Tag: control valves
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27.14 Common Control valve problems
Control valves are subject to a number of common problems. This section is dedicated to an exploration of the more common control valve problems, and potential remedies. 27.14.1 Mechanical friction Control valves are mechanical devices with moving parts, and as such they are subject to friction, primarily between the valve stem and the stem packing. Some degree…
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27.13 Control valve characterization
Control valves are supposed to deliver reliable, repeatable control of process fluid flow rate over a wide range of operating conditions. As we will soon see, this is something of a challenge, as the rate of fluid flow through a control valve depends on more than just the position of its stem. This section discusses…
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27.12 Control valve sizing
When control valves operate between fully open and fully shut, they serve much the same purpose in process systems as resistors do in electric circuits: to dissipate energy. Like resistors, the form that this dissipated energy takes is mostly heat, although some of the dissipated energy manifests in the form of vibration and noise27 . In…
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27.11 Split-range control
There are many process control applications in industry where it is desirable to have multiple control valves respond to the output of a common controller. Control valves configured to follow the command of the same controller are said to be split-ranged, or sequenced. Split-ranged control valves may take different forms of sequencing. A few different modes of…
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27.10 Valve positioners
The reason why a pneumatic control valve’s stem position corresponds linearly to the amount of air pressure applied to the actuator is because mechanical springs tend to follow Hooke’s Law, where the amount of spring motion (x) is directly proportional to applied force (F = kx). A pneumatic actuator applies force as a function of air pressure and…
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27.9 Pneumatic actuator response
A limitation inherent to pneumatic valve actuators is the amount of air flow required to or from the actuator to cause rapid valve motion. This is an especially acute problem in all-pneumatic control systems, where the distance separating a control valve from the controller may be substantial: The combined effect of air-flow friction in the…
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27.8 Actuator bench-set
Valve actuators provide force to move control valve trim. For precise positioning of a control valve, there must be a calibrated relationship between applied force and valve position. Most pneumatic actuators exploit Hooke’s Law to translate applied air pressure to valve stem position. Where, F = Force applied to spring in newtons (metric) or pounds (British) k = Constant…
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27.7 Valve failure mode
An important design parameter of a control valve is the position it will “fail” to if it loses motive power. For electrically actuated valves, this is typically the last position the valve was in before loss of electric power. For pneumatic and hydraulic actuated valves, the option exists of having a large spring provide a…
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27.6 Control valve actuators
The purpose of a control valve actuator is to provide the motive force to operate a valve mechanism. Both sliding-stem and rotary control valves enjoy the same selection of actuators: pneumatic, hydraulic, electric motor, and hand (manual). 27.6.1 Pneumatic actuators Pneumatic actuators use air pressure pushing against either a flexible diaphragm or a piston to move a valve mechanism. The following photograph shows a…
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27.5 Valve seat leakage
In some process applications, it is important that the control valve be able to completely stop fluid flow when placed in the “closed” position. Although this may seem to be a fundamental requirement of any valve, it is not necessarily so. Many control valves spend most of their operating lives in a partially-open state, rarely…
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27.4 Valve packing
Regardless of valve type, all stem-actuated control valves require some form of seal allowing motion of the stem from some external device (an actuator) while sealing process fluid so no leaks occur between the moving stem and the body of the valve. The general term for this sealing mechanism is packing. This mechanical feature is not unlike…
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27.3 Dampers and louvres
A damper (otherwise known as a louvre) is a multi-element flow control device generally used to throttle large flows of air at low pressure. Dampers find common application in furnace and boiler draft control, and in HVAC (Heating, Ventilation, and Air Conditioning) systems. Common damper designs include parallel and radial. Parallel-vane dampers resemble a Venetian blind, with multiple…
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27.2 Rotary-stem valves
A different strategy for controlling the flow of fluid is to insert a rotary element into the flow path. Instead of sliding a stem into and out of the valve body to actuate a throttling mechanism, rotary valves rely on the rotation of a shaft to actuate the trim. An important advantage of rotary control…
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Chapter 27 Basic Principles of Control Valves and Actuators
One of the most common final control elements in industrial control systems is the control valve. A “control valve” works to restrict the flow of fluid through a pipe at the command of a remotely sourced signal, such as the signal from a loop controller or logic device (such as a PLC), or even a manual…