Two different dosing methods can be distinguished for which the magnetic drive gear pump is employed in a control loop: The continuous and the batch dosing process. Continuous dosing is the strong point of this pump. Due to the non-pulsating liquid transfer and direct capacity variation via speed control the pump can be very easily integrated into a PLC system. To enable our pump to be used for batch dosing duty the process must be configured such that the pump is operated with a bypass. If dosing irregularities occur this is frequently due to the selection of inappropriate components, these must be suited for the relevant dosing job and matched to each other.
For continuous dosing service the pump is operated with motor and speed control feature (e.g. frequency converter). Downstream of the pump a flowmeter is arranged which is connected with a speed controller. Via the controller the setpoint is adjusted either manually or externally (e.g. by PLC) and compared with the actual value of the flowmeter. The speed control system will then cause the setpoint and actual value to coincide.
For batch dosing the pump is operated at a prescribed speed which is dependent on the dosing duration. Downstream of the pump a three-way valve is located which causes the conveyed fluid to be either dosed into the process or returned via a bypass to the pumping vessel. Downstream of the valve in the process line a flowmeter with batch controller is arranged which acts on the valve by means of a controller. Before the dosing operation begins the pump is started up and the fluid pumped through the bypass. To start the dosing operation the controller actuates the valve thus clearing the passage to the process piping. The conveyed flow is added up by the batch controller. When the prescribed volume has been reached the controller clears the valve passage to the bypass. With this control configuration the total error results from the flow measurement error, the time delay caused by the actuation of the valve and the after-dripping of the liquid at the end of the pipe. The error may be minimized if the duration of the dosing operation is maximized.
Commercially available flowmeters usually have a measuring range smaller than the capacity range of the GATHER magnetic drive gear pump (up to 1:400). Examples of measuring systems with advantages and limitations are given below.
Measurements performed with a mass flow meter installed into the piping system are based on the mass acceleration principle. The liquid is usually passed through an omega-shaped tube subjected to micro-vibrations. The resulting force (coriolis force) is measured and, depending on the inside diameter of the omega-shaped tube, is indicative of a mass flow. The mass flow meter is capable of measuring flow rates as low as 1.0 g/min (appr. 1.0 ml/min). Due to its design characteristics the mass flow meter has a relatively small measuring range; mass flow or viscosity increases cause the pressure loss across the mass flow meter to rise as well.
As a rule, metering is effected from the balance through the pump into the system. It is the most accurate measuring method offering a broad measuring range. This type of mass flow measurement is widely used in the laboratory field. It is deemed highly sophisticated (especially in hazardous areas) because vibrations must be ruled out that may otherwise impair the measurements.
The volume flow measurement yields results that are similarly precise than those obtained by mass flow measurement. However, this is only true if certain prerequisites are met: Constant temperature and density and no gas inclusions.
Magnet-inductive flow measurement is the most versatile volume flow metering method. Only one prerequisite must be fulfilled: The liquid needs to have a discrete electrical minimum conductance.
The gearwheel meter is frequently used with highly viscous fluids. For starting it requires a minimum fluid velocity to cause the gearwheels to be set in motion. The gearwheel meter is quite expensive because it needs a specific electronic evaluation unit.
The turbine flowmeter is a cost-effective device for inline volume flow measurement. However, it has application-related limitations.
A flowmeter serves to measure the pump capacity directly. The following measuring systems detect the effects the capacity has on the process. The process quality in this case is governed by significant parameters such as pH value, pressure or temperature. These may be directly influenced by capacity variations.
Example: By adding caustic solution a wastewater stream of fluctuating pH value shall be adjusted to a constant pH before the wastewater is admitted to a sewage treatment plant. In such a case the pump must be capable of operating at a control range of 1:100 and above; the pH meter only measures fluctuations around a given value.