Low-Pressure Steam Main Water Hammer Diagnosis
You are the lead steamfitter called to a 1960s low-pressure steam heating system serving a 6-story apartment building. Tenants on the top floors report loud banging at morning start-up, followed by uneven heat. The boiler holds 5 psig. The main is 3-inch Schedule 40, 180 ft long, pitched 1/8 in/ft toward a single F&T trap at the far end. A contractor recently replaced two radiator traps but did not touch the main trap. You measure 210°F at the main trap inlet and 195°F at its outlet; the trap cycles every 45 seconds. A sight glass on the condensate receiver shows the pump running continuously and the receiver level dropping. The maintenance log notes the banging began after a boiler reset to 5 psig from 2 psig. Identify likely causes and the order of checks.
1. List the probable causes of the water hammer in order of likelihood, and state the specific check you would use to confirm or rule out each.
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Start with the main trap because the temperature readings are the strongest clue. An F&T trap that passes steam will show an outlet temperature close to the inlet (195°F versus 210°F) and will cycle frequently as steam reaches the receiver; a normal trap discharges cooler condensate and cycles less often. Confirm by ultrasonic listening, a trap tester, or isolating the trap and watching the receiver level. Next, check the 5 psig reset: higher pressure raises condensate temperature and flash steam, and the trap may be undersized at 5 psig. Compare trap capacity at 5 psig with the connected load. Third, survey main pitch and sag; 1/8 in/ft is a minimum, and any low spot traps water. Fourth, check radiator traps and the receiver level control; continuous pump running with falling level suggests the pump is moving more condensate than the trap returns, consistent with steam blow-through. The order is main trap first, then pressure, then pitch, then branch traps.
2. What immediate action and repair sequence do you recommend before restoring normal operation?
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Immediate action: reduce boiler pressure to the original 2 psig and, if banging persists, isolate the main trap and use manual bypass only long enough to drain the main. Do not leave a bypass open. Repair sequence: lock out the boiler; remove and inspect the main F&T trap and strainer; replace the trap with a same-size or larger-capacity unit rated for 5 psig; verify main pitch with a level and correct any sag; check receiver level control and pump strainer; restart at 2 psig, raise pressure slowly, and observe. If the trap body is sound, a repair kit may be acceptable, but given the age and symptoms, replacement is more reliable. Record trap model, orifice size, and operating pressure.
3. How would you verify the repair and what preventive maintenance would you set?
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After restart, measure trap inlet and outlet temperatures; the outlet should be well below steam temperature and the cycle should be intermittent. Confirm the receiver level is stable and the pump cycles rather than runs continuously. Listen for banging through at least three morning warm-up cycles. An ultrasonic check should show intermittent discharge, not continuous steam flow. Preventive maintenance should include an annual trap survey with ultrasonic and temperature readings, a boiler pressure log, inspection of main pitch and supports, and scheduled replacement of radiator traps. Also verify that any future boiler pressure reset is coordinated with trap capacity.