How to control low flow rates
For controlling low flow rates, the weakest link is usually not the mass flow sensor. A mass flow controller is capable of accurately measuring and controlling the flow rate, at its position in the system. However, there is no absolute confidence that this flow rate is accurate further up- or downstream of the flow controller. If no countermeasures are taken, the exact desired flow rate will not be delivered to the process. As the flow rate minimizes the relative internal volume of system components, such as piping, filters and valves, seem to increase. This affects the dynamics of the system as the response time will slow down resulting in a loss of direct control. So, when a set point is given and this is assumed to reach the process, expectations might not be met.
For instance, a popular setup to force a flow through the system is to make use of pressurized gas. However, gas will dissolve in the liquid to a saturation level proportional with the gas pressure. The dissolved gases appear again as bubbles downstream in the system where the pressure has decreased. If a gas bubble passes the flowmeter or valve or enters the process it disturbs the stability of the flow.
Practically for low flow rate processes, it is sometimes hard to understand why and when the system works correct. And so many questions arise. Is the purity of the media correct? Are the process temperatures as they should be? Is the set rate or dosage correct? Is the pressure stable?
Challenges which can occur within low flow process
For the lowest flow rates it is hard to verify if, at any time, the flow entering the process is as expected. As mentioned, there may be various underlying causes:
- Dissolved gases in the liquid and uncontrolled gas bubble entrapment and release
- Dynamic effects of multiple fluid transmission lines: e.g. in medical multi infusion systems
- Compliance of the system: e.g. in plastic tubing or plastic syringes
- Local heating and fluid expansion: due to the internal volume and power dissipation of solenoid valves
- Ripple on the flow delivery when using pumps