In the previous article we studied the pressure regulating system incorporated within the IG systems on board ships. We saw that pressure regulating valves serve the purpose of regulating pressure when it tends to go beyond certain limits on either side of the desired pressure spectrum. As an additional safety measure there might be pressure vacuum breakers fitted in the lines which prevent the build up of excessive pressure and vacuum in case other systems fail to achieve the same.
Pressure – Vacuum Breakers
The main feature of a pressure vacuum breaker is that it is pretty simple in construction and working principle and therefore requires very little maintenance. The schematic diagram of a typical PV breaker is shown below. as you can see from that diagram it just consists of a an open section of the pipe which would have been open to the atmosphere but for the liquid which is present in the container.
It is easy to imagine that as the pressure inside the region of the T-section increases it pushes the liquid in the container downwards. The more the pressure rise the deeper the level of liquid goes down. This would continue to happen unless the liquid goes down as far as level shown by the red coloured arrow marked at the “Break Point". Any further increase in pressure inside the chamber would push the liquid further and expose the inside of the chamber to the atmosphere by throwing the liquid out of the container thereby easing the excess pressure inside that area.
Similary when vacuum increases it pulls the liquid inside the leg of the T-section and more the vacuum, the higher the level that would rise. If the amount of vacuum rises beyond a certain value the whole amount of liquid would be pulled inside the pipe and again it would be exposed to atmosphere thus breaking the excess of vacuum.
It can be seen that amount of liquid inside the container would depend on several parameters such as the requires values of pressure and vacuum to be maintained, the operating conditions of the IG system as a whole, the settings of various alarms and so forth. The breaker should be carefully calibrated for the required values and should be actually tested for working at required pressure settings.
The liquid used for this purpose is either oil of a specific category or it would be a mixture of water and glycol, the latter being added as a safeguard against extremely low temperatures wherein water would simply freeze. Apart from freezing care should be taken to main the quantity of the liquid as that would change the pressure settings. This quantity could change due to evaporations, spillage, ingress of sea water and so forth.
In the next article we will study about the instrumentation used in the IG System