1 Scope
NOTE This specification covers the complete field installation package that connects a process instrument to the process: the primary process connection, the isolation manifold, the impulse tubing or piping run, and the supports, fittings, and accessories that complete the assembly, for industrial, process, and water and wastewater treatment facilities. (1.1)
NOTE Equipment and materials covered include process taps and root valves, flanged root-valve connections, 2-valve / 3-valve / 5-valve instrument manifolds, stainless, carbon-steel, and alloy impulse tubing and piping, compression and threaded and socket-weld tube fittings, supports and clamps, condensate and seal pots, purge connections, heat-tracing provisions, and the blowdown, vent, and drain valves on the sensing lines. (1.2)
NOTE This standard is the "last mile" of every field instrument: the instrument-selection specification ends at the instrument's process connection, the process-piping specification ends at the root valve, and the run between those two points — the impulse line — is the segment this standard owns. (1.3)
NOTE The boundary of work under this standard begins at the process tap (the branch connection or nozzle on the process pipe or vessel) and ends at the instrument process connection, inclusive of the root valve, the manifold, the tubing, and the line accessories. (1.4)
NOTE A common project failure is to specify impulse tubing only inside the instrument specification, leaving no contractor section that owns the run between the root valve and the instrument; this standard exists to close that gap. (1.5)
NOTE Two distinct construction methods are in scope and are treated separately: small-bore instrument tubing (typically 1/4 in. to 1/2 in. OD, compression-fitted) and instrument piping (1/2 in. to 1 in. NPS, threaded or flanged). (1.6)
1.7Design of the impulse tubing and piping system shall comply with ASME B31.3 except where ASME B31.1 governs the connected service in power-plant applications.
2 Referenced Standards
NOTE Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited. (2.1)
2.2Where conflicts exist between referenced standards, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
| Standard |
Title |
| ASME B31.3 |
Process Piping (governing design code for instrument piping and tubing) |
| ASME B31.1 |
Power Piping (governs sensing lines in power-plant service) |
| ASME B16.5 |
Pipe Flanges and Flanged Fittings |
| ASME B1.20.1 |
Pipe Threads, General Purpose (Inch) — NPT |
| ASTM A269/A269M |
Seamless and Welded Austenitic Stainless Steel Tubing for General Service |
| ASTM A213/A213M |
Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater, and Heat-Exchanger Tubes |
| ASTM A179/A179M |
Seamless Cold-Drawn Low-Carbon Steel Heat-Exchanger and Condenser Tubes |
| ASTM B88 |
Seamless Copper Water Tube (instrument air / non-corrosive sensing) |
| ANSI/ISA-5.1 |
Instrumentation Symbols and Identification |
| ANSI/ISA-5.4 |
Instrument Loop Diagrams |
| ANSI/ISA-77.70 |
Fossil Fuel Power Plant Instrument Piping Installation |
| ANSI/ISA-77.70.02 |
Fossil Fuel Power Plant Instrument Piping Installation — Part 2 |
| ISA-67.02.01 |
Nuclear Power Plant Pressure- and Vacuum-Sensing Line Piping and Tubing Standards |
| PIP PCCIP001 |
Instrument Piping and Tubing Systems Criteria |
| PIP PCSIP001 |
Instrument Tubing Material Specification |
| NFPA 70 |
National Electrical Code (Articles 500-505, area classification of heat-trace) |
| OSHA 29 CFR 1910.119 |
Process Safety Management of Highly Hazardous Chemicals |
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following for the Engineer's review and approval prior to fabrication or installation:
- Instrument installation detail (hook-up) drawings for each instrument type, showing tap location and orientation, root valve, manifold, tubing routing, slope, supports, condensate or seal pots, purge connections, and vent/drain valves
- Bill of materials for tubing, fittings, valves, manifolds, and accessories, identifying material grade, OD and wall thickness, pressure rating, and ANSI class for each item
- Tubing material certifications (mill test reports) per ASTM A269/A213/A179/B88 as applicable, showing chemistry, mechanical properties, and hydrostatic test compliance
- Manifold data sheets identifying valve count (2, 3, or 5), mount style (integral direct-mount or remote), body material, port size and thread form, and pressure-temperature rating
- Root valve data sheets identifying valve type (needle, ball, or gate), ANSI pressure class, body and trim material, and end connections
- Tubing pressure-rating calculation per ASME B31.3 confirming the assembly meets design pressure at design temperature with the required margin
- Heat-trace and insulation design for sensing lines requiring freeze or congealing protection, including circuit type, setpoint, and division-of-responsibility coordination
- Leak-test plan stating test medium, test pressure, hold time, and acceptance criteria per ASME B31.3
- Compression-fitting make-up procedure and the manufacturer's initial make-up specification (turns past finger-tight or torque)
☑ Instrument hook-up (installation detail) drawings per instrument type
☐ Bill of materials with grades, OD/wall, ratings
☐ Tubing mill test reports (ASTM A269/A213/A179/B88)
☐ Manifold data sheets (valve count, mount, ports)
☐ Root valve data sheets (type, class, material)
☐ Tubing pressure-rating calculation per ASME B31.3
☐ Heat-trace and insulation design
☐ Leak-test plan (medium, pressure, hold, acceptance)
☐ Compression-fitting make-up procedure
3.1.2Fabrication and field installation shall not proceed until action submittals have been reviewed and returned.
3.2 Closeout Submittals
3.2.1At substantial completion, the Contractor shall provide the following before the instrument installations are accepted:
- Marked-up as-built hook-up drawings reflecting the installed tap locations, routing, and accessory placement
- Leak-test records for each completed sensing line, identifying the line, test medium, pressure, hold time, and result
- Material traceability records correlating installed tubing and fittings to the submitted mill test reports for PSM-covered processes
- Heat-trace circuit test and continuity records, with the insulation installed only after the trace circuit was verified
- Compression-fitting make-up verification records confirming each connection was made to the manufacturer's initial make-up and field-marked
- Mechanical-integrity inspection baseline records for impulse lines on OSHA 29 CFR 1910.119 covered processes
☑ As-built hook-up drawings
☐ Leak-test records per sensing line
☐ Material traceability records (PSM processes)
☐ Heat-trace circuit test and continuity records
☐ Fitting make-up verification records
☐ Mechanical-integrity inspection baseline (PSM)
4 Quality Assurance
4.1 Installer Qualifications
4.1.1Instrument tubing and piping shall be installed by craftspersons experienced in compression-fitting make-up, tube bending, and small-bore instrument installation.
4.1.2Welded socket-weld and tap connections shall be made by welders qualified under the welding procedure required by Welding Requirements. NOTE Field make-up of compression fittings is deceptively skill-dependent: an installer who has not been trained to the manufacturer's make-up method produces fittings that pass a casual visual check but leak under pressure or vibration. (4.1.3)
4.2 Material Compatibility
4.2.1Tube fittings shall be of the same base material as the tubing they join.
4.2.2Stainless-steel tubing shall not be joined with carbon-steel or brass fittings.
NOTE Mixing stainless-steel tubing with carbon-steel or brass fittings sets up a galvanic cell that corrodes the fitting body from within; the failure is internal and is not visible during inspection until the fitting leaks. (4.2.3)
4.2.4Supports and clamps in contact with stainless tubing shall provide galvanic isolation where the support material is dissimilar.
4.3 Process Safety Management
4.3.1Impulse lines on OSHA 29 CFR 1910.119 covered processes shall be enrolled in the facility mechanical-integrity program, with inspection, testing, and documentation records satisfying the PSM program.
NOTE On a PSM-covered process the impulse line is process-containing pressure boundary, so it carries the same mechanical-integrity and recordkeeping obligations as the process pipe it taps. (4.3.2)
5 Environmental and Service Conditions
5.1 Process Service
Clean liquid (water, treated process liquid)
Steam or high-temperature condensate
Gas or vapor (clean, dry)
Corrosive liquid (acid, caustic, high-chloride)
Slurry or plugging service
Hydrocarbon (refinery, petrochemical)
5.1.1Tubing material, fitting type, slope direction, and accessories shall be selected for the process fluid, its temperature, and its corrosion characteristics.
NOTE Process service drives nearly every selection in this standard: a single tubing material specified regardless of fluid is the root cause of in-service corrosion failures, because copper or carbon steel that is fine for instrument air will fail in chlorinated, acidic, or ammonia service. (5.1.2)
5.2 Ambient and Climate
● None — conditioned indoor, no freeze risk
○ Freeze protection required (water service, ambient below 32°F)
○ Congealing protection required (viscous or wax-forming fluid)
5.2.1Sensing lines exposed to ambient temperatures that could freeze the fill fluid or congeal the process fluid shall be heat-traced and insulated.
5.2.2Freeze protection setpoint for water-filled sensing legs shall maintain a minimum fluid temperature of 40°F (4°C), sized against the local design-day minimum ambient per the project mechanical design.
NOTE A frozen or congealed impulse line is not merely a maintenance nuisance: it isolates the instrument from the process and produces a measurement that is silently wrong, which on a control loop drives the process to a false setpoint. (5.2.3)
5.3 Area Classification
5.3.1Heat-trace wiring and any electrical components on impulse lines in classified (hazardous) locations shall be rated for the area class, division or zone, and gas group per NFPA 70 Articles 500-505.
NOTE Electrical area classification at the instrument location, not at the control room, governs the heat-trace components: an impulse line that runs through a Class I Division 1 area requires explosion-proof or intrinsically safe trace components even when the instrument itself is remote. (5.3.2)
6 Primary Process Connection
6.1 Process Tap and Root Valve
6.1.1Each instrument sensing connection shall be provided with a primary isolation (root) valve at the process tap, between the process and the impulse line.
NOTE The root valve allows the instrument and its impulse line to be isolated from the live process for maintenance, calibration, and removal without shutting down the process. (6.1.2)
6.1.3The root valve pressure class shall meet or exceed the ANSI pressure class of the connected process pipe.
● Needle valve (fine throttling, small-bore sensing)
○ Ball valve (full-bore isolation, quick on/off)
○ Gate valve (full-bore, larger process piping)
ANSI 150
ANSI 300
ANSI 600
ANSI 900
ANSI 1500
ANSI 2500
316/316L stainless steel
304 stainless steel
Carbon steel (A105)
Duplex stainless (UNS S31803 / 2205)
Alloy steel (chrome-moly, high-temperature service)
6.2 Tap Orientation
● Instrument above tap (liquid service, self-venting)
○ Instrument below tap (gas/steam service, self-draining)
6.2.1Tap orientation and impulse-line slope shall be coordinated so that liquid-filled lines self-vent toward the instrument and gas or steam lines drain condensate back toward the process tap.
NOTE Tap orientation is a measurement-integrity decision, not just a routing choice: for a liquid-filled line the instrument must sit above the tap so trapped gas vents back to the process, while for a gas or steam line the instrument must sit below so condensate drains back to the tap. (6.2.2)
NOTE Reversing the correct slope direction traps gas in a liquid leg or liquid in a gas leg and introduces a systematic, offset error that calibration cannot remove. (6.2.3)
6.3 Flanged Connections
6.3.1Flanged root-valve and manifold connections shall comply with ASME B16.5 for flange rating, facing, and bolting.
6.3.2Threaded process connections shall use NPT threads conforming to ASME B1.20.1.
7 Instrument Manifolds
7.1 Manifold Selection
7.1.1Each transmitter shall be provided with an instrument isolation manifold matched to the instrument's measurement type and connection.
NOTE The manifold type is set by the measurement: a single-pressure (gauge) instrument needs a 2-valve block-and-bleed; a differential-pressure instrument needs a 3-valve manifold to equalize and isolate both legs; a DP instrument that requires in-situ calibration with dual-side bleed needs a 5-valve manifold. (7.1.2)
NOTE Omitting the manifold type and mount style from the specification forces a substitution RFI in the field; the Engineer shall state valve count and integral-versus-remote mount explicitly. (7.1.3)
○ 2-valve block-and-bleed (gauge / single-pressure service)
● 3-valve DP manifold (differential-pressure, equalize + isolate)
○ 5-valve DP manifold (DP with dual-side vent/bleed for calibration)
● Integral — direct-mount to transmitter flange
○ Remote — pipe- or bracket-mounted
316 stainless steel
Carbon steel
Duplex stainless (2205)
Alloy (Monel / Hastelloy, severe service)
7.2 Manifold Connections
7.2.1Manifold port size and thread form shall match the connected instrument and tubing, without adapters added in the field.
NOTE Many manifolds ship with 1/2 in. NPT or SAE ports while the specified instrument uses 1/4 in. NPT; an unspecified port size forces field adapters that add leak points, so the port size shall be stated explicitly. (7.2.2)
1/4 in. NPT
1/2 in. NPT
Direct flanged to transmitter (integral mount)
8 Impulse Tubing and Piping
8.1 Tubing Material and Grade
8.1.1Impulse tubing shall be of a material compatible with the process fluid, temperature, and corrosion allowance, conforming to the applicable ASTM specification.
8.1.2Stainless impulse tubing shall conform to ASTM A269/A269M; alloy-steel high-temperature tubing to ASTM A213/A213M; carbon-steel tubing to ASTM A179; and copper instrument-air tubing to ASTM B88.
NOTE The 80% case for process and water/wastewater service is 316L stainless steel, chosen for broad corrosion resistance and ready availability; chloride-bearing service (seawater, brine, high-chloride wastewater) moves to duplex, and high-temperature steam moves to carbon or alloy steel. (8.1.3)
316L stainless steel (ASTM A269)
304 stainless steel (ASTM A269)
Duplex stainless 2205 (chloride service)
Carbon steel A179 (steam service, power plant)
Alloy steel T11/T22 (high-temperature)
Copper Type K (ASTM B88, instrument air / non-corrosive)
8.2 Tubing Size and Wall
8.2.1Tubing OD and wall thickness shall be selected so the tubing pressure rating meets or exceeds the process design pressure at design temperature.
8.2.2A minimum wall thickness shall be specified for each service; thin-wall tubing (0.028 in.) shall not be substituted.
NOTE Some contractors will source cheaper thin-wall tubing when no minimum wall is stated; thin-wall tubing cannot meet the design pressure at elevated temperature and is a latent rupture hazard, so the wall is specified, not left to the supplier. (8.2.3)
● 1/4 in. OD × 0.049 in. wall (general process, default)
○ 3/8 in. OD × 0.065 in. wall (purge / longer runs / bore cleanliness)
○ 1/2 in. OD × 0.083 in. wall (high-viscosity, plugging, steam legs)
NOTE Tubing pressure rating, 316L stainless 1/4 in. OD × 0.049 in. wall, at 100°F is approximately 5,500 psi per ASME B31.3 / PIP PCSIP001, derated by the temperature correction factor above 100°F. (8.2.4)
NOTE Tubing pressure rating, 316L stainless 1/2 in. OD × 0.083 in. wall, at 100°F is approximately 6,600 psi at 100°F. (8.2.5)
Per drawings (deferred by default)
Per drawings (deferred by default)
8.3 Slope and Routing
8.3.1Impulse lines shall be routed without traps, pockets, or high points in liquid service, and without low points in gas or steam service.
8.3.2Liquid-service lines shall slope continuously upward toward the instrument; gas- and steam-service lines shall slope continuously downward toward the process tap.
8.3.3The minimum slope shall be 1 in. per 12 in. (1:12); a slope of 1 in. per 10 in. is preferred.
1 in. per 12 in. (1:12) minimum
1 in. per 10 in. (preferred)
8.3.4Impulse-line length shall be minimized; liquid-service runs longer than 50 ft (15 m) shall be reviewed for response lag, and pneumatic runs over 100 ft (30 m) shall be checked against the transmitter manufacturer's response data.
NOTE A long impulse line adds dead time to the loop; the line is "sensing" the process through a column of fluid, and excess length, traps, or undersized bore slow the response the controller sees. (8.3.5)
8.4 Fittings
8.4.1Tube fittings shall be selected for the service pressure class, vibration exposure, and maintainability.
8.4.2Double-ferrule compression fittings shall be used for general process and water/wastewater service.
8.4.3NPT threaded fittings shall not be used as the primary connection method in high-cycle vibration service (reciprocating compressors and pumps); compression or socket-weld fittings shall be used there.
NOTE Vibration loosens tapered NPT threads over time, so high-cycle vibration service demands the mechanical retention of a compression or socket-weld joint rather than thread engagement alone. (8.4.4)
● Double-ferrule compression (general process, default)
○ Socket-weld (steam / power-plant / high-vibration)
○ NPT threaded (low-pressure, non-vibration only)
8.4.5Compression fittings shall be made up to the manufacturer's initial make-up specification (for example, 1-1/4 turns past finger-tight for a 1/4 in. fitting) and the made-up connection shall be marked and verified at installation.
NOTE Over-tightening a compression fitting collapses the bore and under-tightening allows a leak, so the make-up is a specified, gap-inspectable, marked step — not left to the installer's feel. (8.4.6)
9 Line Accessories
9.1 Condensate and Seal Pots
9.1.1Steam and high-temperature liquid sensing lines shall be provided with condensate pots to establish and maintain a stable liquid reference leg.
9.1.2On a DP circuit, the seal-leg height above the low-pressure tap shall be equal on both legs.
● Not required (ambient liquid or gas service)
○ Condensate pot required (steam / high-temperature sensing)
○ Seal pot with fill fluid required (incompatible or freezing process)
1/2 in. NPT
3/4 in. NPT
1 in. NPT
NOTE Omitting condensate pots on steam or high-temperature sensing lines lets the liquid levels in the two legs of a DP circuit drift unequal, producing a permanent, uncorrectable zero offset in the measurement. (9.1.3)
9.2 Purge Connections
● None
○ Continuous purge (rotameter-regulated)
○ Batch / manual purge
Instrument air
Nitrogen
Treated water
9.2.1Where a purge system is provided, the purge connection shall be made on the impulse line at the point shown on the hook-up detail. purge connection point NOTE Plugging and slurry services benefit from a purge so the sensing line is kept clear, but the purge fluid must be compatible with the process and its flow must not bias the measurement. (9.2.2)
9.3 Blowdown, Vent, and Drain Valves
9.3.1Low points of liquid-service lines shall be provided with drain valves and high points of gas-service lines with vent valves, shown on the hook-up drawings.
9.3.2Plugging-prone lines shall be provided with blowdown valve provisions to clear the line during operation.
NOTE Inability to clear a plugged or air-bound line in service is a recurring maintenance failure; vent valves at high points and drain valves at low points let an operator clear the line without taking the loop out of service. (9.3.3)
10 Supports and Clamps
10.1 Support Spacing
10.1.1Impulse tubing shall be supported at intervals that prevent sag, vibration fatigue, and strain on fittings.
10.1.2Maximum unsupported span for 1/4 in. tubing shall be 36 in. (900 mm); for 1/2 in. tubing, 60 in. (1,500 mm), per PIP PCCIP001.
36 in. (900 mm) — 1/4 in. and 3/8 in. tubing
60 in. (1,500 mm) — 1/2 in. tubing
10.2 Clamp Type
● Rigid clamp (standard, low-vibration)
○ Vibration-isolation clamp (rotating/reciprocating equipment)
10.2.1Clamp and support materials in contact with the tubing shall be compatible with the tubing material, with galvanic isolation provided where dissimilar metals would otherwise be in contact.
NOTE A bare carbon-steel clamp gripping stainless tubing in a wet location is a galvanic couple at the one point the tubing is least inspectable, so dissimilar supports get an isolating liner or sleeve. (10.2.2)
11 Heat Tracing and Insulation
11.1 Heat-Trace Provisions
● None
○ Electric (self-regulating or constant-wattage)
○ Steam tracing
11.1.1Heat-traced sensing lines shall be insulated over the trace, and the insulation shall be installed only after the trace circuit has been tested and verified.
11.1.2Division of responsibility for heat tracing shall be stated explicitly: the mechanical contractor installs the tubing, the electrical contractor powers and tests the trace circuit, and the insulation contractor wraps the line after the trace test.
NOTE Without explicit coordination, insulation is routinely installed before the heat-trace circuit is tested, hiding a dead trace circuit that is then discovered only when the line freezes; the spec sequences the trace test ahead of insulation to prevent it. (11.1.3)
12 Testing
12.1 Leak and Pressure Testing
12.1.1Every completed impulse line assembly shall be leak- and pressure-tested per ASME B31.3 before the instrument is connected and the loop is commissioned.
12.1.2Hydrostatic test pressure shall be 1.5 times the design gauge pressure, held a minimum of 10 minutes for tubing and 30 minutes for welded systems.
12.1.3Where a pneumatic test is approved by the Owner in lieu of hydrostatic, the line shall first be pressurized to 25 psi to check for gross leaks, then tested at 110% of design pressure with visual and bubble inspection.
● Hydrostatic (1.5 × design pressure)
○ Pneumatic (110% design, owner-approved, with 25 psi gross-leak check)
10 minutes (compression-fitted tubing)
30 minutes (welded systems)
NOTE Discovering an impulse-line leak during startup stalls the entire loop checkout, so the line is proven tight as an installation step, well before commissioning depends on it. (12.1.4)
NOTE A pneumatic test stores far more energy than a hydrostatic test, which is why it is the owner-approved exception rather than the default and why the gross-leak pre-check at 25 psi is required first. (12.1.5)
13 Installation
13.1 General
13.1.1Instrument tubing and piping shall be installed in accordance with the approved hook-up drawings, the tubing manufacturer's instructions, and PIP PCCIP001 / ANSI/ISA-77.70.
13.1.2Tubing shall be cut square, deburred, and blown clear before fittings are made up.
13.1.3Bends shall be made with a proper tube bender to the manufacturer's minimum bend radius, without flattening or kinking the tube.
13.1.4Instrument air supply tubing and impulse (sensing) tubing shall be installed as separate systems, with their own materials, ratings, and cleanliness requirements.
NOTE Confusing instrument-air tubing with impulse tubing is a recurring error; the two carry different service, pressure, and cleanliness requirements and shall not share materials or runs even where they parallel each other. (13.1.5)
13.2 Cleanliness
13.2.1Tubing bores shall be kept clean and capped during storage and installation, and shall be free of cutting oil, chips, and debris before connection to the instrument.
NOTE Oxygen and other special-cleanliness services shall be cleaned and handled to the project cleanliness specification before installation. (13.2.2)
14 Delivery, Storage, and Handling
14.1 Protection
14.1.1Tubing, fittings, manifolds, and valves shall be delivered in original protective packaging with end caps and plugs in place.
14.1.2Materials shall be stored indoors or under cover, off the ground, and protected from moisture, dirt, and mechanical damage until installed.
NOTE Stainless tubing shall be stored away from carbon-steel storage that could transfer iron contamination to the stainless surface. (14.1.3)
15 Warranty
15.1 Warranty Period
15.1.1The Contractor shall warrant the instrument installation, tubing, fittings, manifolds, and accessories against defects in materials and workmanship for the project warranty period, minimum one year from substantial completion.
15.1.2Leaks, fitting failures, and corrosion attributable to incorrect material selection or improper fitting make-up within the warranty period shall be corrected at no cost to the Owner.
16 Spare Parts
16.1 Recommended Spares
16.1.1The Contractor shall furnish spare tube fittings, ferrule sets, and manifold seal kits in the quantities listed.
020
Default: 5 % of installed
NOTE Ferrule sets and manifold seal kits are the wear items most likely to be needed for a re-make or a packing replacement, so a small field stock keeps a single leak from becoming a procurement delay. (16.1.2)