Vent Holes in reinforcement pads (Also named Weep holes or Telltale)
You might have noticed a round hole in the reinforcing pad. This is known as a "Weep Hole" and can serve multiple functions. It is sometimes called a "Tell-Tale Hole" or "Vent Hole."
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Step 1: Vent Holes in reinforcement pads (Also named Weep holes or Telltale)
You might have noticed a round hole in the reinforcing pad. This is known as a "Weep Hole" and can serve multiple functions. It is sometimes also referred to as a "Tell-Tale Hole" or "Vent Hole."
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Step 2: According to ASME.
ASME VIII Div 1, Reinforcing plates and saddles of nozzles attached to the outside of a vessel shall be provided with at least one vent hole [maximum diameter 7/16 in. (11 mm)] that may be tapped with straight or tapered threads. These vent holes may be left open or may be plugged when the vessel is in service. If the holes are plugged, the plugging material used shall not be capable of sustaining pressure between the reinforcing plate and the vessel wall.
ASME B31.3 2022 Reinforcing pads and saddles shall have a good fit with the parts to which they are attached. A vent hole shall be provided at the side (not at the crotch) of any pad or saddle to reveal leakage in the weld between branch and run and to allow venting during welding and heat treatment.
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Step 3: Uses of a weep hole:
· Weep holes serve as vents during welding or heat treatment, allowing trapped gases at high temperatures to escape. Without them, the accumulation of welding gases can increase internal pressure between the shell and the reinforcement plate.
· By threading the hole, a test gauge can be installed for an "air/soap" test to check for leakage. Typically, a 1/4" NPT thread is used for this purpose. (weep hole should be monitored during hydro/leak test.).
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Step 4: Inspection of Reinforcing-Plate Welds
According to API 650, Each manhole reinforcing plate shall be provided with a 6 mm (1/4 in.) diameter telltale hole (for detection of leakage through the
interior welds).
After fabrication is complete, but before filling the tank with test water, the Manufacturer shall test the reinforcing plates by applying up to 100 kPa (15 psi) of pneumatic pressure between the tank shell and the reinforcement plate at each opening using the telltale hole. While under pressure, a soap film, linseed oil, or another suitable leak-detection material shall be applied to all attachment welds around the reinforcement to check for leaks. (both internally and externally)
The purpose of the leak test (maintain pressure for several minutes through a weep or vent hole 1/4") is to ensure the seal is tight and no water or oxygen ingress will occur which will lead to corrosion under the pad.
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Step 5: Closed or opened
To prevent internal corrosion weep hole shall be tapped with bolting or grease after the pneumatic test. Since it is a tapped hole, you can use suitable bolts to seal and prevent corrosion.
Threaded bolts with sealing grease is a more normal practice, seal welding these holes may become problematic in the future if you want to use them for corrosion checks.
If the hole is left open, water, chemicals, vapor, or humid air can accumulate beneath the pad, causing hidden corrosion. (Also see Lethal Service)
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Step 6: Lethal Services
Most codes and standards specify more strict requirements (e.g., in PWHT, test and inspection, toughness, material quality) for safety control of pressure equipment and piping containing lethal service.
What is a Lethal Service: “Lethal substances” meant poisonous gases or liquids of such a nature that a very small amount of the gas or the vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled. (ASME Sec. VIII, Div.1)
In the case of lethal service, the weep hole must be sealed to prevent leaks.
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Step 7: Quantity of holes
Reinforcement pads do not have to be manufactured as single pieces. When required or more practical, they can be fabricated in multiple sections, providing greater flexibility in design and installation.
A vent hole shall be provided at each side of any section of the pad
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Step 8: References
Handbook of Engineering Practice of Materials and Corrosion - Jung-Chul (Thomas) Eun
API 650 Welded Tanks for Oil Storage 2020
ASME VIII Div 1 2023
ASME B31.3 2022
