This article provides discussion and suggestions on the role of Coriolis flowmeters in pharmaceutical and related processes. In addition to typical process considerations such as productivity, measurement accuracy and cost consciousness, the industry provides some specific challenges for flow measurement technology. With the variety of processing steps and the need for cleanliness, purity and rigorous accountability to consumers and regulators, there is a need for a flow technology that is reliable with high accuracy and low risk of causing contamination.
The information in this article is based upon research and field testing of Emerson Process Managements Micro Motion Coriolis flowmeters and their capabilities. Where design and applicability similarities exist between manufacturers, the information in this article will be applicable to operation of other brands of Coriolis flowmeter.
Overview of Coriolis Flowmeters
Coriolis flowmeters measure mass flow directly by taking advantage of the Coriolis Effect. Simply stated, the inertial effects caused by a fluid flowing through a tube are directly proportional to the mass flow of the fluid. In a Coriolis flowmeter, vibration is induced in the process-filled flow tube(s), and then the mass flow rate is captured by measuring the time delay between the inlet and the outlet of the sensor.
Coriolis flowmeters also provide a direct and independent density measurement. The density measurement is proportional to the vibrational frequency of the tubes. Coriolis density measurement can be compared with a mass on a spring. As the process fluid becomes denser and heavier, the vibrational frequency of the flow tubes decreases.
Bent Tube and Straight Tube Basics
There are two basic styles of Coriolis flowmeter: (Dual) bent tube and (single) straight tube.
- Bent tube meters offer extraordinary accuracy and turndown capabilities. Two bent tubes in the meter are vibrated relative to one another. The vibration produces a sinusoidal motion along the length of both flow tubes. The sinusoidal motion of the flow tubes is measured at the inlet and outlet of the sensor. The Coriolis affect produces a time delay between the motion at the inlet and outlet that is directly related to mass flow rate.
- Straight tube meters are more rigid than their bent counterparts. The rigidity makes for a less sensitive flow sensor and a tradeoff in accuracy is needed to get improved drainability. The single flow tube is vibrated relative to an external reference tube. The two pieces move in opposite directions similar to the two tubes in the bent tube design. As with the dual tube device, the motion at the inlet is compared to the motion at the outlet. The time delay between the inlet and the outlet motion is a direct measure of mass flow rate. Straight-tube Coriolis flowmeters are less accurate than their bent-tube counterparts, but they are still more accurate than many other technologies.
Coriolis flowmeters are well suited to many areas of pharmaceutical manufacturing. A particular virtue of the technology is natural process isolation. The only part of the flowmeter that is in contact with the process is the inside of the flow tubes, which are made of corrosion-resistant metal alloys. There are no fluids or parts that can be exposed to the process in the event of device failure.
Other technologies use components that can contaminate the process when the device fails. For example, a magnetic flowmeters liner and electrodes provide an entry point for contaminants; and the sensing element of a vortex flowmeter typically contains a fluid that can leak into the process.
Cleanability and Drainability
Most applications in pharmaceutical manufacturing require a high level of cleanability. Cleanliness in a pharmaceutical manufacturing plant is an indicator of product quality. Plants must control product mingling between lots and the growth of foreign organisms in the system. Where systems are cleaned-in-place, drainability becomes a required design consideration. Cleanability best practices rule out many popular measurement options (Vortex, dP, positive displacement) because they can trap materials in the crevices of the meter itself.
Coriolis meters are well suited to pharmaceutical manufacturing because the flow tubes contain all-welded metal surfaces. There are no crevices within the device or synthetic materials to absorb process chemicals.
Some bent-tube flowmeters drain well if they are oriented carefully; others do not drain well no matter how they are installed. Where drainability is critical and the higher accuracy of a bent tube meter is required, the meter should be selected with the help of the manufacturer. System drainability and/or meter performance can be compromised depending upon the orientation of the tube bends. For instance, low points can trap liquids and solids, while high points can trap gases.
Straight-tube Coriolis flowmeters drain well with proper installation. They can be oriented in virtually any direction. When straight-tube Coriolis flowmeters are installed in vertical process piping, the meter is fully drainable; flow should also be up through a vertically installed meter. If straight-tube Coriolis meters are installed in horizontal process piping, they must be sized and installed carefully. The ASME BPE-2005 Bioprocessing Equipment Standard, Part SD-3.12.1, gives guidance for ensuring full system drainability in horizontal piping and in-line measurement equipment, such as straight-tube Coriolis flowmeters.
Accuracy and Pressure Drop
In some applications, accuracy and repeatability are paramount to success. In others, accuracy is not as critical, but unrestricted flow is important to process efficiency. The accuracy of a straight tube Coriolis meter is determined by the material of construction and the sensitivity of the components used to measure time delay. At any flow rate, the sensitivity (accuracy) decreases as the sensor tubes inside diameter increases.