Hot Tapping Essay
Hot tap operation is a combination of many variables, welding is only one of them,though it’s critical for any hot tapping operation. Note Hot Tap could lead to serious fatalities if not done properly. Since you mentioned about Shell’s DEP, please note these DEP’s are well respected in the industry. That does not mean what’s written is absolutely unquestionable, but there’re good rationale on what’s written in the DEP’s. DEP 31. 38. 60. 10-Gen, HOT-TAPPING ON PIPELINES, PIPING AND EQUIPMENT is quite detailed.
Some important extracts are very relevant to your queries and would clear the doubts why the practice of hot tapping is not permitted as a general rule. JUSTIFICATION FOR HOT-TAPPING:-Hot-tapping is potentially hazardous and therefore should only be undertaken when continuity of service is essential or shutdown of the system is impractical and when it is shown to be technically feasible and to offer a clear advantage over alternatives. The advantage may be economic and, in some situations, the safety and environmental risks may be less than those associated with more conventional methods.
A decision on whether hot-tapping is to be applied shall be based on careful considerations including at least the following aspects: * safety; * condition of the pipe/equipment under consideration; * configuration of the connection; * code/statutory requirements; * operating conditions; * technical capabilities of the drilling equipment under the operating conditions (pressure, temperature, nature of product); * related welding problems; * economic aspects; * environmental/pollution aspects.
If hot-tapping is to be applied under conditions approaching the technical or operational limits as defined in this DEP, specialist advice should be sought. MATERIALS:-This DEP is relevant only to hot-tapping on carbon, carbon-manganese and high-strength low-alloy steels (e. g. ISO 3183 L245 to L485 or API 5L grades B to X70). Materials for pipelines and on-plot piping shall comply with the requirements of the piping classes DEP 31. 38. 01. 12-Gen. and DEP 31. 38. 01. 15 Gen. If it is proposed to hot-tap ther materials, specialist materials/welding engineers shall be consulted and a specific procedure shall be developed. Current experience is limited to L450 (X65) materials (see also BS 6990) and therefore special attention should be given to procedures to be developed for higher grade materials. It shall be ensured that during the welding operation the material in the region of the weld pool has sufficient strength to contain safely the internal pressure and avoid a blow-out.
The risk of blow-out is dependent upon a complex interaction of welding conditions, pipe material, pipe thickness, pipe temperature and hoop stress. For materials of yield strength not greater than 450 N/mm2, operating at temperatures of not greater than 350 °C and with a hoop stress of not greater than 72 % of the specified minimum yield stress, blow-out can be prevented during welding provided the minimum pipe thickness is not less than 5 mm. The actual minimum wall thickness shall be determined by ultrasonic testing.
For other materials or conditions, a minimum pipe thickness shall be determined, based on either specific previous experience and/or data from trials. NOTE: For some products, due to chemical reaction, the safe pipe temperature might be much lower than350 °C (See also BS 6990). Fitting and branch material shall be of a type and grade compatible with the material of the run-pipe to be hot-tapped, including sour service requirements where applicable. Hot-tap welding should not normally be performed on materials which require post-weld heat treatment.
However, if stress relieving or post-weld heat treatment is required, a specialist shall be consulted to assess whether it is feasible to make the specific hot-tapping operation with post-weld heat treatment. Note: Stress-relieving may be required if a material is susceptible to stress corrosion cracking and post-weld heat treatment may be specified by the design code if the wall thickness is greater than a certain limit. Welding shall not be performed on lined, clad or internally coated pipe.
Hot-tap welding shall not be performed on the following materials: – any material with a maximum allowable operating temperature greater than 370 °C; – pipelines and on-plot piping where the minimum allowable operating temperature is less than -20 °C. For minimum allowable operating temperatures between -20 °C and 0 °C, additional attention shall be given to ensuring adequate material toughness PROCESS FLUIDS:- Hot tapping shall only be performed under the following conditions: – the contained process luids remain stable when subjected to the high metal temperatures (see 3. 3) during welding; – the process fluids remain stable when in contact with air during drilling and perforation operations; – there shall be no decomposition or chemical reaction between components of the contained fluid (explosion, ignition, exothermic reaction); – there shall be no chemical reaction between the fluid and the hot containment wall(burning, (stress) corrosion, embrittlement).
These requirements lead to a number of situations in which welding operations are prohibited on equipment which contains: – Mixtures of gases or vapours within their flammable range or which may become flammable as a result of heat input in welding operations. – Substances which may undergo reaction or decomposition leading to a dangerous increase in pressure, explosion or attack on metal. In this context, attention is drawn to the possibility that under certain combinations of concentration, temperature and pressure, acetylene, ethylene and other unsaturated hydrocarbons may decompose explosively, initiated by a welding hot spot. – How about LPG- Does it fit into this category?? – Oxygen-enriched atmospheres in the presence of hydrocarbons which may be present either in the atmosphere or deposited on the inside surface of the equipment or pipe. – Compressed air in the presence of hydrocarbons which may be present either in the air or deposited on the inside surfaces of the equipment or pipe. – Gaseous mixtures in which the partial pressure of hydrogen exceeds 700 kPa (ga),except where evidence from tests has demonstrated that hot-tapping can be done safely.
Based on the above, welding on equipment or pipe which contains hazardous substances or conditions as listed below (even in small quantities) shall not be performed unless positive evidence has been obtained that welding/hot tapping can be applied safely. Substances: Acetylene;Acetonitrile;Butadiene;Caustic soda;*Chlorine;Compressed air at a pressure in excess of 3 000 kPa (ga);Ethylene;Ethylene oxide;Fat/lean DEA/MEA;HP steam (pressure in excess of 5 000 kPa (ga));Hydrogen (partial pressure in excess of 700 kPa (ga)); Hydrogen sulphide;*Hydrofluoric acid;Oxygen;Propene;Propene oxide; Sulphuric acid;
I would think if the environment contains both LPG (which is flammable) and 2500 ppm of H2S which could be toxic in case of any leakage, a proper way would be to drain the line purge and then perform hot tapping or weld followed by PWHT as required. Not only that wet H2S could cause typical issues related to wet H2S cracking. Hence you need to establish a welding procedure with controlled hardness. The attached article is a good educative document on hot tapping in sour services.