Improving dosing and metering pump performance

This week I wanted to discuss pumps, dosing pumps in particular. Many of our customers want to combine flow meters with pumps and as a result we frequently get asked the same questions.

Why do we need to have a Coriolis mass flow meter if we use a dosing or metering pump?

Traditionally, and in most cases we see, dosing- or 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 volume. In practice however it will never achieve a high level of accuracy with deviations of 10-15% being normal. Inaccuracies like this are caused by many changing process conditions, such as:

1. Temperature
2. Pressure shifting
3. Air entrapment
4. 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 then multiple each of those factors you can realise quite large measuring errors that create both inaccuracy and inconsistency.
 

What can be done to improve the accuracy?

Option 1) Add a flow meter between the pump and the process
By adding a flow meter between the 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 or similar, from the flow meter into a separate PID controller that compares the real flow signal to the desired flow. Subsequently, the electronic controller can then adjust the speed of the pump to achieve the desired dose or flow.
 
Using this solution will mitigate the issues in the original solution, however it introduces more:

• Slow flow signal due to signal filtering in the PID controller
• Slow pump response due to extra control relay
• Increased complexity with extra components
• Time to achieve stable flow can be long
• Additional price of meter and PID controller

Option 2) Direct mass flow measurement with a flow meter with built in PID control
Now we need to discuss the next possible solution, using a direct Mass Flow measurement device 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 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:

• Direct mass flow control of the flow
• Mass flow dosing is independent of temperature and pressure, in contrast to the volumetric dosing when only a pump is used
• Accurate delivery mitigating normal pump issues
• Alarm functionality of low flow
• Preventative maintenance based on pump performance over time
• Consistent flow measurement based on actual not assumed numbers