HVACR116 – Trade Skills

Pipe SizingSizing Gas Pipe for Low-Pressure

Systems

1) The purpose of the gas piping system is to distribute the gas to the manifold at the correct pressure.a) The gas must reach the appliance regulator with a pressure approximately 6” w. c.

b) At this pressure the regulator will be able to property control the gas pressure at the outlet.c) Any pressure drop below 6 inches w. c. will cause fluctuations in the manifold pressure.

d) The size of the gas pipe distribution system determines the pressure drop.e) If the pipe is sized too small, the pressure drop will be too great.

f) The gas may leave the meter at 7 inches w. c., but would be below the 6 inch w. c. requirement when the gas reaches the appliance regulator.g) If the gas pipe is sized too large, the system would not be economical.

2) The National Fuel Gas Code for gas pipe sizing for low-pressure natural and LP gas is used.a) The following factors must be considered in sizing pipe for gas piping systems.

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Gas Pipe Sizing

• Allowable loss in pressure from point of delivery to equipment.

• Maximum gas demand.• Length of piping and number of fittings.• Diversity factor.

Gas Pipe Sizing

4) The allowable loss in pressure is usually 0.3 inches w. c., but pipe can be sized for 0.5 inches w. c. pressure loss.a) The length of pipe is the distance from the meter to farthest appliance.b) This will be measured in feet of pipe.

Gas Pipe Sizing

c) The pressure drop in the pipe fittings are expressed in equivalent length of pipe.d) Equivalent length of pipe expresses the pressure drop of a fitting as though it were a length of pipe (Fig.9-26).

Gas Pipe Sizing

• EXAMPLE #1 – If you wanted to know what the pressure drop is in a ¾” threaded 90 degree elbow.

a) Refer to figure 9-26, and find the entry for ¾” in the left had column.b) Move to the right and find the entry for a 90-degree screwed elbow.

Gas Pipe Sizing

c) The pressure drop is equivalent to 2.06 feet of pipe.d) This amount can be added to the length of pipe in the system for pipe sizing purposes.e) Valves are also expressed in equivalent feet of pipe.

Gas Pipe Sizing

• Example #2: The diversity factor in sizing pipes involves specific gravity of the gas to be moved.

a) The specific gravity of the gas is important if the gases, other than those in the table are used.b) The heavier the gas, less gas will move in a pipe size.

Gas Pipe Sizing

• EXAMPLE #3(Fig.9-27) shows two tables for sizing natural gas piping.a) One of the tables is for a pressure drop of 0.3 inches w. c. b) The other table is for 0.5 inches of w. c.

Gas Pipe Sizing

• Example #4 : how to determine the gas demand of each appliance.

a) Determine the gas demand of each appliance to be connected to the piping system.b) Calculate the gas demand in terms of cfh (cubic feet per hour) for each piping system outlet.

Gas Pipe Sizing

c) For natural gas it is sufficient to use the appliance input divided by 1,000 btu’s to determine the cfh.d) If a furnace has a input of 120,000 btu per hour, this furnace would use 120 cfh.e) 120,000 / 1,000 = 120 cfh

Gas Pipe Sizing

• Example #5 : Determine;a) The design system pressure.b) The specific gravity of the gas.c) The allowable pressure loss.

Gas Pipe Sizing

d) The design system pressure is usually 7 inches w. c.e) The specific gravity of natural gas is 0.6f) The allowable pressure loss is 0.3 or 0.5 inches of w. c. (0.5 will be used).

Gas Pipe Sizing

• Example #6 : Measuring pipinga) Measure the length of pipe, from the point of delivery (gas meter).b) To the most remote outlet in the building (furthest point).

Gas Pipe Sizing

c) If the system has more fittings than usual us the chart in Fig.9-26.d) A typical system with some elbows and tees and a valve does not require adding the fittings to the pipe length.e) Because we round up appliance capacities and pipe sizes.

Gas Pipe Sizing

• Example #7 – Using Capacity Tablea) Select the column showing the measured length.b) Or the next longer length, (if the table does not give the exact length).

Gas Pipe Sizing

c) Use the vertical column to locate all gas-demand figures for the piping system.d) Starting at the most remote outlet, find the vertical column just for the gas demand for that outlet.

Gas Pipe Sizing

e) If the exact figure of demand is not shown, choose the next larger figure in the column.f) Across from the gas demand figure, in the first column at the left, find the correct size of gas piping.

Gas Pipe Sizing

Example #8 – Pipe sizing using Fig.9-29.a) The house has four appliances.

Gas Pipe Sizing

• Cooking range: 65,000 btu/hr input.• Dryer: 35,000 btu/hr input.• Furnace: 120,000 btu/hr input.• Water heater (50 gallon): 55,000 btu/hr input.

Gas Pipe Sizing

b) Change to CFH;• 65,000/1,000 = 65 cfh• 35,000/1,000 = 35 cfh• 120,000/1,000 = 120 cfh• 55,000/1,000 = 55 cfh

Gas Pipe Sizing

c) Start by measuring the longest run of piping.d) Because there is no excessive use of fittings in this piping, the fittings are disregarded.

Gas Pipe Sizing

e) The longest run of piping is to the water heater (65 ft.).f) Because we will round up, we will use the 0.5 inches of w. c. table for natural gas (0.6 specific gravity).g) From figure 9-27 find the column for 70 ft. (this is rounded up from 65 ft.).

Gas Pipe Sizing

• Note: In the remainder of this problem, the column for 70 feet will be used.

a) Always use the longest run of pipe for the complete problem.

Gas Pipe Sizing

• Example #9 – Follow these steps to determine the branch line pipe size, from the main gas line supplying all appliances.

Gas Pipe Sizing

1) The cooking range has a capacity of 65,000 btu/hr. and consumes 65 cfh.a) Find the column for 70 ft. and move down to 65 cfh, or the next highest value.

Gas Pipe Sizing

b) A value of 61 cfh is shown but is not enough.c) Round up to 125 cfh, and the branch line pipe size of ¾” is found.

Gas Pipe Sizing

2) The dryer uses 35 cfh.a) Move down the column for 70 ft.b) Find 35 cfh or the next highest value which is 61 cfh.c) At 61 cfh the branch pipe line size should be ½”.

Gas Pipe Sizing

3) The furnace uses 120 cfh.a) Move down the column for 70 ft. to 120 cfh or the next highest value.b) We would find 125 cfh as the need value.c) The branch line pipe size is 3/4 inch.

Gas Pipe Sizing

4) The water heater uses 55 cfh.a) Move down the column for 70 feet, to 55 cfh or the next highest number.b) We would find 61 as the needed value.c) The branch pipe size is ½ inch.

Gas Pipe Sizing

1) There are three main lines that serve more than one appliance.a) The same pipe length should be used for each appliance.b) The total gas carrying capacity for each will be used.

Gas Pipe Sizing

• Example #10: 1) The main serving the furnace and dryer must carry 155 cfh of gas.a) Move down the column for 70 ft. to 155 cfh or the next largest number.b) This number is 240 cfh, the pipe size is 1”

Gas Pipe Sizing

2) The main that serves outlets #2, #3, #4 must carry 210 cfh.a) It must have the capacity to carry gas to all three appliances.b) Go to the column for 70 ft. locate 210 cfh or the next highest number, the size needed is 1 inch.

3) The main that carries gas from the meter to all of the appliances must carry 275 cfh.a) Look down the column for 70 ft.b) Find 275 cfh or the next highest number.c) This pipe must be 1 ¼”.