valves typically are used for flow and pressure control in fluid-power cir-
cuits (see Figure 17-9). Flow-control valves regulate the flow of hydraulic fluid or
gases in these systems. Pressure-control valves, in the form of regulators or relief
valves, control the amount of pressure transmitted downstream from the valve. In
most cases, the types of valves used for flow control are smaller versions of the types
of valves used in process control. The major types of process-control valves were dis-
cussed previously. These include ball, gate, globe, and butterfly valves.

Pressure-control valves have a third port to vent excess pressure and prevent it from
affecting the downstream piping. The bypass, or exhaust, port has an internal flow-
control device, such as a diaphragm or piston, that opens at predetermined set points
to permit the excess pressure to bypass the valve’s primary discharge. In pneumatic
circuits, the bypass port vents to the atmosphere. In hydraulic circuits, it must be con-
nected to a piping system that returns to the hydraulic reservoir.

Two Way
A two-way valve has two functional flow-control ports. A two-way, sliding spool
directional control valve is shown in Figure 17-10. As the spool moves back and
forth, it either allows fluid to flow through the valve or prevents it from flowing. In the
open position, the fluid enters the inlet port, flows around the shaft of the spool, and
through the outlet port. Because the forces in the cylinder are equal when open, the
spool cannot move back and forth. In the closed position, one of the spool’s pistons
simply blocks the inlet port, which prevents flow through the valve.

/SPRING

BODY / NO FLOW

IN OUT

FREE FLOW
Figure 17-9 One-way, fluidpower valve.