A COMPLETE GUIDE TO CHECK VALVES

Table of Contents:

Swing Check Valves

This section deals with check valves, which are inherently different from almost all other types of valves in that no outside intervention is required for the valve to perform its designated function. Check valves, which are more generically known as non-return valves, are intended for control of fluid flow just like any other valve, but their contribution to the system is to keep fluid from returning in the direction that it came from. This principle is used widely in nature; the human body contains a number of check valves.

In piping systems, there are a number of ways to prevent backflow without using check valves. Any system that flows by gravity is automatically protected from backflow just by having one portion of the system higher than another. Having an air gap or a spot where the fluid has to fall freely from one part of the system to another is an even better isolation method. This method is often required where absolute separation is needed, for instance, in connections to potable water systems where the clean water may be used for other purposes but where it is unacceptable to have any possibility of contaminated water flowing back into the drinking water.

For the more usual industrial piping system where all parts of the system are pressurised, some type of valve is needed. The earliest type of check valve to be developed was based on the door or gate analogy. Like a door built to open in one direction but to come to a stop against a sill or jam that prevents it from swinging both ways, the swing check valve has the ability to open freely when fluid pushes against it. In one direction the door, or disc, opens wide, and in the other direction the disc swings until it hits the seat.

Many check valves close dependably enough, but in the closed position they really are not closed tight. Fluid can get around a check valve disc a lot more easily than a person can get through a closed door. In most cases that is just the way it is, and there is not much you can do about it. The check valve is there to slow down the flow in the reverse direction, or to cut the mass flow rate down to about 1 or 5 percent in the backward direction of what the forward flow rate is. In some cases, a much better shutoff rate is required, and there are several varieties of check valve that can do that job.

The swing check still has a large portion of the check valve market, although there are many other types of check valves that are an improvement over the swing. Physically, a standard swing check valve looks like most other valves. It has flanges or other standard end connections, and it has a cover that is removable. The cover or cap is just that - there is no stem or other mechanisms to apply force or torque from the outside. The disc is attached to a swing arm or lever that is mounted on a bearing shaft. The arm pivots freely around this shaft, and the fluid swings the disc open or closed.

Actually, although this is the way that almost all swing checks are built, this simplistic picture is not entirely true. Since that shaft usually has to have been inserted from outside the body anyway for mechanical assembly reasons, the end of the shaft can be used for actuation just like any other valve. Often the reversal of flow is still required for the check to begin functioning, but a power actuator can be used to help in closing the valve. Sometimes a weight or a spring on the outside can also be used, as an external but still passive actuation aid. These springs or weights can be used either to assist in closing or to retard closing, depending on how they are connected and why they are needed. Theoretically, a valve built like a swing check could be actuated from the outside and used as a shutoff valve, but because of the torque required to close such a valve against line flow, such a scheme is almost never used.

The outside lever and weight, or lever and spring, requires a packing gland just like any other valve that has an actuation mechanism from the outside. However, since no positive actuation is applied, the force available is still limited to that of the weight or spring. This means that sometimes the force that the packing gland exerts is great enough to bind or restrict the pivoting motion. The outside lever design, therefore, is generally practical only in large valves. It is available in sizes down to about 6-in. pipe size (DN 150). It is most common in transportation and building service piping. When a positive closure is required in refinery piping, it is more common to install a different type of check that inherently possesses better closure characteristics or, alternately, some type of actuated valve that closes on a flow signal, usually in addition to a check valve.

Note that the normal swing check has a pivot that is inserted through the body wall in the same location that this packing gland would be, but there is no gland because the retainer pin does not extend through the wall. There is normally just a standard pipe plug on one side to plug the opening used to assemble the retainer pin into the valve. It is possible to design a swing arm retaining device that is entirely within the body, installed from the top or attached to the cap and retained with some bolted-on or welded-in device. These designs are slightly more expensive and potentially more complicated, but definitely do not provide the leak path to the outside that the plugged opening presents. It might be better if more swing checks were available with this feature, but they normally do not have it except in pressure-seal bonnet and other high-pressure designs. It is true that anything is available if you want it badly enough and are willing to accept the increased cost and (especially) delivery time that a special order implies.

The common swing check has a seat inclined at about the same angle as that of a wedge gate valve, and usually has a stop on the cap or the inside of the body, such that the disc travels about 60 degrees or a little more from fully closed to fully open. One of the reasons for having the inclined seat is that, when the valve is installed in the horizontal with the cap upward, the weight of the disc still exerts some force tending to hold it against the seat. In a vertical line, with the disc wide open, the disc has to remain unbalanced. That is, there still has to be some gravity force acting on the disc to tend to close it when the fluid flow stops. (Note that virtually no check valves, except a few very specialised ones, are capable of functioning in a vertical line with the flow downward.)

Like its gate and globe counterparts, the swing check valve is available in almost any rating, material, and type of end connection and cap design. Physically, the swing check is about the same size as a globe valve from the bonnet flange on down. In the lower pressure classes, then, it is somewhat longer face-to-face than a gate valve is. In higher pressure classes, the potential exists for rather high differential pressures. This can make the disc get heavier and harder to swing, and can slam the disc shut rather forcefully. Therefore high-pressure check valves such as are encountered in steam and high-pressure process services tend to be other designs than the swing type.

In small sizes, the opposite problem exists. Although the fluid flow provides most of the force to close a check valve, gravity is also required most of the time. Small check valves, say under 2-in. pipe size, do not have much mass in the disc and are more prone to some mechanical problem preventing the disc from closing. In stainless steel and bronze bodies, the swing check is still fairly common since corrosion of the swing arm and pivot is not likely, but small carbon steel valves are more likely to be some other type of check. An exception is an interesting little swing check made from the same forging that is used to make a tee, where the entire swing mechanism including pivot is inserted from the downstream end and welded in.

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Tilting-Disc Check Valves

The closest relative of the swing check is the tilting-disc check valve. The tilting-disc moves the same way as the swing, except that the pivot arm runs right through the disc instead of being above the disc. The overall motion of the disc is thus less, and the motion is more rotational than translational. The pivot must still be offset a certain distance from the centre of forces acting on the disc for the disc to open, but the distance is much less in the tilting-disc. The centre of mass of the disc is much closer to the pivot point, and in fact is normally located even closer to the pivot point than the centre of opening forces. This means that the tilting-disc opens more easily (with less resistance to flow and consequent pressure loss) than the swing check. Of course, all these statements apply equally well to closing forces.

The tilting-disc check was thus the first “non-slam" check, and to this day its primary attractiveness is that it can close more gently than the swing check can. It can also be designed to close very fast since the relative positions of the centre of mass and the centre of force can be varied in ways that are very impractical to do with a swing check.

It has certain disadvantages, mostly that it is not repairable in-line since it is not built with a cap. The body joint is along the seat line, at approximately a 45-degree angle to the ends and the pipe centerline. Otherwise, the mass and dimensions of a tilting-disc are comparable to those of the swing check. The angled seat is also one of the reasons for fast closing time: In the open position the disc turns in the direction of fluid flow like a butterfly valve, but in the closed position the disc is 45 to 60 degrees away from the pipe centerline, not 80 to 85 degrees as in a swing check.

The tilting-disc check is a rather specialised valve since to most industries it is used only in applications where its closure characteristics are needed. Probably its most common use is in water lines, such as industrial cooling water or municipal water, as a pump discharge check valve. Water lines tend to be among the largest lines in a plant, and the mass of water being moved can be quite large. When water hammer and other fluid flow disruptions occur in lines this size, the forces imposed on the piping can be enough to knock pumps out of alignment or knock piping off its supports. The tilting-disc check is designed to assist in combating this problem by closing fast, almost as soon as the moving column of water reaches a standstill and before it has a chance to begin moving backwards. One of the contributions to the piping loads is the inertia of the water column as it strikes an immovable object, the check valve disc that has closed in response to the water beginning to move backwards through it. If the valve can close at the precise time that the moving water has reached its lowest average velocity, the least amount of force will result.

There is a second, related niche that the tilting-disc check occupies. The high closing forces that are a problem with large masses of water are also a problem with high-velocity vapour flows subject to sudden fluctuations, such as in compressor discharges. Here the valve is normally configured to respond fairly slowly to the changes in flow rate, to reduce the tendency to slamming, which is highly destructive not only to the valve but to the rest of the piping system subject to such loads.

The tilting-disc check valve is available down to about 4-in. pipe size (DN I00), but it is most common in sizes on the order of 24 in. to 48 in. (DN 600 to DN 1200) for municipal water and pump discharge check service. In these services, the valve is generally cast iron or steel, normally rated for class 125 (PN 10) or class 150 (PN 16). The other situation that the tilting-disc appears in calls for a steel body, high-pressure-rated valve that is often butt-weld with a pressure-seal cover. These valves, by the way, are built more like a conventional swing-check in that the pressure rating makes the split body impractical so they have a bonnet or cover and a pivot arm that is accessible from the top. These two sets of service conditions are quite far apart in their requirements, and it can be very difficult to obtain tilting-disc check valves that fall in between these two extremes, for instance in a class 300 (PN 25) steel body.

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Lift Check Valves

The lift check valve operates on an entirely different principle from the swing and tilting-disc checks. As its name implies, the lift check lifts when fluid pushes on it from below. It is also known as the piston check since the disc moves up and down like a piston.

While the swing check shares the door analogy with the gate valve, the lift check is more closely related to the globe valve. In fact, in the small sizes where the lift check is most common, the body is the same as that of a globe valve, just with a cap instead of a bonnet, and often the disc, or piston, is the same.

The piston moves up and down freely, but some sort of guide is required. The guides consist of an internal guide attached to the cap, inside the piston and occupying the space where the stem attachment would go on a globe valve, or either a cylindrical surface or three or four guide ribs external to the piston and bearing on the cylindrical area outside of the piston above the seating area. Smaller lift checks, 2-in. pipe size (DN 50) and smaller, are generally of the former type, while piston checks larger than 2-in. size (DN 50) are often of the latter type. Larger piston checks are special-purpose designs, often designed with damping devices and used like other special checks for pulsating flows or cases of water hammer.

In sizes 2 in. (DN 50) and smaller, the piston check is by far the most common type. It is available in any material, from brass to high alloy, and in almost any rating and type of end connection. Check valves are not a high-volume item in comparison to block valves of any description, so the ability of the lift check to share the same body as the same-size globe valve brings with it an economy that makes the lift check cheaper than some of its competitors. It does share the double 90-degree change in flow direction with the globe valve, and the consequent pressure loss.

The garden-variety lift check operates by a combination of fluid forces and gravity-gravity being probably the stronger force most of the time. The piston must move in a true up and down orientation, so the cap of the valve must be reasonably close to upright. In a vertical line, the check will not work - the force of gravity holding the piston to one side is too much for the fluid flow to overcome. The piston is normally furnished with a pressure-equalizing hole through its side, above the seating area. This is required for the upstream line pressure to get behind the piston and help push it down.

In order to help the valve operate better, a spring is sometimes installed above the piston, pushing on the underside of the cap. This improves the operability of the valve since it helps overcome any tendency toward sticking in the open position and closes a little more tightly. It does generally mean that a little higher differential pressure is required to open the valve (this is called cracking pressure). Normally this pressure is on the order of a fraction of a psi (0.1 to 0.01 bar) and is not worth considering. A spring-loaded valve might close slightly better in an off-of-vertical position, but it still cannot be expected to close reliably in a vertical line.

There is a piston check that will operate in a vertical line just as well as it would in the horizontal. This is the Y-pattern piston check, and of course, it works equally well in vertical and horizontal lines because the piston and spring area is in exactly the same orientation regardless of which orientation the end connections are in. As a general rule, most small Y-pattern piston checks are spring-loaded. They share the same advantage that Y-pattern globe valves do, that is, they have a straighter flow path and a lower pressure drop than does a comparable straight or T-pattern check. They are, of necessity, larger than the straight pattern. A side benefit of the Y-pattern is that it is a little easier to tell from some distance away which direction the flow in the line is supposed to be since the Y-pattern body "points" downstream. And the Y-pattern check also has the same body and disc as the Y-pattern globe.

Specification engineers would most commonly select the lift check in smaller sizes, up to about I 1⁄2-in. or 2-in. pipe size (DN 40 or DN 50) and swing checks or double- door wafer checks in sizes above that. But lift or piston checks are surprisingly common in larger sizes for several somewhat specialised purposes. One is for pulsating flow, where pressure and volume surges in a line, as from a piston or diaphragm pump, would cause some other types of check valves to bang open and closed with each cycle. Such high-cycle service is almost a guarantee of short life from check valves that depend on nothing but gravity to oppose the fluid flow forces. Since the piston check has to have a connection from the volume space above the disc to the downstream portion of the line anyway, valve designers can make use of the fluid flowing back and forth to dampen the piston's movement. There are a couple of ways to do this; one is to put an orifice into the piston to control the amount of fluid that flows in and out of the area above it and thus control its speed, and another, more adjustable method, is to run the line that connects the two volumes outside the body of the valve and put a small throttling valve into the line to allow adjustment of the piston's response time. Many also have a spring above the disc to help in dampening the opening forces.

Piston checks of this type are used quite often in higher pressures and smaller sizes where the swing check with the outside lever and weight might otherwise be used. This is because the packing gland required in the swing check is not needed here, so there is nothing to bind or obstruct the valve's movement or provide a leak path. High-pressure valves even without the consideration of fluctuating flow rates can make good use of the piston design, since a heavy disc, often with a spring, is a better match for a high opening force often found in such valves. These valves are often built as Y-pattern, which is not often true of the valves designed for pulsating flow. Valves built for high-pressure service also often have special design considerations, such as contours designed with aerodynamic effects in mind and hard-faced guide surfaces to reduce wear caused by the high forces involved.

Another variation on the lift check that is very popular in certain applications is the ball check valve. The ball check is still a lift check, often with exactly the same body. The difference is that the piston has been replaced with a spherical ball, basically a ball bearing. It takes a slightly different guiding system than the piston and does not need to be vented because it never totally blocks in the volume in the top of the valve. The main reasons for using the ball check are that since it has more internal clearance than the piston, in viscous services it will not gum up as easily and that since the ball falls back on the seat in a different orientation each time, it generally wears better in high-cycling services. It is still possible to use a spring to aid in seating.

Valves specially designed to be used in vertical lines are also ball checks, although with a different body construction because the flow around the ball and out through the top of the valve is easy to accomplish. These valves require a union or other joint in the middle for disassembly since there cannot be a cap. They can be installed only where the flow is vertical up since gravity is the closing mechanism. There are some ball checks suitable for either horizontal or vertical up installation.

The same valve that is used as a vertical flow check valve can be placed in the horizontal, where it takes on an entirely different function. It can be used as an excess flow check valve, whose purpose is not to respond to fluid flows when they are of low magnitude. The check becomes active only when a high-velocity flow occurs. Basically, the fluid flow sweeps the ball into its seat and holds it there, thus shutting off the flow. This valve is useful for places where line ruptures or leaks might occur. Of course, since the ball must seat on the downstream side, it must be installed backwards from any check valve.

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Split-Disc Check Valves

Yet another variation on the same theme is the split-disc or double-door or wafer check valve. All these names are commonly used to describe one product, and although the term "wafer check" is probably the most common, it is also misleading in that other types of check can be wafer body. What is discussed here is a design in which there are two discs, each in the shape of a half-circle and hinged on the straight side, which is also the vertical centerline of the valve, such that the two discs open toward each other. These discs are also spring loaded, so that gravity is not required for the valve to operate.

Normally the two discs are mounted on the same hinge pin, which is inserted through the body wall in the same way that the pin of a swing check is. The springs are wrapped around this pin also. Normally there is a second pin, called a stop pin, just downstream of the hinge pin. The springs usually work against this pin to exert closing force in the discs, or doors, as they are more often called. Sometimes one spring pushes against both discs, but more often there are two springs, one for each door. In the open position the two doors normally stop against each other, but some designs use the stop pin for this purpose also. There are small plugs threaded into the outside of the body to cover the points where the pins are inserted. These are made of the same material as the body m· of the pins, which tend to be of the same material as the rest of the valve trim.

With reference to the door analogy again, the fluid force required to open the valve is a great deal less than with a swing check since each door has less than half as far to travel as in a swing check, and not by lifting against gravity (unless the valve happens to be in a vertical line). Pressure drop is thus less than in a swing check. Since the parts are physically smaller, it is practical to install them in a wafer-type body, thus significantly decreasing the mass of the entire valve and thus its cost. All of these factors combine to make the double-door check one of the most efficient check valve designs.

A few valves, mostly in oil field applications but sometimes in high alloy chemical service piping, are built to a swing design but in a wafer pattern. Such valves require the disc to swing beyond the downstream end of tile valve. These valves share the weight advantage of the double door check, but otherwise they are standard swing check valves in operation. Because of the limited space within the body of the valve, many are built with the seat at 90 degrees to the axis of the valve, rather than the 80- to 85-degree inclined seat in a flanged swing check. To compensate for this arrangement, which makes the disc hang loosely next to the seat instead of applying any gravity force to it, an elastomer O-ring is sometimes installed. Basically, the only force available to seat the valve is positive back pressure. These valves tend to be very low in cost. The double-door check is available in almost all materials, and because of its small body mass, in high-alloy construction, its cost can be significantly less than that of the equivalent swing check. It has an economic advantage in large sizes, though, in just about any material, even cast iron. The springs are available in a number of materials, some of which make better springs than others. Inconel, Inconel-X, or some other variation of nickel-chrome-iron alloy is the most common since it has both good springiness and high corrosion resistance. Springs are available out of other corrosion-resistant material and high-temperature alloys also, some of which are good to much higher temperatures than the valve bodies are.

The springs are the source of one of the major disadvantages to the double-door valve-that they can break and go downstream. This unfortunate incident both renders the valve somewhat nonfunctional and potentially can cause damage to some downstream piece of machinery. This problem used to occur more often than it does now, although it is always a danger. When the two springs each push against the stop pin, then if one breaks the other half of the valve is still functional. Having only one-half of the valve working does not make it 50 percent as good as an intact valve though, but perhaps 5 percent if the door with the broken spring does not close. Fluid backflow usually will still close the valve, even without the spring.

The springs also can be varied in springiness to close at greater or slower speeds, which helps for very large valves and for resistance to flutter or slamming. Generally, stiffer springs are used to close the valve faster to counteract slamming (the valve closes before the flow can build up any momentum in the backward direction), and softer springs are used if the valve shuts too forcefully against low flows.

In the double-door check, as in any other valve, a different material is used on seating surfaces for greater resistance to wear and impact loading. However, since the doors have less mass and travel less far than in a swing check, there is less loading in the first place so less protection is often required. Hard facing is rarely used. Often only an increased corrosion resistance is required, to maintain the integrity of the seating surfaces. An 11 to 13 percent chrome is often sufficient for services that require only carbon steel bodies. Since the metal mass in a double-door check is fairly small to begin with, in smaller sizes it is actually cheaper to make the entire body from the chrome steel rather than making a steel body with weld-overlaid seating areas. Conversely, in some very large valves, it may be cheaper to make the doors from carbon steel and overlay the seating surfaces rather than cast the entire piece from the alloy, just as is often done on gate valve wedges.

The wafer check, of course, is a through-bolting type. Extra long stud bolts are used, and the outside diameter of the valve body is the same as the inside of the bolting pattern, which is normally the same as the outside diameter of the gasket seating surface or very close to it. This means that the same casting can be used for a number of different ratings, each one being machined to a different outside diameter. It also means that the bolting lies totally exposed in the region between the flanges and outside the check valve wall, making the bolting more vulnerable to thermal expansion in fires or in high-temperature service. Some users do not permit wafer checks above a certain temperature, but others simply prefer a lug type. In check valves, the lug type serves the same purposes as it would in butterflies, that is to allow the valve itself to be actually bolted to the flange on either side, not just held in between. For purposes of avoiding thermal expansion from external heat sources, it is perfectly adequate to drill the lugs out and pass the studs through them rather than using cap screws; here the purpose of the lugs is merely to shield the bolting.

Where the four plugs used to retain the hinge and stop pins are considered as undesirable leak paths, they can be seal-welded. It is also possible to build a double-door check in which these pins are retained by an inner retainer in which the joint does not lead to the outside atmosphere.

In many piping configurations, a check valve is most likely to be found on a pump discharge and its purpose is to keep the fluid flow from spinning the pump impeller backwards. Often the piping is vertical at this point, so the check valve becomes the lowest point of a volume to be drained. In standard swing checks the body is simply drilled and tapped. The wafer check is perfectly capable of being drilled and tapped also, but by its nature, it is somewhat harder to do. The space available is very small, so in valves below 6-in. pipe size (DN 150) the standard drain size available may be oddball. In addition, the drain connection has to pass between two of the flange bolts. In many cases, a longer than normal pipe nipple is thus required to provide a valved drain. The drain actually comes from the area immediately downstream of the seat plane, and in some cases actually notches into the seat slightly at a point between the doors so it does not interfere with seating. This, of course, is the lowest possible drain point.

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Centred Spring Check Valves

A common type of specialised check valve is the centred spring check. This valve is widely manufactured and is used as a standard type of check valve in building service piping and a number of other industrial applications. This type is also the simplest check valve for very small sizes. But its most significant application is in the prevention of water hammer.

The centred spring check works somewhat like a globe valve or lift check, in that the disc moves directly into the flow. Unlike the globe or lift check, no right-angle turns are required. This movement is accomplished by having a centring device suspended from the sides of the valve, downstream of the seat, holding a short stem or guide that is attached to or part of the disc. The force that moves the disc into the seat is supplied by a spring that wraps around this guide and pushes against it.

The significance of this design is that the spring force directly opposes the force of flowing liquid. As the flow of liquid slows the spring begins to push the disc closed. When the fluid flow stops completely, the disc seats and stays seated until the differential pressure of fluid is enough to lift the disc back off the seat. In many other check valve designs, the movements of the disc or discs are spring-loaded or otherwise guided by a force other than fluid flow, but the two forces are not in line. A swing check, for instance, moves by gravity, but the resultant of its downward movement and the influence of the pivot arm is the actual force that opposes the fluid force. Although these may seem to be insignificant differences, the differences in behaviour are really enough to justify a special check valve where the water hammer problem exists.

It must be noted that in severe water hammer situations, often no one single change will effect a cure. Often, a combination of solutions including special check valves, reorientation of piping to reduce gravitational influence, better anchoring of pumps, adding chambers or piping to allow for diffusing of shock waves, or other changes may be necessary.

An additional advantage claimed for the centred spring check valve is its lack of closure noise. The valve indeed closes silently, or at least with much less noise than a swing check, but in general, the noise is only a symptom of a problem and not a problem itself. In building water piping in a basement, the lack of check valve banging is probably significant. But what really makes this important is the fact that the noise is caused by the impact of the disc onto the seat. When the valve seats gently, as do a number of other check valves under the right conditions, less wear occurs and the valve lasts longer. In many cases, the centred spring check is built as a wafer type, often in iron with bronze trim for water service.

Other types of centred spring check valve are more exotic, available in a wide variety of materials and used in pulsating or widely varying flows. Some of them look like a wide spot in the piping or like a snake that swallowed a rat. This bulging design is very common in Europe, but is also imported to the United States. It is also possible to configure this valve as a control valve by bringing a rotary stem in from one side and adding a rack gear to the sliding stem. With a pinion attached to the rotary stem, a standard rotary actuator can be used to open and close the valve. Because of its streamlined flow path, a very low-pressure drop can be obtained.

These valves are normally built in welded or cast steel, sometimes with a soft-seat component but also with a hard-faced seat. The spring also has to be resistant to temperature and corrosion. Because of their streamlined design, they generally have fairly low-pressure drop and high flow capacity. However, they are also somewhat longer than other valves.

Backflow Preventer Valves Another specialised type of check, known as the backflow preventer, is actually two check valves plus one or two block valves in one. The backflow preventer is a legally mandated device to separate potable water systems from non-potable systems. Generally, the backflow preventer is used whenever there is a hard-piped connection from municipal water systems to an industrial water user such as a building sprinkler system, or in residential water systems to guard against lawn chemicals being sucked back into the household water supply, or in any open water system that has the possibility of being infiltrated with bacteria.

Numerous incidents of sickness and death resulting from contaminated water are behind the requirements for backflow preventers. They are made by a number of manufacturers that specialise in the municipal business and are generally flanged or threaded end, in cast iron or bronze. Health codes generally prescribe where they are required, with some states requiring them more frequently or in more places than others.

The reason for the interconnects in the first place is often that the drinking water supply can be made available for short periods of time for firefighting or water makeup. Often a backflow preventer is not required if no permanent connection exists and a backflow is not possible, for instance, if a source of potable water can flow by gravity into a tank.

The design of the backflow preventer consists of two check valves in series, usually with soft inserts in the disc to seat more tightly, with a drain between them such that when the valves close, the drain opens to prevent any seepage from going back upstream. The block valves, upstream and downstream of the check valves, are there to act as reinforcement to make extra sure that there is no possibility of upstream leakage. In smaller sizes, this entire assembly is made as one solid body casting.

Bodies are usually cast bronze up to 2-in. size (DN 50) and cast iron with bronze trim in larger sizes, with available sizes ranging from 1/2-in. (DN12) to about 10-in. pipe size (DN 250).

There are a number of variants on the backflow preventer, to account for various severities of service. Many household models are designed to fit into the space next to the water meter, or on lines leading to the outdoor faucets. Most household models are designed for low noise during operation. Other, larger ones for industrial use have positive means LO expel the water trapped between the two checks to ensure that there is an air gap between the different water services at all times that positive flow (in the correct direction) is not occurring.

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Other Check Valves

Among the miscellaneous types of check valves in service are the ones used for highly variable flows. These valves must open at relatively low flow rates, yet present low-pressure drops at high flow rates. The most common occurrence of these conditions is in compress or discharge lines, where often there is also a significant pulsation problem.

There are a number of designs of compressor check valve, many made by compressor manufacturers themselves for use between stages of the compressors but also needed on the piping outside of the compressor. There are a few manufacturers that specialise in this valve. Typically these valves have many, very small passageways, which not only help to break up the pulsations but also provide very small flow passages for the check mechanisms to operate across. Some designs use a metal reed of spring steel that snaps shut across each of the flow passages when the flow stops. Others use a plate that contains a number of reeds or fingers, sometimes provided with tiny coil springs in addition, which both break up the flow into many small passages and provide quick closure. This whole assembly generally is built like a wafer, which is then mounted into a housing to be bolted into the pipeline.

All of these valves, and especially the spring-loaded ones, need to be designed for the particular location they are to be installed in. The flow rate, gas density, pressure, and amount of pulsation, in terms of the speed and volume of the cylinder discharges, must be known. The orientation of the valve in the piping is helpful information also. The manufacturers can size these valves and can determine the size of the pulsations from the size and speed of the compressor. This is generally a very complex calculation, almost always computerised.

Standard check valves are not suitable for most compressor discharge lines, and if installed there are almost always doomed to a short life. Reciprocating compressors produce a pressure pulse with every discharge, and the resultant pressure waves produce noise, sometimes even amplified by the piping through resonance, and vibration. The plate type compressor discharge check valves are specifically designed to dampen these pressure pulses without imposing significant pressure drops. They are also an effective solution for discharge lines on centrifugal compressors with widely varying duties, although the pressure pulsation is essentially absent in a centrifugal compressor.

Another useful design is the recirculation check, which is a check valve and flow-splitting valve in one. Its use is on the discharge side of a process pump, where a certain minimum flow through the pump is desirable or necessary. The recirculation check acts like a centred-spring check in reverse, with a spring-loaded disc that is connected to a cage and plug in the centre of the valve, with an additional side outlet. Under full flow conditions, the check is wide open and the entire flow goes down the pump discharge line. As the flow through the valve decreases, the flow passageway leading to the recirculation line starts to open and some of the fluid is diverted to the pump bypass. This allows at least some fluid to be flowing through the pump at all times, even if the discharge line is completely blocked in downstream of the recirculation check.

Yet another type of valve that operates similarly to the pinch valve is the resilient body check. This check operates in a manner similar to the check valves in blood vessels. It is capable of shutting off around solid objects and opens under very small heads of water.

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