Basic Process Control Fundamentals

Описание: Process Control
In almost all industrial process applications, control of process variables is critical to the safe and efficient operation of the process. The most common variables controlled are pressure, level, temperature, and flow.
Even though there are many different methods used to control these processes, this monitoring and control is generically called process control.
Control Types
The type of control needed to maintain a process is dependent upon the type of process involved. Each process has its own unique set of characteristics, depending on the kind of process and the physical components that make it up.
There are two broad categories of processes: self-regulated processes and non self-regulated processes.
We will see what is a Self-Regulating Process is?
Self-regulating processes are processes that are inherently self-regulating. Self-regulated processes have built-in feedback characteristics that cause the process to tend towards self-regulation.
An example of a self-regulating process is a tank of water with an input of water entering the tank and an output of water leaving the tank. Let’s say the water level in the tank is constant at 10 inches.
Water enters the tank at a rate of 20 gallons per minute and leaves the tank at a rate of 20 gallons per minute. As long as this balance is maintained, water level in the tank will remain constant at 10 inches.
What happens if the outlet valve is opened an 1/8 of a turn and water leaving the tank changes to a rate of 25 gallons per minute?
Since this is a self-regulating process, the level will actually stabilize at a new position and maintain that position. Flow out of the tank is proportional to the square root of the differential pressure across the output valve.
As level decreases, the differential pressure will also decrease, causing the rate of drainage to decrease. At some point, the drainage rate will once again equal the fill rate, and the tank will reach a new equilibrium point.

In self regulating process we have
Time Constants
Every self-regulated process has a time constant associated with it. The time constant is the amount of time it takes the process to change 63.2% of the final value of the process. In this example, the process changes by 10%.
The time it takes to change 6.32 inches (63.2% of 10 inches), is the time constant. It takes five time constants in order for the process to complete the total change.
Process Gain
The time constant is affected by the capacity of the process and the process’ resistance to change.
The larger the process capacity, the longer the time constant, and the more resistive elements in the process (small pipes, penetrations, valves, etc), the longer the time constant.
Dead Time
Dead time, by definition, is “the time difference between when a change occurs in a process and when the change is detected.”
Dead time exists in all processes and is a factor in the control loop control, which must be addressed when turning the loop.
Now we will see what is Non Self-Regulating Process is
A non self-regulating process is one where the process does not tend towards self-regulation. These processes have no self-regulating feedback characteristics and will tend towards being unstable if not controlled externally.
Take, for example, the initial scenario. The water level in the tank is constant at 10 inches. Water enters and leaves the tank at a rate of 20 gallons per minute. In this process, instead of having a discharge valve on the tank, a positive displacement pump is used to drain the water. As long as the balance is maintained, water level in the tank will remain constant at 10 inches.
If we increase the discharge rate of the positive displacement pump to 25 gallons per minute, what will happen? Initially, as with the self-regulating process shown before, we are removing 5 more gallons per minute from the tank than we are putting in the tank. This causes the level to drop. However, unlike the self-regulated process, this drop in level does not affect the flow out of the tank.
A positive displacement pump will discharge a set flow rate regardless of head pressure. The pump will continue to discharge at a rate of 25 gallons per minute until the tank is completely empty.