MS & GI Pipes — ERW Structural and Service Pipes

MS & GI Pipes — Electric Resistance Welded Structural and Service Pipes

Mild steel ERW pipes and hot-dip galvanized iron pipes bridge structural and service applications in a way few other construction products do. The same 50 NB Medium class ERW pipe that supports a canopy rafter today is specified as a compressed air distribution main tomorrow. The same 32 NB GI Heavy class pipe that supplies drinking water in a residential building is the branch line in a commercial fire-fighting system.

Understanding the distinction between pipe categories — and ordering the right class, finish, and size for each duty — is where procurement experience prevents costly site-level errors.

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MS ERW Pipes — Structural and General Engineering

Electric Resistance Welded mild steel pipes manufactured to IS 1239 (Part 1) cover sizes from 15 NB to 150 NB in Light, Medium, and Heavy class. IS 3589 covers sizes above 165 NB outside diameter for large-bore structural and process applications.

Weight Classes Explained

The four weight classes share the same nominal bore but differ in wall thickness. The wall thickness determines the pipe's pressure rating, structural capacity, and suitability for each application:

Class	Wall Thickness (relative)	Typical Use
Light	Thinnest	Drainage, conduit, low-pressure non-critical services
Medium	Standard	Structural hollow sections, compressed air, general plumbing
Heavy	Thick	Fire-fighting mains, gas distribution, moderate-pressure process
Extra Heavy	Thickest	High-pressure process lines, specialist structural members

Over-specifying wastes material cost without adding safety. Under-specifying creates a compliance and safety liability on pressure-rated systems. The selection table in the inquiry section helps match class to duty.

Applications — MS ERW Pipes

Structural hollow sections are the fastest-growing application for ERW pipes in building construction. Circular hollow sections (CHS) in Medium class offer an attractive combination of structural efficiency, clean appearance, and ease of connection for canopy frames, entrance gates, handrail systems, and furniture frameworks.

Agricultural irrigation — 65 NB to 150 NB Medium class ERW pipes are the distribution pipe of choice for drip and sprinkler irrigation systems. The pipe's smooth internal bore minimises friction losses, and the plain-end mill finish is compatible with both threaded fittings and mechanical couplings.

Scaffolding and falsework — 48.3 mm OD (approximately 40 NB) Heavy class ERW pipe is the standard scaffold tube diameter used throughout the Indian construction industry. The 4.0 mm wall thickness in Heavy class gives the working load capacity required by IS 1139.

Cable conduit — 20 NB to 50 NB Light class ERW pipe, cut into conduit lengths and threaded at both ends, protects electrical cables in civil infrastructure projects where flexible conduit is unsuitable for the burial depth or mechanical impact risk.

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GI Pipes — Corrosion-Protected Service Pipes

Hot-dip galvanized pipes start life as ERW mild steel tubes. After forming and welding, the pipe is cleaned by pickling, fluxed, and immersed in a bath of molten zinc at approximately 450 °C. The zinc alloys with the steel surface and solidifies as a continuous metallurgically bonded coating that protects sacrificially — the zinc corrodes preferentially, protecting the parent steel even if the coating is scratched.

The coating weight on IS 1239 galvanized pipes is 460 g/m² minimum for bore sizes up to 40 NB, rising to 610 g/m² for larger bores, providing 30 to 50 years of corrosion protection in typical atmospheric exposure.

Applications — GI Pipes

Potable water supply — GI Medium and Heavy class pipes to IS 1239 remain the preferred internal water distribution pipe in budget-constrained residential and commercial construction. The zinc coating is safe for drinking water contact at the concentrations encountered in normal service, and the pipe's mechanical strength resists damage during concealment in masonry walls.

Fire-fighting sprinkler systems — IS 15105 (the national fire sprinkler standard) specifies GI Heavy class pipe for wet pipe systems in light and ordinary hazard occupancies. The heavy wall thickness provides the pressure rating required for sprinkler system working pressures of 12 to 16 bar, and the galvanized coating resists internal corrosion from the standing water in a wet pipe system.

Gas distribution — IS 1239 GI Heavy class pipe is the most widely specified material for natural gas and LPG distribution in residential buildings. Its threaded joints, mechanical strength, and resistance to galvanic corrosion from soil contact (when buried) make it suitable for meter risers, service connections, and internal distribution.

Borewell casings — GI Medium class pipe in 100 NB to 150 NB is used as casing for shallow borewells in agricultural and municipal projects. The galvanized coating resists the corrosive soils and groundwater encountered in many Indian aquifer zones.

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Technical Specifications

IS 1239 — Dimensions for Common Sizes

Nominal Bore	Outside Dia (mm)	Light Wall (mm)	Medium Wall (mm)	Heavy Wall (mm)
15 NB	21.3	2.0	2.6	3.2
20 NB	26.9	2.3	2.6	3.2
25 NB	33.7	2.6	3.2	4.0
32 NB	42.4	2.6	3.2	4.0
40 NB	48.3	2.6	3.2	4.0
50 NB	60.3	2.9	3.6	4.5
65 NB	76.1	3.2	3.6	4.5
80 NB	88.9	3.2	4.0	4.85
100 NB	114.3	3.6	4.5	5.4
125 NB	139.7	4.0	4.85	5.4
150 NB	165.1	4.0	4.85	5.4

End Conditions Available

• Plain end (PE) — for welded joints, mechanical couplings, and press-fit systems
• Threaded and socketed (T&S) — BSP taper thread to IS 554, with socket one end
• Bevelled end (BE) — 30° bevel for butt-welded pipe joints in process piping

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Selecting the Right Class — Quick Reference

Application	Recommended Class	Standard
Drainage and waste water	Light	IS 1239
Compressed air (< 8 bar)	Medium	IS 1239
Structural hollow sections	Medium	IS 1239
Irrigation distribution	Medium	IS 1239
Potable water internal	Medium / Heavy	IS 1239
Fire-fighting (sprinkler)	Heavy	IS 1239 / IS 15105
Gas distribution	Heavy	IS 1239
High-pressure process	Extra Heavy	IS 1239 / IS 3589
Scaffold tube	Heavy (48.3 mm OD)	IS 1139

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Frequently Asked Questions

What is the difference between NB (nominal bore) and OD (outside diameter)? Nominal bore is a conventional pipe size designation that approximately corresponds to the internal bore. The outside diameter is fixed for a given nominal bore regardless of wall class — only the wall thickness changes. A 50 NB pipe always has a 60.3 mm OD whether it is Light, Medium, or Heavy class.

Can GI pipes be welded? Welding burns off the zinc coating in the heat-affected zone, exposing bare steel and generating toxic zinc oxide fumes. GI pipes are designed for threaded or grooved mechanical joints, not welding. If a welded joint is required, use plain MS ERW pipe and galvanize or paint the joint after welding.

What is the shelf life of GI pipes stored outdoors? Hot-dip galvanized pipes stored off the ground, in the shade, with adequate drainage and airflow have a practical storage life exceeding 24 months without significant coating degradation. White rust (zinc oxide deposits) may form on surfaces that trap moisture — this is superficial and does not compromise corrosion protection.

Do you supply square and rectangular hollow sections in the same sizes? Yes. Square hollow sections (SHS) and rectangular hollow sections (RHS) are available in standard sizes from 20 × 20 mm to 250 × 250 mm (SHS) and 50 × 25 mm to 300 × 200 mm (RHS), wall thicknesses 1.6 mm to 10 mm. These are roll-formed ERW sections, not pipes, and are supplied to IS 4923.