CONTROL LOOP DIAGRAMS Process ControlsLubricating oil reduces friction between moving mechanical parts and also removes heat fromthe components. As a result, the oil becomes hot. This heat is removed from the lube oil by acooler to prevent both breakdown of the oil and damage to the mechanical components it serves.The lube oil cooler consists of a hollow shell with several tubes running through it. Coolingwater flows inside the shell of the cooler and around the outside of the tubes. Lube oil flowsinside the tubes. The water and lube oil never make physical contact.As the water flows through the shell side of the cooler, it picks up heat from the lube oil throughthe tubes. This cools the lube oil and warms the cooling water as it leaves the cooler.The lube oil must be maintained within a specific operating band to ensure optimum equipmentperformance. This is accomplished by controlling the flow rate of the cooling water with atemperature control loop.The temperature control loop consists of a temperature transmitter, a temperature controller, anda temperature control valve. The diagonally crossed lines indicate that the control signals are air(pneumatic).The lube oil temperature is the controlled variable because it is maintained at a desired value (thesetpoint). Cooling water flow rate is the manipulated variable because it is adjusted by thetemperature control valve to maintain the lube oil temperature. The temperature transmittersenses the temperature of the lube oil as it leaves the cooler and sends an air signal that isproportional to the temperature controller. Next, the temperature controller compares the actualtemperature of the lube oil to the setpoint (the desired value). If a difference exists between theactual and desired temperatures, the controller will vary the control air signal to the temperaturecontrol valve. This causes it to move in the direction and by the amount needed to correct thedifference. For example, if the actual temperature is greater than the setpoint value, thecontroller will vary the control air signal and cause the valve to move in the open direction.This results in more cooling water flowing through the cooler and lowers the temperature of thelube oil leaving the cooler.(B) in Figure 9 represents the lube oil temperature control loop in block diagram form. The lubeoil cooler is the plant in this example, and its controlled output is the lube oil temperature. Thetemperature transmitter is the feedback element. It senses the controlled output and lube oiltemperature and produces the feedback signal.The feedback signal is sent to the summing point to be algebraically added to the reference input(the setpoint). Notice the setpoint signal is positive, and the feedback signal is negative. Thismeans the resulting actuating signal is the difference between the setpoint and feedback signals.IC-07 Page 12 Rev. 0
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