Drain line

The drain line is a safety measure for pumps, radial motors, and cylinders in hydraulic systems. It serves to keep the pressure within the housings of these components constant.

Necessity of the drain line

Highly loaded hydraulic components are technically very difficult to seal. Internal leakages occur in many areas. This oil collects in the housing and must be discharged. If it is not, the internal pressure will burst the seals and the housing. For this reason, openings are provided on the components through which the leakage oil can escape. The line that carries away the leakage oil is called the drain line.

The paths of the leakage oil

Leakage oil does not differ in any way from hydraulic oil. It is part of the normal fluid that is initially lost through leakage. For this reason, leakage oil must be included in the calculation of volumetric losses. These leakages, for example at piston rings or non-sealable components such as pump gears, are technically unavoidable. The oil exits through the holes provided for this purpose and initially runs through the drain line through a leakage oil filter. This is required to filter out dirt particles and abrasion from the hydraulic oil.

Since the leakage oil flows past narrow gaps under high pressure, it has a strongly abrasive effect on its surroundings. These particles of rubber, bronze, or carbon can be very destructive and can also remain unnoticed for a long time. Therefore, a drain line always includes a corresponding filter. From the leakage oil filter, the oil flows back into the hydraulic tank via another drain line. There it mixes again with the normal hydraulic oil. The circuit is closed.

Different methods for calculating the drain line

Calculating the drain line is complex. A distinction is made between three applications:

• Leakage flow at the flat gap
  The “flat gap” occurs at straight housing edges.
• Leakage flow at the concentric annular gap
  The “concentric annular gap” is a typical application for seals on piston rods and cylinders.
• Leakage flow at the eccentric annular gap
  The “eccentric annular gap” is the calculation basis for pumps with an eccentrically rotating center shaft.

Damage to the drain line

Damage to the drain line is noticed very quickly. The oil escapes, runs down the outside of the hydraulic power unit, and collects on the floor. Major damage or a failure of the entire system is not immediately expected in the event of a defective drain line. The return flow of the leakage oil is comparatively low.

However, escaping hydraulic oil is fundamentally a disruption of operating processes that endangers machine safety and the environment. If defective drain lines are noticed, they should be replaced immediately. Since these lines are not under pressure, such a repair may be possible during operation. For safety reasons, however, the hydraulic system should be shut down during any repair.