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A.3 Use of Capacity Tables
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A.3.1 Longest Length Method
This sizing method is conservative
in its approach by applying the maximum operating
conditions in the system as the norm for the system and by
setting the length of pipe used to size any given part of the
piping system to the maximum value.
To determine the size of each section of gas piping in a system within the range of the capacity tables, proceed as follows (also see sample calculations included in this Appendix):
Where a large number of piping components (such as elbows, tees and valves) are installed in a pipe run, additional pressure loss can be accounted for by the use of equivalent lengths. Pressure loss across any piping component can be equated to the pressure drop through a length of pipe. The equivalent length of a combination of only four elbows/tees can result in a jump to the next larger length row, resulting in a significant reduction in capacity. The equivalent lengths in feet shown in Table A.2.2 have been computed on a basis that the inside diameter corresponds to that of Schedule 40 (standard- weight) steel pipe, which is close enough for most purposes involving other schedules of pipe. Where a more specific solution for equivalent length is desired, this can be made by multiplying the actual inside diameter of the pipe in inches by n/12, or the actual inside diameter in feet by n (n can be read from the table heading). The equivalent length values can be used with reasonable accuracy for copper or brass fittings and bends although the resistance per foot of copper or brass pipe is less than that of steel. For copper or brass valves, however, the equivalent length of pipe should be taken as 45 percent longer than the values in the table, which are for steel pipe.
To determine the size of each section of gas piping in a system within the range of the capacity tables, proceed as follows (also see sample calculations included in this Appendix):
- Divide the piping system into appropriate segments consistent with the presence of tees, branch lines and main runs. For each segment, determine the gas load (assuming all appliances operate simultaneously) and its overall length. An allowance (in equivalent length of pipe) as determined from Table A.2.2 shall be considered for piping segments that include four or more fittings.
- Determine the gas demand of each appliance to be attached to the piping system. Where Tables 402.4(1) through 402.4(24) are to be used to select the piping size, calculate the gas demand in terms of cubic feet per hour for each piping system outlet. Where Tables 402.4(25) through 402.4(37) are to be used to select the piping size, calculate the gas demand in terms of thousands of Btu per hour for each piping system outlet.
- Where the piping system is for use with other than undiluted liquefied petroleum gases, determine the design system pressure, the allowable loss in pressure (pressure drop), and specific gravity of the gas to be used in the piping system.
- Determine the length of piping from the point of delivery to the most remote outlet in the building/ piping system.
- In the appropriate capacity table, select the row showing the measured length or the next longer length if the table does not give the exact length. This is the only length used in determining the size of any section of gas piping. If the gravity factor is to be applied, the values in the selected row of the table are multiplied by the appropriate multiplier from Table A.2.4.
- Use this horizontal row to locate ALL gas demand figures for this particular system of piping.
- Starting at the most remote outlet, find the gas demand for that outlet in the horizontal row just selected. If the exact figure of demand is not shown, choose the next larger figure left in the row.
- Opposite this demand figure, in the first row at the top, the correct size of gas piping will be found.
- Proceed in a similar manner for each outlet and each section of gas piping. For each section of piping, determine the total gas demand supplied by that section.
Where a large number of piping components (such as elbows, tees and valves) are installed in a pipe run, additional pressure loss can be accounted for by the use of equivalent lengths. Pressure loss across any piping component can be equated to the pressure drop through a length of pipe. The equivalent length of a combination of only four elbows/tees can result in a jump to the next larger length row, resulting in a significant reduction in capacity. The equivalent lengths in feet shown in Table A.2.2 have been computed on a basis that the inside diameter corresponds to that of Schedule 40 (standard- weight) steel pipe, which is close enough for most purposes involving other schedules of pipe. Where a more specific solution for equivalent length is desired, this can be made by multiplying the actual inside diameter of the pipe in inches by n/12, or the actual inside diameter in feet by n (n can be read from the table heading). The equivalent length values can be used with reasonable accuracy for copper or brass fittings and bends although the resistance per foot of copper or brass pipe is less than that of steel. For copper or brass valves, however, the equivalent length of pipe should be taken as 45 percent longer than the values in the table, which are for steel pipe.
A.3.2 Branch Length Method
This sizing method reduces
the amount of conservatism built into the traditional Longest
Length Method. The longest length as measured from the
meter to the furthest remote appliance is only used to size the
initial parts of the overall piping system. The Branch Length
Method is applied in the following manner:
- Determine the gas load for each of the connected appliances.
- Starting from the meter, divide the piping system into a number of connected segments, and determine the length and amount of gas that each segment would carry assuming that all appliances were operated simultaneously. An allowance (in equivalent length of pipe) as determined from Table A.2.2 should be considered for piping segments that include four or more fittings.
- Determine the distance from the outlet of the gas meter to the appliance furthest removed from the meter.
- Using the longest distance (found in Step 3), size each piping segment from the meter to the most remote appliance outlet.
- For each of these piping segments, use the longest length and the calculated gas load for all of the connected appliances for the segment and begin the sizing process in Steps 6 through 8.
- Referring to the appropriate sizing table (based on operating conditions and piping material), find the longest length distance in the first column or the next larger distance if the exact distance is not listed. The use of alternative operating pressures and/or pressure drops will require the use of a different sizing table, but will not alter the sizing methodology. In many cases, the use of alternative operating pressures and/or pressure drops will require the approval of both the code official and the local gas serving utility.
- Trace across this row until the gas load is found or the closest larger capacity if the exact capacity is not listed.
- Read up the table column and select the appropriate pipe size in the top row. Repeat Steps 6, 7 and 8 for each pipe segment in the longest run.
- Size each remaining section of branch piping not previously sized by measuring the distance from the gas meter location to the most remote outlet in that branch, using the gas load of attached appliances and following the procedures of Steps 2 through 8.
A.3.3 Hybrid Pressure Method
The sizing of a 2 psi (13.8
kPa) gas piping system is performed using the traditional
Longest Length Method but with modifications. The 2 psi
(13.8 kPa) system consists of two independent pressure
zones, and each zone is sized separately. The Hybrid Pressure
Method is applied as follows:
The sizing of the 2 psi (13.8 kPa) section (from the meter to the line regulator) is as follows:
The sizing of the 2 psi (13.8 kPa) section (from the meter to the line regulator) is as follows:
- Calculate the gas load (by adding up the name plate ratings) from all connected appliances. (In certain circumstances the installed gas load can be increased up to 50 percent to accommodate future addition of appliances.) Ensure that the line regulator capacity is adequate for the calculated gas load and that the required pressure drop (across the regulator) for that capacity does not exceed ^{3}/_{4} psi (5.2 kPa) for a 2 psi (13.8 kPa) system. If the pressure drop across the regulator is too high (for the connected gas load), select a larger regulator.
- Measure the distance from the meter to the line regulator located inside the building.
- If there are multiple line regulators, measure the distance from the meter to the regulator furthest removed from the meter.
- The maximum allowable pressure drop for the 2 psi (13.8 kPa) section is 1 psi (6.9 kPa).
- Referring to the appropriate sizing table (based on piping material) for 2 psi (13.8 kPa) systems with a 1 psi (6.9 kPa) pressure drop, find this distance in the first column, or the closest larger distance if the exact distance is not listed.
- Trace across this row until the gas load is found or the closest larger capacity if the exact capacity is not listed.
- Read up the table column to the top row and select the appropriate pipe size.
- If there are multiple regulators in this portion of the piping system, each line segment must be sized for its actual gas load, but using the longest length previously determined above.
- Determine the gas load for each of the connected appliances.
- Starting from the line regulator, divide the piping system into a number of connected segments or independent parallel piping segments, and determine the amount of gas that each segment would carry assuming that all appliances were operated simultaneously. An allowance (in equivalent length of pipe) as determined from Table A.2.2 should be considered for piping segments that include four or more fittings.
- For each piping segment, use the actual length or
longest length (if there are sub-branchlines) and the
calculated gas load for that segment and begin the
sizing process as follows:
- Referring to the appropriate sizing table (based on operating pressure and piping material), find the longest length distance in the first column or the closest larger distance if the exact distance is not listed. The use of alternative operating pressures and/or pressure drops will require the use of a different sizing table, but will not alter the sizing methodology. In many cases, the use of alternative operating pressures and/or pressure drops can require the approval of the code official.
- Trace across this row until the appliance gas load is found or the closest larger capacity if the exact capacity is not listed.
- Read up the table column to the top row and select the appropriate pipe size.
- Repeat this process for each segment of the piping system.
A.3.4 Pressure Drop Per 100 Feet Method
This sizing method is less conservative than the others, but it allows the designer to immediately see where the largest pressure drop occurs in the system. With this information, modifications can be made to bring the total drop to the critical appliance within the limitations that are presented to the designer.
Follow the procedures described in the Longest Length Method for Steps (1) through (4) and (9).
For each piping segment, calculate the pressure drop based on pipe size, length as a percentage of 100 feet (30 480 mm) and gas flow. Table A.3.4 shows pressure drop per 100 feet (30 480 mm) for pipe sizes from ^{1}/_{2} inch (12.7 mm) through 2 inches (51 mm). The sum of pressure drops to the critical appliance is subtracted from the supply pressure to verify that sufficient pressure will be available. If not, the layout can be examined to find the high drop section(s) and sizing selections modified.
Note: Other values can be obtained by using the following equation:
For example, if it is desired to get flow through ^{3}/_{4}-inch (19.1 mm) pipe at 2 inches/100 feet, multiply the capacity of ^{3}/_{4}-inch (19.1 mm) pipe at 1 inch/100 feet by the square root of the pressure ratio:
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm.
Follow the procedures described in the Longest Length Method for Steps (1) through (4) and (9).
For each piping segment, calculate the pressure drop based on pipe size, length as a percentage of 100 feet (30 480 mm) and gas flow. Table A.3.4 shows pressure drop per 100 feet (30 480 mm) for pipe sizes from ^{1}/_{2} inch (12.7 mm) through 2 inches (51 mm). The sum of pressure drops to the critical appliance is subtracted from the supply pressure to verify that sufficient pressure will be available. If not, the layout can be examined to find the high drop section(s) and sizing selections modified.
Note: Other values can be obtained by using the following equation:
For example, if it is desired to get flow through ^{3}/_{4}-inch (19.1 mm) pipe at 2 inches/100 feet, multiply the capacity of ^{3}/_{4}-inch (19.1 mm) pipe at 1 inch/100 feet by the square root of the pressure ratio:
PRESSURE DROP PER 100 FEET IN INCHES W.C |
PIPE SIZES (inch) | |||||
^{1}/_{2} | ^{3}/_{4} | 1 | 1^{1}/_{4} | 1^{1}/_{2} | 2 | |
0.2 | 31 | 64 | 121 | 248 | 372 | 716 |
0.3 | 38 | 79 | 148 | 304 | 455 | 877 |
0.5 | 50 | 104 | 195 | 400 | 600 | 1160 |
1.0 | 71 | 147 | 276 | 566 | 848 | 1640 |
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