14.1 Process Flowsheets and Instrumentation

Process Flowsheets and Instrumentation

Process design begins with flowsheets that visually represent the chemical process. The level of detail increases as the design progresses, transitioning from block flow diagrams (BFDs) to process flow diagrams (PFDs) and eventually to piping and instrumentation diagrams (P&IDs). For the FE Chemical exam, candidates must be proficient in interpreting these diagrams, understanding instrumentation symbols under the ISA-5.1 standard, and identifying various control loop configurations.

Process Flow Diagrams (PFDs) vs. Piping and Instrumentation Diagrams (P&IDs)

A Process Flow Diagram (PFD) provides a high-level overview of the process. It focuses on the main process path, showing major equipment (reactors, columns, heat exchangers, pumps), process streams, and basic control schemes. PFDs typically include a stream table detailing mass and energy balances, operating temperatures, pressures, and flow rates. PFDs omit utility lines, detailed piping, valve specifications, and safety alarms.

A Piping and Instrumentation Diagram (P&ID) is a highly detailed schematic. It displays the entire physical plant design, including all process and utility piping, valve types and sizes, pipe specifications, insulation, instrumentation, safety devices (such as pressure safety valves (PSVs) and rupture discs), alarms, and connection lines to distributed control systems (DCS). P&IDs do not show physical scales, piping layouts, or stream tables.

Instrumentation Symbols (ISA-5.1)

The Instrument Society of America (ISA) S5.1 standard defines the symbols and letters used for instrumentation. A circular symbol, or bubble, represents an instrument. The lines inside and surrounding the bubble indicate its location and accessibility:

• No horizontal line: Field-mounted instrument (located near the physical equipment in the plant).
• Single solid horizontal line: Board-mounted or control-room instrument, accessible to the operator at the main control panel.
• Double solid horizontal line: Auxiliary control panel, accessible to the operator.
• Single dashed horizontal line: Instrument located behind the panel or in an inaccessible location.
• Bubble inside a square: Shared display and control (e.g., Distributed Control System (DCS) or Programmable Logic Controller (PLC) interface).

Instrument Tag Numbers and Letter Codes

Each instrument has an alphanumeric tag (e.g., TIC 101). The letters represent the function, and the numbers represent the loop.

• First Letter (Measured Variable):
  • F: Flow
  • T: Temperature
  • P: Pressure
  • L: Level
  • C: Concentration or Composition
  • H: Hand (manual action)
• Succeeding Letters (Readout or Output Function):
  • A: Alarm
  • C: Controller
  • I: Indicator
  • T: Transmitter
  • V: Valve
  • Y: Calculation/Relay
  • S: Switch

For example, FIT 202 is a Flow Indicating Transmitter in loop 202. LCV 305 is a Level Control Valve in loop 305. PAH 401 is a Pressure Alarm High in loop 401.

Flowsheet Connection and Signal Lines

Different lines represent the physical connection between instruments and process equipment:

• Thick solid line: Major process piping.
• Thin solid line: Minor process or utility piping (e.g., steam, cooling water).
• Dashed line: Electrical signal (usually 4–20 mA or 1–5 V, represented by a dashed line or a dashed line with single diagonal ticks).
• Solid line with double diagonal slashes: Pneumatic signal (usually 3–15 psig air).
• Solid line with Xs: Capillary tubing (filled with system fluid).
• Dotted line or line with bubbles: Software or data link (DCS communication, fieldbus).
• Sine wave line: Electromagnetic, sonic, or optical signal.

Control Loop Configurations

Control loops are categorized based on their structural design:

• Feedback Control: The controller measures the controlled variable (e.g., temperature) and compares it with the setpoint. It adjusts the manipulated variable (e.g., steam valve opening) based on the error. This is reactive and only corrects disturbances after they affect the process.
• Feedforward Control: The controller measures a major disturbance stream before it enters the process. It calculates the necessary adjustment to the manipulated variable to cancel out the disturbance's impact. Feedforward is proactive but requires a detailed process model and is rarely used alone; it is typically paired with feedback control.
• Cascade Control: Two controllers are nested. The primary (outer) controller measures the main process variable (e.g., reactor temperature) and calculates a setpoint for the secondary (inner) controller (e.g., jacket cooling water flow controller). Cascade control responds rapidly to disturbances in the utility line (jacket flow) before they can affect the primary process variable (reactor temperature).
• Ratio Control: Maintains a constant ratio between two stream flow rates. Typically, one stream is designated as wild (uncontrolled) and its flow rate is measured to determine the setpoint for the controlled stream's flow controller. This is common in fuel-air combustion systems and reactor feed blending.
• Split-Range Control: A single controller output signal is split between two final control elements. For example, a temperature controller might open a cooling water valve when the output is 0–50% and open a steam valve when the output is 50–100%.

Regulatory and Design Standards

Chemical process design must conform to national and international standards to ensure safety, quality, and environmental compliance:

• OSHA 29 CFR 1910.119 (Process Safety Management - PSM): Mandates that facilities handling highly hazardous chemicals maintain highly accurate, up-to-date P&IDs and Process Safety Information (PSI). The P&IDs serve as the baseline for mandatory Process Hazard Analyses (PHAs), such as Hazard and Operability (HAZOP) studies.
• ISO 10628 (Flow Diagrams for Process Plants): Establishes general rules for the drawing and structure of PFDs, P&IDs, and block diagrams.
• ASTM Standards: Specify material properties, testing methods, and fabrication limits for piping, pressure vessels, and flanges.
• ISA-5.1: Governs the standard symbols and identification codes used for process instrumentation.

Worked Example: Interpreting a Control Loop on a P&ID

Consider a shell-and-tube heat exchanger heating a cold process stream using condensing steam on the shell side. A temperature transmitter (TT 102) is mounted on the process outlet line. The TT 102 sends an electrical signal (represented by a dashed line) to a temperature indicating controller (TIC 102), which is shown as a circle inside a square with a single horizontal line. The TIC 102 compares the temperature to a setpoint and sends an electrical signal to a current-to-pressure transducer (TY 102). The TY 102 converts the electrical signal into a pneumatic signal (represented by a line with double slashes) that actuates a control valve (TV 102) on the steam utility inlet line.

Let's analyze each component of this system:

1. TT 102: A field-mounted temperature transmitter (circle with no horizontal lines) that senses the temperature of the exiting stream.
2. TIC 102: A temperature indicating controller located in the main control panel (circle inside a square with a single solid horizontal line). The circle inside the square indicates that it is a shared display DCS instrument.
3. TY 102: A signal converter or transducer. In this case, it is an I/P (current-to-pressure) transducer, converting the controller's electrical current output (e.g., 4–20 mA) to a pneumatic pressure signal (e.g., 3–15 psig) to drive the physical diaphragm of the steam valve.
4. TV 102: The temperature control valve on the steam supply line.

This represents a classic feedback control loop. If the outlet temperature drops below the setpoint, the controller senses the error, and increases the steam valve opening. If a disturbance occurs in the steam supply pressure, the feedback loop cannot react until the outlet temperature actually shifts, demonstrating the limitation of simple feedback systems compared to cascade configurations.