Metering and dosing pump operation. Any positive displacement pump can be used as a metering or dosing pump. The purpose of these pumps is to inject a known volume of liquid into a process stream. The primary requirement is that over a given period of time the pump consistently delivers the same amount of liquid. Keyword: gear pump, piston pump, helical rotor pump, diaphragm pump, peristaltic (hose) pump, additive, dosing rate.
Types of Metering and Dosing Pumps
Positive displacement pumps used for dosing duties include diaphragm pumps, peristaltic (hose) pumps, helical rotor pumps, gear pumps and piston pumps. Piston pumps are the type commonly known as metering or dosing pumps. The degree of accuracy in the delivered quantity is low with diaphragm pumps, better with hose pumps, good with rotor pumps, excellent with gear pumps and the best with piston pumps. The cost of each pump type rises in the same order.
Pumps from the lower order of the list suffer from back pressure efficiency losses. When they work against a high pressure some of the liquid in the pump does not actually get ejected from the pump. The higher the pressure the less liquid is ejected. This is exactly the opposite of what is required from a metering pump. But for low pressure, low accuracy applications they are cost effective.
Helical rotor and gear pumps perform well when dosing is required over longer periods of time. The most accurate metering pumps are the piston pumps. Small chambers in the pump are squeezed empty by a solid piston moved mechanical or hydraulically. The stroke length and/or the stroke frequency are adjustable.
Accurate dosing leads to less waste of expensive additives and translates to lower costs as only the exact amount is used.
Dosing System Design and Installation
The components of a dosing system are the supply tank, the dosing pump, pressure relief device, the flow control equipment and the injector. Figure No. 1 shows a drawing of a simple dosing system.
Measuring the quantity to be dosed can be done by putting the required amount into a calibrated buffer tank and pumping it dry or by installing a level detector in the supply tank and controlling the pump’s stop/start automatically. Using a flow meter to measure the pump discharge is usually too inaccurate for the small flows and quantities involved. The surging flow from pump-action pumps makes accurate flow measurement impractical.
It is necessary to include a means to adjust the flow rate from the pump. This can either be built into the flow control system or it is a function offered with the pump. Motor frequency controllers and regulating valves with a by-pass back to the tank are common flow adjustment methods.
If surge flows from the pump cause problems a pulsation dampener can be installed in the discharge line. This is a small pressure vessel with a pressurised space inside. As a surge arrives the pressurised space compresses and absorbs the impulse into the dampener. The pressured void applies a constant pressure on the liquid and the discharge from the dampener is constant and steady.
When installing the dosing pump provide room for inspection and maintenance access to the equipment. Keep the discharge pipe length from pump to injector as short as possible though positive displacement pumps can generate high pressures and will force liquid a long distance. Long discharge pipes should be one size larger diameter than the connection to the pump outlet. Keep the suction line short and one diameter larger than the inlet connection to reduce the chance of pipe friction losses causing low pressures leading to pump cavitation.
Piston pumps can produce high accelerations in the suction line as the piston strokes and the sudden fast movement can cause cavitation. Though positive displacement pumps will draw liquid from a distance, it is best to maintain a pressurised suction. Mount the pump below the lowest level the liquid falls to in the tank and insure the suction line slopes downward all the way to the pump.
The injection point into the process should project into the tank or the pipe. On a large diameter pipe inject the dosed chemical into the center of the pipe to provide the best chance of mixing thoroughly. Injecting into the middle of the flow is not so important on small-bore pipes because the flow streams in the pipe are close together and tend to mix. Be aware of the dilution effects when an additive is injected. Effects such as increased corrosion rates, temperature rise and vapour release from chemical reactions can occur when the additive contacts the process liquid.
Issues with Each Type of Pump
Summarised below is a list of some of the issues that affect the various types of dosing pumps.
|Peristaltic (hose) Pump||Good for batch dosing Conceptually simple|
|Helical Rotor Pump|
Pumping a known quantity of liquid into the process stream and measuring the time taken is how a dosing pump is calibrated. A clear, graduated column of sufficient volume is installed to supply the pump. The column is filled with liquid and the pump is started. The drop in level over a period of time is recorded. From the time difference and the change in level the flow rate can be calculated. Changes to the flow rate are done by adjusting discharge valve positions or by changing the pumping speed/stroke.
Mike Sondalini – Equipment Longevity Engineer