Diagrams and Instrumentation

Read Piping Schematics

Flow diagrams describe in a schematic drawing format the flow of fluids and gases through a unit or an entire plant. By using symbols to represent various pieces of equipment, the flow diagram provides the piping designer with an overall view of the operation of a facility.

The flow diagram used in this chapter is representative of the types used by many companies in the piping industry. While actual symbols may vary slightly from one company to the next, the "look and feel" of flow diagrams is the same throughout the piping industry.

Students must become familiar with the piping, equipment, instrumentation symbols and abbreviations used on flow diagrams, in order to be able to "read" and interpret them.

One of the most difficult concepts for students to comprehend is the absence of scale in the preparation of flow diagrams. The flow diagram should be laid out in a very simplistic and logical order and be read from left to right. It guides the drafter and designer in the same manner a road map guides a traveler.

USES OF FLOW DIAGRAMS

The flow diagram is used by the piping group to develop and lay out the plot plan. When developing the plot plan, the arrangement of the equipment in the facility reflects, in part, the logical sequence of flow depicted on the flow diagram. However, many other factors such as code requirements, client standards and preferences, worker safety, and cost also influence the positioning of equipment.

Once the plot plan is finalized, the piping designer routes the pipe between two vessels as indicated by the flow diagram using piping specifications and accepted design practices. The flow diagram is usually "yellowed out" as each line is completed and incorporated into the design.

TYPE OF FLOW DIAGRAMS

Process engineers are responsible for developing flow diagrams. In many large engineering firms, an entire department is dedicated to the development of flow diagrams. Today almost all flow diagrams are laid out with CAD, using third-party piping packages such as Pro-Flow or individually developed company packages.

Process Flow Diagram

The process flow diagram is the first flow diagram developed by the flow diagram department. It includes the following:

1. major equipment

2. main piping

3. direction of flow

4. operating pressure and temperature

5. major instrumentation

The process flow diagram will denote the following:

• Conditions to be used for the design of various pieces of equipment (fractionation columns, pumps, heaters, etc.) required for facility operation.

• Operating and design conditions under which a particular unit or piece of equipment will normally operate. Design conditions establish the limits that equipment used in the facility can withstand. Design pressure is calculated to be at least 10% above the maximum operating pressure or 25# greater (whichever is largest). The design temperature will be at least the maximum operating temperature, but should be at least 25 degrees above the normal operating temperature.

• Composition of the commodities used in the process sequence as they enter and leave the unit.

Figure 7-1 shows a sample process flow diagram. Mechanical Flow Diagram

From the process flow diagram, the mechanical group develops the mechanical flow diagram. The mechanical flow diagram provides much more detailed data than the process flow diagram. Many companies refer to the mechanical flow diagram as the "P & ID" (process and instrument diagram). Often referred to as the bible of the design process, this drawing provides the pipe drafter with the design criteria for the unit. Mechanical flow diagrams include the following:

1. pipe line numbers and direction of flow

2. pipe specifications and line sizes

3. all equipment

4. all valves

5. all instrumentation with controlling devices

The mechanical flow diagram defines the exact sequence in which all equipment, valves, instrumentation, connections, etc., are to be made on each pipe throughout the facility.

Figure 7-2 shows a sample of the mechanical flow diagram.

The Utility Flow Diagram

The utility flow diagram shows the piping, valves, and instrumentation for the basic plant utilities. Utilities are services that are essential to the proper function of the plant. These utilities correspond to some of the same utilities used in a typical house, such as water, gas, and sewer drains.

Some of the common plant utilities are:

steam fuel oil instrument air drainage systems condensate utility air cooling water flare system

REBOILER DEPROPANIZER OVERHEAD REFLUX

CONDENSER ACCUMULATOR

REBOILER DEPROPANIZER OVERHEAD REFLUX

CONDENSER ACCUMULATOR

Overhead Sewer System Diagram

PRODUCT AND REFLUX PUMPS

Figure 7-1 shows a sample of the process flow diagram.

PRODUCT AND REFLUX PUMPS

Figure 7-1 shows a sample of the process flow diagram.

E—101 FRACnONATOR REBOILER

V-101

FRACTIONATOR 48TOD. X 52'—OTT/T

P101A/B PRODUCT & REFLUX PUMPS

E—102 OVERHEAD CONDENSER

V—102 REKUJX ACCUMULATOR 4€T(ffl. X 15'—Ö"T/T

Depropanizer Reflux Pump Flow Diagram

drain

Figure 7-2 shows a sample of the mechanical flow diagram.

drain

Figure 7-2 shows a sample of the mechanical flow diagram.

The flow diagram is a dynamic document. It may be revised and updated during the project to reflect the client's changes or modifications imposed by governmental regulations. Figure 7-3 shows a sample utility flow diagram.

FLOW DIAGRAM INSTRUMENTS

Instruments function by sensing changes in the variables they monitor. The four basic instrument groups are: Flow (F)

Pressure (P)

Temperature (T) The types of instruments used to sense, control and monitor these variables are:

Controller (C)

Indicator (I)

Recorder (R)

By learning these nine terms, students will be able to understand most of the instrument symbols found on a mechanical flow diagram.

Figure 7-4 illustrates a combination of the symbols and abbreviations used to represent an instrument's function on flow diagrams. The first letter in the symbol indicates the instrument group, and the second and/or third letters indicate the instrument type.

To indicate a change or to control the flow, level, pressure, or temperature, an instrument must first sense a change in the variable. Once a change has been detected, the instrument then transmits this information via mechanical, electronic, or pneumatic means to a control panel where it can be observed and recorded. At the same time, the instrument may activate other devices to affect and change process conditions in the facility. Some instruments are read in the plant at the instrument's actual location. Others are displayed on a control panel located in an operator's control room.

Instrument Types

Gauges. Gauges are instruments that measure the liquid level inside a vessel or the temperature and/or pressure in the piping system. Level, temperature, or pressure gauges are locally mounted to enable plant operators to obtain a visual reading.

Utility Flow Diagram
Figure 7-3 shows a sample of the utility flow diagram.

LOCALLY MOUNTED INSTRUMENT

BOARD MOUNTED INSTRUMENT

FLOW INSTRUMENTS

FA J FLOW ALARM

FE FLOW ELEMENT

FE FLOW ELEMENT

Fl FLOW INDICATOR

Fl FLOW INDICATOR

FR FLOW RECORDER

FR FLOW RECORDER

FRC FLOW RECORDING CONTROLLER

LEVEL INSTRUMENTS

LA LEVEL ALARM

LA LEVEL ALARM

LAH LEVEL ALARM HIGH

LAH LEVEL ALARM HIGH

@ LEVEL ALARM LOW

LC LEVEL CONTROLLER

LC LEVEL CONTROLLER

LG LEVEL GLASS

LG LEVEL GLASS

LI LEVEL INDICATOR

LI LEVEL INDICATOR

TEMPERATURE INSTRUMENTS

TAJ TEMPERATURE ALARM

Tl ) TEMPERATURE INDICATOR

TR ) TEMPERATURE RECORDER

TEMPERATURE RECORDING CONTROLLER

TW ) TEMPERATURE WELL

TW ) TEMPERATURE WELL

PRESSURE INSTRUMENTS

PC PRESSURE CONTROLLER

PI J PRESSURE INDICATOR

PI J PRESSURE INDICATOR

PR ) PRESSURE RECORDER

PR ) PRESSURE RECORDER

PRESSURE INDICATING CONTROLLER

PRESSURE RECORDING CONTROLLER

PRESSURE SAFETY VALVE

RV ) RELIEF VALVE

PIC PRC

RV ) RELIEF VALVE

MISCELLANEOUS SYMBOLS

LIC) LEVEL INDICATING CONTROLLER TRANSMITTER (OR)

(LRC) LEVEL RECORDING CONTROLLER (HCv) HAND CONTROL VALVE

Figure 7-4 Flow diagram instrument symbols.

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