Pinch valves, the simplest valve design, include any valve with a flexible elastomer body that can be pinched close to cut off flow, using a mechanism or fluid pressure. They are linear motion valves that can be used to start, stop and throttle media through a system. Pinch valves are low maintenance, low weight, and can be used in systems requiring explosion-proof line closure. While the design of pinch valves provides extensive advantages for use in sterile lines and in situations where product purity is a high priority, these same design features do create some disadvantages. Due to their elastomeric bodies, pinch valves are not viable in situations where the transport media is at a high temperature. They are also not recommended for services that require high-pressure flow, and for use with gases.
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Cannot be used in high temperature applications
Cannot be used in high pressure applications
Cannot be used with gas media
Low airborne contaminants
Can be used in explosion-proof lines
Industrial valves can be classified in a number of different ways. Pinch valves are full bore, linear action valves that can be used in both an off / on the manner or in a variable position or throttling service.
Method of Control
Pinch valves use a liner motion method of control. The disc is a flexible material liner, similar to a diaphragm. The stem of the pinch valve has a free-moving connection to a moving closure bar, called a compressor, which is located directly above the liner. As the handwheel is turned, the compressor lowers the liner to squeeze the liner against the bottom support to close the valve. The pressure of the media in the valve can help in starting or stopping the flow.
Pinch valves can be used for On/Off, as well as throttling services.
Pinch valves are ideal for on/off services because of their straight through design that allows for uninhibited flow. Straight through designs also have very little pressure drop between the inlet and outlet.
The effective throttling range of a pinch valve is usually between 10% and 95% of the rated flow capacity. The The best flow control is at the last 50% of the stroke. This is because the smooth walls and resilience of the liner do not provide a significant pressure drop until at least 50% of the stroke has been achieved. To avoid using the ineffective half of the full stroke some pinch valves are designed for maximum opening at 50%. It is not recommended that media with sharp particles are throttled close to shutoff. The particles can scratch the liner, causing grooves that could potentially tear.
Image Credit: Valve Handbook
Pinch valves can be used for liquid, solid and slurry applications. The valve can effectively control the flow of both abrasives and corrosives, as there is no contact between metal parts and the transport media. Pinch valves have a liner that keeps all moving parts outside of the flow stream so large particulates can be trapped in the seal area of the valve without breaking the seal. Due to the isolation of the valve parts from the media, these valves can also be used in applications where corrosion or metal contamination of the fluid, slurries, sand-entrained water systems, sewage treatment, and process food might be a problem.
In liquid applications, the liquid must be kept moving so the displacement of the fluid by the sealing area does not cause additional strain on the liner and cause it to burst.
Most pinch valves are constructed so that the compression pressure is from the top only allowing the valve to drain thoroughly in all positions except upside down. Additionally, many have a straight-through design that allows for a high rate of flow with minimal turbulence. Both of these features call for low air consumption, allowing for the system to stay relatively closed, reducing the introduction of airborne contaminants. There are three body designs for pinch valves:
The open body design has no metal casing and relies upon a skeletal metal structure which consists of two cross-bars fastened to metal flange supports. Rubber liner is placed between the two halves of the metal flange during assembly and the two halves are connected with top and bottom supports. The top support is threaded to accept the threaded handwheel stem. The open body design is simple and inexpensive. The open design allows for easy inspection of the liner for failures. The disadvantage of this design is that the liner is exposed to the media and environment of the system. Exposure could result in early failure of the liner. When the body is exposed, the valve can also be used in limited vacuum applications because the sleeve can collapse when a vacuum is applied.
The enclosed-body pinch valve uses a protective casing for the liner. The design looks similar to most flow-through globe valves and the closure mechanism is similar to the open-body pinch valve. The difference is the compressor is enclosed in the body above the liner. An alternative body design is to use an integral bar cast into the bottom of the casing, called a weir. Not all enclosed-body pinch valves have the weir bar; in this design the liner compresses against the bottom of the valve to stop flow. The body is split along the axis of the flow passage to allow for assembly of the liner. The two sides are bolted together and a drain included in the bottom servers as an indicator for liner failure. The advantage of enclosed-body designs is the outside fluid or pressure can be introduced into the casing to help the liner stay open or closed. This is especially helpful during vacuum applications. For this type of valve body, flow is controlled with the conventional wheel and screw pinching device, hydraulically, or pneumatically with the pressure of the liquid or gas within the metallic body forcing the sleeve walls together to shut off flow.
The pressure-assisted pinch valve uses outside fluid pressure to close the valve. This design doesn't use a manual operator. Pressure-assisted valves are similar to an enclosed-body design but don't have a closure mechanism or operator. Fluid is introduced to the inside of the casing (but outside the liner) through tapered connections. When the pressure of the additional fluid overcomes the inside media pressure, the liner closes and stays closed until either the system pressure increases or the introduced pressure decreases. While this body type is very inexpensive, it can only be used for on/off applications.
Pinch Valves: Open body | Enclosed body | Pressure-assisted body. Image Credit: Valve Handbook.
The valve body is a molded sleeve of rubber or other synthetic material. The body also consists of the pinching mechanism, however the rest of the operating portions are external to the valve and do not come into contact with the media. The valve body is reinforced with fabric. The liner has smooth walls that cause gentle turns in the fluid to minimize turbulence and line vibration. The liner also allows for easy bubble-tight shut off. Packing is not required in pinch valves.
The stem is responsible for the movement of the liner for opening or closing the valve. It is usually forged and connected to the valve hand-wheel, actuator, or the lever by threading. The stem moves the disc in a linear movement to open or close the valve. There are five types of valve systems depending on the application.
Rising stem with outside screw and yoke-The exterior of the stem is threaded, while the portion of the stem in the valve is smooth. The stem threads are separate from the flow medium by the stem packing. This type of valve is common for larger valves.Rising stem with inside screw– The threaded part of the stem is inside the valve body, and is in contact with the flow medium. When rotated, the stem and the hand-wheel rise together to open the valve.Non-rising stem with inside screw– The valve disc travels along the stem, like a nut as the stem is rotated. Stem threads are exposed to the flow medium so this model is appropriate when space is limited to allow linear movement, and the flow medium does not cause erosion, corrosion, or abrasion of the stem material.Sliding stem– The valve stem slides in and out of the valve to open or close the valve. This design is for hand-operated, rapid opening valves, and control valves that operate by hydraulic or pneumatic cylinders.Rotary stem– This is a commonly used model in ball, plug, and butterfly valves. A quarter-turn motion of the stem opens or closes the valve.
Image Credit: Pipe Fittings and Flanges
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Material of Pinch Valve Construction
The body of the pinch valve should be made of a lightweight material for easy handling. The material of the body does not need to be compatible with the media since generally they do not come into contact. The pinch valve liner must be manufactured of natural or synthetic rubbers and plastics which have good abrasion resistance. This is important because if the material of the sleeve is damaged by the media, the flow will be interrupted. If the material is not flexible enough it can develop a memory and will no longer fully open. This causes the material to protrude into the media stream. By interrupting the media stream the pressure drop is increased and turbulence and abrasion of the liner wall is increased.
Common liner material includes polytetrafluoroethylene, Neoprene, Buna-N, and Viton.
Selection Tip: Pinch valve should not be used in pulsating flows because the liner continuously expands and contracts, causing premature failure.
Pinch Valve Actuator
The valve actuator operates the stem and disc to open and close the valve. Pinch valves are available with several different types of actuators. Pinch valves may be closed either by manual means, or fluid actuation. Electromechanical closure is effected by actuating a solenoid, which then lowers a bar or gate onto the sleeve, cutting off the flow. With fluid actuated pinch valves, the pinching action is accomplished by air or hydraulic pressure placed directly on the elastomer sleeve.
Manual/ hand operated actuators use a hand-wheel or crank to open or close the valve. They are not automatic but offer the user the ability to position the valve as needed. The hand-wheel is connected to a threaded bonnet and threaded stem. This is used to adjust the height of the compressor when operating the valve.
Solenoid operated valves use hydraulic fluid for automatic control of valve opening or closing. A solenoid is a designed electromagnet. When an electric current is applied, a magnetic field is generated around the wire. An iron “T" or plunger is put in the center of the coil to concentrate the magnetism. The movable “T" acts as the actuator of the valve. Solenoid valves can be arranged such that power to the solenoid either opens or closes the valve. Solenoid actuated valves operate faster than pneumatic valves because electrons can flow into the solenoid coil very quickly to induce a magnetic field. This type of actuator can be very loud.
Electric motor actuators permit manual, semi-automatic, and automatic operation of the valve. The high speed motor is usually reversible and used for open and close functions. The relationship between the energized and non-energized time of an electric pinch valve is expressed as a percentage called the duty cycle. The percentage is calculated by (ON time)/ (ON time +OFF time). The actuator is operated either by the position of the valve or by the torque of the motor. A limit switch can be included to automatically stop the motor at fully open and fully closed.
Pneumatic operated pinch valves can be automatic or semi-automatic. They function by translating an air signal into valve stem motion by air pressure acting on a diaphragm or piston connected to the stem. Pneumatic actuators are fast-acting for use in throttle valves and for open-close positioning. The plunger of a pneumatic pinch valve encounters a resistant spring force which increases linearly as the plunger advances toward closing. Therefore the plunger decelerates as it advances, but this dampens the sound of the actuator in the system.
Hydraulic actuators provide for semi-automatic or automatic positioning of the valve. With no fluid pressure, the spring force holds the valve in the closed position. Fluid enters the chamber, changing the pressure. When the force of the hydraulic fluid is greater than the spring force, the piston moves upward and the valve opens. To close the valve, hydraulic fluid (such as water or oil) is fed to either side of the piston while the other side is drained or bled. Hydraulic actuators are available in a wide range of sizes and are economical to use in a valve system as well as with a single valve.
Actuation of a pinch valve. Video Credit: AKOUKValves / CC BY-SA 4.0
Due to the wide variety and variations in valves, the actuator must be sized to the specific valve in the system. If the actuator is undersized, it will be unable to overcome the forces against it, causing slow and erratic stroking. If the actuator is not stiff enough to hold the closed position, the closure element will slam into the seat, causing a pressure surge. If the actuator is oversized, it will cost more, weigh more, and be more sluggish in terms of speed and response. Larger actuators may also provide a higher thrust that will damage internal valve parts. Actuators tend to be oversized because of safety factors but smaller sizes function just as well when built-in safety factors are considered.
Pinch valves should be designed to fit between flanges of the connecting piping to prevent the need for extra space for compression of the flange sleeves. This makes it easier to remove the valve for service. Optional sterility features include end flange configurations that connect flush with the transport tubing. In situations where the tubing does not connect flush, seals in both the valve and fittings are needed to eliminate particle entrapment and facilitate in-line cleaning.
The liner of the valve can have extended hubs and clamps designed to slip over a pipe end. Pinch valves can also use a flanged end with standard dimensions.
Pinch valves are generally used for applications with a maximum operating temperature close to 250°F. At this temperature, operating pressure varies from 100 psig for a 1-inch diameter valve and decreases to 15 psig for a 12-inch diameter valve. For temperature ranges of -100°F to 550°F and operating pressures of 30 psig, special pinch valves should be used.
“Non-wetted" and “Wetted"
Non-wetted and wetted are terms used to describe the body and stem design. Most pinch valves have few wetted parts. Generally the liner is the only wetted component. This is important if the media is corrosive or needs to be kept sterile.
- Non-wetted valves have the stem and body isolated from the media in the system. Therefore, the stem and body do not need to be made of a corrosive resistant material
- Wetted valves leave the stem and body exposed to the media in the line
Normally Open Vs. Normally Closed
Pinch valves can be normally open or normally closed in the system. Normally closed (N/C) means that the valve blocks flow by pinching the tubing when the valve is de-energized. Normally open (N/O) means that the valve is open to allow flow when the valve is de-energized. Solenoid valves are normally open, while pneumatic valves can be normally open or normally closed.
The pinch force is the force exerted by the pinch valve on the tubing in the closed state. The force must be strong enough to occlude flow. The pinch gap is the distance between the two pinching surfaces when the valve is closed while the total opening is the distance between the opposed pinching surfaces when a pinch valve is open. This is the total of pinch gap and stroke.
The valve flow coefficient is the number of U.S. gallons per minute of 60°F water that will flow through a valve at a specified opening with a pressure drop of 1 psi across the valve. The coefficient is used to determine the size that will best allow the valve to pass the desired flow rate, while providing stable control of the process fluid. For a control valve, the flow rate is related to the opening of the valve. There are two relationships available to determine flow rate.
- Linear– The flow rate is directly proportional to the amount the disc travels. If the disc is open 50%, the flow rate is at 50% of maximum flow.
- Equal percentage– The flow rate is related to the percent the valve opening changed in an incremental manner. For example, if the valve changed from 20% open to 30% open and produced a 70% change in flow rate, changing the valve from 30% to 40% open would increase the flow rate another 70%.
Image Credit: Enggcyclopedia.com
Variables for flow calculations
Pinch valves can only effectively be used in low-pressure systems because of the elastomer materials they are made from. The pressure limit can be increased by utilizing liners or body designs with metal mesh woven into the rubber or by injecting an outside fluid (under pressure) around the liner to offset the fluid pressure. Since pinch valve liners are commonly made from the same material as rubber hoses, rubber-hose pressure ratings are used in lieu of valve pressure ratings.
The pinch valve might fail if the pressure inside the system moves towards a vacuum or there is a high pressure drop. Collapsing of the disc can happen if the valve is in the open position and not attached to the closure mechanism. The pressure drop is the pressure change between the inlet and outlet of the system. The formula is as follows:
ΔP = G (q/FpCv)2
If the pressure drop is too high, a larger valve or a valve with a higher Cv can be used to lower the pressure.
The valve flow coefficient is the number of U.S. gallons per minute of 60°F water that will flow through a valve at a specified opening with a pressure drop of 1 psi across the valve. The coefficient is used to determine the size that will best allow the valve to pass the desired flow rate, while providing stable control of the process fluid. It can be used to compare flow capacities of valves of different sizes, types, and manufactures. The flow coefficient is different for gases, liquids, and steam and is also dependent on the pressure drop across the valve. The Cv calculated will apply to either the opening or closing depending on the function.
For liquids: Incompressible media- The flow rate only depends on the difference between the inlet and outlet pressures so the rate stays the same as long as the change in pressure remains the same.
Cv= q/Fp √ (G/ΔP)
If Cv calculated is too small the valve will be undersized and the process system may be starved for fluid. This also causes a higher pressure drop across the valve causing cavitation or flashing. If Cv is too high the valve will be too big leading to a waste of money and the machine being too difficult to maneuver. A larger Cv can also be a problem for throttling because the flow cannot be effectively controlled at the openings. The location of the closure element leads to the possibility of creating a high pressure drop and faster velocities causing cavitation, flushing or erosion.
The flow characteristic describes the relationship between the flow coefficient and the valve stroke. It is inherent to the design of the selected valve. For example, as the valve is opened, the flow characteristic allows a certain amount of flow through the valve at a particular percentage of the stroke. This is especially important for throttle control because it controls the flow in a predictable manner.
The science behind valve sizing is in determining the flow through the diameter of the valve. Valve sizing is almost always done for throttling valves and should also be considered for open/close valves.
If the pinch valve will be used for open/close applications, the valve is expected to pass 100% of the flow without a significant drop in pressure. This process does not throttle the flow so the openings are generally the same size. If the valve is too small, the flow will be restricted, defeating the point of the on/off valve. A large valve will cost more because increasers will need to be installed.
Throttle pinch valves are expected to produce a certain amount of flow at certain levels of opening to create a pressure drop. Throttle valves work best when the valve uses the full range of stroke while producing desired flow characteristics and maximum flow output.
Oversizing a valve happens more frequently than under-sizing because the manufacturer adds safety factors to the specifications they receive from the user, which are generally the maximum specifications of the system. Having an over-sized valve is more manageable and safer than undersized valves
The use of increasers or reducers to create nonstandard piping configurations can be corrected in the Cv equation. In order to determine the piping geometry factor, Fp, the inside diameter of the pipe is required. “d" is the inside diameter and “D" is the outside diameter.
Piping- geometry factor for valves with reducers and increasers on both ends. Table Credit: Valtek International
Pinch valves can be used for a wide variety of manufacturing and industrial applications. Some typical applications for pinch valves are medical, pharmaceutical, wastewater, slurries, pulp, powder and pellets. They are the preferred valve in many industries and can be used for harsh medias.