Hot Tap Location and Design Considerations
By Vincent A. Carucci
An earlier article introduced what a hot tap is, when it should not be
done, and when it may be done with special precautions. This article
discusses hot tap location and design considerations.
Selecting the Hot Tap Site
The hot tap site should be chosen considering both mechanical
reliability and personnel safety. It is normally possible to adjust the
exact position of the hot tap as needed to avoid potential problem areas
in the pipe being tapped. Consider the following items:
- Safe personnel access, egress, and working conditions.
- Clearance required for the hot tap nozzle, valve, and machine.
Also remember that the required clearance increases with pipe
- Thermal displacement effects. Remember that the hot tap is being
done with the pipe (or equipment) in service. Therefore, consider the
direction and amount of thermal movement when the system shuts down.
- Drainage of liquid from the lines.
- Hot tap angle. The hot tap should ideally be perpendicular to the
pipe and on the top.
- Distance between the hot tap and any flange, threaded connection,
or welded seam (including the longitudinal seam of welded piping). Hot
tapping too close to such joints could increase the possibility of
- Rotating equipment or control valves located downstream of the hot
tap location. Cuttings from the hot tap operation will go into the
pipe and could damage downstream equipment. The hot tap coupon could
even be lost and proceed into the flow stream.
- Pipe surface cleanliness, soundness, and curvature. Surface
roughness or excessive pipe out-of-roundness could affect nozzle and
hot tap machine installation.
Hot Tap Design Considerations
The hot tap design must meet the applicable Code, local engineering
practices, and local governmental requirements. The following highlights
several design considerations.
- Inspect the area to be hot tapped before starting the design to
confirm the material of construction, wall thickness, and freedom from
- Ensure that there is adequate flow through the pipe during the hot
tap welding and cutting operations. Both minimum and maximum permitted
flow rates must be established, and these depend on whether the fluid
is a gas or liquid. A minimum flow is needed to help dissipate the
welding heat. Maximum flow limits must also be observed in order not
to quench the weld and to not “spin” the coupon (which could cause it
to disengage from the cutter).
- Most non-air hardenable materials that are normally fabricated by
welding can be hot tapped provided the correct conditions of
temperature and pressure exist.
- Nozzle and reinforcement materials should be the same nominal
chemistry and strength as the pipe or equipment being hot tapped.
- PWHT is normally not possible during hot tapping. However,
specific cases may be evaluated to determine if the pressure can be
reduced enough to permit online PWHT, or if special welding procedures
may be used instead of PWHT.
- All new valve and piping components should be in accordance with
the local Piping Material Specifications applicable for the piping
- Lined piping or equipment. If a hot tap is made into lined pipe or
equipment, the immediate vicinity of the hot tap will become
unprotected. This might be tolerable for short periods of operation in
- Hot taps into stainless steel lines or equipment typically require
using special cutters and procedures.
- A maximum metal temperature [typically about 700°F (370°C)] must
be set considering the design temperature of the hot tap machine and
personnel protection requirements.
- The minimum acceptable temperature while performing a hot tap is
governed by the material to be welded, the hot tap equipment, and
welding conditions. Temperatures below the dew point can cause welding
problems due to the formation of moisture or frost on the metal
- The wall thickness of the header pipe or equipment must be
confirmed to be acceptable. This must consider the design conditions,
potential burning through the wall when welding, and pressure test of
the nozzle assembly before cutting (which imposes an external pressure
on the pipe).
- Weld details and procedures must be specified and be qualified for
the specific installation.
- Test pressure. Per API RP-2201, a hydrostatic test should be done
at a test pressure that is at least equal to the operating pressure of
the line or vessel to be tapped. Local code requirements and the
possibility of local buckling of the pipe wall must also be
considered. Calculations may be required to ensure that buckling does
- Hot tap fitting. This may be a welded-on pipe stub, an integrally
reinforced set-on type connection, or a prefabricated hot tap fitting.
A bolted fitting may also be considered in special cases (e.g., when
welding cannot be done with the system in operation).
- Hot tap valve. This must have a full-round opening that is at
least 1/8 in. (3 mm) larger than the specified drill or cutter OD. A
regular, API-600 gate valve may be used. Special hot tap valves
(commonly called “sandwich valves”) are available. Their
face-to-face dimension is smaller than conventional gate valves, and
may be required if the hot tap is done in a confined location.
- Orientation. Hot tap connections should be oriented at a 90° angle
to the pipe, in the vertically up direction, and be from the top of
the pipe. This reduces the probability of valve seat damage due to chips from the cutting operation entering the
valve. Any variations from this position must be carefully evaluated.
- Size-on-size connections. Size-on-size connections should be
avoided. Additional design and procedural items (e.g., full
encirclement reinforcement) must be considered when a size-on-size
connection must be used.
- Piping system stresses. Design the connections to prevent
overstressing the pipe being tapped due to the weight of the hot tap
machine, application of test pressure to the hot tap nozzle, or
thermal displacement of the added piping. Also consider the pipe
flexibility stresses for the case when the system is shut down.
Remember, the new line is initially at ambient temperature when it is
added to a system that is in operation.
- Supports. Use adequate bracing and support to minimize vibration or
impact on the line during cutting.