A. What is Slip on flange?

B31.3, Section 308.2.1:
(a) A slip-on flange shall be double-welded as shown below when the service is:
(1) subject to severe erosion, crevice corrosion, or cyclic loading
(2) flammable, toxic, or damaging to human tissue
(3) under severe cyclic conditions
(4) at temperatures below −101°C (−150°F)

(b) The use of slip-on flanges should be avoided where many large temperature cycles are expected, particularly if the flanges are not insulated.

B. Note for Slip-on flange (ASME B31.3):

1. This flange will not resist shock and vibration as well as a weld-neck flange
2. Need for venting space between welds
3. Should be avoided where many large temperature cycles are expected, particularly if the flanges are not insulated.
4. If slip-on flanges are single welded, the weld shall be at the hub

The note No. 2 is very important!

ASME B31.3 2004 Appendix F – Precautionary Considerations

F308 FLANGES AND GASKETS
F308.2 Specific Flanges
Slip-On Flanges: The need for venting the space between the welds in double-welded slip-on flanges should be considered for fluid services (including vacuum) that require leak testing of the inner fillet weld, or when fluid handled can diffuse into the enclosed space, resulting in possible failure.

Appendix F is precautionary, not mandatory.

The drilled vent hole is also helpful to ensure that you don’t blow out a bubble in the closing (back) fillet due to expansion of trapped gas in the space between the two fillets.

The drill hole same with “tell-tale hole” on double plate!

Document for refer:

ASME B31.3