Improving metering pump performance with flow meters

January 18, 2022 Ferdinand Luimes
Coriolis mass flow meter in modular dosing system
Coriolis mass flow meter in modular dosing system

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 a flow meter. In this blog, I will share the details with you.

Why do you need a flow meter if you use a metering pump?

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 realise quite large measuring errors that create both inaccuracy and non-repeatability.

flow meter between the metering pump and the process

What can you do to improve the accuracy & repeatability of your metering pump?

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.
flow meter with built in PID control

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.

Where can you apply a metering pump with flow meter?

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 main stream 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 a random skid.

Dosing solution flow meter with pump
Dosing solution flow meter with pump
Pump with flow meter for low liquid flows.

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.

Are you interested to learn more about how flow meters can help you optimise your pump performance? Check out our webinar video.

Would you like more information about flow meter pump combinations?

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