Natural gas measurement by DP transmitter

Improving accuracy, reducing costs of natural gas flow measurement over operating ranges previously considered unobtainable: Foxboro DP50 units with Accelabar achieve turndown of 150:1

Improving accuracy, reducing costs of natural gas flow measurement over operating ranges previously considered unobtainable.

The natural gas industry involves a complex network of production, distribution, and delivery to meet the growing global demand for more than 100 trillion cubic feet per year.

Of the myriad procedures involved with supplying this natural resource, flow measurement is one critical constant.

From producer to consumer, accurate flow measurement is the keystone for delivery and product accountability.

However, a traditional challenge, which has enormous financial stakes, is accurate flow measurement at the extremes of both high and low flow rates.

A Minnesota-based energy company faced this situation at their Liquid Natural Gas (LNG) storage facility.

The company stores LNG in two 12,000,000 gallon tanks and uses gas-fired heaters to vaporise it as required to meet customer demand.

For most of the year demand is low, however during the coldest winter months gas consumption jumps from 1,000 to 60,000 standard cubic feet per hour (SCFH), representing an enormous flow rate change (a "turndown" ratio of 60:1).

The plant had attempted to measure this flow rate with a turbine meter sized for the maximum flow rate, but could not get accurate flow readings at the low end of the measurement range, making it impossible to determine actual usage during the off-peak periods.

In addition to accuracy limitations, turbine meters have moving parts, which wear and require expensive maintenance.

An option they considered was using a differential pressure (DP) transmitter with an orifice plate, a flow measurement method commonly used in the gas industry.

But given the wide span of flow rates, a single meter could not accurately measure the high and low extremes.

And use of multiple meters proved too costly with the required bypass manifold piping, valves, and complete monitoring electronics necessary to allow seamless change-over between meters.

Another option was to change the orifice plate sizes seasonally to measure widely changing flow rates.

However, this approach involved intensive field labour in severe weather conditions for manual changing of the plates and re-ranging of the transmitter.

This Minnesota natural gas supplier found its solution in a flowmeter developed by Veris, that incorporates Foxboro d/p cell transmitters.

Named the Accelabar, the new Veris meter features a Veris-patented toroidal nozzle design with Verabar flow rate measurement technology.

The nozzle provides a straight run "settling distance" that accelerates and stabilises the velocity profile and significantly increases the DP captured by the Verabar located within the nozzle.

The result is a very accurate and predictable DP over an exceptionally wide flow range.

From Invensys, the Foxboro IDP50 Premium Performance d/p cell was selected.

The IDP50 features improved characterisation and compensation techniques to achieve industry-leading accuracy and resistance to environmental effects over a very wide measurement range.

Two sensor selections are available within the IDP50 group, one with an upper range limit (URL) of 1000 inchesH2O, and the other with a URL of 200 inchesH2O.

Both selections are accuracy-rated +/-0.025% of span for spans as small as 1/10 of maximum span and no more than +/-0.05% of span even for spans as small as 1/80 of maximum span.

Because of the exceptionally wide range of flow rates in this natural gas application, the engineers chose to use two high-accuracy IDP50 transmitters, one to measure DPs from 302.8 down to 2.5 inH2O with +/-0.025% of span accuracy, and the other to measure DPs from 2.5 down to 0.34 inH2O (and ultimately even lower) with +/-0.050% of span accuracy.

"When designing this system, we immediately turned to Invensys Foxboro for DP transmitter expertise.We've worked with Invensys Foxboro through the years and continue to be impressed with their level of technology and proven product performance.

For this application we combined two leading edge technologies in one flowmeter to achieve operating ranges never before attainable," says Jerry Gallo, national sales manager at Veris.

RESULTS.

The LNG producer has now installed the Veris Accelabar flowmeter with Foxboro IDP50 transmitters in a pipeline with no straight runs, and with valves, strainers, and pipe expansions and reductions ahead of it.

Even in this demanding configuration, the Accelabar flowmeter is meeting all performance requirements.

The system provides flow rate measurement accuracy of +/-0.75% over the entire 60:1 flow operating range and has actually exceeded expectations by accurately measuring flow rates with turndowns as high as 150:1 (from 60,000 SCFH all the way down to 400 SCFH).

"The system proved to be everything we wanted and more.

We were so confident it would meet our customer's stringent requirements that Veris and Foxboro jointly took performance responsibility and gave the customer a 5-year performance warranty.

This is unheard of in this industry, but, until now so was obtaining accurate flow measurement in the operating ranges we have here," says Gallo.

Also, because both the Accelabar and the IDP50 have no moving parts to wear or seize, maintenance is minimal.

And, the natural gas supplier has found that the flow metering system is user friendly and easy to operate, especially since DP flow measurement is one of the most easily understood of any technology available.

For the LNG provider, this translates into improved material accountability and lower operating costs, both of which improve profitability.

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