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This content is provided for information only. The authors make no representation or warranty regarding the accuracy, completeness or currency of the content. No information in this whitepaper should be construed as medical advice. Readers should seek appropriate professional guidance before acting on any information contained in this document. The authors expressly disclaim all liability for any direct or indirect loss or damage arising from the use of or reliance on this information.

Introduction

This paper sets out the wear and failure modes of washer-disinfector components under heavy use in hospital central sterile services departments (CSSDs), and the maintenance and condition monitoring practices that control them. It applies to instrument washer-disinfectors to ISO 15883-1 and ISO 15883-2 running thermal disinfection cycles, typically at 90 deg C [194 deg F].

Heavy Use Conditions

Definition: In this paper, heavy use means operation across two or more shifts, typically 16 h or more per day, with cycles run back to back. At 16 cycles per day, 7 days per week, a machine completes about 5,800 cycles per year. Record cycle count and operating hours for each machine, and schedule maintenance on these values rather than calendar time alone.

Total Lifecycle: Plan a total lifecycle of 10 years for a washer-disinfector in high use. At heavy use, 10 years is about 58,000 cycles, each a thermal cycle between ambient and disinfection temperature that fatigues welds, joints and seals. Over the same period, controller electronics and spare parts become obsolete, and the machine falls behind current editions of ISO 15883. Require spare parts and service support for at least 10 years from installation in the purchase contract. Review condition, failure history and spare parts availability from year 8. Replace earlier where failure rate, repair cost or parts availability make continued operation uneconomic or non-compliant. Extend beyond 10 years only where condition, failure history and parts availability all remain acceptable.

Machine Class: Practice-class washer-disinfectors are designed for lower annual operating hours than CSSD-class machines. At 16 h per day, a machine accumulates about 5,800 operating hours per year. Specify CSSD-class machines for heavy use, and obtain the manufacturer's rated operating hours or cycles before purchase.

Stress Mechanisms: Heavy use increases the annual number of thermal cycles between ambient and disinfection temperature, pump starts and running hours, door operations, valve actuations and dosing pump revolutions. It reduces cooling time between cycles and increases the debris load on filters and nozzles.

Requirements: AS 5369:2023 requires planned preventive maintenance by a competent person to documented procedures, with the frequency of each task and the maximum permitted delay specified. Equipment must not return to use until maintenance is complete and documented. Requalify at least annually and after any major repair that could affect the process. Size the CSSD so that peak demand is met with one washer-disinfector out of service.

Components and Wear Modes

The following table lists the main washer-disinfector components, their function and their wear and failure modes under heavy use.

High-Value Items: Pump and motor assemblies, heating elements and controllers typically have the highest replacement cost and longest lead time. Hold critical spares based on lead time and failure history. Gaskets, valves and dosing tubing are low cost but stop the machine when they fail. Keep them in stock and replace them at defined intervals.

Failure Modes

Corrosion and Chemical Degradation

Chloride causes pitting, crevice corrosion and stress corrosion cracking of austenitic stainless steel, and the rate increases with temperature. Iron, copper and manganese stain stainless steel. AS 5369:2023 limits chloride to 120 mg/L in wash stage water and 10 mg/L in final rinse water, and final rinse pH to 5.5 to 8.0. Detergent residue, overdosing and incompatible chemicals attack metals and elastomers. Heat and process chemicals harden, crack and swell gaskets and hoses.

Mitigation: Treat water to the AS 5369:2023 limits and test at the required frequencies. Use detergents compatible with the machine materials and dose to the IFU. Confirm elastomer compatibility with each process chemical with the machine and chemical manufacturers. At each preventive maintenance visit, inspect the chamber, racks, welds and sump for discolouration, pitting and rust, and identify the source of any deposit.

Mechanical Wear and Fatigue

Pump and Motor: Starts and running hours drive bearing and mechanical seal wear. Seal leakage, increased noise or vibration, rising motor current and falling spray pressure indicate wear. Motor windings overheat where ventilation is restricted.

Door Mechanism: Hinges, latches, alignment pins, door drives and interlock switches wear with door cycles, most on pass-through machines with powered doors. Failure prevents sealing or interlock engagement. Test the door interlock at every preventive maintenance visit.

Spray Arms and Load Carriers: Spray arm bushings and bearings wear and cause wobble or seizure. Racks and load carriers deform and crack under thermal and mechanical load, and injector connections for lumened devices lose alignment.

Structure: Thermal cycling stresses welds, seals and fixings. Check fixings, joints and chamber integrity at each preventive maintenance visit.

Mitigation: Check spray arm rotation daily. Lubricate moving parts and replace bearings and seals at the IFU interval. Install the machine level and load to the IFU. Check load carrier alignment and the spray system at least annually.

Hose Clamps and Fittings: Under heavy use, check and retighten hose clamps and fittings every 2 months. Elastomer hoses undergo stress relaxation and compression set that increase with temperature. Their thermal expansion is about ten times that of stainless steel, so each heat cycle works the joint and reduces clamp preload. Two months at 5,800 cycles per year is about 1,000 thermal cycles. Use constant-tension clamps where the manufacturer permits.

Clogging and Blockage

Filters and Drains: Bone fragments, labels, lint and soil collect in chamber and sump filters. Blocked filters cause poor drainage, pump cavitation and fault stops. Clean filters at least daily, and each shift where the soil load requires it.

Spray Nozzles: Scale, soil and biofilm reduce orifice area and spray force. Descale spray arms and nozzles on a schedule set from supply hardness and inspection.

Inlet Strainers: Sediment blocks inlet valve strainers and reduces fill rate. Clean strainers at the IFU interval and whenever fill time increases.

Dosing Lines: Dried or crystallised chemical blocks lines and injectors and stops dosing. Verify dosing volumes quarterly, as AS 5369:2023 requires for instrument washer-disinfectors.

Condensers and Coolers: Vapour condensers and waste water coolers foul with lint and scale. Include them in the descaling schedule.

Clogging rarely destroys components directly, but it causes cycle faults, raises pump load and reduces cleaning efficacy. Treat repeated filter or flow faults as an early indicator. Where the machine monitors spray pressure or flow, trend the values and act on a falling trend.

Electrical, Thermal and Operational Faults

Electronics and Sensors: High ambient temperature, blocked ventilation, moisture ingress and supply disturbances cause controller and sensor faults. Relays and switches wear with cycle count. Keep cabinet ventilation clear, and hold a spare controller where its lead time exceeds the acceptable downtime.

Controller Backup Battery: Replace the controller backup battery annually, or at the IFU interval if shorter. Chemical degradation rates roughly double for each 10 deg C [18 deg F] rise in temperature, and controller enclosures beside heated chambers run above room temperature. A failed battery can corrupt the clock, cycle records and configuration, and the battery costs little compared with the resulting downtime.

Thermal Load: Back-to-back cycles reduce cooling time for motors, electronics, wiring and door seals. Follow any duty cycle limit in the IFU, and confirm that plant room and CSSD ambient temperatures are within the equipment specification. Confirm that steam and hot water supply capacity meets peak simultaneous demand.

Operator Error: Incorrect loading jams spray arms and blocks spray patterns. Running with a failed fill exposes heating elements. Train operators to the IFU and verify the low-level heater interlock at preventive maintenance.

Maintenance Strategies

• Usage-Based Preventive Maintenance: Quarterly preventive maintenance is commonly recommended for heavy-use machines, and annual maintenance for low-use machines. The basis is a constant number of cycles between visits. At about 1,500 cycles per visit, a machine running 5,800 cycles per year needs quarterly service, and a machine running 1,500 cycles per year needs annual service. Set intervals by cycle count or operating hours using the IFU and failure history. Where the IFU specifies both a time and an operating hour limit, service at whichever comes first. Daily, clean filters and check spray arm rotation, chemical levels and leaks. Monthly, inspect nozzles, inlet strainers and fittings, and clean sensors. Quarterly, verify dosing volumes and replace wear parts that are due. Annually, recalibrate instrumentation at an interval not exceeding 12 months, carry out the electrical safety check required by AS 5369:2023, and requalify.

• Condition Monitoring: Trend cycle data, including heat-up time to disinfection temperature, spray pressure, fill and drain times, pump motor current and total cycle time. A rising heat-up time indicates scaled elements or reduced heating capacity. Measure pump vibration and bearing temperature against a baseline recorded at commissioning. Review controller fault logs at each visit for recurring non-critical faults. Fit temperature and vibration loggers to older machines without built-in monitoring.

• Water Quality: Test supply water hardness and chloride, and final rinse conductivity, monthly. Final rinse limits include conductivity not greater than 30 µS/cm, chloride not greater than 10 mg/L and hardness not greater than 10 mg/L as CaCO3. Service the treatment plant when results trend towards the limits.

• Planned Replacement: Replace low-cost wear parts, e.g. door gaskets, spray arm seals and dosing tubing, at fixed intervals set from the IFU and failure history. For pumps, heating elements and controllers, record cycles to failure and schedule replacement before the shortest observed life, e.g. at 17 months where heating elements have failed at 18 months.

• Test Results as Condition Indicators: A failed cleaning efficacy test indicates blocked nozzles, low spray pressure, dosing faults or misaligned load carriers. A thermometric deviation in the disinfection stage indicates heating, circulation or sensor faults. Thermal disinfection is verified thermometrically, not with biological indicators.

• Operator Reporting: Train operators to report noise, vibration, leaks, fault messages, residue on processed items and spray arms that do not rotate. AS 5369:2023 requires equipment to be removed from use immediately where a fault could affect device quality or operator safety.

• Records: For each maintenance task or repair, record the reason, date, model and serial number, location, work done, parts replaced, the person or company doing the work, and the person releasing the equipment. Use a computerised maintenance management system and import cycle counts from the machine where available. Analyse failure history to set intervals.

Maintenance and Replacement Guidelines

The following maintenance and replacement intervals are guidelines for heavy use. They are not manufacturer-validated limits. Use the IFU interval where it is shorter, and adjust each interval using site failure records.

Circulation Pump: Each cycle imposes a cold start, a thermal shock between fill water and disinfection temperature, and brief low-flow running at start and drain. These wear mechanical seal faces in proportion to cycle count rather than running hours. For ball bearings, basic rating life is proportional to the cube of the ratio of dynamic load rating to equivalent bearing load (ISO 281). Cavitation, imbalance or misalignment that doubles bearing load reduces bearing life to one eighth. At 0.5 h of pump running per cycle, 5,000 cycles is about 2,500 running hours. Rebuild at about 4,500 cycles, 90 % of the expected life, in a planned maintenance window.

Heating Elements: Scale growth is proportional to hardness and the volume of water heated. Each 1,000 L [264 US gal] of water at 150 mg/L hardness carries 150 g [5.3 oz] of hardness as CaCO3. Scale thermal conductivity is about 1 to 2 W/m.K [0.6 to 1.2 BTU/h.ft.deg F], compared with about 16 W/m.K [9.2 BTU/h.ft.deg F] for stainless steel. At a heat flux of 5 W per square centimetre [32 W per square inch], 1 mm [0.04 in] of scale raises sheath temperature by 25 to 50 deg C [45 to 90 deg F]. Element oxidation and insulation breakdown accelerate with temperature, so a rise of this size shortens element life several-fold. Under heavy use with hard water, plan element replacement at about 18 months. With softened water, plan 3 to 5 years and adjust based on inspection of the element surface.

Valves and Doors: At about 5 fill and drain stages per cycle, inlet and drain valves actuate about 29,000 times per year at heavy use. Each door completes about 5,800 open and close cycles per year. Elastomer diaphragms and seals fatigue with each actuation and age faster in hot water, so replace them every 2 to 3 years. Replace door drive components every 3 to 5 years.

Sensors: Thermal cycling and fouling cause sensor drift. AS 5369:2023 limits the recalibration interval to 12 months. Verify temperature sensors quarterly against a reference instrument to detect drift between calibrations.

Contactors: Contact erosion is proportional to switching operations under load. Heater contactors switch several times per cycle, so inspect contacts annually, scan connections thermographically, and replace contactors at their rated electrical life.

Drying Air Filters: Filters load with particles and their pressure drop rises, which reduces drying airflow. AS 5369:2023 requires air filters to be checked and changed as the IFU requires. Under heavy use, replace them at least every 12 months or at the manufacturer pressure drop limit.

Conclusion

Heavy use increases the thermal, mechanical and chemical load on every washer-disinfector component. Pumps, heating elements, spray systems, valves, door seals, dosing systems and sensors are the main wear points. Control wear by scheduling maintenance on cycle count, trending cycle and condition data, holding water quality to AS 5369:2023, replacing wear parts at defined intervals, and analysing failure records to refine those intervals. Plan a total lifecycle of 10 years in high use, with a condition review from year 8.

Sources

1. Standards Australia. AS 5369:2023 Reprocessing of reusable medical devices and other devices in health and non-health related facilities.
2. ISO 15883-1:2024 Washer-disinfectors, Part 1: General requirements, terms and definitions and tests.
3. ISO 281 Rolling bearings, Dynamic load ratings and rating life.
4. AS/NZS 3500.1 Plumbing and drainage, Part 1: Water services.