Installation of Impulse Lines for Instruments

Explain basic I&C systems installation requirements, such as design consideration for the installation of sensing lines, use of valve manifolds, accessibility for maintenance and calibration, etc.


Installation of Sensing Lines

The following is taken from Los Alamos National Laboratory, LANL Engineering Standards Manual ISD 341-2. Instrument sensing lines should be installed in accordance with the guidance presented in LANL Engineering Standards Manual ISD 341.2.

The safety classification of piping for instrument tubing systems should be, at a minimum, consistent with the requirements of the process system to which the instrument is connected. Sensing lines should not be installed in a manner that would interfere with or prevent maintenance and/or operational activities. Minimum headroom clearance of known and identified passageways should be 7 feet. Whenever practical, sensing lines should be routed along walls, columns, or ceilings, avoiding open or exposed areas.

Structural channels or a track should be installed to protect sensing lines in exposed locations subject to accidental crushing or damage. This type of protection, however, should not render the tubing and fittings inaccessible. Instrument sensing lines should be routed separately from process lines and equipment where vibration, abnormal heat, or stress could affect the lines. Tubing and piping that must be connected to vibrating equipment should be fabricated with adequate flexibility.

Instrument sensing lines should not come in contact with structural steel and concrete surfaces of building members. In no case should tubing be installed in direct contact with painted or unpainted concrete surfaces, except for penetrations requiring closure.

Grout should only be used for tubing with temperatures below 200°F. Instrument sensing lines routed through penetrations, shield walls, or other barriers where visual contact is impaired or lost, should be labeled with a permanent tag attached securely on each side of that barrier displaying the corresponding instrument identifier.

The spacing around sensing lines should always be wide enough to allow each tube to expand independently at all turns without striking adjacent tubes or other equipment. Heat tracing should be used for sensing lines containing liquid that may freeze or become viscous, or wet gas from which moisture may condense. The heat tracing should provide enough heat to prevent freezing or condensation, but not great enough to boil the liquid in the sensing line.

Sensing lines should have continuous slope to promote their being kept either full or free of fluid. The preferred slope is 1 inch per foot, however, 1/4 inch per foot is acceptable. For instruments sensing steam at pressures up to 20 pounds per square inch, absolute (psia), the instrument lines should slope a minimum of 2 inches per foot. Minimum slope will begin after the root valve and terminate at the instrument valve inlet.

The sensing lines may be level through and on each side of a valve manifold or instrument connection shut off valve for a distance of up to 4 inches. Instrument sensing lines may be level through and on each horizontal leg of a vertically oriented tee or cross connection for a distance of up to 4 inches, and through a penetration for a total cumulative length of 12 inches outside the ends of each penetration. Bends, rather than tube or pipe fittings, should be used to change the direction of sensing lines.

The cold bending method is advised for all bends. A minimum bend radius of at least two and one quarter (2 1/4) times the tubing outside diameter should be employed for bends in stainless steel tubing and copper tubing. The minimum bend radius for capillary tubing, aluminum, and plastic tubing should be per manufacturer recommendations. Where fittings must be used, and an installation detail document specifies the size and type of fitting, a combination of fitti
ngs of other sizes may be substituted if the specified part is unavailable or it is more convenient to use the combination.

The fittings must, however, be of the same equivalent type and produce the same or better overall effect. A weld fitting may replace a threaded or flareless joint; however, a threaded or flareless connection should not be used if welded fittings are required. Flareless fittings should be installed using the manufacturer’s assembly instructions.

For threaded connections of stainless steel to stainless steel, lubrication should be applied to prevent seizing and galling. Low or no chloride content lubricants should be used with stainless steel. Although Teflon tape is allowed in many applications, it should not be used as a sealant or lubricant on threaded instrument connections.

The use of compound or lubricant on threads should consider the potential reaction with either the service fluid or the piping material. Sensing lines should be blown clear of any foreign material with clean, oil free, dry air or nitrogen before the system is placed in operation.

Demineralized water may be used to flush tubing, provided the process system to which the tubing is connected will also be flushed or hydro-tested with water in accordance with applicable construction procedures. Open lines, fittings, or valves should be sealed after being blown clear. Instrument tubing between the manifold valve and the instrument is not required to be blown down or flushed if visual inspection is performed prior to final connection and tightening of fittings.

Capillary tubes sealed to the instrument by the manufacturer should not be opened or cut during or after installation unless specifically required by the installation drawing or manufacturer’s instruction. Slope requirements do not apply to capillary tubing. Manufacturer’s installation requirements, including those relating to minimum bend radius, should be followed.

Excess lengths of capillary tubing should be neatly coiled in protected enclosures. The maximum amount of unprotected capillary should be no more than 6 inches at any one location, except at capillary enclosures, process connections, instrument connections, and penetrations.

At the entrance and exit of capillary enclosures, process connections and instrument connections, the maximum unprotected capillary should be 18 inches. Capillaries in trays should be tied down or clamped every three feet. At the entrance and exit of penetrations, the maximum unprotected capillary should be 12 inches.

Primary sensing lines at local panels and racks should be neatly arranged with easy access to test, drain, and vent connections, instruments valves, and manifold. Primary tubing between the instrument valve or manifold and the instrument should be arranged in accordance with the vendor’s instruction and with adequate flexibility to avoid undue strain to the instruments.

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