GUIDANCE NOTE

Compressed Natural Gas (CNG) Flow Measurement Technologies

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Compressed Natural Gas (CNG) Flow Measurement Technologies

Introduction

This Guidance Note provides a brief review of the key issues associated with flow measurement of Compressed Natural Gas (CNG).

Compressed Natural Gas (CNG)

CNG is predominantly methane that has been compressed to between 200 and 300 bar for storage. The primary application of CNG is a fuel for vehicles, and the principal motivation for using CNG instead of petroleum fuels is the relatively low pollution.

There are almost 5 million natural gas vehicles (NGVs) in use worldwide, with around 500 CNG vehicles on the road in the UK and about 30 refuelling stations.

Currently NGV vehicle usage in the UK is at best static and may even be in decline, but there is a long-term view that the eventual adoption of compressed gaseous fuels will be required to replace declining supplies of petrol and diesel.

CNG Flow Measurement

In a CNG vehicle filling station, gas from the national distribution network is compressed and stored using a cascade system in which typically there are low, medium and high pressure storage vessels. The low pressure vessel is used at the start of the vehicle filling cycle, and the high pressure vessel is used at the end as the vehicle tank pressure rises towards the typical maximum of 200 bar. This type of system is designed to provide a “fast fill”.

The low-pressure gas flow from the transmission network is measured using a conventional gas meter. The compressed gas is delivered to a vehicle using a dispenser incorporating a flow meter which must handle conditions of rapidly and widely varying flow, pressure and temperature during the course of a fill cycle.

There is also a “slow fill” option in which a compressor pumps the compressed natural gas directly into the vehicle tank without intermediate storage. The only measurement involved would be the volume flowrate from the low pressure gas supply meter.

Coriolis meters are regarded as the technology offering the greatest accuracy and reliability for fast-fill CNG dispensers. This means that mass flow rather than volume flow is measured, even though measurement of the gas flow from the distribution network is by volume.

In outline, the main requirements for flow meter calibration, verification and testing are:

Initial calibration of the dispenser flow meter under laboratory conditions Initial verification of the dispenser over typical vehicle fill cycles Subsequent verification of the dispenser

Dispenser verification can be achieved either gravimetrically or using a “master meter”. In the latter case there is a need for high accuracy calibration of the master meter.

Flow Meters and Calibration

Several manufacturers of Coriolis meters provide models specifically designed for use in CNG dispensers. A key calibration issue for both a master meter and a dispenser meter is the use of a fluid other than CNG to calibrate the flow meter.

Previous Flow Programme research, and other research in the USA implies that water calibrations are directly transferable to natural gas. However these studies are limited to gas pressures of at most 36 bar, and it must be recognised that there are no independent published data to confirm transferability of water calibrations to CNG at higher pressures.

Larger sizes of Coriolis meters, normally 1-inch and above, are affected by pressure, with higher pressures causing an increasing negative bias in the meter output. This effect is said to be well known and repeatable between meters of the same design.

Coriolis meters for CNG are smaller than 1-inch and no pressure correction is apparently required.

Figure 1 - Coriolis Flow Meter for CNG Dispensers

June 2010

The GTI Precision Meter Test Facility in the USA is claimed to provide precision measurement of high-pressure gas meters, dispensing, and storage systems for natural gas, hydrogen, and other fuels. It is the only known facility capable of operation with natural gas at typical CNG storage pressures of 200 bar and above.

The GTI facility consists of a high-precision scale equipped with high-pressure CNG cylinders. The high-resolution scale has a weighing range of 0-1,360 kg with resolution capability of ±2.3 grams.

All testing is conducted within a Class I, Division 2 explosion-proof room. High-pressure natural gas and hydrogen are available from full-size multi-bank high-pressure cascade storage systems. The natural gas cascade holds natural gas at pressures up to 345 bar. The facility provides the capability to test stand-alone meters and complete dispensers.

Other Measurement Issues

The low pressure gas flow to a CNG refuelling station is measured by a conventional gas meter. It is believed that this type of meter may lose accuracy due to the stop-start cycling of the compressor, with the possibility of generating a reading that is too high. The conventional gas meter on the low pressure side provides the measurement on which fuel duty is based, and there is also the problem of reconciling the amount of gas supplied to the station with that dispensed.

OIML Recommendation

The OIML International Recommendation “Compressed Gaseous Fuel Measuring Systems for Vehicles, 3rd Committee Draft, December 2002” specifies the metrological and technical requirements that apply to dispensers for vehicle re-fuelling.

The document sets down maximum permissible errors for the meter alone and for the complete measuring system. The maximum permissible relative errors on mass indications at pattern approval are:

±1% of measured quantity for the meter alone

±1.5% of measured quantity for the complete measuring system

These values also apply to initial verification tests that are performed in a laboratory.

The maximum permissible relative errors on mass indications at initial verification under rated operating conditions on-site or at subsequent verifications are ±2% of the measured quantity for the complete measuring system. The maximum possible errors that apply to the minimum measured quantity are twice the above values.

The maximum permissible errors are greater than corresponding values recommended by OIML for petrol, diesel and LPG, which presumably reflects the particular uncertainties associated with the dynamic conditions of a CNG fast fill.

Other relevant statements from the document are:

Coriolis mass flow meters for natural gas can be calibrated using nitrogen with no need for corrections to be applied.

The ratio between the maximum and minimum flowrate shall be at least 15.

Annex B of the OIML document provides a detailed test procedure for the initial verification of the complete metering system under simulated dynamic conditions of a typical fast fill. The procedure takes into account:

sizing of the meter in relation to the maximum flow obtainable with the refuelling system to which it is fitted whether the fill is a large one with the vehicle cylinder at low pressure, or a small one with the cylinder initially at an intermediate pressure the relative contribution of the low flow rate portions of the refuelling process, at which meter accuracy is likely to be oorer

shocks, flow acceleration and deceleration resulting from the action of the control device for switching between storage banks at different pressures.

Figure 2 - Typical Flowrate Profile During a “Fast Fill”

The test procedure involves using a test rig to replicate the conditions of a fast fill using three storage banks. The working fluid is discharged into a test receiver that represents the vehicle fuel storage cylinder and can be weighed. The performance of the meter and the measuring system are checked against the allowable errors.

The standard leaves the policy for subsequent verification to the appropriate national authority, and it is stated that the procedure may be identical to that of the initial verification.

Compressed Natural Gas (CNG) Flow Measurement Technologies

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 NEL, East Kilbride, GLASGOW, G75 0QF, UK

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

www.tuvnel.com

© TUV NEL Ltd 2010

Re-issued 2010This publication is to provide outline information only which (unless agreed by TUV NEL 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.

Other Compressed Gas Fuels

HCNG is a blend of hydrogen and natural gas with typically 15-30% hydrogen by volume, and has the advantage of lower emissions than CNG. HCNG requires a dispenser that mixes the fluids to the required ratio and therefore involves flow measurement of both hydrogen and CNG.

There is a view that HCNG is a key element in the development of a refuelling structure for future transition to hydrogen powered vehicles.

Biogas is produced by the anaerobic digestion or fermentation of organic matter, and it is composed mainly of methane and carbon dioxide. When it is cleaned up it has very similar characteristics to natural gas. Biogas is seen as a more promising route in some countries than natural gas towards the use of compressed gas fuels for vehicle transport.

The small number of compressed hydrogen vehicle fuelling stations that have been prototyped or constructed use a Coriolis flow meter in the dispenser.

Summary

CNG usage is relatively small in the UK at present.

CNG dispensers in filling stations incorporate Coriolis meters for mass flow measurement. Several manufacturers offer meters suitable for CNG dispensers.

Independent research studies indicate that Coriolis meters can be calibrated using water and the calibration satisfactorily transferred to gas, though there are no data to confirm this beyond gas pressures of about 50 bar. Coriolis meters used in CNG dispensers are typically 0.5 inch, and apparently do not require the static pressure corrections to the meter bias that have to be applied to larger meters.

Further Reading

Review of CNG Flow Measurement Technologies and Measurement issues, Report 2006/297, TUV NEL Ltd. East Kilbride, Glasgow.