Answer EKG Question 42

Q42. With reference to hydraulic steering gears, sketch and describe each of the following:
(A)Single failure concept;

(B)100% redundancy.

Answer:
Four Ram electro-hydraulic steering gear system:-

Automatic Isolation system:

Automatic isolation system is designed and manufactured in accordance with the requirements of classification society and/or SOLAS regulation. Automatic isolation system shall be provided in order to detect and isolate the defective system automatically when working oil have been lost from one system, and make the other sound system remain fully operational. In this case max steering torque is reduced to half of the rated torque and ship's speed should be reduced under half of max service speed.

If it takes long time to repair the defective part. It would be better to operate manually the automatic isolating valves according to the operating instruction plate in order to isolate the defective part and switch off the automatic isolation system

Detailed Description of Main Parts

Automatic Isolating Valve.

In lieu of manual isolating valve for standard steering gear, automatic isolating valve (solenoid control pilot operated valve) is provided on the valve unit mounted on each oil tank in order to isolate the defective system automatically

The automatic isolating valve is controlled by the signal of oil level switch through control panel.

Namely, when solenoid is de-energized, pilot pressure is blocked at solenoid valve and hydraulic change over valve is located in neutral position by the spring force. On the other hand, when solenoid is energized. The pilot pressure is led to the hydraulic change over valve through the solenoid valve and moves the main spool against the spring

Oil Level Switch

In lieu of oil level switch sensing one oil level (for alarm) for standard type steering gear. Oil level switch sensing two oil levels (for "Low and "Low-Low") is mounted on each oil tank.

The signal of oil level switch shall be transmitted to the control panel.

System Test Bar

The system test bars are mounted on the each oil tank to confirm the operation of automatic Isolation system.

By pushing these test bars, oil level switch is actuated.

Signal from oil level switch is transmitted to the control panel and operate the automatic isolation system in sequence as shown above.

Control Panel

Control panel is installed in the steering gear room in order to operate the automatic isolation system in regular sequence according to the condition of steering gear.

Operation of Automatic Isolation System:

At normal steering (when all are in normal condition), the conditions of oil level switch and automatic isolating valve are as follows.

Oil level switch---closed (non-activated)

Automatic isolating valve--off (de-energized)

Sequence of operation:-
 when working oil have been lost from one system. The automatic isolation system shall be operated in the following sequence and the steering can be maintained

Failure Mode in Both Power Units Operation

When both the pumps are running, the hydraulic oil is being used from both the tanks, thus the isolation of the system follows the tank level which goes down first.   
Isolation of defective part takes place in the following manner.
(refer to the operation sequence and operating instructions)

When oil level in No.1 oil tank goes down firstly:

1. The oil level in No.1 oil tank goes down to "Low" position and audible and visual alarms are given on the navigation bridge and in the machinery space.

2 At the same time No.1 automatic isolating valve (IV-1) is automatically energized and the hydraulic system associated with No.2 power unit is isolated.

3. If the oil loss is in the hydraulic system associated with No.2 power unit the level in No.2 oil tank will reach "Low-Low" position

Then No.2 power unit is automatically stopped. So steering is being carried out by No 1 power unit and its two related cylinders (No. 3& No. 4) giving 50% torque

4. On the other hand. If the oil loss is in the hydraulic system associated with No.1 power unit, the oil level in No.1 oil tank goes down to "LOW-LOW" position and No.1 automatic isolating valve (IV-1) is automatically de-energized. And also No.1 power unit is automatically stopped and No.2 automatic isolating valve (IV-2) is automatically energized.

Then the hydraulic system associated with No.1 power unit is isolated. Steering is now being carried out by No.2 power unit and its two related cylinders (NO.1& No 2) giving 50% torque.

When oil level in No.2 oil tank goes down firstly:

1. The oil level in No.2 oil tank goes down to "Low" position and audible and visual alarms are given on the navigation bridge and in the machinery space

2. At the same time No.2 automatic isolating valve (IV-2) is automatically energized and the hydraulic system associated with No.1 power unit is isolated. If the oil loss is in the hydraulic system associated with No.1 power unit. The oil level in No.1 oil tank will reach "Low-Low" position. Then No.1 power unit is automatically stopped. So steering is being carried out by No.2 power unit and its two related cylinders (No.1& No.2) giving 50% torque.

3. On the other hand. If the oil loss is in the hydraulic system associated with No 2 power unit. The oil level in No.2 oil tank goes down to "LOW-LOW" Position and No.2 automatic isolating valve (IV-2) is automatically de-energized. And also No.2 power unit is automatically stopped and No 1 automatic isolating valve (IV-1) is automatically energized. Then the hydraulic system associated with No.2 power unit is isolated. Steering is now being carried out by No.1 power unit and its two related Cylinders (No.3 & No.4) giving 50% torque.

Inspection of Automatic Isolation System:

lf the automatic isolation system has been in abnormal condition. The automatic isolation of defective system shall not be carried out in an emergency. Therefore it is necessary to inspect before voyage and periodically the operation of automatic isolation system in the Following procedure

.1. Start the electric motor(s)

.2. Push down the system test bar "LOW" or "LOW-LOW"

Confirm that each element is operated in regular sequence as shown in the check list below.

After inspection is carried out. Remove the oil pans replenish the working oil in the oil tank and operate the system test valves as they were (valve (1): open, valve (2): shut)

Failures when one power unit is operating:-
In this section we will see the sequence of isolation of the defective part and automatic taking over of system by the healthy part of power unit.

1. Failure mode in No.1 Power Unit Operation

.1. The oil level in No.1 oil tank goes down to "Low" position and audible and visual alarms are given on the navigation bridge and in the machinery space.

.2. At the same time. No.1 automatic isolating valve (IV-1) is automatically energized and the hydraulic system associated with No.2 power unit is isolated.

.3. If the oil loss is in the hydraulic system associated with No. 2 power unit, steering is being carried out by No. 1 power unit and its two related cylinders (No.3& No.4) giving 50% torque.

.4. On the other hand. If the oil loss is in the hydraulic system associated with No.1 power unit, the oil level in No. 1 oil tank goes down to "LOW-LOW" position and No. 1 automatic isolating valve (IV-1) is automatically de-energized. And also No.1 power unit is automatically stopped, No. 2 automatic isolating valve (IV-2) is automatically energized and No.2 power unit is automatically started then the hydraulic system associated with No. 1 power unit is isolated. Steering is now being carried out by No.2 power unit and its two related cylinders (No.1 & No.2) giving 50% torque

2. failure Mode in No.2  Power Unit Operation:-

.1. The oil level in No.2 oil tank goes down to "Low position and audible and visual alarms are given on the navigation bridge and in the machinery space.

.2. At the same time, No.2 automatic isolating valve (IV-2) is automatically energized and the hydraulic system associated with No.1 power unit is isolated.

.3. If the oil loss is in the hydraulic system associated with No.1 power unit steering is being carried-out by No.2 power unit and its two related cylinders (No.1 & No.2) giving 50% torque.

.4. On the other hand if the oil loss is in the hydraulic system associated with No.2 power unit, the oil level in No.2 oil tank goes down to "LOW-LOW" position and No.2 automatic isolating valve (IV-2) is automatically de-energized. And also No.2 power unit is automatically stopped. No.1 automatic isolating valve (IV-1) is automatically energized and No.1 power unit is automatically started. Then the hydraulic system associated with No.2 power unit is isolated. Steering is now being carried out by No.1 power unit and its two related cylinders (No.3& No.4) giving 50% torque.