What are Flat Flanges or Slip-On Flanges?

Flat Flanges (Slip-On Flanges) have a bore slightly larger than the pipe OD so the pipe slides through the flange. The flange is then fillet-welded to the pipe on both the inner and outer faces — two fillet welds total. They are the most economical and easiest-to-install flange type, suitable for low-to-medium pressure water supply, HVAC and general process piping service.

What sizes of Flat Slip-On Flanges are available?

I.S. Engineering Co. manufactures Flat Slip-On Flanges in sizes from 15mm NB to 600mm NB in Flat Face (FF) and Raised Face (RF) face types to IS:6392 and ANSI B16.5 standards. Available in PN6, PN10, PN16 and Class 150/300 pressure ratings.

What gasket should be used with Flat Flanges?

Flat Face Slip-On Flanges require a full-face gasket that covers the entire flange face including the bolt holes. Common gasket materials: natural rubber (NR) for water service; neoprene (CR) for oil service; EPDM for chemical service; compressed asbestos fibre (CAF) or graphite for general industrial service. Never use a raised face ring gasket with a flat face flange.

Flat Slip-On Flanges Manufacturer Mumbai – M.S. Flat Flanges IS:6392 ANSI B16.5 Low Pressure

IS:6392 / ANSI B16.5 · Low–Medium Pressure Service

Flat / Slip-On Flanges

Name: Flat / Slip-On Flanges
Brand: I.S. Engineering Co.
Availability: InStock

M.S. Flat & Slip-On Flanges Manufacturer & Supplier – Mumbai, India · Since 1999

IS:6392 / ANSI B16.5Low-Medium PressureEasy InstallationDirect ManufacturerPAN India Supply

M.S. Flat Flanges (also called Slip-On Flanges or SO Flanges) are the most economical and widely used flange type for low-to-medium pressure water supply, HVAC, utility and general process piping. The pipe slides through the flange bore and is fillet-welded on both the inner and outer face — simple to align, install and replace. Manufactured from IS:2062 mild steel at our Darukhana, Mumbai factory to IS:6392 and ANSI B16.5 standards. Available in sizes 15mm NB to 600mm NB with flat face (FF) and raised face (RF) options in PN6, PN10 and PN16 ratings.

Quick Specifications

StandardIS:6392 / ANSI B16.5

Size Range15mm NB – 600mm NB

Pressure RatingPN6 / PN10 / PN16 / Class 150 / 300

Weld TypeFillet Weld (Inside + Outside Pipe)

Face TypesFlat Face (FF) / Raised Face (RF)

Material GradeIS:2062 Mild Steel (MS)

Lead Time3–7 Days (Standard)

SupplyPAN India | Darukhana, Mumbai

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25+Years Mfg.

600mmMax NB Size

Most EconomicalFlange Type

What Are Flat / Slip-On Flanges?

Flat Flanges (Slip-On Flanges / SO Flanges) have a bore slightly larger than the pipe OD so the pipe slides through the flange body. The flange is then fillet-welded to the pipe on both the inner face (at the pipe end) and the outer face (behind the flange hub). Two fillet welds are required for full structural integrity.

Flat/Slip-On Flanges are the most affordable flange type due to their simple geometry and easy manufacture. They are also the easiest to install because the pipe simply slides through and alignment is straightforward. However, their two-fillet-weld design and stress concentration at the flange-pipe junction make them unsuitable for high-pressure, high-temperature or fatigue-prone service.

When to Use Flat/Slip-On FlangesLow-to-medium pressure systems (PN6–PN16 / Class 150–300)
Non-critical water supply, HVAC and utilities piping
Where pipe alignment is difficult and ease of installation is important
When budget is a primary consideration and service is not cyclic or high-pressure
Not recommended for high-pressure gas service, cryogenic, cyclic thermal service or critical process piping

Why Choose I.S. Engineering Co.?

25+ Years Experience

Manufacturing Flat / Slip-On Flanges in Mumbai since 1999. Trusted by 500+ clients across India.

Factory-Direct Pricing

No middlemen. Buy direct from the manufacturer at the most competitive prices in Mumbai.

100% Quality Inspected

Every Flat / Slip-On Flanges undergoes dimensional and material inspection before dispatch from our Mumbai factory.

PAN India Delivery

Supplying to Mumbai, Pune, Delhi, Bangalore, Chennai, Hyderabad, Ahmedabad and all major cities across India.

Flat / Slip-On Flanges – Dimensional Specifications

NB Size	OD (mm)	Bore (mm)	PCD (mm)	No. Bolts	Thickness (mm)
15mm	95	21	65	4	12
25mm	115	34	85	4	14
50mm	165	61	125	4	16
80mm	200	90	160	8	18
100mm	220	114	180	8	18
150mm	285	168	240	8	20
200mm	340	220	295	8	22
250mm	395	273	350	12	24
300mm	445	325	400	12	24

* Dimensions are indicative. Custom sizes available on request. Contact I.S. Engineering Co. at +91 83558 15350 for specific sizes, tolerances and full specification sheets.

Flat / Slip-On Flanges – Industries & Applications

Water Supply Systems

Most common flange type for municipal and building water supply mains. Lower cost and easy installation make flat flanges the standard choice for non-critical water distribution piping.

HVAC & Chilled Water

Used in HVAC chilled water systems, cooling water distribution, air handling unit connections and thermal storage tank piping. PN6–PN10 typically sufficient.

Fire-Fighting Systems

Used in fire-fighting ring mains and sprinkler system mains where IS:1538 E Table or flat face flanges are specified. PN10–PN16 for fire water systems.

Sewage & Drainage

Used in low-pressure sewage mains, drainage pumping stations and wastewater collection systems. Low service pressure makes flat flanges economically attractive.

Irrigation & Agriculture

Used in irrigation distribution networks, canal outlet structures and agricultural water supply systems. Flat flanges are cost-effective for large quantities in irrigation projects.

General Process Piping

Used in utility process piping, cooling water systems and service water distribution in industrial plants where pressure is low and service is non-critical.

Flat / Slip-On Flanges – Frequently Asked Questions

What are Flat / Slip-On Flanges?

Flat Flanges (Slip-On Flanges) have a bore slightly larger than the pipe OD so the pipe slides through the flange. The flange is then fillet-welded to the pipe on both the front face (at pipe end) and the back face (behind the hub) — two fillet welds total. They are the most economical and easiest-to-install flange type, suitable for low-to-medium pressure service.

What is the difference between Flat Flanges and Welding Neck Flanges?

Flat/Slip-On Flanges: Pipe slides through bore; two fillet welds; lower cost; suitable for PN6–PN16 / Class 150–300; not for cyclic, high-temperature or high-pressure service. Welding Neck Flanges: Butt welded; superior stress distribution; suitable for all pressure classes; preferred for critical, high-pressure and fatigue-prone service. Choose flat flanges for economy in non-critical low-pressure service.

What pressure ratings are available for Flat Flanges?

Flat/Slip-On Flanges are available in: PN6, PN10, PN16 (IS:6392 standard) and Class 150, Class 300 (ANSI B16.5). For higher pressure classes (Class 600 and above), Welding Neck Flanges are recommended as slip-on flanges are not reliable at high pressures due to their fillet weld joint.

When should I use Flat Flanges instead of Welding Neck Flanges?

Use Flat Slip-On Flanges when: (1) Service pressure is low to medium — PN6 to PN16 or Class 150 to 300; (2) Service is non-cyclic and non-critical — water supply, HVAC, sewage, irrigation; (3) Budget is a constraint and higher performance of WN flanges is not required; (4) Pipe alignment in the field is difficult. Do not use Flat Flanges for high-pressure gas service, cryogenic service, fatigue-prone or critical process piping.

Are Flat Flanges the same as Slip-On Flanges?

Yes. Flat Flanges and Slip-On Flanges are the same product. The terms are used interchangeably in the Indian industrial market. In IS:6392, they are called Flat Flanges. In ANSI B16.5, they are called Slip-On Flanges (SO). Both describe the same flange design — pipe slides through the bore, fillet-welded front and back.

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