Marine Engineering: Air Starting Valves Used In Ships

Introduction

 

What do you do when you want to start your car or bike? Well you simply turn your ignition key and the battery of the vehicle turns the engine with the help of a starting motor and the firing process starts which continues till the engine is able to run on its own, of course this process takes very short time. But how do you turn the gigantic engine like that of a ship's main propulsion plant? The answer lies in the use of compressed air for starting the engine, hence it is also known as the starting air.

Starting air admittance valves or air starting valves are provided in the cylinder head mountings of marine diesel engines. Their main function is to admit the starting air into the cylinder for starting the engine by air and closes when the engine picks up speed and starts running on fuel oil. In this article we will take a look at the construction and working principle behind these air admittance valves. Just take a close look at the picture given below which shows the full constructional details of the valve before proceeding to study further.

Construction

 

The air starting valve is operated by pressurised air from the air bottle. Each air admittance valve is equipped with its own distributor and Ahead-Astern cam.

These cams have been adjusted for their opening times in such a manner, so as to ensure overlapping for consecutive cylinders for a certain time period, based on the firing order of the engine

Each cylinder is equipped with a starting valve fitted at the lateral side of the cylinder cover. The housing for the valve is integrally cast with the lower part of the two piece cylinder cover.

The valve chest is made out of a steel casing. The miter faced valve is carried at one end of the spindle, and the other end is provided with the guide piston.

The control piston is screwed with the extension of the valve spindle. The valve guide piston is fitted in a bush and is equipped with a number of seal rings.

A store spring keeps the valve firmly seated. A control piston with its cylinder is secured with the valve body by two studs, thus the body makes a gas tight joint with the cylinder cover.

Operational Details

The starting air valve is PNEUMATICALLY operated to admit starting air at the instant of starting.

Basically two types of air are used in the operation of the air admittance valve namely - pilot air and the actual starting air. The pilot air acts to control the entry of the main air which in turns actually starts off the engine and the procedure is described as follows.

The opening of the starting air valve depends upon the PILOT AIR which is send by the distributer; the pilot air is delivered from the control air which passes through the hydraulic interlocks and air starting handle.

The distributer valve is pressed down by the PILOT AIR when the roller stands against the inwardly depressed segments of the cam profile.

This is the starting phase of the cylinder, pilot air get passage and acts on the control piston and opens the starting air valve, thus the starting air is admitted into the cylinder.

In the up coming articles we can see the consequence of leakage of air starting valve and starting air line explosion
.