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Control Valve
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Here are some more information for Control Valve:
Many industries use specific valves, called pinch valves, to control fluids, wastewater, slurries, abrasives and corrosive chemicals. Pinch valves are constructed of a heavy-duty, rugged pinch mechanism that positions a sleeve. The valve offers maximum durability and ensures precise flow control in many industrial processes.
Inside every pinch valve is a sleeve that is crucial to the performance success of the valve. This sleeve enables a pinch valve to have a bi-directional, drop tight shut off with which to control flow. Most pinch valve sleeves are made of flexible elastomer compounds, many of which resemble rubber in composition and feel. Because a sleeve's composition is crucial to the success of the valve's application, valve manufacturers offer different elastomer options from which the sleeve can be constructed.
The many uses of the pinch valve
Because pinch valves are useful in a wide range of industries, the ability of a purchaser to choose the desired sleeve composition is paramount. Commonly used in the mining industry, a pinch valve's tough construction makes it ideal for use on harsh and abrasive mining slurries. Pinch valves can also be useful in demanding wastewater treatment applications. For example, a pinch valve's accurate, repeatable linear flow control and drop tight, bi-directional shut off make this valve type a good choice for sludge and raw sewage control.
In the power industry, pinch valves are an important component used in lime and ash handling - two tough services commonly found within a power plant. The abrasion-resistant, self-cleaning, and flexible elastomer sleeve of a pinch valve won't scale, bridge, plug or freeze on slurries. Pinch valves can also work in the treatment and handling of corrosive chemicals. Because there is no packing to maintain and no cavities, seats, or cam action to bind valve operation, pinch valves are often used in corrosive chemical applications.
Pinch valves can also be used in pulp and paper mills. Pulp stock, coating and recycled paper lines are some of the most difficult applications for a valve. The flexible elastomer sleeve of a pinch valve are able to withstand these abrasive services. Pinch valves can also be a great asset in food and beverage plants, cement/sand/silica factories, refineries, pneumatic conveying, and scrubbing.
A sleeve elastomer for every application
The multitude of uses for pinch valves require that purchasers be able to choose specific sleeve compositions and styles for various applications. A pure gum rubber sleeve, for example, is not suitable for the handling of strong acids, but might be appropriate for food manufacture or alcohol distillation. The application of a particular pinch valve, including the temperature range to which it will be exposed, dictates which sleeve elastomer a valve customer should choose.
Pure gum rubber sleeves are resilient and abrasion-resistant. They can handle an incredibly wide temperature range, from -50 F to 180 F, and they are extremely flexible, as well as non-marking. They are composed of natural rubber, and are most suitable for organic (or, carbon-based) acids, and mild chemical compounds, including alcohols. Strong acid and basic solutions, oils, and solvents are some of the chemicals that this type of pinch valve sleeve is ill-equipped to handle.
Neoprene pinch valve sleeves are synthetic rubber sleeves composed of chloroprene polymers. They handle a slightly wider temperature range than pure gum rubber sleeves do, working well with temperatures up to 220 F. Neoprene is chemically inert, and works best in the handling of moderate acids and other chemicals. Unlike pure gum rubber, it is strong and resilient enough to handle products containing ozone, as well as some oils and fats. Neoprene is more resistant to abrasion than is gum rubber; however, it can be eroded or weakened by oxidizing acids, ketone, ester, and chlorinated hydrocarbons.
Sleeves composed of chlorobutyl elastomers, or chlorinated butyl rubber compounds, handle major temperature ranges, from -60 F to 300 F. These sleeves are known for their good abrasion resistance and their ability to handle animal and vegetable fats. However, other types of oils are too heavy for chlorobutyl elastomers to handle -- as are solvents, which can break down the rubber.
The trademarked name of nitrile (a triple-bonded carbon-nitrogen compound), Buna-N can be used to make sleeves suitable for -40F to 240 F temperature ranges. These types of sleeves can be useful in chemical manufacture and chemical engineering because they can handle chemicals and solvents. They are not suitable, however, for ozone, ester, ketone, or nitro/chlorinated hydrocarbons. Meanwhile, Hypalon, a trademarked name for a chlorosulfonated polyethylene compound, is used to construct sleeves suitable for temperature ranges between -60 F and 275 F. This durable compound can handle quite strong acids and bases, as well as freon, ozone, alcohol compounds, and alkalines. This type of pinch valve sleeve should not be used with ketone, ester, or various aromatic and chlorinated hydrocarbons. Hypalon resists weathering quite well, however.
EPDM (or ethylene propylene diene monomer) valve sleeves are excellent for use with fats and oils. These valves -- made of the material used to seal vehicle doors, windows, and the like -- can withstand temperature ranges between -60F and 300F. They should not be used, however, to work with mineral oils or solvents. They also weather well. Viton, a type of synthetic rubber used commonly in O-rings, can also be used with animal and vegetable oils, as well as with acids. Though this type of sleeve performs poorly at temperatures lower than -10 F, it performs quite well at temperatures up to 400 F. Viton sleeves also display excellent tensile strength.
Each of the many uses of pinch valves requires careful consideration of the valve sleeve that is most appropriate to the process for which the valve will be used. With the variety of sleeve elastomer options available, it is no wonder that pinch valves are applicable in so many situations and for so many processes.
Red Valve, in Pittsburgh, Pennsylvania, manufactures custom pinch valves, including air operated, control, and manual pinch valves, along with pinch valve sleeves. For more information, visit Red Valve's Web site.
Control Systems
Copyright (c) 2008 Matt Ridler
A HVAC control system is a device or set of devices to manage, command, direct or regulate the behavior of other devices or systems.
There are two common classes of HVAC control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls. There is also fuzzy logic, which attempts to combine some of the design simplicity of logic with the utility of linear control. Some devices or systems are inherently not controllable.
The term "control system" may be applied to the essentially manual controls that allow an operator to, for example, close and open a hydraulic press, where the logic requires that it cannot be moved unless safety guards are in place.
An automatic sequential control system may trigger a series of mechanical actuators in the correct sequence to perform a task. For example various electric and pneumatic transducers may fold and glue a cardboard box, fill it with product and then seal it in an automatic packaging machine.
In the case of linear feedback systems, a control loop, including sensors, control algorithms and actuators, is arranged in such a fashion as to try to regulate a variable at a setpoint or reference value. An example of this may increase the fuel supply to a furnace when a measured temperature drops. PID controllers are common and effective in cases such as this. Control systems that include some sensing of the results they are trying to achieve are making use of feedback and so can, to some extent, adapt to varying circumstances. Open-loop control systems do not directly make use of feedback, but run only in pre-arranged ways.
Pure logic control systems were historically implemented by electricians with networks of relays, and designed with a notation called ladder logic. Today, most such systems are constructed with programmable logic devices.
Logic controllers may respond to switches, light sensors, pressure switches etc and cause the machinery to perform some operation. Logic systems are used to sequence mechanical operations in many applications. Examples include elevators, washing machines and other systems with interrelated stop-go operations.
Logic systems are quite easy to design, and can handle very complex operations. Some aspects of logic system design make use of Boolean logic.
For example, a thermostat is a simple negative-feedback control: when the temperature (the "measured variable" or MV) goes below a set point (SP), the heater is switched on. Another example could be a pressure-switch on an air compressor: when the pressure (MV) drops below the threshold (SP), the pump is powered. Refrigerators and vacuum pumps contain similar mechanisms operating in reverse, but still providing negative feedback to correct errors.
Simple on-off feedback control systems like these are cheap and effective. In some cases, like the simple compressor example, they may represent a good design choice.
In most applications of on-off feedback control, some consideration needs to be given to other costs, such as wear and tear of control valves and maybe other start-up costs when power is reapplied each time the MV drops. Therefore, practical on-off control systems are designed to include hysteresis, usually in the form of a deadband, a region around the setpoint value in which no control action occurs. The width of deadband may be adjustable or programmable.
Linear control systems use linear negative feedback to produce a control signal mathematically based on other variables, with a view to maintaining the controlled process within an acceptable operating range.
The output from a linear control system into the controlled process may be in the form of a directly variable signal, such as a valve that may be 0 or 100% open or anywhere in between. Sometimes this is not feasible and so, after calculating the current required corrective signal, a linear control system may repeatedly switch an actuator, such as a pump, motor or heater, fully on and then fully off again, regulating the duty cycle using pulse-width modulation.
About the Author
Control Systems are used for all types business big or small. For more information vist Pulse Services Ltd.
what will happen if my vac control valve is disconnected?
hi,
on a 93 vw cabrio...
i installed a cold air intake and didnt hook up the vac control valve, is this why it wont run properly?
actually it runs faster with that hose disconnected. notice why your cold air intake doesnt have that hook up, its because it adds the power there by taking less from your intake
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On CTV.ca: When drugs fail, new surgical procedures and devices can help.
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