Welding a Slip On Flange: Installation Steps, Weld Checks and Common Mistakes

Welding a slip on flange looks simple because the flange slides over the pipe before welding. In real projects, however, the quality of the joint depends on pipe preparation, flange position, bolt-hole alignment, weld size, gasket face protection and final bolting. A small fit-up error can create leakage, flange stress or field rework.

This guide explains how to install a flange when the design uses a slip on type. It covers preparation, alignment, flange slip on welding, inspection and common mistakes to avoid in industrial piping work.

What Is a Slip On Flange?

A slip on flange is a pipe flange with a bore large enough to slide over the pipe outside diameter. After positioning, the flange is normally attached with fillet welds. Many projects use an outside fillet weld, and some specifications require both outside and inside fillet welds for added strength and sealing support.

The design is popular for water systems, low to moderate pressure piping, HVAC, utility lines, fire protection, general process service and equipment connections where installation speed and cost control matter.

Before Installation: Check the Parts

Before any slip on flange weld work begins, confirm the flange standard, pressure class, material, bore, pipe schedule, face type and gasket requirement. The pipe outside diameter must match the flange bore with proper clearance. The flange face should be clean and protected from weld spatter.

Also check the welding procedure, filler metal, preheat requirement and inspection level. Carbon steel, stainless steel and alloy steel flanges may require different welding controls.

Step 1: Prepare and Cut the Pipe

Cut the pipe square to the required length. Remove burrs, heavy scale, oil, paint and rust from the weld area. A poor pipe cut can tilt the flange and make bolt-hole alignment difficult. Clean metal at the weld zone helps produce a sound fillet weld.

If the pipe end will sit near the flange face, keep enough setback so the weld does not interfere with gasket seating. The exact spacing should follow the project specification or applicable standard practice.

Step 2: Slide the Flange Onto the Pipe

Slide the flange over the pipe carefully. Do not damage the raised face, flat face or gasket surface. For raised face flanges, protect the sealing face from scratches because even small marks can affect gasket compression.

At this stage, many installers lightly mark the pipe so the flange can be positioned consistently around the joint. This is useful when several slip on flanges must match the same spool drawing.

Step 3: Align the Flange and Bolt Holes

Alignment is one of the most important parts of how to install a flange. The flange face should be square to the pipe centerline, and bolt holes should match the mating flange, valve or equipment nozzle. Use a level, square, flange alignment pins or temporary bolts as needed.

For pipe spools, check both face-to-face length and bolt-hole orientation before tack welding. A flange that is welded in the wrong clock position can cause major field installation delays.

Step 4: Tack Weld and Recheck

Apply small tack welds at balanced positions around the pipe. After tack welding, recheck face alignment, rotation and pipe projection. Heat from tacks can pull the flange slightly, so this inspection step prevents a small error from becoming permanent.

Do not place large tack welds where they will create lumps, undercut or poor tie-in with the final weld.

Step 5: Complete the Slip On Flange Weld

The main slip on flange weld is usually an outside fillet weld around the hub where the pipe exits the flange. The weld should be continuous, smooth and sized according to the approved procedure. Some jobs also require an inside fillet weld between the pipe end and flange bore.

For flange slip on welding, control heat input so the flange face does not warp. Weld in a balanced sequence when possible. Stainless steel slip on flanges may need extra care to avoid excessive oxidation, distortion and contamination from carbon steel tools.

Step 6: Clean and Inspect the Weld

After welding, allow the joint to cool naturally unless the procedure states otherwise. Clean slag, spatter and discoloration from the weld area. Inspect for cracks, undercut, lack of fusion, porosity, overlap and uneven weld size.

Depending on the service, inspection may include visual testing, liquid penetrant testing, magnetic particle testing, radiographic testing, pressure testing or dimensional checks. At minimum, the gasket face and bolt holes should be clean and undamaged before assembly.

Step 7: Install Gasket and Bolts

Place the correct gasket between mating flanges. Insert bolts and tighten them in a cross pattern so gasket load is even. Do not use the bolts to pull badly misaligned flanges together; this can overload the weld and distort the sealing faces.

Final bolt torque should follow the gasket supplier, project standard or engineering instruction. After pressure testing or initial service, some systems require rechecking bolt tightness according to the maintenance plan.

Common Mistakes in Slip On Flange Installation

• Welding the flange before checking bolt-hole orientation.
• Scratching the raised face or flat face during handling.
• Using the wrong flange bore for the pipe outside diameter.
• Leaving weld spatter on the gasket surface.
• Skipping inside welding when the specification requires it.
• Forcing misaligned flanges together with bolts.
• Using poor tack welds that pull the flange out of square.

When Is a Slip On Flange a Good Choice?

Slip on flanges are practical for many low and moderate pressure services because they are easy to fit and often cost less than weld neck flanges. They are common in water treatment, air lines, cooling water, fire protection, HVAC, general utility piping and some chemical systems.

For high pressure, high temperature, severe cyclic loading or critical service, engineers may prefer weld neck flanges because their tapered hub provides better stress transition. The final choice should follow the design code and project conditions.

FAQ About Welding a Slip On Flange

What is the correct slip on flange weld?

A typical slip on flange weld is a continuous outside fillet weld around the pipe-to-flange connection. Some specifications also require an inside fillet weld. Always follow the approved welding procedure.

How do you install a flange on a pipe?

Prepare the pipe, slide the flange into position, align the face and bolt holes, tack weld, recheck alignment, complete the weld, inspect the joint, then install the gasket and bolts.

Can a slip on flange be welded inside and outside?

Yes. Many slip on flange assemblies use an outside fillet weld, and some use both inside and outside welds when the specification or service requires additional strength.

Need Slip On Flanges for Industrial Piping?

Songhai Flange supplies slip on flanges, weld neck flanges, blind flanges, lap joint flanges and custom flanges for industrial piping, chemical equipment, marine systems, water treatment and OEM projects. If you need support with flange selection, drawings, material certificates or export packing, our team can help.