Clocking a Gas Meter (Natural Gas)

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For our first article, we will be discussing the procedure to identify the actual horsepower of a boiler in operation. This is useful for a couple of reasons. First, it is important to know that the BTUs being consumed match the rated input of the equipment for your specific application. Second, boiler efficiency increases when a boiler is properly sized for the application. Can you identify other reasons in the comments below?

The only true way to clock a meter is to convert the fuel usage to horsepower at the highest firing rate. The process might seem complicated, but it is far simpler than you might expect. Though we will not be diving into how to do the math behind the calculations, we will identify the horsepower using an example based on readings, so you can see that it works. Let’s get to it then!

Your first step is to isolate the gas meter so that any readings are only for the specific equipment being identified. If your totalizing meter is on the exterior of your building and you run an entire facility, you won’t be able to clock the meter and identify a single boiler at this location easily. You’ll need to install a gas meter at the boiler or specific piece of equipment so you are only measuring the flow of gas to that unit. Isolation is the key!

Next, you’ll need to start the boiler and bring it to the highest firing rate. Once there, you need to maintain this for a period of time while you perform your readings. It is important to know how to read a gas meter, which this article does not cover. Essentially, you need to identify how much fuel is being consumed over a period of time. Remember this simple rule: The more time allowed, the more accurate the calculation. You will also need to know at what pressure the gas is under.

Now, using a manometer, measure the gas pressure at the inlet of the meter to identify the correction factor you’ll need later. Then use a stopwatch to time how long it takes your gas meter to measure a specified volume of fuel. Most meters will have a standardized dial system, so measure the time it takes to complete at least one revolution. You’ll want to record that volume and the time it took to get there. Once you have the gas pressure, gas volume, and length of time, you have all the information you need to make your calculation!

Important: Convert the volume of gas to cubic feet (cf). Convert the time to hours (h).

Using the following example of measured data, what is our identified horsepower:

Gas pressure = 5psi (equates to a correction factor of 1.34)
Gas volume = 350 cubic feet (Assuming 1,000btu/cf for Natural Gas)
Time = 103 seconds (equates to 0.0286 hours)

This data tells me our gas flow is approximately 12,233cfh @ 5psi. With a little math, if you calculated the boiler horsepower would be 390HP, you’ve got it down. This seems reasonable, if our boiler is rated for 400HP. Voilà!

Here are some things to remember…

The calculated input HP rating should be within 5{609b0bb56d9e1b35f335d90d178507d48b6b5c3ba769b393145ab19033b6ca1d} of the data plate.
For a more accurate calculation, identify the actual amount of Btu’s per Cubic Foot from the local gas company.

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