Answer EKM Question 42

Q42. Explain the term “cascade control’ and sketch such a system suitable for use with a main engine jacket cooling water system. Show the variation of pressure and temperature at major points of the system. 

Answer: Cascade control is useful when there are more than one variable that influence the single parameter which is to be controlled.

The sketch shows a lubricating oil cooling system with cascade control.

The system consists of a pump drawing lubricating oil from the main engine sump and pumping it through the cooler where the oil is cooled by circulating sea water through the cooler tubes. The outlet oil from the cooler is led to the main engine bearing oil and piston cooling oil system.
From proper functioning of the system, it is desired to maintain a constant oil temperature of 45 deg cel. at the engine inlet. The control is by means of a three-way valve that allows a certain amount of oil to bypass the cooler. If the engine inlet temperature is too high, more oil will be passed through the cooler and if too low, more oil will bypass the cooler.
There are two main variables in this case that can influence the oil inlet temperature - the  engine load and the sea cooling water inlet temperature - forming two loops:
An outer loop that measures the engine oil inlet temperature and passes the information for further processing to a controller which is called the master or primary controller; and
An inner loop that measures the sea water inlet temperature and passes the information to a second controller called the slave or secondary "controller The secondary controller processes the signals from the primary controller and the second sensor and sends an appropriate signal to the 3-way valve to control the oil temperature at inlet to the engine.
The main advantage of the cascade system is that it provides a quicker response and better management of the controlled parameter. When the sea water temperature changes, an immediate signal will be sent to the slave controller for adjustment of the 3-way valve even before the actual oil inlet temperature begins to change. The response is therefore faster.