Instrument tubing

Tube is not pipe

Pipe is sized by a nominal size and a schedule: NPS under the American system, DN in Europe, or NB under the old British system — in Australia you will usually hear "50NB Schedule 40S". Nominal sizes are not the real OD, and the systems differ: size 65 pipe is 73.0 mm OD under the American system but 76.2 mm under the British one.

Tube is sized by its actual OD and wall thickness. Quality depends on OD and ID tolerance, concentricity, hardness, material and wall thickness — every one of them affects whether a compression fitting grips and seals.

How instrument tube is made

CDS — cold drawn seamless

Drawn from a hollow billet, then supplied annealed and pickled (AP) or bright annealed (BA). The preferred choice for high-pressure, corrosive instrument service.

AW / AWA

As welded straight off a continuous tube mill; AWA adds a final anneal.

CW / CWA

Welded, then the internal weld bead is cold worked smooth; CWA adds a final anneal.

WD / WDA

Welded tube redrawn through dies and mandrels for close OD and wall tolerance; WDA adds a final anneal.

Pickling & passivating

Acid treatments that remove heat-treatment scale and surface contamination so the protective chromium-oxide film can re-form.

A typical 316 tube specification

• Annealed, cold drawn seamless to ASTM A213 / A269 (average wall).
• TP316/316L with a minimum 2.5% molybdenum — extra corrosion resistance for Australia's coastal, chloride-laden onshore and offshore sites.
• Hardness not above about 80 HRB, so the ferrules can bite.
• Pickled finish, fixed 6 m lengths, certified to EN 10204 3.1.
• Continuously marked with manufacturer, heat number, size, wall, grade and Mo content.

Check it when it arrives

Read the roll-stamping: material grade, minimum Mo content, heat number, vendor, OD, wall and seamless or welded. If the marking is incomplete or unreadable, measure OD and wall with callipers, and trace the heat number to the material certificate before using the tube.

What happens as you turn the nut

1. 0
   Finger-tight

   The ferrules only touch the tube. Nothing is set yet.

2. ¾
   First ¾ turn — front ferrule sets

   The rear ferrule pushes the front ferrule into the body taper, making the seals to the tube and to the body. The front ferrule grabs the tube and drives it to the base of the sizing angle.

3. 1¼
   Last ½ turn — rear ferrule sets

   With the tube bottomed, the rear ferrule (the hardest part of the system) sets and grips the tube to resist pull-out and vibration.

Tell metric from fractional

• Body hex: straight bodies with a metric end have a machined step on the tube side of the hex.
• Nut: metric nuts have a short shank on the threaded end and are stamped mm.
• Front ferrule: metric brass and 316 front ferrules have a groove in the shoulder; other materials show "mm" after the material code.
• Shaped fittings: elbows, tees and crosses are stamped mm on the side.
• Packaging: blue boxes mean metric parts and accessories.
• Special alloys: 316 nuts have two machined shoulders and a raised front-ferrule shoulder; alloy parts (6Mo, C276, Ti, Monel, Inconel, Incoloy, duplex) have one shoulder, a wedge-shaped front ferrule, and a suffix such as -6MO, -625 or -825.

Read a part number

Fractional sizes are in sixteenths of an inch. Tees are coded run (ports 1 and 2) then branch (port 3): a TTM has tube at 1 and 2 with a male pipe thread on the branch; a TFT has tube at 1 and 3 with a female thread at 2. Confirm codes in the current catalogue.

Accessories that remove leak points

• Tube adaptors — solve thread-alignment problems without backing off (leaks) or over-tightening (thread damage, stress cracking) a threaded joint.
• Port connectors — join two fittings directly without cutting a short tube stub; a union that is easy to break.
• Reducers and reducing tees — connect different or metric/fractional tube sizes without extra adaptors.
• Plugs and caps — blank off an unused port or tube end, ready for a future extension.
• Safety changers (nut and ferrule sets) — replace ferrules without handling them, so they can't go in the wrong way round.
• Tube inserts — support soft plastic tube (PVC, Tygon®) inside a compression fitting.

How much thread should engage?

Because the standard allows ±1 pitch, a fully assembled ASME fitting may have only 2–3 threads engaged, leaving 4–5 visible. Tighter-tolerance fittings (the HOKE thread specification is 62.5% tighter) typically engage 5–6 threads, leaving 1–2 visible — a stronger joint that better resists vibration and corrosion.

Site gauging requirement (example)

The site-example specification tightens the ASME L1 gauging tolerance to L1 + ¼ to L1 + 1 turn:

• Male threads: hand-tight in the ring gauge, engage L1 + ¼ to L1 + 1 turn past the first scratch mark on the chamfer.
• Female threads: hand-tight, the plug gauge engages L1 + ¼ to L1 + 1 turn below the last scratch on the chamfer.
• Hand-tight assembly should give 4–6 full turns of engagement without tape.
• Wrench-tight with approved tape adds a further 0.5–1.5 turns.
• Thread-profile compliance to ASME B1.20.1 is part of the quality-assurance requirements.

Instrument tappings have traditionally used a piping-class double block and bleed (DBB) assembly with extra small-bore instrument valves screwed on. A typical conventional 1 in, 1500# multi-valve DBB tapping:

• 45 kgweight
• 1.5 mheight
• 8leak points
• 6gaskets & bolt sets

It also needs a support bracket and on-site assembly, is prone to vibration fatigue, and puts a bending moment on the branch weld.

Better options for new and replacement tappings

• Close-coupled / modular valves — Keyblok (ball or globe needle valves in an integrally forged body), Monoflange (slimline forged body) and root valves welded or threaded directly to the vessel.
• Flange adaptors — one-piece forged bodies with a compression-fitting outlet, for connecting impulse lines to piping and vessels in any pressure class. Some project specifications require them offshore.
• Integral tube connections — the compression-fitting port is machined directly into the valve or manifold body, removing the NPT/BSP joint, the tape and the paste. Safer for high-pressure, steam and sour service; fully field-maintainable.
• Minimise threaded connections — don't stack adaptors to "make it fit".

Leave enough straight tube at the fitting

The tube must run straight far enough into the fitting to bottom fully, give correct ferrule action, leave room for the nut to travel and allow easy disassembly. A bend that starts too close to the fitting stops the tube seating and loads the ferrules. Never bend tube after it has been inserted into a fitting.

GYROLOK® planning values. Check your bender and fitting manufacturer's data for other brands and wall thicknesses.

Bends instead of fittings

• Replace elbow fittings with tube bends wherever you can — every fitting is a potential leak path.
• Keep the number of joints to a minimum and impulse lines as short as possible.
• Plan expansion loops, and put break (make-break) connections where maintenance needs them.
• Tube that is oval after bending won't enter the nut and ferrules easily — don't force it. Cut it out and bend again.
• Repeated bending or vibration work-hardens tube. Don't re-bend a finished bend.

Routing rules

• Keep impulse lines as short as practical, and keep both DP legs the same length and at the same temperature.
• Slope continuously — a minimum fall of about 1 in 12 is common in specifications — with no unintended high or low points.
• Leave room for thermal expansion and vibration with a bend or loop — not a tight straight run between fixed points.
• Keep clear of hot surfaces, walkways and access routes. Never use tubing as a step or handhold.
• Label lines at both ends with tag and service.

Indicative support spacing

In the offshore site example, tube runs longer than 1000 mm are also supported at least every 1000 mm, and within 250 mm of any bend. Support a fitting close by on both sides where there is vibration or heavy components. Project specifications take precedence.

Clamps and supports

• Use support clamps compliant with DIN 3015 (Stauff type or company-approved equivalent).
• Clamp bodies are two profiled halves of heavy-duty, electrically insulating material such as polypropylene — keeping stainless tube isolated from carbon-steel steelwork and preventing galvanic and fretting corrosion.
• Clamps are hydrophobic and self-draining, designed so water can't sit against the tube by capillary or seepage action — a key cause of chloride pitting and crevice corrosion.
• Clamp materials are non-corrosive: AISI 316 and/or flame-retardant plastic.
• Run tube on trays or ladders, adequately supported and braced.

Plan for access and maintenance

• Offset joints (raised about 30 mm) and stagger them in parallel runs so each nut can take a wrench.
• Leave every fitting as accessible as possible for inspection, leak checks and remakes.
• Plan expansion loops and make-break connections at equipment that will be removed for maintenance.
• Don't run tube in front of equipment, access panels or instruments that need to be reached.
• Consider flexible hose where pressure pulses, temperature cycling or vibration would fatigue rigid tube.

Inspection and leak-test regime — site example (OT&FMS 9.4)

• Inspect new and modified instrument fitting installations and record the results on the site tubing & fitting inspection test record.
• Sample 10% of installed fittings. If any defect is found, increase to 100% unless the Instrument TA-2 agrees otherwise.
• Leak-check every completed fitting.
• Lines normally carrying pressurised air: snoop (leak-detection fluid) check.
• Toxic, steam or hydrocarbon service: hydrostatic test at 150% MAWP or pneumatic test at 110% MAWP, held for at least 10 minutes.

Service conditions that shorten tube life

Impulse and shock

Pressure cycling and temperature cycling apply sudden, violent stress. Consider flexible hose.

Vibration

Adds stress and work-hardens tube over time. Support close to fittings; consider flexible hose.

Temperature

Metal strength falls as temperature rises, lowering allowable working pressure — apply derating.

Elastomers

Harden and crack at low temperature, and shrink far more than metal when cold — seals made at ambient may leak in cryogenic service.

Metals

Select on media, temperature, pressure and safety factor. External environment matters as much as the process — take care with lower-grade stainless for non-wetted parts.