Multisafe Process Pumps
The strictness of demands imposed by legislators and operators on plants and systems employed in the process engineering industry in general and in aseptic process engineering in particular, continue to intensify. Whilst terms such as reliability, availability, life cycle costs, or diagnostic capabilities were used when it came to the characterisation of process pumps previously, cost and energy-efficiency are nowadays increasingly regarded as priority factors.
Author: Heinz M. Nägel
A great number of different pump types are typically applied in process engineering systems, such as centrifugal, progressive cavity or positive displacement pumps. Although centrifugal pumps have market dominance, positive displacement pumps tend to be specified for high pressure applications. In many low pressure applications, however, hydro abrasion and chemical attack results in elevated wet end wear, which in turn culminates in correspondingly high operating expense (OPEX). Prior to considering a final decision in terms of pump selection, it is highly recommended to focus on criteria other than solely capital expenditure (CAPEX) and carefully balance other important, individual and contributing factors, such as pump design, configuration, footprint, driving power, efficiency, speed, flow path, number of wetted/wearing parts, lifetime expectancy, wearing parts accessibility, pollution control and finally, cost and energy-efficiency.
With traditional diaphragm piston pumps, wet end and drive end are separated by a flat diaphragm. Apart from the diaphragm, check valves and piping, the product is also in contact with the pump casing which, as a result, tends to be manufactured from expensive special materials that are chemically and abrasively resistant to the slurry. In the event of a diaphragm failure, the slurry – that is in many cases of an aggressive nature – penetrates and thus contaminates the hydraulic control area and can culminate in considerable expense, as well as induce unplanned and inconvenient pump downtime for decontamination and repair.
Hydraulically actuated double hose-diaphragm pumps offer decisive advantages vs. traditional piston diaphragm pumps (Fig. 1). They are designed to eliminate flat diaphragms and their obligatory clamping rings which is the typical construction of most diaphragm piston pumps. Instead, they are provided with twin cylindrical hose-diaphragms (primary and a secondary) although the pump only requires just one of these to be operational and can function normally in the event of a failure of either. These tubular diaphragms avoid all of the disadvantages of traditional diaphragm piston pumps, as aforementioned. They fully enclose the product and provide for double hermetic sealing between the wet and drive ends. Moreover, the pair of hose-diaphragms ensures that the slurry will not come into contact with the pump casing. Should one of the hose-diaphragms fail, the second will ensure that the product neither contaminates the pump casing nor the hydraulic drive area. For this reason the pump casings are not required to be manufactured from expensive special materials. Unlike peristaltic hose pumps, double hose-diaphragms are not subject to mechanical squeezing, but are actuated by the piston, utilising hydraulic fluid as an operating medium.
One of the decisive advantages of double hose-diaphragm pumps is the linear flow path from the suction to the discharge. However, the hose-diaphragm design does not just influence the flow path, but also the efficiency of the pump. Traditional diaphragm pumps are characterised by their circular design pump casings. Particularly with high flow designs, this round diaphragm housing is subject to minor movement in step with the pulses caused by the piston stroke, which in turn reduces the overall efficiency of the unit (Fig. 2). In comparison, tubular hose-diaphragms allow for cylindrical casings and a correspondingly small footprint. Fabrication of such housings, therefore, is much easier to control and this advantage allows for an increase in hydraulic efficiency of some 5 % (Fig. 1).
Double hose-diaphragm pumps are designed to avoid sudden deviance from admissible working conditions and unplanned downtime. For additional back-up of its failsafe characteristics, they utilise an overall diagnostic system for permanent condition monitoring of essential components and parameters (Fig. 3).
One of the decisive criteria of condition monitoring is to recognise even the merest indication of wear, or any other variance from set-points early enough to permit the operator to closely monitor any further developments. This allows the opportunity to introduce preventative maintenance into the process as appropriate, in order to avoid unscheduled shutdown of the system. Since the leak-free sealing of check valves plays a decisive role, an innovative system for the early detection of wear in check valves has been developed for double hose-diaphragm pumps in cooperation with a well-known German electronics manufacturer. The measuring principle of these purpose-built sensors is based on the analysis of solid-borne sound and is capable of detecting leaks between valve seat and ball or cone respectively, at a time when the loss of flow is still less than 1.5 %. Multiple options are available for the transmission of the measuring results by means of a dry contact (such as Internet or Intranet). Intrinsically, this provides the operator the opportunity of well-directed advance planning of maintenance or repair action, as well as the precise determination of MTBR values. As an added advantage, it avoids loss of energy, because any decrease in output resulting from valve wear, is usually automatically compensated by the necessity of increased pump speed.
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