oil tankers of 150 gross tonnage and above shall be provided with effective oil/water interface detectors approved by the Administration for a rapid and accurate determination of the oil/water interface in slop tanks and shall be available for use in other tanks

Control

.fail safe

P action:

Proportional action is the when the controller output signal is proportional to the deviation of the measured value from the set value. Alternatively, the rate of change of output signal is proportional to the rate of change of deviation.

Proportional band:

The proportional band is defined as the amount of change in input (or deviation), as a percent of span, required to cause the control output to change from 0% to 100%

Proportional Gain is the ratio of output change (%) over the measured variable change (%) that caused it.

If proportional band is 25% then the proportional gain is 4.

I action:

Integral action time: The time a integral controller take to produce the same output as a P controller.

D action:

The rate of change of input(deviation) is proportional to controller output.

It has a stabilizing effect but can not be used alone.

The derivative or differential controller is never used alone. With sudden changes in the system the derivative controller will compensate the output fast. The long term effects the controller allow huge steady state errors.

Fail safe in a pneumatic control systems ?

On failure of control air supply, the pneumatic actuator may be arranged to move to a position that allows the plant to continue to operate safely, in other words, fully opened or fully closed. This is known as fail safe. For example, in the case of a jacket cooling water system, on failure of control air, the actuator opens fully to allow jacket water to the cooler without bypass. On the other hand a fuel oil control valve for boiler closes completely on such a failure. This ensures safety of the plant.

Fail-Safe condition:

In boiler auto combustion control system,

“Fail-safe” conditions are ensured by shutting off fuel oil to burners under certain fault conditions relating to loss of power supplies arising in the ACC.

They are ensured as follows:

1. Loss of ACC electric power supply: A sol- operated 3- way v/v de-energizes & vents the actuator air from FO control v/v.

2. Loss of actuator power to FO Control v/v:

ATO type FO control v/v shuts automatically.

3. Loss of Level Transmitter power (pneumatic or electric): A solenoid operated 3-way v/v de-energizes & vents the actuator air from FO control v/v.

Case 1: Pneumatic Level Transmitter

Case 2: Electronic Level transmitter

Fail set

.fail set

In some other systems, control air supply failure locks the position of valve at that time of failure. This is called fail set. The advantage of this system is that the plant gets stable and have time for normal shutdown or can wait for reestablishment of control air supply for some time. Example for such a system is boiler water level control.

Hysteresis

.hysteresis

The difference between up scale and down scale results in instrument response when subjected to the same input but approached from the opposite direction

Example: A control valve has a stroke of 1.0 inch and we give the valve a 9 psig signal. The valve trave0ls 0.500 of an inch. We then give the valve a 12 psig signal, and the valve travels to 0.750 an inch. When the valve is then given a 9 psig signal, the stroke is measured at 0.501. That represents hysteresis.

Hysteresis can be caused by packing friction, loose linkage, pressure drop, etc.