In continuation of my series on piping and instrumentation diagram tutorials, we shall continue with the development of P&IDs when given some information about a process or control system. Let us take a look at the tutorial question below:
It is desirable to have a small control system to control liquid flow and consequently level in an open tank. The description of the control system is as follows:
(a) A flow control valve will be used to regulate flow. This flow control will be based on flow measurement in an orifice meter
(b) We want to automatically adjust the setpoint of the flow controller with the aid of a level control loop. As level is being measured, the set point of the flow control valve is adjusted automatically. If the level goes up, the set point of the flow control valve should be lowered and vice versa
(c) The Orifice meter should have a secondary device to transmit a 4 – 20mA signal to the control room. The secondary device should be able to indicate flow rate locally at the Orifice meter.
(d) The secondary device on the Orifice meter is required to send this 4 - 20mA electronic signal to a controller in a central control room. The flow rate should be indicated on this controller
(e) The control room will send a 4 – 20mA signal from the controller to the control valve. At the control valve, we will use an I/P converter to provide pneumatic signal to control our valve. The flow control loop will have a loop number 100.
(f) The level of the tank will be measured using a transmitter, with local indication on the transmitter.
(g) We also want to send a 4 – 20mA level signal to a level controller in the control room. This controller will display the level of the tank in the control room.
(h) The level control instrumentation in the tank will make provision for activating high and low level alarms seen in the control room whenever the level goes too high or too low
(i) The tank should have a local sight glass or gauge for indicating level locally for plant operators
(j) The level controller will also send the level signal via wire to the flow controller in the control room, where the setpoint for the flow control valve will be adjusted. The level control loop will have the loop number 101
From the information provided above, develop the piping and instrumentation diagram (P&ID) for this control system.
To develop the P&ID from the information given above, you need to be familiar with most of the symbols used for representing flow and level since we are basically dealing with only flow and level control here. You also need to be familiar with the various instruments and control functions encountered in most instrumentation systems. You also need to refresh your mind about the various abbreviations used in denoting instruments and control functions in piping and instrumentation diagrams. To get useful facts about all I have just mentioned, Please go through:
Below is the piping and instrumentation diagram I have developed from the all the information provided above in our tutorial question:
Let us now examine the information we used to develop the piping and instrumentation diagram above in detail:
(a) A flow control valve is required to regulate flow. On the developed P&ID, FV 100 is the control valve provided to regulate flow
(b) The setpoint of the flow controller is to be adjusted by a level control loop. In the P&ID above, LIT 101 and LIC 101 make up the level control loop that helps to adjust the setpoint of the flow controller FIC 100
(c) The Orifice meter is required to have a secondary device that can transmit a 4 – 20mA signal to the control room and should also indicate flow locally in the plant. In the above P&ID, the Orifice meter is shown to have ,FIT 100, which is a flow indicator and transmitter. The indicator indicates flow locally in the plant and the transmitter transmits the required 4 – 20mA signal to the control room.
(d) The secondary device on the Orifice meter (in this case, FIT 100) is required to send a 4 – 20mA electronic signal to a controller in the control room. In the piping and instrumentation diagram I have developed above, FIT 100 is sending a 4 – 20mA signal to flow indicator and controller, FIC 100, located in the control room. The controller has an indicator function to indicate flow in the control room.
(e) The flow controller in the control room is required to send a 4 – 20mA signal down to the flow control valve. On the flow control valve, an I/P (current to pneumatic converter) will then provide the pneumatic signal required to actuate the control valve. In the P&ID developed, the 4 – 20 mA signal sent to the I/P converter from FIC 100, is converted into pneumatic signal that is used to control the flow control valve, FV 100.
(f) For level measurement, it is required that the open tank should have a transmitter with a local indication function. In the piping and instrumentation diagrams that I have developed above, LIT 101 is a level indicator and transmitter that is measuring the level of the tank and indicating it locally in the plant. The transmitter helps to transmit the measured level signal to the control room.
(g) A 4 – 20mA level signal is required to be sent to a level controller in the control room. In the P&ID above, LIT 101 is sending the required 4 – 20mA level signal to level controller and indicator , LIC 101. Note that because it is required that level should be displayed in the control room, LIC 101 has an indicator function.
(h) The level instrumentation is required to have alarms for high and low levels in the tank. In our P&ID, the level signal from LIT 101 is used for indicating high and low level alarms respectively via LAH 101 (level alarm high) and LAL 101 (level alarm low).
(i) A level gauge or sight glass is required for rough level indication. This is provided by LG 101 in the above P&ID
(j) The level controller LIC 101 is required to send a level signal via wire to our flow controller, FIC 100 so as to adjust the setpoint of this controller. In the P&ID above, LIC 101 is sending the measured level signal to FIC 100. This signal is being used to adjust the setpoint of the flow controller FIC 100. If the level in the tank becomes too high, the setpoint of FIC 100 is automatically reduced and if it becomes too low, it is increased.
Hope you have found this tutorial useful. Please note that on the P&ID developed, there are arrows on the electrical signals indicating the direction where they are being sent to. On actual piping and instrumentation diagrams, this is not often the case. I have done this for the purpose of learning and driving home my point. Also note that the piping and instrumentation diagram is rather simplistic. This is done to facilitate understanding of the basic concepts involved in understanding piping and instrumentation diagrams.