Densitometers Calibration Sorted? But what about in Service? Dr Norman F Glen NEL

Agenda Introduction Background Technical approach Results The way forward

Background

Background Most liquid measurement systems in the North Sea still make use of volume flowmeters Allocation is by mass in shared transportation systems fluid density is required to convert from volume to mass Most common device is vibrating tube densitometer traceable calibration required

Background Identification of offset errors in densitometer constants arising from calibration procedures Working group reviewed issues and recommended research into practical methods for the in-situ calibration of densitometers greater understanding of the effects on densitometer performance of variations in product density, pressure and temperature JIP formed to address these issues

Project Objective and Approach Key objective - to provide the oil and gas industry with confidence in densitometer performance by developing a calibration procedure which links traceably to National Standards. Approach - detailed characterisation of two densitometers in wellcontrolled conditions, making use of four accuratelycharacterised transfer standard fluids which cover the density and viscosity ranges which are encountered across the full temperature and pressure range of operation of UK offshore oil facilities

JIP Members

Technical Approach

Densitometer Characterisation Calibration Characterisation Calibration simply using defined procedures + transfer standards to determine correction terms Characterisation examination of device characteristics (using transfer standards) development of working equations and calibration procedures

Densitometer Calibration Use fluids of accurately known density fluids must cover full operational density range of device fluids must also cover full range of other relevant properties (viscosity, speed of sound etc) fluid properties must be known across full temperature and pressure range of device fluids are therefore acting as transfer standards

Condition Survey Parameter Full range Most common range Temperature / C 8-105 20-80 Pressure / bar 5-140 5-50 Density / kgm -3 530-1180 700-850 Viscosity / cp 0.3-40 2-8

Low Viscosity High Transfer Standard Fluids Fluids must be of defined composition X X be readily available cover the full range of other fluid properties X Low Density X High

NEL Primary Liquid Densitometer Single-sinker magnetic suspension densitometer Archimedes buoyancy principle m s m * s V U = 0.015% (at k = 2) across the temperature range from -40 to 150 C at pressures up to 300 bar s

100 ( exp - calc) / calc / % Comparison with Literature Data - 1 0.025 0.020 NIST - 2008 eqn PTB - 2002 eqn 0.015 0.010 0.005 0.000-0.005-0.010-0.015 780 800 820 840 860 880 900 Density / kgm -3

Industrial Densitometer Calibration Results

Densitometer Calibration Facility Thermostated enclosure with fluid pressurisation

100( - ref )/ ref / % Densitometer No. 354583 Deviations All Fluids 0.25 0.20 0.15 0.10 0.05 20 C toluene iso-octane sebacate iso-butanol 100 C toluene iso-octane sebacate iso-butanol 0.00-0.05-0.10-0.15-0.20-0.25 0 20 40 60 80 100 120 Pressure / bar

100( - ref )/ ref / % Densitometer No. 355118 Deviations All Fluids 0.80 0.70 0.60 0.50 0.40 20 C toluene iso-octane sebacate iso-butanol 100 C toluene iso-octane sebacate iso-butanol 0.30 0.20 0.10 0.00-0.10 0 20 40 60 80 100 120 Pressure / bar

Oscillatory Densitometer Theory

Basic Densitometer Theory - 1 Model as simple spring mass oscillator f 1 2 K M If K, M 1 and V (the volume of fluid in the test section) are constant then K 0 K 2 where K 0 = -M 1 / V and K 2 = K / 4 2 V 2

Basic Densitometer Theory - 2 Modify basic equation to try to include second-order effects temperature pressure speed of sound fluid viscosity flow rate 0 K0 K1 K2 2 T 0 1 K18 T T0 K19 T T0 2 T, P T 1 K20A p p0 K20B p p0 K21A p p0 K21B p p0 2

The Way Forward

Transfer Standard Fluids Equations now available for four transfer standard fluids toluene iso-octane di-(2ethylhexyl) sebacate iso-butanol covering temperature range from 20 to 100 C and pressure range from 1 to 300 bar with expanded uncertainties of the order of 0.01%

Transfer Standard Fluids If used in a suitable facility with temperature control of the order of 0.05 C and pressure control of the order of 0.1 bar, this will give δρ 0.015% In view of cross-coupling between τ, ΔT and ΔP, calibration should ideally be carried out over the full operational range of the densitometer Existing form of equation can still be used but consideration should be given to a simpler form with less cross-coupling

Transfer Standard Fluids Fluids chosen are all readily available at required purity but consideration should be given to periodic checks against Primary Standard Liquid Densitometer Could also consider checking of transfer standard densitometers against Industrial Densitometer Calibration Facility

JIP Recommendations 1. Densitometers should be calibrated at their anticipated operating conditions, i.e. simultaneously at temperature and pressure, using one or more transfer fluids, the density of which has been determined across the required temperature and pressure range with an uncertainty not exceeding 0.01%, directly traceable to national standards. Interpolation routines or ρ, p, t models used to calculate transfer standard fluid density at calibration conditions must produced a calculated fluid density with a combined uncertainty (arising from the experimental data for the transfer standard fluid and the fitting routine) not exceeding 0.015%.

JIP Recommendations 2. The calibration facility should be capable of maintaining the temperature of the transfer standard fluid in the densitometer to 0.02 ºC and measuring it to an uncertainty not exceeding 0.05ºC The calibration facility must be capable of maintaining the pressure of the transfer standard fluid in the densitometer to 0.05 bar and measuring it with an uncertainty not exceeding 0.10 bar. 3. The current form of the equation used to calculate density from densitometer period may still be used providing optimised values of the coefficients K 18, K 19, K 20A, K 20B, K 21A and K 21B have been determined in a calibration laboratory that meets the requirements specified above.

DECC Position - 1 On 1 st March 2010, DECC issued a statement that, based on the JIP findings It is therefore proposed that from July 2011, densitometers used in fiscal applications should be calibrated following the procedures set out the JIP report.

DECC Position - 2 On 30 th September 2010, DECC issued a further statement DECC has been in discussion with Mobrey Measurement regarding the full implementation of the JIP s recommendations. 3 stages in the implantation of the JIP recommendations are envisaged: Immediate (using existing calibration facility) Interim (using new calibration facility) Long-Term (using new calibration facility)

DECC Position - 2 Immediate (using existing calibration facility) A generic constant, which is capable of being applied within the existing format of equations and which takes some account of densitometer performance at elevated pressure and temperature Interim (using new calibration facility) The existing format of densitometer equations will be maintained, so that no software modifications will be necessary. For each application, a temperature range will be cited by the Operator (within 5 C), which together with one of the pressure ranges will allow a K21A figure to be calculated for each densitometer unit. Long-Term (using new calibration facility) A new calibration facility, to be available from mid-2011, will be capable of producing a single set of constants, applicable across the full operating range of each densitometer, taking account of the combined temperature and pressure response of the individual densitometer unit. The implementation of the calibration results will require software modifications to existing flowcomputer systems.

Single Fluid Recalibration

100( - ref )/ ref / % Densitometer No. 355118 Deviations All Fluids 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 20 C toluene iso-octane sebacate iso-butanol 100 C toluene iso-octane sebacate iso-butanol 0.25 0.20 0.15 0.10 0.05 0.00-0.05-0.10-0.15-0.20-0.25 0 20 40 60 80 100 120 Pressure / bar

100( - ref )/ ref / % Densitometer No. 354583 Deviations All Fluids 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00-0.05 20 C toluene iso-octane sebacate iso-butanol 100 C toluene iso-octane sebacate iso-butanol -0.10-0.15-0.20-0.25 0 20 40 60 80 100 120 Pressure / bar

Single-fluid Recalibration

Effect on Flow Rate Uncertainty Source Units Value Expanded Relative Divisor Standard Sensitivity Output Uncertainty Uncertainty U Uncertainty U* Uncertainty u Coefficient c Uncertainty u.c Squared (u.c) 2 (%) kg/m 3 Volumetric flowrate m 3 /s 0.010 0.00003 0.250 2.00 0.00001 850.0 1.063E-02 1.129E-04 Density kg/m 3 850 0.42500 0.050 2.00 0.21250 0.0100 2.125E-03 4.516E-06 Overall Uncertainty m 3 /s 8.500 2.167E-02 0.255 2.00 1.084E-02 1 1.084E-02 1.174E-04 Uncertainty Source Units Value Expanded Relative Divisor Standard Sensitivity Output Uncertainty Uncertainty U Uncertainty U* Uncertainty u Coefficient c Uncertainty u.c Squared (u.c) 2 (%) kg/m 3 Volumetric flowrate m 3 /s 0.010 0.00003 0.250 2.00 0.00001 850.0 1.063E-02 1.129E-04 Density kg/m 3 850 0.21250 0.025 2.00 0.10625 0.0100 1.063E-03 1.129E-06 Overall Uncertainty m 3 /s 8.500 2.136E-02 0.251 2.00 1.068E-02 1 1.068E-02 1.140E-04 Uncertainty Source Units Value Expanded Relative Divisor Standard Sensitivity Output Uncertainty Uncertainty U Uncertainty U* Uncertainty u Coefficient c Uncertainty u.c Squared (u.c) 2 (%) kg/m 3 Volumetric flowrate m 3 /s 0.010 0.00003 0.250 2.00 0.00001 850.0 1.063E-02 1.129E-04 Density kg/m 3 850 0.85000 0.100 2.00 0.42500 0.0100 4.250E-03 1.806E-05 Overall Uncertainty m 3 /s 8.500 2.289E-02 0.269 2.00 1.144E-02 1 1.144E-02 1.310E-04

Conclusions Initial (factory) calibration of densitometer requires use of three fluids basic form of equation has three unknowns (K 0, K 1 and K 2 ) Single-fluid recalibration at a range of conditions (T nom 10 C, p nom 10 bar) will be sufficient for most densitometers Existing form of equation can still be used optimise K 21A and K 18 No reprogramming of flow computers required for majority of installations

Uncertainty in Use

Overall Uncertainty ESTIMATE OF UNCERTAINTY IN ULTRASONIC FLOWMETER Source Value Units U U *(%) k u c u.c (u.c ) 2 Rank Calibration Uncertainty 421.30 m 3 /hr 1.264 0.300 2 0.632 1 0.632 0.399 3 Repeatability 421.30 m 3 /hr 0.843 0.200 1.732 0.486 1 0.486 0.237 4 Drift between calibrations 421.30 m 3 /hr 2.107 0.500 1.732 1.216 1 1.216 1.479 1 Installation effects 421.30 m 3 /hr 2.107 0.500 1.732 1.216 1 1.216 1.479 1 Flow Computer 421.30 m 3 /hr 0.421 0.100 1.732 0.243 1 0.243 0.059 5 USM Field Conditions 421.30 m 3 /hr Temperature correction 421.30 m 3 /hr Pressure correction 421.30 m 3 /hr Signal computation 421.30 m 3 /hr Combined Uncertainty 421.30 m 3 /hr 3.823 0.907 2 1.911 1 1.911 3.653

Overall Uncertainty ESTIMATE OF UNCERTAINTY IN LIQUID DENSITOMETER Source Value Units U U *(%) k u c u.c (u.c ) 2 Rank Calibration Uncertainty 800.00 kg/m 3 0.400 0.050 2 0.200 1 0.200 0.040 3 Repeatability 800.00 kg/m 3 0.080 0.010 1.732 0.046 1 0.046 0.002 4 Drift between calibrations 800.00 kg/m 3 0.400 0.050 1.732 0.231 1 0.231 0.053 2 Installation effects 800.00 kg/m 3 1.600 0.200 1.732 0.924 1 0.924 0.853 1 Temperature correction 800.00 kg/m 3 Pressure correction 800.00 kg/m 3 Signal computation 800.00 kg/m 3 Combined Uncertainty 800.00 kg/m 3 1.948 0.244 2 0.974 1 0.974 0.949

Installation Effects

What Next? Implement revised calibration procedures Quantify installation effects

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