7 Points to Consider When Metering Batches of Liquid
Metering batches of liquid may seem like a very simple task, but not when obstacles arise. Engineers should be aware of the problems that can occur during the process, and understand how to assess them to ensure successful set-up. Typically, the easy process of metering fixed volumes of liquid entails putting a flow meter in the product line, connecting it to an instrument, controlling the measurement with on-board software, wiring in a valve, entering the required volume and pressing a button, and waiting for the vessel to have the right amount of liquid added. To prevent any issues from happening during this otherwise simple process, consider the following points:
- System repeatability is an essential aspect of metering liquid batches. There are multiple uncertainties that can change the liquid amount being delivered, as well as cause a negative effect on the system repeatability. If the batching arrangement becomes hydraulically locked with no place for gaseous fluids to accrue, there may be a full or empty air pocket. This occurrence may cause a burst of a dynamic flow and high flow situation for part of the dispense, or it may have no effect at all if the line is full. If you are using a volumetric flowmeter, any air passing through would be seen as part of the batch, causing a low delivery.
- The process conditions should be constant if at all possible. Changes in temperature, pressure, or flow rate may cause the flow meter to behave in an unpredictable dynamic way, or operate on a different part of its calibration curve.
- The overall flow should be constant, since tuning the pulsation damper depends on the flow rate. Therefore, pulsating flow should be avoided. Selection of the pump type is important! Problems, such as the introduction of large variations in flow over an operating cycle from the use of aggressive chemicals in pumps, can be minimized with a pulsation damper and pressure regulator.
- If the measurement point is immediately before the dispense valve, the flow meter will perform much better. Doing so makes the resulting delivery accurate by eliminating contained fluid volume and inertia.
- Always be aware of your flow meter resolution. For instance, a water meter, which gives 1 pulse per liter, is never going to deliver 10 liters with an accuracy and repeatability of better than 10%, unless some very clever electronics or calibration are involved.
- Ask yourself which type of shut-off valve should be used in the process. Consider which method is best for having the valve stop signal made before the batch size is reached. It may entail a solenoid or an alternative valve.
- If you are using an electronic-based flow meter for the measurement of rapid batch cycles, be aware that it may cause potential time delay during the process. Some standard electromagnetic flowmeters may have an internal cycle time of perhaps 50 Hz, which means that the flow throughput is only updated every 20ms; therefore, a 2-second dispense could be 1% out regardless of meter linearity. Using a special batching flow meter may be more beneficial, as they have the ability to predict when the stop signal should be sent before the amount of liquid has been delivered.
By taking these precautions when specifying and designing a system, you can be sure that the measurement will be extremely accurate. We supply various measurement instruments such as the turbine flow meter and inline flow meter. For more details, visit us at http://jlcinter.wpengine.com.
For further demonstration, we’ve included some results from a custom Atrato ultrasonic flow meter designed purely for batching a 3-second delivery for a 150ml bottle filling trial: