01953 609930Applications
How do you achieve accurate variable flow on a sanitary or pharmaceutical pump skid?
Changing pump speed is the simplest way to vary flow, but it does not guarantee that the process receives exactly what has been commanded. For dosing, blending and validated duties, measuring the real flow and correcting it through a closed control loop can provide a more accurate and repeatable result.
Why can commanded pump speed and actual flow differ?
We regularly encounter applications where a pump has been correctly selected for the maximum duty, but the relationship between speed and actual process flow becomes less predictable towards the bottom of its operating range.
The difference may be small enough to ignore in a straightforward transfer application. In dosing, blending or validated processes, however, even a relatively small variation may affect repeatability or the amount of product delivered.
Open-loop pump control versus closed-loop flow control
The system assumes the expected flow
The PLC commands the pump to run at a given speed and assumes that this corresponds to the required flow.
- Simple to implement
- Suitable where conditions remain stable
- Relies on pump repeatability and calibration
- Does not automatically correct process variation
The system measures and corrects the real flow
A flowmeter provides feedback to the controller, allowing the pump, valve or both to be adjusted continuously.
- Responds to changing process conditions
- Controls around the measured flow
- Supports improved accuracy and repeatability
- Provides recorded process feedback where required
Open-loop control is often perfectly adequate where the pump and process are stable. Closed-loop control becomes more valuable where accuracy, verification or changing operating conditions make assumed pump output insufficient.
How can variable flow be controlled?
Control the pump directly
Vary the pump speed from the PLC and use a flowmeter for verification or as the feedback signal. This can be the cleanest approach where the selected pump has good speed-to-flow linearity across the required range.
Control flow with a sanitary valve
Use the pump to provide a stable supply while a responsive sanitary valve modulates the actual process flow. This separates the pumping duty from the fine-control function.
Combine pump and valve control
For a particularly wide operating range, pump speed can provide coarse capacity control while the valve provides fine regulation around the required process setpoint.
The correct approach depends on the pump technology, process dynamics, pressure conditions, required response and the difference between minimum and maximum flow.
Where does an Equilibar FD valve fit?
The Equilibar FD Series sanitary valve can be configured for automated flow control by combining it with a flow transmitter and an electro-pneumatic pilot regulator.
The flow signal is compared with the required setpoint and the pilot pressure is adjusted to move the FD valve accordingly. Its diaphragm and multiple-orifice design provides a wide controllable flow coefficient range and a fast response across changing process conditions.
The exact control philosophy, valve action and fail-safe behaviour must be designed for the individual application. Increasing pilot pressure moves the FD diaphragm towards the orifices and reduces process flow.
Pump selection remains fundamental
A control valve cannot compensate for a pump that is fundamentally wrong for the application. The pump must still provide the required maximum flow and differential pressure, remain compatible with the fluid and operate reliably at the expected duty cycle.
Valveless piston pumps
Can provide highly repeatable low-volume metering and dispensing where fine resolution is required.
Peristaltic pumps
Keep the fluid contained within the tubing and can be suitable for contamination-sensitive transfer or dosing.
Other positive-displacement pumps
May be more appropriate for larger sanitary duties, different viscosities or higher process-flow requirements.
The control strategy should therefore be considered at the same time as the pump principle, rather than after the pump and skid layout have already been fixed.
Do you need a pulsation dampener as well?
Potentially. Closed-loop flow control can correct the average flow rate, but it does not necessarily remove the rapid pressure ripple created by a pulsating pump.
These are two related but different problems
If pulsation is affecting the flowmeter signal, triggering alarms or disturbing a filter or chromatography column, the Equilibar PD sanitary pulsation dampener may form part of the wider control strategy.
What information is needed at the design stage?
Accurate component selection depends on understanding all the operating modes rather than one nominal duty point.
Talk to PCT before the pump is fixed into the design
If you are developing a sanitary or pharmaceutical skid, send PCT the complete flow range, pressure conditions, fluid details and required control performance. We can help review the pump, flow measurement, pressure stability and valve-control approach as one coordinated system.
Find this interesting, talk to us more about it
We aim to respond within 1 hour for customer enquiries
Call us
01953 609930
Email us
sales@pctflow.com