Maintenance Flashcards

A centrifugal pump is cavitating intermittently even though suction pressure readings appear normal. After ruling out air leaks and checking impeller clearance, what is the MOST likely remaining cause?

Answer: The fluid temperature is near its vapor pressure at the suction conditions

Cavitation occurs when local fluid pressure drops below the fluid's vapor pressure, causing vapor bubbles to form and collapse. When suction gauge readings look normal but cavitation persists, the fluid temperature being close to its vapor pressure point is a classic culprit — the available NPSHa (Net Positive Suction Head available) is being consumed by temperature, not pressure loss. Discharge back-pressure would reduce flow but not directly cause cavitation. Misalignment and voltage drop affect mechanical and electrical performance but not the thermodynamic trigger for cavitation.

A 3-phase motor draws equal current on all three phases at no load but shows a 12% current imbalance under full load. Which condition BEST explains this pattern?

Answer: One stator winding has a developing turn-to-turn short

A turn-to-turn short in a stator winding reduces the effective impedance of that winding. At no load, the effect is minimal because current is low and the winding differences are masked. Under full load, the shorted winding draws disproportionately higher current, producing the observed imbalance that worsens with load. Voltage imbalance from the utility would cause current imbalance even at no load. Bearing failure adds mechanical load symmetrically across all phases. True single-phasing would cause dramatic imbalance (or shutdown) at no load as well.

A hydraulic cylinder extends slowly and lacks full force even though system pressure at the pump outlet reads correctly. The cylinder retracts normally at full speed and force. What is the MOST probable cause?

Answer: There is a worn or cracked piston seal allowing internal bypass

A worn piston seal allows hydraulic fluid to bypass from the high-pressure cap end (extension side) to the low-pressure rod end during extension. This internal leakage reduces the effective force and slows extension, while the pump pressure gauge still reads system pressure since the pump is compensating. During retraction, pressure is on the rod-side annular area and bypass direction reverses — the piston seal bypass now has less impact because the rod-side area is smaller and the movement direction is reversed, allowing normal retraction. Rod seal leakage would cause external leakage. A sticking spool or clogged filter would affect both directions.

When using a megohmmeter to test motor winding insulation, a technician records 800 MΩ at the start of a 10-minute test and 950 MΩ at the 1-minute mark, but the reading drops back to 820 MΩ at 10 minutes. What does this pattern MOST likely indicate?

Answer: Surface contamination (moisture or carbon tracking) is present on the windings

In a clean, dry winding, insulation resistance typically increases steadily over a 10-minute test as the dielectric absorption phenomenon takes effect (the Polarization Index test relies on this). A reading that rises briefly then falls back suggests that surface leakage currents — caused by moisture, oil contamination, or carbon tracking — are initially displaced but then re-establish conduction paths as the test voltage drives current through the contaminated surface layer. A developing ground fault would typically show a steadily declining or low reading. A failing megohmmeter battery would produce erratic, not systematic, behavior.

A pneumatic system uses an FRL (Filter-Regulator-Lubricator) unit. After maintenance, a new solenoid valve begins chattering rapidly at 85 PSI even though it is rated for 60–120 PSI operation. The system pressure is stable at the compressor. What should the technician check FIRST?

Answer: The regulator output — a failing regulator diaphragm can cause pressure oscillation downstream

Solenoid valve chatter in a pneumatic circuit is typically caused by pressure instability at the valve inlet, not by the steady-state pressure value. A failing or worn regulator diaphragm can create pressure oscillations (hunting) downstream — the regulator overshoots and undershoots the set point rapidly. Even though the system pressure at the compressor is stable, the regulator is the component that most directly determines the local pressure the solenoid sees. Low lubrication causes sticking, not chatter. AC coil chatter is a valid consideration for electrical solenoids but this is a pneumatic-side symptom. A clogged filter reduces pressure rather than creating oscillation.

A maintenance tech is torquing a flanged joint with spiral-wound gaskets on a steam line. The specified torque is achieved, but during the subsequent pressure test, the flange leaks at a bolt near a pipe support bracket. No bolts are broken or stripped. What is the MOST likely root cause?

Answer: The bolt near the support bracket is carrying additional bending stress from pipe strain, reducing its effective clamp load

Pipe support brackets can introduce bending moments and axial pipe strain into adjacent flanges. A bolt near such a bracket can experience external tensile load from the pipe trying to pull apart or bend at that support point — this added load effectively reduces the net clamping force the bolt exerts on the gasket, causing a leak at that specific location. This is a well-known problem in piping systems called 'flange rotation' or bolt load loss due to pipe strain. Gasket installation error would cause a full-face leak pattern, not a single-bolt location. An uncalibrated wrench would statistically affect random bolts, and gasket material creep would show a uniform or temperature-zone pattern.