Metering out a dose of technology

Reciprocating positive displacement process pump technology is a proven method for the in-line proportioning and dosing of liquids, offering the highest levels of accuracy and repeatability available. The factors affecting the choice of a metering or dosing pump, as opposed to other types of pump, are normally associated with precision over a range of flows and the fact that the capacity is not influenced by process pressure fluctuations.

Although the basic technology is well proven, users are now turning their attention to issues such as improved safety, greater efficiency and lower running costs. The first two have been addressed by the development of diaphragm pumpheads, as these prevent product leakage. This in turn contributes to higher levels of safety and more economic use of ingredients. However, users are also becoming more conscious of the overall life-cycle costs, taking into account known factors such as the initial investment and subsequent spares, alongside the less obvious factors of maintenance, downtime effect, power consumption and efficient use of the product being handled.

The metering pump is very adaptable and ideal for the continuous, in-phase metering of single or multiple liquid systems at varying rates. It is the only pump with two flow adjustment methods - speed and stroke length - that can be used independently or simultaneously to provide optimum performance. For instance in a process where an ingredient has to be added accurately to a varying main flow, the proportion can be maintained by speed adjustment, while any corrections necessary to maintain product quality can be made by adjustment of stroke length. This means that the metering pump can be finely tuned to the requirements of the system. Manual, pneumatic or electric control options are available, including flameproof designs for pumps sited in hazardous areas.

It is, of course, the application that will always determine the type of pump to be used. Having established the nature of the medium, it is necessary to consider other parameters such as delivery volume and pressure, temperature, flow characteristics, vapour pressure, solids content, corrosiveness, toxicity and hygiene. The correct material for the pumphead is selected according to the properties of the pumped medium. For toxic materials or exacting applications where security and safety are priorities, a diaphragm pumphead is usually the first choice. For less demanding applications, a plunger pumphead will normally suffice.

The standard plunger pumphead, consisting of a housing, displacement plunger, seal arrangement, pump valves and suction/discharge side connections, is commonly used where high pressures and high flow rates are encountered. The simple design and small number of components allows a low dead volume and high volumetric efficiency.

Mechanically coupled and hydraulically actuated diaphragm pumpheads are one of the areas of focus for new developments in metering pump technology. They are typically specified where toxic materials are being handled and leak-proof performance is required. Available in a wide variety of materials, diaphragm pumpheads are now capable of accommodating hazardous, toxic and dangerous liquids at variable flow rates and pressures. As greater emphasis is placed on safety and environmental factors, the market is moving towards diaphragm and away from the traditional plunger design.

Typifying these trends, Bran + Luebbe's hydraulically actuated double diaphragm pumpheads incorporate diaphragms made from either ptfe or stainless steel, and are capable of handling pressures of up to 1000 bar and above. All these pumphead designs incorporate a diaphragm failure indicator to give warning of a problem, although this design of leak-free pumphead will continue to provide continuous service even in the event of one diaphragm failing.

Successful switch

In Fife, Scotland, the Akzo Nobel subsidiary Diosynth has switched from plunger type pumps to double diaphragm metering pump technology for handling the aggressively reactive and corrosive chemical thionyl chloride. The decision to change to Bran + Luebbe's double diaphragm technology was based on the grounds of greater safety by removing the possibility of product leakage. The replacement metering pump has a hydraulically actuated, double diaphragm pumphead with Halar coating and ptfe valves. In addition to eliminating leakage, this pump resists chemical attack and provides the highest levels of metering accuracy.

Traditionally the options for high pressure and hazardous proportioning and metering applications have been limited to reciprocating fixed stroke positive displacement, high speed, or multi-stage type centrifugal pumps. All these have their limitations, not least in respect of regular maintenance, frequent replacement of worn parts, product leakage and high running costs.

The introduction of a hydraulically actuated, double diaphragm pump capable of handling pressures up to 1000 bar and above leak-free, is changing this situation. The Novaplex pump, for example, contains modular phased pumpheads that provide a smooth product flow across a wide operating range, including low speeds. Comprising various gear sizes in combinations between one and seven, a choice of diaphragm materials and variable flowrate, its modular build accommodates the widest possible range of duties for all critical processes.

Process and metering pump technology is not standing still, of course, and Bran + Luebbe continues to invest considerable R&D resources to improve overall performance and efficiency. In addition to its work in double diaphragm development, it has been concentrating on making gears much more efficient.

The spin-off from this is that less power is required to drive them, enabling users to reduce their energy costs. On some of the big pumps now being used, savings can be very significant particularly when life cycle costs are also taken into account.The recently developed H4, for instance, provides users of traditional B+L solutions with a new high efficiency horizontal gearbox containing state-of-the-art sealing technology. This new gear design offers the benefits of low oil temperature, reduced leakage potential, stroke adjustment forces and life cycle costs.

The introduction of off-line diagnostics allows maintenance engineers to look at the pump while in use to determine its performance and position with regard to wear and possible repair.

All Bran + Luebbe diaphragm pumps are supplied with diagnostics fittings, consisting of a pressure sensor and gearbox mounted impulse device as standard. A p-V diagram of each pump cylinder is produced, via the sensor probe linked to a laptop PC with PCMCIA data acquisition card, for analysis of the pressure curve versus time, or piston stroke on the diagram.

The changing requirements in pharmaceuticals and chemicals manufacturing is forcing pump manufacturers to examine current technologies with a view to developing new techniques and making use of more exotic materials for components. At the same time, there is a push for more economical and efficient dosing and proportioning systems. The market for dosing and proportioning is very diverse and the demands of each industry are different. However, it is possible to draw on the experience gained in one industry and successfully adapt it to another.

Steve Digby is general manager of the UK pumps division of Bran + Luebbe