Pressure Drop in a Long Gas Run
You are installing a new gas supply to a detached house. The meter is at the front of the property, and the boiler is at the rear. The total equivalent length of the pipe run, including fittings, is 25 m. The boiler is rated at 24 kW. The pipe will be run in 22 mm copper tube. The gas is natural gas (NG) with a calorific value of 38.7 MJ/m³ and a relative density of 0.6. The meter outlet pressure is 21 mbar. The boiler requires a minimum inlet pressure of 18 mbar to operate correctly. The customer wants to know if the 22 mm pipe will be sufficient. You have a copy of the relevant pressure drop tables. For NG, the pressure drop per metre for 22 mm copper at a flow rate of 2.5 m³/h is approximately 0.05 mbar/m. The flow rate for a 24 kW boiler is calculated as: (24 kW × 3600 s/h) / (38.7 MJ/m³ × 1000 kJ/MJ) = 2.23 m³/h. The pressure drop for 25 m would be 25 × 0.05 = 1.25 mbar. The meter outlet pressure is 21 mbar, so the pressure at the boiler would be 21 - 1.25 = 19.75 mbar, which is above 18 mbar. However, you must also consider the pressure drop across the meter and any other appliances. The house also has a gas hob rated at 8 kW. The total load is 32 kW. The flow rate for 32 kW is 2.98 m³/h. The pressure drop for 22 mm at 2.98 m³/h is higher, perhaps 0.08 mbar/m, giving 2.0 mbar drop, resulting in 19 mbar at the boiler, still above 18 mbar. But you need to check the tables for the actual flow rate. The pipe run may have additional fittings not accounted for. The customer is concerned about the boiler not working.
1. Assess whether the 22 mm copper pipe is adequate for the total gas load, considering pressure drop. Show your calculations and state any assumptions.
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First, calculate the total gas flow rate. Total heat input = 24 kW (boiler) + 8 kW (hob) = 32 kW. Using NG calorific value of 38.7 MJ/m³ (which is 38,700 kJ/m³), flow rate Q = (32 kW × 3600 s/h) / 38,700 kJ/m³ = 2.98 m³/h. Next, determine the pressure drop for 22 mm copper tube at this flow rate. From standard tables, for NG (relative density 0.6), the pressure drop per metre for 22 mm copper at 3 m³/h is approximately 0.08 mbar/m (this is an estimate; actual tables may vary). For a 25 m equivalent length, pressure drop = 25 × 0.08 = 2.0 mbar. The meter outlet pressure is 21 mbar. The pressure at the boiler would be 21 - 2.0 = 19 mbar. The boiler requires 18 mbar minimum, so 19 mbar is acceptable. However, we must also consider the pressure drop across the meter (typically 1 mbar) and the allowance for the gas hob (which may have its own pressure drop). The total pressure drop from meter to boiler should not exceed 21 - 18 = 3 mbar. Our calculated drop is 2.0 mbar, leaving 1 mbar for meter and other losses, which may be tight. Also, the equivalent length may be underestimated if there are more fittings. Therefore, 22 mm may be marginal. It would be safer to use 28 mm pipe for the main run, especially if the boiler is sensitive. So the answer: 22 mm is likely adequate but marginal; recommend upgrading to 28 mm to ensure reliable operation. Assumptions: pressure drop figures from tables, no additional appliances, meter pressure stable.
2. If the boiler fails to operate due to low pressure, what steps would you take to diagnose and rectify the problem?
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If the boiler fails to operate, first check the inlet pressure at the boiler test point using a manometer. If it is below 18 mbar, confirm the pressure at the meter (should be 21 mbar). If the meter pressure is correct but boiler pressure is low, the pressure drop in the pipe is excessive. Check for blockages, partially closed valves, or incorrect pipe sizing. Also check the gas rate at the meter to ensure it matches the appliance demand. If the pipe is undersized, the solution is to replace the pipe with a larger diameter (e.g., 28 mm) or reduce the load (e.g., by not using the hob simultaneously). Additionally, check the meter governor; if it is faulty, it may not maintain pressure. If the pressure drop is due to additional fittings not accounted for, recalculate the equivalent length. After rectification, re-test the pressure and gas rate. Ensure all work is compliant with regulations.