Metering pumps are a positive displacement chemical dosing device with the ability to vary capacity manually or automatically as process conditions require.
It features a high level of repetitive accuracy and is capable of pumping a wide range of chemicals including acids, bases, corrosive or viscous liquids, and slurries.
The pumping action is developed by a reciprocating piston, plunger or diaphragm which is either in direct contact with the process fluid or is shielded from the fluid by a diaphragm.
Diaphragms may be activated by direct mechanical link or by hydraulic fluid. Metering pumps are generally used in applications where
one or more of the following conditions exist:
- Low flow rates are required
- High system pressure exists
- High accuracy feed rate is demanded
- Dosing is controlled by computer, microprocessor, DCS, PLC, or flow proportioning
- Corrosive, hazardous, or high temperature fluids are handled
- Viscous fluids or slurries need to be pumped
Metering pump characteristics
1. The pumping action is developed by the reciprocating action. This reciprocating motion develops a flow sine wave. Actual flow rate is determined by the formula below in Figure 1.
2. Unlike centrifugal pumps, flow rate is not greatly affected by changes in discharge pressure (Figure 2).
3. The metering pump flow vs stroke characteristic curve is linear (Figure 3). It is not however, necessarily proportional in that 50 per cent stroke setting may not equal 50 per cent flow. This is due to the fact that the calibration line may not pass through 0 on both axes simultaneously. By measuring flow at two stroke settings, plotting both points and drawing a straight line through them, other flow rates vs stroke can be accurately predicted.
The steady state accuracy of a correctly installed industrial grade metering pump is generally +/- 1.0 per cent or better.
Although a metering pump can generally be adjusted to pump at any flow rate between 0 and its maximum capacity, its accuracy is measured over a range determined by the pump’s turndown ratio.
Most metering pumps have a turndown ratio of 10:1 which simply means that the pump is within its accuracy rating anywhere between ten per cent and 100 per cent of capacity.
Some newer designs of metering pumps feature higher accuracy, and a greater turndown ratio of 100:1.
Therefore, this design will accurately dose anywhere between one per cent and 100 per cent of capacity.
Article courtesy of Kelair Pumps Australia “When Pump Knowledge Matters”. Phone: 1300 789 466 or visit www.kelairpumps.com.au.