-14-November-2005
Metering and dosing pumps
Every industrial process where fluids are moved requires flow monitoring, flow metering and possibly flow control. Patrick Deniau of Dosapro Milton Roy reviews the options that can be employed.
The most common pump type is the centrifugal pump, where the rotation of an impeller gives the fluid its speed. The output flow of the centrifugal pump is highly dependant on the pump discharge head and the fluid characteristics (specific gravity, viscosity, suspension), and is characterized by its flow curve. This characteristic makes centrifugal pumps unsuitable for flow metering when used on their own. A control device must be added, the most common one being a control valve.
Flow metering
With the centrifugal pump, a high level of accuracy cannot be expected on the measured flow, for any change in the pumping parameters can alter the flow. For example, a change in fluid temperature alters the viscosity and impacts on the pump discharge head, therefore ultimately changing the measured flow if no other corrective device is employed. Generally, flow metering with a centrifugal pump can only be done around a given working point.
Volumetric pumps
Metering flow is more accurate when using a volumetric pump. Being either rotary or of an alternative design, the pump in this case injects a volume of fluid per revolution or stroke. Unlike centrifugal pumps, volumetric pumps do not have a flow curve. If the delivered flow is dependent on the discharge pressure, the phenomena is linked to the pump design (dead volume ratio, internal and external leaks), the compressibility of the sealing devices and the pumped fluid compressibility itself. It appears to be a hydraulic efficiency issue and the flow sensitivity to pressure is very limited, as shown in Figure 1. As a volumetric pump delivers media by volume, it cannot be operated in conjunction with a control valve. Whilst the gradual closing of a control valve does not change the pump injection rate and flow significantly, it does create the potential for over pressure.
In processes not requiring high levels of accuracy, a by-pass valve run in conjunction with a volumetric pump may be encountered, but this arrangement is becoming less frequent today. It is now more common to use the variable speed technique, this very much due to its competitive price and the increasing reliability of the technology. In a volumetric pump, the pumping rate depends upon the geometric design of the pumping cavities, and is a function of the pump speed in rotary pumps or pump stroking speeds in reciprocating pumps. The greater the speed, the more volume injected per unit of time. The relationship between pump speed and output pump flow is relatively linear and is only marginally affected by pump volumetric efficiency.
Metering pumps
Where the required level of accuracy is of a high order, the metering pump concept comes into its own. A metering pump is a reciprocating volumetric pump that has by design a swept volume adjustment function. The flow delivered is, therefore, a function of both the pump flow adjustment and pump speed. Swept volume adjustment can be achieved basically by two conceptual designs.
In the first concept, a mechanical subsystem allows the amplitude of the pump stroke to be varied between 0 and 100%. Different designs exist, including variable eccentric (the most common), inclinable crank, beam and pivot (Figures 2a and 2b) and all are frequently used in process industries. More simple designs based on a piston mechanical lost motion are also available but, as in the case of the cam and spring return versions, they are limited to light duty applications
Pump Industry Analyst
Sealing Technology
World Pumps © Copyright 2008, Elsevier Ltd, All rights reserved.