Hang-up (Deposit Sticking)

What it is. Some mechanical seal faces are designed to move axially a few micrometers during operation, tracking wear and small thermal shaft movements — that's how the seal keeps the faces in proper contact over its service life. Hang-up happens when solid deposits form in the region where that face should slide freely — typically coke (from hydrocarbons thermally degrading near hot surfaces) or crystals (from salts that precipitate as the fluid cools or partially evaporates) — and these deposits stick and block the face's axial movement. The seal loses its ability to adjust, and the result is leakage or face damage from lost tracking.

How to recognize it in the field. It's most common in services with fluids that coke (heavy oil, asphalt, refinery residues) or crystallize (saline solutions, some chemical processes) — especially after prolonged shutdowns, when the stagnant fluid has time to cool/settle and form a deposit without the continuous flow that normally keeps it suspended or dissolved.

Why it matters. It's an industry/service-specific failure mode — not every application is exposed, but where it is, it's recurring and seasonally tied to maintenance shutdowns, not constant wear. Typical prevention involves keeping the line heated/cooled even while stopped, preventing the fluid from reaching the condition that forms the deposit.

Mechanism — two paths to the same sticking

Coking: heavy hydrocarbons, exposed to heated surfaces (proximity to friction faces, or process heating), undergo thermal degradation and form hard, adherent carbonaceous deposits — exactly in the face's axial-movement region, the worst possible place for a rigid deposit to form.

Crystallization: fluids with dissolved salts near saturation can precipitate crystals when temperature drops (solubility typically decreases with cooling) or when a fraction of the fluid evaporates at the seal's atmospheric face, concentrating the remaining salts to saturation. The classic pattern: it happens more during shutdowns (stagnant fluid, time to precipitate) than during continuous operation (constant flow keeps the solution).

Framework A — diagnosis

Mixed/Complex: the deposit accumulates progressively with exposure time to risk conditions (β>1 in the buildup), but the formation trigger (a prolonged shutdown, a specific temperature drop) is operational and not always predictable on a calendar — IT-MNT-001 would treat this case as a candidate for FMEA decomposition: the deposit itself is wear-out, but the event that triggers formation is situational.

Framework B — prescription

CBM with two fronts: monitoring the symptom (start-up vibration/noise, once sticking already exists) and — of higher value — attacking the cause by keeping the line heated/cooled even during shutdowns, so the fluid never reaches the deposit-forming condition. It's one of the few component failure modes where operational strategy (thermal management of the stopped line) matters more than any inspection of the seal itself.

Why shutdowns are the highest-risk moment

In continuous operation, constant flow through the sealing chamber tends to keep particles suspended and dilute local salt concentrations near the face — even in fluids with crystallization risk, flow "washes" the region before local saturation is reached. During a shutdown, that flow stops: the fluid in the chamber becomes stagnant, exposed to ambient temperature variation for hours to days, enough time for the slow precipitation/coking processes to complete exactly at the seal's most sensitive geometry.

The design lesson: thermal piping plans

API 682 Part 5 includes specific heating plans (steam trace/electric on the chamber) and cooling plans (Plans 21/23, which recirculate fluid through a heat exchanger) — the existence of these plans in the normative catalog confirms that temperature-driven hang-up is a recognized, design-addressed failure mode, not a rare anomaly. The correct audit when specifying a new seal explicitly asks: "does this fluid coke or crystallize under any plausible shutdown condition?" — if so, the thermal plan is not optional.

Relation to other modes

A seal that suffered hang-up and had its face forced beyond its normal tracking travel can develop localized face damage analogous to abrasion — the rigid deposit, if fragmented during an attempted movement, itself becomes an abrasive particle trapped at the interface. A post-mortem on a seal with recurring hang-up should investigate both damage patterns.