Best Practices for Instrumentation on P&IDs

1.0 Introduction & Application

This guide outlines the minimum requirements and general philosophies for applying instrumentation on Piping & Instrumentation Diagrams (P&IDs). These principles are intended for use during basic, front-end, and detail engineering phases.

When modifying or expanding existing facilities, the established philosophy and past experience of that specific plant should always be taken into consideration.

2.0 Flow Instruments

Selection Criteria

• Differential Pressure (Orifice): The default choice for flow measurement.
  • Permanent pressure drop typically ranges from 0.15 kg/cm² (utilities) to 0.3 kg/cm² (process).
  • Use segmental orifice plates for liquids containing solids.
  • Use quarter-circle or conical entrance plates for viscous liquids.
• Small Line Sizes (≤ 50mm): Integral orifice meter run assemblies are commonly used.
• Venturi Meters: Use for high-capacity applications or where a very low pressure drop is required.
• Pitot Tube / Annubar: May be used in very large lines, such as cooling water headers.
• Variable Area (Rotameters): Generally used in piping (< 40mm) where only local indication is needed. They are suitable for clean process/utility services and are particularly well-suited for nitrogen purging applications.
• Ultrasonic Meters: A non-contact option suitable for fluids containing solids.
• Mass Flow (Coriolis): Use for critical flow measurements. They accurately measure mass flow despite varying fluid density, offer wide rangeability, and have minimal pressure loss.
• Magnetic Meters: Can be used for general process/utility service. They offer an unobtrusive, crevice-free installation.
  Limitation: This type can *only* be used on conductive liquids (not suitable for high-purity water or solvents).
• Positive Displacement (PD) Meters: Suitable for simple local batching applications. This meter provides high accuracy for clean liquids (free from entrained vapors), including low-flow and viscous applications.
• Turbine Meters: Another common option for flow measurement.

Special P&ID Requirements for Flow

• Diaphragm Seals: When a process fluid could cause improper operation or damage an instrument, diaphragm seals must be provided. This is common with high viscosity, corrosion, coking, or sedimentation. The fill fluid must be suitable for the maximum process operating temperature.
• Flashing Prevention: When an orifice measures liquid flowing directly from a vessel, it must be placed sufficiently below the vessel. This static liquid leg ensures that no flashing occurs across the orifice. A P&ID note should specify this (e.g., "Equipment elevation must be 'X' meters above orifice assembly to prevent flashing").
• Rotameter Installation: Must be installed vertically with flow moving upwards. A typical assembly includes a block valve upstream, a globe valve downstream (for throttling), and a globe valve in a bypass line.
• PD Meter Installation: The installation must ensure the meter is continuously flooded. A strainer and drain must be provided upstream of the meter.
• Local Indication: If field operations using control valve handwheels or bypasses are expected, local indication (a local gauge or a DCS repeater) may be necessary in addition to the control room value.

3.0 Level Instruments

Selection Criteria

• General Measurement: Differential Pressure (DP) type is the most common.
• Interface Level: Displacer type instruments are preferred for liquid-liquid interface.
• Slurry Service: A dip tube (bubbler) type device may be used.
• Large Storage Tanks: Float-and-cable or Servo-type gauges are commonly used.
• Underground Tanks / Toxic Service: Magnetic level instruments are a good choice.
• Radar Type: Shall be used in highly viscous or slurry applications.
• Level Gauges (Visual):
  • Reflex Type: Used for most services.
  • Transparent Type: Required for interface level, or for liquids containing sediments or solids.
• Level Switches:
  • Ball Float: Used for non-critical, non-hazardous services (e.g., raw water).
  • Displacer Type: Used for most liquid applications (can be external cage or internal).
  • Solids: Vibrating tuning fork, capacitance type, or paddle type switches.

Special P&ID Requirements for Level

• Diaphragm Seals: As with flow, use diaphragm seals on DP level instruments if the fluid can cause issues (viscosity, corrosion, coking, solids). For slurry service, consider a pad-type diaphragm seal.
• Standpipes: Use standpipes when two or more instruments are used to measure the same level (e.g., a level gauge and a level transmitter).
  • Standpipes should *only* contain level instrumentation.
  • A 2-inch standpipe should only be considered if the fluid is clean, non-viscous, and does not crystallize or contain solids.

4.0 Temperature Instruments

Selection Criteria

• Primary Element:
  • Resistance Type (RTD): For temperatures below 600°C.
  • Thermocouple (TC): For temperatures beyond 600°C.
• Local Gauges: Bi-metallic type.

Special P&ID Requirements

• All temperature elements (thermowells) must have a minimum immersion length (e.g., 80mm). In piping below this size, the line must have local expanders/reducers.
• The temperature of a mixed stream must be measured at a sufficient distance from the mixing point to ensure a homogenous reading (typically 10 pipe diameters downstream).
• In tray columns, temperature should be measured in the downcomer region to ensure an accurate liquid-phase temperature measurement.

5.0 Pressure Instruments

• Pressure transmitters are generally referenced to atmospheric pressure.
• Diaphragm Seals: Must be provided if the fluid can cause improper operation or damage (due to viscosity, corrosion, coking, or solids).
• Transmitters vs. Switches: Transmitters are preferred over pressure switches because the measurement loop is continuously monitored for fault conditions.
• Pulsating Service: On services with pulsation (e.g., positive displacement pump discharge), a damper or snubber must be fitted in the input connection. This service should be noted on the data sheet.

6.0 Control & On-Off Valves

General Philosophy

• Control Valve Selection Preference:
  1. Globe Valve
  2. Rotary (e.g., eccentric plug or segmented ball)
  3. Butterfly Valve
  4. Ball Valve
  5. Special Types
• On-Off Valves: Ball or gate type are standard.
• Large Control Valves (> 200mm): May be provided with a handwheel, in which case a block and bypass may not be necessary.
• Fail Action: The valve's action on air failure must be clearly depicted on the P&ID:
  • FO: Fail Open
  • FC: Fail Closed
  • FL: Fail in Last Position
  • FLDC: Fail Last, then Drift to Close
  • FLDO: Fail Last, then Drift to Open

Valve Type Specifics

• Globe & Rotary Valves: Shall be used for all clean services. They should not be used in fluids that can coke or that contain solid particles. If TSO is required, a dedicated TSO valve (like a ball valve) must be installed in series.
• Butterfly Valves: Use in cases where pressure drop limitations prevent using a globe valve, or for corrosive services where a lined globe valve is economically unattractive.
• Ball Valves: Shall be considered for On-Off service. V-Notch type ball valves may be used for slurry control applications.
• Angle Valves: Consider for erosive applications (slurries), hydrocarbon services where coke may form, or high-pressure drop applications (e.g., boiler blowdown).
• Diaphragm / Plug Valves: Generally used only in specialized applications like crystalline or slurry services.
• Gate Valves: Used *only* for remote-operated on-off operations. They are normally equipped with an electric (MOV) or hydraulic actuator and do not typically have a fail-safe action.

Self-Actuated Valves (Regulators)

• Pressure Regulators: Shall be used only in clean fluid services and for pressure regulation of utility supplies or gas blanketing. They are not recommended for general process applications.
• Temperature Regulators: Shall be considered for simple, non-safety, non-critical applications where utilities (like instrument air) are not available.
• A local means of pressure or temperature indication must be provided to facilitate regulator adjustment.

7.0 ESD & Interlocks

• Interlocks shall be described in a separate "Trip Schedule" or "Cause & Effect Diagram." No logic description shall be provided on the P&ID itself.
• Interlocks shall be referenced on the P&ID to match the numbering in the trip schedule. A general note should be added to the P&ID (e.g., "For interlock description, see Trip Schedule Doc. No. XXX").
• ESD-actuated valves (and any control valves actuated by an ESD) must be provided with limit switches to indicate their tripped position.