Instrument Piping Systems Installation Guidelines

The right installation ensures the long term instrument system reliability. In this article we look at the general guidelines for the installation of instrument piping systems.

Piping Systems/Tubing Runs

Before we can proceed, it is important to state the difference between piping and tubing; tubing can be regarded as thin-walled seamless pipe that cannot be threaded and which is joined by compression fittings as opposed to piping which can be threaded or welded.

All instrument piping or tubing runs should be routed to meet the following requirements:

• Kept as short as possible.
• Shouldn’t cause any obstruction that would bar personnel or traffic access.
• Shouldn’t interfere with the accessibility for maintenance of other items of equipment.
• Located with adequate clearance to permit lagging which may be needed on adjacent pipework.
• Avoid hot environments or potential fire-risk areas.
• All piping and tubing should be effectively supported along its entire length from supports attached to firm steelwork or structures (not handrails).
• The number of joints should be kept to a minimum consistent with good practice.

Air Supplies Piping

The air supplies to instrument should be clean, dry and oil free. Air is usually distributed around a plant from a high-pressure header (around 6-7 bar), ideally forming a ring main. This header, normally of galvanized steel, should be sized to cope with the maximum demand of the instrument air users being serviced and an allowance should be made for possible future expansion or modifications to its duty.

The branch headers should be provided to supply individual instruments or groups of instruments. Once more, sufficient spare tappings should be allowed to cater for future expansion. Branch headers should be self-draining and have adequate drainage/blow-off facilities.  This may be achieved on small headers by using air filter/regulators.

Every single instrument air user should have an individual filter regulator. Piping and fittings installed after filter regulators should be non-ferrous.

Pneumatic Transmission Signals Tubing

Pneumatic transmission signals are typically in the range 0.2 – 1 bar (3 – 15 psi). For these signals, copper tubing is the most commonly used, preferably with a PVC outer sheath. Other materials are sometimes used, depending on environmental considerations such as alloy tubing or stainless steel. Although expensive, stainless steel tubing is the most durable and can withstand the most demanding service conditions.

Plastic tubing should be used only within control panels. A number of problems should be factored in when utilizing plastic tubes on a plant site, since they are very vulnerable to damage unless sufficiently protected; they generally cannot be installed at sub-zero temperatures and they can be considerably weakened by exposure to hot surfaces and lastly, they can be totally be destroyed in the event of a fire.

Pneumatic tubing should be run on a cable tray or similar supporting steelwork for its entire length and securely clipped at regular intervals. Where a number of pneumatic signals are to be routed to a remote control room they should be assembled in a remote junction box and the signals conveyed to the control room via multitude bundles. Such junction boxes should be carefully positioned in the plant in order to minimize the lengths of the individual run tubes.

Impulse Lines

These are lines containing process fluids which run between the instrument impulse connection and the process tapping point, and are typically made up from piping and pipe fittings or tubing and compression fittings. The pipe materials must be compatible with the process fluids.

Impulse lines should be designed to be as short as possible and should be installed so that they are self-draining for liquids and self-venting for vapors or gases. If necessary, vent plugs or valves should be located at high points in liquid-filled lines and similarly, drain plugs or valves should be fitted at low points in gas or vapor-filled lines. All in all, it should be ensured that there provisions for isolation and depressurizing of instruments for maintenance purposes. Additionally, filing plugs should be provided where lines are to be liquid scaled for chemical protection and on services which are prone to plugging, rodding out connections should be provided close to the tapping points.

Related articles:

• How to Connect a DP (Differential Pressure) Flow Sensor to a DP Transmitter
• What are Control and Instrumentation Cables?
• The Ultimate Practical Guide to Industrial Flow Measurement Instruments
• How to Minimize Water Vapor in Instrument Air/Pneumatic Systems
• How to Perform Mass flow Measurements with DP sensors