Types , Principles ,And Application
Introduction
Check valves are designed to prevent the reversal of flow in a piping system. These valves are
activated by the flowing material in the pipeline. The pressure of the fluid passing through the
system opens the valve, while any reversal of flow will close the valve. Closure is accomplished
by the weight of the check mechanism, by back pressure, by a spring, or by a combination of
these means. The general types of check valves are swing, tilting-disk, piston, butterfly, and
stop.
Valve Types
There are essentially four major types of check valves:
· Swing Check Valves
· Tilting Disk Check Valves
· Lift Check Valves
· Dual Disk In-Line Check Valves
1- Swing Check Valves
A swing check valve is illustrated in Figure 1. The valve allows full, unobstructed flow and automatically closes as pressure decreases. These valves are fully closed when the flow reaches
zero and prevent back flow. Turbulence and pressure drop within the valve are very low.
A swing check valve is normally recommended for use in systems employing gate valves because
of the low pressure drop across the valve. Swing check valves are available in either Y-pattern
or straight body design. A straight check valve is illustrated in Figure 1. In either style, the disk and hinge are suspended from the body by means of a hinge pin. Seating is either metal-tometal or metal seat to composition disk. Composition disks are usually recommended for services where dirt or other particles may be present in the fluid, where noise is objectionable, or where positive shutoff is required.
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| FIG 1 |
Straight body swing check valves contain a disk that is hinged at the top. The disk seals against the seat, which is integral with the body. This type of check valve usually has replaceable seat rings. The seating surface is placed at a slight angle to permit easier opening at lower pressures, more positive sealing, and less shock when closing under higher pressures. Swing check valves are usually installed in conjunction with gate valves because they provide relatively free flow. They are recommended for lines having low velocity flow and should not be used on lines with pulsating flow when the continual flapping or pounding would be destructive to the seating elements. This condition can be partially corrected by using an external lever and weight.
2- Tilting Disk Check Valves
The tilting disk check valve, illustrated in Figure 2, is similar to the swing check valve. Like
the swing check, the tilting disk type keeps fluid resistance and turbulence low because of its
straight-through design
Tilting disk check valves can be installed in horizontal lines and vertical lines having upward Figure 2 Operation of Tilting Disk Check Valve flow. Some designs simply fit between two flange faces and provide a compact, lightweight installation, particularly in larger diameter valves.
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| FIG 2 |
on the flow. Disk stops built into the body position the disk for optimum flow characteristics.
A large body cavity helps minimize flow restriction. As flow decreases, the disk starts closing
and seals before reverse flow occurs. Back pressure against the disk moves it across the soft seal
into the metal seat for tight shutoff without slamming. If the reverse flow pressure is insufficient
to cause a tight seal, the valve may be fitted with an external lever and weight.
These valves are available with a soft seal ring, metal seat seal, or a metal-to-metal seal. The
latter is recommended for high temperature operation. The soft seal rings are replaceable, but
the valve must be removed from the line to make the replacement.
3- Lift Check Valves
A lift check valve, illustrated in Figure 3, is commonly used in piping systems in which globe
valves are being used as a flow control valve. They have similar seating arrangements as globe
valves.
Lift check valves are suitable for installation in horizontal or vertical lines with upward flow.
They are recommended for use with steam, air, gas, water, and on vapor lines with high flow
velocities. These valves are available in three body patterns: horizontal, angle, and vertical.
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| FIG 3 |
is raised within guides from the seat by the pressure of the upward flow. When the flow stops
or reverses, the disk or ball is forced onto the seat of the valve by both the back flow and
gravity.
Some types of lift check valves may be installed horizontally. In this design, the ball is
suspended by a system of guide ribs. This type of check valve design is generally employed in
plastic check valves.
The seats of metallic body lift check valves are either integral with the body or contain
renewable seat rings. Disk construction is similar to the disk construction of globe valves with
either metal or composition disks. Metal disk and seat valves can be reground using the same
techniques as is used for globe valves.
3-1 Ball Type Lift Check Valves
Ball check valves are very similar in design to piston check valves, except the piston closing element is replaced with a spherical ball. The advantage of using a ball as the closing device is that the ball can rotate with flow, utilizing the entire spherical area as a seating surface. Used in viscous services and services where particulates are present, the increased available seating surface and the rotation during flow help to keep the spherical
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| FIG 4 |
3-2 Piston Type Lift Check Valve
Generally, piston style lift check (also commonly referred as a piston check valve) valve bodies are of Y-pattern design, with the flow coming up underneath the piston closing element. Note that this arrangement is similar to a globe valve. The closing element is normally contained in a cage-type guiding element to control lateral movement of the piston, assuring alignment during opening and closing.
Manufacturers frequently design piston lift check valves with springs above the piston to increase the closing speed. Seat contact between the piston seat and the valve seat is generally conical. The seat angles and the degree of angular mismatch between the piston and valve seat are varied between manufacturers and the applications for which the valves are designed.
One of main advantages of piston lift check valves is the fast closure response to reverse flow. Lift check valves are very sensitive to fluctuations in flow and offer an extremely
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| FIG 5 |
quick closure time. Due to the fact that they have very few moving parts (normally just the piston and in some cases a spring), wear due to impact and bearing loads is minimized. However, due to their quick closure time, lift check valves can be susceptible to repeated disk slam in pulsing flow applications, such as downstream of a positive displacement pump or compressor. This application has been addressed by the development of a “nonslamming” lift check valve design.
Generally, nonslamming lift check valves are designed with vents between the upper bonnet chamber and the downstream pressure by means of machined orifices through the upper guiding surface of the piston. These vents act to dampen the impact of the piston against the bonnet and the seat by increasing the response time to differential pressure reversals. The design of the vents are varied, some even utilizing miniature ball check valve in the piston vent orifices to provide slow piston movement in only one direction. The piston is sealed against the cage or body by very tight machining tolerances or by seal rings installed in machined grooves on the piston guiding surfaces. The purpose of the seal rings is to ensure controlled dissipation of fluid in the bonnet chamber through the piston orifices and the prevention of leak-by between the piston and the guiding surface.
The main disadvantages of piston lift check valves are small Cvs (large pressure drops for given flowrates), difficulty of maintenance and susceptibility to “sticking” in fluid systems containing particulates. The low Cv is caused by the tortuous flowpath through the valve body. These valves may also have higher cracking pressures due to the disk weight, especially in larger valve sizes. Additionally, any particulates in the system fluid can cause disk to become bound in the disk chamber due to the tight tolerances between the piston and the guiding surfac
4- Piston Check Valve
A piston check valve, illustrated in Figure 6, is essentially a lift check valve. It has a dashpot consisting of a piston and cylinder that provides a cushioning effect during operation. Because of the similarity in design to lift check valves, the flow characteristics through a piston check valve are
essentially the same as through a lift check valve. Installation is the same as for a lift check in that the flow must enter from under the seat. Construction of the seat and disk of a piston check valve is the same as for lift check valves.
Piston check valves are used primarily in conjunction with globe and angle valves in piping systems experiencing very frequent changes in flow direction. Valves of this type are used on water, steam, and air systems.
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| FIG 6 |
The third type of lift check valve is the straight through poppet lift check valve. These valves are designed such that the disk is centered in the flow stream such that the full disk area is always available to the fluid pressure during both forward and reverse flow. The disk is attached to a “rod” which passes through guiding sleeves designed to provide alignment of the disk to the valve seats. A spring is installed on the downstream side of the disk to increase the closing speed during the initiation of reverse flow and to provide a slightly positive cracking pressure. The spring selection is analyzed for the particular valve application.
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| FIG 7 |
Poppet check valve designs range from the very simple and inexpensive foot valves (such as those used in vertical sump pumps in non-critical commercial applications), to the highly engineered “nozzle check valve” design. Poppet lift check valves are extremely good for fast closure applications due to their short disk travel, low minimum velocity and generally low disk mass. These attributes make poppet lift check valves especially good in systems were water hammer is a concern. Another advantage of this valve design is that the minimum required velocity is less than that of swing, tilting disk, piston lift and ball check valves.
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| FIG 8 |
6- Butterfly Check Valves
Butterfly check valves have a seating arrangement similar to the seating arrangement of butterfly valves. Flow characteristics through these check valves are similar to the flow characteristics
through butterfly valves. Consequently, butterfly check valves are quite frequently used in systems using butterfly valves.
In addition, the construction of the butterfly check valve body is such that ample space is provided for unobstructed movement of the butterfly valve disk within the check valve body without the
necessity of installing spacers.
The butterfly check valve design is based on a flexible sealing member against the bore of the valve body at an angle of 45o.
The short distance the disk must move from full open to full closed inhibits the "slamming" action found in some other types of check valves. Figure 9 illustrates the internal assembly of the butterfly check valve.
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| FIG 9 |
As with butterfly valves, the basic body design lends itself to the installation of seat liners constructed of many materials. This permits the construction of a corrosion-resistant valve at less expense than would be encountered if it were necessary to construct the entire body of the higher alloy or more expensive metal. This is particularly true in constructions such as those of titanium.
Flexible sealing members are available in Buna-N, Neoprene, Nordel, Hypalon, Viton, Tyon, Urethane, Butyl, Silicone, and TFE as standard, with other materials available on special order. The valve body essentially is a length of pipe that is fitted with flanges or has threaded, grooved, or plain ends. The interior is bored to a fine finish. The flanged end units can have liners of various metals or plastics installed depending upon the service requirements. Internals and fasteners are always of the same material as the liner.
7- Stop Check Valves
A stop check valve, illustrated in Figure 10, is a combination of a lift check valve and a globe valve. It has a stem which, when closed, prevents the disk from coming off the seat and provides a tight seal
(similar to a globe valve). When the stem is operated to the open position, the valve operates as a lift check. The stem is not connected to the disk and functions to close the valve tightly or to limit the travel of the valve disk in the open direction.
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| FIG 10 |
A dual disk check valve is actually a type of swing check valve that has been specially modified for piping systems where a low face-to-face profile is necessary due to constraints.
The dual disk design contains a disk hinge similar to that in a swing check valve. Two “half-circle” shaped disks are connected and hinged by a single “disk shaft” (analogous to a swing check valve’s hinge pin). The disk shaft is a hardened metal rod inserted through a bore in the body wall. A threaded retaining plug seals the body bore hole. Normally, two torsion springs are placed in contact with the two disks, with the disk shaft running through the inside diameter of the torsion springs. Each spring has two legs, one leg contacting each disk. The contact point between the disk and the spring is usually located beyond the centroids of the disks in order to minimize seat wear. As the disks move to the open position, the springs are placed in torsion. The purpose of the springs is to increase the closing speed and to provide a slight positive cracking pressure. Dual disk check valves normally incorporate a backstop into the valve design to prevent each disk from traveling more than 85° from the seated position. The purpose of limiting the travel is twofold:
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| FIG 11 |
· It ensures that enough of the downstream disk surface area is available to sense a reversal of flow,
· It prevents one of the disks from opening more than 90° and being pinned open by the reverse flow.
Without a backstop, the valve may not close fast enough or at all. The disk travel limiting attribute of the dual disk design is either integral to the geometry of the disks or is performed by a backstop component, normally a shaft running through the body and placed such that interference occurs at a disk angle < 85°.
Valve Sizing
When choosing a check valve for a particular application, the flow of the system must be analyzed to determine the correct size and type of valve. There are two primary areas of concern when sizing a check valve:
- The pressure drop across the valve
- The minimum required velocity required to keep the disk in the full open position.
Pressure Drop Calculation Equations
The pressure drop across a check valve can be calculated by the following equations:
Fp = piping geometry factor (dimensionless), obtained by calculation
K = resistance coefficient (dimensionless), obtained by manufacturer
m = mass flow rate (lb/sec), obtained by measurement
𝜌 = fluid density (lb/ft³), obtained by physical properties tables
d = inside pipe diameter (in), obtained by piping tables
w = mass flow rate (w = m x 3600) (lb/hr), obtained by calculation or measurement
Cv = flow coefficient (dimensionless), obtained by manufacturer
S = specific gravity of a liquid relative to water (dimensionless), obtained by physical properties table
q = volumetric flow rate (gpm), obtained by calculation
Minimum Velocity Calculation Equations
necessary to calculate the minimum required velocity for a specific valve to ensure that the valve will be fully open during operation. If system flow velocities are less than the minimum required velocity specified by the valve manufacturer for a specific valve size and model, stable operation of the valve will not be maintained. Excessive wear due to disk oscillations could result. The minimum required flow velocity is calculated by the following equation
C = minimum required velocity constant (dimensionless), obtained by manufacturer
𝛖 = specific volume (ft³/lb), obtained by physical properties tables
The following calculation can be used to calculate the flow through a valve with a Cv
value obtained from the Vendor and a measured differential pressure:
Seat leakage can be the result of a number of adverse conditions. The most common are:
· Seat erosion due to cavitation, abrasive service or high velocity flow
· Corrosion of the seating elements
· Particulates or other seat contact obstructions
· Excessive wear/binding
· Misapplication, and
· Inadequate seat contact force
Advantages of Check Valves
They are self-actuated and require no external means to actuate the valve either to open or close. They are fast acting.
Disadvantages of Check Valves
The following are some of the disadvantages that are attributed to check valves:
1. Since all moving parts are enclosed, it is difficult to determine whether the valve is open or closed. Furthermore, the condition of internal parts cannot be assessed.
2. Each type of check valve has limitations on its installation configurations.
3. Valve disc can stick in open position.
Application of Check Valves
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