Understanding Overspeed Trips

Introduction

The main aim of any diesel engine governor is to keep the speed of the engine within reasonable limits without getting affected by the changing load on the engine. In case the governor is not able to keep the speed of the engine within specific limits, the engine might speed up to such a level that it can damage the engine itself. In engines wherein the governor does not fail safe, over speed trips are used.

Overspeed Trip

The main function of an over speed trip is to cut the fuel supply to the cylinders in case the engine speed rises above the pre-set level. Generally fitted on steam turbo alternators or generators, over speed trip prevents the engine from getting damaged because of extremely high speeds.

How Does An Over Speed Trip Work?

The design of an over speed trip consists of a heavy bolt with a comparatively heavy head. The bolt is fitted at the forward end of the engine shaft where no engine torque is transmitted and is bored throughout the diameter of the shaft. Just like any other conventional bolt, the over speed trip bolt is held in place by using a nut and a spring.

At normal speeds, the bolt stays at the same position; however if the speed of the engine rises beyond the pre-set range, the centrifugal force generated by the heavy head of the bolt makes it fly out of the spring and nut’s compression pressure. The bolt eventually comes out of the bore, getting rid of the spring and washers.

Moreover, the arrangement is made in such a manner that the bolt head that comes out of the bore hits a trip lever which shuts the supply of fuel to the engine cylinders, eventually stopping the engine. The mechanism for stopping the fuel supply varies according to the type of the ship and engine. For e.g. in motor ships having steam turbo alternator, the over speed trip is controlled with the lubricating oil low pressure cut-out and the steam stop valve; whereas in few other ships the over speed trip is controlled using a system of levers and linkages that are connected by the turbine steam stop valve.

The steam stop valves actually controls the supply of fuel going to the cylinders. When the over speed trip operates, the pin holding the steam stop valve in the open position is released, closing the valve and stopping the flow of both, the steam and the fuel. The set speed for operating the over speed trip can be adjusted by adding washers below the spring and then adjusting their compression pressure.

Apart from over speed trips, permanent magnet alternators from engine camshafts along with rectifiers can also be used to prevent engine from racing at high speeds. However, this technique can only be used for electronic governors. As the speed of the engine crosses the pre-set speed, the voltage automatically increases, making the governor to reduce the fuel supply to the engine cylinder and lowering its speed.

References

Lambs Question and Answers - Stanley G. Christensen