Hydraulic Top Bracing System for Main Engine
Introduction
Hydraulic top bracing cylinders are placed between the main engine and the ship’s hull to detune the natural frequencies that may arise during the engine’s regular operation. These cylinders are designed to accommodate longitudinal and transverse movements caused by thermal expansion and various loading conditions of the ship. Typically, they are installed on the starboard side of the engine, but if more convenient, installation on the port side is possible. Ideally, installation on both sides is preferable. The standard configuration includes two or four top bracing cylinders, depending on the size of the engine and the number of cylinders.
Components
The hydraulic top bracing system is integrated with the following components:
(1) Top Bracing Cylinders
Each cylinder is equipped with an adjustment indicator, non-return valves at the oil inlet, and flexible connections to the pipe system.
(2) Relief Valve Block
Each cylinder contains a relief valve block that includes two relief valves: one for high oil pressure and one for low oil pressure. The high pressure setting of the high relief valve is 1.7 MPa, while the low pressure setting of the low relief valve is 0.2 MPa.
(3) Pump Station
The pump station consists of an oil tank, two pumps, a level switch for alarm, a control valve, and other necessary accessories. Within the control valve assembly, there is an oil filter, a pressure control valve, two pressure switches, three ball valves, and a pressure gauge. The oil filter has a fineness of about 10μm and includes a differential pressure switch. When the alarm is triggered on this switch, indicating a pressure difference, the filter requires cleaning. The pressure switch is responsible for regulating the system’s pressure and managing the changeover between the two pumps.
(4) Control box
The control box, which includes switches and power/alarm indicators, is connected to the ship’s control system.
(5) Oil accumulator
Each pair of cylinders is equipped with an oil accumulator to stabilize the pressure of the system.
Working Principle
The hydraulic top bracing system consists of single-acting units with automatic return. During regular operation, the pump station delivers oil at a pressure of approximately 0.5 MPa to the inlet side of the system. This pressure serves as the minimum workload for the positive stroke of the hydraulic pistons, occurring when the distance between the hull and the engine increases. A non-return valve on the inlet side of the cylinders generates rising pressure during the negative stroke of the hydraulic piston, which happens when the distance between the hull and the engine decreases. The high-pressure relief valves within the relief valve blocks open at around 1.7 MPa, redirecting the oil to the outlets on the back of the pistons. The oil then returns to the tank through the return pipe system.
The oil accumulator facilitate rapid oil supply to the top bracing to prevent significant pressure fluctuations when the hydraulic cylinders experience heavy vibrations. When the engine and ship structure begin to vibrate, the oil pressure rapidly increases. The forces are transmitted through the top bracing cylinders, which act as rigid connections, thereby detuning the natural frequencies.
Upon the ship’s arrival at port, when the engine receives a telegraph order to “finish with engine,” the pump station automatically stops. Simultaneously, the solenoid valves in the relief valve block shift each top bracing cylinder to “low” mode. This action immediately reduces the work pressure of the top bracing cylinders from a maximum of 1.7 MPa to approximately 0.2 MPa.
Before startup, when the engine receives a telegraph order to “stand by,” the pump station starts automatically, and the solenoid valves in the relief valve block are deactivated. Each top bracing cylinder shifts to “high” mode, and the workload in the top bracing cylinders builds up to approximately 0.5 MPa at standstill, reaching a maximum of 1.7 MPa when the engine is started.
Maintenance
During regular navigation, the position indicator should point to the green band. A position between the green and red bands might be acceptable if the current loading condition of the vessel is not considered “normal.” However, when the vessel returns to a “normal” loading condition, the indicator should return to the green band. It is crucial to never operate the top bracing cylinder when the position indicator is in the red band. Operating in this position could potentially damage the hydraulic piston.
Reference: HYDRAULIC TOP BRACING SYSTEM OF M/E MANUAL