National Grid Gas Transmission, Northern Gas Networks, SGN and National Grid Gas Distribution
NGGD Andy Finch – Project Manager (lead GDN) and NGGD Andy Newton – Portfolio Manager
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Gas Distribution Networks and Gas Transmission Networks
Orifice plate meters form a substantial part of National Transmission System (NTS) to Local Distribution Zone (LDZ) custody transfer and in order to conform to the orifice plate metering standard, ISO 5167, the plates need to satisfy numerous criteria such as flatness, edge squareness, surface finish, thickness of size of orifice bore and bevel angle, any deviation from the standard can lead to metering errors which can cause under registration of mass flow.
During normal operation, orifice plate meters are subjected to a differential pressure across the plate, design maximum pressures of up to 1000 mbar have been used in these installations. A correctly designed orifice plate will deflect or deform elastically. At higher differential pressures, the plate will reach a point beyond which is will permanently deform, as an orifice plate deforms elastically there are increasing flow errors which
must remain below 0.1% to conform with the standard, additionally the plate flatness under load must not exceed 1%. The equations used to predict orifice plate deformation are in question.
Modern computational techniques make it possible to revisit orifice plate deformation using a combination of CFD and FEA. Typical data taken from offtake metering can be used to generate a bespoke and allencompassing solution for calculating the deformation of orifice plates in natural gas systems.
To improve the measurement of volume and energy flow through orifice plate metering systems by ensuring that orifice plate deformation is calculated correctly.
The achievements of this project will enable the calculation of orifice plate deformation to be carried out more accurately, there is currently not a single agreed industry method and with the advantages of modern computational techniques, it is believed that this can now be achieved; The ultimate aim is to recommend this method as the Industry Standard.
The technical achievements are anticipated to be:
- Survey of existing technical literature to establish current status of orifice plate deformation calculation
- Establish the causes of the differences between the Jeplast routine within HPMIS and other equations
- Use computational fluid dynamics (CFD) to calculate the actual load distribution on the orifice plate for a worst possible case
- Use FEA to calculate the behaviour of the orifice plate under the load distribution calculated from the CFD for a worst possible case
- Repeat the CFD and FEA calculation method established in stage 2 for a range of plate sizes, beta ratios, seal and mounting types
- Recommend a method of calculating orifice plate deformation that can be implemented.