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Metering without compromises

Drive concept for optimum metering quality, process reliability and efficiency
Metering without compromises

Today, magnetic drives, motor drives and in part also stepper motors are used for the purpose of driving diaphragm-type metering pumps in process control and water treatment applications. Up until now, choosing the right type of drive for the job inevitably always involved striking a compromise between metering quality, cost efficiency and process reliability. The drive technology optoDrive now makes metering without compromises possible. Prominent offers this innovative technology for the first time in its new delta series.

Dr.-Ing. Karl-Michael Nigge

Ever more demanding and diverse requirements are placed on diaphragm-type metering pumps for use in chemical feed, process control and water treatment applications. Precision and reliability in process control systems rely heavily on extremely high degree of metering accuracy and, more often than not, efficient flow rate monitoring. In addition, the metering characteristics must be capable of catering for ever more specific application requirements. For instance, very short metering pulses are required in filling operations that follow a fast machine operating cycle. On the other hand, low-pulsation, virtually continuous metering flow is occasionally advantageous for ensuring effective mixing of products or for avoiding vibration damage in metering lines.
In addition to these aspects of metering quality, today it is expected of metering pumps, serving as intelligent field devices, that they increase process reliability by monitoring their process environment. A specific case in point is the need to avoid excess pressure damaging system or plant installations as the result of blocked metering points or to prevent continued metering in the event of a break in feed lines without the need for separate equipment, which in turn also drives up costs. High metering quality and process reliability must therefore not be achieved at the expense of cost efficiency in procurement and operation.
High level of additional technical equipment
The various types of diaphragm-type metering pumps currently available on the market mainly differ in terms of the type of the drive used and satisfy the most diverse range of conflicting tasks regarding metering quality, process reliability and efficiency using a variety of different methods – none of these technologies, however, can excel equally well in all three disciplines.
The very efficient magnetic drives for diaphragm-type metering pumps operate overload-proof and with virtually no wear as they have no gear mechanisms susceptible to damage. Their pulsating metering characteristics with defined stroke length are required in many applications, such as when filling operations need to be precisely synchronized with a fast machine operating cycle. In many other applications the pulsating operating mode is not a disturbing factor as it is damped by the flexible hose lines frequently used in such applications or it is taken into account in favor of the high degree of efficiency. Only in a relatively small number of applications is the use of pulsation dampers required, especially in long or inflexible metering lines.
Motor-driven diaphragm-type metering pumps are predominantly used in the high output ranges as from a delivery rate of approx. 50 l/h – a range which is less suitable for magnetic drives due to their operating principle. Stepper motors, which are relatively expensive in terms of their manufacture, have recently been used in chemical feed and metering for the purpose of facilitating virtually continuous metering in the lower output ranges. The price paid for such metering characteristics is the higher wear susceptibility compared to magnetic pumps owing to the higher number of the moving parts. Separate overflow valves must therefore be installed to protect the drive unit from damage as the result of excess pressure. Metering pumps with stepper motor drive units are not suitable for fast-cycle metering operations because the pressure stroke takes too long and the stroke length cannot be adjusted.
A high price has to be paid for increased process reliability and metering accuracy in connection with all diaphragm-type metering pumps known to date. Although detection of blocked metering points or line breaks as well as compensating for backpressure fluctuations in the metering line are possible, they require, however, a high level of additional technical equipment. For instance, separate overflow valves and pressure measuring instruments or even pressure sensors have to be used in connection with conventional diaphragm-type metering pumps – with corresponding effects on the overall system costs.
The optoDrive drive concept
The optoDrive drive technology combines optimum metering quality with maximum cost efficiency. It additionally incorporates functionalities of hydraulic process monitoring, which to date could be realized only with separate accessories and components.
The flow rate characteristics can be precisely adapted to suit the requirements of the specific application. For instance, the user can set a slow pressure stroke for virtually continuous metering or a fast stroke for fast-cycle filling operations. The intake stroke can be optionally slowed down in both operating modes (fig. 1). In this way it is possible to avoid the main cause of inaccurate metering in connection with high-viscosity fluids, i.e. incomplete filling. The slow intake stroke prevents cavitation and therefore increases the metering accuracy in connection with outgassing media.
The drive automatically compensates fluctuations in backpressure in the metering line, which often leads to unwanted variations in the metered quantity in other types of diaphragm-type metering pumps. This drive technology achieves levels of metering accuracy that are otherwise attainable only with intricate control circuits.
A monitoring facility for the hydraulic metering parameters is also integrated in the drive system. Blocked metering points or broken metering lines are detected automatically. Separate overflow valves and pressure sensors are rendered superfluous and the environment is protected against continuous metering in the event of a broken line. Likewise, air or gasses trapped in the metering head (airlock) are also detected. This facility prevents the metering of incorrect quantities and therefore increases process reliability. The corresponding messages are shown in plain text on the pump display. Depending on the type of fault, the system operator can determine whether a message is sent to the process control system via the field bus or fault indicator relay and whether metering is stopped automatically.
Despite optimum metering quality and process reliability, the drive concept optoDrive reduces the procurement costs of metering systems as this innovative drive technology renders separate flow monitoring, overflow protection facilities and pulsation dampers as well as pressure sensors unnecessary (fig. 2). Also in operation, the drive system impresses through its efficiency: it is based on overload-proof design and guarantees operation virtually free of wear with very low noise emission levels thanks to the very small number of moving parts.
Diaphragm-type metering pump series delta
Prominent offers the drive concept optoDrive for the first time in its delta series (fig. 3). The delta series includes dia-phragm-type metering pumps designed for delivery rates of 12 l/h at 16 bar and up to 80 l/h at 2 bar. The lowest possible delivery rate is 0.0003 l/h. In view of the very wide adjustment range – both the stroke length as well as the stroke rate can be varied – this output range is covered by only four different pump sizes. This reduces the variety of replacement parts and therefore the operating costs. The pumps are easy to operate as already known from the gamma and sigma series. Prominent therefore offers a consistent operating concept for low-pressure metering pumps over the output range from 1 to 1,000 l/h.
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