Definitive Guide to Pressure Independent Control Valves
Short Description
Definitive Guide to Pressure Independent Control Valves...
Description
The Definitive Guide to Pressure Independent Control Valves Edition 1.1 Written by Chris Parsloe, Nick Martin and Martin Lowe
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This guide will explain how the valves work, their operational limits and the control options available. The aim is to educate designers on how to select the appropriate PICV solution for their particular application, and how to design systems to ensure the best performance from the valves.
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t 'MPXDPOUSPMFOBCMJOHNPEVMBUJOHDPOUSPMPG IFBUJOHDPPMJOHPVUQVUT t 'MPXSFHVMBUJPOFOBCMJOHnPXSBUFTUPCFTFUBU their specified design values t %JGGFSFOUJBMQSFTTVSFDPOUSPMoFOTVSJOHBDPOTUBOU differential pressure across control valves regardless of changes in pump speed or valve closures elsewhere in the system
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This means that each PICV replaces up to three separate valves that would otherwise be required (i.e.regulating valve, two port control valve, plus a differential pressure control valve).
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tŚĂƚ͛ƐƚŚĞĚŝīĞƌĞŶĐĞďĞƚǁĞĞŶ ĂW/sĂŶĚW/s͍ A Pressure Independent Control Valve (PICV) is a valve that enables modulating control of flow rate but where the flow characteristic (i.e. the relationship between valve closure and flow rate) may vary depending on the pressure differential across the valve, the flow setting of the valve or the actuator fitted.
A Pressure Independent Characterised Control Valve (PICCV) gives more accurate modulating control of flow rate because the valve itself has a protected characteristic. Hence, the characteristic is unaffected by operating conditions.
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PICVs also have the additional benefit of being much easier to commission. The traditional exercise of proportional balancing flow rates through branches is eliminated and instead the task becomes one of merely setting the required flow rate at each PICV. This procedure is explained in CIBSE COMMISSIONING CODE W, WATER DISTRIBUTION SYSTEMS and BSRIA GUIDE BG2/2010 COMMISSIONING WATER SYSTEMS.
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PICVs resolve all of these problems thereby improving thermal comfort in the building, whilst maximising energy savings from the pump. Advice on how to design systems with PICVs is provided in CIBSE KNOWLEDGE SERIES GUIDE KS7 VARIABLE FLOW PIPEWORK SYSTEMS.
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Each section of the valve works as follows. The inlet to the valve houses the differential pressure regulator. This comprises a flexible rubber diaphragm which flexes against a spring simultaneously varying the size of the opening to flow. One side of the diaphragm is in contact with water from the inlet to UIFWBMWFBUBQSFTTVSF1 XIFSFBTUIFPUIFSTJEF is in contact with water from the outlet to the valve at a pressure P3. This means that if there is any DIBOHFJOUIFEJGGFSFOUJBMQSFTTVSF1UP1 UIF position of the differential pressure regulator will also change. The result will be that the differential pressure P2 to P3 (i.e. from downstream of the differential pressure regulator to the valve outlet) will remain constant at all times regardless of changes in UIFPWFSBMMEJGGFSFOUJBMQSFTTVSF1UP1)FODFUIF UFSNiQSFTTVSFJOEFQFOEFOUwoJUEPFTOUNBUUFSIPX external pressures may be varying, the performance and function of the valve will be unaffected providing JUTXJUIJOJUTXPSLJOHSBOHF In the central section, there is an actuated 2 port NPEVMBUJOHDPOUSPMWBMWF'PSFYBNQMF UIFPQFOJOH of the valve can be varied by the actuator depending on a signal from the control system to achieve the required temperature in the occupied space. At the outlet to the valve body there is a flow setting device. This enables the valve to be adjusted to achieve the required design flow rate, as specified by the designer. The required flow rate can be set using the flow setting dial incorporated in the valve
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UIFOBTUIFnPX setting is adjusted some of the travel of the control valve is used up in regulating the flow. Modulating flow control is only available across the remaining travel of the valve, after the flow has been set. Pressure tapings built into the valve allow the overall QSFTTVSFEJGGFSFOUJBM1UP1UPCFNFBTVSFE to ensure that the valve is operating within the NBOVGBDUVSFSTTUBUFEQSFTTVSFEJGGFSFOUJBMSBOHF
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tŚĂƚǁŝůůĂW/sĚŽ͍ dŚĞǁĂLJW/sƐǁŽƌŬŵĂŬĞƐƚŚĞŵŝĚĞĂůĨŽƌƵƐĞŝŶ ǀĂƌŝĂďůĞŇŽǁƐLJƐƚĞŵƐ͘ *UDBOCFTFFOUIBUBOZDIBOHFTJOUIFQSFTTVSF1 (as might be caused by changes in pump speed or by valve closures in other parts of the system) will automatically be compensated for by the action of the differential pressure regulator. The controller will TJNQMZJODSFBTFUIFWBMWFTSFTJTUBODFJG1JODSFBTFT PSSFEVDFJUJG1SFEVDFT 'VSUIFSNPSF UIFBCJMJUZPGUIFSFHVMBUPSUPNBJOUBJOB constant pressure differential between P2 and P3 has two important implications. 'JSTUMZ XJUIUIFDPOUSPMWBMWFGVMMZPQFOBOEUIFnPX setting device set to its required value, the valve effectively becomes a constant flow regulator (since a constant pressure differential across a constant resistance will result in a constant flow rate). Hence, if installed without the actuator, the valve can be used as a flow limiting valve, to maintain the flow within a fixed pre-settable value, regardless of changes in other parts of the system. This can be useful in bypass circuits.
Secondly, by maintaining a constant pressure differential across the control valve and flow setting device, the authority of the control valve is maximised. The authority of a valve is an indication of how accurately the valve will be able to modulate flow as it opens and closes. To achieve good authority, the pressure differential across the valve should be at MFBTUPGUIFUPUBMQSFTTVSFEJGGFSFOUJBMJOUIFQJQF branch or circuit for which it is controlling flow. Such a valve would be considered as having an authority PG*UJTPGUFOJNQPTTJCMFUPTJ[FDPOWFOUJPOBM port control valves with such good authority because the controlled circuit may include terminal unit and QJQFXPSLMPTTFTCBDLUPBSFNPUFQVNQPS%1$7*O many applications, this necessitates 2 port valves with impractically high resistances. Hence, valve BVUIPSJUJFTBTMPXBTBSFDPNNPOCVUGBSGSPN ideal. In a PICV, authority is improved because the pressure differential P2 to P3 across the control valve and flow setting device is effectively the circuit for which the valve is controlling flow. This means that terminal unit and pipework pressure losses do not need to be considered for valve selection and the valves can be selected purely on flow rate.
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ƋƵĂůƉĞƌĐĞŶƚĂŐĞĐŚĂƌĂĐƚĞƌŝƐƟĐ "WBMWFTiDIBSBDUFSJTUJDwJTUIFSFMBUJPOTIJQCFUXFFO the flow through the valve relative to its degree of closure. The valve characteristic is a feature of the design of either the valve itself, or the valve and actuator combination. Typical valve control DIBSBDUFSJTUJDTBSFEFTDSJCFEBTPOPGG RVJDLBDUJOH
linear and equal percentage. These are illustrated HSBQIJDBMMZJO'JHVSF
&ŝŐƵƌĞϮ͘ŽŶƚƌŽůǀĂůǀĞĐŚĂƌĂĐƚĞƌŝƐƟĐƐ 'PSiGPSDFEDPOWFDUJPOwDPJMTXIFSFBGBOCMPXT air across the coils, the best solution is an equal percentage characteristic. This is because for these types of coil the heating or cooling output gradually stabilises as water flow increases until a point is reached where the output becomes unresponsive to GVSUIFSJODSFBTFTJOnPX5IJTJTJMMVTUSBUFEJO'JHVSF In order to achieve good modulating control of the sensible heating or cooling output from the coil, the control valve needs a characteristic that mirrors the performance of the coil. It can be seen that the equal percentage characteristic does this. Equal percentage valve characteristics are so called because as the valve opens, for each percentage increment in valve travel, the flow increases by an equal percentage. Hence, they produce small changes in flow when the valve is nearly closed, and large changes in flow when the valve is nearly open.
&ŝŐƵƌĞϯ͘,ĞĂƚƚƌĂŶƐĨĞƌǀĞƌƐƵƐƉĞƌĐĞŶƚĂŐĞĚĞƐŝŐŶŇŽǁƌĂƚĞ
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'JHVSFTIPXTUIFXBZUIBUUIFIFBUUSBOTGFS characteristic of the heating or cooling coil is modified by various control valve characteristics. It can be seen that an equal percentage characteristic gives the best control with each change in heat transfer being equal to each change in valve opening. It can also be seen that a linear characteristic valve with perfect authority is not as good as an equal percentage valve. As previously explained, all PICVs achieve close to perfect authority, but there is a marked difference in the stability of offcoil air temperatures between coils controlled by equal percentage characteristic valves and linear or on-off characteristic valves.
ƵƚŚŽƌŝƚLJ In the case of a PICV, the authority is calculated by comparing the pressure lost across the flow control element with the control pressure diffrential; these two values are nearly equal SFTVMUJOHJOBOBVUIPSJUZDMPTFUP%FQFOEJOH on the design of the PICV, the authority of the flow control element may change as the valve is regulated. This loss of authority is exhibited BTBDIBOHFJOUIFWBMWFTDIBSBDUFSJTUJDDVSWF 'JHVSFTIPXTUIFFGGFDUPGMPTTPGBVUIPSJUZPO an equal percentage valves.
&ŝŐƵƌĞϰ͘,ĞĂƚƚƌĂŶƐĨĞƌǀĞƌƐƵƐǀĂůǀĞŽƉĞŶŝŶŐ
ĐƚƵĂƚŽƌƐĞůĞĐƟŽŶ Even if a valve has an intrinsically equal percentage characteristic this can be negated UISPVHIBQPPSDIPJDFPGBDUVBUPS'PSFYBNQMF if the stroke of the actuator does not match the stroke of the valve then the characteristic may be deformed. This is particularly important when the stroke of the valve is being changed in order to regulate the maximum flow rate of the valve. 'JHVSFTIPXTIPXDIPPTJOHUIFXSPOHBDUVBUPS can affect the intrinsic characteristic of the valve.
&ŝŐƵƌĞϱ͘>ŽƐƐŽĨĂƵƚŚŽƌŝƚLJŽŶĞƋƵĂůƉĞƌĐĞŶƚĂŐĞĐŽŶƚƌŽůǀĂůǀĞ
*GUIFWBMWFIBTBOJOIFSFOUMZMJOFBSPSPOPGG characteristic then this can be improved in some cases by using a characterising actuator to change the intrinsic curve to a more acceptable one. However, it is important to match the characterising action of the actuator to the valve to which it is fitted.
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ĐƟǀĞĐŽŶƚƌŽůŽĨƌŽŽŵ ƚĞŵƉĞƌĂƚƵƌĞ The importance of achieving an accurate equal percentage control characteristic and good valve authority becomes clear when the resulting variations in off-coil supply temperatures are considered. 8IFOJOVTF UIFDPOUSPMWBMWFGPSNTUIFPVUQVU part of a closed loop controller, changing its opening in response to changes in the measured room or return air temperature. In such systems it is particularly important to ensure effective modulating control of the offcoil temperature, and it is the function of the control valve to achieve this. Inaccurate control will result in hunting whereby the controlled temperature in the occupied space repeatedly over-shoots or under-shoots its set point value. This can make the space uncomfortable for the occupants and wastes energy. 'PSUIJTSFBTPO JUJTIJHIMZSFDPNNFOEFEUIBU the PICV and its actuator deliver an accurate equal percentage characteristic.
&ŝŐƵƌĞϲ͘ĞŐƌĂĚĂƟŽŶŽĨǀĂůǀĞĂƵƚŚŽƌŝƚLJ;ůŝŶĞĂƌĐŚĂƌĂĐƚĞƌŝƐƟĐͿďLJ ĮƫŶŐŽĨŝŶĐŽƌƌĞĐƚĂĐƚƵĂƚŽƌƚŽƐƚƌŽŬĞůŝŵŝƚĞĚǀĂůǀĞ
'JHVSFDPNQBSFTUIFTVQQMZUFNQFSBUVSFJOUPB space with a poorly characterised valve, performing BTPOPGG BOEBDPSSFDUMZQFSGPSNJOHFRVBM percentage control valve.
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&ůŽǁĂĐĐƵƌĂĐLJĂŶĚ ƌĞƉĞĂƚĂďŝůŝƚLJ͍ All PICVs will exhibit variations in the accuracy and repeatability of their flow rate settings. The reasons for this can be understood by considering a plot of UIFWBMWFTnPXSBUFSFMBUJWFUPQSFTTVSFEJGGFSFOUJBM "UZQJDBMHSBQIJTTIPXOJO'JHVSF It can be seen that each valve has a minimum and maximum pressure differential value below or above which the valve will not control flow. If the pressure differential is less than the minimum value, the spring inside the pressure regulator remains fully extended, whereas at pressure differentials greater than the maximum value, the spring is fully compressed. Under both of these conditions the pressure control
element in the valve acts as a fixed resistance; the valve can only control flow when the spring is under some degree of partial compression. The “operating range” of the valve is the range of differential pressures for which control is possible. 8JUIJOJUTPQFSBUJOHSBOHF UIFnPXUISPVHIUIFWBMWF TUBCJMJTFT BMUIPVHIBTDBOCFTFFOJO'JHVSF FWFO in this range the flow rate is not constant. If the pressure across the valve is allowed to vary between its minimum and maximum operating pressures, its nPXNBZWBSZCZVQUPßGSPNJUTTFUQPJOUWBMVF The degree of flow variation exhibited by a valve operating within its recommended operating range JTTPNFUJNFTSFGFSSFEUPBTUIFWBMWFTiQSPQPSUJPOBM band”. The smaller the proportional band the more accurately the valve will maintain its set flow rate.
&ŝŐƵƌĞϴ͘&ůŽǁZĂƚĞǀĞƌƐƵƐŝīĞƌĞŶƟĂůWƌĞƐƐƵƌĞ;dLJƉŝĐĂůW/sͿ
,LJƐƚĞƌĞƐŝƐ To further complicate things, the accuracy with which the flow rate setting is maintained also depends on whether the pressure differential across the valve is SJTJOHPSGBMMJOH*UDBOCFTFFOGSPN'JHVSFUIBU there are distinct rising and falling pressure curves. The difference between the two curves is often SFGFSSFEUPBTUIFWBMWFTiIZTUFSFTJTw5IFIZTUFSFTJT effect is caused by the sealing elements in the pressure regulating part of the valve, although the spring and elastic membrane may also have some influence. This hysteresis effect can be seen in all TFMGBDUJOHTQSJOHPQFSBUFE1*$7TBOE%1$7T
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%VFUPIZTUFSFTJT UXPSFQFBUBCMFnPXSFBEJOHTDBO be obtained depending on whether the pressure differential across the valve has risen or fallen to the value when the measurement is taken. Since the valves are factory tested on their rising pressure curves, the flow setting device indicates flows that correspond to a rising rather than decreasing pressure differential. 'PSUIFSFBTPOTFYQMBJOFE UIFWBMWFTQSPQPSUJPOBM band and hysteresis may cause flow values to vary from their set values. These effects can be minimised by ensuring that systems are: t %FTJHOFETVDIUIBUXIFOB1*$7PQFOTUPJODSFBTF the flow rate to a terminal unit, its pressure differential simultaneously increases rather than decreases.
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t$PNNJTTJPOFETVDIUIBUXIFOB1*$7JTTFUUP its required flow rate, the pressure differential across the valve is as close as possible to its final operating value. Both of these objectives can be easily achieved by ensuring that during commissioning and subsequent system operation, pump pressure always reduces as PICVs close. The best way to achieve this is to set the pump speed controller such that a constant pressure differential is maintained at a differential pressure sensor located towards the index PICV i.e. the PICV located furthest from the pump. A single sensor located two thirds of the way along the index branch is satisfactory in systems with a uniform load pattern; alternatively multiple sensors across the most remote PICV controlled terminal branches can be used in systems with an unpredictable and varying load pattern.
Unless unavoidable, pump speed should be controlled such that pump pressure is maintained constant. This solution inevitably results in large increases in pressure differential across PICVs as they close, resulting in the largest possible variations from set flow rate values, much better than standard two ports. The use of remote sensors for pump speed control will enable PICVs to perform as accurately as possible. This solution also gives the best possible energy savings from the pump as recommended in $*#4&,OPXMFEHF4FSJFTHVJEF,47BSJBCMFnPX QJQFXPSLTZTUFNTBOE#43*"#(&OFSHZ Efficient Pumping Systems - a design guide.
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&ƵůůƐƚƌŽŬĞŽƌƐƚƌŽŬĞůŝŵŝƚĞĚ ǀĂůǀĞƐ PICVs where the control and flow setting functions BSFTFQBSBUFEXJUIJOUIFWBMWF 'JHVSFB BSFLOPXO as “full stroke” control valves because the full stroke of the control valve is available for control. This type of valve can be fitted with a programmable actuator which can be set to provide an equal percentage characteristic. The actuator always operates over the full stroke of the valve so it never needs to modify its programmed characteristic. The main benefit a full stroke valve offers is that the actuator can be driven through its full range which is of particular importance on valves where the stroke is quite short (e.g. in UIFNNTJ[FSBOHF 8IFSFUIFDPOUSPMBOE regulation elements of the valve are separated there is a risk that the control characteristic may change as the valve is regulated although this can be mitigated by the control element having an intrinsic equal percentage characteristic and by proper selection of the actuator. PICVs where the control valve and flow setting devices are combined in a single component are LOPXOBTiTUSPLFMJNJUFEwDPOUSPMWBMWFT 'JHVSF C 5IJTJTCFDBVTFQBSUPGUIFDPOUSPMWBMWFT stroke will be used up in regulating the flow to its required setting. In stroke limited valves a significant proportion of the valve stroke may be taken up during flow regulation. This limitation of stroke is most apparent with linear characteristic valves, since JOPSEFSUPSFHVMBUFBWBMWFUPPGJUTNBYJNVN nPX PGUIFDPOUSPMTUSPLFJTBMTPMPTU*UJT therefore essential that stroke limited valves have an intrinsic equal percentage characteristic; if an equal QFSDFOUBHFWBMWFJTTJNJMBSMZSFHVMBUFEUPPG JUTNBYJNVNnPXSBUFUIFOPOMZBSPVOEPGUIF stroke of the valve will be lost. Stroke limited valves can be of the lift and lay type, multi-turn type or characterised ball type. Lift and lay types have a rising stem which the actuator pushes
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ZŽƚĂƌLJW/sǁŝƚŚĐƚƵĂƚŽƌ against to close the valve. Multi-turn or characterised ball types have a rotating stem which the actuator must turn to close the valve. In the case of lift and lay valves an actuator which can compensate for the lost stroke as the valve is mechanically regulated must be supplied otherwise the control characteristic of the valve will be adversely affected. Multi-turn valves are usually supplied with a matched actuator so this problem is avoided. In the case of rotary type valves, where the control and regulation device is a characterised slot in a ball valve, the valve can be driven to the regulated position by scaling the output from the controller to the actuator. There are also special actuators available that can be used where the scaling cannot be accomplished by the controller. Both full stroke and stroke limited valve solutions are capable of providing effective modulating control of flow rate. Although both types of valve will inevitably exhibit some loss in controllability when flow setting devices are regulated to their minimum flow settings, the level of control achieved is invariably better than that achievable from equivalent 2 port control valves operating against varying pressure differentials.
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^ŚƵƚͲŽī /ŶƚĞƌŶĂƟŽŶĂůƐƚĂŶĚĂƌĚIEC 60534-4ĚĞĮŶĞƐǀĂƌŝŽƵƐ ĐůĂƐƐĞƐŽĨƐŚƵƚͲŽīĂŶĚƚŚĞŵĞƚŚŽĚƐƚŽƚĞƐƚƚŚĞ ƐŚƵƚͲŽīĐĂƉĂďŝůŝƚLJŽĨĂĐŽŶƚƌŽůǀĂůǀĞ͘DŽƐƚƉƌĞƐƐƵƌĞ ŝŶĚĞƉĞŶĚĞŶƚĐŽŶƚƌŽůǀĂůǀĞƐĂƌĞĚĞĐůĂƌĞĚĂƐďĞŝŶŐ ůĂƐƐ/sǁŚŝĐŚƌĞůĂƚĞƐƚŽĂůĞĂŬĂŐĞƌĂƚĞŽĨϬ͘Ϭϭй ŽĨƚŚĞǀĂůǀĞ͛ƐŶŽŵŝŶĂůŵĂdžŝŵƵŵŇŽǁƌĂƚĞ͘dŚŝƐŝƐ ĞƋƵŝǀĂůĞŶƚƚŽƚƌĂĚŝƟŽŶĂůĐŽŶƚƌŽůǀĂůǀĞƐ͘ One of the concerns with any traditional control valve is the maximum shut-off differential pressure, this is the maximum differential pressure that the actuator could close the valve against. The close-off load that the actuator must overcome is the product of the differential pressure acting on the closing element of
the valve and the surface area of this closing element (globe, sleeve or ball). In a pressure independent control valve the differential pressure acting on the closing element PGUIFWBMWFJTDPOUSPMMFE 1JO'JHVSFBBOEC meaning that the close-off pressure of the valve is constant throughout the working range.
DĂŶƵĂů^ĞƫŶŐ Setting most PICV valves is as simple as turning the setting hand wheel to the specified position. Most often the hand wheel of a PICV will be graduated with a scale showing the set flow rate as a percentage of the valves maximum flow rate.
ZĞŵŽƚĞΠŽŵŵŝƐƐŝŽŶŝŶŐ /ŶĂĚĚŝƟŽŶƚŽŵĂŶƵĂůƐĞƫŶŐ͕ƐŽŵĞW/sƐĐĂŶ ďĞƵƐĞĚŝŶĐŽŶũƵŶĐƟŽŶǁŝƚŚĂD^ĐŽŶƚƌŽůůĞƌ ƚŽƌĞƚƵƌŶƚŚĞǀĂůǀĞĂĐĐƵƌĂƚĞůLJƚŽĂŐŝǀĞŶƉƌĞͲ ƉƌŽŐƌĂŵŵĞĚŇŽǁƐĞƫŶŐ͘hƐŝŶŐĂƌĞŵŽƚĞD^ ĐŽŶƚƌŽůůĞƌ͕ƚŚŝƐĐĂŶďĞĂĐŚŝĞǀĞĚǁŝƚŚŽƵƚƚŚĞŶĞĞĚ ƚŽǀŝƐŝƚĞĂĐŚǀĂůǀĞĂŶĚŵĂŶƵĂůůLJƐĞƚƚŚĞƌĞƋƵŝƌĞĚ ŇŽǁƌĂƚĞƐƌĞƐƵůƟŶŐŝŶďĞƩĞƌƵƐĞŽĨƚŚĞĂǀĂŝůĂďůĞ ĐŽŵŵŝƐƐŝŽŶŝŶŐƟŵĞ͘ A standard BMS controller with proprietary strategy can control both heating and cooling outputs for individual terminal units. Each controller can be pre-programmed with the heating and cooling valve references and maximum design flow rate values for each valve.
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%VSJOHDPNNJTTJPOJOH FBDISPUBSZDPOUSPMWBMWF is set to a position which will achieve its specified design flow rate. The controller is factory pre-set to the required design flow rates although these can be overridden on site using a commissioning computer. The required setting for each valve is determined based on the known relationship between flow rate and valve setting, as measured on a test rig. A “trim factor” is available in the control software to enable flow settings to be adjusted for greater accuracy. This approach provides a flexible method for establishing design flow rates during commissioning. Once set, flows will be maintained within the limits of accuracy dictated by the hysteresis limits of the TQSJOH BTFYQMBJOFEPOQBHF
Setting the valves in this way brings advantages for buildings that are being seasonally commissioned, in other words where commissioning may have to be repeated in an occupied building. 'PSDSJUJDBMBQQMJDBUJPOT FWFOHSFBUFSnPXBDDVSBDZ can be achieved by incorporating flow sensors in the pipework feeding terminal units. The controller is then able to adjust its setting until the specified flow rate is achieved at the flow sensor. This solution provides near perfect flow control since it can correct flow variations caused by the aforementioned hysteresis effect.
WƌĞĨĂďƌŝĐĂƚĞĚǀĂůǀĞĂƐƐĞŵďůĞLJŝŶĐŽƌƉŽƌĂƟŶŐϮƐĞĐƟŽŶƌŽƚĂƌLJW/s
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