A contractor recently asked us a question that comes up more often than you might expect: "Can we skip the inside weld on the slip-on flange to save time? The outside weld looks plenty strong."
It's an understandable question - especially under schedule pressure. But the answer is an unambiguous no, and the reason matters far more than a simple code citation. This article explains the engineering logic behind the two-weld requirement, what actually happens when only one weld is used, and how to get it right the first time on-site.
ANSI Slip On Flange Welding
What the Code Actually Says - and Why
ASME B16.5 is the governing standard for pipe flanges in the NPS ½" to 24" range. It defines not just the flange geometry, but also how the flange must be attached to the pipe. For any ANSI Slip On Flange, the standard is explicit: two fillet welds shall be furnished - one on the outside (at the hub end) and one on the inside (at the flange face end).
This is not bureaucratic caution. Each weld has a distinct mechanical function, and neither can substitute for the other.
| Weld Position | Primary Function |
|---|---|
| External fillet weld (hub end) | Bears axial load, bending moment, and pipe pull-out force |
| Internal fillet weld (bore end) | Seals the annular gap between pipe OD and flange bore; prevents crevice corrosion and fluid ingress |
Remove the internal weld, and you leave an unsealed annular crevice - a direct path for process fluid to migrate between the pipe and the flange body. Under cyclic pressure or thermal expansion, that crevice acts as a stress amplifier, progressively working against the single remaining external weld. Remove the external weld instead, and the flange is essentially hanging by a seal weld with near-zero bending resistance. Either scenario ends the same way: premature fatigue cracking.
ASME B31.3 Process Piping Code (Section 308.2.1) makes double welding mandatory in specific service categories - severe erosion, crevice corrosion risk, and cyclic loading environments. These aren't edge cases; they describe a large share of real industrial piping.
CNCJ Ansi Slip On Flange
The Physics of a Single-Weld Failure
Understanding why single-weld installations fail requires a basic grasp of fillet weld fatigue - the tendency of angular welds to crack from the root or toe under repeated loading.
A full-penetration butt weld, like the one used on a Weld Neck Flange ASME B16.5, fuses pipe and flange into a continuous, smooth metal cross-section. Stress flows through it evenly. A fillet weld, by contrast, creates an abrupt geometric change - a notch effect - at both the root and the toe. Under fatigue loading (pressure cycles, vibration, thermal expansion), these stress concentration points accumulate damage.
When only the external fillet weld is present, internal pressure in the pipe exerts a prying moment on the flange. Think of the unsupported bore edge as a lever: internal pressure tries to "open" the gap between the pipe and the flange face, loading the external weld eccentrically rather than axially. This prying action significantly amplifies the actual stress on the weld root - by as much as a factor of four compared to a properly double-welded installation, per fatigue assessment data cited in engineering practice under ASME B31 frameworks.
The result: what appears to be an adequate external weld can crack from the root in a fraction of its expected service life.
This is one of the core reasons that slip-on flange connections, even when correctly double-welded, are rated at approximately two-thirds the pressure strength of an equivalent weld neck flange, and roughly one-third the fatigue life. These are not theoretical penalties - they are engineered-in limits that must be respected in service selection.
The Hidden Risk: The Interstitial Cavity
There is a subtler problem that even experienced piping engineers sometimes overlook.
When a slip-on flange is double-welded, the two fillet welds create an enclosed annular cavity between the pipe OD and the flange bore. Under most conditions this is benign. But in certain service environments - particularly fluid systems where the medium can diffuse through metal or permeate micro-discontinuities - this trapped space becomes a liability.
ASME B31.3 Appendix F (a precautionary, non-mandatory appendix) specifically notes that for fluid services including vacuum service, or where the process fluid can diffuse into enclosed spaces, the interstitial cavity between the two welds should be considered for venting. In practice, this means specifying a small vent hole in the flange hub in applicable services, so that the cavity does not become a pressure trap during hydrostatic testing or service startup.
This is rarely discussed in generic slip-on flange guides - but for engineers specifying flanges in hydrogen service, cryogenic applications, or systems requiring full leak testing of the bore weld, it is a real engineering consideration.
Weld Size Requirements: Not Left to the Welder's Judgment
Beyond weld count, the size of each fillet weld is also code-defined. This is another area where field practice sometimes diverges from standard requirements.
Per ASME B31.1 (Power Piping) and B31.3 (Process Piping), Figure 328.5.2B:
- Internal fillet weld leg size: shall be the lesser of the pipe wall thickness (T) or 6 mm
- External fillet weld leg size: shall be the lesser of 1.4 × T or the hub thickness
A common field error is oversizing the external weld - either for appearance or because the welder is working by feel rather than procedure. Oversized fillet welds do not improve joint strength; they increase heat input, can introduce distortion, and may generate residual stresses that actually reduce fatigue performance. The minimum specified size is also the target size.
The 2009 update to ASME VIII-1 (Section UW-21) revised the minimum external fillet weld size upward for standard B16.5 slip-on flanges used as pressure vessel nozzles - a change that required CRN (Canadian Registration Number) updates in some provinces. This regulatory evolution reflects ongoing code refinement based on field performance data, not conservative over-engineering.
Slip-On vs. Socket Weld: Why the Weld Count Differs
A frequent point of confusion: if socket weld flanges are also a fillet-welded connection, why do they only require one external weld?
The answer is geometry. A socket weld flange has a machined counterbore (socket) into which the pipe is inserted. The pipe end bears against a stop inside the socket, and a single external fillet weld is made at the hub. The pipe-to-socket fit is tight enough that there is no meaningful annular gap to seal - the socket wall itself provides the mechanical engagement and partial sealing function that the internal weld provides in a slip-on configuration.
Socket weld flanges are therefore used for small-bore high-pressure applications (typically NPS ≤ 2" to 4"), where bore access for an internal weld would be physically impractical. The trade-off is a potential crevice corrosion zone at the pipe end inside the socket - which is why socket weld connections are generally avoided in corrosive or crevice-sensitive services.
Understanding this distinction prevents misapplication: an ANSI Slip On Flange is not a socket weld flange with the stop removed. The internal weld is a functional requirement, not an optional addition.
Summary: What "Correct Installation" Actually Means
For piping engineers, QC inspectors, and contractors working with slip-on flanges, correct installation comes down to four verifiable requirements:
- Two welds, always. External fillet at the hub, internal fillet at the bore end. No exceptions for schedule, access difficulty, or perceived redundancy.
- Correct weld size. Follow ASME B31.1/B31.3 Figure 328.5.2B minimum leg dimensions - do not oversize.
- Check the cavity in sensitive services. If the process fluid can diffuse or if vacuum testing is required, consult B31.3 Appendix F on interstitial cavity venting.
- Match the flange type to the service. For Class 600 and above, cyclic service, or high-temperature applications, evaluate whether a weld neck flange is the correct specification rather than a slip-on.
These are not paperwork requirements - they are the engineering conditions under which the published pressure and fatigue ratings of the connection are valid. Deviate from them, and you are operating outside the basis of the design.
FAQ
Q: Is it ever code-compliant to use only one fillet weld on a slip-on flange?
In standard ASME B16.5 installations under B31.3 and B31.1, no - two fillet welds are required. Some non-code or light-duty applications (general structural service, low-pressure utilities outside pressure piping codes) may permit a single weld, but this should be explicitly documented and approved, not assumed.
Q: How do you inspect the internal fillet weld once the pipe is installed?
Visual inspection during welding is the primary method - the bore is accessible before the external weld is complete. Post-installation, dye penetrant testing (PT/DPT) can be used on accessible weld surfaces. Radiographic testing (RT/X-ray) is not practical for fillet welds; this is one of the inherent inspection limitations of slip-on flanges versus weld neck flanges, where the butt weld can be fully radiographed.
Q: What's the maximum pressure class for ANSI slip-on flanges?
ASME B16.5 lists slip-on flanges in Classes 150 through 1500 for sizes NPS ½" to 24" (Class 1500 only up to 2½"). However, in practice most industry specifications limit slip-on flanges to Class 300 or Class 600, reserving higher classes for weld neck or socket weld designs due to fatigue and inspection limitations.
Q: Can a slip-on flange be used in steam service?
For low-pressure steam (Class 150/300, non-cyclic), slip-on flanges are used, but they require careful evaluation. For high-pressure steam, elevated-temperature steam, or any steam line subject to thermal cycling, most piping codes and many plant specifications mandate weld neck flanges. The fatigue life penalty of the double fillet weld configuration - approximately one-third that of a butt-welded connection - becomes critical in cyclic thermal environments.
Q: How does Chengjiu (CNCJ) ensure slip-on flanges are manufactured to the correct bore and hub dimensions for proper double-weld fit-up?
At CNCJ's facility in Quzhou, Zhejiang, all ANSI B16.5 slip-on flanges are machined on dedicated CNC lathes with bore tolerances verified against B16.5 dimensional tables before production runs. The bore-to-pipe OD clearance is controlled to ensure proper fit-up for both fillet welds without excessive gap. Each production batch is dimensionally inspected flat-laid with 100% piece coverage, and material is PMI-verified against the specified grade (A105, SS304, SS316L, etc.). MTCs to EN 10204 3.1 are supplied as standard with every order.
CNCJ (Zhejiang Chengjiu Pipeline Co., Ltd.) has manufactured stainless steel and carbon steel flanges to ANSI B16.5, EN 1092-1, GOST, JIS, and other international standards since 2012. Our product range covers NPS ½" to 80", pressure classes 150 lb to 2500 lb, with ISO 9001:2015 certification and third-party inspection by BV, SGS, TÜV, and Lloyd's available on request.