By its performance rather than its noise. Flow falls, discharge pressure will not reach the setpoint, and priming becomes slow, while the pump sounds and looks normal. Comparing current flow and pressure against the original duty is the reliable test, which makes commissioning records valuable.

Usually yes. Fitting new screws into a worn liner restores only part of the clearance and the pump underperforms from the day it goes back together. Since the clearance is formed between the two, replacing them as a set is what actually restores the original duty.

Abrasive contamination in the fluid, dry running, and fluid with inadequate lubricity - particularly on triple-screw pumps where the idlers depend on the fluid film. Rebuilding without addressing the filtration or the fluid condition simply buys another interval of the same wear.

On timed twin-screw designs, critically - they hold the screws apart exactly as a lobe pump's gears hold its rotors apart, and if backlash lets the screws touch, the screw set is destroyed. Triple-screw pumps have no timing gears, since the idlers are turned by the fluid.

Deflection. Worn bearings let the shaft move under load, which closes or misaligns the running clearance between the screws and the liner. The symptom presents exactly like ordinary clearance wear, so replacing the screw set without addressing the bearings repeats the failure within a short interval and at full cost.

Generally yes while the casing bore is serviceable, since screws, liners, seals and bearings are all replaceable. The decision usually turns on whether the liner can be renewed - once the casing itself is worn beyond a liner fit, replacement is normally the better option.

The filtration and, where relevant, the fluid specification, because those caused the wear. Also fit or verify dry-run protection if the pump has lost suction in service. A rebuild that does not address the cause returns the pump to the same wear rate it had before.