Seals Replaced, Still Leaking? Uncover The Real Cause of Frequent Leaks: Abnormal Case Drain Pressure!

Still Leaking After Multiple Seal Changes? The Real Culprit Might Not Be the Seal, But Case Pressure!

During the maintenance of hydraulic equipment, we often encounter this frustrating scenario: hydraulic motors or pumps persistently leak oil. Even after replacing multiple sets of new seals, the problem recurs. Many maintenance technicians instinctively suspect "poor seal quality," leading them to switch brands, materials, or even suppliers – often to no avail.

However, frequent leakage isn't always the seal's fault. The hidden issue is often excessively high case drain pressure.

1. Why Does High Case Drain Pressure Cause Leaks?
Hydraulic components (such as motors, pumps and valves) inherently generate internal oil leakage during operation. To prevent this oil from accumulating inside the casing, manufacturers design dedicated drain ports. These allow leakage oil to flow back to the tank via a drain line.

However, problems within the drain system can cause abnormal pressure to build up inside the casing:

Clogged drain line: Restricted flow path prevents timely oil discharge.

Undersized line diameter: Restricted flow increases velocity and pressure.

Excessive return line backpressure: Especially when multiple components share a common return line, pressures can compound.

Drain port connected directly to main return line: Main return pressure is transmitted back to the casing ("backpressure transfer" effect).

Excessive bends or excessive line length: Increased flow resistance exacerbates pressure buildup.

Persistent high case pressure causes two critical failures:

Seal Extrusion: Pressure exceeding design limits forces seals out of their grooves, causing permanent deformation or tearing, leading to seal failure.

Lubrication Film Breakdown & Premature Component Wear: Abnormal pressure disrupts the vital oil film lubrication between bearings and sliding surfaces. This results in metal-to-metal contact (dry friction) and premature wear or even seizure.

2. Case Study: Typical Failure Analysis
Customer Report: A hydraulic travel motor frequently leaked oil from the main shaft. Three sets of imported seals were replaced over a short period，but this did not improve the situation.

3. Technical Team Findings:

Drain line connected to the middle section of the main return line, where system backpressure consistently measured ~1.0 MPa.

Drain line: 6-mm diameter tubing, over 3 meters long.

Disassembly revealed seals extruded from their grooves, with O-rings showing significant deformation.

Conclusion: The seals showed no material defects. The root cause was excessive case pressure due to improper drain system design.

3. How to Correctly Address Case Pressure Issues?

Independent Drain Line: Route the case drain line directly back to the tank, avoiding connection to the main return line.

Ensure Unrestricted Drain Flow: Regularly inspect drain lines for kinks, crushing, or blockages. Consider increasing line diameter.

Install Backpressure Control Valves or Pressure Relief Devices: Prevent abnormal pressure transmission back to the casing.

Minimize Drain Path Length & Bends: Simplify the drain circuit to reduce flow resistance.

Monitor Case Drain Pressure: Install pressure sensors on critical hydraulic components for real-time monitoring.

4. Conclusion: Stop Blaming the Seals Outright!
Frequent leaks ≠ Poor seal quality. Seals are the system's "last line of defense." If case pressure exceeds their design limits, even premium seals will fail prematurely.

Therefore, when tackling hydraulic leaks, investigate the entire system's design and operating conditions rather than simply blaming the seals themselves.

Only through comprehensive, systematic diagnostics can we accurately identify the root cause ("prescribe the right cure") and enhance equipment reliability and service life.