Sierra Instruments, Inc.

Advantages of flexible flow measurement in sub-metering of natural gas

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Courtesy of Sierra Instruments, Inc.

The ability to measure gas flow more accurately with sub-metering based on a new technology, can result in substantial savings.

Natural Gas: An Abundant Resource

Unlike oil, natural gas exists in abundance in the United States. Of the natural gas consumed in the United States in 2011, 95% was produced domestically. The U.S. Energy Information Administration projects U.S. natural gas production to increase from 23.0 trillion cubic feet in 2011 to 33.1 trillion cubic feet in 2040, a 44% increase. Almost all of this increase in domestic natural gas production is due to projected growth in shale gas production, which will grow from 7.8 trillion cubic feet in 2011 to 16.7 trillion cubic feet in 2040.

This natural gas boon highlights the importance for utility companies to provide homes, businesses, and mid-to-large size facilities with accurate natural gas billing. Traditionally, utility companies have used diaphragm meters to measure natural gas usage and then charged customers their monthly energy bill. Although diaphragm meters are a trusted billing mechanism by utility companies, they may not provide customers with the most accurate natural gas bill. Diaphragm meters have limitations in natural gas measurement. If the diaphragm meter does not accurately measure the natural gas, this can result in utility companies overcharging customers.

To avoid this problem, many mid-to-large-size facilities use flow meters to sub-meter their natural gas usage to validate the diaphragm meters’ readings and more efficiently allocate energy. Thermal flow meters have traditionally been an accurate device for sub-metering natural gas. But these meters are unable to compensate for the composition and pressure changes inherent in natural gas measurement. However, recent advancements in thermal dispersion technology offer facility operators more advanced flow metering options to drive their energy management programs. By taking control of their energy management instead of relying on utility companies, facilities can save thousands of dollars in natural gas usage charges and improve energy allocation.

What is Natural Gas?

Natural gas is the cleanest burning fossil fuel, producing by-products of mostly carbon dioxide and water vapor. It is used extensively in power generation, industrial and commercial applications, and for home utility and heating. Natural gas is primarily a mixture of methane, ethane and propane, but those components can vary greatly. Table 1 shows the typical composition of gas delivered by Pacific Gas and Electric (PG and E).

One British thermal unit (BTU) is equal to the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit at atmospheric pressure. One standard cubic foot of natural  gas has an average heating capacity of approximately 1000 BTUs. This value varies with gas composition, and since utilities use gases from different sources at different times of the day, the heating value changes as well. The heating value for PG and E natural gas is required to range between 990 and 1,050 BTU/ft3 and will normally range from 1,000 to 1,030 BTUs per standard cubic feet.

Dealing with Changing Compositions and Delivery Pressure

The most common type of gas meter, seen in almost all residential and small commercial installations, is a diaphragm meter. Utility companies use diaphragm meters to measure the flow rate of natural gas and monetize the usage for billing (see Figure 1).

Within the meter there are two or more chambers formed by movable diaphragms. With the gas flow directed by internal valves, the chambers alternately fill and expel gas, producing a near continuous flow through the meter. As the diaphragms expand and contract, levers connected to cranks convert the linear motion of the diaphragms into the rotary motion of a crank shaft, which serves as the primary flow element. This shaft can drive an odometer-like counter mechanism or it can produce electrical pulses for a flow computer (a smart meter).

Diaphragm gas meters are positive displacement meters. These gas meters measure a defined volume, regardless of the pressurized quantity or composition of the gas flowing through the meter. Temperature, pressure and heating value compensation must be made to measure the actual amount and value of gas moving through a meter. These fixed compensation variables used by utility companies can yield inaccuracies and overcharging of utility bills. For example, the diaphragm meter typically measures the natural gas volume in hundreds of cubic feet (CCF); however, the consumer is billed in therms, where one therm is equal to 100,000 BTUs. Customers are billed by taking the gas meter reading in cubic feet, converting this value to therms, then applying a multiplier that is the product of the heat value of the gas (composition dependent) times the gas density (pressure dependent). A typical home or business may receive natural gas at .7 psig with a heating value of around 1030 BTU/ft3. Applying a correction factor for the heating value of 1030/1000 equals 1.025 and a correction factor for the density of (0.7 + 14.7)/14.7 equals 1.048. The overall correction factor is 1.025 x 1.048 = 1.074. This multiplier is applied to therms consumed and the consumer is billed according to the result.

Ideally this multiplier will be 1.000. The energy value of the gas in BTUs is normally 1000 BTU/ ft3 when averaged over time, but this assumes the utility company is tracking this value throughout the year and making the necessary adjustments in the multiplier. The density portion of the multiplier is where more variability is seen, as pressure in delivery lines can vary with the demand placed on them or may be controlled with poor pressure regulation. As an example, if the pressure in the gas delivery line drops from 0.7 psig to 0.25 psig the correction factor changes from 1.048 to (0.25 + 14.7)/14.7 = 1.17. That is a 12% increase in the consumer’s bill, potentially putting the customer in a higher usage tier. As mentioned, the diaphragm meter is a volumetric meter, while natural gas is sold on the basis of mass. Volumetric meters cannot account for changes in gas composition nor deal with changes in pressure and temperature (and hence density). Such changes must be corrected for, and the consumer is at the mercy of the utility company to make those corrections.

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