
SIZING NATURAL GAS PIPING
DAVID R. OLSON, PE
Things are different when you are over a mile above sea level. There’s less air at this altitude. The density of the atmosphere at sea level is 0.075 pounds per cubic foot. That means that every cubic foot of air weighs 0.075 pounds. In Denver, Colorado with an elevation of 5,280 feet above sea level, the air density is 0.0617 pounds per cubic foot. That means that there is only 82.3% of the air in Denver as there is at sea level locations. The air density in the Colorado mountains is even less. The altitude of Aspen, Colorado is 7,928 feet above sea level. Consequently, the air density there is just less than 0.0554 pounds per cubic foot, or about 73.9% of that found at sea level. The reduced air density experienced at high elevations does not just make it more difficult for individuals to breathe; it impacts the operation and performance of all sorts of mechanical equipment. This article will focus on the impacts of altitude on the sizing of natural gas piping.
Natural gas piping utilized in building construction projects is governed by the International Fuel Gas Code. This code provides sizing charts and formula’s applicable to sizing natural gas piping at various supply and delivery pressures. Currently, the State of Colorado and many individual building departments within Colorado are enforcing the 2012 IFGC when reviewing submittals for permitting for new or remodel construction involving natural gas piping systems. Chapter 4 of this code provides the requirements for sizing and installing natural gas piping systems.
At sea level, the btu content of natural gas is generally considered to be 1,000 btu/cubic foot of gas. The altitude above sea level alters the btu content of natural gas, similar to that of the surrounding atmosphere. In Denver, the natural gas provided by Xcel Energy has a btu content of 830 btu/cubic foot. Note that this btu content is proportional to the relative density of the air at elevation compared to that found at sea level. Therefore, at Aspen, the btu content of natural gas supplied to consumers would be 740 btu/cubic foot.
Section 402.2 of the 2012 IFGC includes the following language:
The volumetric flow rate of gas to be provided, in cubic feet per hour, shall be calculated using the manufacturer’s input ratings of the appliances served adjusted for altitude.
This means that a natural gas burning appliance installed in Denver will produce about 83% of the heat produced by the same appliance if installed at sea level. The btu input and output ratings from all gas burning appliance manufacturers are rated at sea level. For instance, a natural gas fired heating furnace that is rated at 100,000 btu/hour input by a manufacturer will actually operate at an derated input of 83,000 btu/hour at a mile high elevation such as that found in Denver, Colorado. The output of this same furnace can be determined by reducing the input rating by the percentage inefficiency which is published by the furnace manufacturer. A furnace that is 80% efficient at sea level will still be about 80% efficiency at altitude. Therefore in my example, the 100,000 btu/hour input furnace will deliver 80,000 btu/hour at sea level, but only 66,400 btu/hour at Denver’s altitude of 5,280 feet above sea level. This furnace burns just as much gas, at the same level of efficiency, but produces only 83% of the heat cataloged by the manufacturer. Similarly, if this same furnace were installed in Aspen, the heat output of the operating furnace would only equal 59,200 btu/hour.
It is very important to note and understand that despite the reduced input and output capacity of an appliance installed at an elevation high above sea level, the flow of gas through the appliance burner is the same as it would be at sea level – measured in cubic feet per hour. In other words, the 100,000 btu/hour furnace burns 100 cubic feet of gas per hour (100,000 btu/hour ÷ 1,000 btu/cubic foot). At altitude, the same 100,000 btu/hour furnace burns 100 cubic feet of gas per hour (100 cubic feet x 830 btu/cubic feet = 83,000 btu/hour). This firing rate is based upon the orifices installed by the manufacturer at the factory. Therefore, despite the gas fired appliance being derated for altitude, the rate of fuel delivery necessary to obtain the reduced amount of heat delivery equals the same amount necessary at sea level. Fuel piping systems must be sized based up natural gas delivery measured in cubic feet per hour.
It is possible to replace the gas orifices in a natural gas (or propane) appliance installed at high elevation to deliver the rate of heat identified on the appliance nameplate. It is simply a matter of removing the factory installed “sea level” orifice and installing an appropriate “high altitude” orifice in its place. In this manner, a higher quantity of gas is delivered to the burner(s) allowing the appliance to fire at the firing rate indicated on the appliance nameplate. For this reason, it is important to size the natural gas piping to deliver the nameplate rated heat output, regardless of the elevation of the installed appliance. Using our 100,000 btu/hour input furnace example, at mile high elevation, the natural gas required to deliver 100,000 btu/hour input would equal 100,000 btu/hour ÷ 830 btu/cubic foot = 120.5 cubic feet/hour. This means that the gas piping must be sized appropriately to deliver roughly 20% more natural gas at Denver altitude in order to deliver the same amount of heat that this same appliance would deliver at sea level.
It is my belief that the meaning of 2012 IFGC section 402.2 is intended to take the possibility of reorificing natural gas appliances into consideration. The language of the above referenced code section describes, the “volumetric” flow rate of gas should be based upon the manufacturers input rating of the appliance served “adjusted for altitude”. Admittedly, this code language is not entirely clear. Some may argue that the meaning of this section is the “volumetric” flow rate of gas based upon “the altitude adjusted manufacturer’s input ratings of the appliances served”. While this sizing would result in satisfactory derated gas supply to appliances knowingly delivering less heat at high altitude installations, the resultant gas pipe sizing would not be sufficient if and when the appliance were reorificed in order to deliver that actual nameplate rating output capacity. For this reason, I recommend that when sizing natural gas piping for a Denver, Colorado installation you should adjust the volumetric flow of gas anticipated upward by 20.5%. When sizing for elevations greater than 5,280 feet, adjust the fuel piping estimated volumetric flow using a multiplier consistent with the altitude of the actual installation. This insures a sufficient supply of natural gas if at some point in time an HVAC technician determines that a larger orifice is needed to deliver the rated output of heating. At worst, this sizing methodology results in a more conservative 20% safety factor for fuel pipe sizes if the orifice(s) remain factory sized for sea level operation.
The 2012 IFGC bases fuel pipe sizing on the equivalent length of the distance between the gas meter outlet and the connection to the farthest appliance. It is the designers choice to determine the equivalent length of the longest run and size all gas piping in the building based upon this length, or to determine the actual length of all individual branch piping runs and base the sizing of each branch on its equivalent length. In my experience as a designer and plans examiner, I find that most designers determine the equivalent length of the longest run and size all gas piping based upon these conditions.
The equivalent length method of determining gas piping length is a manner to estimate the impact of valves and fittings on the pressure drop experienced by the gas flowing within the piping system. To determine the equivalent length, take the actual estimated physical length of piping from the meter to the furthest appliance requiring gas. Then add up the number of fittings of each type, such as elbows, tees and ball valves (assumed to be open during flow conditions). Using the equivalent length factors included within 2012 IFGC Table A2.2, add the equivalent length of all fittings and valves to the actual furthest length to obtain an estimated equivalent length of the piping system.
The 2012 IFGC allows determination of piping sizes based upon either sizing tables or mathematical formulas. Natural gas is normally delivered from a gas meter at a delivery pressure of either 7” w.c. (1/4 psig), 14” w.c.(1/2 psig), 2 psig or 5 psig (note that 28” w.c. = 1 psig). The code includes tables for anticipated delivery pressures of “Less than 2 psig”, 2 psig, 3 psig and 5 psig. [Note that the “g” in “psig” stands for “gauge”. This is the pressure that is read off of a pressure gauge and it ignores the atmospheric pressure available at any site as opposed to “absolute pressure” (psia) which equals atmospheric pressure plus the elevated pressure within a piping system. All pressures indicated in the 2012 IFGC are identified as “psi”, but these are actually intended to be psig, not psia.] Pressures of 1.5 psig or less are considered “low pressure”, while pressures 2 psig or greater are “high pressure”. Unless certain conditions are met (see 2012 IFGC section 402.6), the code prohibits the use of natural gas in buildings at pressures greater than 5 psig. Most gas appliances used within buildings have factory regulators that require a minimum delivery pressure of 3.5” w.c. at the outlet of the regulator. Therefore, the pressure delivered to the appliance regulator must be sufficient to overcome the pressure drop through the regulator and maintain at least that 3.5” w.c. at the burner.
Gas piping systems within buildings can be reduced in diameter, and often cost, by using higher gas distribution pressures, and reducing the pressure at the appliance or at particular locations within a piping network. There are specific requirements included within the code for installation and venting of gas pressure regulators. Most residential and commercial gas appliances are limited to a maximum inlet pressure to the factory installed gas regulator included with the appliance. The maximum pressure is always indicated on the appliance nameplate. These pressures are typically 10” w.c. or 11” w.c. If the delivery gas pressure at an appliance exceeds this value, then an external gas pressure regulator is needed in addition to the built in factory regulator. If you plan to use a gas delivery pressure greater than ¼ psig (7” w.c.) be careful to check with the local gas provider to ascertain that gas delivery pressures exceeding 7” w.c. are actually available for consumer use. For example, there are some areas within MetroDenver that allow a maximum 7” w.c. delivery pressure. This is due to advanced age of the fuel piping delivering natural gas to certain communities. The gas company is concerned about leakage or failure with older piping, and consequently keeps the gas pressure down to ½ psig (“pounds low”).
I prefer to utilize the mathematical formulas given in 2012 IFGC for code compliant gas pipe sizing. Section 402.4 includes a formula for low pressure gas sizing (<1.5 psig) and a formula for high pressure gas sizing (>2 psig). The two formulas and descriptions of the variables are as follows:
Low pressure gas sizing:
D = Q^{0.381}/19.17(ΔH/C_{r} x L)^{0.206}
High pressure gas sizing:
D = Q^{0.381}/18.93[(P_{1}^{2 – }P_{2}^{2}) x Y/C_{r }x L]^{0.206}
where:
D = Inside diameter of pipe in inches
Q = Input rate of appliances in cubic feet per hour at 60°F and sea level atmospheric pressure (14.7 psia)
P_{1} = Absolute upstream pressure (psia) (14.7 + psig at sea level) [12.09 + psig for Denver]
P_{2} = Absolute downstream pressure (psia) (14.7 + psig at sea level) [12.09 + psig for Denver]
L = Equivalent length of pipe (longest run) in feet
ΔH = Pressure drop in inches of water column (“w.c.)
For natural gas sizing: For propane sizing:
C_{r} = 0.6094 C_{r} = 1.2462
Y = 0.9992 Y = 0.9910
By using the above formulas, I can quickly determine the required size of gas piping once the equivalent length is calculated, I know the supply gas pressure downstream of the gas meter and the required gas pressure at the appliances. I often use a value of 3” w.c. for ΔH in the low pressure formula when the meter pressure equals 7” w.c. Don’t forget to utilize the altitude adjusted value for “Q”.
Returning to the 100,000 btu/hour input furnace example, let’s assume that the equivalent length of proposed gas piping from the gas meter to the furnace equals 150 feet and the installation is in Denver. Filling in the appropriate quantities into the 2012 IFGC low pressure formula reveals:
D_{req’d} = (100,000/830)^{0.381}/19.17(3/150 x 0.6094)^{0.206} = 0.65”
Therefore, rounding up to the next standard pipe diameter, the required gas line would be ¾” diameter.
Using the same length and project location, but assuming a 2 psig meter delivery pressure, and the appropriate 2012 IFGC high pressure gas sizing formula determines:
D_{req’d} = (100,000/830)^{0.381}/18.93[(14.09^{2} – 12.34^{2}) x 0.9992/150 x 0.6094]^{0.206} = 0.38”
Again, rounding up to the next standard pipe diameter, the required gas line would be ½” diameter. This installation would require either a labeled gas regulator with an approved vent limiting device or a vented gas pressure regulator at the appliance. The required vent would need to be run full size to the building exterior, constructed with an approved gas piping material. In either case, the 2012 IFGC also requires a capped tee fitting to be installed immediately upstream and downstream of the gas pressure regulator (see 2012 IFGC section 410.2), and a full size shutoff valve to be installed immediately upstream of the regulator also (see 2012 IFGC section 409.4). Since the code usually requires a sediment trap upstream of the appliance gas connection as well (see 2012 IFGC section 408.4), the piping can be arranged so that this sediment trap serves as one of the capped tee fittings. These capped tees allow the operation of the gas pressure regulator to be tested. The code also requires an appliance shutoff valve within 6 feet of the appliance gas connection (see 2012 IFGC section 409.5) so if the piping and regulator are arranged appropriately, then the same gas valve can satisfy both the required shutoff for the regulator and for the appliance.
Note: This article is intended for the convenience, clarification and use by qualified individuals and plumbing designers only. All gas pipe sizing and installation must comply entirely with the governing edition of the International Fuel Gas Code adopted by the Authority Having Jurisdiction (AHJ) or other similar fuel gas codes adopted by the AHJ. David R. Olson, PE and Integrated Mechanical Systems, Inc. does not accept liability for gas pipe sizing or installation deficiencies which result from use of this article by less than fully qualified individuals and/or failure to follow the adopted building and fuel gas codes within any jurisdiction where a construction project including natural gas piping may occur. All fuel gas piping shall be designed under the strict guidance and responsible charge of a licensed professional engineer and fully licensed plumbing contractors in the jurisdiction of the project.