Instrumentation Diagrams

Instrumentation Diagrams

Instrumentation diagrams are the descriptive diagrams made following standardized methods and rules. However, the scope of instrumentation is so wide that a single form of diagram is not sufficient to capture all what is required to be represented. The different types of instrumentation diagrams which are commonly used are (i) process flow diagram (PFD), (ii) loop diagrams (loop sheets), (iii) process and instrument diagrams (P&ID), and (iv) functional diagrams.

At the highest level, the interest is in the interconnections of process vessels, pipes, and flow paths of process fluids. The proper form of diagram to represent this ‘big picture’ of a process is called a process flow diagram. Individual instruments are sparsely represented in a PFD, because the focus of the diagram is the process itself.

At the detailed level, the interest is in the interconnections of individual instruments, including all the wire numbers, terminal numbers, cable types, and instrument calibration ranges etc. The proper form of diagram for this level of fine detail is called a loop diagram. Here, the process vessels and piping are sparsely represented, because the focus of the diagram is the instruments themselves.

Process and instrument diagrams (P&IDs) lie somewhere in the middle between PFDs and loop diagrams. A P&ID shows the layout of all relevant process vessels, pipes, and machinery, but with instruments superimposed on the diagram showing what gets measured and what gets controlled. Here, one can view the flow of the process as well as the ‘flow’ of information between instruments measuring and controlling the process.

Functional diagrams are used for an entirely different purpose. These diagrams show the strategy of a control system. In a functional diagram, emphasis is placed on the algorithms used to control a process, as opposed to piping, wiring, or instrument connections.

When troubleshooting a complex control system, it is often required to switch between different types of diagrams. Normally, there is large amount of detail is needed and everything cannot be shown on any one diagram. Even if the page of the diagram is large enough, a ‘show everything’ diagram becomes so congested with details that it becomes difficult to focus on any particular grouping of details which is required to be seen. The narrowing of scope with the progression from PFD to loop diagram can be visualized as a process of ‘zooming in’, as though the process is being viewed through the lens of a microscope at different powers. The viewing of the process first begins with a PFD or P&ID to get an overview of the process, to see how the major components interact. Then, once this has been identified then one looks for the instrument ‘loop’ which is required to be examined. The appropriate loop diagram is to be seen for the inter-connection details of the needed instrument system to know where to connect the test equipment and what signals to expect to find when the test equipment is connected.

The PFD, P&ID, and loop diagrams are explained below practically through an imaginary process example of a compressor control system. In this imaginary process, water is being evaporated from a process solution under partial vacuum (provided by the compressor). The compressor then transports the vapours to a ‘knockout drum’ where the vapours condense hack into the liquid form.

Process flow diagram

The PFD of the above imaginary process is given in Fig 1. This diagram shows the major interconnections of process vessels and equipment, but omits details such as instrument signal lines and auxiliary instruments.

Fig 1 Example of process flow diagram

From a PFD, it is only possible to guess the instrument interconnections based on the labels of the instruments. For example, a good guess can be that the level transmitter (LT) on the bottom of the knockout drum is sending the signal which eventually controls the level valve (LV) on the bottom of that same vessel. The guess can also be that the temperature transmitter (TT) on the top of the evaporator is the part of the temperature control system which lets the steam into the heating jacket of that vessel. Based on this diagram alone, one is hard-pressed to determine the control system, if any exist, for the control of the compressor itself. All that the PFD show is which is relating directly to the compressor is a flow transmitter (FT) on the suction line. This level of uncertainty is perfectly acceptable for a PFD, since its purpose is merely to show the general flow of the process itself, and only a bare minimum of control instrumentation.

Process and instrument diagram

P&ID is also sometimes called as ‘piping and instrument diagram’. Either way, it means the same thing. A P&ID provides the next level of details. Here, there is a ‘zooming in’ of the scope from the whole evaporator process to the compressor as a unit. The evaporator and knockout vessels almost fade into the background, with their associated instruments absent from the view. However it is to be noted that the ‘zooming in’ of scope in a P&ID does not necessarily mean the scope of other areas of the process is to be ‘zoomed out’. In fact, it is rather typical in a P&ID that the entire process system is shown in finer detail than in a PFD, but not all on one page. In other words, while a PFD depict a process in its entirely on one piece of paper, a comprehensive P&ID typically span multiple sheets of the drawing, each sheet detailing a section of the process system. An example of P&ID is given in Fig 2.

Fig 2 Example of process and instrument diagram

From the P&ID, it can be seen that there is more instrumentation associated with the compressor than just a flow transmitter. There is also a differential pressure transmitter (PDT), a flow indicating controller (FIC), and a ‘recycle’ control valve allowing some of the vapour coming out of the compressor’s discharge line to go back around into the compressor’s suction line. Additionally, there is a pair of temperature transmitters (TTs) reporting suction and discharge line temperatures to an indicating recorder.

Some other important details also emerge in the P&ID. These details show that the flow transmitter, flow controller, pressure transmitter, and flow valve all bear a common number ‘loop number’ 42. This common loop number indicates these four instruments are all part of the same control system. An instrument with any other loop number is part of a different control system, measuring and/or controlling some other function in the process. Examples of this include the two temperature transmitters and their respective recorders, bearing the loop numbers 41 and 43. The differences in the instrument ‘bubbles’ as shown on this P&ID are to be noted. Some of the bubbles are just open circles, where others have lines going through the middle. Each of these symbols has meaning according to the ISA (Instrumentation, Systems, and Automation society) standard. Instrument bubbles are shown in Fig 3. The type of bubble used for each instrument shows  something about its location. This, obviously, is quite important when working in a facility with many thousands of instruments scattered over a large facility area, structures, and buildings.

Fig 3 Instrument bubbles

The rectangular box enclosing both temperature recorders in the P&ID diagram indicates that they are part of the same physical instrument. In other words, this indicates there is really only one temperature recorder instrument, and that it plots both suction and discharge temperatures (most likely on the same trend graph). This suggests that each bubble may not necessarily represent a discrete, physical instrument, but rather an instrument function which can reside in a multi-function device. Details which are not seen on the P&ID include cable types, wire numbers, terminal blocks, junction boxes, instrument calibration ranges, failure modes, power sources, and the like. To examine this level of detail, the loop diagram is required to be seen.

Loop diagram

Example of a loop diagram (sometimes called a loop sheet) for the compressor surge control system (loop number 42) is given in Fig 4. In the diagram, the details of all the instruments in this control ‘loop’, which the P&ID is not showing, are seen. The loop diagram shows that not only there are two transmitters, a controller, and a valve, but also there are two signal transducers. Transducer 42a modifies the flow transmitter’s signal before it goes into the controller, and transducer 42b converts the electronic 4 to 20 mA signal into a pneumatic 3 to 15 PSI air pressure signal. Each instrument bubble in a loop diagram represents an individual device, with its own terminals for connecting wires.

Fig 4 Example of a loop diagram for the converter surge control

It can be seen that the dashed lines now represent individual copper wires instead of whole cables. Electrical terminals, where these wires connect to, are represented by squares with numbers in them. Fluid ports on instruments are also represented by labeled squares. Cable numbers, wire colours, junction block numbers, panel identification, and even grounding points are all shown in the loop diagram. The only type of diagram for this system more detailed than a loop diagram is to be an electronic schematic diagram for an individual instrument, which of course only shows details pertaining to that one instrument. Thus, the loop diagram is the most detailed form of diagram for a control system as a whole, and as such it is required to contain all the details which are omitted in the PFDs and P&IDs alike.

To the beginner in instrumentation, it can seem excessive to include such details as wire colours in a loop diagram. To the experienced instrument personnel who has had to work on systems lacking such documented details, this information is highly valued. The more detail is put into a loop diagram, the easier it makes the inevitable job of maintaining that system at some later date. When a loop diagram shows the person what wire colour to expect at exactly what point in an instrumentation system, and exactly what terminal that wire is to be connected to, it becomes much easier to proceed with any troubleshooting, calibration, or upgrade task.

Loop diagrams are fairly constrained in their layout as per the ISA standard. Field instruments are always placed on the left-hand side, while control-panel or control-room instruments are located on the right-hand side. Text describing instrument tags, their ranges, and notes are always placed on the bottom. Unlike PFDs and P&IDs where component layout is largely left to the practice followed by the designer drawing the diagram, loop sheets offer little room for creativity. This is intentional, as creativity and readability are mutually exclusive in cases where there is an immense amount of technical detail embedded in a diagram. It is simply easier to find details being looked for when they are known exactly where they are supposed to be.

An interesting detail seen on theis loop diagram is an entry specifying ‘input calibration’ and ‘output calibration’ for each and every instrument in the system. This is actually a very important concept to keep in mind when troubleshooting a complex instrumentation system. Every instrument has at least one input and at least one output, with some sort of mathematical relationship between the two. Diagnosing, where a problem lies within a measurement or control system, often means testing various instruments to see if their output responses appropriately match their input conditions, so it is important to document these input and output ranges.

For example, one way to test the flow transmitter in this system is to subject it to a number of different pressures within its range (specified in the diagram as 0 to 100 inches of water column differential) and seeing whether or not the current signal output by the transmitter is consistently proportional to the applied pressure (e.g. 4 mA at 0 inch pressure, 20 mA at 100 inches pressure, 12 mA at 50 inches pressure, etc.).

Given the fact that a calibration error or malfunction in any one of these instruments can cause a problem for the control system as a whole, it is important to know there is a way to determine which instrument is to blame and which instruments are not. This general principle holds true regardless of the instrument’s type or technology. One can use the same input-versus-output test procedure to verify the proper operation of a pneumatic (3 to 15 PSI) level transmitter or an analog electronic (4 -20 mA) flow transmitter or a digital (field bus) temperature transmitter alike. Each and every instrument has an input and an output, and there is always a predictable (and testable) correlation from one to the other.

Another interesting detail seen on this loop diagram is the direction of action of each instrument. One can notice a box and an arrow (pointing either up or down) next to each instrument bubble in the loop diagram in Fig 4. An ‘up’ arrow represents a direct-acting instrument i.e. one whose output signal increases as the input stimulus increases. A ‘down’ arrow represents a reverse-acting instrument i.e. one whose output signal decreases as the input stimulus increases. All the instruments in the loop diagram in Fig 4 are direct-acting with the exception of the pressure differential transmitter PDT-42.

In PDT 42, the ‘down’ arrow indicates that the transmitter outputs a full-range signal (20 mA) when it senses zero differential pressure, and a 0 % signal (4 mA) when sensing a full 200 PSI differential. While this calibration can seem confusing and unwarranted, it serves a definite purpose in this particular control system. Since the transmitter’s current signal decreases as pressure increases, and the controller is to be correspondingly configured, a decreasing current signal is to be interpreted by the controller as a high differential pressure. If any wire connection fails in the 4-20 mA current loop for that transmitter, the resulting 0 mA signal is to be naturally ‘seen’ by the controller as a pressure over-range condition. Excessive pressure drop across the compressor is considered dangerous since it can lead to the compressor surging. Compressor ‘surge’ is a violent and potentially self-destructing action experienced by a centrifugal compressor if the pressure drop across it becomes too high and the flow rate through it becomes too low. Surging can be prevented by opening up a ‘recycle’ valve from the compressor’s discharge line to the suction line, ensuring adequate flow through the compressor while simultaneously unloading the high pressure differential across it. Hence, the controller is naturally to take action to prevent surge by commanding the anti-surge control valve to open, because it ‘thinks’ the compressor is about to surge. In other words, the transmitter is intentionally calibrated to be reverse-acting such that any break in the signal wiring is naturally to bring the system to its safest condition.

Functional diagrams

Functional diagrams are a unique form of technical diagram for describing the abstract functions comprising a control system (e.g. PID controllers, rate limiters, manual loaders). This form of document finds wide application in several industries to document control strategies. Functional diagrams focus on the flow of information within a control system rather than on the process piping or instrument interconnections (wires, tubes, etc.). The general flow of a functional diagram is top-to -bottom, with the process sensing instrument (transmitter) located at the top and the final control element (valve or variable-speed motor) located at the bottom. No attempt is made to arrange symbols in a functional diagram to correspond with actual equipment layout. These diagrams are all about the algorithms used to make control decisions, and nothing more.

A sample functional diagram appears at Fig 5 (i) shows a flow transmitter (FT) sending a process variable signal to a PID controller, which then sends a manipulated variable signal to a flow control valve (FCV). A cascaded control system, where the output of one controller acts as the set point for another controller to follow, appears in functional diagram form at Fig  5 (ii). In this case, the primary controller senses the level in a vessel, commanding the secondary (flow) controller to maintain the necessary amount of flow either in or out of the vessel as needed to maintain level at some set point.

Fig 5 Examples of functional diagrams

Functional diagrams can show varying degrees of detail about the control strategies they document. As an example, one can see the auto/manual controls represented as separate entities in a functional diagram, apart from the basic PID controller function. In the example at Fig 5(iii), one can see a transfer block (T) and two manual adjustment blocks (A) providing a human operator the ability to separately adjust the controller’s set point and output (manipulated) variables, and to transfer between automatic and manual modes. Rectangular blocks such as the triangle, P, I, and D shown in this diagrams (Fig 5) represent automatic functions. Diamond-shaped blocks such as the A and T blocks are manual functions (i.e. set by a human operator).

The functional diagram at Fig 5 (iv) shows more details, and indicates the presence of set point tracking in the controller algorithm, a feature which forces the set point value to equal the process variable value any time the controller is in manual mode. Here one can see a new type of line, dashed instead of solid. This too has meaning. Solid lines represent analog (continuously variable) signals such as process variable, set point, and manipulated variable. Dashed lines represent discrete (on/off) signal paths, in this case the auto/manual state of the controller commanding the PID algorithm to get its set point either from the operator’s input (A) or from the process variable input (the flow transmitter, FT).

Instrument and process equipment symbols

There are many instrument symbols found in different types of technical diagrams used to document instrument systems. Some of the standard symbols are shown in the Fig 6 to Fig 11. Regular pneumatic and electrical line symbols can represent either continuous or discrete states.

Fig 6 Line types and line connections

Valve status may or may not be shown in a process diagram. If solid-coloured valve symbols are there anywhere in a diagram, then the status is being represented. If there are no solid-coloured valves anywhere in the diagram, either all valves are shown open or else status is not represented at all.

Fig 7 Types of process valves, valve actuators, and valve failure mode

Fig 8 Liquid level and flow measurement devices

Fig 9 Process equipment and functional diagram symbols

Fig 10 Single line electrical diagram symbols

 Fig 11 Fluid power diagram symbols

Instrument identification tags

Instrumentation diagrams, reference is made to different instruments by lettered identifiers such as TT (temperature transmitter), PDT (pressure differential transmitter), or FV (flow valve) etc, without formally defining all the letters. These are standard identifiers and are explained below.

Each instrument within an instrumented facility normally have its own unique identifying tag consisting of a series of letters describing the instrument’s function, as well as a number identifying the particular loop it belongs to. An optional numerical prefix typically designates the larger area of the facility in which the loop resides, and an optional alphabetical suffix designates multiple instances of instruments within one loop. As an example, if an instrument bearing the tag FC-135 means that it is a flow controller (FC) for loop number 135. In a large manufacturing facility with multiple processing ‘unit’ areas, a tag such as this can be preceded by another number designating the unit area. As an example, the hypothetical flow controller might be labeled 12-FC-135 means flow controller for loop number 135, located in unit number 12. If this loop happened to contain multiple controllers, then there is a need to distinguish them from each other by the use of suffix letters appended to the loop number (e.g. 12-FC-135A, 12-FC-135B, and 12-FC-135C etc.).

Each and every instrument within a particular loop is first defined by the variable that loop seeks to sense or control, regardless of the physical construction of the instrument itself. As an example, the flow controller FC-135 can be physically identical to the level controller in loop number 72 (LC-72), or to the temperature controller in loop number 288 (TC-288). FC-135 is a flow controller because of the fact that the transmitter sensing the main process variable measures flow. Likewise, the identifying tag for every other instrument within this loop is to begin with the letter ‘F’ as well. This includes the final control element as well. In a level control loop, the transmitter is identified as an ‘LT’ even if the actual sensing element works on pressure (because the variable that the loop strives to sense or control is actually level, despite the fact that liquid level is being inferred from pressure), the controller is identified as an ‘LC’, and the control valve throttling fluid flow is identified as an ‘LV’. Every instrument in that level-controlling loop serves to help control level, and so its primary function is to be a ‘level’ instrument.

Standard letters recognized by the ISA for defining the primary process variable of an instrument within a loop are shown in the Tab 1. It is to be noted that the use of a modifier defines a unique variable. As an example, a ‘PT’ is a transmitter measuring pressure at a single point in a process, whereas a ‘PDT’ is a transmitter measuring a pressure difference between two points in a process. Likewise, a ‘TC’ is a controller controlling temperature, whereas a ‘TKC’ is a controller controlling the rate-of-change of temperature.

Tab 1 Standard letters for defining process variables
Letter VariableModifier
AAnalytical (composition)
BBurner or combustion
FFlowRatio or fraction
HHand (manual)
KTime or scheduleTime rate-of-change
PPressure or vacuum
QQuantityTime-integral or total
SSpeed or frequencySafety
WWeight or force
YEvent, state, or presenceY-axis
ZPosition or dimensionZ-axis

A ‘user-defined’ letter represents a non-standard variable used multiple times in an instrumentation system. As an example, an engineer designing an instrument system for measuring and controlling the refractive index of a liquid can choose to use the letter ‘C’ for this variable. Thus, a refractive-index transmitter is to be designated ‘CT’ and a control valve for the refractive index loop is to be designated ‘CV’. The meaning of a user-defined variable need only be defined in one location (e.g. in a legend for the diagram).

An ‘unclassified’ letter represents one or more non-standard variables, each used only once (or a very limited number of times) in an instrumentation system. The meaning of an unclassified variable is best described immediately near the instrument’s symbol rather than in a legend. Succeeding letters in an instrument tag describe the function which instrument performs relative to the process variable. As an example, a ‘PT’ is an instrument transmitting a signal representing pressure, while a ‘PI’ is an indicator for pressure and a ‘PC’ is a controller for pressure. Many instruments have multiple functions designated by multiple letters, such as a TRC (temperature recording controller). In such cases, the first function letter represents the ‘passive’ function (usually provided to a human operator) while the second letter represents the ‘active’ (automated) control function. Tab 2 shows the letters representing passive functions, active functions and the modifiers.

Tab 2  Letters representing passive and active functions and modifier
LetterPassive function Active functionModifier
EElement (sensing)
GGlass or view port
KControl station
MMiddle or intermediate
PTest point
VValve, damper, louver
YRelay, compute, convert
ZDriver, actuator, or unclassified final control element

A variety of other letter combinations are often used to identify details not standardized by the ISA. For example, chemical analyzer instruments often have their sample tube connections represented by the letter combination ‘SC’, although this does not appear anywhere in the standard.

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