Page 222 - Root Cause Failure Analysis

210 Root Cause Failure Analysis

Simply installing a valve with the same inlet and discharge size as the piping used in
the process is not acceptable. In most cases, the valve must be larger than the piping to
compensate for flow restrictions within the valve.

Operafing Methods
Operating methods for control valves, which are designed to control or direct gas and
liquid flow through process systems or fluid-power circuits, range from manual to
remote, automatic operation. The key parameters that govern the operation of valves
are the speed of the control movement and the impact of speed on the system. This is
especially important in process systems.

Hydraulic hammer, the shock wave generated by the rapid change in the flow rate of
liquids within a pipe or vessel, has a serious, negative impact on all components of the
process system. For example, instantaneously closing a large flow-control valve may
generate in excess of 3 million foot-pounds of force on the entire system upstream of
the valve. This shock wave can cause catastrophic failure of upstream valves, pumps,
welds, and other system components.
Changes in flow rate, pressure, direction, and other controllable variables must be
gradual enough to permit a smooth transition. Abrupt changes in valve position should
be avoided. Neither the valve installation nor the control mechanism should permit
complete shutoff, referred to as deadheading, of any circuit in a process system.

Restricted flow forces system components, such as pumps, to operate outside of their
performance envelope. This reduces equipment reliability and sets the stage for cata-
strophic failure or abnormal system performance. In applications where radical changes
in flow are required for normal system operation, control valves should be configured to
provide an adequate bypass for surplus flow in order to protect the system.

For example, systems that must have close control of flow should use two proportion-
ing valves that act in tandem to maintain a balanced hydraulic or aerodynamic system.
The primary, or master, valve should control flow to the downstream process. The sec-
ond valve, slaved to the master, should divert excess flow to a bypass loop. This mas-
ter-slave approach ensures that the pumps and other upstream system components are
permitted to operate well within their operating envelope.

FLUID POWER
Fluid power control valves are used on pneumatic and hydraulic systems or circuits.

Configuration
The configuration of fluid power control valves varies with their intended application.
The more common configurations include one way, two way, three way, and four way.