Coriolis meter density errors induced by ambient air and fluid temperature differentials

https://doi.org/10.1016/j.flowmeasinst.2020.101754Get rights and content

Highlights

  • Temperature differentials will produce errors in existing compensation algorithms used by Coriolis flow meter manufacturers.

  • Extent of error is also dependant on fluid properties present within the device.

  • Results obtained at the United Kingdom's national standard for fluid flow and density measurement.

Abstract

Coriolis metering technology is widely applied throughout industry. In addition to the mass flow rate, a Coriolis meter can measure fluid density based on the resonant frequency of the flow tube vibration. There is currently increasing interest in utilising this density measurement capability as the primary process value in applications such as precision control for fluid property conditioning, and fluid contamination monitoring.

However, within these applications, ambient temperature variation can be significant.

This paper details research data obtained using NEL's ‘Very Low Flow’ single-phase facility. The rig was modified to include a programmable temperature enclosure in which a Coriolis meter was installed. Two commercial meter models from the same manufacturer were tested. Both meters showed fluid density errors when subjected to fluctuations in the surrounding ambient air temperature. The fluid properties of the test medium were confirmed to be stable using NEL's UKAS standard reference instrumentation.

Previous temperature effects research for Coriolis meters have focussed on the process fluid temperature and there is little published data on the effects of ambient temperature.

Introduction

The continued development of Coriolis flow metering technology (Fig. 1) has been well documented and summarised in Ref. [[1], [2], [3]]. During the evolution of this technology, a largely consistent device design has emerged. While manufacturer and application-specific variations exist, the common design principle entails a single or dual flow tube, which is manufactured in either a straight or curved configuration. The flow tube is mechanically driven to oscillate at its natural frequency. Displacement (or more usually velocity) sensors located upstream and downstream of the centre of the flow tube are used to determine the extent of Coriolis force exerting twist. The time delay measured by these sensors is proportional to the mass flow rate passing through the meter. If no mass flow is present there will be no Coriolis force, and therefore no time delay is detected between the upstream and downstream sensors.

As a secondary output, a Coriolis meter is also capable of determining the density of the fluid present within the vibrating pipe sections. This process value is determined from the resonant frequency of the flow tube and is defined in Ref. [4] asfrf=(1/2π)·(C/m)1/2m=mtb+mfmf=(ρf)·(Vf)Where

  • -

    frfis the resonant frequency

  • -

    Cis the mechanical stiffness/spring constant

  • -

    m is the total mass

  • -

    mtb is the mass of the oscillating flow tube

  • -

    mfis the mass of fluid within the oscillating flow tube

  • -

    Vfis the volume of fluid within the oscillating flow tube

  • -

    ρfis the density of the fluid

To calculate the density of the fluid within the flow tube, the following equation can be derived from equations (1), (2), (3):ρf={C[Vf(2πfrf)2]}mtb/mf

The phenomenon of ambient air temperature affecting the quality of Coriolis meter measurements has been noted in earlier research. In Ref. [5], an examination of ‘zero drift’ highlighted ambient temperature variation as a contributing factor. In Ref. [6], where the suitability of Coriolis technology was assessed for a specific industrial application, it was again observed that ambient air fluctuations, which were intentionally introduced into the system by the research team, caused a detectable drift in the meter mass flow rate.

It should be noted that [5,6] do not address the effects of ambient air temperature on the fluid density output from Coriolis meters. It is this gap in knowledge that this research intends to address.

The diagnostic capabilities of Coriolis transmitters which are responsible for analogue signal interpretation, digitisation and process value correction have been discussed previously [7]. Significant research has also been conducted with respect to developing the capabilities of the transmitter. In particular, the research described in Ref. [8] developed a self-validating sensor, capable of fault detection and data correction to ensure measurement quality is upheld.

An initial investigation into the effects of air temperature on the density measurement from a Coriolis flow meter is reported in Ref. [9].

The results in Ref. [9] were presented to a Coriolis manufacturer and a partnership was formed, the research objective being to develop a new temperature correction model that would significantly reduce errors and which could readily be implemented in a conventional commercial transmitter [10,11].

Correct measurement and interpretation of data output from flow metering technologies is key to production forecasting, custody transfer and fiscal metering [[12], [13], [14], [15], [16]].

Section snippets

Test design

To ensure fine control over all variables, NEL's Very Low Flow Facility (VLFF) was used. The 8 mm pipe bore supports good temperature control on a minimised mass of fluid, compared to NEL's larger flow loops. The VLFF is housed in a small (4 m × 3 m x 2 m) laboratory, reducing the potential for uncontrolled ambient temperature fluctuation. Details of the VLFF and the test matrix are described in sections 2.1 Facility layout and equipment, 2.2 Test procedure.

Reference measurements and data analysis

Fig. 3, Fig. 4 show the test meter's ambient air temperature changes as measured by the enclosure thermocouple (Fig. 2) for tests 1 and 2 respectively. The reference fluid temperature as measured by the test section central PRT (Fig. 2) is also trended. The reference fluid temperature increased by 1 °C and 0.8 °C for water and kerosene respectively.

Fig. 5, Fig. 6 show the corresponding room temperatures. The intention was to maintain the room temperature to within ±1 °C of its initial value.

Meter B results

The experiments of section 3 were repeated using a different meter model from the same manufacturer. In addition, this meter was exposed to extreme changes in ambient air temperature. This was designed to be representative of conditions that may be encountered in the field, such as sudden increases and decreases in air temperature due to sunlight. The results of these experiments are presented and discussed below.

Discussion and conclusions

The results presented here have demonstrated the potential for error in Coriolis meter calculated density. The errors were induced by ambient air temperature changes. Specifically, as the differential between the flowing fluid and test meter ambient air temperature increased, the error in both the uncompensated and compensated fluid density was shown to increase. These results combined with our preceeding research [9] indicate that limitations exist within the temperature compensation models

CRediT authorship contribution statement

Gordon Lindsay: Conceptualization, Methodology, Software, Resources, Investigation, Validation, Formal analysis, Data curation, Writing - original draft, Visualization, Project administration, Funding acquisition. Norman Glen: Conceptualization, Validation, Resources, Writing - review & editing, Supervision. John Hay: Conceptualization, Validation, Resources, Writing - review & editing. Seyed Shariatipour: Conceptualization, Validation, Resources, Writing - review & editing, Supervision. Manus

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was funded by the Department of Business, Energy and Industrial Strategy (BEIS). Project number - FPRE05.

References (19)

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