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.

  1. 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 line(s).
  2. Hot taps into piping or equipment in any of the following services may be prohibited or additional requirements may apply:
    Acetylene Air Amines (MEA/DEA)
    Butadiene Ethylene Reactive Sediment
    Highly Toxic Wet H2S/CN Highly Corrosive
    Acid “No-flow” lines Chlorides
    Caustic Oxygen enriched Peroxides
    Hydrogen Combustible Mixtures H2S
    Sulfur Flammable Mixtures Ammonia
    Vacuum Flare lines LPG
  3. 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.
  4. 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.
  5. 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?
  6. 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.
  7. 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 valves.
    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.
  8. 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 burn-through.
    c. Between 0.25” and 0.5”, some or all these requirements apply.
  9. 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.
  10. 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 zones.
  11. 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 Allowance.
    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 UT readings.
    d. Pipe to face of flange dimension, valve length, gasket and bolting requirements.
    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.
  12. 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 required.
    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.
  13. 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.
  14. Finally, the hot tap drawing should be reviewed and signed by all parties and then filed.