- Pressure Indicator
- 2 Control Valves
- Pressure Controller
This is a typical split range feedback control loop
- Measured variable - Pressure in tank
- Manipulated variables
- Flowrate of nitrogen
- Flowrate of material through vent
- Disturbance - Flowrate of nitrogen
The pressure in the tank is measured. If it is too low, due to a decrease in the flowrate of nitrogen, then the vent line is closed and the N2 valve is opened and the pressure builds back up. If it is too high, due to an increase in N2 flowrate, then the N2 valve is closed and the vent opened and the pressure allowed to fall.
Pressure Control Systems
In gas or vapour systems we regulate inventory as pressure. A typical system is shown below. Both the inlet and outlet are gas or vapour. Therefore if the control valve is shut then the pressure in the tank will rise and vice versa.
In principle we might, like the level control system, have the valve either upstream or downstream of the tank. In practice in gas systems it is more likely to be downstream for the following reason.
Raising the pressure of a gas requires energy, and normally this energy is imparted by some mechanical device, such as a compressor. Both the compressor itself, and the energy to drive it, are expensive.
To minimise the first cost we try to minimise the number of compressors in a process. Where possible we would use only one, locate it at the front of the process, and perform any subsequent manipulations to obtain the required pressure by downstream valves.
The energy used in compression is expensive, and throttling through a control valve throws this energy away. Therefore in proesses where compressor costs are very significant we may sometimes avoid such valves and manipulate the compressor speed in order to maintain the system at the required pressure. This control system is shown in the diagram below.
When we have vapour we usually also have liquids. Regulating pressure in two phase systems can be somewhat different.