Common practices that must follow for Instrument installation


For mechanical installation such as Instrument mounting, source connection details, installation of impulse pipe and other accessories, auxiliaries, and supporting details, etc., have to follow some standard/common practices, also in line with ISA Guideline, Which are listed below:

Process tap:

When the pressure is<62.5 kg/cm2 and the temperature is <426C, 15 nominal bore (NB) pipe is used. If the pressure or temperature exceeds those limits, then 20 NB pipe is recommended. The material for process pipe, fittings, and root valves should be a material compatible with the process fluid.

Impulse line:

The process design stress is at saturation temperature for subcritical steam facilities. The line is intended at process stress and temperature for other facilities.

Commonly used tubing materials:

SS tubing:

For noncorrosive tubing ASTM A213 GR TP 304 (for temperature up to 537C) and ASTM A213 GRTP 316 (for temperatures up to 648°C) and ASTM A213 GR TP 316L (for temperatures up to 450C).

In larger plants, where pressure and temperature ratings are high, ASTM A213 Gr. TP 316 L and 316H are recommended.

Carbon steel:

For noncorrosive use ASTMA210 GR A1 (temperature up to 412C).

Copper tube:

ASTM B88 or B75 or annealed soft temper (maximum temperature of 205C) mainly used for air and gas services.

Impulse line inside diameter (ID) and thickness:

To avoid plugging and sufficient mechanical strength, minimum ID >9.14 mm and thickness 1.25 mm are normally accepted.

Line drain/vent:

High-point vent and low-point drain or a combination should be used to purge unwanted gas/liquid.

Preferred location of tapping point on pipe:

In a circular pipe cross-section tapping can be done at any position in the plane, but there are certain preferences in locating the tapping points based on the services

For steam, air, and gas services: Preferred tapping point location could be the top point, but it can be at any other position (upper half) with 45 on both sides of the vertical line.

For liquid services: Preferred tapping point locations are on either side of the horizontal line, but can be at any other position (lower half) with 45 on both sides of the horizontal line.

Valve manifold:

This is a very important accessory for installation. Two-valve manifolds are used for pressure instruments. Three-valve manifolds are used for DP instruments. Five-valve manifolds are used also for DP measurements where there is a provision for a line drain.

Valve manifolds used with tools are the forged form and are usually attached attached to the toos, particullarly for DP instruments. Normally, the valves are screened 1.5 times the peak process stress they must resist.

Source point:

Tapping socket, nipple, and root valve mainly constitute the source point of most instruments.

A piping engineer’s terminal point (instrument take-off point) is a socket with a temperature element plug and a root valve(s) for tapping points that are relevant to other measurements.

If there is insulation, the pipe root valve(s) should be outside of the insulation. Normally in power plants any line where line pressure >40 kg/cm2 (especially for steam service), two root valves are used. In lower pressure applications a single root valve would sufficient.

Mounting location of instruments:

Mounting of instruments is done on the basis of the functional, operational, and maintenance facility.

In power plants, to facilitate maintenance, a number of instruments (mainly transmitters and switches) in nearby locations are grouped together and kept in a separate enclosure referred to as a local instrument enclosure or local instrument rack.