Verifying gas meter accuracy

By Russ Kratowicz, ExecutiveEditor

The genesis of this story lies in a conversation with Bob Houle, the controller at Sunlaw Cogeneration Partners, an independent producer of electric power. Sunlaw generates electric power using gas-fired turbines that are modified versions of aircraft engines. The company sells the electricity it makes to its local utility.

This cogeneration facility runs year-round. The utility bill for natural gas is certainly not insignificant--about 1 million dollars per month. This corresponds to about half of the operating cost of the plant. This amount of gas consumption generates a great deal of combustion products, oxides of nitrogen being one.

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Sunlaw injects water into the combustion chamber of the turbines to reduce the emissions of NOx. The theoretical amount of water needed per unit of fuel comes from standard engineering calculations. However, the injection system must be dynamic--water rate must vary with the firing rate. The logical way to connect water rate to fuel rate is with the output signal from the gas meter the local utility installed at the plant. However, Sunlaw discovered that it was unable to control the NOx emission effectively in this seemingly straightforward manner.

Further calculations indicated that the source of the problem might be the signal from the meter. To get around this uncertainty, Sunlaw installed a Coriolis meter in series with the utility-furnished meter. Even after accounting for pressure and temperature corrections, there was an obvious discrepancy in the meter readings. Simply having differing measurements of the same quantity of gas is insufficient to determine which meter, if any, is accurate.

The next step was making additional calculations to try to account for the difference. This round of calculations seemed to confirm that readings from the Coriolis meter were more accurate than the readings from the utility-supplied meter used for billing purposes.

There is a lesson here

The point is that resolving meter problems might be an opportunity for improved plant financial performance. According to Ed Bowles, Section Manager of The Flow Measurement Group at the Southwest Research Institute, an independent, nonprofit, applied engineering and physical sciences research organization, part of the regulations that apply to the gas industry concerns itself with the possible and probable gas losses in the process of transporting it from the wellhead to your combustion chamber. These "losses" are rolled into the rate structure under which your plant compensates the local utility for the gas it provides.

The degree to which the utility can reduce actual losses below the allowance provided for in the regulations represents revenue that drops straight to the utility's bottom line. There is an incentive for the gas company to be as accurate as possible in passing gas around the neighborhood. So, why don't the local utilities adopt as standard the latest and greatest in highly accurate flow measuring technology?

In general, said Bowles, the companies involved in moving gas from the well to your burners are risk averse. They exhibit a degree of conservatism that embraces only those measuring techniques generally accepted by the industry and referenced in the standards under which they operate. Any technological breakthrough that produces highly accurate flow measurements must demonstrate a real-world track record before it gets incorporated into the industry standards As a result, the regulatory and operational standards tend to lag behind the best technology available in the marketplace.

However, the accurate measurement of flow is not the only issue to consider. Custody transfer is based on BTUs delivered, not the volume or mass. Since gas meters do not measure BTU content directly, custody transfer is based on a BTU value derived from a volumetric measurement and a chromatographic analysis of diluents and other constituents that affect the heat content being delivered. Which returns us to the issue of measuring flow accurately. Among producers, the meter of choice is that which the two parties agree to use. As consumers, though, we have no say in the selection of the meter to be used as a basis for billing purposes--we must use the meter the utility provides.

Hot tips when burning gas

In an ideal world, you pay only for the exact amount of gas you consume. Unfortunately, there is a real possibility that the meter reading exhibits error in either direction. But, does it even pay to start investigating?

Before you spend countless hours in checking into this matter, start with the assumption that your gas meter reads a few percent over actual consumption. Calculate the dollar value of that discrepancy and compare it to an estimate of the cost of labor and material required to eliminate the discrepancy.

Obviously, if your plant burns little gas, then it probably does not make sense to waste a dollar trying to save a dime.

On the other hand, if your plant consumes a great deal of gas, it makes sense to verify the accuracy of the meter. Start by getting your own house in order. Go through the effort of running a heat balance or efficiency calculation for your gas-burning equipment. Document every action you take and every result you find.

Analyze and quantify the heat content per unit volume of natural gas flowing into the plant. Although this step requires analysis by a qualified independent laboratory, it is relatively inexpensive when compared to what is at stake. Compare the results to the correction factor printed on your gas bill.

Find out when the meter was last calibrated. You have a right to a utility meter that is reading accurately.

Your utility should provide the recalibration service at little or no cost.

Another step is to install a gas meter that is approved by the American Gas Association. Mount this meter in series with your existing meter to verify the readings are identical.

Spend time researching meter accuracy. The American Gas Association, the Gas Research Institute, the Institute of Gas Technology and the Southwest Research Institute each maintain a Web site that provides useful background. Don't rely on only one source for a conclusion. Cross-check against these and other sources you may discover in your searching.

If your basic research indicates your meter is in error, contact your utility. Having an accurate meter is a right dictated by law. If you cannot resolve the issue with the utility, there is always litigation with its own cost/benefit analysis. If you sue for recovery of excess payments, remember that the statute of limitations is different in each state.

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