Module 4 Scenario Cards: Materials, Seals, Flanges & Interfaces

R1-A Component Reference

SC-M04-01: The Gradual Seal Degradation — O-Ring Failure Symptoms

Module: M04 Rig Config: R1-A Competency: M04-COMP-02 Indicators Assessed: M04-IND-02.02, M04-IND-01.04

System State

State Name: ISOLATED (rate-of-rise test in progress) One-line description: R1-A has been roughed down and isolated. Rate-of-rise data collected over the past three months shows a progressive deterioration in seal performance.

Background Information (Provided to Students)

R1-A has been in regular use for eight months. For the first five months, the system performed within specification.

Over the past three months, the maintenance technician has recorded rate-of-rise data after each weekly pump-down. The data shows a clear trend.

The system is always tested under the same conditions: pump down to base pressure, isolate (R1-V-ISO closed, R1-V-VENT closed, pump off), and record pressure at 0, 1, 5, and 10 minutes.

No maintenance has been performed on any seals since installation. No process gases have been used — the system has only pumped air. The chamber has not been opened between tests.

Valve Positions

Gauge Readings — Three-Month Trend

Month 1 (system age: 6 months):

Month 2 (system age: 7 months):

Month 3 (system age: 8 months):

Pump Status

Media Placeholder

[Media: SC-M04-01 Three-Month Rate-of-Rise Trend]

• Three overlaid rate-of-rise curves (Month 1, Month 2, Month 3) showing progressive worsening
• Month 1 curve: low rates, clearly concave (outgassing pattern)
• Month 2 curve: higher rates, flattening toward constant
• Month 3 curve: highest rates, nearly constant (leak pattern)
• Annotations showing the transition from "outgassing signature" to "leak signature"
• Priority: P2-STRONGLY RECOMMENDED

Student Prompt

Teaching Points (Facilitator Notes)

Expected student observations:

• Month 1: Decreasing rate (0.020 to 0.006 mbar/min) — classic outgassing, normal behaviour
• Month 2: Rate barely decreases (0.040 to 0.034 mbar/min) — the pattern is shifting toward constant, suggesting a growing leak component alongside outgassing
• Month 3: Rate is essentially constant (0.070 to 0.068 mbar/min) — now dominated by a real leak; the outgassing component is masked by the much larger leak signal
• The base pressure at isolation (starting pressure) has also worsened: 0.05, 0.06, 0.08 mbar — the pump is fighting a progressively larger gas load

Key learning moments:

• Compression set is progressive: The O-ring does not fail suddenly — it gradually loses elastic restoring force over months of continuous compression. The seal weakens incrementally, producing a growing leak that transitions the rate-of-rise pattern from outgassing-dominated to leak-dominated.
• Trend data is more powerful than a single test: A single rate-of-rise test at Month 2 might be ambiguous (is the near-constant rate a large outgassing source or a small leak?). The three-month trend removes the ambiguity — the progressive worsening confirms a degrading seal, not a transient contamination event.
• Preventive vs reactive maintenance: Waiting until the seal fails completely risks an unplanned shutdown. Replacing the O-ring now (during a scheduled maintenance window) is cheaper and less disruptive than an emergency repair.

Model escalation note: "Rate-of-rise testing on R1-A shows progressive seal degradation over three months. The rate pattern has shifted from a decreasing (outgassing) signature in Month 1 (0.020 to 0.006 mbar/min) to a near-constant (leak) signature in Month 3 (0.070 to 0.068 mbar/min).

This is consistent with compression set on the chamber or valve O-rings — seals that have been under continuous compression for 8 months without replacement. Additional information needed: visual inspection of the O-rings on R1-V-ISO, R1-V-VENT, and the chamber flange would help confirm whether compression set is present and identify which seal is the primary contributor."

Common student errors:

• Treating each month's data in isolation rather than seeing the trend
• Identifying the Month 3 data as a "leak" without explaining why Month 1 was normal (missing the progressive degradation story)
• Not connecting the failure mode (compression set) to the physical mechanism (loss of elastic restoring force)
• Recommending "investigate the leak" without specifying that the likely cause is seal ageing, not a new mechanical failure

SC-M04-02: The Wrong Material Inside the Chamber

Module: M04 Rig Config: R1-A Competency: M04-COMP-02 Indicators Assessed: M04-IND-02.01, M04-IND-02.03

System State

State Name: ROUGHING (stalled) One-line description: R1-A is being roughed but the pump-down has stalled well above the normal base pressure. A plastic fixture has been placed inside the chamber for a demonstration.

Background Information (Provided to Students)

A new instructor is preparing R1-A for a demonstration on vacuum gauge behaviour. To give students something visible inside the chamber, the instructor has placed a plastic equipment tray (approximately 200 mm x 150 mm x 30 mm, made from ABS plastic) on the chamber floor. The tray contains several small metal samples.

The system was in VENTED state (both valves closed, pump off, chamber at ~950 mbar) before the demonstration. The instructor closes the chamber, opens R1-V-ISO, starts R1-P-RP, and begins roughing.

The bulk gas removal (950 to 1 mbar) proceeds normally in about 2 minutes. Below 1 mbar, the pump-down slows dramatically.

After 45 minutes of continuous pumping, R1-G-CH reads 0.40 mbar and is still dropping, but very slowly. The normal base pressure for R1-A is 0.05 mbar.

The instructor, puzzled, isolates the system and performs a rate-of-rise test.

Valve Positions (During Rate-of-Rise Test)

Gauge Readings — Rate-of-Rise Test

Pump Status

Additional Information

• The metal samples on the tray (stainless steel coupons) are clean and have been used in the vacuum chamber before with no issues
• The plastic tray is new — purchased from a general laboratory supplier, not a vacuum equipment vendor
• The chamber and all seals were serviced two weeks ago with no issues found
• The system reached 0.05 mbar normally during post-maintenance testing (without the plastic tray)

Media Placeholder

[Media: SC-M04-02 Pump-Down Comparison]

• Two curves: "normal pump-down" (reaching 0.05 mbar in ~8 minutes) and "today's pump-down with plastic tray" (stalling at 0.40 mbar after 45 minutes)
• Curves identical from 950 to ~1 mbar; diverge dramatically below 1 mbar
• Annotate the "outgassing plateau" where today's curve flattens
• Priority: P2-STRONGLY RECOMMENDED

Student Prompt

Teaching Points (Facilitator Notes)

Expected student observations:

• Normal bulk gas removal (950 to 1 mbar) means the pump works and there is no major leak
• Stall at 0.40 mbar means a large gas load source is present in the sub-1 mbar range
• Decreasing rate of rise (0.180 to 0.052 mbar/min) confirms outgassing, not a leak
• The plastic tray is the only change to the system — it is the gas source

Key learning moments:

• Plastics are catastrophic in vacuum. ABS plastic absorbs water, solvents, and atmospheric gases during manufacturing and storage. Under vacuum, these molecules desorb over hours to days. The outgassing rate is orders of magnitude higher than stainless steel.
• Types of gas released by ABS plastic under vacuum: 1. Adsorbed water vapour (the dominant component initially — ABS is hygroscopic) 2. Residual manufacturing solvents and plasticisers 3. Volatile organic compounds (monomers, additives) 4. Absorbed atmospheric gases (nitrogen, oxygen trapped in the porous structure)
• Why extended pumping has limits: The stall pressure represents an equilibrium where the outgassing rate from the plastic equals the pump's throughput at that pressure. Extended pumping will slowly lower this equilibrium as the plastic gradually dries out — but the time required could be hours to days, and the plastic will re-absorb moisture as soon as the system is vented. The plastic will never behave like metal.
• The solution is removal, not patience. The correct approach is to remove the plastic from the chamber and use vacuum-compatible fixturing (stainless steel or aluminium).

Model observation summary: "The plastic equipment tray is outgassing heavily under vacuum, which is preventing R1-A from reaching its normal base pressure. The evidence indicates that the tray is the sole source of the elevated gas load — the system performed normally without it.

Metals such as stainless steel or aluminium have outgassing rates orders of magnitude lower than plastics. As a general rule, no plastic, rubber, adhesive tape, paper, or other organic material should be placed inside a vacuum chamber unless it has been specifically rated for vacuum use."

Common student errors:

• Blaming the pump ("the pump cannot handle the extra volume" — volume is irrelevant; the issue is gas load, not volume)
• Suggesting a leak despite the clearly decreasing rate-of-rise pattern
• Not identifying the plastic as the gas source (looking for contamination elsewhere)
• Underestimating the outgassing impact ("it's just a small tray" — surface area and material porosity matter far more than physical size)

SC-M04-03: The Flange Identification Exercise

Module: M04 Rig Config: R1-A Competency: M04-COMP-01 Indicators Assessed: M04-IND-01.02, M04-IND-01.03, M04-IND-01.04

System State

State Name: VENTED (system at rest, under inspection) One-line description: R1-A is in VENTED state. Students are given a written description of each connection point and must identify the flange type, seal type, and relevant hardware at each location.

Background Information (Provided to Students)

A walkdown inspection of the R1-A vacuum system is being conducted. The system is at rest in VENTED state (both valves closed, pump off, chamber at ~950 mbar). Each connection point is being documented for a maintenance record.

Manufacturer labels and part numbers are not visible. Each connection type must be identified from its visible features: the hardware used to hold it together, the seal visible between mating surfaces, and the overall size and appearance.

Connection Descriptions

Connection A — Chamber-to-Isolation-Valve (R1-CH to R1-V-ISO): Two machined metal flanges are bolted together with a circle of six evenly spaced hex bolts.

The mating surfaces have a polished groove running around the circumference. Between the flanges, a flat metal ring (copper-coloured) is visible. The flange diameter is approximately 70 mm.

Connection B — Foreline Junction (R1-V-ISO outlet to R1-L-FL): Two flat metal flanges are held together by a single hinged clamp with a wing nut.

Between the flanges, a metal ring is visible that holds a black rubber O-ring. The connection is approximately 25 mm bore. Assembly appears quick — the clamp can be released by hand.

Connection C — Foreline-to-Pump (R1-L-FL to R1-P-RP): A flexible corrugated metal tube approximately 300 mm long connects the end of the foreline to the pump inlet. At each end, the corrugated tube terminates in a flat flange held by a single clamp (similar to Connection B). Black O-rings are visible in the centering rings at each end.

Connection D — Vent Line (R1-V-VENT to R1-FLT-VENT): A small-bore tube (approximately 10 mm internal diameter) connects the vent valve to the vent filter. The connections at each end appear to use compression fittings with ferrules rather than flanges — no clamps or bolts are visible. The tube is rigid stainless steel.

Student Prompt

Teaching Points (Facilitator Notes)

Expected identification table:

Key learning moments:

• Visual identification skills: Students practise identifying connection types from physical descriptions rather than labels. In the field, labels are often missing or illegible — flanges are identified by their features.
• Connection A (CF) vs B/C (KF) trade-off: CF gains: metal seal (zero permeation, UHV-compatible, bake-out compatible). CF sacrifices: assembly speed (multiple bolts vs one clamp), gasket reusability (copper is single-use), and cost. If R1-A's chamber port uses CF, it suggests the chamber was designed for potential future upgrade to higher vacuum performance even though the current roughing-only configuration does not require it.
• Connection C (bellows): The corrugated metal tube is a flexible bellows. Functional reasons: (1) Vibration isolation — the roughing pump generates mechanical vibration that could stress the foreline connections and chamber if transmitted through rigid tubing. (2) Alignment tolerance — the bellows accommodates minor misalignment between the foreline and pump inlet without stressing the flanges. (3) Thermal expansion — if the pump or foreline changes temperature during operation, the bellows accommodates dimensional changes.
• Connection D (compression fitting): This is a small-bore utility connection, not a vacuum-performance-critical joint. Compression fittings are adequate for the vent line because: (a) the vent line only carries gas at or near atmospheric pressure, (b) sealing requirements are minimal (the line is only open during controlled venting), and (c) the small bore makes flanged connections impractical.

Model maintenance log note: "During walkdown inspection, observed early-stage degradation on the O-ring in the R1-V-ISO outlet KF25 centering ring (Connection B): slight flattening and a faint circumferential crack on the outer surface. System is currently sealing normally but the O-ring shows signs of compression set and possible thermal or age-related degradation. Additional information needed: a rate-of-rise test baseline and the O-ring's installation date would help assess remaining service life and determine urgency."

Common student errors:

• Confusing ISO flanges with CF flanges (both can use bolt circles, but ISO uses O-ring seals while CF uses copper gaskets — the copper-coloured gasket is the key identifier for CF)
• Not recognising the corrugated tube as a bellows (describing it as "a flexible pipe" without identifying its vacuum function)
• Identifying Connection D as a KF connection (compression fittings are a different category entirely — no centering ring, no clamp)
• For the escalation question: either ignoring the crack ("system is working fine") or overreacting ("shut down immediately") — the appropriate response is documented observation with scheduled preventive replacement

End of Scenario Cards — Module 4