Introduction to Check Valves
Check Valves, also known as Non Return Valves (NRV) or One Way Valves, are automatic valves that normally allows fluid (liquid or gas) to flow through it in only one direction. Check valve open with forward flow and close against reverse flow. Check valves differ considerably from other members of the valve family and the prime function of a check valve is to protect mechanical equipment in a piping system by preventing reversal of flow by the fluid. Check valves are entirely automatic in their operation having no requirement for any operator or external control; so most do not have any valve handle or stem. Check valves are powered by the fluid flow and the pressure difference that exists between their inlet and outlet ports.
Flow reversal in fluid-handling systems is possible and can be caused by accidental
equipment stoppage or automatic fail-safe devices. For this reason, check valves are installed in pipelines to prevent such a back flow and to permit the flow of product in only one direction. When the flow is moving in the proper direction, the valve remains open. When the flow reverses, the valve closes automatically. Use of check valve is particularly important in the case of pumps and compressors, where back flow could damage the internals of the equipment and cause an unnecessary shutdown of the system and in severe cases the complete plant.
Few Important Technical Terminologies Used in Check Valve Operations;
Cracking Pressure – Refers to the minimum differential upstream pressure between inlet and outlet at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure. Cracking pressure is also known as unseating head (pressure) or opening pressure.
Reseal Pressure – Refers to the pressure differential between the inlet and outlet of the valve during the closing process of the check valve, at which there is no visible leak rate. Reseal pressure is also known as sealing pressure, seating head (pressure) or closing pressure.
Back Pressure – Refers to the pressure differential between the inlet and outlet of the check valve when the pressure is higher at the outlet of the valve than that at the inlet or a point upstream of the valve.
Check Valve Working Principle
A check valve requires a minimum upstream pressure (pressure differential between inlet and outlet), also known as cracking pressure, to open the valve and allow fluid flow through it. If the upstream pressure ever falls below the cracking pressure or there is a back pressure (flow attempting to move from the outlet to the inlet), then the check valve will close automatically. Depending on the check valve design, the closing mechanism can change and the closing process can be assisted by a spring or gravity. The back pressure pushes the closing mechanism (a gate, ball, diaphragm, or disc) against the orifice and seals it.
Types of Check Valves
Check valves may be grouped according to the way the closure member moves onto the seat. Few basic and most used types of check valves are listed and detailed out below:
- Swing Check Valve – The closure member swings about a hinge, which is mounted outside the seat.
- Piston or Lift Check Valves – The closure member travels in the direction normal to the plane of the seat.
- Ball Check Valve – The closure member consists of a ball, which travels in the direction normal to the plane of the seat and rotates constantly.
- Tilting-Disc Check Valve – The closure member tilts about a hinge, which is mounted near, but above, the center of the seat.
- Diaphragm Check Valve – The closure member consists of a diaphragm, which deflects from or against the seat.
- Stop Check Valve – Is a combination of a lift check valve and a globe valve. It can either be used as a check valve or as an isolation (stop) valve like a globe valve.
- Spring Loaded Check Valve – The closure member consists of a center guided stem-disc assembly along with a compression spring.
- Duckbill Check Valve – The closure member consists of a soft tube of which the end has a natural flattened shape.
Swing Check Valve
The Swing Check Valve is the most popular of all check valves, offering very little resistance to the flow when it is wide open. The closing element (disc) swings open and shut like a trap door about a hinge outside the seat. A replaceable composition washer may be fitted on the disc and takes most of the wear. A loose pin hinge ensures the disc closes when the flow diminishes to zero.
Swing check valves should be mounted in the horizontal position, but may also be mounted in the vertical position, provided the disc is prevented from reaching the stalling position. Swing check valves are companions to gate valves and are commonly used on all lines where fluid-friction loss is of prime importance. They are recommended for applications where the reversal of flow is not so frequent, to prevent “valve chattering”. Dirt and viscous fluids cannot easily hinder the rotation of the disc around the hinge.
As the size of swing check valves increases, weight and travel of the disc eventually become excessive for satisfactory valve operation. For this reason, swing check valves larger than about NPS 24 are frequently designed as multi-disc swing check valves, and have a number of conventional swing discs mounted on a multi-seat diaphragm across the flow passage in the valve. Double Disc Swing Check Valve, as shown in adjoining figure, design reduces the length of the path along which the center
of gravity of the disc travels and it also reduces the weight of such a disc by about 50%, compared with single-disc swing check valves of the same size.
Lift or Piston Check Valves
The Lift Check Valves (also known as Piston Check Valves) have a guided plug which is lifted by the higher pressure of inlet or upstream fluid to allow flow to the outlet or downstream side. When the inlet pressure is no longer higher, gravity or higher downstream pressure will cause the disc to lower onto its seat, shutting the valve to stop reverse flow. Lift check valves have an advantage over most other types of check valves in that they need only a relatively short lift to obtain full valve opening.
In the majority of lift check valves, the closure member moves in a guide to ensure that the seating’s mate on closing. However, such guiding also has a disadvantage in that dirt entering the guide can hang up the closure member, and viscous liquids will cause lazy valve operation or even cause the closure member to hang up. These types of lift check valves are therefore suitable for low viscosity fluids only, which are essentially free of solids. This type of check valve does not have the tendency to slam as does the Swing Check Valve and is preferred for installations having irregular or frequent reversals of flow.
The flow through the lift check valve changes direction twice as it passes through a horizontal section upon which the disc seats. The disc moves vertically upward to allow the flow to take place and moves downward to close if the flow should reverse. A dashpot is used to cushion the action of the disc. Lift check valves are manufactured also in vertical designs for vertical mounting on piping.
Ball Check Valve
Ball Check Valve is a is a check valve in which the closure member is ball shaped and allowed to travel without being closely guided. When the valve closes, the ball-shaped closure member rolls into the seat to achieve the required alignment of the seating’s. In some ball check valves, the ball is spring-loaded to help keep it shut. For those designs without a spring, reverse flow is required to move the ball toward the seat and create a seal.
The ball check valve is intended to handle viscous fluids and services where scales and sediments are present. This design prolongs the life of the valve, cutting wear on the ball and seat to the minimum. Ball check valves are quiet in operation and are recommended for applications of rapidly changing fluid flows. The disc has a true spherical shape and is accurately guided to the seat. The seats are usually replaceable, and both seat and ball can be removed without opening the pipe.
Ball check valves should not be confused with ball valves, which is a different type of valve in which a ball acts as a controllable rotor to stop or direct flow.
Tilting-Disc Check Valve
Tilting-Disc Check Valves have a disc-like closure member that rotates about a pivot point between the center and edge of the disc and is offset from the plane of the seat. The disc drops thereby into the seat on closing, and lifts out of the seat on opening. Because the center of gravity of the disc halves describes only a short path between the fully open and the closed positions, tilting-disc check valves are potentially fast closing. This particular valve is, in addition, spring-loaded to ensure quick response to retarding forward flow.
Tilting-disc check valves have the disadvantage of being more expensive and also more difficult to repair than swing check valves. The use of tilting disc check valves is therefore normally restricted to applications that cannot be met by swing check valves.
Diaphragm Check Valve
Diaphragm Check Valves use rubber flexing diaphragms to control backward or reverse flow. The self-centering flexible diaphragm disc is the working part of diaphragm check valve. The valve body keeps the disc centered over the seating area or sealing surface of the valve. The sealing surface is on the inlet side of the check valve body. It has a fine, smooth surface that is usually concave or curved back towards the inlet port. There is an opening at its center. Backward or reverse flow causes the flexible rubber disc to lie down across the curved sealing surface. This closes the opening to the inlet port preventing upstream flow back through the inlet port.
A floating disc diaphragm check valve has a particular design on the outlet side of the valve body. Here there is usually some sort of two-level valve seat with built in channels. Positive flow towards the outlet or exit port causes the disc to lie against this seat. The media then flows around the disc, through the channels and out through the exit port of the check valve. High viscosity or high specific gravity liquids may cause them to malfunction.
Stop Check Valve
Stop Check Valves are usually constructed similar to a normal check valve but have an additional external control mechanism (such as an actuator, lever, or hand-wheel) that enables the valve to be deliberately closed regardless of flow pressure. Stop check valves are a modified version of the standard globe stop valve, which has the valve stem permanently affixed to the globe disc. In the stop check configuration, the stem head floats in the globe disc (i.e., it is not attached). The internal disc, which is not attached to the stem, performs as a lift check allowing it to freely move up and down when the stem is raised to adjust the opening and closing. This controls the flow rate, but when backward flow occurs, the disconnected disc functions as a piston check and quickly closes, thus preventing reverse flow into the boiler. If needed, the stem can be manually lowered for flow to be stopped or completely shut off.
Stop check valves have two main purposes:
- As a globe valve, they isolate or regulate flow.
- Modified as a check valve, they prevent reverse flow.
In other words, they are generally used as a globe valve to start or stop the flow of the media, but they also act as a check valve to automatically close thus preventing backward flow that could possibly cause damage to equipment such as boilers or pumps. Because of this, having a stop check valve is like having two valves in one.
Spring Loaded Check Valve
Spring Loaded Check Valves (also known as Nozzle Check Valves or Silent Check Valves) typically incorporate a center guided stem-disc assembly along with a compression spring. This means the disc stays in the flow stream. The media flows around it and does not require manual or automated assistance to operate. When flow enters the input port of the valve, it has to have enough pressure (force) to overcome the cracking pressure and the spring force. Once overcome, it pushes the disc, opening the orifice and allowing flow to move through the valve. When the input pressure is no longer high enough, or there is a back pressure, then the back pressure and spring push the disc against the orifice and seal the valve shut.
The spring, along with the short travel distance for the disc, allows for quick reaction time for closing. This valve design also prevents against pressure surges in the line, and therefore, also prevents a water hammer from occurring. They can be installed in a vertical or horizontal orientation. However, since they are in-line to the system, they have to be fully removed from the line to be inspected and/or perform maintenance.
Duckbill Check Valves
Duckbill Check Valves are unique, one-piece, elastomeric components that act as backward flow prevention devices or one-way valves or check valves. Duckbill check valve enable flow to proceed through a soft tube that feeds into the downstream side of the valve wherein back pressure collapses the tube and cuts off the flow. Duckbill check valves are of a flexible sleeve design made of quality fabric like reinforced elastomer materials for maximum resistance to corrosion and wear caused by continuous operation with abrasive slurries, sludge, or effluent.
Duckbill check valves have following charaterstics;
- No mechanical parts.
- Will not corrode.
- Self Draining which eliminates standing water.
- Low Head loss.
- Low Cracking Pressure.
- Non Slamming silent operation.
Check Valve Flow Direction
As Check Valve only works in one direction, it is crucial to understand the correct installation orientation. Usually there is an arrow on the valve housing to signal the flow direction. Otherwise, one will need to examine the valve to ensure it is installed in the intended flow direction. If it is backwards, flow will not be able to move through the system and a build-up of pressure could cause damage.
We must make sure that the particular check valve type will work in the said installed position. For example,
- Not all check valves will work in a vertical line with fluid flow downward.
- Not all conventional or 90-degree piston check valves perform in a vertical line without a spring to push the disc back into the flow path.
- The disc in some check valves extends into the pipeline when the valves are fully open. This could interfere with the performance of another valve bolted directly to the check valve.
Check Valve Applications
- Check vales are used in the case of pumps and compressors, where back flow could damage the internals of the equipment and cause an unnecessary shutdown of the system.
- Centrifugal pumps, the most common type of water pumps, are not self-priming, and therefore check valves are essential for keeping water in the pipes.
- Check valves may also be used in lines feeding a secondary system in which the pressure can rise above that of the primary system. This pressure difference may cause reverse flow.
- Check valves are often used when multiple gases or fluids are mixed into one stream. A check valve is installed on each of the individual fluid streams to prevent mixing of the fluid in the original source causing contamination.
- Other than with steam traps discharging to atmosphere, check valves should always be inserted after a steam trap to prevent back flow of condensate flooding the steam piping.
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