Volume 26, number 1, March 2012

Sizing natural gas piping

During the design of a new facility in which several natural gas appliances were to be installed, one of the difficulties encountered was sizing the natural gas piping. This involved achieving a balance between the pressure at the entrance to the building, the flow, the pressure required for the appliances, and the cost of the piping. There are several methods of calculation (including Annexes to Code B149.1 and software). In order to simplify the decision-making process, the principal reference will be Annex A of Code B149.1.

When calculating the natural gas piping, several important parameters have to be taken into account, including the pressure of the natural gas available, the loads to be connected, the applicable standards, the type of material used, the length of piping required, and the routing (number of fittings).

Natural gas piping

The first objective is to be able to supply natural gas where it is needed. The piping must be of a sufficient size to provide a supply of gas to meet the volume and pressure requirements at the point of use. (Clause 6.3.1)

Choice of material

All-purpose steel piping can be found in residential, commercial and industrial projects. According to Clause 6.2.3, it must be at least Schedule 40. If the operating pressure exceeds 125 psi (860 kPa), or if the threaded piping is below NPS 1/2 (Clause 6.3.8), the piping must be at least Schedule 80. Copper tubing is often used in the residential market and, according to Clause 6.2.4, it must be Type G, K or L. Corrugated stainless steel tubing, according to Clause 6.2.20, must comply with CSA 6.26 or with the CSA document “CGA Certification Laboratory Requirement LAB-009.” This type of tubing may not be used for appliance connections (Clause 6.2.21). Lastly, if plastic piping is used, Clause 6.2.13 stipulates that it must comply with CSA-B137.4. Plastic piping may only be used underground and outside buildings. (Clauses 6.2.16, 6.2.19)

Sizing of piping based on material chosen

Clause 6.3.2 tells us which Annex A table to consult to easily select the minimum diameter of piping based on the pressure required, the allowable pressure drop, and the type of material used.
Note: For pressures up to 2 psi (14 kPa), an allowance of 20 % for fittings loss is included in the corresponding tables. For higher pressures, data in Table A16 has to be taken into account.

Tables A1 and A8 are used for a supply pressure of less than 7 in w.c. (1.75 kPa) and a maximum allowable pressure drop of 0.5 in w.c.; Tables A2 and A9 are used for a supply pressure between 7–14 in w.c. (1.75–3.50 kPa) and a maximum allowable pressure drop of 1 in w.c.; Tables A3 and A4, and Tables A10 and A11 are used for a supply pressure of 2 psi (14 kPa); Tables A5–A7, Tables A12–A14 and Table A16 (fittings losses) are used for supply pressures of 5 psi (34 kPa), 10 psi (70 kPa), and 20 psi (140 kPa), respectively.

Calculation method

All the calculations for the values indicated in the tables are taken from the formulas in Annex A. (Clause A3.5)

Tables A1–A4 and Tables A8–A11 take into account a factor of 1.2 (20 % is allowed) for a reasonable number of bends and fittings. Tables A5–A7 and Tables A12–A14 do not include any correction factor for bends and fittings and use a factor of 1.0. Table A16 gives the equivalent lengths. The relative density used for natural gas is 0.6. If another relative density is required, the multipliers in Table A15 can be applied to make the necessary corrections.

Annex E gives a detailed example of how to calculate the sizing of piping using the tables. A certain methodology is recommended to make the calculations easier. For example, a survey needs to be made of all the appliances and the distances (installation diagram). Then the pressures and regulators need to be specified. The longest route taken by the natural gas has to be determined for each pressure. Then the established route is sub-divided, based on the loads at each branch. Use the table that allows you to determine the diameters (Annexes A and E of Code B149.1). Lastly, the diameter of the piping can be determined for each section.

Richard Meunier, Eng.