Overview of Piping – Instrument Interface (With PDF)


Piping has regular interface with the Instruments for

  • Scope Break
  • Process Connections
  • Materials and BOM
  • Orientation

The interfaces are mostly standardised, however should be discussed with C&A team at early stage of the project, preferably.

This presentation covers some common instrument items. This presentation does not cover the interface with respect to layouts.

Some instruments give desired performance when installed in particular position / orientation. (ex: Coriolis meter).

When in doubt the functionality of the instrument should be discussed with the C&A engineer.

Types of instruments

In line instruments: In-line instruments are considered to be all instruments and components direct-mounted in or on process and utility lines or equipment and are subjected to the pressures and temperatures of the piping systems or equipment in or on which they are installed. Examples of few inline instruments are given below.

Types of Inline Instrument
Fig. 1: Types of Inline Instruments

On line instruments: On-line instruments are all instruments and components connected to process and utility lines or equipment via small (maximum DN 50 or 2 in) primary isolation valves. They are subjected to the pressures of the piping systems or equipment on which they are installed.

Off line instruments: Off-line instruments are considered to be all instruments and components, which are not in direct, contact with any process medium or which are not connected to any process/utility line or equipment

Typical examples of online instruments are impulse line components, transmitters, pressure gauges, monoflange assemblies, analyser sampling systems etc.

Typical examples of offline instruments are thermocouples, resistance elements and bi-metallic thermometers in themowells, signal converters, local receiving indicators, etc.

Some general points which need to be remembered are mentioned below:

  • Datasheets for ESD valves if not standardized the Plant ESD valve datasheet must be given to C&A
  • Material details for the control valves, Nuts, Bolts and Gaskets should be considered by piping. Flanges as per ASME B 16.5
  • A process to instrument valve unit is a means of interfacing between process piping and instrumentation systems.
  • Instrument specifications apply downstream the last joint of the last process to instrument valve or valve assembly, defined for the instrument connection in the mechanical piping class.
  • The philosophy is to use single block valve up to Class 600 and double block for Class 900 and above.
  • If the pipe is insulated, then the design shall incorporate an “over the insulation design” or mount the transmitter remotely.

Flow Instruments:

  1. Coriolis Flow meter (Fig. 2):
  • Should be installed in downside In Liquid Service and upside in Gas service
  • Flow direction should be marked on vendor drawings
  • Orientation should be shared with C&A.
Coriolis Flow Meter
Fig. 2: Coriolis Flow Meter

2. Vortex / Ultrasonic Flow meter / Restriction orifice (Fig. 3)

  • Straight lengths should be maintained.
  • Tappings on the straight length should be avoided


Flow Meter
Fig. 3: Flow Meter

3. Flow Transmitters / Restriction orifice (Fig. 4)

  • Break at the isolation valve ( DBB /SBB).
  • End connection ( instrument side) should be discussed. ( generally threaded)


Flow Transmitters
Fig. 4: Flow Transmitters

Analytical Instruments

  1. Red Eye Meter (BSW meter-Fig. 5)
  • Orientation to be ensured in such a way that the instrument is always flooded.
  • Flow direction should be marked on vendor drawings
  • Orientation should be shared with C&A
Red Eye Meter
Fig. 5: Red Eye Meter

Pressure Instruments

  • Pressure Gauge / Pressure Transmitter-Break at the isolation valve ( DBB /SBB).

Level Instruments

Level Gauge / Level Transmitter:

  • Break at Isolation valve
  • Drain / Vent valve under piping scope
  • Level gauge drawings by piping
  • Transmitter details by C&A

Temperature instruments

Thermowells :

  • Scope break at the flange.
  • Gaskets, studs, bolts by piping
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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