Does the Pressure Gauge Needle Hunt? Watch out for Air-in-Oil.


Technical Tip:

Shipboard staffs require frequent engineering tips to brush up and hone their skills.  As a professional Superintendent or a Vessel Manager, you can offer a quick tip as you visit the ship so that they can grasp it quickly and remember it forever.

One such tip is all about observing pressure gauges in the hydraulic systems.  Whenever there is a continuous jump or spikes in the pressure gauge (especially on the pump’s discharge or downstream of the system), then it is most likely due to air in the hydraulic system.

In such cases, it is essential to release the air from the hydraulic system.  Such entrapment of air or gas will be released into the expansion tank over a period as the system operates.  However, if there is a continuation of such jump in the pressure gauge, it is most likely that air is being sucked into the system.  The air ingress in the system can also be confirmed by an observable change in sound while the pump is running.

What can be done when air is being sucked into the hydraulic system?

  1. Spray some lubricating oil on the pump’s shaft seal as a test of air ingress past the seal.
  2. You should hear a momentary change (quieting) in the pump’s sound, indicating less aeration.
  3. Alternatively you can consider applying a thin coat of grease on the shaft seals.
  4. Grease can also be used on the filter covers/joints and pipe flanges to check for air ingress.  It is to be noted that the hydraulic lines are usually seamless lines with minimal bends and adjoining flanges.

Contaminants of hydraulic oil are broadly defined as any substance that impairs the proper functioning of the fluid.  Air fits this definition, and whenever air gets entrained within the oil, corrective action is required to prevent damage to both the oil and the other components in the hydraulic system.

Air can be present in four forms:

  1. Free air – such as a pocket of air trapped in part of a system.
  2. Dissolved air – hydraulic oil contains between 6 and 12 percent by volume of dissolved air.
  3. Entrained air – air bubbles typically less than 1 mm in diameter dispersed in the oil.
  4. Foam – air bubbles typically greater than 1 mm in diameter that congregate on the surface of the oil.

Of these four forms, entrained air is the most problematic.  Pre-filling components and proper bleeding of the hydraulic system during start-up will usually eliminate free air.  Small amounts of foam are cosmetic and do not pose a problem.  However, if large volumes of foam are present, it’s sufficient enough to cause the reservoir to overflow.  For example, this can be a symptom of a more serious air contamination and/or oil degradation problem.  This problem is not associated with hydraulic systems alone, but also with any positive displacement pumps used on board the ship.

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  1. My husband works on a ship which is about to get a new stainless steel pressure gauge. So he was just hoping to learn a little about how to read one so that if he’s ever around their new one, he can help. So thank you for saying that when there is continuous jumping in the pressure gauge, it is probably because of air in the hydraulic system.


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