We have already studied about the general overview of steering gears and construction and components of a steering gear arrangement in our previous articles. We also saw how steering gear testing is carried out on board various types of ships. In this article we will continue to study about steering gears and talk about the various safety devices which are fitted on them such as the liquid level switch and other arrangements so as to ensure safe operation of the entire system.
SWITCH LEVEL 1:
The steering system has two lube oil tank system with 2 rams functioning on each system. The level switch 1 gives an initial alarm following a loss of oil from either system. In normal operation one power unit provides hydraulic power to all four rams.
The causes for the loss of oil is mainly due to broken hydraulic pipes and damage in the ram, thus the leakage oil drains to the bilge well.
SWITCH LEVEL 2:
When no action is taken immediately upon the previous alarm, the loss of oil continues and over a period of time this loss of oil intiates one or both of the level switches 2. This leads to the energization of a solenoid operated servo valve causing a combined isolating valve and bypass valve to operate. Thus splitting the system such that each power unit supplies power to 2 rams only .
At the same time this switch automatically starts the other standby helesaw pump to assist the turning and building up the pressure in the rams.
SWITCH LEVEL 3:
This switch will get activated when there is still no further improvement in the loss of oil on the faulty side. In such a situation the steering continues un-interupted if one unit is stopped and thus the rudder is turned with the help of the other unit system of 2 rams only, isolating the faulty one, and this is called as emergency steering.
Thus the steering system is designed to work at half of the maximum torque on the steering system.
The defective system is put out of action and isolated.
A relief valve is fitted in the system to prevent over pressure in the hydraulic system due to shock loading of rudder. Shock loading of the rudder could occur in a variety of cases such as bad weather.
MANUAL BY PASS VALVE:
This valve is only operated when there is a failure of the one of the systems and the rudder stock has to be turned mechanically, so that the ideal fluid can flow from the high pressure side to low pressure side.
LOW PRESSURE VALVE :
Low pressure valve is a NON RETURN VALVE fitted in the main hydraulic pressure line, which opens for oil filling from a header tank if a low pressure situation occurs in the system .
It stops the hydraulic pump when the position of the rudder turning is at an angle of 36o. It automatically cuts of the supply because it is the maximum turning angle of rudder. (It normally operates at 35o angle).
In case of electrical stopper fails, the mechanical stopper is provided so that it stops the tiller arm after 36o angle of rudder
Electrical motor overload alarm
When ever hydraulic motor or helesaw pump motor draws excessive current during bad weather, the steering should not fail. Instead it gives an alarm about excessive current being drawn by the motor. Continued recurrence of this alarm could mean that the electrical motor might burn up.
POWER FAILURE ALARM:
The full power failure alarm will be raised if the power fails in black out condition. Emergency power should be arranged with in 45 seconds and the emergency generator must be capable to give power for at least 30 minutes for a big ship and 10 min in case of small ships (ships<10,000 ton diaplacement).
- Author's experience and expertise