Emerson Process Management 53eA instruction manual Time Proportional Control TPC Mode Code

MODEL 54eA	SECTION 15.0
	CALIBRATION - CONTROL

15.2TIME PROPORTIONAL CONTROL (TPC) MODE (Code -20)

In the TPC mode, you must establish the following parameters which will determine how the Model 54eA controller responds to your system (see Section 5.7):

•Setpoint

•Time period

•URV point (or 100% on)

•LRV point (or 0% on)

•Proportional

•Integral

•Derivative

The following describes how TPC works. In the exam- ple, pH is to be controlled.

The setpoint is the pH you want to control the process to. Time period is programmed in seconds and defines the interval during which the controller compares the pH input from the sensor with the setpoint. In the TPC mode the controller divides the period up into pump on- time (feed time) and pump off-time (blend time).

The URV setting determines how far the pH must devi- ate from the setpoint to get the pump to be on for the entire period. The LRV setting determines how close the pH must be to the setpoint for the pump to be off for the entire period. The LRV setting should always be set at zero. When the error (the pH minus the setpoint) is between the URV and LRV values, the relay will be energized for some portion of the time period. As the pH value approaches the setpoint, the pump will be feeding for shorter and shorter intervals, and the chem- icals will be allowed to mix for longer and longer inter- vals of the period. This relationship is illustrated in Figure 15-3.

The exact amount of on time and off time per period is determined by the settings for proportional, integral, and derivative bands. The proportional band (P) in % is a separate adjustment that narrows (or widens) the range of the TPC 0-100% action. Smaller values are used for more control response. For a setpoint of 7 pH, a URV of 2 pH, and P=100%, a pH reading of 8 would result in a relay on (8-7)/((2-0)*(100%)) or 50% of the time. If P was changed to 50%, the same relay would be on (8-7)/((2-0)*50%) or 100% of the time.

The integral band is set in seconds and acts to increase the controller output as more time is spent away from the setpoint. A smaller value in seconds will result in faster integration response. Too low a value will result in excess oscillation.

FIGURE 15-3. Time Proportional Control

The derivative band is set in % and acts to prevent changes in the reading. This setting should generally be set to zero for pH applications.

TPC offers precise control by forcing the pump to feed chemical for shorter periods of time as you approach the desired setpoint. If the process faces a large upset, TPC mode forces the pump to feed chemical for longer periods of time as the process deviates further from the setpoint. This action continues until the pump is feed- ing all the time, providing a speedy recovery from large up-sets.

The controller can be programmed to be direct or reverse acting, depending on the pH (or temperature) value selected for URV. For example, if the controller is direct acting based on pH, such as in caustic chemical addition control, the pH will rise as chemical is added, so the URV value will be below the LRV (i.e. below zero). As the pH rises toward the control point value, the pump will be on for gradually less time. Conversely, if the controller is reverse-acting based on pH, such as in acid addition for control, the pH will drop as acid is added, and the URV value will be positive. The pH will fall toward the control point value, and the pump will be on for gradually less time.

Complete TPC configuration is explained and typical settings for these parameters are listed in Section 5.0. After startup, the operator needs to adjust only the 0% On to maintain the desired chemical concentration.

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