Pump-Down Before Opening a Strainer
Maintenance must clean the strainer on the liquid line feeding a freezer evaporator. The line normally runs at 2.0 barg suction-side pressure with the plant at 60% load. The technician proposes to close the hand valves either side of the strainer, crack the strainer cover, and 'let the little bit of ammonia out' while wearing safety glasses and nitrile gloves. The nearest eyewash is 30 m away down a corridor. The machine room ammonia detector is functioning. The technician has no respirator and no second person present.
1. Evaluate the technician's plan against ammonia safety practice and state what must change.
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The plan is not acceptable. Cracking a strainer on a line containing ammonia and 'letting it out' is a controlled release done with no respiratory protection, no second person, and eye protection that is inadequate (safety glasses and nitrile gloves do not protect against an ammonia spray or vapour). The eyewash 30 m away is too far for an eye exposure.
What must change: isolate and pump down or evacuate the section so it is at or near atmospheric before opening; verify isolation (double block and bleed or blind); wear the PPE the site procedure requires, including chemical splash goggles and face shield, and respiratory protection if residual ammonia is possible; have a second person present; and stage an eyewash or water supply at the work point. Only then open the strainer.
2. Why is 'a little bit of ammonia' a poor assumption on a suction-side line, and how would you verify the section is safe to open?
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A suction-side liquid line at 2.0 barg holds liquid ammonia that flashes to vapour as pressure drops, so a small crack can release far more vapour than expected and the line can keep feeding liquid from the accumulator. 'A little bit' is an assumption, not a measurement.
Verify safety by isolating the section, bleeding it to a safe location (not the room), and confirming pressure is at atmospheric on a gauge at the section. Confirm no liquid is trapped between valves (check for a bleed or relief). Use the detector or a suitable indicator to confirm no residual. Only a verified, depressurised, isolated section is safe to open; the gauge reading is the evidence, not the technician's expectation.