Irrespective of the great diversity of pump types, the amount of delivered liquid can be adjusted precisely only in volumetric pumps with a special control system. In pumps with the liquid pumping principle based on centrifugal or inertial forces (centrifugal pumps, vortex pumps, screw pumps and others), the delivered liquid flow control is insufficiently precise and delayed; the running flow is measured, the flow rate is analyzed, and the feedback procedure is used to send the control signal to the actuator (the control valve, the hydraulic clutch, the variator or the motor variable frequency driver) that implements successive iterative variations to control the flow. In pumps implementing the volumetric operation principle, the exactly fixed liquid volume is moved in each complete pumping cycle; this volume is a function of the effective pump operating volume, i.e. the difference between the operating space volume at the time when the suction process is complete and the residual operating space volume at the time when the liquid discharge process is complete. As a result, metering is possible with high degree of accuracy; for this purpose, frequency of operating cycles or effective operating volume of the metering pump shall be adjusted. In volumetric pumps (unlike the pumps in which the centrifugal or inertial operation principle is used), the metered volume does not depend significantly on external process factors such as pumped liquid viscosity variations or pressure variations at the suction inlet or at the metering chamber (these variations arise from time to time due to changes of raw materials, process disturbances or variations of external temperature or process temperature). Also, because the parameters can be calculated precisely and remain stable irrespective of the external factors, these pumps can be integrated into the systems with high level of automation and high precision of metering. These factors are the backgrounds for wide application of volumetric pumps for metering purposes.
Peristaltic pumps are successfully used in laboratories and small-scale manufacturing facilities with low flow rates, low pressures and low automation levels. These pumps are very effective to pump small amounts of aggressive and caustic liquids, because the operating medium is completely separated from the actuator, and there are no seals that are the major sources of leaks during operation; to a large extent, reliability and safety of a peristaltic metering pump depend on the material of hoses and on their resistance to the pumped liquid and to multiple deformations.
Diaphragm (membrane) metering pumps are less effective than peristaltic ones but they are more convenient for operation and more reliable. In a membrane pump, the operating volume of the pump is in the chamber with a membrane, and this volume is limited by the suction check valve and the injection check valve. The minimum operating volume is the chamber volume available at the time when the valve is closed at the suction inlet side; the maximum operating volume is the chamber volume available at the time when the check valve is closed at the pressure side, i.e. when the metering pump membrane displaces the metered liquid from the chamber in a maximum extent and starts to move backwards. The effective operating volume in membrane pumps is significantly less than in peristaltic pumps because the diaphragm does not press the total volume from the operating space and, also, because small stray flows exist when the check valves operate. The factors determining the characteristics of membrane metering pumps are the area of a membrane (a diaphragm), the membrane movement amplitude and the frequency of injection cycles. For existing pump models, frequency of injection cycles or membrane movement amplitude shall be adjusted to control the metering.
The advantages of diaphragm metering pumps are as follows:
Plunger (piston) metering pumps are the most common in industry because their design is capable to reach high pressure and high flow rate; however, these pumps have several drawbacks in terms of their operation. Similar to diaphragm metering pumps, the effective operating volume is a difference between the operating chamber volumes: the maximum volume existing at the time when the check valve is closed at the pressure side minus the minimum volume existing at the time when the valve is closed at the suction inlet side; however, the volume-forming operating element in this type of pumps is not a membrane but a piston or plunger. Due to the piston or plunger used, the operating space can be used more effectively than for the membrane; however, stray flows also exist while the check valves opening or closing is in progress. The factors determining the characteristics of plunger metering pumps are the piston diameter and stroke, the number of pistons (single-, double-, three- or five-plunger pumps) and the frequency of operating cycles. For existing pump models, frequency of piston stroke cycles or piston stroke length shall be adjusted to control the metering.
The advantages of plunger (piston) metering pumps are as follows:
Metering pumps’ application examples
These pumps are widely used in metering plants for various branches of industry such as:
We are convinced that our company ENCE GmbH will become your reliable and efficient distributor (authorized representative) of your metering pumps in the Russian market.
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