Improving metering pump performance

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.
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.

Option 1) Add a flow meter between the metering pump 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

In all applications where the performance of a metering pump is critical, I advise you to add an additional flow meter. Especially in processes where a liquid is dispensed into a container or bottle or into the mainstream of a process. It offers many benefits: accuracy, repeatability, traceability, documented certainty, less waste, advanced data analysis, no recalibration and savings on pump costs.
Adding a flow meter to a metering pump can be relevant in a chemical lab setup, in a food pilot plant or even in household chemicals like detergents and cleaning products.

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 control checking of your product with reduced human impact.

Additives or performance chemicals are usually precious and have a huge impact on the result of the process. The significant 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.