Onboard ships, many automation systems are used to ensure that the machinery runs efficiently and smoothly. However, machines continue to malfunction, owing to the crew’s lack of expertise about the system in question. As a result, it is critical to properly install and maintain the system in order to avoid any harm to the machinery.
What is Spark Erosion?
When two current-carrying dissimilar metals come into contact, sparks flow through the point of contact, eroding the tiny metal and creating a hollow.
A propeller, hull, bedplate, crankshaft, bearing, and other metals are used to construct a vessel. The current from the cathodic protection system is often present in these sections, creating the ideal environment for spark erosion.
A minor galvanic current flows through an anodic section of a ship’s hull constructed of steel that is immersed in seawater, causing corrosion and erosion.
Effects of spark erosion
An Impressed Current Cathodic Protection system is used to prevent galvanic corrosion, notably near the stern of the ship where the propeller is located. To ensure a continuous circuit and avoid malfunction, the propeller shafting is earthed.
The stern tube, propeller shaft, and bearings are all in touch when the propeller is at rest. The primary engine bearing and journal are also in contact with each other, ensuring circuit continuity. The shaft gets somewhat electrically insulated when the ship is running due to the rotation of the propeller and lubricating oil coating. It could also happen on the tail shaft, where a non-metallic bearing serves as an insulator.
The propeller at the aft is a huge area of exposed metal that collects protective cathodic current, which discharges from the lubricating film and generates an arc. This causes bearings to ignite, which can exacerbate the problem if the lubrication oil is contaminated with seawater.
If these effects persist for a long time, they may cause overheating of main engine bearings due to incorrect lubrication caused by cavities generated by spark erosion. It could also result in the creation of oil mist, an engine shutdown, or, in the worst-case scenario, a crankcase explosion.
Reasons for Spark Erosion
Some of the main reasons which result in Spark erosion related problems on ships are
- The shaft earthing system is broken or inadequately installed.
- The current system setting for cathodic protection is incorrect.
- The hull covering is thicker than necessary, which will accelerate shaft galvanic corrosion.
- The earthing device’s slip rings and brushes are worn out.
- The connection between the shaft and the earth device is dirty.
For the primary engine’s shaft, two earthing devices are recommended. One is for grounding, and the other is for connecting to the voltmeter to measure the potential difference between the ship’s shaft and hull.
If the potential difference is less than 50 mv, the effect of spark erosion will be minimal.
Did you subscribe to our newsletter?
It’s free! Click here to subscribe!
Source: Marine Insight
