Compressed Air in Hydraulic Tanks Can Indicate System Imbalance or Design Intent

[MikePhua]

The Role of Hydraulic Tanks in Mobile Excavators
Hydraulic tanks serve as reservoirs for fluid used in powering cylinders, motors, and valves across mobile equipment. In machines like the Badger 4435, which is a rail-capable excavator designed for off-track maintenance and lifting operations, the hydraulic system is central to boom articulation, swing, travel, and auxiliary tool control. The tank not only stores fluid but also allows for thermal expansion, degassing, and filtration.
While most hydraulic tanks are vented to atmosphere through breather caps or filters, some systems intentionally use pressurization to improve pump priming, reduce cavitation, and stabilize fluid flow under high demand. However, unintended air pressure buildup can signal a malfunction or oversight.
Terminology Notes

• Cavitation: The formation of vapor bubbles in hydraulic fluid due to low pressure, which can damage pumps and valves.
  
• Breather cap: A vented cap that allows air exchange while filtering contaminants.
  
• Charge pressure: A low-level pressure applied to the hydraulic tank to assist fluid delivery.
  
• Return line aeration: Air introduced into the tank via turbulent fluid returning from actuators.
  

Is Compressed Air in the Hydraulic Tank Normal
In some hydraulic systems, especially those with high-mounted pumps or long suction lines, a small amount of positive pressure—typically 5 to 15 psi—is intentionally maintained in the tank. This helps ensure consistent fluid delivery and prevents vapor lock during startup or high-speed operation. Such systems use pressure-rated caps or nitrogen-charged accumulators to regulate tank pressure.
However, if compressed air is present without a designed pressure system, it may indicate:

• A blocked breather or vent filter
  
• Excessive return line turbulence introducing air
  
• Thermal expansion without adequate venting
  
• Internal leakage from a pressurized circuit into the tank
  
• Improper sealing during maintenance or refilling
  

A Story from the Midwest Rail Corridor
In 2019, a maintenance crew operating a Badger 4435 noticed a hiss when removing the hydraulic tank cap. Initially dismissed as residual pressure, the issue escalated when the boom began to jerk during feathering operations. Investigation revealed a clogged breather filter and a return line elbow that had loosened, allowing air to enter the tank. After replacing the filter and resealing the fitting, the system stabilized. The technician remarked, “It wasn’t the pump—it was the pressure behind the scenes.”
Inspection and Diagnostic Recommendations
To assess compressed air presence:

• Remove the tank cap slowly and listen for pressure release
  
• Inspect breather caps for clogging or damage
  
• Check return lines for aeration points or loose fittings
  
• Monitor fluid level and temperature during operation
  
• Use a low-pressure gauge to measure tank pressure if designed for it
  

If pressure is unintended:

• Replace or clean breather filters
  
• Inspect seals and gaskets around the tank lid
  
• Verify that no pressurized lines are routed into the tank
  
• Check for foaming or milky fluid, which indicates air entrainment
  
• Drain and refill the tank if contamination is suspected
  

Preventive Measures and System Optimization
To prevent future issues:

• Use OEM-rated breather caps with proper micron filtration
  
• Install baffles or diffusers in return lines to reduce turbulence
  
• Maintain proper fluid levels and avoid overfilling
  
• Inspect tank vents quarterly, especially in dusty or humid environments
  
• Train operators to report unusual noises or pressure signs during servicing
  

Some advanced systems include tank pressure sensors and alarms to alert operators of abnormal conditions. Retrofitting such sensors can improve reliability and reduce downtime.
Badger Equipment and the 4435 Platform
Badger Equipment, based in Winona, Minnesota, has specialized in rail and off-road maintenance machines since the 1940s. The 4435 series is known for its dual-mode capability, allowing operation on rail tracks and conventional terrain. Its hydraulic system powers a full range of attachments, including tie tampers, grapples, and buckets, making it a versatile tool for railroads and contractors.
The hydraulic tank in the 4435 is designed for high-cycle operation, and any deviation from normal pressure behavior should be investigated promptly to avoid damage to pumps, valves, or actuators.
Final Thoughts
Compressed air in a hydraulic tank can be either a design feature or a warning sign. In machines like the Badger 4435, where hydraulic precision is critical to safe lifting and rail work, understanding the source and behavior of tank pressure is essential. Whether it’s a clogged breather or a deliberate charge system, the key is knowing what’s normal—and acting quickly when it’s not. In hydraulic systems, silence is golden, but a hiss can speak volumes.