Piping Hot Taps
By Doug Stelling
A hot tap is a branch connection installed on a pipe while the system is
in service. Hot taps can also be used to add nozzles to pressure vessels
and storage tanks. The hot tap machine is a hole cutter that operates
through a packing gland and a valve so that the cutting operation can be
done while the pipe is pressurized with a fluid. After the hot tap machine
is unbolted, piping can be connected to the new hot tapped nozzle. Figure
1 shows a typical hot tap set-up. This article covers some basics in the
design and installation of hot tapped nozzles in piping systems. Later
articles will discuss hot tapped nozzles for pressure vessels and tankage.
All hot taps should be designed, fabricated, and inspected and tested in
accordance with the applicable Codes and Standards. The applicable
standard for piping is API-570 “Inspection, Repair, Alteration, and
Re-rating of In-Service Piping Systems.” The design of the hot tap nozzle
should also be in accordance with the original Code of Construction for
the piping system, such as ASME B31.3. In addition, API RP 2201
“Procedures for Welding or Hot Tapping on Equipment in Service” is an API
Recommended Practice which should also be followed.
- The first step after receiving a request to design a hot tap is to
review and verify the information provided. The design pressures,
design temperatures, pipe material, pipe thickness, corrosion
allowance and any restrictions associated with the service should be
checked. This usually entails checking the piping drawings, piping
line list, and piping materials specifications for the existing
- Hot taps into piping or equipment in any of the following services
may be prohibited or additional requirements may apply:
- If the piping is Post Weld Heat Treated, refractory lined, weld
over-laid, clad, or impact tested, hot taps may not be permitted or
additional restrictions may apply.
- Will there be problems if the coupon is lost or if metal shavings
travel down the pipe? This is a concern if the hot tap is located
upstream of a pump, compressor, control valve, or instrument.
- The following process flow requirements are normally considered:
a. Will there be sufficient fluid flow to ensure that burn-through
does not occur?
b. Will the flow be too high such that quenching of the branch weld
may be a concern, or could the coupon be spun and lost?
- Are there limitations on the hot tap pressure/temperature?
a. Check the flange pressure-temperature ratings of the existing pipe
and the new nozzle.
b. Check the pressure-temperature rating of the hot tap machine.
- Are there size limitations for hot tap branch connection?
a. Is the hot tap too small? In some cases, small hot taps ≤ NPS 1 can
be a problem since the hot tap machine can get “hung-up” in very small
b. Is the hot tap connection too large? In many cases, a reduced size
hot tap may be satisfactory from a pressure drop standpoint although a
larger full line size hot tap was requested.
- Can burn-through occur during welding the hot tap nozzle?
a. RP 2201 indicates that burn-through is usually not a concern when
the actual pipe wall thickness is over 0.5”.
b. Below 0.25” thickness, RP 2201 recommends special welding
electrodes [e.g., small diameter (3/32”), low hydrogen (E7018), etc.]
and welding procedures (stringer beads) to minimize the possibility of
c. Between 0.25” and 0.5”, some or all these requirements apply.
- Next, design the hot tap branch connection per the appropriate
design Code. Even if a reinforcing pad is not required for internal
pressure, reinforcing the hot tap is advisable and may be required for
piping loads (i.e., weight, thermal expansion, wind/earthquake loads).
In addition, loads on the nozzle due to weight of the hot tap machine
should be considered.
a. For small connections (≤ NPS2) integrally reinforced branch welding
fittings are typically used.
b. If the hot tap is over NPS 2, a pipe type nozzle with a reinforcing
pad is usually used.
c. If the branch is over NPS 2 and less than NPS 8, integrally
reinforced branch welding fittings can also be used.
d. For larger hot taps, sleeve type reinforcing or special split-tee
type fittings may be used.
- After the hot tap nozzle is designed, UT measurements should be
made in all areas of the pipe where welds will be made. This is to
determine if there is adequate pipe wall thickness, and that no
laminations or step wise cracking are present. A detailed sketch
should be given to the inspector to clearly define the hot tap weld
- The UT information is then reviewed and the design finalized. A
detailed hot tap fabrication drawing should be prepared and include:
a. Design Pressure, Design Temperature, Fluid Service, Corrosion
b. Any restrictions to limit the MAOP during welding, pressure test or
hot tapping. Such restrictions should also be discussed with
maintenance, process, and operations personnel.
c. Minimum thickness required for welding on the line, and the actual
d. Pipe to face of flange dimension, valve length, gasket and bolting
e. Maximum cutter diameter for the branch or valves used. Check the
hot tap machine manufacturer’s catalog for standard diameters. The
maximum cuter diameter should be at least 1/8” less than the minimum
bore of any component. This may require using a reduced size cutter.
- Indicate inspection requirements for the hot tap welds.
a. For non-hazardous (ASME B31.3, Category D) services, visual
examination of root and final passes is the minimum inspection
b. For general process plant services, MT (or PT for non-magnetic
materials) inspection of root and final passes is normally specified.
c. For large branches (≤ NPS 8), the root of the weld inside of the
branch should be visually inspected. Back gouging and a seal pass from
the inside can be used for repair in non “Wet H2S” services.
d. If the hot tap will not be leak tested in accordance with the Code,
the weld neck flange to branch weld should be 100% RT examined along
with MT/PT of the root and final weld passes.
- Indicate the test pressure for the hot tap.
a. It is desirable to test the branch attachment weld before cutting
the hot tap in accordance with the original Code of construction. But
there may be concern that too high a pressure applied externally to a
run pipe could cause buckling. API RP 2201 recommends that the branch
be tested at a pressure at least equal to the operating pressure of
the line or vessel being tapped, but not more than 10% above the
operating pressure present.
b. A 15 psig leak test of the reinforcing pad is typically performed.
- Finally, the hot tap drawing should be reviewed and signed by all
parties and then filed.