GUIDANCE NOTE

Installation and Set-up of Coriolis Mass Flow Meters

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Introduction

Coriolis flow meters have various applications and are used in many different industries such as oil & gas, food & beverage, chemicals, and pharmaceuticals amongst others. In industry, mass flow measurement is often preferred to volume flow due to mass being independent of the process conditions (density, pressure and temperature). One of the main advantages of Coriolis flow meters are that they provide a direct measurement of the mass flow and product density, with no compensation for changing temperature, viscosity, and pressure required.

Another key benefit of Coriolis flow meters is that they are claimed to be unaffected by installation effects. However, Coriolis flow meters still require compliance with the manufacturer’s instructions to ensure optimum performance. These instructions can vary depending on the flow meter design and also the flow meter manufacturer.

Operating Principle

The measuring principle for Coriolis flow meters is based on the controlled generation of Coriolis forces. These forces are present when both translational and rotational movement occurs simultaneously. The amplitude of the Coriolis force depends on the moving mass, its velocity in the system, and thus on the mass flow.

When fluid travels through a Coriolis flow meter, Coriolis forces are exerted by the moving fluid which causes the vibrating flow tube to twist (Figure 1). Sensors are used to measure the magnitude of twist, which increases with increasing mass flow.

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ter Pockets

Figure 1: Coriolis Meter Operational Principle

The density measurement is derived from the natural frequency of oscillation of the flow tube, which varies with mass. A change in the mass results in a change in the frequency. As the volume of the flow tube is constant, the oscillation frequency is a function of fluid density.

Coriolis Flow Meter Designs

There are three main Coriolis flow tube designs available: Split Tube (Figure 2), Continuous Tube (Figure 3) and Straight Tube (Figure 4). The installation of the Coriolis flow meter will depend upon which design is selected by the end user for the specific application.

Figure 2: Split U-Tube Coriolis Sensor

Figure 3: Continuous Tube Coriolis Sensor

Figure 4: Straight Tube Coriolis Sensor

Split U-Tube and continuous Coriolis flow meters can be quite large and cumbersome for line sizes above 50 mm. They require adequate installation space due to the arrangement of the flow tubes. If space is at a premium, then a straight tube Coriolis flow meter may be a more suitable selection as it a less obtrusive and more compact design.

Installation

Most manufacturers claim that their Coriolis flow meters are not adversely affected by installation effects. As such, they should not require large lengths of straight length pipe work upstream and/or downstream of the flow meter. However, compliance with the manufacturer’s guidance is necessary. This guidance can vary depending on flow tube design and Coriolis flow meter manufacturer.

Installation and Set-up of Coriolis Mass Flow Meters

August 2010

Coriolis flow meters are not usually affected by swirling fluid or non-uniform velocity profiles resulting from upstream or downstream piping configurations. However, good practice should be observed at all times, by ensuring that the flow meter is installed with adequate upstream and downstream straight pipe work.

Although not overly affected by installation effects, Coriolis meters should not be installed close to valves that will regularly open and close as this can cause pulsations and vibrations in the flow. This has the potential to lead to significant mismeasurement of the mass flow.

Coriolis flow meters can be adversely affected by stress applied from the adjoining pipe work, and by vibrations acting on the flow meter. As such, manufacturers specify that their flow meter must be fixed firmly to the pipeline. To achieve this, the adjoining pipe work should be clamped securely both upstream and downstream of the flow meter.

Cavitation is a serious consideration and can occur in the process line if the system pressure drops below the vapour pressure of the fluid. To prevent this from occurring, the back pressure should be kept at a sufficient level. If the fluid is highly viscous or if the meter is smaller than the line size, the pressure drop across the Coriolis flow meter may be an issue.

The fluid pressure drop through the Coriolis flow meter is extremely important and is often the key parameter considered when sizing the device. To avoid a large pressure drop, some users select a Coriolis flow meter that has a larger bore than the line size. However, oversized meters are not only more expensive, they are also less accurate. This is because the flow rates will be at the lower end of the flow meter’s operating range, resulting in a higher flow measurement uncertainty.

Direct sunlight can adversely affect the flow meter’s measurements due to the effect the temperature can have on the flow tubes. An elevated temperature heats the flow tube and thus the stiffness of the tube changes due to the Young’s modulus. As such, most Coriolis manufacturers specify that their flow meter should be installed in a shady location, avoiding direct sunlight.

Orientation

Unlike some other devices, Coriolis flow meters can be installed both horizontally and vertically. This is extremely beneficial if the flow meter is being installed in a process line where space is at a premium.

Highly viscous fluids such as food stuffs, paints and viscous oil, readily entrain gas bubbles, which can potentially cause the Coriolis flow meter to misread the flow or even stop operating completely. If the prospect of air entrainment exists, to potentially avoid the meter from malfunctioning, the Coriolis flow meter should be mounted with the measuring tube directed downwards (Figure 5).

Figure 5: Coriolis Flow Meter Downwards Mounted Micro Motion Coriolis Flow Meter

(Ref: http://www.emersonprocess.com/micromotion/tutor/images/CMF300.gif)

If solids are present in the flow, the flow meter should be installed with the measuring tube directed upwards. This ensures that the solids do not accumulate in the flow tubes and cause the flow meter to misread (Figure 6).

Figure 6: Coriolis Flow Meter Upwards Mounted Micro Motion Coriolis Flow Meter

(Ref: http://www2.emersonprocess.com/SiteCollectionImages/News%2Images/micromotion_elitehc3_hires.jpg)

Set-up

All Coriolis flow meters are affected by the fluid in use and also the installation. It is good practice to ‘zero’ the flow meter upon installation. However, for some flow meter designs, it is specified that the zero is checked prior to installation. This is to confirm whether the flow meter actually requires zeroing. The zeroing process establishes the base conditions at which the flow meter will indicate that no flow is present.

When zeroing the flow meter, the fluid conditions should be as close to the process operating conditions as possible. The process line must be full and the flow should be completely stopped. It is important that the flow meters ‘low flow cut-off is disabled and set to zero. The Coriolis flow meter will then establish a zero flow condition from which the flow meter can operate. Most modern Coriolis flow meters will indicate when they have established a zero.

Outputs

The electronic outputs from a Coriolis flow meter can either be current, pulse or as part of a communications package such as PROFIBUS, MODBUS or FIELDBUS. The current and pulse outputs can be either active or passive and can provide the mass flow or the density reading from the flow meter. The communications packages can be used for both the mass flow and density measurements and also additional flow meter measurements and diagnostics.

Effects of Liquid/Gas Flow

The main impediment to accurate measurement under two-phase flow conditions using a Coriolis meter is a dramatic rise in the flow tube damping. Mechanical energy is lost in the interactions between compressible bubbles, fluid and flow tube walls, and the drive energy required to maintain oscillation increases. Not only does the damping rise, but it varies rapidly due to the chaotic nature of the phase distribution. Similarly, the frequency and amplitude of oscillation exhibit much greater variation than for single phase flow [1].

Traditional Coriolis metering systems were unable to supply high enough drive current (due to intrinsic safety requirements) and drive gain to maintain the tube oscillation under two-phase conditions. They were also unable to respond fast enough to rapid changes in the natural frequency of resonance causing the sensor to stall and the transmitter to go into fault.

Research is continuing into the operation of Coriolis flowmeters under two-phase conditions and it is expected that these devices will continue to improve in terms of reliability and accuracy. Due to the inherent complexity of meter operation under two-phase conditions most major manufacturers are mainly concentrating their efforts into the area of entrained gas rather than the wider and more complex full range of two-phase flows.

Recent testing at NEL shows that there have been dramatic improvements in Coriolis electronics. They are now able to respond fast enough to rapid changes in the natural frequency

and will give values for mass flow rates in the presence of gas. However, the errors in mass flow rate can be as high as 40% which is significant when the expected uncertainty for a Coriolis mass flow meter is ±0.15%.

Note: In addition to measuring mass flow rate and density, Coriolis meter software can also record a number of other parameters such as tube frequency and drive gain. Using these additional measurements, particularly the drive gain, can help the end-user detect the presence of gas in liquid streams.

References

1 Henry M, Duta M, Tombs M, Yeung H, Mattar W, “How a Coriolis Mass Flowmeter Can Operate in Two-Phase (Gas/Liquid) Flow” ISA EXPO 2004 Technical Conference

Further Reading

BS ISO 10790:1999, Measurement of fluid flow in closed conduits — Guidance to the selection, installation and use of Coriolis meters. London, BSI

Ross A. and Harvey R, The performance of Coriolis meters in two-phase liquid/gas flows. North Sea Flow Measurement Workshop, 2011

The purpose of this Guidance Note is to provide, in condensed form, information on measurement methods and technologies. It was produced as part of the UK Government’s National Measurement System.

For further information, contact:

TUV SUD NEL Ltd, East Kilbride, GLASGOW, G75 0QF, UK

Tel: + 44 (0) 1355 220222 Email: info@tuvnel.com

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© TUV SUD NEL Ltd 2011This publication is to provide outline information only which (unless agreed by TUV SUD NEL Ltd in writing) may not be reproduced for any purpose or form part of any order or contract or be regarded as representation relating to products or services concerned.

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