Hot Tap Guidelines
By Vincent A. Carucci
Hot tapping is the technique used to attach a
branch connection to a pipe while the system is in service, and then
creating an opening in that pipe by drilling or cutting. While hot taps
are most often done on pipe, they may also be done on pressure vessels and
storage tanks. The figure illustrates a typical hot tap installation.
As an example, Carmagen engineers developed the design details and
procedures that were used to hot tap two nozzles into a crude tower. In
this case, the nozzles were needed to install bypass lines around plugged
distributor trays, and were located about 150 ft. above grade.
Hot taps permit adding connections without depressurization or disruption
of process operations. They may also be used to make piping connections
where it would be inconvenient to prepare the system for hot work. Hot
tapping is also used to isolate pipe sections for maintenance by plugging
or stoppling the line.
Hot taps are successfully completed every day. However, remember that a
hot tap is an inherently dangerous operation. Therefore, hot taps should
generally be used only when it is impractical to take the system out of
service. When specifying hot taps, care is required in inspection, design,
and testing to ensure that this operation is done in a safe and reliable
manner. Therefore, a hot tap should be considered only after other options
are evaluated and rejected. Each hot tap should be properly designed, the
hot tap location thoroughly inspected, and the installation procedures
Sometimes A Hot Tap Should Not Be Done
Hot taps may be made in most cases without difficulty. However, hot
tapping is not recommended in some cases and requires extra precautions in
others. Each case must be evaluated based on the particular service, pipe
material, and hot tap location.
Hot tapping is not recommended under any of the following conditions:
- Piping containing a combustible or flammable mixture.
- Piping containing acids, chlorides, peroxides, or other chemicals that
are likely to decompose or become hazardous from the heat of welding
(e.g., sulfuric acid or acetylene).
- Piping containing caustic.
- Piping that contains pure oxygen or chlorine.
- Air lines where the absence of hydrocarbon cannot be assured.
- Monel piping handling sulfur compounds.
- Stainless steel piping containing catacarb solution.
You Can Hot Tap, But ...
Services or conditions where hot tapping may be permitted, but where
special precautions are necessary, are listed below.
- Systems with no flow, unless the fluid is not dangerous (e.g., cooling
water) or the pipe is open to another system or equipment item.
- Hydrogen service. It should first be confirmed that the pipe has not
experienced hydrogen attack (e.g., by confirming that it has not operated
above the Nelson Curve limits).
- Flammable or combustible liquid below atmospheric pressure. The concern
here is that a flammable or combustible mixture may form if air enters the
system during the hot tap.
- H2S or other toxic materials. Additional safety precautions may be
- Air lines that are free of hydrocarbon, but with greater than 23.5%
- Butadiene, ethylene, or wet H2S services.
- Piping operating at a process temperature above 750°F (400°C), due to
potential concerns with creep cracking at attachment welds.
- Equipment that meets a Level 3 brittle fracture assessment per API RP
579 (i.e., grandfathered) requires special welding precautions to prevent
- Stainless steel systems subject to chloride stress corrosion cracking
should be inspected first to confirm that cracking is not already present.
- Hot taps that will be made at low temperatures [e.g., carbon steel below
40°F (4°C)]. Special welding procedures will be required to prevent
- Services that may produce carburization, nitriding, or other forms of
embrittlement of the material to be welded. Special welding procedures may
be needed to avoid embrittlement. In addition, confirm that there is
adequate remaining thickness of unembrittled material.
- Services in which aqueous or room temperature hydrogen fissuring may
occur. Perform NDE to confirm that the material is sound.
- Piping requiring PWHT. However, it may still be possible to hot tap,
depending on the material involved and the reason for the PWHT.
- For piping with internal linings, cladding or weld overlay. The internal
material will be damaged by the hot tap, and the potential consequences of
this must be evaluated.
- Concrete or refractory lined pipe. The consequences of potential lining
damage or detachment must be evaluated.
- Underground pipe.
- Large diameter lines where it might be necessary to reinforce the coupon
to prevent it from becoming flat and causing the cutter to bind.
This article provided an introduction to what a hot tap is, when it should
not be done, and when it may be done with special precautions. Other
articles will discuss hot tap location, design, installation, inspection,
and safety considerations.
Typical Hot Tap Installation