Check Valves : International Site for Spirax Sarco

8/10/2019 Check Valves : International Site for Spirax Sarco

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09/10/2014 11:42Check Valves : International site for Spirax Sarco

Page 3 sur 12http://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/check-valves.asp

Swing check valves produce relatively high resistance to flow in the open position, due to the weight of thedisc. In addition, they create turbulence, because the flap 'floats' on the fluid stream. This means that there istypically a larger pressure drop across a swing check valve than across other types.

With abrupt changes in flow, the disc can slam against the valve seat, which can cause significant wear of theseat, and generate waterhammer along the pipe system. This can be overcome by fitting a dampingmechanism to the disc and by using metal seats to limit the amount of seat wear.

Wafer check valvesBoth lift and swing check valves tend to be bulky which limits their size and makes them costly. To overcome

this, wafer check valves have been developed. By definition wafer check valves are those that are designedto fit between a set of flanges. This broad definition covers a variety of different designs, including disc checkvalves and wafer versions of swing or split disc check valves.

Disc check valvesThe disc check valve consists of four main components: the body, a disc, a spring and a spring retainer. Thedisc moves in a plane at right angles to the flow of the fluid, resisted by the spring that is held in place by theretainer. The body is designed to act as an integral centring collar that facilitates installation. Where a 'zeroleakage' seal is required, a soft seat can be included.

Fig. 12.3.3 A disc check valve

When the force exerted on the disc by the upstream pressure is greater than the force exerted by the spring,the weight of the disc and any downstream pressure, the disc is forced to lift off its seat, allowing flow throughthe valve. When the differential pressure across the valve is reduced, the spring forces the disc back onto itsseat, closing the valve just before reverse flow occurs. This is shown in Figure 12.3.4. The presence of thespring enables the disc check vale to be installed in any direction.

Fig. 12.3.4 Operation of a disc check valve

The differential pressure required to open the check valve is mainly determined by the type of spring used. Inaddition to the standard spring, there are several spring options available:

No spring - Used where the differential pressure across the valve is small.

Nimonic spring - Used in high temperature applications.


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Heavy-duty spring - This increases the required opening pressure. When installed in the boilerfeedwater line, it can be used to prevent steam boilers from flooding when they are unpressurised.

As with all wafer check valves, the size of the disc check valve is determined by the size of the associatedpipework. This usually ensures that the valve is correctly sized, but there are cases where the valve is over orundersized.

An oversized check valve is often indicated by continuous valve chatter, which is the repeated opening andclosing of the valve that occurs when the valve is only partially open. It is caused by the fact that when thevalve opens, there is a drop in the upstream pressure; if this pressure drop means that the differential

pressure across the valve falls below the required opening pressure, the valve will slam shut. As soon as thevalve shuts, the pressure begins to build up again, and so the valve opens and the cycle is repeated.

Oversizing can usually be rectified by selecting a smaller valve, but it should be noted that this will increasethe pressure drop across the valve for any one flow. If this is not acceptable, it may be possible to overcomethe effects of chatter by reducing the closing force on the disc. This can be done either by using a standardspring instead of a heavy-duty one, or by removing the spring altogether. Another alternative is to use a softseat; this does not prevent the chatter but rather, reduces the noise. Care must be taken however, as thismay cause excessive wear on the seat.

Undersizing results in excessive pressure drop across the valve and, in the extreme, it may even preventflow. The solution is to replace the undersized valve with a larger one.

Disc check valves are smaller and lighter than lift and standard swing check valves and subsequently costless. The size of a disc check valve is however limited to DN125; above this, the design becomes

complicated. Typically, such a design would include a cone shaped disc and a small diameter spring that isretained and guided along the centre line of the cone, which is more difficult and expensive to manufacture.Even then, such designs are still limited in size to DN250.

Standard disc check valves should not be used on applications where there is heavily pulsating flow, forexample, on the outlet of a reciprocating air compressor, as the repeated impact of the disc can lead to failureof the spring retainer and high levels of stress in the spring. Specifically designed retainers are available forsuch applications. These designs typically reduce the amount of disc travel, which effectively increases theresistance to flow and therefore increases the pressure drop across the valve.

The design of disc check valves allows them to be installed in any position, including vertical pipelines wherethe fluid flows downwards.

Swing type wafer check valvesThese are similar to the standard swing check valves, but do not have the full-bodied arrangement, instead,

when the valve opens, the flap is forced into the top of the pipeline. Subsequently, the flap must have asmaller diameter than that of the pipeline, and because of this, the pressure drop across the valve, which isoften high for swing type valves, is further increased.

Swing type check valves are used mainly on larger pipeline sizes, typically above DN125, because onsmaller pipelines the pressure drop, caused by the disc 'floating' on the fluid stream, becomes significant.Furthermore, there are significant cost savings to be made by using these valves on larger sizes, due to thesmall amount of material required for the construction of the valve.

There is however one problem with using larger size valves; due to their size, the discs are particularly heavy,and therefore possess a large amount of kinetic energy when they close. This energy is transferred to theseat and process fluid when the valve slams shut, which could cause damage to the seat of the valve andgenerate waterhammer.


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Fig. 12.3.5 Swing type wafer check valve

Wafer check valve applicationsWafer check valves are becoming the preferred type of check valve for most applications, due to theircompact design and relatively low cost. The following is a list of some of their most common applications:

Boiler feedlines - The check valve is used to prevent boiler water being forced back along thefeedline into the storage tank when the feedpump stops running. Furthermore, a disc check valve witha heavy-duty spring and a soft seat can be fitted in the boiler feedline to prevent flow under gravityinto the boiler when the feedpump is shut off.

Fig. 12.3.6 Boiler feedline applications

Steam traps - Other than with steam traps discharging to atmosphere, check valves should alwaysbe inserted after a steam trap to prevent back flow of condensate flooding the steam space. Thecheck valve will also prevent the steam trap from becoming damaged by any hydraulic shock in thecondensate line. It should be noted that when using blast discharge type steam traps, the check valveshould be fitted at least 1 m downstream of the trap.


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Fig. 12.3.7 Steam trap applications

Hot water circuits -A check valve should be installed after each pump to prevent reverse flowthrough the pump when it has been shut off (see Figure 12.3.8).

Fig. 12.3.8 Duplex pump set

Vacuum breakers - Check valves can be used as vacuum breakers, by fitting them in reverse. Whena vacuum is created, the valve opens, allowing air to be drawn in from the atmosphere (see Figure12.3.9).


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Fig. 12.3.9 Steam injection into a tank

Blending -A check valve should be fitted in each supply line to prevent reverse flow along thedifferent lines which will lead to contamination. A common blending application is the mixing of hotand cold water to provide hot water (see Figure 12.3.10).

Fig. 12.3.10 Blending applications

Pipeline fitting protection - Check valves are used to prevent damage to equipment such asflowmeters and control valves, all of which can be damaged by reverse flow. Check valves also stopthe contents of strainers from being deposited in upstream pipework by back flowing fluid.

Multiple boiler applications -A check valve must be inserted on the outlet of each boiler to preventany steam flowing into boilers, which may be on hot stand-by (see Figure 12.3.11).

Fig. 12.3.11 Multiple boiler applications

Blowdown vessels - When a blowdown vessel receives blowdown from more than one boiler, awafer check valve should be installed on each separate blowdown line. This will prevent theblowdown from one boiler flowing back into another boiler. In many countries, this is a statutoryrequirement.

Flash vessels -A wafer check valve is installed at the flash steam outlet from the flash vessel; thisensures that steam from any make-up valve does not flow back into the flash vessel (see Figure12.3.12). A check valve is also installed after the steam trap that drains the flash vessel.


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Fig. 12.3.12 Flash vessel applications

Split disc check valvesThe split disc check valve or dual plate check valve is designed to overcome the size and pressure droplimitations of the swing and disc type wafer check valves. The flap of the swing check valve is essentially splitand hinged down its centre, such that the two disc plates will only swing in one direction. The disc plates areheld against the seat by a torsion spring mounted on the hinge.

In order to hold the hinge in the centre of the flow path, externally mounted retainer pins can be used. Theseretainer pins are a common source of leakage from the valve. An improved design secures the hingeinternally, and as the valve mechanism is entirely sealed within the body, leakage to atmosphere is prevented

(see Figure 12.3.13).

Fig. 12.3.13 A split disc check valve (retainerless design)

The valve is normally closed, as the disc plates are kept shut by the torsion spring. When fluid flows in theforwards direction, the pressure of the fluid causes the disc plates to hinge open, allowing flow. The checkvalve is closed by the spring as soon as flow ceases, before any reverse flow can occur.


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Fig. 12.3.14 Operation of a split disc check valve

The frequent opening and closing of the split disc check valve would soon cause seat damage if the heels ofthe disc plates were allowed to scuff against the seat during opening. To overcome this, the heel of the discplates lift during the initial opening of the valve and the plates rotate purely on the hinge as opposed to theseat face.

The split disc type of check valve has several advantages over other types of check valves:

The split disc design is not limited in size and these valves have been produced in sizes of up toDN5400.

The pressure drop across the split disc check valve is significantly lower than across other types.

They are capable of being used with lower opening pressures.

Split disc check vales can be installed in any position, including vertical pipelines.

Other check valve typesThe above mentioned types of check valve are the most commonly encountered types in steam, condensate,and liquid systems. However, several other types are also available. The three types listed below are mainlysuited to liquid applications and subsequently may be found in condensate systems:

Ball check valve - This consists of a rubber-coated ball that is normally seated on the inlet to thevalve, sealing off the inlet. When pressure is exerted on the ball, it is moved off its seat along a guiderail, allowing fluid to pass through the inlet. When the fluid pressure drops, the ball slides back into itsposition on the inlet seat. Note: Ball check valves are typically only used in liquid systems, as it isdifficult to obtain a tight seal using a ball.

Diaphragm check valve -A flexible rubber diaphragm is placed in a mesh or perforated cone withthe point in the direction of flow in the pipeline (see Figure 12.3.15). Flow in the forwards directiondeflects the diaphragm inwards, allowing the free passage of the fluid. When there is no flow or abackpressure exists, the diaphragm returns to its original position, closing the valve. Note: Thediaphragm material typically limits the application of the diaphragm check valve to fluids below 180Cand 16 bar.

Fig. 12.3.15 A diaphragm check valve

Tilting disc check valve - This is similar to the swing type check valve, but with the flap pivoted infront of its centre of pressure and counterweighted or spring loaded to assume a normally closed


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position (see Figure 12.3.16). When flow is in the forwards direction, the disc lifts and 'floats' in thestream offering minimum resistance to flow. The disc is balanced so that as flow decreases, it willpivot towards its closed position, closing before reverse flow actually commences. The operation issmooth and silent under most conditions. Note: due to the design of the tilting disc check valve, it islimited to use on liquid applications only.

Fig. 12.3.16 Operation of a tilting disc check valve

Pressure loss chartsAs most types of check valve are suitable for use on both liquid and gas systems, manufacturers typicallyshow the pressure drop across a valve in the form of a pressure loss chart for water. A typical pressure loss

chart is shown in Figure 12.3.17. It shows the pressure drop across a particular check valve for a given valvesize and water flowrate in m/h.

Fig. 12.3.17 A typical manufacturers pressure loss diagram

In order to determine the pressure drop across the check valve for other liquids, the equivalent water volumeflowrate needs to be calculated, this is done using the formula in Equation 12.3.1:

Equation 12.3.1

Once the equivalent water volume flowrate has been determined, the pressure drop across the valve can beread off the chart using the same method as for water, selecting the equivalent water volume flowrate insteadof the actual volume flowrate.

It should be noted that the volumetric flowrate (in m/h) is typically quoted for liquid applications, whereas, insteam applications, the mass flowrate (in kg/h) is normally used. To convert from kg/h to m/h, the mass


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flowrate is multiplied by the specific volume (in kg/m) for the particular working pressure and temperature(see Equation 12.3.2).

Equation 12.3.2

Alternatively, if the Kvvalue of the valve is specified, the pressure drop across the valve can be determined

using the method outlined in Tutorial 12.2.

Example 12.3.1Determine the pressure drop across a DN65 check valve passing 1200 kg/h of saturated steam at 8 bar g.Use the pressure drop characteristics shown in Figure 12.3.17.

Solution:The first step is to calculate the volumetric flowrate:From steam tables at 8 bar gauge g= 0.2149 m!/kg

Using Equation 12.3.2

Using Figure 12.3.18, the pressure drop across the valve would be approximately 0.085 bar.

Fig. 12.3.18


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Check Valves : International Site for Spirax Sarco