REVIEW OF LPG FLOW MEASUREMENT TECHNOLOGIES AND MEASUREMENT ISSUES

Transcription

1 Executive Summary REVIEW OF LPG FLOW MEASUREMENT TECHNOLOGIES AND MEASUREMENT ISSUES A Report for National Measurement System Programme Unit Department of Trade & Industry 151 Buckingham Palace Road London SW1W 9SS Project No: FFRE31 Issue 1 Date: November 2006

2 The work described in this report was carried out under contract to the Department of Trade & Industry ( the Department ) as part of the National Measurement System s Flow Programme. The Department has a free licence to copy, circulate and use the contents of this report within any United Kingdom Government Department, and to issue or copy the contents of the report to a supplier or potential supplier to the United Kingdom Government for a contract for the services of the Crown. For all other use, the prior written consent of NEL shall be obtained before reproducing all or any part of this report. Applications for permission to publish should be made to: Contracts Manager NEL Scottish Enterprise Technology Park East Kilbride G75 0QF jduff@nel.uk Tel: +44 (0) NEL 2006

3 NEL East Kilbride Glasgow G75 0QF Tel: Fax: REVIEW OF LPG FLOW MEASUREMENT TECHNOLOGIES AND MEASUREMENT ISSUES A Report for National Measurement System Programme Unit Department of Trade & Industry 151 Buckingham Palace Road London SW1W 9SS Prepared by: Richard Paton Approved by: Jane Sattary Date: November 2006 for Mr M Valente Director Project No: FFRE31 Page 1 of 22 November 2006

4 SUMMARY Liquefied Petroleum Gas (LPG) refers to the light hydrocarbon products propane and butane, and to commercial blends consisting of mixtures of the two. LPG is used as an automotive fuel (autogas), domestic and commercial heating fuel, chemical feedstock, refrigerant, and aerosol propellant. LPG is stored and traded in liquid form either refrigerated or at elevated pressure. The positive displacement meter is the most commonly utilised method for measuring LPG when loading and discharging road tankers. It is also almost exclusively used in fuel dispensers for filling vehicles. Although it is known that these meters give problems with wear and reliability, they are seen as the mainstay for measurement for some time to come. Coriolis meters are being introduced for loading systems and also on larger trucks as delivery meters. The relatively high cost of the meter has so far prevented it from entering the dispenser market to any significant extent. LPG in the industrial and bulk transfer market is being measured in a number of ways but in general the current measurement and accounting practices are perceived as satisfactory. Currently tank level gauging is the common method of accounting for large (ship) bulk deliveries. It is however clear that the introduction of more direct dynamic metering would add efficiencies to the measurement process and allow the verification of transfers to be carried out between tank gauging and ship loading figures, with the flowmeter providing a potentially more accurate measure of the transaction. Both Coriolis meters and ultrasonic meters could be considered for these large bulk transfers. Ultrasonic meters would only become attractive for pipelines above 8 inch while Coriolis meters best meet the desire to provide a mass flow measurement. Both technologies can be used for pressurised and refrigerated transfers. Small volume pipe provers are used for flowmeter calibration in refineries and large facilities where metering is routinely employed for fiscal or custody transfer metering. With dedicated densitometers they provide both volume and mass based references. The provision of calibration facilities is the obvious weakness in the traceability chain. For dispenser calibration, UK practice is to use Coriolis reference meters calibrated in water. This practice can also be applied to truck meters although these are more often calibrated or verified by filling a truck and weighing. The use of volumetric proving tanks for LPG calibration is common in the US but although used in Australia is reported as being unreliable for routine work and reference PD meters are being employed. NWML is currently testing an LPG proving tank for use in the UK and as a primary standard. Comparison with direct weighing remains a recognised method for calibration in the field but requires care in the implementation. Weighing does however provide a common verification route for flowmeters. Weighing is also used for trading and deliveries of LPG. Conversion from mass to volume and calculation of standard volume require accurate knowledge of the properties of LPG mixtures. Measurement difficulties with LPG arise in understanding and controlling the fluid handling process. The fluid is stored at saturation conditions and therefore control when transferring fluid is vital. Project No: FFRE31 Page 2 of 22 November 2006

5 Pressure drop can cause flashing and give rise to two phase conditions. Similarly heat input due to, for example solar radiation, can also cause bubble generation. Pressure changes due to filling and emptying storage tanks can require the use of vent and pressurisation lines. Flow through these has to be accounted for. The traded quantity quoted for transactions varies depending on the application. In the UK actual volume is used for retail deliveries, which is only acceptable due to the current low price of the product. From a measurement perspective actual volume gives rise to significant over or under reporting of the mass depending on the fluid temperature. Accounting using standard temperature should be encouraged. For larger deliveries, standard volume, mass or calculated weight in air are used. There is little information available on the expected accuracy of mechanical and lower accuracy compensation devices, and more sophisticated corrections will depend on the accuracy of the input information on temperature and composition. Project No: FFRE31 Page 3 of 22 November 2006

6 CONTENTS Page SUMMARY 2 1 INTRODUCTION LIQUEFIED PETROLEUM GAS 2.1 Introduction Overview of Applications Properties LPG Storage and Transportation Overview of Quantity Measures FLOW MEASUREMENT 3.1 Introduction Mass Measurement and Flowmeters Volumetric Flowmeters CALIBRATION 4.1 Introduction Volumetric Proving Tasks Pipe Provers Reference Meters 14 5 OTHER MEASUREMENT ISSUES 5.1 Density Vapour Return and Balancing Lines UNCERTAINTY EXPECTATIONS STANDARDS AND GUIDANCE DOCUMENTS 18 APPENDIX 1: REPORTING QUANTITIES 20 Project No: FFRE31 Page 4 of 22 November 2006

7 1 Introduction This report provides a state-of-the-art review of the current position and technologies used for the measurement, accounting and metering of Liquefied Petroleum Gas (LPG). Some of the issues and technology will also cover NGL (Natural Gas Liquids). This review complements similar reviews covering LNG (Liquefied Natural Gas) [1] and CNG (Compressed Natural gas) [2]. 2 Liquefied Petroleum Gas 2.1 Introduction Liquefied Petroleum Gas (LPG) generally refers to the light hydrocarbon products propane and butane, and to commercial blends consisting of mixtures of these two products. Any such product will also contain trace components of other light hydrocarbons. LPG is stored and transported as a liquid at pressure or under refrigeration. The liquid is returned to gas for use as a fuel normally by reducing the pressure. Storage in liquid form provides an increase in energy density of a factor of 250 compared to gas. Other refined products traded as LPG in the industrial field also include propylene and butylenes. Although not generally classed as LPG, many of the measurement and process control issues are common to the measurement of pentanes, ethane and derivatives and the basic raw feedstock classified as condensate or Natural Gas Liquids (NGL). These are effectively mixtures of hydrocarbons with variable composition varying between ethane and pentane. Table 1 shows information provided by the DTI on LPG usage in the UK. In summary, overall usage of LPG (Propane and Butane) within UK has shown a small rise between 2004 and Around 3 million Tonnes of LPG was used in UK in 2005 of which about 1 million Tonnes was transferred by export or import. Ethane Propane Butane LPG Propane + Butane Supply Production 5 1, ,462 Other sources 1, ,358 Imports Exports ,298 Marine bunkers Stock change Transfers Total supply 1,402 2, ,323 Statistical difference (3) Total demand 1,459 2,521 1,071 3,592 Table 1: Extract from DTI Commodity Balance 2005 (DTI: Project No: FFRE31 Page 5 of 22 November 2006

8 On the assumption of an end user price of some 30p a litre, LPG will conservatively have a value in UK of over 1,000 Million with a significant proportion of this going to the Treasury. 2.2 Overview of Applications LPG is supplied for use as a vehicle fuel where it has advantages of clean burning with lower emissions in comparison with petrol and Diesel. In this use it is particularly suited to cars and light commercial vehicles with spark assisted engines. Currently LPG has a distinct financial advantage to the motorist in the UK where the lower level of tax applied gives a retail price of between half and two thirds the price of petrol. LPG is used as an alternative to mains (natural) gas for domestic and commercial heating and cooking purposes. This is used occasionally in specific installations where a price advantage may be derived, but more commonly where mains gas is unavailable. For this purpose LPG is used extensively in remote areas where it is available from bulk storage or from cylinders. Many chemical and plastics production processes utilise LPG as a feedstock. LPG is also used as a propellant for aerosols and as a refrigerant gas. Although natural gas (methane) cannot be replaced directly by LPG due to the differences in calorific value, Synthetic Natural Gas (SNG) is produced by adding air and inert gas to LPG to provide an acceptable Wobbe index. This allows SNG it to be accepted as a natural gas substitute particularly when blended with natural gas. LPG is also used to blend with natural gas to establish the correct calorific value and supplement supplies at times of high demand. 2.3 Properties Some properties of propane and butane are summarised in Table 2 below. Name Propane Butane Composition C 3 H 8 C 4 H 10 (N and I forms) Molecular weight Boiling point at 1 bar(a) -45 C -2 C Pressure at 0 C 4.5 bar 0.5 bar Pressure at 38 C 14.5 bar 4.8 bar Cu C / C / C Cu C / C / C Cu C / C / C Table 2: Some properties of propane and butane The cubical thermal expansion coefficient of liquid propane is relatively large. The value of /K at 20 C means that the volume changes by around 0.3% per C change in temperature. The implication of this is that a volume delivered will have 3 per cent more or less mass, and therefore energy, than that same volume delivered if the temperature changes by 10 C. Project No: FFRE31 Page 6 of 22 November 2006

9 Propane and butane may be marketed as commercially pure products or may be blended to suit the application. All commercial products will also contain additives and trace products. Commercial propane or butane are in excess of 90% pure with butane or propane and trace products of pentane making up the remainder. Butane will be present as n-butane and i-butane and the relative composition affects the density of the product. Odour in the form of an additive (usually Ethyl mercaptan) is added to aid detection of leaks of fuel gas. Other proprietary compounds and hydrocarbon blends may be added to increase the efficiency of the product especially when used as a road fuel. 2.4 LPG Storage and Transportation LPG is supplied to customers in small cylinders or via bulk delivery to a local pressure tank. Both butane and propane can be supplied to customers in bulk and in cylinders. Commercially, and at refineries, LPG is stored in large spherical or horizontal cylindrical tanks. These may be pressurised tanks at ambient temperature or refrigerated storage at ambient pressure. Transport tanks, truck or rail mounted, are normally cylindrical pressure storage systems. Ship transport tanks can be cylindrical or spherical built into specialised vessels and are normally refrigerated. In all cases the product is stored at saturation pressure and temperature, either pressurised or refrigerated. An advantage of LPG as a fuel when compared with lighter hydrocarbons (methane) is that it can be transported as a liquid at moderate pressure. The calorific value is however less than that of petrol and diesel fuel but higher than that of methane. Propane is the main (90%) component of LPG for automotive use. It is also the main fuel for domestic and commercial applications. Butane is used extensively as a fuel gas for many industrial applications and some domestic installations (e.g. camping gas). Butane has a lower saturation pressure at ambient temperatures and hence requires lighter containers than propane. LPG is stored in cylinders or pressure vessels and the pressure at ambient temperature within the container will be the saturation pressure corresponding to the temperature. The container will be partially filled with liquid with a vapour space above. Alternatively LPG can be stored and transported as a refrigerated liquid at ambient pressure. From Table 2 propane will be stored at around -45 C and butane at -2 C, which means that there is inevitably heat gain to the system. This generates a production of boil-off gas. In LPG storage this gas is captured and either compressed back to liquid for pressure storage or more commonly refrigerated and re-injected back to the storage tank. In fact the generation of boil-off gas maintains the bulk liquid at saturation. Latent heat is absorbed from the liquid as the vapour is generated, compensating for the heat transfer from the surroundings and providing a self cooling effect. In pressure storage, as the fluid is withdrawn, either as gas or liquid, evaporation will occur to maintain the vessel at the saturation pressure corresponding to the temperature in the vessel. Project No: FFRE31 Page 7 of 22 November 2006

10 The evaporation of the liquid during withdrawal of large amounts of gas will cause a temperature reduction due to the absorption of the latent heat. This reduction in temperature may be seen by condensation or even frosting on cylinders when large amounts of fluid are withdrawn. Similarly a change in the vessel temperature changes the saturation pressure and will cause LPG vapour to form or condense to maintain saturation conditions within the storage vessel. LPG containers should always be filled to leave an appropriate vapour space and must never be completely filled with liquid. 2.5 Overview of Quantity Measures When LPG is supplied in cylinders the quantity sold is mass (or weight). The LPG weight is obtained by weighing the cylinder and subtracting the weight of the empty cylinder. Weight is used and not mass as the difference is insignificant relative to the value of the product. Bulk transfers to storage tanks for domestic and commercial use are transacted in volume terms and are generally measured using an appropriate flowmeter. LPG for vehicle fuel (auto gas) is delivered to a vehicle through a dispenser similar to a petrol dispenser, and measurement is by volume. This volume may be quoted at actual conditions or at standard conditions of temperature. Delivery to the forecourt is based on standard volume. Transfers to and from bulk storage and for small pressurised deliveries (retail) are generally carried out using volume expressed at standard conditions. Large commercial bulk transfers may be carried out using standard volume, mass (in vacuo) or weight in air. Mass measurement is used for mass balance, loss control and process efficiency. Where standard conditions are used, it is normal to correct the measured volume to the volume that would exist at 15 C or 60 F. Generally volume is expressed in m 3 (or litres) at 15 C or US Barrels (or gallons) at 60 F depending on the jurisdiction or contract. For general trade and fuel dispensing, temperature correction to standard conditions is applied. For large bulk transfers using flowmeters, correction to a standard pressure is strongly recommended in standards and guideline documents. It is conventional to use the equilibrium pressure at the prevailing temperature of the fluid as the standard pressure. This is of course variable depending on the properties of the LPG. The reporting of refrigerated bulk cargos is identical to that of the pressurised product. Further details on reporting quantities are given in Appendix 1. Project No: FFRE31 Page 8 of 22 November 2006

11 3 FLOW MEASUREMENT 3.1 Introduction The main specific issues to be considered in LPG flow measurement are: Fluid density and composition Potential for flashing due to pressure drop or heating Low liquid viscosity and hence lubricity Flow measurement of LPG is employed in autogas dispensers and during bulk deliveries from road tankers to storage facilities. On the larger scale a visit to the Shell NGL plant at Mossmorran, Fife, revealed that tank gauging is typically used to measure volumes delivered to ships at export terminals. 3.2 Mass Measurement and Flowmeters LPG is frequently traded based on measurement by weighing. Cylinders or transport containers (truck or rail) are weighed before and after the transaction and this provides the measure of quantity. There are also a number of road tanker delivery systems delivering large and small batch quantities based on weighing using a load cell system fitted to the tanker. If volume or energy is required as the delivery quantity, the conversion is carried out using a pre-determined density or calorific value. This method is reported to be providing more consistent results than the equivalent delivery systems based on volumetric truck mounted meters. Coriolis mass flowmeters have become well established as a reliable method of dynamically measuring LPG. The lack of moving parts and the primary output of mass means that Coriolis meters are particularly suited to dynamic measurement of LPG transactions. Where volume is required for the transaction, the integral measurement of density can be utilised to provide volume measurements from the Coriolis meter and additionally the internal calculations can provide this figure corrected to actual or standard conditions. Coriolis meters are used extensively used in truck loading and ship and pipeline applications. For ship loading and pipelines the limited range of sizes may require a number of meters to be connected in parallel. Modern designs are adapted to operate for refrigerated transfers. Project No: FFRE31 Page 9 of 22 November 2006

12 Figure 1: Example of an Endress and Hauser Coriolis meter on a road tanker application Coriolis meters are also used for mobile truck-mounted applications as shown in Figure 1 above. Generally the high cost of the meters makes Coriolis uneconomic for retail sales from trucks though even in this application some lower cost designs are being utilised. There is concern that truck vibration and poor pipework design when retro-fitting can cause measurement inaccuracies. Coriolis meters are still too expensive for general installation in forecourt dispensers. Only one manufacturer, Kraus-Global in Canada, was found to offer this option. However Coriolis meters are being used as reference calibration devices for meters in service including the calibration (verification) of forecourt dispensers as well as for larger flow applications. This is currently the preferred method in UK although not yet accepted in many other countries e.g. Australia and the USA. In this application they are configured to indicate volume flow using the meter s internal density measurement capability and temperature corrections from test to reference meter applied externally. Accuracy depends on maintaining a single phase liquid flow through the meter. The density measurement capability can be used to indicate and alarm the presence of gas in the flow. Project No: FFRE31 Page 10 of 22 November 2006

13 Although the presence of gas can be detected, this capability is not normally available directly to the user and so large unpredictable errors can be introduced if the liquid phase is not maintained. This can be a concern in LPG transfers close to equilibrium conditions in both pressurised and refrigerated systems where gas can flash within the meter due to localised pressure drops or temperature rises. For ship transfers, especially for refrigerated systems, the current relatively small sizes of Coriolis meter (8 inch is currently the largest commonly available meter) causes a limitation. One potential solution is to install parallel meters in a manifold, but there may be then be problems in ensuring that all parts of the manifold remain at the correct temperature to maintain liquid conditions particularly when the flowrate is low. 3.3 Volumetric Flowmeters Introduction Volumetric flowmeters of various types have traditionally been used in the measurement of LPG, and may be used in conjunction with a densitometer to provide mass measurement. Some common features which have to be addressed in all volumetric flowmeters are wearing due to the relatively poor lubricity of LPG, incorrect reading due to the presence of vapour, pressure drop causing vapour flashing, and damage due to high velocity gas passing through the meter Turbine flowmeters Turbine flowmeters are employed for bulk transfers at both pressure and refrigerated conditions. They are less commonly used in truck or rail loading applications. The turbine meter has to be correctly designed to allow for the low lubricity of the LPG, and avoid pressure drop causing local flashing of the product. The installation must be designed with adequate flow conditioning and a high enough back pressure to avoid flashing. Vapour eliminators can be used but have to be installed upstream of the flow conditioning sections Positive displacement meters Positive displacement (PD) meters are more commonly employed for loading and delivery from road and rail cars. Small positive displacement types, both rotary piston and reciprocating piston types, are used in fuel dispensers. Generally PD meters do not require flow profile conditioning and hence, though bulkier and heavier than turbine meters, they can be installed in a more restrictive space. For trade application meters are usually fitted with vapour eliminators upstream. Meters of all types may be equipped to output volume flowrate or fitted with electronic or mechanical temperature compensation to provide volume flowrate at standard conditions. Project No: FFRE31 Page 11 of 22 November 2006

14 3.3.4 Other flowmeter meter technologies Vortex meters have been employed for LPG applications and are frequently employed for NGLs. The lack of moving parts and simplicity, and the range of electronic calculation options, provide a distinct advantage. Although the accuracy is considered to be somewhat poorer than that expected from other types of meters, it is often adequate for process control and monitoring and allocation of product where fiscal standards are not required. Differential pressure meters such as orifice meters and Venturi meters are not normally used for LPG as the pressure drop can be a concern and the accuracy of other types of meter is normally superior. However the simplicity and traditional acceptance of this technology means that many installations are found in LPG process and pipeline applications. Ultrasonic meters meet the requirement for low pressure drop, but no reference has been found for extensive use in LPG metering. Suitable products are available, and it is suggested that they would work well for LPG particularly in pipeline and ship loading applications where the large throughputs would justify the expense of the meter. 4 CALIBRATION 4.1 Introduction Flowmeters used for LPG service can be calibrated using volumetric proving tanks, pipe provers, or using another reference flowmeter calibrated to a high accuracy. Calibration normally takes place in an industrial environment or at the final location of the meter. World-wide there were no laboratories identified as having primary dedicated test or calibration facilities for LPG. 4.2 Volumetric Proving Tanks Volumetric proving tanks are used extensively for the calibration of lighter hydrocarbon oils. Conventionally these tanks are shaped to provide a large body sized to contain the relevant volume of oil and a narrow neck where the volume can be measured with an adequate resolution. Normally these tanks are open to atmosphere. Specialist companies manufacture volume proving tanks for LPG. These consist of a large cylindrical body combined with a narrow top and bottom neck. Volume is measured to an adequate resolution by observing the level of liquid in the neck. Project No: FFRE31 Page 12 of 22 November 2006

15 Figure 3: Single and twin proving tanks on trailers for mobile use (Courtesy of Arrow Tank and Engineering Co) Unlike similar tanks for non-volatile liquids, an LPG prover tank must be sealed and is built as a pressure containment vessel. For propane this will require 20 bar pressure containment and all the associated certification and pressure relief precautions. The handling and control in filling and emptying the tank and ensuring pressure is maintained without loss of product provides the main challenge. Vent lines and pressure relief valves have to be shown to be leak free. Extreme care has to be taken to ensure that the vessel is carefully purged of all air before use and all vapour purged before a tank is opened. The volume of the proving tank is determined by either volumetric or gravimetric water filling. Proving tanks are not commonly used in the UK and only two or three are known to be in use. Two tanks, one at 100 litres and one at 250 litres have been identified as being owned by Kent County Council trading standards department. The 100 litre tank is currently being renovated on behalf of NWML for assessment as a UK primary standard. Tanks are the standard recommended method for calibration of LPG dispensers in the USA. Their use has however been discontinued in Australia due to measurement stability problems. In general tanks are only available in smaller sizes, from 50 to 500 litres. A larger tank would become extremely heavy due to the pressure containment requirements. 4.3 Pipe Provers Pipe provers may also be used for the calibration of larger meters. In practice ball provers have been found to be less reliable for LPG duties because the low lubricity of the fluid can cause sticking and wear on the sphere displacers. Sticking of the sphere also causes uneven movement giving rise to poor repeatability or even gas breakout. For this reason ball provers are not commonly used for LPG. Straight piston provers are constructed from a straight honed bore pipe and a piston type displacer. This design is preferred for LPG and other light hydrocarbons where the tight sealing at low friction provides reliable performance. A pipe prover of this type could operate at pressure or for refrigerated product. Project No: FFRE31 Page 13 of 22 November 2006

16 Care is needed to ensure the chosen materials of construction are correct for the duty especially if large temperature changes are going to occur. Due to the large size and cost of a conventional piston prover these are not common. Figure 5: Trailer mounted Small Volume Prover (Emerson Process Management, Brooks) A variant of the piston prover, the Small Volume Prover (SVP), has however become the calibration method of choice for larger LPG meter installations. This device can calibrate turbine meters and Coriolis meters volumetrically and if combined with a densitometer a mass calibration can also be achieved. SVPs can be mounted on trucks or trailers for mobile use or fixed as part of a metering installation. The SVP must be specified for LPG applications, since carbon steel may lose strength when exposed to low temperatures experienced during venting. A trailer mounted SVP manufactured by Emerson Process (Brooks) is shown in Figure Reference Meters The master meter calibration method has become the method of choice for many small meter applications such as truck meters and forecourt dispensers. A number of companies use standard or specially adapted PD meters as reference meters and these can be packaged as mobile units. It is unclear at this time how the traceability of these meters is established without access to an LPG primary standard. In the UK, Coriolis meters are the favoured method for calibration of forecourt meters and there are a number of bespoke meter assemblies in service with contracting companies and local authorities for this purpose. Although larger meters can be utilised for proving of truck meters and loading meters, no examples were found during the study. Coriolis meters are relatively unaffected by the transport properties of the metered fluid as long as it is maintained in the single phase state and the manufacturers advice on pressure correction is followed. For this reason Coriolis meters are often calibrated using water as the reference fluid. This is currently perceived to be adequate for the purposes of dispenser calibration, though there is little evidence in the public domain that this theory has been fully proved across a range of meter types and installations. Project No: FFRE31 Page 14 of 22 November 2006

17 4.5 Weighing methods Weighing is recognised as a common method of calibrating and verifying LPG flowmeters. This is often carried out by weighing a transport tank, truck or rail, before and after filling. The method is sound as long as all precautions are taken to accurately carry out the weighing process and avoid the measurement being affected by the environmental conditions or changes in the weight of the vehicle between weight determinations. It also has to be assured that all losses through vapour balance lines can be accounted for. The conversion between mass and volume has to be carried out accurately and the small buoyancy correction applied to the weighing. Generally weighing calibrations are carried out on an ad hoc basis using equipment installed for other purposes, e.g. general purpose weigh bridge and the normal transport vehicle, truck or rail. There is however one example identified where the truck mounted weight delivery system of load cells has been modified and upgraded to provide accuracy commensurate with that required to calibrate flowmeters on delivery systems and truck mounted meters. 5 OTHER MEASUREMENT ISSUES 5.1 Density The density of the LPG is required to allow conversion from mass flow to volume flow measurements. It is also required as an input to correction algorithms for temperature expansion. Density can also be utilised to determine the energy value through empirical relationships relating the component mixture to density. Density can be measured using an on-line densitometer. A densitometer installation must be designed to ensure that it measures liquid only and reflects the temperature and pressure at the flowmeter if the values are to be used for mass/volume conversion. For this reason the temperature and pressure at the densitometer must be known. Two novel techniques have been identified as being applied to determine density, and hence LPG blend quality for use in retail dispensers and truck meters. The first is offered by an Australian Dispenser manufacturer, LPG Measurement Technology Pty Ltd. ( This company uses permittivity measurement of the LPG in the flow to measure the density and hence indicate the blend. They estimate that density is measured to an accuracy of ±5 g/cm 3 The second technique is being utilised by Compac Industries Ltd from New Zealand ( This company is installing ultrasonic transducers to provide a measure of density. Both companies use the density measurement to select volume correction algorithms, indicate blend quality and to detect gas bubbles within the product. They have targeted their products to fuel dispensers and truck delivery systems. It is well documented that ultrasonic flowmeters can utilise the speed of sound to give a density measurement. This is not normally suitable for fiscal applications but is certainly good enough for control and indication purposes and over the limited range of LPG densities may be able to provide resolution for higher accuracy purposes. Project No: FFRE31 Page 15 of 22 November 2006

18 Normally for bulk LPG transactions, a sample is withdrawn from the delivery line, and the density established using an offline densitometer working at the reference temperature. The sample will also be used to establish the actual composition of the product hence assuring quality and energy content. The measurement is carried out using gas chromatography. The techniques for gas chromatography are well established, though care must be taken to obtain the representative sample and avoid preferential selection of the lighter components during filling or transferring the sample bottle contents. Gas chromatographs are frequently installed and used online where a continuous sample is drawn through the chromatograph during a delivery. Again great care must be taken to ensure that the sample is truly representative and does not degrade through preferential extraction of lighter components between the sample point and the gas chromatograph. 5.2 Vapour Return and Balancing Lines As a safety provision, particularly in older transfer systems, pressure balancing lines are connected between the receiving and delivery storage tanks. As a transfer takes place, vapour can pass back across the vapour line, effectively returning product in gas phase back to the supplier. This vapour return may be considered to be an acceptable loss in low value transactions as the mass is 250 times less than the same volume of liquid being delivered. It is still however a very significant quantity of product. Although modern systems retain return lines for safety reasons, they are designed to remove or reduce the need for product to flow through this line during a delivery. When a delivery is made to a tank, the liquid level rises and compresses the vapour. As a result there is the potential for a large pressure and temperature rise in the receiving tank before equilibrium between liquid and vapour is established. The pressure rise has to be relieved or equalised through the return line. This effect is minimised in modern systems by introducing the liquid into the receiving tank via a spray into the vapour space in order to rapidly establish equilibrium. In large bulk transfers, gas flow in the vapour return may be metered using a turbine meter, vortex meter, orifice plates or, increasingly, a Coriolis flowmeter. It is also possible to feed the released gas to a refrigeration compressor and introduce it back to the receiving tank, hence avoiding any return gas. In refrigerated LPG systems, there are a number of flow paths which have to be measured and the product accounted for during a transaction, especially when tank gauging rather than dynamic measurement is employed. Storage tanks will produce gas and this is either lost to the transaction by being directed to another tank or process. Alternatively the gas is captured, refrigerated and returned to the storage tank, but with the risk of interfering with measurement. Project No: FFRE31 Page 16 of 22 November 2006

19 6 UNCERTAINTY EXPECTATIONS The expected uncertainties for LPG measurements are higher than for other liquid hydrocarbon fuels. In the UK, LPG fuel dispensers for retail are not prescribed instruments and hence there is no formal legal accuracy requirement specified. This is also the case for road tanker loading and deliveries by flowmeter. Deliveries by weight may however be prescribed through regulation on weighing instruments. All measurements should however have to meet the requirements of the Weights and Measures Act for fair and equitable trade. In most other countries LPG meters are prescribed and they fall within the scope of the Measuring Instruments Directive (MID) in Europe. The MID requirements are applied to instruments prescribed within a country and supersede any existing national regulations. Within the MID the accuracy requirements for LPG dispensers follow the requirements of OIML R117: Liquids other than water. This recommendation classes LPG meters as accuracy class 1.0, which means that an LPG dispenser has to meet an accuracy of 1.0% for type (or pattern) approval and for initial and subsequent verification. An LPG meter when assessed alone has to meet an accuracy of 0.6% for pattern approval and initial verification. OIML R117 is the standard which would apply to dispensers and truck deliveries as well as truck and rail car loading. It has been indicated that, for bulk transfers, an uncertainty of 0.25% overall would be desired. However there is some evidence that this uncertainty level is found to be challenging. Project No: FFRE31 Page 17 of 22 November 2006

20 7 STANDARDS AND GUIDANCE DOCUMENTS Some published documents of particular relevance to LPG flow measurements are listed below. EI: IP: HM 47: Recommended Operational Practice for Proving LPG Meters (Formerly PMM Part X, Section 13) Date: 2003 ref: / OIML: R117 Measuring systems for liquids other than water (1995) LPGA: Technical Memorandum No 77 Selection and operation of Coriolis meters for proving LPG metering equipment by volume. NWML: Liquid Petroleum Gas: An investigation into the practical testing issues. programme reports/report lpg.pdf - Last Modified: 18/10/ CONCLUSIONS The positive displacement flowmeter is the most commonly utilised method for measuring LPG in Autogas dispensing and road tanker loading and dispensing. Coriolis meters are being introduced for loading systems and also on larger trucks as delivery meters, and are also entering the dispenser market. Larger scale bulk transfers of LPG are often measured by tank gauging. Calibration of LPG flowmeters is usually performed using water. LPG dispensers are typically tested and verified using a Coriolis meter calibrated to a high accuracy on water. This practice can also be applied to truck meters although these are more often calibrated or verified by filling a truck and weighing. Small volume pipe provers are used for calibration in refineries and large facilities where flow measurement is routinely employed for fiscal or custody transfer metering. With dedicated densitometers they provide both volume and mass based references. The absence of LPG calibration facilities is the obvious weakness in the traceability chain. NWML is currently commissioning an LPG proving tank for use in the UK, and completion of this exercise will provide a primary standard. However it is likely that many LPG flowmeters will continue to be calibrated using water, and there is therefore a need to establish the transferability of calibrations from water to LPG. Project No: FFRE31 Page 18 of 22 November 2006

21 9 ACKNOWLEDGEMENTS NEL would like to acknowledge the help and assistance given during the development of this report from numerous contributors within the industry and other NMIs throughout Europe.Particular appreciation is given to the following companies: Calor Gas Ltd John Wigfull and Co Ltd Shell Exploration and Production REFERENCES 1 Review of LNG Flow Measurement Technologies and Measurement Issues, NEL Report 2006/ Review of CNG Flow Measurement Technologies and Measurement Issues, NEL Report 2006/297. Project No: FFRE31 Page 19 of 22 November 2006

22 Appendix 1: Reporting Quantities The measurement and reporting options are summarised below. Mass: Mass is defined as being the quantity of substance and is by far the most reliable and sensible method of reporting LPG transactions. Mass can easily be used to calculate the energy content in combination with density, calorific value and component breakdown. Mass can be derived by weighing LPG in a closed container. This weighing will provide a measure of the Conventional Mass of the product. The conventional mass has to be corrected for the buoyancy of the calibration weights to give the true mass. This correction is the ratio of air density to the weight density and is equal to the constant Although the correction is small, it is required in some applications, especially high accuracy bulk transfers. Mass is therefore expressed as: M = W OR W = M (1) where M is the mass, W is the measured weight (in air) Accounting in terms of mass means that measurement and accounting for any gas transfers via boil off or pressure balancing lines can be more easily added or subtracted from the liquid transfer. Weight in air: In many shipping and large bulk contracts for LPG the quantity is traded as Weight in air. This is the apparent quantity of LPG which would balance a (conventional) mass when weighed in air on the basis that it displaces its own volume of air from its container and is therefore subject to buoyancy. This is the equivalent to the buoyancy correction applied to filling an open container to give the mass. Weight in air is a common convenient quantity to report for other heavier hydrocarbon liquids where an open tank can be filled and weighed without difficulty, and this reporting practice has been continued for LPG contracts. The relationship between the mass M and the weight W a of a fluid obtained from weighing in air is through the application of the Buoyancy correction factor B: where M = BW a (2) 1 1 B = 1 + ρ a. (3) ρl ρ weights ρ a The conventional air density (1.2 kg/m 3 ) ρ l The density of the fluid. ρ weights The conventional density of the weights used to balance the weighing or calibrate the weighing machine. (8000 kg/m 3 ) Project No: FFRE31 Page 20 of 22 November 2006

23 Conventionally, and for trade purposes, buoyancy correction factors are given in the ASTM petroleum measurement tables Table 56: Short table. These allow for conversion of weight in air to mass and mass to weight in air. It is important when using the tables that the density at reference temperature (15 C or 60 F depending on the table) is used rather than the actual density of the fluid in the weighing container. Density at 15 C should be used if replicating the tables by calculation hence maintaining the convention for trade. In the case of LPG, the weight in air cannot be realised by measurement since weighing is carried out in closed containers. The measured weight is therefore very close to the mass. Thus, when weighing a quantity of LPG, the mass is first calculated from the measured weight by including the buoyancy correction factor for the reference weights (equation 1). Equations (2) and (3) are then used to obtain the weight in air from the derived mass. Volume: The volume, as measured at the actual conditions, may be used for trade and transactions but this is not recommended for any applications other than small volume and low value retail transactions. The obvious transaction where actual volume provides the measurement is dispensing propane (Autogas) as a fuel for vehicles. Actual volume is allowed in most countries for this transaction and most UK retail sales of auto-gas are carried out at actual volume. All recommendations and standards specifically suggest that more accurate expressions of quantity are required for bulk transfers and hydrocarbon accounting purposes. Standard Volume Temperature: The cubical thermal expansion coefficient of LPG is approximately /K at 20 C. This means that with LPG the volume will change by around 0.3% per C change in temperature. The implication of this is that a volume delivered will have 3 per cent more or less mass, and therefore energy, than that same volume delivered if the temperature changes by 10 C. The butane expansion coefficient is smaller. All bulk trade and hydrocarbon volumetric based accounting transactions are carried out using standard temperature. The choice of reference temperature depends on the relevant convention, contract conditions and jurisdiction. Most European transactions are referenced to 15 C and US transactions referenced to 60 F. Unfortunately in the case of LPG the density varies non-linearly, and depends on the composition of the product. A correlation between the expansion factor and the product mixture as a function of density (or specific gravity) exists and is utilised in the corrections applied. For bulk trade purposes the correction factor Ctl (correction factor for temperature effect on liquid volume) is used to relate a volume at measured temperature to that at reference conditions. The measured volume is multiplied by Ctl to give the standard volume. For liquid crude and refined oils Ctl is conventionally obtained through the use of the ASTM / API / EI / ISO petroleum measurement tables. For LPG additional tables, 24E and 23E, have been produced and published by The Gas Processors Association (GPA) under the designation TP25. These are also available from ASTM and API. The TP25 document describes a calculation procedure that is best implemented using a computer program. Project No: FFRE31 Page 21 of 22 November 2006

24 However tables have been produced from the calculations and are used in industry where no computer is available. The results from the computer algorithm provided by Table 23E give the density of the product based on its specific gravity at reference condition and interpolation from reference fluids at both standard and actual temperature. If the specific gravity is not known at reference conditions, only at actual conditions, Table 24E gives a method of calculating the specific gravity at the reference condition. It should be noted that Ctl calculated in this way represents a ratio of densities at the saturation pressures corresponding to the actual and standard temperatures. Many industry users where not constrained by contract terms specifying TP25, calculate the volume correction factor by the ratio of the densities of the fluid at actual and reference conditions. The densities are calculated from properties of fluids software (e.g. IUPAC formulations accessed via PPDS from NEL or REFPROPS from NIST). As part of the project, calculations were carried out comparing TP25 with the NEL PPDS package for propane butane and a 80/20 propane butane mixture. It was found that for butane the agreement is within ±0.03% between -50 C and +50 C and within ± 0.01% between 0 and +30 C. However for propane agreement is within ±0.01% between -50 C and +20 C but rises to almost 0.2% at +50 C. Although the differences are perhaps not considered significant at ambient conditions, they may be as large as 0.06% at 30 C. Project No: FFRE31 Page 22 of 22 November 2006