Basic of Reading Drawings

8/13/2019 Basic of Reading Drawings

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The scale of a drawing is usually presented as a ratio and is read asillustrated in the

following examples.

1" = 1" Read as 1 inch (on the drawing) equals 1 inch (on the actualcomponent or system). This can also be stated as FULL SIZE in the scaleblock of the drawing. The measured distance on the drawing is the actual

distance or size of the component.

3/8" = 1' Read as 3/8 inch (on the drawing) equals 1 foot (on the actualcomponent or system). This is called 3/8 scale. For example, if acomponent part measures 6/8 inch on the drawing, the actual componentmeasures 2 feet.

1/2" = 1' Read as 1/2 inch (on the drawing) equals 1 foot (on the actualcomponent or system). This is called 1/2 scale. For example, if acomponent part measures 1-1/2 inches on the drawing the actualcomponent measures 3 feet.

Drawing Scale


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Valves are used to control the direction, flow

rate, and pressure of fluids.

Globe and gate valves will often be depicted by

the same valve symbol. In such cases,information concerning the valve type may be

conveyed by the component identification

number or by the notes and legend section of

the drawing: however, in many instances eventhat may not hold true.


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Some valves are provided with actuators to

allow remote operation to increase mechanical

advantage, or both.

Note that although each is shown attached to agate valve, an actuator can be attached to any

type of valve body.

If no actuator is shown on a valve symbol, it may

be assumed the valve is equipped only with a

handwheel for manual operation.


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The combination of a valve and an

actuator is commonly called a control

valve.

The most commonly found configurations

are to manually control the actuator from a

remote operation station, to automatically

control the actuator from an instrument, orboth.


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In the example, air to the process air-

operated control valve is controlled by the

solenoid-operated, 3-way valve in the air

supply line. The 3-way valve may supplyair to the control valves diaphragm or vent

the diaphragm to the atmosphere.


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Note that the symbols alone do not provide thereader with enough information to determine

whether applying air pressure to the diaphragmopens or closes the process control valve, orwhether energizing the solenoid pressurizes orvents the diaphragm,

Also, it is incomplete in that it does not show theelectrical portion of the valve control system nordoes it identify the source of the motive force(compressed air).

It only informs the reader of the types ofmechanical components in the control systemand how they interconnect, it does not provideinformation to determine how those components

react to a control signal.


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Control valves operated by an instrument

are symbolized in the same manner as

shown previously, except the output of the

controlling instrument goes to the valveactuator.


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Figure 4 shows a level instrument(designated LC) that controls the level in

the tank by positioning an air-operateddiaphragm control valve. It does not contain

enough information to enable the reader to

determine how the control valve respondsto a change

in level.


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An additional aspect of some control

valves is a valve positioner, which allows

more precise control of the valve. This is

especially useful when instrument signalsare used to control the valve.

An example of a valve positioner is a set

of limit switches operated by the motion ofthe valve.


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A positioner is symbolized by a square boxon the stem of the control valve actuator.

The positioner may have lines attached formotive force, instrument signals, or both.

Figure 5 shows two examples of valves

equipped with positioners. Still does not have info to fully determine

response of the control valve to a changein control signal.


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Control Valve Designations

A control valve mayserve any number offunctions within a fluidsystem.

To differentiatebetween valve uses,a balloon labelingsystem is used to

identify the function ofa control valve, asshown in Figure 6.


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The common convention is that the first letter usedin the valve designator indicates the parameter to

be controlled by the valve. For example: F = Flow

T = Temperature

L=Level

P=Pressure

H=Hand (manually operated valve)

The second letter is usually a C and identifies the

valve as a controller, or active component, asopposed to a hand-operated valve.

The third letter is a V to indicate that the piece ofequipment is a valve.


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Piping Systems - may contain more than a single medium.

For example, although the main processflow line may carry water, the associatedauxiliary piping may carry compressed air,inert as, or hydraulic fluid.

Also, a fluid system diagram may alsodepict instrument signals and electricalwires as well as piping.


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Figure 7 showscommonly used

symbols for indicatingthe medium carried bythe piping and fordifferentiating between

piping, instrumentationsignals, and electricalwires.

Note that, although the

auxiliary piping symbolsidentify their mediums,the symbol for theprocess flow line doesnot identify its medium.


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The diagram may

also depict theindividual fittings

comprising the

piping runs

depending on its

intended use.

Figure 8 shows

symbols used todepict pipe fittings.


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Instrumentation

One of the main purposes of a P&ID is to

provide functional information about how

instrumentation in a system or piece of

equipment interfaces with the system or apiece of equipment.

Because of the above, a large amount of

the symbology appearing on P&Ids depictsinstrumentation and instrument loops.


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The symbols used to represent instruments and theirloops can be divided into four categories. Each of thefour uses the component identifying (labeling) scheme

below:


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The first column of Table 1 lists the letters usedto identify the parameter being sensed ormonitored by the loop or instrument.

The second column lists the letters used toindicate the type of indicator or controller.

The third column lists the letters used to indicatethe type of component.

The fourth column lists the letters used toindicate the type of signals that are beinmodified by a modifier.


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Sensing Device and Detectors

The parameters of any system are

monitored for indication, control, or both.

To create a usable signal, a device must

be inserted into the system to detect thedesired parameter.

In some cases, a device is used to create

special conditions so that another devicecan supply the necessary measurement.

Figure 9 shows the symbols used


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Figure 9 shows the symbols used

for various sensors and detectors


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Modifiers and Transmitters

Sensors and detectors by themselves are not

sufficient to create usable system indications.

Each sensor or detector must be coupled with

appropriate modifiers and/or transmitters. The exceptions are certain type of local

instrumentation having mechanical readouts,

such a bourdon tube pressure gages and

bimetallic thermometers.


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Figure 10 also

indicates common

notations used to

indicate the locationof an instrument,

i.e., local or board

mounted.


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Transmitters are used to convert the signal

from a sensor or detector to a form thatcan be sent to a remote point for

processing, controlling, or monitoring.

The output can be electronic (voltage or

current), pneumatic, or hydraulic.


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Modifiers may only be identified by the

type of input and output signal (such as I/P

for one that converts an electrical input to

a pneumatic output) rather than by themonitored parameter (such as PM for

pressure modifier).


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Indicators and Recorders

Are instruments that convert the signal

generated by an instrument loop into a

readable form.

The indicator or recorder may be locally orboard mounted, and like modifiers and

transmitters this information is indicated by

the type of symbol used.

Figure 11 provides examples of the symbols used for


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g p p y

indicators and recorders and how their location is

denoted.


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Controllers

Controllers process the signal from an

instrument loop and use it to position or

manipulate some other system

component. Generally they are denoted by placing a

C in the balloon after the controlling

parameter as shown in Fig. 12.


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There are controllers that serve to processa signal and create a new signal.

These include proportional controllers,

proportional-integral controllers, andproportional-integral-differentialcontrollers. The symbols for thesecontrollers are in Figure 13.

Note that these types of controllers arealso called signal conditioners.


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Simple Instrument Loops Figure 14 shows a

temperature transmitter (TT),which generates two electricalsignals.

One signal goes to a boardmounted temperature

recorder (TR) for display. The second signal is sent to a

PID, the output of which issent to a current-to-pneumaticmodifier (I/P).

In I/P modifier, the electricsignal is converted into apneumatic signal, commonly 3to 15 psi, which in turn

operates the valve.


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The function of the complete loop is to modifyflow based on process fluid temperature.

Note that there is no info to determine how flow

and temperature are related and what thesetpoint is stated on a P&ID.

Knowing the setpoint and purpose of the system

will usually be sufficient to allow the operation of

the instrument loop to be determined.


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The pneumatic level transmitter (LT) in Fig.14(B)senses tank level.

The output of the level transmitter is pneumaticand is routed to a board-mounted level modifier

(LM).

The level modifier conditions the signal (possiblyboosts or mathematically modifies the signal)and uses the modified signal for two purposes.

The modifier drives a board-mounted recorder(LR) for indication, and it sends a modifiedpneumatic signal to the diaphragm-operatedlevel control valve.

Components


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Components

Figure 15 shows the

engineering symbols

for the most common

major components.


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Miscellaneous P&ID Symbol


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Unless otherwise stated, P&Ids indicate valvesin their normal position.

This is usually interpreted as the normal orprimary flowpath for the system.

An exception is safety systems, which arenormally shown in their standby or non-accidentcondition.

3-way valves are sometimes drawn in theposition that they will fail to instead of always

being drawn in their normalposition. This willeither be defined as the standard by the systemof drawings or noted in some manner on theindividual drawings.


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Identify the following components by letters


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y g p y

or numbers Centrifugal pump

Heat exchanger Tank

Venturi

Rupture disc

Relief valve

Motor-operated valve Air-operated valve

Throttle valve

Conductivity cell

Air line

Current-to-pneumatic converter

Check valve

A locked-closed valve

Closed valve

Locked-open valve

Solenoid valve


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What is the controlling parameter forvalves 10 and 21?

Which valves would need to change

position in order for pump B to supply flowto only points G and H?

Which valves will fail open? Fail Closed?

Fail as is?


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2. Temperature as sensed by the temperature elements (TE)

3. Open 18 and/or 19

4. Fail open : 2 and 3

Fail closed : 10 and 21

Fail as is: 7 and 16


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Documents

Basic of Reading Drawings