Maintenance Fundamentals Episode 2 part 5 docx

Repeatability Repeatability is perhaps the most important performance criterion

of a process-control valve. This is especially true in applications in which precise

flow or pressure control is needed for optimum performance of the process

system.

New process-control valves generally provide the repeatability required. How￾ever, proper maintenance and periodic calibration of the valves and their actu￾ators are required to ensure long-term performance. This is especially true for

valves that use mechanical linkages as part of the actuator assembly.

Installation

Process-control valves cannot tolerate solids, especially abrasives, in the gas or

liquid stream. In applications in which high concentrations of particulates are

present, valves tend to experience chronic leakage or seal problems because the

Figure 13.8 High-torque electric motors can be used as actuators.

Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 5:56pm page 274

274 Maintenance Fundamentals

particulate matter prevents the ball, disk, or gate from completely closing

against the stationary surface.

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.

Operating 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, or the shock wave generated by the rapid change in the flow

rate of liquids within a pipe or vessel, has a serious and 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 mech￾anism should permit complete shut off, 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 catastrophic failure or abnormal system performance. In applications in

which radical changes in flow are required for normal system operation, control

valves should be configured to provide an adequate bypass for surplus flow to

protect the system.

For example, systems that must have close control of flow should use two propor￾tioning 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 second valve, slaved to the master, should divert excess flow to a

bypass loop. This master-slave approach ensures that the pumps and other up￾stream system components are permitted to operate well within their operating

envelopes.

Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 5:56pm page 275

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