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Extending The Boundaries Of Fluid Flow Control

Monday, November 7, 2022

Understanding the two fundamentally different flow control methods

Conventional automated flow control has historically been achieved via two main methods. The first is via a network system with a flow meter taking the measurement and transmitting that reading to the Distributed Control System (DCS) where the actual value is compared to the desired value. The DCS then sends a corresponding control signal to position a separate valve. The second method is via an Electronic Flow Controller or Mass Flow Controller whereby the measurement, PID control and valve are engineered as one device. All the user is required to do is provide a set-point and even that can sometimes be “on-board”.

Why would one flow method be used as opposed to the other?

One could argue that it is merely a matter of scale – it would not make sense for an oil refinery to instal a single device with a 24” flowmeter and control valve mounted onto a single block. It would be impractical, prohibitive in terms of cost and a nightmare for maintenance. Conversely, compact MFC’s within a digital wiring loom make perfect sense for bench top analytical equipment.

“The reality however is that the requirements of “big industry” are completely different to those of scientists and equipment designers with such elements as safety monitoring, cyber security and accountability being merely the start.”

There is, however, a cross-over zone between the two whereby a separate flow meter, control electronics and valve can overcome challenging applications. Examples can be seen as:

  • highly dynamic systems where flow needs to be controlled from high to very low rates
  • high pressure systems and especially high differential pressure applications
  • either very low or very high temperatures
  • multi-phase fluids involving liquids, gas and solids
  • aggressive or corrosive fluids requiring the cost-effective use of exotic materials
  • special fluids that could be non-Newtonian or that must not be sheared
  • larger bore applications where single device instruments do not exist

An example of where the two flow methods cross over

Flow Control System

Flow Control System with an Alicat CODA Flow Meter and Equilibar Flow Control Valve

One very good practical example can be seen within the following video. The combination of an Alicat CODA Coriolis Meter, an Equilibar Control Valve and EQ-DA electro-pneumatic control to demonstrate the 1000:1 control is easily achieved. All these devices are available from Premier Control Technologies.

An application example follows:

Application example: CIP Cleaning Cycle Precision Flow Control

Central clean in place systems (CIP) are crucial to many large biopharmaceutical plants as they disperse the cleaning processes across all the skids located around the manufacturing facility. These process skids are diverse in nature, with bottle filling, ultrafiltration (UF), tangential flow filtration (TFF) and chromatography being but four. CIP fluids are supplied within the pressurized and flowing system to all the different parts of the biopharmaceutical manufacturing plant as required for regulated sanitization.

The requirement for CIP cleaning within sanitary processes is based upon the fluid reaching a specific velocity (ft/sec) for a specified period of time. In small diameter pipework this is relatively easy as the internal volume of the pipe runs is equally small. In larger diameter pipework consideration needs to be given to the flow profile to ensure that the much larger internal volume maintains the correct velocity throughout. It is evident therefore that the design of control systems for each individual skid must be adequate for these highly variable flow regimes.

Equilibar FDO Control Valve

Equilibar FDO Control Valve

The Equilibar FDO fluid control valve offers an ideal solution

Equilibar FDO fluid control valves are specifically designed for sanitary applications and are particularly helpful in processes such as these where precise control across a wide range of flow rates is required. The FDO can control a range of flow rates at a ratio of 100:1, whereas many traditional flow control valves (FCV) control in a range as low as 15:1. Additionally, the Equilibar FDO can be set up to control precise pressure or flow depending on how the feedback loop is set up.


Schematic: The CIP Delivery System

As can be seen in the schematic above, the CIP delivery system is primarily a recirculation loop driven by a pump taking suction from the reservoir, through an Equilibar FDO, and then back to the reservoir. Depending upon how the feed-back loop is oriented the system can either be pressure dependent or flow rate dependent. Based with feedback from a high-resolution flow meter and a pressure transducer. As described towards the beginning of this article a central computer controls the process for easy automation. During a production shift the CIP loop recirculates water through the heat exchanger with the possibility of this being controlled within a desired pressure. Then, as different skids across the manufacturing plant finish their cycle, they request CIP cleaning with the result that the supply from the CIP system must ramp up quickly to meet the demand. A precision sanitary flowmeter provides the feedback and the Equilibar FDO with its multiple orifice design and supple diaphragm control adjusts in milliseconds to the new flow setpoint.

When one skid finishes a cleaning cycle, another cleaning cycle begins potentially with a new flow rate setpoint to reach the proper fluid velocity. Traditional valves may take time to reach the desired setpoint during transitions, but because the Equilibar valve can transition from flow control to pressure control with very simple control logic, it can reduce the period of unstable flow. Shorter transition times between cleaning cycles can save money and reduce cleaning time. Importantly single-use systems can also be fulfilled, for example with the Equilibar SD Series.

Equilibar SD

Equilibar SD Series

From the schematic above PCT has expertise in house to supply the pump, the pressure transmitter, the flowmeter, the Equilibar back pressure regulator and the electro-pneumatic controller.

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