Process flow diagram

Author: Tammy Wong ^[2015]

Stewards: Jian Gong and Fengqi You

Introduction

The process flow diagram (PFD) is a critical component of process design. It is absolutely necessary that chemical engineers know how to read process flow diagrams because it is the primary method of detailing the process and design information. Additionally, the most effective way of relaying information about a process design is the use of process flow diagrams. The PFD shows the sequence of flow through a system through the various equipment (such as piping, instrumentation, and equipment design) and details the stream connections, stream flow rates and compositions and operating conditions through the plant layout. The PFD differs from a block flow diagram (BFD) in that the PFD is more detailed and conveys more information than the BFD, which only gives a general sense of flow of information.

Overview

Essential Information That Must Be Included In A Process Flow Diagram

Process vessels and equipment (such as heat exchangers, pumps, distillation columns, and reactors)
Process piping
Process and Utility Flow Lines
Full heat and material balances
Composition, flow rate, pressure and temperature of every stream
Stream enthalpy
Location of every control valve
Sizing of pumps and compressors
Bypass and recycle streams

Optional Information That Can Be incorporated In A Process Flow Diagram

Molar percentage composition and/or molar flow rates
Physical property data
Mean values for stream
Stream names

Categorization of Information In a Process Flow Diagram

The information that a process flow diagram conveys can be categorized into one of the following three groups:

Process topology
Stream information
Equipment information

Process Topology

Process topology is defined as the interactions and locations of the different equipment and streams. It includes all of the connections between the equipment and how one stream is changed to another after it flows through a piece of equipment. It should be noted on a separate table following the PFD, the equipment on the must be labeled (it can be labeled with a number) and followed by a short description so that the engineer who is trying to understand the PFD will have a easier time following along.

Naming Equipment

Typical names for the equipment include a letter followed by a set of numbers. The letter usually corresponds to the first letter of the equipment. For example, the first pump in the PFD is typically labeled P-101. The following table displays the convention of naming the letters for process equipment:

C-Compressor or Turbine
E-Heat Exchanger
H-Fired Heater
P-Pump
R-Reactor
T-Tower
TK-Storage Tank
V-Vessel
Y means that it is a specific area in the plant
A/B identifies parallel units or backup units not shown on a PFD

Additionally, it should be noted that in a plant, certain equipment will need to be replaced. Typically, the new equipment will take the old equipment's name but an additional letter or number will be added onto the new equipment to indicate that there was a modification.

Symbols for Process Technology

For process equipment, there are a few standard symbols that should be recognized by chemical engineers. These symbols are displayed in the following figure: (Source Towler and Sinnott, Chapter 2, page 36)

Utility Streams in Process Topology

Utilities are necessary for the plant to keep running. The purpose of the utilities is usually to add or remove heat to the equipment so that the temperature can be controlled. There are many different types of utilities that can service a plant:

Electricity
Compressed Air
Cooling Water
Refridgerated Water
Steam
Condensate Return
Inert Gas
Flares

Stream Information

In small PFDs, stream information, including flow rates, temperatures, pressures, and compositions, are shown directly next to the PFD on a table. The corresponding number on the stream will be translated onto the table. The table is usually divided into two sections - one section for the essential information and one for the optional information.

Equipment Information

In addition to the stream information, there should also be a table detailing equipment information. This table can be helpful for the economical analysis of the plant. The equipment information table should include a list of all of the equipment that is used in that particular flow diagram along with a description of size, height, number of trays, pressure, temperature, materials of construction, heat duty, area and other critical information.

Process Flow Diagram Example

Example 1: Polymer Production

In the following figure about polymer production (taken from Towler and Sinnott, Chapter 2, page 38), the PFD contains a few pieces of equipment so that the corresponding streams can be placed on the figure itself instead of on a separate table. Please note that all streams are labeled with the temperature, the flow rate and amount of each composition, and on a separate table, all of the equipment is clearly defined with their names.

However, not all process flow diagrams are as simple as the previous example. In fact, many are complicated processes that may span multiple pages. Therefore, a better example would be the following one.

Example 2: Simplified Nitric Acid Process

In the following figure, air enters a filter while ammonia enters a vaporiser to eventually combine in a reactor and form nitric acid. Each stream is labeled with a number and the compositions of the streams are labeled in a separate table. Additionally, in the separate table that follows directly below the PFD (standard convention), the pressures and temperatures of the streams are also listed. The one improvement on this PFD that can be made is to name the equipment and define those names on a separate table instead of writing the name of the equipment on the PFD. This way, there is less clutter on the PFD and it may be easier to follow along when all the names of the equipment are placed on the same table.

Sources