by Steve Digby, SPX Flow Technology

For the non-expert in pump technology, selecting the most appropriate pump technology for a specific application or process system can be challenging because many of today's pumps are so versatile that they are suitable for a considerable range of applications. Rotary positive displacement pumps, peristaltic pumps and reciprocating positive displacement pumps all cross application boundaries where there is a requirement to introduce  a specific volume of liquid into a process system. Selecting which of these is the best option requires careful consideration and should always involve the expertise of the pump manufacturer.

Where an application has a requirement for the in-line proportioning and dosing of liquids, the reciprocating positive displacement process pumphas historically dominated the market and continues to do so. The issues affecting the choice of a metering or dosing pump as opposed to other types of pumps are normally associated with precision over a range of flows and the fact that the capacity is not influenced by pressure fluctuations within the process, which is the case with rotary pumps. It can handle a wide range of fluid types at varying temperatures, can operate at high pressures in excess of 1,000bar if required and provide high levels of accuracy and repeatability.

The breadth of applications for these pumps is considerable, with market sectors ranging from toiletries and detergents, to pharmaceuticals, water and waste treatment, food and beverages, power generation, chemical processing, petrochemicals and oil and gas recovery. The metering pump is equally at ease with dosing fragrances and colours in toiletries as it is with pumping hydrocarbon condensate offshore and liquid CO2 in cryogenic duties.

Cross section of a hydraulically actuated diaphragm pump head

Selecting the Correct Pump

The most common issues facing users are improved safety, greater efficiency and lower running costs. The first two issues have been addressed by the development of diaphragm pump heads, which prevent product leakage and contribute to higher levels of safety and more economic use of ingredients. Users are also conscious of the overall life -cycle costs, which consider known factors such as the initial investment and subsequent spares, maintenance, downtime, power consumption and efficient use of the product being handled.

The application usually determines the pump to be used so it is important to clearly understand the fluid's characteristics,like viscosity, solids content, abrasiveness, corrosiveness, toxicity and hygiene. It is also necessary to consider other parameters including, delivery volume and pressure, temperature, flow characteristics and vapour pressure. The correct material for the pump head must be selected according to the fluid's characteristics. For toxic materials or exacting applications where security and safety are priorities, a diaphragm pump head is usually the first choice. For less demanding applications, a plunger pump head will normally suffice.

Since metering pumps are typically employed as part of an integrated processing system, several factors influence what typeis appropriate. These could include NPSH (Net Positive Suction Head) calculations, a review of inlet and discharge pipe diameters and lengths and whether any constraints exist within the system. The environment  also plays a part in the selection of the pump materials. Metering pumps are used in some of the most arduous environments and conditions, ranging from offshore oil and gas platforms where sub-zero temperatures are frequent, in remote deserts where temperatures are high and the ingress of sand is a distinct possibility or in tropical areas where the presence of moisture has to be eliminated. Every type of working environment will influence the material is used for the pump components.

Only when a complete profile of the medium to be pumped and the processes involved are fully documented, can the type and design of metering pump be specified. In compiling this profile, it is always sensible to review any previous pumping experience with the fluid being handled.

Multiple pump head arrangement for metering several ingredients