Natural Gas Piping Pressure Drop After Appliance Addition
A restaurant adds a 200,000 BTU/hr combi oven to an existing natural gas system. The service regulator delivers 2 psig to a 1-inch Schedule 40 steel header that runs 80 ft to a second-stage regulator at the kitchen, then 1/2-inch pipe 25 ft to the appliances. Existing load is 350,000 BTU/hr. The new total is 550,000 BTU/hr. At full load, the manifold pressure at the farthest appliance drops to 3.2 in w.c. with all appliances running; the appliance nameplate requires 4.0 in w.c. minimum. The utility gas has a heating value of 1,000 BTU/ft³ and specific gravity 0.60. The installer suggests merely adjusting the second-stage regulator to 5.0 in w.c. You must decide whether that is acceptable and what the correct solution is.
1. Is adjusting the second-stage regulator to 5.0 in w.c. an acceptable solution? Explain why or why not.
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No. The 3.2 in w.c. at the farthest appliance is a downstream pressure-drop problem, not a regulator setpoint problem. The 1/2-in line is carrying the entire 550,000 BTU/hr load for 25 ft; at that load the velocity and friction loss are far beyond what a 1/2-in line can handle at low pressure. Raising the second-stage regulator to 5.0 in w.c. may temporarily raise the reading but will not restore 4.0 in w.c. at full load, and it may exceed the appliance nameplate maximum, which is often 5.0 in w.c. or less. It can also cause regulator lockup and erratic burner operation and masks an unsafe condition. The correct fix is to upsize the downstream piping and verify regulator capacity.
2. What calculations or checks would you perform to size the replacement piping?
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Use NFPA 54 or CSA B149.1 sizing tables for natural gas at 0.60 specific gravity and 1,000 BTU/ft³. The total load is 550,000 BTU/hr. The available drop from the regulator outlet to the appliance manifold is from about 7 in w.c. down to the 4.0 in w.c. minimum, so allow about 3 in w.c. Check the 1-in header at 2 psig for 550,000 BTU/hr over 80 ft; it is likely adequate, but confirm with the table. Then size the 25-ft branch for a 3 in w.c. drop. A 1/2-in branch will be far too small; 3/4 in or 1 in will be required depending on the table. Also check the second-stage regulator capacity at 2 psig inlet and 7 in w.c. outlet, and check the appliance connector and manifold size.
3. What code and safety requirements apply to the repair?
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The repair must be done by a licensed gas fitter under permit, with the gas isolated and purged. Requirements include approved pipe and fittings, proper support, a regulator vent that terminates outdoors and is protected, maintained bonding and grounding, and a leak test of the new piping at the required pressure. After restart, measure manifold pressure at each appliance with all appliances running, verify combustion air and venting, and check for carbon monoxide. Do not rely on a single gauge at the regulator; test at the farthest appliance.