Trevor Forster is Managing Director of Titan Enterpises. His experience in fluid handling, and specifically in this case – metering batches of liquid & fluids, dates back to the mid 1960’s when he started working on rotating seals and flow meter design for third party clients. Drawing upon over 40 years of using innovative flow meter design and production techniques, Trevor and his team have been able to produce elegant flow metering solutions for organisations around the globe. In this issue – Trevor offers you useful technical tips on metering batches of liquid:
Metering Batches of Liquid & Fluid
Metering batches of liquid & fluid is a very simple thing to do… or is it? Typically metering fixed volumes of liquid into a process e.g. adding water to a mixer for food production is easy. It requires that you put a flow meter in the product line, connect it to an instrument, control the measurement with on-board software, wire in a valve, enter your required volume and press a button and wait for your vessel to have the right amount of liquid added. With all these steps in the process a lot of problems can occur along the way and needless to say not that easy. The following points must be considered when designing and commissioning the flow system set-up.
1. Flow system repeatability is the key when metering batches of liquid & fluid. The batching arrangement must have be hydraulically locked with no place for gaseous fluids to accumulate. If there is an air pocket this can be full or empty. Consequently it may cause a burst of high flow and a dynamic flow situation for part of the dispense or have no effect at all if the line is full. With a volumetric flow meter any air passing through would be seen as part of the batch and cause a low delivery. These uncertainties will change the amount of liquid delivered and will negatively affect system repeatability.
2. Ideally the process conditions should be constant when metering batches of liquid & fluid. Changes in pressure, temperature or flow rate may cause the flowmeter to operate on a different part of its calibration curve or behave in an unpredictable dynamic way.
3. Pulsating flow should be avoided if possible so the selection of the pump type is important. Aggressive chemicals frequently use pumps which introduce large variations in flow over an operating cycle. These problems can be limited with a pressure regulator and pulsation damper. The overall flow should be constant as the tuning of the pulsation damper is flow rate dependant.
4. The flowmeter will perform better if the measurement point is immediately before the dispense valve. This eliminates inertia and contained fluid volume making the resulting delivery more accurate.
5. Be aware of your flowmeter resolution. For example a water meter which gives 1 pulse per litre is never going to deliver 10 litres with an accuracy/repeatability of better than 10% unless some very clever electronics and/or calibration is involved.
6. Consider the type of shut-off valve that is used for metering batches of liquid. A solenoid is excellent with a fast open/close time but is not practical for larger pipe diameters and higher flow rates. Alternative valves will have relatively long open and close times and these must be accommodated in the batch cycle time. The valve stop signal has to be made before the batch size is reached this can be done in a number of ways. The usual method is to calibrate the system by measuring the volume dispensed for a given setting and adjusting the volume to accommodate errors in the start/stop times. The more sophisticated solution is to use a “batch overrun” program. This measures the actual volume dispensed by the flow meter and then adjusts the stop signal accordingly. It may take 3 or four runs to reach the required volume. A typical set of records may go something like this:
Run 1 ? 10.13 Litres
Run 2 ? 9.95 Litres
Run 3 ? 10.02 Litres
Run 4 ? 10.00 Litres
This relies on the meters linearity and is not as efficient with a non-linear meter. Some instruments have 2 stage shut-off so a large valve for the first stage and a small fast one for the final shut-off.
7. Be aware of electronic based flow meters with rapid batch cycles. A standard electromagnetic flowmeter may have an internal cycle time of perhaps 50 Hz. This means that the flow throughput is only updated every 20ms which means a 2 second dispense could be 1% out regardless of meter linearity. Special batching flowmeters accommodate this potential time delay by predicting when the stop signal should be sent before the amount of liquid has been delivered.
By taking these simple precautions when designing or specifying a system extremely good accuracy can be achieved.
Below are some results obtained from a custom Atrato ultrasonic flowmeter designed purely for batching a 3 second delivery for a 150ml bottle filling trial.
Enquire here for further info or to discuss your flow metering requirements