- Using flow meters improves performance by verifying and dosing liquids more accurately than through the use of a metering pumps alone
- Deviations of 10-15% are considered normal conditions when only using metering pumps, which can be greatly reduced by integrating a flow meter to measure and correct flow rate
- This improved mass flow dosing system is independent of liquid, temperature and pressure, and reduces wear and tear
A metering pump can be used to move a volume of liquid in a specific period of time. Users often believe this provides them an accurate pumped volume. However, there are some disadvantages of using only metering pumps without flow meters.
Why Do You Need Flow Meters if You Use Metering Pumps?
Traditionally, metering pumps are believed to be accurate because the theory is that a known pump head displacement will move a known volume over a known time giving a known delivered flow. In practice, however, it will never achieve a high level of accuracy.
Coriolis mass flow meter in modular dosing system
Deviations of 10-15% are considered normal. In specific cases a metering pump can deviate even more than 50%. Inaccuracies like this are caused by the many changing process conditions, such as:
- Temperature changes
- Back pressure changing
- Gas entrapment
- Wear of components
These factors can each be the cause of an inaccuracy in the expected volume of displacement from a pump head movement. If you multiply each of those factors you can realize quite large measuring errors that create both inaccuracy and non-repeatability.
Improve Accuracy & Repeatability of Metering Pumps
Option 1) Add flow meters between metering pumps and the process
By adding a flow meter between the metering pump and the process, you can take information from the flow meter to adjust the speed of the pump. Traditionally, this would be managed with an analogue output signal, 4….20 mA. Another option is to transfer the signal from the flow meter into a separate controller that compares the real flow signal to the desired flow. Subsequently, the electronic controller can then adjust the speed of the metering pump to achieve the desired dose or flow.
Using this solution will mitigate the issues in the original solution, however it introduces other disadvantages:
- Slow control loop due to signal filtering in the controller
- Increased complexity with extra components
- Additional price of meter and separate controller
Option 2) Direct flow measurement with a flow meter with built in PID control
Another solution is using a direct liquid independent flow meter with built in PID control that can drive a pump to achieve the desired dose or flow.
With this solution, you do not need to include the metering pump in the control system. Just give a set point demand to the mass flow meter and it will drive the pump to achieve the desired dose or flow. This solution will give you several advantages, such as:
- Accurate and precise control of the flow
- Mass flow dosing is independent of liquid, temperature and pressure, wear and tear, in contrast to volumetric dosing when only a pump is used
- Accurate delivery mitigating normal pump issues
- Alarm functionality of low flow to prevent running dry of the system
- Preventative maintenance based on pump performance over time
Pump control can offer accurate dosing solutions for house hold chemicals like detergents and cleaning products. Learn more about a flow meter & pump combination.
Dosing solution flow meter with pump
In all applications where the performance of a metering pump is critical, it’s advised to add an additional flow meter. This is especially true in processes where a liquid is dispensed into a container or bottle or into the main stream of a process. This system offers many benefits: accuracy, repeatability, traceability, verification, documented certainty, less waste, advanced data analysis, no recalibration, and savings on pump costs.
Commonly, quality control tests on liquids are carried out randomly on a small percentage of the dosings to ensure general compliance. If you use a flow meter to control the dosage of your liquid, you can achieve up to 100% quality.
Additives or performance chemicals often have a significant impact on the result of a process. The substantial efficiency improvement of this process will then lead to a high-performance result and less waste. This is of a huge advantage in many applications, time-wise and cost-wise.