Root cause analysis of filter element failures in hydraulic systems: pressure drop, bypass valves and contamination spikes

1. Introduction

At one heavy engineering enterprise in Ukraine, an emergency shutdown of the main hydraulic drive of a heavy press occurred. During a scheduled oil change, operators discovered a sharp pressure drop in the control line and metal shavings in the drain tank. The initial inspection showed complete destruction (disassembly) of the high‑pressure filter element. An additional risk arose due to the blocking of the feedback loops of the feed motors with a Siemens 6FX8002-2EQ10-1BG0 cable assembly, which transmitted distorted data because of electromagnetic interference caused by an emergency overheating of adjacent hydraulic lines. Similar incidents lead to production stoppages lasting from 36 to 72 hours and significant financial losses. The purpose of this investigation is to determine the true causes of the accident based on ISO and EN standards.

2. Component Overview

High‑pressure filter element is located in the pressure line of the hydrostatic system upstream of the sensitive axial‑piston pumps and proportional valves. It is intended to protect precision components from solid particles in accordance with cleanliness class ISO 4406 (target rating 16/14/11).

Standard operating conditions:

• Working pressure: up to 320 bar.
• Operating fluid temperature: from +15 °C to +65 °C (mineral oil ISO VG 46)
• Maximum allowable pressure drop ($\Delta P$): 5.0 bar for standard microfibre elements.
• Nominal filtration fineness: 10 microns ($\eta_{10} \ge 75$)

3. Failure evidence

Laboratory and visual analysis of the damaged element revealed the following data:

• Deformation of bellows: The star-shaped structure of the folds is completely destroyed inside the supporting perforated pipe. The bellows have merged into a solid dense mass.
• Filter material rupture: Multi‑layer material made of synthetic fibers has longitudinal tears along the weld line.
• Pressure drop measurement: SCADA sensors recorded a peak pressure drop of 8.4 bar two minutes before the pump station shutdown.
• Vibration monitoring: Vibration sensors on the filter housing recorded an increase in the high‑frequency spectrum (from 1 to 5 kHz) of 350% compared with the baseline level, indicating cavitation and flow pulsation.

4. Root cause analysis

To determine the event chain, the Fault Tree Analysis method and the 5 Whys analysis were used. The investigation ruled out a manufacturing material defect after spectral and microscopic testing according to series ISO 2941 standards (resistance to rupture under pressure drop).

5. Identified root causes

Based on the expert assessment, three key factors were identified, arranged by likelihood:

1. Elevated fluid viscosity during cold start (Probability: 55%). Equipment start-up was performed at oil temperature +5 °C. The dynamic viscosity of ISO VG 46 oil exceeded 900 cSt, creating a resistance across the mesh of over 6.0 bar.
2. Failure of the bypass valve (Probability: 30%). The bypass valve spring seized due to accumulation of lacquer deposits. The valve did not open at the nominal setting pressure of 5.0 bar, directing the entire flow through the clogged element.
3. Sudden discharge of contaminants from the hydro tank (Probability: 15%).Destruction of the tank's internal protective coating led to abrasive particles larger than 100 мкм entering the system, which instantly blocked the filter pores.

6. Corrective Actions

Measures of operational and long-term nature have been developed to eliminate the detected defects:

7. Quick Diagnostic Card (For Technicians on Tablet)

Use this checklist directly in the workshop near the equipment:

• [ ] Check the pressure gauge readings before and after the filter (normal $\Delta P$ < 2.5 bar).
• [ ] Read the current oil temperature on the operator panel (should be within +35...+55 °C).
• [ ] Inspect the filter housing for leaks and signs of mechanical damage.
• [ ] Check the contamination indicator status (visual and electrical signal on PLC).
• [ ] Carry out an express analysis of the lubricant using a portable particle counter (cleanliness class).
• [ ] Ensure the absence of unusual noise or high vibration of pump housings.
• [ ] Check the operation of the heating system before starting the main drive during the winter period.
• [ ] Check the article of the installed element against the manufacturer's specification (prohibition of using uncertified equivalents).

8. Prevention Strategy

To ensure the mean time between failures (MTBF) at a level of not less than 15 000 hours, the following scheduled procedures are implemented at the enterprise:

• Condition Monitoring: Installation of stationary humidity and particle concentration sensors in real time (compatibility with IO-Link protocol).
• Maintenance schedule: Filter element replacement every 2 500 hours of operation or upon reaching $\Delta P = 4.0 ext{ bar}$, whichever occurs first.
• Standardization of spare parts: Use of exclusively certified components complying with European quality and safety standards CE.

9. Conclusion

Filter element failure due to pressure drop is a result of the combined effect of low temperatures, failure of the protective valve, and accumulation of harmful contaminants. Compliance with the technical maintenance schedule, control of the working environment cleanliness according to ISO standards, as well as timely replacement of consumable materials ensure trouble-free operation of industrial equipment. For selection of quality certified components and filter elements, use UNITEC-D E-Catalog.

10. References

Нормативні документи та технічна література:

1. ISO 2941:2009 — Hydraulic fluid. Filter elements. Resistance to rupture under pressure drop.
2. ISO 4406:2021 — Hydraulic fluid. Method of coding contamination level with solid particles.
3. ISO 3968:2017 — Hydraulic fluid. Filters. Determination of pressure drop characteristics as a function of flow rate.
4. EN 60204-1:2018 — Safety of machinery. Electrical equipment of industrial machinery. Part 1: General requirements.
5. DSSTU EN ISO 12100:2016 — Machinery Safety. General Principles of Design. Risk Assessment and Risk Reduction.