1/16 The benefits of system solutions for valve users 1. Introduction 2. System types 2.1. Pressure control 2.2. Temperature control 3. Technical process design 3.1. Checking of operating data 3.2. Selection of system type 3.3. Description and dimensioning using a pressure reducing station as an example 4. Documentation 5. Certification 6. Summary 7. Bibliography 3.3.1. Pressure reducer PREDU 3.3.2. Globe valves FABA / orifice plate 3.3.3. Safety valve SAFE 3.3.4. Strainer / seperator 3.3.5. Steam trap CONA 3.3.6. Other components
2/16 1. Introduction The correct functioning and right interplay of widely varying components e.g. valves in general installations, power plant stations, chemicals and petrochemicals and the processing industry, are essential for economical and smooth operation. The number of valves introduced runs from a few in small systems to several thousand in large and complex ones. These large systems must nevertheless be seen in respect of every detail not as a whole, but are able to be divided into individual subsystems. Since the functioning of these subsystems must also be able to be accurately and individually defined, it is possible to take these parts entirely out of the overall system and to project them on the basis of operating data to be accurately laid down. The planning department, which is responsible for the entire installation, is able to concentrate on the important main functions and so ensure that the overall position fulfils the actual principal task, for example energy conversion, or the production of a chemical base material. The detailed design and provisions of optimum coordination of individual components are the responsibility of the supplier of the subsystem. The subsystems can be delivered by the supplier in one of two different forms, depending on the position of incorporation, either as a complete assembly of the subsystem at the supplier s, or as components, which are then assembled at their destination. In either case, the subsystem is dimensioned by the supplier and the covering documents are created. 2. System types Standard systems for pressure and temperature control are set out below and can be varied individually as desired. 2.1. Pressure control A steam pressure reducing station without auxiliary energy is shown in Fig 1. Pressure control takes place via a special control valve, the pressure reducer PREDU (Pos. 1). This selfacting valve requires no auxiliary energy. For the operation of a pressure reducer, a series of other auxiliary and monitoring valves are required. The concept of "steam pressure reducing station" embraces all these components, including pipes. Two distinct sets of pipes are recognised, the main pipe divided into the upstream and downstream pipe, as well as the bypass pipe. The steam flows first of all through the upstream pipe, then the globe valve FABA (Pos. 4), the strainer (Pos. 5) and the steam trap (Pos. 3) before then reaching the pressure reducer. Following pressure reduction in the pressure reducer, it reaches a further globe valve (Pos. 6) in the downstream pipe to the station output, the safety valve SAFE (Pos. 2) being directly connected to this area. The nominal diameter of the pipe depends on the maximum admissible speed of flow.
3/16 Fig. 1: Steam pressure reducing station without auxiliary energy The exact dimensioning is described under point 3. Technical process design". On the admission of steam, the control pipe and the water seal pot (Pos. 7) must be filled with water. The diaphragm of the pressure reducer hanging downwards together with the actuator is thus protected from the high steam temperatures. The bypass pipe is required for the manual operation of the downstream system in the case of maintenance of the strainer and the pressure reducer. For this purpose, globe valves in front and behind the pressure reducer are closed and the globe valve in the bypass pipe (Pos. 8) is opened. Watching the manometer (Pos.10), the operation is to be maintained manually, with the safety valve performing the task of pressure protection in this case as well. In steam operation, condensate constantly forms in the pipes and must removed via the steam trap. Fig 1 clearly shows the diversion in the pressure pipe via the steam trap CONA -S (Pos. 13). The upper globe valve (Pos. 11) is normally open and is closed only in the case of a maintenance operation on the trap, the lower globe valve (Pos.14) being provided for the removal of sludge and is normally closed. Using the sight glass (Pos. 12) it is possible to observe the flow of condensate and thus to control the functioning of the trap. Condensate also forms in the downstream pipe. In particular in the case of a small flow and a almost closed pressure reducer, the condensate level can rise. In the case of a pressure reducer suddenly opened by increased output demand, water hammer occurs, which in principle is to be avoided. Fig. 1 shows the outflow using a bimetallic steam trap CONA -B (Pos. 17).
4/16 For controlling the reducing station, gauges in front and behind the pressure reducer are useful and the inlet pressure between the strainer and the pressure reducer should be measured, since a blockage of a strainer can be recognised by a pressure drop. For simplifying the setting and an easier recognition of any disruption, it is useful to arrange the measurement of the minimum pressure in the proximity of the pressure outlet point of the control pipe. Fig 2 shows a pressure reducing station with auxiliary energy, where instead of a pressure reducer, a pneumatic control valve (Pos.1) is introduced. In place of a outlet pressure pipe with a water seal pot, the pressure to be controlled is measured by a sensor and taken to an SPS control. Here, the actual measured value is compared with the preset value and the resulting control signal (e.g. 4-20 ma) is taken to the positioner of the control valve. In addition to the electrical energy for the SPS control and the positioner, the control valve requires a further source of auxiliary energy, in this case compressed air. In contrast with the pressure reducing station, the remaining valves are the same without auxiliary energy and are also needed to perform the same functions. Fig. 2: Pressure reducing station with auxiliary energy
5/16 2.2. Temperature control Fig. 3 shows a temperature regulating station without auxiliary energy, with steam as heat transfer medium and a tube bank heat exchanger as the heat transfer device (Pos. 21). The temperature regulating valve TEMPTROL (Pos. 1) functioning as the central component of the installation is shown in Fig 4. It functions without auxiliary energy on the principle of liquid expansion. Temperatures on the sensor bring about a change in volume in the capillary tube system, which are converted into a lift change in the actuator. The set value can be accurately set on the display unit. The requirements for temperature regulators are given in [2]. Fig. 3: Temperature regulating station without auxiliary energy After having passed the globe valve FABA (Pos. 4), the strainer (Pos. 5) and the seperator (Pos. 3) the steam is taken to the temperature regulating valve. After passing through a further globe valve (Pos. 6) it reaches the heat exchanger. Here, the steam is completely condensed via heat transfer to the secondary circuit
6/16 Adjusting knob Indicator unit Set point indicator Valve Actuator Capillary tube Sensor Fig. 4: Temperature regulator TEMPTROL condensate collection system via the steam trap CONA-S (Pos.17). By reason of its construction, this trap adapts itself immediately to the quantities of condensate formed according to the opening of the temperature regulating valve. The drained condensate from the seperator is collected as in the case of the pressure reduction station, but due to its higher temperature, it is taken via a nozzle tube (Pos. 22) to the condensate collection system. For starting the installation, a vent (Pos. 23) is arranged between the temperature control valve and the input of the heat exchanger unit. The vacuum breaker (Pos. 24) next to the vent prevents an inadmissible vacuum, which may formed during the cooling of the installation. The temperature control station shown in Fig 5 differs from that shown in Fig 4 only through the use of a control valve with an electrical actuator and an integrated electronic temperature regulator (Pos. 1). This system accordingly functions with electrical auxiliary energy, a sensor (e.g. PT100) being used for temperature detection.
7/16 Fig. 5: Temperature regulating station with auxiliary energy 3. Technical process design 3.1. Checking of operating data An accurate design of a system is not possible without knowledge of exact and complete operating data. To ensure a rapid and correct design, the use of a checklist such as that shown in Fig 6 is useful. Mistakes can be avoided and the time needed for delivery shortened. 3.2. Selection of the system type The selection of the system type is determined in the first place by the principal requirements placed on the subsystem of the complete plant e.g. pressure control, temperature control and the like. The decision on "without auxiliary energy" or "with auxiliary energy" is governed by several factors. First of all, it is necessary to answer the question whether the required auxiliary energy is available in the location in question. Next, the requirements placed on control characteristic and accuracy must be checked. In the case of systems with auxiliary energy, the characteristic and accuracy can be
8/16 altered according to the regulators employed. Systems without auxiliary energy work with proportional regulators, where options of varying accuracy are limited. Behaviour, which may be required in the case of a failure of auxiliary energy is also a codeterminant of selection of installation type; for example, control valves with a safety function in the event of energy failure may be required. Fig. 6: Checklist for project planning
9/16 3.3. Description and dimensioning, using the example of a pressure reducing station The basic procedure for the design of installation systems is described in detail below, using the example of a steam pressure reducing station (Fig 1) for individual valves/components. The dimensioning of the valves is given on the basis of the following installation data Medium : Saturated steam p 1 : 12 bar(g) p 2 : 8 bar (g) PS 1 : 14 bar(g) PS 2 : 10 bar(g) Capacity : 3000 kg/h It should be noted that the pressures p 1 and p 2 reproduce the normal operating conditions, whereas PS 1 is the protecting pressure of the feed system and PS 2 the protecting pressure of the system components behind the pressure reducer = set pressure of the safety valve of this pressure reducing station. 3.3.1. Pressure reducing valve PREDU The functioning of the pressure reducer is explained by means of the diagram (Fig 7). The minimum pressure reaches the diaphragm in the actuator and is converted into a Upstream pressure Downstream pressure Seat Water seal pot Disc Spring Actuator Diaphragm Control line Fig. 7: Pressure reducing valve PREDU
10/16 force, which is directed against the force of the spring. By means of setting it, it is possible to alter the pre-tension of the spring in such a way that at the desired minimum pressure the two forces are in balance. If the quantity of steam being extracted is then changed, the plug is moved until balance is again reached. To protect the diaphragm from excessive steam temperatures, the control pipe and the water seal pot are filled with water. The quick and accurate calculation of the pressure reducer PREDU is made possible through the computer programme ARI-VASI [3]. After the input of the before mentioned data and the selection of pipes according to the maximum speed of 25 m/s for saturated steam (D 1 : DN 80; D 2 : DN 100), a K v value of 22.7 is obtained. Taking into account a correction factor of 1.25, the programme selects a pressure reducer DN 50 with K vs 32. According to the catalogue [1], a choice of set value ranges of 4.5-10 bar and 8-16 bar is available for this pressure reducer. Here, the range 4.5-10 bar should preferably be selected, since the control accuracy will then be smaller. 3.3.2. Globe valves FABA / orifice plate The nominal sizes of the globe valves FABA (Pos. 4, 6) in front and behind the pressure reducer are made equal to the nominal diameters of the pipes described in 3.3.1. The size of the bypass pipe is dimensioned as the pipe in front of the pressure reducer. In the event of the pressure reducer being taken out of service, the globe valve in the bypass pipe must allow the required capacity to pass through. Since, however, this valve is chosen to be of the same size as the bypass pipe, its output is usually greater than required; in the concrete case, the globe valve FABA DN 80 has a K vs value of 89.5 [1] and hence as against the required value of K vs = 22.7, an approximately 4 times output. The maximum flow-through quantity must accordingly by reduced by means of a orifice plate arranged at the output of the valve. The calculation gives a orifice plate diameter of 24.3 mm. To ensure an exact setting, this valve is to be equipped with a regulating plug. In order to prevent misuse, the hand wheel should be secured with a sealed cap. 3.3.3. Safety valve SAFE The safety valve SAFE serves for the protection of the downstream pressure installation part behind the pressure reducer, corresponding to the maximum possible pressure on the associated components and pipes. The arrangement must ensure that the downstream pressure pipe, the pressure reducer actuator and the bypass pipe are directly connected to the safety valve (see Fig 1, Pos. 2). Reference must be made here to the required blow-off pipe behind the safety valve, which for reasons of clarity of representation is not however shown in Fig 1. Like all steam pipes, this too must be drained and the drained condensate safely led away. The size of SAFE valve is determined as follows. First, the set pressure is laid down, using the general guide p 2 + 25%, but < PN actuator and/or < PN minimum pressure
11/16 system, in this case PS 2 = 10 bar(g). The maximum blow-off is obtained from the flowthrough capacity of the pressure reducer (see 3.4) calculated with ARI-VASI [4] (PS 1 +10% = 15,4 bar(g)/ps 2 = 10 bar(g) with Q = 5326 kg/h. The size determination of SAFE using the programme ARI-VASI gives figure 902 DN 50. The safety valve of this size is nevertheless only correctly dimensioned, if the bypass valve is not simultaneously open to the pressure reducer. Since, however, both pipes are able to be opened, the flow-through capacities of the pressure reducer and the bypass pipe must be added together. In this case, the blow-off output, which is to be removed, rises to 9191 kg/h and the required SAFE to DN 65. Since with the bypass pipe closed, the SAFE is overdimensioned, it is in this case recommended to introduce a second SAFE. The set pressure of the first SAFE should then be somewhat lower. The requirements placed on safety valves are defined in [4,5]. 3.3.4. Strainer/ seperator For the protection of the plug and the seat of the pressure reducer, the addition of a strainer (Pos. 5) is required. To prevent accumulations of condensate, the strainer should be mounted with the sieve lying on the side. The nominal width chosen should be the same as the diameter of the pipe. To remove the condensate drops carried by the flow, which may lead to erosion in the plug/seat area of the pressure reducer and its diversion, a seperator (Pos. 3) is placed in front of the pressure reducer. The associated drying of the steam has a certain simultaneous cleaning effect on any finely-divided dirt, which cannot be removed by the screen of the strainer. 3.3.5. Steam trap CONA The condensate, which collects in the seperator, must be removed without delay. This takes place via the steam trap CONA -S (Pos. 13). The size of this trap depends on the quantity of condensate in the area to be drained in front of the pressure reducing station. The determination of size will not be discussed here. The area behind the pressure reducer should also be drained and Fig 1 shows a bimetallic steam trap CONA -B (Pos. 17) used for this purpose. 3.3.6. Other components Further components are required for the monitoring of the functioning of the pressure reducing station (see Fig 1). The functioning of the steam trap can be controlled visually with the help of sight glasses (Pos. 12, 18). From time to time, it may become necessary to blow out the sludge, which may have collected in the condensate supports by the opening of the sludge valves (Pos. 14, 16), where a safe removal must be ensured. Using the gauges (Pos. 9, 10) in front and behind the pressure reducer, it is possible to check the control and possibly to correct the minimum pressure setting by means of moving the adjusting plate on the pressure reducer. Further information concerning the steam pressure reducing station and the valves for steam applications is given in [6,7].
12/16 4. Documentation To ensure perfect delivery, correct installation and commissioning, smooth service and many years of operation requiring only limited maintenance of an installation or of a system, a correct and complete documentation is needed. This includes first of all a general data sheet containing all the important information such as for example, versions and dimensions and the like as shown in Fig 8. Fig. 8: Data sheet for ARI pressure reducing station
13/16 Fig. 9: System specification sheet for ARI pressure reducing station
14/16 The current station is described in the installation specification sheet containing exact product data (see Fig 9). In addition, every installation has an operating and installation instruction, with all data concerning assembly and commissioning, including safetyrelevant instructions (Fig. 10). Fig. 10: Operating and installation instructions for ARI pressure reducing station
15/16 5. Certification A plant system, which is to be supplied and installed in Europe, is subject to various European guidelines. The most important is the "Pressure equipment directive" (PED) 97/23/EC [8], which specifies various conformity assessment procedures according to size of installation and the potential dangers. The assessment of an installation is governed by the highest category of an installation component (article 10 paragraph 2), with safety function valves such as safety valves not being taken into account. If the limits defined in article 3 paragraphs (1) and (2) (PS x l or PS x DN) are not exceeded, the installation is to comply with to article 3 paragraph (3) "Sound engineering practice" and does not receive any EC mark or conformity declaration. It is useful to document this by a manufacturer s certification. If these limits are exceeded, then various modules are to be chosen according to category. Starting with category I, a manufacturer s self-certification according to module A is provided, the installation receives an EC mark and a conformity declaration. In the case of higher categories the conformity assessment procedure and the certification are to be carried out alongside a nominated organisation. If, however, a part of the installation such as a pressure reducing station is not furnished by the supplier completely assembled, but in individual parts and only assembled at its destination, only the individual parts are to be assessed and possible certified according to the guideline. The certification of the subsystem is only given by the manufacturer of the complete installation. Depending on the type of the installation system, other European guidelines must also be observed; these will not be dealt with in detail here and include - Electromagnetic compatibility directive 89/336/EEC - Low voltage equipment directive 73/23/EEC - Machinery safety directive 98/37/EC
16/16 6. Summary In large systems the number of components to be obtained separately can be reduced considerably if parts of the system such as pressure reducing or temperature regulating stations are delivered in one unit. The planner then neither needs to deal with the details of component sizing and selection, as in the case of valves and pipes, for example, nor to verify that they comply with all rules and regulations, with the result that he can concentrate on his main task, the system. Once all requirements for the subsystem have been defined, the supplier is responsible for component sizing and selection, ensures correct certification, and compiles all the necessary documentation. The subsystem should preferably be delivered fully assembled, or in separate parts for on-site assembly. 7. Bibliography [1] ARI-Armaturen: Manufacturer s catalogue, 2005 [2] DIN 3440: Thermostats, temperature limiters and thermal cut-offs for heat generating systems [3] ARI Armaturen: Computer calculation programme ARI-VASI [4] TRD 421: Safety valves against excessive pressure safety valves for steam boilers [5] EN ISO 4126-1: Safety devices for protection against excessive pressure Part 1: safety valves [6] Stork, E.: Steam pressure reducing station for process and plant engineering. Industriearmaturen 2/2000, Vulkan-Verlag, Essen [7] Stork, E.: Valves for steam facilities. Industriearmaturen International Edition 2004, Vulkan-Verlag, Essen [8] Pressure equipment directive (PED) 97/23/EC