nrv philosophy and application

The Steam and Condensate Loop 12.3.1

Block 12 Pipeline Ancillaries Check Valves Module 12.3

Module 12.3Check Valves

The Steam and Condensate Loop12.3.2


Check Valves

Check valves, or non-return valves, are installed in pipeline systems to allow flow in one directiononly. They are operated entirely by reaction to the line fluid and therefore do not require anyexternal actuation. In this text, the expected, or desired direction of flow is termed �forward flow�,flow in the opposite direction is �reverse flow�.

There are a number of reasons for using check valves, which include:

o Protection of any item of equipment that can be affected by reverse flow, such as flowmeters,strainers and control valves.

o To check the pressure surges associated with hydraulic forces, for example, waterhammer.These hydraulic forces can cause a wave of pressure to run up and down pipework until theenergy is dissipated.

o Prevention of flooding.

o Prevention of reverse flow on system shutdown.

o Prevention of flow under gravity.

o Relief of vacuum conditions.

Although check valves can effectively shut off reverse flow, they should never be used inplace of an isolation valve to contain live steam, in a section of pipe.

As with isolation valves, there are a number of different check valve designs, each suited tospecific applications. The different types of check valve and their applications are discussed inthis module, along with the correct sizing method.

Lift check valvesLift check valves are similar in configuration to globe valves, except that the disc or plug isautomatically operated. The inlet and outlet ports are separated by a cone shaped plug that restson a seat typically metal; in some valves, the plug may be held on its seat using a spring. Whenthe flow into the valve is in the forward direction, the pressure of the fluid lifts the cone off itsseat, opening the valve. With reverse flow, the cone returns to its seat and is held in place by thereverse flow pressure.

Fig. 12.3.1 A lift check valve

Forwardflow



If a metal seat is used, the lift check valve is only suitable for applications where a small amountof leakage, under reverse flow conditions, is acceptable. Furthermore, the design of a lift checkvalve generally limits its use to water applications, subsequently, they are commonly used toprevent reverse flow of condensate in steam traps and on the outlets of cyclic condensate pumps.

The main advantage of the lift check valve lies in its simplicity, and as the cone is the only movingpart, the valve is robust and requires little maintenance. In addition, the use of a metal seat limitsthe amount of seat wear. The lift check valve has two major limitations; firstly, it is designed onlyfor installation in horizontal pipelines, and secondly, its size is typically limited to DN80, abovewhich, the valve would become too bulky.

The piston-type lift check valve is a modification of the standard lift check valve. It incorporatesa piston shaped plug instead of the cone, and a dashpot is applied to this mechanism. Thedashpot produces a damping effect during operation, thereby eliminating the damage caused bythe frequent operation of the valve, for example, in pipeline systems, which are subject to surgesin pressure, or frequent changes in flow direction (one example would be a boiler outlet).

Swing check valvesA swing check valve consists of a flap or disc of the same diameter as the pipe bore, which hangsdown in the flow path. With flow in the forwards direction, the pressure of the fluid forces thedisc to hinge upwards, allowing flow through the valve. Reverse flow will cause the disc to shutagainst the seat and stop the fluid going back down the pipe. In the absence of flow, the weightof the flap is responsible for the closure of the valve; however, in some cases, closure may beassisted by the use of a weighted lever. As can be seen from Figure 12.3.2, the whole mechanismis enclosed within a body, which allows the flap to retract out of the flow path.

Fig. 12.3.2 A full-bodied, swing check valve

Swing check valves produce relatively high resistance to flow in the open position, due to theweight of the disc. In addition, they create turbulence, because the flap �floats� on the fluidstream. This means that there is typically a larger pressure drop across a swing check valve thanacross other types.

With abrupt changes in flow, the disc can slam against the valve seat, which can cause significantwear of the seat, and generate waterhammer along the pipe system. This can be overcome byfitting a damping mechanism to the disc and by using metal seats to limit the amount of seatwear.

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 arethose that are designed to fit between a set of flanges. This broad definition covers a variety ofdifferent designs, including disc check valves and wafer versions of swing or split disc checkvalves.

Cover

Hinge pin

Disc

Seat ring

Body

Forward flow



Disc check valvesThe disc check valve consists of four main components: the body, a disc, a spring and a springretainer. The disc moves in a plane at right angles to the flow of the fluid, resisted by the springthat is held in place by the retainer. The body is designed to act as an integral centring collar thatfacilitates installation. Where a �zero leakage� 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 bythe spring, the weight of the disc and any downstream pressure, the disc is forced to lift off itsseat, allowing flow through the valve. When the differential pressure across the valve is reduced,the spring forces the disc back onto its seat, closing the valve just before reverse flow occurs. Thisis shown in Figure 12.3.4. The presence of the spring enables the disc check vale to be installedin any direction.

Fig. 12.3.4 Operation of a disc check valve

Forward flow

Forwardflow

Reverseflow

Open Closed

Spring retainer

Spring

Disc

Body

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

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

o Nimonic spring - Used in high temperature applications.

o 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 theassociated pipework. This usually ensures that the valve is correctly sized, but there are caseswhere the valve is over or undersized.

Seat

Disc

Spring



An oversized check valve is often indicated by continuous valve chatter, which is the repeatedopening and closing of the valve that occurs when the valve is only partially open. It is caused bythe fact that when the valve opens, there is a drop in the upstream pressure; if this pressure dropmeans that the differential pressure across the valve falls below the required opening pressure,the valve will slam shut. As soon as the valve shuts, the pressure begins to build up again, and sothe valve opens and the cycle is repeated.

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

Undersizing results in excessive pressure drop across the valve and, in the extreme, it may evenprevent flow. 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 subsequentlycost less. The size of a disc check valve is however limited to DN125; above this, the designbecomes complicated. Typically, such a design would include a cone shaped disc and a smalldiameter spring that is retained and guided along the centre line of the cone, which is moredifficult 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 pulsatingflow, for example, on the outlet of a reciprocating air compressor, as the repeated impact of thedisc can lead to failure of the spring retainer and high levels of stress in the spring. Specificallydesigned retainers are available for such applications. These designs typically reduce the amountof disc travel, which effectively increases the resistance to flow and therefore increases the pressuredrop across the valve.

The design of disc check valves allows them to be installed in any position, including verticalpipelines where the fluid flows downwards.

Swing type wafer check valves

Fig. 12.3.5 Swing type wafer check valve

There is however one problem with using larger size valves; due to their size, the discs areparticularly heavy, and therefore possess a large amount of kinetic energy when they close. Thisenergy is transferred to the seat and process fluid when the valve slams shut, which could causedamage to the seat of the valve and generate waterhammer.

These are similar to the standard swing checkvalves, but do not have the full-bodiedarrangement, instead, when the valve opens, theflap is forced into the top of the pipeline.Subsequently, the flap must have a smallerdiameter than that of the pipeline, and becauseof this, the pressure drop across the valve, whichis often high for swing type valves, is furtherincreased.

Swing type check valves are used mainly on largerpipeline sizes, typically above DN125, becauseon smaller pipelines the pressure drop, causedby the disc �floating� on the fluid stream, becomessignificant. Furthermore, there are significant costsavings to be made by using these valves on largersizes, due to the small amount of materialrequired for the construction of the valve.

Forwardflow



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

o 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 checkvalve with a heavy-duty spring and a soft seat can be fitted in the boiler feedline to preventflow under gravity into the boiler when the feedpump is shut off.

o Steam traps - Other than with steam traps discharging to atmosphere, check valves shouldalways be inserted after a steam trap to prevent back flow of condensate flooding the steamspace. The check valve will also prevent the steam trap from becoming damaged by anyhydraulic shock in the condensate line. It should be noted that when using blast dischargetype steam traps, the check valve should be fitted at least 1 m downstream of the trap.

Fig. 12.3.6 Boiler feedline applications

Fig. 12.3.7 Steam trap applications



o 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 mixingof hot and cold water to provide hot water (see Figure 12.3.10).

Fig. 12.3.10 Blending applications

o 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).

o Vacuum breakers - Check valves can be used as vacuum breakers, by fitting them in reverse.When a vacuum is created, the valve opens, allowing air to be drawn in from the atmosphere(see Figure 12.3.9).

Fig. 12.3.8 Duplex pump set

Fig. 12.3.9 Steam injection into a tank

Water

DCV

SteamDisc check valve fitted as a vacuum breaker

Injector

Tank

Cold water supply

Hot water supply

Mixing valve

Check valve

Check valve

Blended water



o 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 alsostop the contents of strainers from being deposited in upstream pipework by back flowingfluid.

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

Fig. 12.3.11 Multiple boiler applications

o Blowdown vessels - When a blowdown vessel receives blowdown from more than one boiler,a wafer 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.

o Flash vessels - A wafer check valve is installed at the flash steam outlet from the flash vessel;this ensures that steam from any make-up valve does not flow back into the flash vessel (seeFigure 12.3.12). A check valve is also installed after the steam trap that drains the flash vessel.

Fig. 12.3.12 Flash vessel applications

On line On line On stand-by

Check valve

Steam

Condensate and steam

Condensate



Split disc check valvesThe split disc check valve or dual plate check valve is designed to overcome the size and pressuredrop limitations of the swing and disc type wafer check valves. The flap of the swing check valveis essentially split and hinged down its centre, such that the two disc plates will only swing in onedirection. The disc plates are held 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 beused. These retainer pins are a common source of leakage from the valve. An improved designsecures the hinge internally, 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 fluidflows in the forwards direction, the pressure of the fluid causes the disc plates to hinge open,allowing flow. The check valve is closed by the spring as soon as flow ceases, before any reverseflow can occur.

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 ifthe heels of the disc plates were allowed to scuff against the seat during opening. To overcomethis, the heel of the disc plates lift during the initial opening of the valve and the plates rotatepurely on the hinge as opposed to the seat face.

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

o The split disc design is not limited in size and these valves have been produced in sizes of upto DN5400.

o The pressure drop across the split disc check valve is significantly lower than across othertypes.

o They are capable of being used with lower opening pressures.

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

Forwardflow

Reverseflow

Open Closed



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 typeslisted below are mainly suited to liquid applications and subsequently may be found in condensatesystems:

o Ball check valve - This consists of a rubber-coated ball that is normally seated on the inletto the valve, sealing off the inlet. When pressure is exerted on the ball, it is moved off itsseat along a guide rail, allowing fluid to pass through the inlet. When the fluid pressure drops,the ball slides back into its position on the inlet seat. Note: Ball check valves are typically onlyused in liquid systems, as it is difficult to obtain a tight seal using a ball.

o Diaphragm check valve - A flexible rubber diaphragm is placed in a mesh or perforatedcone with the point in the direction of flow in the pipeline (see Figure 12.3.15). Flow in theforwards direction deflects the diaphragm inwards, allowing the free passage of the fluid.When there is no flow or a backpressure exists, the diaphragm returns to its original position,closing the valve. Note: The diaphragm material typically limits the application of thediaphragm check valve to fluids below 180°C and 16 bar.

Fig. 12.3.15 A diaphragm check valve

o Tilting disc check valve - This is similar to the swing type check valve, but with the flappivoted in front of its centre of pressure and counterweighted or spring loaded to assume anormally closed position (see Figure 12.3.16). When flow is in the forwards direction, the disclifts and �floats� in the stream offering minimum resistance to flow. The disc is balanced so thatas flow decreases, it will pivot towards its closed position, closing before reverse flow actuallycommences. The operation is smooth and silent under most conditions. Note: due to thedesign of the tilting disc check valve, it is limited to use on liquid applications only.

Fig. 12.3.16 Operation of a tilting disc check valve

Open Closed Closed

Full forward flow Low flow Reverse flow

Forward flow Reverse flow

Open Closed



Pressure loss chartsAs most types of check valve are suitable for use on both liquid and gas systems, manufacturerstypically show the pressure drop across a valve in the form of a pressure loss chart for water. Atypical pressure loss chart is shown in Figure 12.3.17. It shows the pressure drop across a particularcheck valve for a given valve size and water flowrate in m3/h.

Fig. 12.3.17 A typical manufacturer�s pressure loss diagram

Equation 12.3.1

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

Where:Vw = Equivalent water volume flowrate (m³ / h)r = Density of the liquid (kg /m³)

V = Volume flowrate of liquid (m³ / h)

Once the equivalent water volume flowrate has been determined, the pressure drop across thevalve can be read off the chart using the same method as for water, selecting the equivalent watervolume flowrate instead of the actual volume flowrate.

It should be noted that the volumetric flowrate (in m3 / h) is typically quoted for liquid applications,whereas, in steam applications, the mass flowrate (in kg /h) is normally used. To convert fromkg /h to m3/h, the mass flowrate is multiplied by the specific volume (in kg /m3) for the particularworking pressure and temperature (see Equation 12.3.2).

Equation 12.3.2

Where:V = Volume flowrate (m³ /h)

m = Mass flowrate (kg /h)

n = Specific volume (m³ /kg)

!"

!!"!! !

#$$$

Wat

er f

low

rate

(V

w)

m3/h

Pressure loss in bar

DN100

DN80

DN65

DN50

DN40

DN32

DN25

DN20

DN15

0.5

0.7

1

2

3

5

7

10

20

30

50

70

100

200

0.01 0.02 0.05 0.1 0.2 0.5 1

0.2

0.3

0.5

1

2

3

5

10

20

30

50

Water flow

rate (Vw

) I/s



DN100

DN80

DN65

DN50

DN40

DN32

DN25

DN20

DN15

0.5

0.7

1

2

3

5

7

10

20

30

50

70

100

200

0.01 0.02 0.05 0.1 0.2 0.5 1

0.2

0.3

0.5

1

2

3

5

10

20

30

50

0.085 bar

17.6 m³/h

!"# $%

Alternatively, if the Kv value of the valve is specified, the pressure drop across the valve can bedetermined using the method outlined in Module 12.2.

Example 12.3.1Determine the pressure drop across a DN65 check valve passing 1 200 kg /h of saturated steamat 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 n = 0.214 9 m³ / kg

Using Equation 12.3.2

V =

V = 1 200 kg /h x 0.214 9 m³ / kg

V = 257 m³/h

The next step is to calculate the equivalent water volume flowrate:

Using Equation 12.3.1:

Since n = 0.214 9 m³ /kg, the density, r = = 4.65 kg /m³

Vw = 17.6 m³/h

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

! !

!!"!!

#$$$

Fig. 12.3.18

Pressure loss in bar

Water flow

rate (Vw

) I/s

!"#

$%%&%%'#(% %&%)(!"%* +,

)%---

Wat

er f

low

rate

(V

w)

m3/h



Questions

1. Which of the following is not a suitable application of a check valve?

a| To prevent waterhammer ¨

b| To isolate a heat exchanger for upstream maintenance ¨

c| To prevent damage to a flowmeter ¨

d| To divert flow in a blending operation ¨

2. Which of the following can be used to prevent the problems associated withswing check valves, namely waterhammer and seat wear?

a| Limit the velocity of the fluid, by increasing the pipe diameter ¨

b| Replace the metal seat with a soft (PTFE) seat ¨

c| Fit a damping mechanism to the flap ¨

d| Fit a wafer swing check valve ¨

3. A thermodynamic steam trap is used to drain a steam main.How far downstream of the trap should a check valve be fitted?

a| Less than 1 m ¨

b| At least 1 m ¨

c| As close to the outlet as possible ¨

d| It is not necessary to fit a check valve in this situation ¨

4. What advantage does a split disc check valve have over other types ofwafer check valves?

a| It is not limited in size ¨

b| The pressure drop across the valve is lower ¨

c| It can be used with lower opening pressures ¨

d| All of the above ¨

5. Which of the following may be used to eliminate the effects of valve chattercaused by oversizing a disc check valve?

a| Use a spring with a lower spring force ¨

b| Use a soft seat ¨

c| Replace the oversized valve with a smaller valve ¨

d| All of the above ¨

6. A disc check valve with the pressure loss diagram shown in Figure 12.3.17 is useddownstream of a control valve. The downstream pipeline has a diameter of 32 mm,and passes 200 kg/h of saturated steam at 5 bar g.Determine the pressure drop across the check valve?

a| 0.05 bar ¨

b| 0.25 bar ¨

c| 1.55 bar ¨

d| 5.00 bar ¨

1: b, 2: c, 3: b, 4: d, 5: d, 6: a Answers

nrv philosophy and application

Documents

check valvescheck valves

standard lift check

different types of check

pistontype lift check

check valvesthe steam

forward flow

o prevention of flow

reverse flow conditions