valves for steam facilities - ari...

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Dipl.-Ing. Erhard Stork, ARI-Armaturen Albert Richter GmbH & Co. KG, D-33756 Schloß Holte-Stukenbrock Tel.: +49 5207/994-0, Fax: +49 5207/994-297, E-Mail: [email protected], Internet:: http://www.ari-armaturen.com t000009294.doc 16.12.2003

Valves for Steam Facilities 1. Introduction

2. Valve constructions

3. Steam and the demands it makes on valves

4. Design solutions

4.1. Seat/plug area 4.2. Stem guide 4.3. Connections

5. Valve executions

5.1. Stop valves 5.2. Control valves 5.3. Pressure reducers 5.4. Safety valves 5.5. Steam traps 5.6. Strainers/water separators 5.7. Distributors/collectors 5.8. Other valves

6. Selection and sizing

6.1. Finding the right type 6.2. Defining sizes 6.3. Important information for correct product selection

7. Installation, operation and maintenance

8. Summary

9. References

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1. Introduction Because steam has good heat transfer properties, installations that use steam as the energy carrier are widespread in power stations, plant manufacturing, the chemical and petrochemical industries and the manufacturing industry. In addition to the main components such as boilers, heat exchangers and pipes, the valves for isolation, control, safety and steam trapping the medium are also highly important. Some, such as regulating valves and steam traps are indispensable for standard service, others, such as safety valves, are only needed in an emergency. Stop valves, for example, are needed for manual operation, compensation procedures, inspections and maintenance work. As well as high temperatures, the steam medium puts specific demands on the valves, as in a later physical state, it is almost always found simultaneously as condensate and as liquid.

2. Valve constructions There are various styles of valve that can actively affect flow and the one described here is based on the valve principle. Figure 1 shows this in diagrammatic form. A movable closing component, developed as a plug or a disc, is mounted in a housing. This prevents the medium applied to the inlet from exiting. Intermediate positions lead to throttling or control, thus affecting the flow. The plug can be operated manually, via actuators driven by a separate power supply or by the medium itself. A great variety of valve styles are derived from this basic type and these are described below.

Figure 1: Basic valve type

3. Steam and the demands it makes on valves In vapour facilities, steam almost always exists simultaneously in two physical states, "gaseous" and "liquid", as the maximum insulating strength of all installation components is limited and condensate constantly forms on the cooler inner walls of the components and the pipes. When hot steam meets greater collections of condensate, there is sudden evaporation. The changes in volume associated with this sometimes cause violent water hammers and thus powerful pressure surges that can far exceed the operating pressure. Water hammers also occur if vapour bubbles

Closing component

Body

OutletInlet

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are absorbed in the condensate, which is cooler in comparison with the vapour, so that condensation causes the bubbles to implode all of a sudden. With valves, there is then the risk of interior damage or even total operational failure, seals can malfunction and in the worst-case scenario, rupture the valve body. Another pressure situation that puts great stress on the valves is evaporation of the boiling hot condensate. If this is diverted from the vapour system and the pressure reduces, the severity of evaporation increases, the greater the pressure difference and the higher the temperature of the incoming condensate. As this drop in pressure chiefly takes place in the narrowest cross-section of the valves, the stress is also at the maximum there. The risk of washout and material abrasion is then apparent if the components are not designed accordingly. The condensate is not pure water. If the boiler water is not adequately conditioned, this may result in residual acids, salts and bases. If air and therefore oxygen gets into the vapour facility, for example, when it is not being used, there are highly aggressive chemical reactions with the ferrous products of the installation components, such as the pipes and the valves. The associated material abrasion of all the installation component surfaces that come into contact with the media causes particles of rust to form that then pollute the entire installation. Larger particles are carried along in the media flow and cause wear on the valve guides and seals. At points where the flow is diverted, there is erosion and washout. The finer particles turn into corrosion sludge that builds up and leads to blockages in the finer channels and gauging holes.

4. Design solutions 4.1. Seat/plug area The plug (or disc) as the closing element, stops the medium applied to the inlet (steam/condensate) from getting to the outlet, by pressing on the seat. As well as this "main task", in many cases the plug/seat unit also has a second important function to fulfil, that of throttling the flow. Whereas during isolation, the requirements are mainly concentrated on a permanently good seal, during throttling, the stability of the plug movement and the flow characteristic come to the fore. The seat/plug area is also exposed to the greatest stresses in the valve, as this is where the flow is at its fastest because of the minute cross-section of the flow. The associated flow forces working on the plug must be safely intercepted. The saturated steam still mostly contains condensation droplets or these reform on the cooler walls and at these high flow rates, lead to erosion. Plugs and seats damaged in this way then cannot close tightly, with dirt accumulation and deposits also working to the greatest detriment with regard to leakage.

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In vapour facilities, because of the high temperatures, the most commonly found type of valves are those with a metallic plug/seat seal in the "flat seat" (Figure 2) and "marginal seat" (Figure 3) types.

Figure 2: Flat seat Figure 3: Marginal seat Flat seats are used to isolate and are less suitable for regulating and throttling functions, as when there is little change in the travel, the flow increases very rapidly shortly after the valve starts to open. If valves with flat seats are operated rarely, deposits and encrustations can lead to the valve no longer being able to close tightly. The minimum width of the seat with the plane-parallel bearing makes it more difficult for this accumulation of dirt to be removed, even if great force is applied. The associated risk of pressure point formation and thus damage to the facings means that it is no longer possible for the valve to close without leakage. Marginal seats with their conical seating are better for flow and more suitable for throttling and regulating functions. Dirt accumulation in the area of the seal, which occurs after longer periods of operation with few or even no closing actions (encrustation/deposits), can be removed so well with this type of seating that a leakproof closure is obtained. The valve seat shown in Figure 4 has been in use over a long period of time and the deposits on the seat are clear to see. The narrow sealing edge of the marginal seat is easily recognised as the light line.

Figure 4: Marginal valve seat with deposits

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To further increase the service life, it is usual to harden the plug, as this receives the greatest exposure to the particles carried along by the flow. This is further improved by armouring or stelliting the seat and the plug. This procedure is frequently used in the regulating valve area.

4.2. Stem guide The second important function of the valves, ensuring that there is a leakproof seal keeping the medium in, is given by the body components, the seals and the stem guide. Here the latter makes the most stringent demands on the design and construction of the valves. The body and the accompanying seals are static components, movement does not occur at the seals. The stem guide, on the other hand is subjected to dynamic stresses caused by the movement of the stem, which can be axial, radial or a combination of the two. Manual stop valves are usually not operated very often, regulating valves, on the other hand are used anything from frequently to constantly. As far as the stem seal is concerned, there are three different designs for use in steam and condensate applications: - gland packing - V-ring unit - stainless steel bellow seal Gland packing, the oldest type of seal for valve stems is shown in Figure 5. The packing material made of pure graphite is pretensioned using packing fixtures and distributes the resultant pressure on all sides, thus sealing the gap between the stem and the upper part of the body (the bonnet) towards the outside world. Leakages caused by the easing of the packing pressure can be combated by retightening, which automatically involves a certain amount of maintenance expenditure. If there is too much pretensioning, this can also cause a great deal of friction at the stem, which inhibits its movement. If these forces become too great, it is no longer possible to operate a manual valve or the function of the regulating valves is restricted. Figure 5: Figure 6:

Gland packing V-ring unit stem seal The V-ring stem seal shown in Figure 6, provides an improvement with regard to maintenance, as the PTFE V-ring units are permanently pretensioned by means of

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springs, thus ensuring a constant seal. The frictional forces are also defined by this and are not dependent on the manual force applied by the particular fitter. One disadvantage is the limited temperature of max. 220°C. This is why it is necessary to know the operating parameters exactly and application is restricted mainly to the control valve sector. The bellow seal shown in Figure 7 provides a permanently leakproof and maintenance-free stem seal, even at high temperatures. The material most commonly used for the bellow seal is austenitic stainless steel 1.4541 or 1.4571, which also shows adequate corrosion resistance even with an aggressive condensate. The sealing does not cause any additional frictional forces and when used in manually operated or power-operated valves, the rigidity of the spring- constant makes the forces negligible. With valves that work automatically, without auxiliary power, such as pressure reducers and safety valves, rigidity is already taken into account in the design and remains constant throughout the life of the valve. Depending on its design, the bellow seal simultaneously protects the stem guide from the medium and prevents wear from dirt particles.

Figure 7: Bellow stem seal Figure 8: Flange connection 4.3. Connections The valve is connected to the pipe in the steam and condensate area by means of a flange or it is directly welded on. The valve connections must be developed accordingly, Figure 8 shows the flange connection and Figure 9 the welded end. Flange connections have the advantage of being easier to dismantle when it is necessary to replace a valve. The weak point is the seal, which may fail, as a result of changes in temperature, for example. The sudden blowing of a seal also represents an increased risk potential thanks to the high temperatures present in the steam and condensate. This problem does not occur if the valve is welded directly to the pipe and this is also a more cost-effective solution, as it does away with

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Figure 9: Welded end Figure 10: Shoed end

the need for two welded flanges including fitting for each valve, with the need to create weld seams in both cases. But should this need to be removed, higher expenditure is involved. As the material of the valve is normally different to that of the pipe, welding the valve to the pipe puts greater demands on execution (welding technique, post-treatment, such as annealing, etc.). For this reason, valves are available with shoed ends in the material of the pipe (Figure 10). Thus the highly demanding (different materials) welding operation is moved from the building site to the valve factory. On site at the plant, the same materials are welded together and expenditure is sometimes far less.

5. Valve executions 5.1. Stop valves Should flows of steam and condensate need to be isolated, the "ARI-FABA für Energien" type (Figure 11) is particularly suitable, thanks to the combination of

Figure 11: Stop valve "ARI-FABA für Energien" [1]

Hand wheel

Bonnet

Stem

Gland packing

Bellows seal

Plug

Seat

Body

OutletInlet

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special design solutions. Depending on the nominal diameter, the body of the valve can also be made of the ductile materials spheroidal graphite iron GGG 40.3 or cast steel 1.0619+N (>DN 50). The plug sealing to the edge of the seat is made by a marginal seat, with the plug being hardened to improve the resistance to wear. The stem guide is double-sealed, once by a stainless steel bellow seal that simultaneously protects the guide from dirt and secondly, through a secondary sealing gland packing, which, should the bellow seal become damaged, takes over the sealing function for the transitional period until the valve is replaced. The enclosure of the threaded stem in the handwheel area stops dirt getting onto the thread, which would make it run sluggishly. On the other hand, a bare, greased thread would greatly consolidate the local dirt and there is usually no cleaning before the next operation. With this design, the handwheel is also "non-rising" and thus, even at different settings, always remains at the same height relative to the valve body. The extra-fine thread stem produces a better force ratio with the advantages of being smooth running and having greater plug pressure acting on the seat.

5.2. Control valves Vapour facilities are seldom only operated at full load, usually it must be possible to set all the operating states between 0 and 100% for the individual areas of the entire installation independently of one another at any time. This is achieved by throttling the vapour flow by means of control valves. Figure 12 shows the valve that is particularly suited to this task, the "ARI-STEVI 470" valve, that is

Figure 12: Control valve "ARI-STEVI 470" [1]

Actuator

Yoke

Stem

Stem guide

V-ring unit

Mounting bonnet

Plug

Seat

Body

Outlet Intlet

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operated by an electric actuator. The marginal seat plug has a parabolic contour, which makes it possible to regulate even the smallest flow rates precisely and accurately. The seat is screwed and because of the vast number of different diameters, as well as the usual nominal diameter selection, there is also a fine graduation in the flow output and thus the option of accurate sizing. The plug stem guide allows precise regulation and the hardened bush makes this execution particularly resistant to wear. The valve shown has a V-ring unit seal for the stem guide. If the temperature of the medium is higher than the permitted level, an execution with a stainless steel bellow seal is also available. Further details are given in [1].

5.3. Pressure reducers In contrast to control valves, pressure reducers operate without auxiliary power and are driven solely by the medium. Their function is reducing a high pressure (upstream pressure) to a lower pressure (downstream pressure). At the same time, the downstream pressure is automatically kept constant during changes in the flow rate or fluctuations of the admission pressure. A pressure reducer of the "ARI-PREDU" type, as shown in Figure 13, is particularly suited to meet the demands of the steam medium. A marginal seat is screwed into the transitional body. The plug has a small parabolic lug, that keeps the regulating process safe from vibration at the lowest flow rates. In the same way as with the control valve, a spindle guide with a hardened bush is also available here. This pressure reducer has two stainless steel bellow seals. The lower one serves to seal the spindle against the outside world. The upper one is the compensating bellow seal, whose function is to equalise the forces at the plug. To do this, the admission pressure goes through a hole in the plug in the interior space to the outside of the bellow seal. The inside of the bellow seal is connected to the downstream pressure side via openings. As the effective surface of the bellow seal is the same size as the seating, the differential forces are compensated for and fluctuations in the admission pressure have very little effect.

Figure 13: Pressure reducer "ARI-PREDU" [1]

Seat

Body

Plug

Compensating bellow

Spring

Bellows seal

Actuator

Water seal pot

Downstream pressure

Upstream pressure

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The pressure reducer is driven by a diaphragm actuator. The downstream pressure to be regulated gets to the actuator diaphragm via the water seal pot and the control line with a hydraulic seal (protection against high temperatures) and is converted to a pressure that acts in the opposite direction to the spring pressure. The pre-tensioning of the spring can be adjusted in such a way that at the desired downstream pressure, both forces are counterbalanced. If the vapour flow then changes, this leads to an adjustment of the plug until equilibrium is restored. Further details, design tips and data on the "PREDU" can be found in [1] and [2]. 5.4. Safety valves Safety valves are necessary to protect the components of vapour facilities, such as steam boilers, deaeraters and installation areas after the pressure reducers from inadmissibly high pressure. Figure 14 shows the type suitable for this, the "ARI-SAFE" type, that has an angle body.

Figure 14: Safety valve "ARI-SAFE" [1]

The disc (plug) isolates the pressurised medium applied to the inlet from the usually atmospheric pressure prevalent at the outlet. The pre-tensioned spring in the spring bonnet transfers the pressure to the disc via the spindle. The pretensioning of the spring can be varied with the adjusting screw, thus altering the set pressure. The hardened plug is flexibly mounted and an even set pressure is ensured even if it has been idle for a long time, thanks to the sensitive flat seat lapping. As the safety valve is usually always closed, the seating surface is covered by the disc to protect it against deposits. At high steam temperatures, the open spring bonnet prevents too much heating, as if the temperature of the spring is too high, this simultaneously lead to a lessening of force and a reduction in the set pressure. When used in deaerators and in the condensate area, on the other hand, an enclosed execution is required. The drainage opening closed with a screw can be used if steam trapping via the

Lifting device

Adjusting screw

Spring

Bonnet

Plug

Seat

Drain plug

Body

Outlet

Inlet

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exhaust line is insufficient. For further details and information on safety valves, see [1, 3].

5.5. Steam traps Steam traps are valves that automatically divert the condensate in vapour systems, but which retain the vapour. With the "ARI-CONA" product line, you have available an extensive range for a great variety of applications. The distinction is made here between two groups: firstly there are the traps that continuously carry away all the accruing condensate without delay, like the "CONA S" shown in Figure 15, that works on the float principle. As soon as any liquid flows into the cover, the ball floats, the valve opens and the condensate can flow away. The vapour arising from the steam trap is then retained.

Figure 15: Float steam trap Figure 16: Bimetallic steam trap "CONA S" [1] "CONA B [1] On the other hand, the second group functions at a specific undercooling of the condensate, an example of which is shown in the bimetallically driven "CONA B" in Figure 16. The metallically packaged controller opens below the saturated steam temperature, to ensure that only condensate is removed. The buildup of condensate that this produces is sometimes also intentional, if the heat of the condensate is to be used. The temperature at which the valve opens can be varied according to the choice made from the controllers available. Additional types that also remove the condensate below the saturated steam temperature and thus delay condensate removal, are the enclosed diaphragm "CONA M" trap and the thermodynamic "CONA TD" trap. With all the traps shown here (apart from the CONA TD), the starting dewatering and deaerating function, that is to say the immediate removal of large amounts of condensate when drying out the installation, is integrated. Below the operating temperature, the valve is fully open, it is only when this temperature is reached that the trap works in its designated function. As steam trapping as a whole is an extremely complex area and there are a vast number of products variants available, we refer you to the additional information at this point [1, 4, 5].

Body

Controller Hood

Body

Controller

Cover Inlet

Outlet

OutletInlet

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5.6. Strainers/water separators In its two physical states, the medium of steam is in practice only rarely free of impurities and accumulated dirt. Strainers (Figure 17) with interior screens are ideal for filtering out coarse matter, such as welding residue and particles of rust. Screens are available in various mesh sizes. When making the selection, you have to find a compromise between good separation and a permissible maintenance interval / loss of pressure. At high steam speeds, droplets of condensate can cause erosion and material abrasion.

Figure 17: Strainer [1] Figure 18: Water separator Fine matter in the vapour can be separated together with these condensate droplets by means of water separators (Figure 18). These comprise a housing with an internal, spiral-shaped sheet, which uses centrifugal forces to separate the droplets and the contaminants. The condensate must be diverted via a steam trap at the lower dryer connection. To protect the seats and the plugs against damage and to minimise wear, strainers and if possible a water separator should always be arranged before the control valves and the pressure reducers.

5.7. Distributors/collectors To supply the individual consumers in vapour facilities, the pipes are normally distributed from a central main to the individual lines. The reverse is true for the condensate, this occurs locally in the individual lines at the consumers and has to be brought together again centrally by means of collecting mains. This used to mean complicated welded structures consisting of lengths of pipe, dished boiler ends, connecting sleeves and stop valves. The variable modular design of the compact distributor "ARI-CODI" (Figure 19) with integrated stop valves vastly reduces this expenditure. The functioning parts of the valves can be replaced without having to remove the entire distributor from the pipe. The individual valves have a stainless steel bellow seal to seal the stem and have available a safety return seal which, if the bellow seal is damaged and the valve is fully open, provides a temporary seal until it is possible to effect replacements. For more information on the compact distributor, see [1, 6].

Body

Cover

Screen

OutletInlet

Outlet Inlet

Spiral plate

Trap connection

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Figure 19: Compact distributor "ARI-CODI" [1]

5.8. Other valves In addition to the valves described here, there are other types of valves that are useful and in special cases essential for the efficient and trouble-free operation of vapour facilities. These include venting valves (vacuum breakers), flow indicators, condensate operating temperature limiters, automatic starting dewatering machines and return temperature limiters. These valves are described in more detail in [1, 5]. 6. Selection and sizing 6.1. Finding the right type of valve If the required type of valve is not stipulated when product selection begins, a rough selection has to be made on the basis of your knowledge of the plant and the parameters. The following points should be of assistance for this: - The requisite function (e.g. isolation, control, safety, steam trap) - Combining different functions in one valve - Permissible limits/tolerances (e.g. control performance, loss of pressure) - The medium (steam only, condensate only, etc.) - The demands made on steam quality (e.g. pure steam for sterilisation) - Fitting position - Type of connection (e.g. flange, welded end) - Remote maintenance/diagnostic options These two examples should make things clearer: If a higher pressure is to be reduced to a lower one, the simplest option is simply static throttling, for example, by using a manual valve, where the required lower pressure is set once. At a steady flow and constant admission pressure, this option is certainly adequate. If the conditions are not constant, then a pressure reducer driven by the intrinsic medium can be used, which will automatically adjust to the various operating states. However,

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because they regulate proportionally, there is a certain deviation with pressure reducers. Although if there are stringent demands for control accuracy, control valves must be used, which meet these requirements in conjunction with appropriate electronic or pneumatic controllers. A further example is the selection of steam traps. Knowledge of the plant is necessary for this, for example, is immediate trapping required before the regulating valves. On the other hand, a defined buildup of condensate is desirable with steam-heated devices, to take advantage of the heat of the condensate.

6.2. Defining sizes The size of the chosen valve is determined by the flow rate of the steam or the condensate for which it is intended. If the selected valve is too small, it provides too much resistance, which could cause subsequent consumers to be undersupplied or could allow too much condensate to build up. But there can also be problems with valves that are too big, as well as the associated higher costs. If the selected regulating valves are too big, the control properties worsen in line with the degree of oversizing. For example, if a control valve with a linear flow characteristic is chosen twice as big as the maximum required, this achieves the full flow required at approx. 50% of the travel distance. This means that the positioning accuracy and thus the accuracy of control, is worse. Safety valves that are oversized tend to vibrate and can hammer. These associated mechanical stresses do not only affect the seat and the plug and thus the quality of the seal. In extreme cases this can also result in the valve actually breaking. Complete and correct valve sizing requires knowledge of the operating parameters, such as the pressures, the temperatures and the flow rates. Various sizing aids are available to the user, such as tables, charts and calculation slides. The computer sizing program "ARI-VASI" is a fast, reliable and accurate option for size measurement. As well as pure sizing, it is also possible to select the valves and specify the precise type and manage it to suit the project. The calculation formulae necessary for sizing are stipulated mainly by a vast number of specified standards and regulations, as well as the pertinent technical documentation. For example, [8] is applicable to stop valves and control valves and [9, 10, and 11] apply to safety valves. They apply to the situation where the steam medium remains in the same physical state and there is no phase change. With steam trapping, on the other hand, there is usually evaporation, as the boiling hot or only just below saturated steam temperature condensate loses pressure over the trap. There are not yet generally valid, protected regulations defined by the specified standards for this partial evaporation. The charts available for steam traps are therefore based on measurements.

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6.3. Important information for correct product selection In addition to simply calculating the sizes of the valves, there are also additional points to take into account for correct selection and determination. The choice of material also involves correctly determining the nominal pressure level subject to the temperature, to apply to all the valve types. For example, Table 1 shows the following temperature/pressure assignment for flanges made of cast steel 1.0619+N:

Table 1: Pressure/temperature classification for flanges made of cast steel 1.01619+N (DIN EN 1092-1) The type of stem seal, e.g. bellow seal or V-ring unit, must also be stipulated. With control valves, once you have selected the valve, there are various options available for choosing the method of actuation, but this is not covered by this essay.

7. Installation, operation and maintenance For a steam valve to function properly, not only must the correct valve be chosen and the size be right, it must also be fitted correctly. The list below reflects the most important possible errors, although others can be found in the relevant operating and installation instructions: - The shipping braces and protective caps on the inlet and outlet have not been removed - Attention has not been paid to the direction of flow and the mounting position - The pipe is not properly supported, the forces and the torque fall on the valve - The flange seals are not fitted centrally and are constricting the media flow path Commissioning is also described in the operating instructions, possible errors here include: - Installation not rinsed before valve is used for the first time - Strainer not subsequently cleaned - Hydraulic seal for pressure reducers not filled - Any test gags fitted for the safety valves have not been removed - The blow-off line for safety valves is not connected Maintenance includes cleaning the strainers and regularly "desludging" the condensate collector supports. Leaky gland packing on the stem guides should be tightened, if possible. Safety valves must be vented from time to time, to test that they work. However, this should not be done too often, as this will wear the sensitive lapping on the facings, as it is not possible to have strainers before the safety valves.

max. perm. pressures in bar at temperature 20°C 100°C 150°C 200°C 250°C 300°C 350°C 400°C 450°C

PN 25 25 bar 23.3bar 21.7 bar 19.4 bar 17.8 bar 16.1 bar 15 bar 14.4 bar 13.9 barPN 40 40 bar 37.3 bar 34.7 bar 30.2 bar 28.4 bar 25.8 bar 24 bar 23.1 bar 22.2 bar

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8. Summary Vapour facilities make particular demands on the valves, as the medium of steam occurs in the two physical states of steam and condensate. The most important valve types among the great variety of variants that are required to operate these plants are described, design details are explained in more detail and the advantages and disadvantages of use with this special medium are shown. Following selection and sizing some actual possible errors that may arise through incorrect installation and commissioning are presented.

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9. References [1] ARI-Armaturen: Manufacturers’ Catalogue 2003 edition [2] Stork, E.: Dampfdruck-Reduzierstation für die Prozess- und Anlagentechnik. Industrial Valves Magazine Issue 2/2000 [3] Stork, E.: Das Edelstahl-Sicherheitsventil als Hauptkomponente zur Druckabsicherung von Anlagen mit korrosiven Medien. Industrial Valves Magazine Issue 4/1996 [4] Dr. Urbanek, H.; Böhm, L.: Eine neue Generation thermischer Bimetallkondensatableiter. Industrial Valves Magazine Issue 2/1996 [5] ARI-Armaturen: Condensate management advisor (in progress) [6] Dr. Urbanek, H.; Weiß, W.: Neues Sammel- und Verteilsystem für Wärmeträgermedien. Industrial Valves Magazine Issue 3/2002 [7] ARI-Armaturen: Computer valve sizing program ARI-VASI [8] DIN EN 60534: Stop valves for process control [9] TRD 421: Safety valves for overpressure prevention - safety valves - for steam boilers [10] DIN EN 12952-10: Water tube boiler – the demands made on safety devices to prevent excess pressure [11] DIN EN 12953-8: Large waterspace boilers – the demands made on safety valves for overpressure prevention

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