Question 12 QMED05 - Machinist-Pump Technician

The Correct Answer is D. ### Explanation for why Option D is Correct Option D ("first tightened, and then backed off, start pump, and while running under normal conditions; tighten evenly and slightly, over time") describes the standard and correct procedure for properly seating new packing material in a centrifugal pump's stuffing box. 1. **Initial Tightening and Backing Off:** New packing is often inserted slightly loose. Initially tightening the gland nuts firmly (often hand-tight plus a quarter turn) compresses the rings, squaring them up in the stuffing box, and ensuring they are uniformly seated. The nuts are then immediately backed off (loosened) significantly—often until they are just finger-tight or slightly looser—to allow the shaft to turn freely and to permit adequate leakage for cooling and lubrication. 2. **Starting the Pump (Running Under Normal Conditions):** When the pump is started with the gland slightly loose, there will be noticeable leakage. This initial heavy leakage is essential because it prevents the new, dry packing from overheating and burning up immediately due to friction. 3. **Gradual and Even Tightening:** The final step is crucial. While the pump is running and the packing is being lubricated by the pumped fluid (the leakage), the gland nuts must be tightened gradually, evenly, and slightly, over time (often over several minutes or hours). The goal is to reduce the leakage to a necessary minimum (a steady drip, typically 60-120 drops per minute) while ensuring the packing material doesn't generate excessive heat. This gradual process allows the packing rings to fully form, stabilize, and achieve the required sealing without overheating the shaft or the packing itself. ### Explanation for why Other Options are Incorrect **A) loosened until the gland clears the stuffing box:** This action is performed **before** installing the new packing (to remove the old packing). Leaving the gland completely loose after installing new packing would result in excessive, uncontrolled leakage when the pump is started, potentially flooding the area or starving the packing of cooling fluid necessary for controlled bedding in. **B) left in that position:** "That position" presumably refers to the position *after* the old packing was removed or the position immediately *after* the new packing was inserted. New packing must be deliberately compressed (tightened) first to seat it correctly, and then deliberately loosened to allow for run-in and cooling. Simply leaving the nuts in an arbitrary position will almost certainly result in either premature packing failure (if too tight) or excessive leakage (if too loose). **C) tightened an additional 10% to compress the packing:** While initial compression is required (as part of step D), leaving the packing tightened too much—even by a specific percentage like 10%—before starting the pump is dangerous. New, dry packing creates very high friction. If the gland is tightened down before the pump is run and the packing is cooled by leakage, the friction will rapidly generate heat, possibly destroying the new packing rings, scoring the shaft or sleeve, and causing the pump to seize. Initial tightening must always be followed by loosening for the run-in period.