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Recording the heart beat of diaphragm
pumps Systems for condition monitoring are used successfully for many investment goods. So positive experience on the use of such systems e.g. for piston compressors in an industrial production plant (process gas plant) is reported /3, 4/. The development of on-line systems for diaphragm metering pumps still is at an early stage, whereas off-line systems have been used successfully for years already. Fig. 1 shows a process diaphragm pump in triplex design as an example. A monitoring system, per definition, is a system which allows an exact statement on the current as well as the future operating condition of a machine or a unit by recording and interpretation of suitable, sensitive physical values. For diaphragm pumps different signals can be evaluated depending on the requirements of the system, refer to Fig. 2. One distinguishes between off-line and on-line systems. On-line systems are systems installed locally with permanent data recording and evaluation. They definitely are more complicated than mobile off-line systems with temporary data recording and manual data evaluation. The development of on-line monitoring systems still is at the beginning last not least also due to its high investment costs but it is foreseeable that the demand will increase especially for difficult applications. A report about a successful installation is given in /5/. A process diaphragm pump installed in Spain is monitored on-line and the data can be evaluated worldwide via Internet. This allows technical experts to give their sound advice on the evaluation of the measuring results at any time and independently from any local conditions. Off-line systems have been used successfully in operation as well as in research and development for several decades already. Whether on-line- or off-line systems are more suitable must be decided for each individual case. Experiences are available for both systems /5/. However, monitoring systems for diaphragm pumps with automatic early failure diagnosis are unknown to date. The primary aim of every monitoring system is the increase of the machine availability at reduced operating costs. The resulting economic advantages are obvious: Developing failures can be detected at an early stage and repair work can be scheduled. Reports can be carried out precisely aimed at the problem which shortens the down time and reduces the costs caused by production losses. These systems naturally increase the availability of the unit. How successful they are in practice depends on the technical boundary conditions of each individual case, on the expenditure for a suitable measuring and, if required, an evaluating system and finally on the knowledge and the experience of the supplier. The basis of monitoring system for diaphragm pumps is an indicator diagram (p-V-diagram) which describes the pressure in a diaphragm pump via the plunger movement. It virtually shows the heartbeat of the diaphragm pump. For this a pressure transducer and a triggering signal as reference for the plunger position is required. The complete plunger kinematics are then calculated by the measuring software. The pressure transducer is installed in the hydraulic part of the diaphragm pump and therefore is not in contact with the fluid conveyed. Alternatively to the indicator diagram a time related pressure recording can be used if it is guaranteed that the speed of the pump is constant. Otherwise the evaluation of the measurements is problematic. Figure 3 shows an example of the time related pressure recording of a properly functioning diaphragm pump. For later evaluation of further signals a „fingerprint" of the pump must be taken at the beginning as reference for faultless operation during actual operation. In the diagram the suction- and discharge stroke, the opening time of suction- and discharge valve, the compression- and decompression phase as well as the opening period of the leakage replenishing valve in the hydraulic part of the pump (snifting valve) are clearly recognisable. Contrary to this, figure 4, 5 and 6 shows the time related pressure recordings of the same pump in faulty operation is shown. So the snifting valve in the hydraulic part is open for a longer period of time in case of leakages, this can be recognised at the extended snifting phase. The opening period of the snifting valve is therefore a measurement for the leakage in the hydraulic part. If the suction valve is defective (fig. 5), the compression starts later and lasts longer whereas the decompression takes place earlier. This can be associated to the fact that fluid is forced back into the suction line if the suction valve does not close properly, although the pump produces the operating pressure required. In the case of a defective discharge valve (fig. 6) the compression is faster and the decompression is slower. The reason for this is the existing pressure in the discharge line which supports the compression resp. maintains the pressure for a longer time period during the decompression. For the detection of defective suction and discharge valves by means of pressure recordings or indicator diagrams physical limits are set however. If the discharge pressure is too low the deviation in the compression and decompression flanks is no longer significant. In the individual case an oscillation acceleration transducer can help here. In Figure 7 for example the reference acceleration signal for a process pump is compared to the signal of the same pump with defective suction and discharge valves. Monitoring systems help to optimise the life cycle costs of diaphragm pumps. Depending on the application the detectable failures for example are:
The quality of a monitoring system is established by the interaction of hard- and software as well as by the know-how of the pump manufacturer. It must be clearly understood however that the best monitoring system is no real protection against possible failures. Therefore the best guarantee still is a high quality and availability of the pump itself.
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