Sizing a natural gas system for commercial and industrial buildings is an essential task to ensure the proper supply of fuel to equipment and appliances. When designing a natural gas system, it’s important to consider the gas demand of the equipment and appliances that will be using it, as well as the distance of the equipment from the gas supply, the gas pressure, and the allowable pressure drop. In this blog post, we will provide a step-by-step guide on gas pipe sizing and provide an example for a 100,000 square foot warehouse with 4 unit heaters, assuming the longest length is 1000 feet, an inlet pressure of 2 PSI, and an allowable pressure drop of 1 PSI.

Equation Method

The equation method is a technique used to determine the size of a gas pipe needed to supply natural gas to equipment and appliances in a building. This method involves calculating the inside diameter of the pipe based on the gas demand, pressure drop, length of pipe, and gas characteristics such as specific gravity and viscosity. The equation method is more precise than using gas sizing tables, as it takes into account various factors that can affect the gas flow in the pipe.

Step 1: Determine the Gas Demand

The first step in sizing a natural gas system is to determine the total gas demand of the equipment and appliances that will be using it. In this example, we have 4 unit heaters with a gas load of 250,000 BTUH each. The total gas demand for the unit heaters would be:

4 unit heaters x 250,000 BTUH per unit heater = 1,000,000 BTUH

Step 2: Calculate the Gas Flow Rate

To calculate the gas flow rate, we need to divide the total gas demand by the heating value of natural gas, which is typically around 1,000 BTU per cubic foot (BTU/CF). The gas flow rate for the unit heaters would be:

1,000,000 BTUH ÷ 1,000 BTU/CF = 1,000 CFH

Step 3: Determine the Pressure Drop

The next step is to calculate the pressure drop due to the distance between the gas supply and the unit heaters. In our example, we assume a distance of 500 feet between the gas supply and the unit heaters. The pressure drop can be calculated using the following equation:

Pressure Drop = P1 – P2

Where:

• P1 = Upstream Pressure (PSI)
• P2 = Downstream Pressure (PSI)

We assume an inlet pressure of 2 PSI and an allowable pressure drop of 1 PSI.

Step 4: Determine the Gas Pipe Size

Now that we have the gas flow rate and pressure drop, we can calculate the pipe size needed to supply the gas to the unit heaters. The inside diameter of the pipe can be determined using the following equation:

Where:

• D = Inside Diameter (in) of Pipe
• Q = Gas Input Rate (CFH)
• P1 = Upstream Pressure (PSI)
• P2 = Downstream Pressure (PSI)
• L = Length of Pipe
• Cr = 0.6094 for Natural Gas
• Y = 0.9992 for Natural Gas

Calculator

Flowrate, pipe length and pressure significantly impact the required pipe size. To see how these variables relate we have provided a calculator below:

Pipe Length (ft):
Inlet Pressure (PSI):
Outlet Pressure (PSI):
Atmospheric Pressure (PSI):
Cr:
Y:
Gas Input Rate (CFH):

After filling all options, press "Enter".
Pipe Diameter (inches):

Conclusion

Sizing a natural gas system is a critical step in ensuring safe and efficient operation of equipment and appliances in commercial and industrial buildings. The gas pipe sizing must be appropriately chosen based on factors such as the gas demand, pressure, and allowable pressure drop. In this blog post, we provided a step-by-step guide on how to size a natural gas system using the equation method for a 100,000 square foot warehouse with four unit heaters, each with a gas load of 250,000 BTUH.

Using the equation method, we determined that the inside diameter of the pipe required to supply the natural gas to the four unit heaters would be approximately 1.335 inches. It’s important to note that each application may have unique requirements that need to be considered when sizing a natural gas system. Therefore, it’s essential to consult with a qualified professional to ensure the natural gas system is designed correctly and safely for your specific application.

Whether using the gas sizing tables or the equation method, it’s crucial to ensure that the gas system is designed to provide a safe and efficient supply of natural gas to the equipment and appliances. Proper gas system design can lead to increased equipment and pipe lifespan. It’s also important to regularly maintain the natural gas system and to ensure that it meets all safety codes and standards.

Resources

If you prefer to use the sizing chart check out our companion guide here: 5 Steps to Size a Natural Gas System: A Comprehensive Guide for Commercial and Industrial Buildings

Did you know you can access the International Fuel Gas Code and other International Codes for Free? Here is a link to our page with all the codes: Free Building Codes!

Interested in learning more about gas systems? You can access the International Fuel Gas Code here: https://codes.iccsafe.org/content/IFGC2018