Limitations of Hot tap – What Is Piping

What is Hot Tapping? Hot Tap or Hot Tapping is a proprietary method for adding a branch connection to a “hot” or operating line. Hot Tapping process allows a connection into a pressurized in-service operating system, by drilling or cutting. The hot tap procedure involves attaching a branch connection and valve on the outside of an operating pipeline, and then cutting out the pipe-line wall within the branch and removing the wall section through the valve. In general, hot taps are done in piping system. But, it can also be done on pressure vessels and storage tanks to add nozzles. Hot tapping is a highly dangerous operation. Therefore, hot tapping must only be used as the last resort when it is impractical to shut down the system. While hot tapping specification, care need to be exercised in inspection, design, and testing. It must be ensured that this operation is done in a safe and reliable manner. Each hot tapping need to be properly designed, the location of hot tapping thoroughly inspected, and the installation procedures reviewed before starting the operation. A proprietary device, to be used in conjunction with a hot tap that blocks a “hot” line is called “Stopple”. Refer Fig. 1. A typical hot tapping system consists of Tapping fittings.Isolation valve.Hot tapping machine which includes the cutter, and housing. Drilling machine or Cutter: The drilling machine usually consists of a mechanically driven telescoping boring bar that controls a cutting tool. A pilot hole into the pipeline wall is bored by the cutting tool in order to center a hole saw that cuts out the “coupon,” or curved section of pipeline wall. Fitting: Connection to the existing pipe is made within a fitting, which can be a simple welded nipple for small (e.g., one inch) connection to a larger pipeline, or a full-encirclement split-sleeve tee for extra support when the branch is the same size as the parent pipeline. The tee wraps completely around the pipeline, and when welded, provides mechanical reinforcement of the branch and carrier pipe. Valve: The valve on a hot tap connection can be either a block valve or a control valve for the new connection, and must allow the coupon (section of pipeline wall cut out by the drilling machine) to be removed after the cutting operation. Suitable valves include a ball or gate valve, but not a plug or butterfly valve. Limitations of Hot Tapping Hot tap connections shall not be made in any of the following cases: Limited by Fluid Types: Lines carrying the following shall not be hot tapped. AcidCausticElemental sulfurOxygenChlorineAmmoniaPotentially toxic materialsAny material which may cause metallurgical damage when heated. Limited by Flammability of Fluid: Hot taps are not suitable for pipe or vessels containing materials that are flammable below atmospheric pressure or mixtures that will support combustion. Fig. 1: Hot Tapping at site Limited by Material Degradation: Hot taps are not suitable if the pipe material to be welded may suffer metallurgical or physical degradation from the heat of welding. Limited by PWHT Requirement: Piping requiring post-weld heat treatment, for any reason, shall not be hot tapped. Limited by Pipe Cladding or Pipe Lining: Hot tapping shall not be performed in piping, or vessels that are clad or lined Limitation due to Minimum Temperature: Lines with surface temperatures at or below 0 deg. C is not suitable for hot tapping. Some other limitations are Equipment Availability: There must be a hot tapping machine available for the temperature pressure and size of the shell.Tanks: Hot taps on tanks shall be located at least 1 m below the liquid level.Compressed Air: Compressed air, when there is any possibility of hydrocarbon contamination, shall not be hot tapped, without thorough internal cleaning, or the introduction of inert gas, prior to welding.Debris Damage: Hot taps shall not be made upstream of any equipment that may be damaged by chips, shavings, and other debris introduced into the line by the hot tapping process. Hot tapping shall not be allowed until systems have been put in place that will positively trap foreign matter.Minimum Wall: The minimum wall for hot tapping is 5 mm. Hot Tap Design Dimensions of the Connection: The dimensions of the connection, the hot tap valve, and the clearances shall be within the limits specified for the hot tapping equipment to be used. This requires data from the most likely contractor or specification of a list of allowable hot tapping machines for that project. The minimum bore of the valve used shall be large enough to pass the cutter or stopple plug. Size to Size Taps: A tap equal to the nominal size of the header, such as required for the stoppling of a pipeline, shall be made only when the accurate positioning of the cutter can be guaranteed. This is accomplished by the use of a shop-fabricated split tee. In all other cases, the taps are only allowed at least one pipe size smaller than the pipe to be tapped. Welding on Pipelines under Pressure: Before welding on lines under pressure, the existing pipe wall shall be determined. A maximum working pressure shall be calculated in accordance with the appropriate ASME B31 code, using the actual wall minus 2.5 mm. The 2.5 mm wall thickness takes into account the molten and heat-affected portion of the base metal which does not contribute to pressure containment. Control of Cuttings: When using gate valves for hot tapping, orient the valve so that chips from the cutting procedure will not fall into areas where the action of the valve is inhibited. If this is not possible, the hot tap valve shall be provided with a valved drain of not less than 3/4 inch NPS to permit flushing of the valve if needed. Removal of Hot Tap Valve: When the hot tap valve is to be removed after tapping, as in the case of stoppling, a Lock-O-Ring flange and plug, or approved equal, shall be used unless the equipment can be depressurized and drained prior to removal of the valve. Guide Bars: When hot taps are made in pipelines which require scraper bars, a Lock-O-Ring, or approved equal, flange and plug and matching insert with bars attached, shall be installed Hot Tap Fitting Installation Positioning the Split Tee and Cutter The welded branch or split tee shall be accurately positioned so that the axis of the cutter will intersect the axis of the pipe or vessel being tapped at a 90-degree angle unless an angled tap has been specifically approved in the hot tapping procedure. The branch position shall be verified by the assigned inspector prior to making the cut. Safety Procedures The hot tapping contractor shall submit safety procedures with his proposal. The installation, pressure testing, welding, and cutting, shall be in accordance with the approved procedures. Pressure Testing Test Valve: The hot tap valve shall have a pressure test applied to the seats and body to ensure no leaks prior to use. Test Fluid: Special attention shall be given to the possibility of boiling or flashing of the test liquid if the surface to be tapped is hot. Hot Tap Test: When the full assembly is complete, and prior to cutting the coupon, the assembly shall be pressure tested. Test Pressure: The test pressure for the hot tap connection shall be in accordance with the appropriate ASME B31 code, however, not exceeding any of the following: (i) The minimum pressure in the pipe or vessel being tapped, while the test is in progress, plus a calculated differential pressure. The differential pressure shall be 1.25 times the allowable external pressure calculated per the ASME SEC VIII D1, paragraph UG-28. The value of L, for this calculation, shall be the total length of the split tee or the inside diameter of the welded nozzle. (ii) The maximum test pressure of branch connection flange, or the wall thickness. (iii) The test pressure of the hot tap connection may be lower than the original hydrostatic test pressure of the pipe or vessel being tapped. This is acceptable since the purpose of the test is to provide some assurance of the integrity of the connection weld. It is not a proof test of the connection. The system being tapped should not be derated because of the lower test pressure of the hot tap connection. Pad Vent Hole The reinforcing pad of a welded branch shall be provided with a vent hole. The hole shall be plugged with heavy grease before leaving the job. Inspection In addition to the hydro test, the reinforcing pad welds may be inspected by magnetic particle examination. Alternatively, dye penetrant examination may be used provided surface preparation is made as required. Few more useful resources for you… Piping Design and Layout Basics Piping Stress Analysis Basics Piping Material Basics Piping Design Software Piping Interface Articles Hot Tapping Operation Video Refer the following video to visualize the hot tapping operation. https://www.youtube.com/watch?v=gaGIDmrUgAU Hot Tap Video by Pro-Pipe Detailed Course on "Hot Tapping in Pipeline and Piping Industry" If you still have doubts or you want to get more insights on the topic, you can join the detailed online course on "Hot Tapping in Pipeline and Piping Industry" by clicking here