When a mechanical design engineer is given the job of drawing up piping plans for the distribution of natural gas in a building, the designer may be confronted by a few challenges. These may be monetary issues (choice between soldered or non-soldered piping) or even issues involving the optimization of new or existing piping to which natural gas appliances are to be added.
In this article, we will examine two possible solutions to the design challenges: provide for a loss of pressure a little greater than that normally permitted for piping; consider the coincidence factor of loads inÂ buildings.
As a follow-up to the Informa-TECH article of March, 2012, Volume 26, No. 1, which explained the methods of calculation according to CodeÂ B149.1 (Natural Gas and Propane Installation Code), we will look in more detail at the latitude permitted under the Code.
The article described the approaches to sizing piping with the help of various tables in Annex A of CodeÂ B149.1, as well as the method for calculating high and low pressure according to the equations in ClauseÂ A.3.5.
Essentially, the total installed capacity of the appliances (in Btu/h or kW) and the distance between the GazÂ MÃ©tro meter and the applications determine the sizing of the piping. The permitted loss of pressure is also an important factor to be considered.
In the case of the following pressures, Code B149.1 determines the maximum allowable pressure drops:
|Piping supply pressure||Maximum admissible pressure drop|
|Less than 7 in w.c. (1.75 kPa)||0.5 in w.c. (0.125 kPa)|
|from 7 in w.c. (1.75 kPa) to 14 in w.c. (3.5 kPa)||1 in w.c. (0.25 kPa)|
(Code B149.1, Table 6.1 and Clause 6.3.2)
According to the tables in the Code, for other supply pressures, a pressure loss of 50% from the initial pressure is permitted, except for Table A.4, where the permitted pressure loss is 75% (1.5Â psi) for supply at 2 psig.
Still based on the tables, is it possible that a pressure loss greater than the 50% recommended might be required? In some cases, it is. The use of the equations in the Code proposes this solution, which thus facilitates optimizing the natural gas network. However, it must be remembered that the principal design criteria the designer must take into account in order to ensure a reliable distribution network are the following:
- allowable pressure drop at the appliance connection;
- maximum natural gas demand (in Btu/h or kW);
- length of piping and number of connections;
- relative density of natural gas (0.6);
- flow speed in the piping;
- diversity (or coincidence) factor.
Diversity or coincidence factor
What does diversity factor mean? It is a measure of the probability that a particular appliance will be in operation at the same time (coincidentally) as another appliance. Thus, for a diversity factor of 100%, all of the same types of appliances are running at the same time at full capacity. For a diversity factor below 100%, the appliances are not operating at full capacity at the same time, which will thus reduce peak demand on the distribution circuit and so the piping can be of a smaller diameter.
For example, the QuÃ©bec Electrical Code requires the following diversity factors for space heating (Section 62-116)Â :
The diversity factor will be 100% for Power P less than or equal to 10 kW + 75% for power above 10kW (if the heating appliances have an automatic thermostatic control); if not, 100% of the heating load.1
Another example comes from Washington Gas, operating under meteorological conditions different from those in QuÃ©bec; these diversity factors are used for designing a building connection. (Table 8351.2)
|Number of apartments||Diversity factor for stove||Diversity factor for individual water heater||Diversity factor for individual furnace|
|5 – 9||0.44||0.44||0.90|
|10 – 14||0.34||0.34||0.85|
|15 – 24||0.30||0.30||0.83|
|25 – 99||0.27||0.24||0.80|
|100 – 149||0.20||0.18||0.75|
|150 – 199||0.18||0.17||0.70|
|200 and above||0.18||0.16||0.65|
Source: Washington Gas, Engineering and Operating Standards, 2009.
As can be seen, the higher the number of apartments, the lower the diversity factor.
In QuÃ©bec, since a diversity factor is not defined, Code B149.1 allows the designer to establish one and to submit it to the RÃ©gie du bÃ¢timent du QuÃ©bec for approval. ClauseÂ 6.4.3 indicates that if such a diversity factor meets with the approval of that authority, a percentage of the total daily natural gas volume may be used to determine the diameter of the piping.
A natural gas distribution circuit can thus be optimized by using the equations to determine the desired pressure loss, while taking into account the diversity factors inherent in the operation of a building.
Marc Beauchemin, Eng.
1. Editorâs translation