Better think different
Hydraulic diaphragm pumps with applications in metering

An unconventional type of pump, the Wanner Hydra-Cell, is increasingly being chosen for dosing applications in chemical industry. Smaller size and lower cost are the main advantages over API 675 metering pumps.

By no means every pump used for dosing needs to meet the stringent specifications of what many regard as the “true” type of dosing pump: a hydraulic diaphragm pump built in conformance with the API 675 standard. Nevertheless, despite their relatively high cost, such pumps may be chosen for their flow/pressure capabilities, their compatibility with difficult media and for their accuracy, which is not subject to the developing problems of seal wear and internal leakage associated with some other positive-displacement pumps.
In practice, process engineers use a variety of pumps in dosing applications. An understandably conservative approach to pump selection, however, means that their first thoughts tend to be of traditional pump types. Rotary-lobe pumps, peristaltic pumps, piston and gear pumps, single- and multi-stage centrifugal pumps have all been chosen, though each type has its limitations.

Similar, but different
In recent years a less-conventional type of pump, the Wanner Hydra-Cell, has shown itself to have several advantages over the pumps commonly chosen for this work. The Hydra-Cell is thoroughly proven in process industry applications, but is not yet widely known in the special context of metering and dosing. Among the leaders in applying Hydra-Cell pump technology to metering and dosing are two of Germany’s major chemical manufacturers.
In the Hydra-Cell design, hydraulically-actuated diaphragms pump the liquid and also isolate it from the drive mechanism. This feature and some other characteristics are shared with API 675 pumps. Both types can safely handle a wide range of difficult media, including non-lubricating liquids and those with suspended solids. They can operate at low or high pressures (up to 170 bar in the case of Hydra-Cell pumps, depending on the model chosen). Both are seal-less designs, so neither type is exposed to the consequences of seal wear.
There are also major differences. Some pumps to API 675 metering specification are capable of flows up to 50 m3/h or more. Maximum flow of the biggest Hydra-Cell pump is only 7.6 m3/h. But there is a startling contrast between the physical sizes of the two types for equivalent flow and pressure performance (see Figure below).
API 675 pumps have certain inbuilt safeguards and other features, such as stroke adjustment. Consequently they tend to be massive in construction, even in flow ranges below 10 m3/h. Their size, sophistication and high specifications all contribute to high procurement cost.
Unstressed at all pressures
Hydra-Cell pumps, though well-engineered and robust, are compact and simple in construction. Each diaphragm closes off a hydraulic cell (single-cell, three-cell and five-cell arrangements are used) and the cells are pressurized in sequence, flexing the diaphragms and displacing the liquid to be pumped. Liquid pressures on either side of the diaphragm automatically balance, so the diaphragms work unstressed at all pressures.
The API 675 pump works slowly, delivering a large volume of liquid per stroke. A fundamental of the Hydra-Cell concept is that the pump is designed to work at high speeds, delivering small volumes of liquid at high frequency. Not only is pulsation reduced; output is easily and accurately controlled by altering pump speed. And turndown ratios as high as 15:1 allow flow to be controlled over a wide band of performance.
An important factor in the increasing viability of Hydra-Cell pumps for metering and dosing has been the development of smaller, cheaper and more efficient variable-frequency drives. Hydra-Cell pumps are well placed to take advantage of this, because the relationship between pump speed and output flow is linear, varying only minimally with changes in pressure. If flow in the dosed system changes, the pump can respond instantly. Flow can be adjusted and re-adjusted freely from one set point to another with repeatable accuracy.
Practical applications
Presenting an unusual combination of features, Hydra-Cell pumps are increasingly being chosen in preference to more conventional pumps for metering applications. For example, a German chemical plant installed two Hydra-Cell G10 pumps for metering a 30 percent titanium dioxide slurry. They were chosen instead of a much bigger three-piston diaphragm pump. Benefits included less pulsation, space savings and reduced costs (25 percent on procurement and lower energy consumption). Flow data from the receiving pipeline is continuously processed to control motor speed and ensure the Hydra-Cell pumps deliver titanium dioxide at the appropriate rate. Inspected six months after installation, the pumps showed no trace of internal wear.
Low-viscosity foaming substances such as freon, pentane and cyclopentane are volatile and prone to leak past pump seals. For precise dosing of these liquids at the foaming head, a manufacturer of expanded polyurethane has been able to avoid the expense of traditional metering pumps by installing Hydra-Cell pumps with variable-frequency speed control. Performing reliably and not subject to internal leakage, the pumps have achieved levels of accuracy comfortably within the maximum allowable dosing error of ±3%.