Flow Diagrams

 Flow Diagrams:

Flow diagram describe in a schematic drawing format the flow of fluids and gases through a unit or an entire plant.

The flow diagram is used by the piping group to develop and lay out the plot plan.

The piping designer routes the pipe between two vessels as indicated by the flow diagram using piping specifications and accepted design practices.

Process engineers are responsible for developing flow diagrams.

Types of Flow Diagram:

Types of flow diagrams are

• Process Flow Diagram (PFD)
• Piping and Instrumentation Diagram (P&ID)
• Utility flow Diagram

1. Process Flow Diagram (PFD):

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 temperature and Pressure

5. Major Instruments

Click here to know about Process Unit Symbology.

A process flow diagram generally will include following reasons:

• Flow rate of each stream in case of continuous process or quality of each reactant in case of a batch process.
• Composition streams. 
• Operating conditions of each stream such as pressure , temperature, concentration, etc.
• Heat added or removed in a particular equipment.
• Flows of utilities such as stream, cooling water,  brine, hot oil,  chilled water, thermal fluid, etc.
• Major equipment symbols, names and identification.
• Any specific information which is useful in understanding the process. For example, symbolic presentation of a hazard,  safety precautions, sequence of flow, etc.   

Sample Process Flow Diagram:

2. Piping & Instrumentation Diagram (P&ID):

• From Process Flow Diagram the Piping and Instrumentation (P&ID) is developed
• P&ID contains more detail than PFD.
• It also referred to as Mechanical Flow Diagram.
• P&ID includes

  i.  pipe line numbers and direction of flow

  ii.  pipe specifications and line sizes

  iii. all equipment

  iv.  all valves

  v.   all instrumentation with controlling devices
• P&ID defines the exact sequence in which all equipments, valves, instrumentation, connections etc.

Sample P&ID Diagram:

3. 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.
• Some of the common plant utilities are:

  steam

  condensate

  fuel oil

  utility air

  instrument air 

   cooling water

  drainage systems 

  flare system

Sample Utility Flow Diagram:

Flow Diagram Instruments:

Instrument function by sensing changes in the variables they monitor. The four basic instrument groups are:

Flow (F), Level (L), Pressure (P), Temperature (T)

The type of instruments used to sense, control and monitor these variables are:

Controller (C), Indicator (I), Gauge (G), Alarm (A), Recorder (R)

Gauge: Instruments that measure the liquid level inside a vessel or the temperature or pressure in the piping system. Temperature or Pressure gauges are locally mounted to enable plant operators to obtain a visual reading.

Controllers: Devices used to maintain a specified liquid level, temperature, pressure, or flow inside a vessel or piping system. they activate the control valve that regulates the level, temperature, pressure, and flow in and out of the vessel.

Alarms: Signal via lights or horns that indicate the liquid level, temperature, or pressure inside a vessel is too high or too low or that there is no flow or reversal flow.

Indicators: Devices used to indicate the liquid level, temperature, pressure, or flow rate inside a piping system.

Recorders: Devices used to record the liquid level, temperature, pressure, and flow rate inside a vessel or piping system throughout a certain shift or period of time.

Click here to know about Process Unit & Instrumentation Numbering.