High-Precision Calibration and Prover System for Natural Gas Meters

October 15, 2016

 

The oil & gas industry has a massive global presence as billions of units of their products go into our houses and cars every year. While often considered a messy, heavy industry, it might surprise you to hear that some parts of the industry require accuracy and precision to levels that would make other industries jealous!  When it was time to upgrade a proving and calibration system for natural gas meters, one of the largest companies in the world came to Cyth Systems for help.

 

Background

A natural gas meter measures the volume of natural gas that flows into the pipes of your home. Customers expect those systems to be as accurate as possible, and so does the gas company and regulators! Companies use systems originally designed in the early 1900’s that consist of large copper drums moving an accurately measured distance. Like a giant syringe, if the drums move a known distance they displace a known amount of gas volume.  These systems have been improved over time with computer-based controls, but they were all operating on Windows XP and they all had to be upgraded. So once the choice was made to upgrade, the team decided to create the most accurate and modern prover ever designed. 

 

The Solution

The design of the new system was based around a giant steel syringe big enough to fit a typical sedan inside!  The drum was precision machined to a tolerance of 0.001” in diameter throughout its length to ensure a very accurate volume.  A servo motor and screw was designed with a 100-to-1 gear ratio which multiplied the servo encoder to have 100x the accuracy of the internal encoder.  That servo motor also had to be able to move a 2” thick steel piston weighing about the same as a car, and it had to achieve large flow rates which meant moving that piston very rapidly and at an accurate speed.  Yet in addition to that, a 20-foot glass linear encoder was added which is common to CNC machines to ensure precision to 0.00001” linear measurement.  Yet to increase the accuracy further still, 10 pairs of precision ground metrology standard gauge blocks were added to the system, which could be detected with a small-diameter laser.  By combining those three methods of measurement, an unprecedented level of accuracy and precision could be achieved.  And by doing a calibration run every day, an ongoing calibration history could be tracked for traceability to the metrology standards.

 

Making an automated precision gas flow system calls for a handful of measurement and control systems.  The system contained six gas flow valves, each one controlled from 0-100% flow control, with an encoder to detect position, and a sensor at each end to protect the valves from over rotation.  Several single-ended and differential pressure sensors measured pressure inside the piston, and at key points along the flow path.  A few safety valves, an oil sensor, and oil refill pump, several temperature sensors and numerous other inputs and outputs totaled about 45 signals at the piston.

 

At the operator interface, the product had to be loaded and clamped by heavy pneumatic actuators to hold the part without leaks.  To protect the operator, two touch sensors were used to engage the part without the possibility of injuring the hands.  Numerous laser-based sensors were placed to detect rotation of the main shaft both before and after their main gear reduction.  Another array of pressure, temperature, and flow valves were outfitted to the operator interface area as well, totaling an additional 30 signals.

 

Challenges

One big challenge was that the operator interface and the product were about 20 feet away from the piston.  To minimize wiring and make the system easier to install, the system was controlled by two National Instruments Compact-RIO (cRIO) systems, connected by a single ethernet cable.  Yet all the measurements including pressure, temperature, flow, and the precision distance measurements all need to be synchronized between the two instruments.  To start the data gathering process on both machines, and to provide accurate timestamps, two methods were used.  First, a TCP command and reply mechanism was developed using LabVIEW code on both ends for the operator side system to tell the piston what time the test will begin within 0.1 milliseconds, and gave both systems a countdown clock to begin their operations.  To make sure they shared a common clock, the systems were synchronized using NI-Sync, a time synchronization protocol that runs in the background to keep the systems using the same clock.  With those in place a test that included emitting a commanded pulse on both systems showed them synchronized to less than 0.1 millisecond.

 

The Result

After installation the system went through two Factory Acceptance tests and a handful of small upgrades before it was ready for production use.  Regulators from TUV as well as test ISO auditors tested for performance against standards.  Since there is no easy way to get a gas volume standard, the process was done by verifying all the measurements such as temperature and distance, while using traceability standards to confirm gas flow.  With all the primary and secondary measurements confirmed, one of the most precision gas meter provers went into full production use and has been making the meters in our homes and businesses better ever since.

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