Instrument Air Tubing Material & Sizes:

All instrument air tubing and fittings shall be imperial sizes, expressed in nominal outside diameter (OD) and all threads shall be NPT Tubing and fittings shall be of 316 SS to ASTM A269 unless otherwise specified.
The minimum size shall be ¼inch OD. Tubing runs shall be supported and protected. Tube fittings shall be of double ferrule, pressure seat, no torque type for use on heavy or thin wall tubing, and shall be of reputable makes (such as Swagelok or Parker). Ferrule and nut shall be of the same material as the fittings. Flare type fitting shall not be used.
Stainless steel tubing shall be soft annealed seamless type 316 SS as per ASTM A269.

Sizes of Tubing

Sizes shall be as follows:
S. No. Application Size 1 High-pressure hydraulic lines
¼” O.D. X 0.035inch W.T.
3/8″ O.D. X 0.035inch W.T. &
3/8inch O.D. X 0.049inch W.T.
2 ESD/FSD loop 3/8inch O.D. X 0.035″ W.T.
3 Pneumatic supply to shutdown valves 3/8‖ O.D. X 0.035‖ W.T. 316 SS tube.
4 Process impulse lines 1/2‖ O.D X 0.065‖ W.T. 316 SS tube.
For instruments on API 5000# class piping, the impulse lines and fittings shall be as per piping specifications. Monel tubing shall be used on sea water and produced water service and shall be fully annealed seamless as per ASTM B-165 with size ½‖
O.D. X 0.035″ W.T.
Threaded connections shall be NPT for all components and piping and tubing systems for process and utilities connections. TFE threads sealant shall be used on all threaded connections. Tape shall not be used. Tubing shall be supported and protected by stainless steel angle / channel or ladder / tray along the complete length of each run and shall be fastened with stainless steel saddles at a maximum of 1 m intervals on straight runs. Channel or tray support for tubing runs shall be sized for a minimum capacity of 30% greater than that required. All pneumatic exhaust ports and breathers shall be fitted with bug screens, installed facing downwards. Hardness for tubes shall not exceed RB 70 –
79 and hardness for fittings (ferrules) shall be such that, there is a minimum hardness difference of 5 to 10 between tube and fittings for better sealing.

Tubing Testing Requirements:

Testing requirement for tube & tube fittings (Both Hydraulic & Pneumatic):
a) The tests shall be according to ASTM A450.
b) The tests specified below shall be carried out on samples drawn at random.
c) The test fluid for hydraulic testing shall be mineral oil having viscosity grade of ISO
VG46 and ISO VG32.
d) Tests to be carried out are:

Please read also: Method of Pressure leak test of Impulse tubing and Air line

c.1) Proof Pressure Test

Test assemblies (tubes and fittings) selected at random shall be subjected to a pressure of 1.5 times the maximum working pressure applied at the rate of 200 kg/cm2per minute and maintained at the final pressure for five minutes without

c.2) Dismantling and Re-assembly Test

A minimum of three test assemblies that have successfully completed `Proof Pressure Test‘ as above shall be disassembled and assembled ten times after which they must pass the Proof Pressure Test‘ again.

c.3) Hydraulic Impulse Cum Vibration Test

A minimum of three test assemblies that have successfully completed the `Proof Pressure Test‘ shall be mounted on a hydraulic impulse cum vibration test bench. They shall be subjected simultaneously to pressure impulse of 1.5 times the maximum working pressure at 30 to 100 cycle per minute and vibration in two mutually perpendicular planes at 1300 to 2820 cycles per minutes for a minimum of 5 X 10 pressure impulses and 20 X 10 vibration cycles. The only re tightening is after the first 1000 pressure impulses to allow for bending. During the testing the leakage shall not exceed 1cm3/Hr/Assembly.

c.4) Vacuum Test

Proof Pressure Test‘ shall be subjected a negative pressure of 700m Bar and then disconnected from Vacuum Pump. The assembly must maintain the vacuum
pressurefor fifteen minutes.

Tubing Installation:

The primary instrument block valves for all instruments shall be as per piping specifications. All the drain valves shall be in 316 SS/MONEL. All impulse lines shall be run with a slope not less than 1 in 12 except where otherwise specified. Direction of slope is to be downward from the process for liquid service and upward from the process for gas service.
Tubing shall be joined by compression fittings.
Instrument lines shall be run in the vertical plane as far as possible and shall be run with the minimum number of changes of directions consistent with good practice and neat appearance. All pipe and tubing shall be run in horizontal and vertical planes only.


Tubing shall be bent with correct size tubing bender where required to avoid the use of fittings. Tubing cutter shall always be used to cut tubing. The use of short lengths of tubing in long runs shall be avoided, to avoid the use of fittings. All tubing shall be run in such a manner as to give the maximum protection against mechanical damage. Tubing runs shall be grouped together and clamped where possible.
Tubing shall be arranged so that couplings can be tightened without distorting lines. Instrument tubing shall not run on trays intended for cables and shall not share the same transit with cables. Tubing run in permanent enclosures shall not have joints, except at special junction boxes provided for this purpose. Where tubing is run in permanent enclosures, the
Contractor shall ensure that entry and exit of such enclosures is clean and smooth. Pipes or tubes installed but not connected, shall have the ends closed in approved fashion to prevent the entry of foreign material by suitable caps or plugs. All reasonable precautions shall be taken to prevent foreign material entering tubing / pipelines before
and during erection.  No pipe or tube shall be left with mechanical strain on it. Where the length of transmission tubing exceeds 60 Mtrs. (200 Ft.) the Company should be consulted as to the necessary of installing signal booster relays.
Where permanent enclosures are left with space for instrument tubing to be pulled in at some future date, a galvanized pull wire of adequate size shall be left in the tray.
On tubing to pipe connections and in making of screwed pipe joints, Teflon tape shall beapplied to make thread in a manner to insure tape is not over the end of the male fittings.

Piping Supports:

Piping and tubing shall be adequately supported and fixed at distances not exceeding those in the following table:
Single Tubing Max. Distance Between Supports
3/8″ O.D. and less Continuous
½” to ¾” Nom. Size 2 Mtrs. (6 ft.)
¾” to 1″ Nom. Size 3 Mtrs. (9ft.)
Multi tube Bundles 3 Mrrs. (9ft.)
All field-mounted instrument air tubing shall be supported with galvanized steel angles or channels of minimum 1/8inch thickness fabricated to present a neat appearance.
All instrument tubing supports shall be galvanized prior to installation. All mechanical ferrule seater shall be used on tubing for 140 kg/cm2(2000 psi) and over. Pipe bushing shall not be used. Plugs shall be of bar-stock with hex heads. Signal transmission shall be through ¼inch O.D. X 0.035‖ W.T. tubing run singly or in bundles. If extended lengths of multiple tubing are to be run, multiple bundles and junction boxes with weatherproof entries may be used to the best advantage.
Manufacturer or fabricator supplying skid mounted equipment‘s or vessels with instrumentation, which provides, or utilizes pneumatic, offside alarms, shutdowns, or control functions shall tube signals to a central bulkhead, near skid boundary, available for hook up by the Contractor for connection to offside equipment.
Location of the bulkhead shall be noted on vendor drawings. Differential or static pressure sensing lines shall not exceed 6 Mtrs. (20 Feet) for directconnected or locally mounted instruments.

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