Tuesday, 9 June 2015


The primary concern in most process control systems is the degree of accuracy in responding to process changes and the time it takes to achieve the desired correction.
Control accuracy is dependent upon the proper selection, calibration, and operation of the instruments and control systems.  Factors affecting the degree of accuracy (process response time) are:
S    Process Lag.
S    Measurement Lag.
S    Transmission Lag.
S    Response Lag.
Lag is a process term for delay.  In other words, delays in measurement, transmission of control signals, and the process itself to respond to changes.  All effect the process response time.  The degree of accuracy of any process control system can be maximised by reducing these lags.
When the response time for the system is not suitable for the process requirements, design changes are usually made.  These design changes can include the addition of booster relays, or modifying the control loop to maximise the degree of system accuracy.

Process Lag

Process Lag is the time difference between the control elements set point being changed and the amount of time it takes for the process variable (PV) to change in response to the change.
As shown in Figure 2-1, a heat exchanger is operating at a temperature of 200°C.  If the operator wishes to increase its temperature to 300°C, he must allow more steam (or Heat Transfer Fluid) to flow through the exchanger.  However, the temperature in the heat exchanger will not rise to 300°C immediately after he increases the steam flow.
The steam flow changes from one steady value to another quite quickly but the change in the process fluid temperature is gradual.  The process characteristics that cause the process fluid temperature to change gradually instead of instantly are the resistance, capacity and dead time of the process fluid (see below).  This time difference is the process lag.
Figure 2-2 shows how the response curve for the process fluid temperature "lags" behind the response curve for the steam flow.
Process Resistance
Process resistance can be defined as the opposition to the transfer of energy or flow through the process.
Capacity of Process
Process capacity is the ability of a process to hold energy (or a quantity of material).
Dead Time
Dead time is the interval from the time that a control element change takes place until its effect is felt on the control variable as shown in Figure 2-2.
The higher the capacity of a process, process resistance and dead time, the higher will be the process lag and vice versa.



Measurement Lag

Measurement lag is the time it takes the measuring device to give a signal which accurately represents the process variable.
In most cases, measurement of flow, level, and pressure are not greatly affected by measurement lag.  This is because most of the devices used to measure these variables respond quickly to process changes.
The measurement of temperature, however, is often subjected to time delay, or measurement lag.  This measurement lag is usually due to the way in which heat is transferred to the process material and through the process equipment.  It is also due to the way in which heat is transferred through the sensing element.
The net effect of these factors is that there will usually be a time difference from the moment the process temperature changes, to the moment the sensing element transmits this information.


In this case, let us consider that anything slowing the flow of heat from the process to the fluid in the filled bulb is a resistance.  Each resistance between the process material and the fluid in the bulb will have to be overcome before the element can sense any temperature change.
As you can see in this example, the process temperature must overcome four resistances before the temperature can be sensed by the filled thermal element.
·                 First, the temperature change must pass through the walls of the thermowell.
·                 Then it must pass through the space between the thermowell and the filled bulb.
·                 Then it must pass through the walls of the bulb.
·                 Finally, it must change the temperature of the fluid in the bulb.
Once the temperature begins to change in the fluid, the filled thermal element will transmit the process change to a temperature transmitter.
Transmission Lag

Transmission Lag is the time interval between a signal being transmitted from the detecting element to the controller then to the final control element.
Transmission lag is more evident in pneumatic control systems than in electronic control systems.  This is because pneumatic signals travel through the control loops at a slower rate than electronic signals.  Factors affecting transmission lag in pneumatic control loops are:
a)     distance between the instruments in the loop
b)    size of the pneumatic tubing
c)     signal pressure
You can see from Figure 2-4, that the distance between a transmitter in the field and the control room will affect the time it takes for a pneumatic signal to reach the controller.  The same factor applies when the controller sends its output signal to a control valve in the field.  The greater the distance, the longer it will take for the controller to receive the signal.
To correct this problem, pneumatic control loops can be designed to minimise transmission lag.  This is usually accomplished by increasing the diameter of the pneumatic tubing and increasing the transmission signal pressure for long distance tubing runs.  Booster relays can also be used to compensate for pressure losses on long tubing runs.  In this way transmission lag can be kept to a minimum in pneumatic control loops.


Response Lag
This is the time interval between the arrival of the signal to the control valve and the time the control valve responds to the signal (moving towards opening or closing).
This lag is affected by many factors.  Some of these factors are:
a)     The strength of the spring in the valve actuator.
b)    The friction between the valve stem and the packing.
c)     The fluid pressure on the valve plug.
Response lag can be minimised by increasing the air pressure to the actuator.  This is achieved by using a valve positioner on the actuator.

     Effects of Load Change on a Control System

Having studied the different lags that can occur in a control system, let us now see how they affect the system as a whole during a process load change.
These changes will usually be due to changes in the process condition, or changes in the operation of the process.  How quickly the control system responds to these load charges will depend upon the various control lags in the system.
Figure 2-5 shows a typical example of how a control loop responds to process load changes.


Let us see how the process temperature increases:-
Because it takes time for the temperature to increase throughout the entire volume of the heat exchanger, there will be some dead time before the temperature increase reaches the sensing element.  Then, there will be the process lag for the temperature to reach a new value.
Once the temperature change reaches the sensing element, there will be a measurement lag as it overcomes the various resistances of the thermowell and the filled thermal element.  When the change is finally sensed, it is then sent to the transmitter where it is converted to a 4 to 20 mA electronic (0.2 to 1.0 bar pneumatic) signal.  This signal is then transmitted to the temperature controller.
The controller output signal is then transmitted to the control valve.  Both the transmitter signal and the controller output signal takes time to pass through the transmission lines.  This is the transmission lag.  When the signal reaches the control valve, the control valve will take some time to respond to the new signal by opening or closing.  This is the response lag.
Due to all these delays, the correction of the process fluid outlet temperature to the set-point will take time.  For this reason the process temperature (the controlled process variable) fluctuates up and down for some time before it returns to the set point.




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