Lessons Learned During Use of Hot Taps For In-Service Natural Gas Pipeline Connections
Table of Contents
The process of transmission and distribution of natural gas involves an extensive pipeline network. Establishing this network requires making new connections and repairing the old ones. To perform these operations, the usual practice is to shut down the portion of the system and exhale the gas into the atmosphere to work safely. This conventional operation has a lot of drawbacks including loss of product, supply interruptions, and high cost of operation. A new method of hot tapping has resolved this issue in which a new connection or repair is made while the pipeline remains in operation. Hot tapping is carried out by making a branch connection and a valve attached to the outside of the pipeline. Then the pipeline is cut within the branch section. With this method, the loss of product, gas emission, and supply disruption is prevented. It has also reduced several other complications related to the procedure. It must be mentioned here that hot tapping is not a new technique but it has been in practice for several years. However, some design changes have helped in reducing the complications associated with this process (McElligott et al. 1998).
2 Hot Tapping – An Introduction
In common practice, the procedure of shutdown is slightly changed that depends on the pressure of the system. At the interconnection sites, where pressure is high, the interconnecting valves are closed to separate the pipeline segment on which the operation is being carried out. Stopples are used next to the valves to protect against gas leakages and to ensure safe working conditions. When the pressure is low, the length of the pipes is kept shorter for the shutdown. In this condition, the system is not shut down but stoppels are used to isolate the working system. however, in both conditions, purging is required to vent the pipeline for the safety of the workers. The methane emission from this activity results in economic and environmental impacts (Cola et al. 1992).
To prevent these issues, the first lesson learned is the use of a hot tap. This is an alternate method that makes a connection without shutting down the pipeline system and no venting of gas in the open air. Hot tapping, line tapping, pressure tapping and pressure cutting are different names for the same techniques. In the hot tapping process, branch connections are made by cutting holes in the pipeline. The flow of natural gas from the pipeline is not interrupted while this operation is performed. Hot tapping allows new ties in the system and the use of any device in the pipeline with no impact on the gas flow. Hot tapping is also easy because its equipment is available for any pipe size and material. The basic tools required for hot tapping are drilling machines, branch fitting and valves for connections. (Sabapathy, Wahab, and Painter 2001)
3 Justification for Using Hot Taps from the Lessons Learned
It is a highly safety intensive process and it should not be performed as a routine task. Hot tapping in the natural gas pipeline should be used only if shutting down the gas pipeline is not the option (Sabapathy et al. 2000).
Safety of the process: As hot tapping is associated with high-temperature operations such as welding, a hot work risk assessment is required. Safety should be taken as the most important element in this operation. Safety is also associated with a hazardous situation such as the release of hazardous gases if hot tapping fails.
Condition of the pipeline: the condition of the pipeline should be carefully examined before starting the hot tapping process. Also, the equipment used in the hot tapping process should have a fitness certificate issued by the HSE officer.
Proper system configuration: the hot tapping process should be completed in a single go, therefore the configuration of the system should be made such that the process is not halted due to any reason. If there is any accessibility limitation to the pipeline, the process should not be started.
Suitable operating conditions: specific operating conditions are needed for hot tappings such as temperature, flow rate, and pressure. If these parameters are not within the operational limits, hot tapping cannot be performed.
Suitable equipment: a hot tapping machine should be suitable for the operation. Machine selection is important as failure to select a proper machine can lead to failure of the entire operation. The selected machine should be suitable for the operating conditions of the pipeline including pressure, temperature and flow rates.
An economic evaluation of the operation: hot tapping should not be carried out where the cost of hot tapping is greater than shutdown.
Welding possibility: in certain conditions where the material of the pipeline is such that the welding procedure cannot be performed, in such case hot tapping should not be carried out.
Regulatory follow-up: hot tapping process should be carried out in light of the laws/regulations of the company as well as the industry codes given by the administration of the area. In case these laws are not followed, hot tapping should not be performed.
4 Lessons for Safety Restrictions
The American Petroleum Institute (API) has developed detailed safety manuals and procedures for the hot tapping operation. American Society of Mechanical Engineers (ASME) has also developed safety procedures for the drilling and welding pipelines for different parameters. For safety purposes, these manuals should be referred to to get information related to the welding process, burn prevention, information related to metal thickness, fittings, valve, heat treatment, metal temperature and welding designs.
Hot tapping has evolved to be a safe procedure, yet several safety restrictions are still mandatory to initiate the operation. Hot tapping should be avoided in the following cases (Hutt et al. 1995):
Pipelines where a mixture of flammable gas and air is present. Hot tapping in this case can result in an immediate fire event.
Where a pressure vessel or a heat exchanger is present.
Pipelines where a post welding heat treatment is required.
Pipelines in which unsaturated hydrocarbons are present. These hydrocarbons undergo exothermic decomposition reactions due to the high temperature caused by welding.
Areas where cladding has been done on the pipeline.
In the pipelines where the gas temperature is 0 degrees or 200 degrees Celsius. Low temperature is achieved in gas pipelines around valves where compression causes cooling of the gas.
Figure 2 A safe way to perform hot tapping
5 Lessons for the documentation required for hot tapping
The hot tapping procedure should be initiated after detailed planning and design operations. The following documents should be prepared before the hot tapping operation (Felix, Bruce, and Threadgill 1991):
Design documentation: It includes pipe material selection, pipe thickness, resistance to corrosion and other related details of the pipeline.
Drawings: it includes pipeline network drawings, drawing of the area to be hot tapped and section design.
Material take-off: It is the list of materials to be procured for the hot tapping procedure including valves, bolts, nuts, pipes and connectors.
Transportation: It includes the transportation of procured material, hot tapping machine, and manpower to the desired location.
Site preparation: it includes clearance of the site, especially in the congested areas. In case the pipeline is in a remote area, the path leading to the pipeline should be cleared of any hurdles in the transportation of the hot tapping materials.
Hot tapping procedure: the entire hot tapping procedure should be documented before the initiation of the process.
Welding procedure: the entire hot welding procedure should be documented before the initiation of the process.
Non-destructive testing: includes the information related to testing the welded joints for their reliability and safety.
Closure report: a closure report should be submitted to the concerned department after the hot tapping procedure has been completed.
6 Lessons from the Hot Tapping Procedure on Gas Pipelines
Hot tapping is the most common procedure used for tapping the gas pipelines. Following are the basic requirements for hot tapping (Goodfellow, Belanger, and Journal 2001):
The material selected for hot tapping should be similar to the material with which the gas pipeline has been made.
Only those pipes and fittings should be used that have equal or higher ratings than the pipeline being repaired.
Before the welding operation, the welding procedure should be verified by a certified welding engineer to make sure that the welding process will be successfully executed.
Hot tapping should be only used in a vertical or horizontal position. If it has to be used in an angular position, a hazard analysis should be carried out by the technical team to ensure the success of the operation.
Figure 3 Basic steps in hot tapping procedure
Hot tapping shall not be carried out on gas flares or gas vent lines as these points are mixed with oxygen from the air and thus the gas becomes flammable. When hot taps on such lines are to be used, the gas pipeline should be made free from any presence of oxygen. In case there are chances of oxygen ingress, inert gas should be continuously purged. To ensure process safety, gas detectors can be frequently used to find the presence of any inflammable gas mixture in the pipeline.
6.1 Welding requirements
For welding, a specific procedure should be developed. The welders should be qualified for operating with natural gas pipelines. As it is a safety intensive process, preference should be given to those welders who have prior experience in performing such tasks. All welds should be thoroughly inspected after the welding procedure has been completed.
Figure 4 A welded pipeline
6.2 Testing of welded joints
All welded joints should be verified with specific testing techniques. These include dye penetrating tests, ultrasonic and magnetic inspection, and pressure testing. As a rule of thumb, the testing pressure for hydraulic testing should be 10 percent more than the operating pressure to ensure welding strength.
6.3 Gas velocity requirement
Hot tapping should not be carried out on a pipeline in which the gas velocity is either low or completely zero. In such gas, hazardous or flammable gases can replace the gas itself which can cause an explosion due to high heat generated through hot tapping and welding.
6.4 Pressure requirements
All hot tapping machines have pressure ratings that determine how well they work. Before beginning hot tapping, the machine’s pressure rating must be checked. The operating pressure of the pipeline must be less than the hot tap machine’s maximum pressure rating.
6.5 External Loads
When doing hot tapping, external loads are a major concern. External load effects on the pipeline must be thoroughly investigated and mitigated. When hot tapping, the weight of the machine and the people doing the work may be distributed evenly or unevenly on the pipeline, resulting in pipeline failure. To mitigate the influence of external loads, all lines to be hot tapped must be suitably supported. Additionally, the corroded or as-is state of the pipeline should be used when doing this analysis.
7 Types of hot taps that can be used
Different types of hot taps are available including verticle, horizontal, or any other angle suitable for a pipe according to the room available for the installation of the valve, fitting or a tapping machine. Now, there is a flexible choice of taps depending on their diameter, composition, and material of the tapping machines, as new lightweight tapping machines are more suitable for these operations (Dunđerski and Antić 2007).
8 Lessons for using different equipment in hot tapping:
Following is a detail of the equipment that should be used in hot tapping:
8.1 What is the Purpose of a Drilling Machine
In hot tapping, the drilling machine includes a telescope boring bar to control a cutting tool. The purpose of the cutting tool is to bore a hole in the pipeline that aids the hole saw in cutting the pipeline at the curved section of the pipe.
8.2 What Type of Fittings Should be Used
Fittings should be used to connect the pipes. Fittings can be used either to simply weld a small dia pipe using a welded nipple or to connect larger pipes, full encirclement sleeve tee or to provide extra support to the branching pipeline. tee wraps are particularly useful in welding as they completely wrap around the pipeline and provide high enforcement support to the pipe.
8.3 Type of Valves to be Used
Two types of valves should be used in hot tapping namely a block valve or control valve for a new connection. Most frequently valves used are ball or gate valves. However, plug or butterfly valves are not used.
Figure 4 Schematic hot tapping machine profile
9 Benefits of Using Hot Tapping Procedure
The hot tapping procedure includes the following benefits drawn as compared to using other procedures like the complete shutdown of the pipeline:
The uninterrupted operation of the pipeline using hot tapping provides an economic edge to the pipeline operators.
Atmospheric pollution and loss of resources are prevented by no release of gas in the atmosphere.
Hot tapping is simpler and does not require extensive planning and paperwork.
Hot tapping provides more safety to the workers.
With hot tapping, cutting, realignment and re-welding of the pipeline are prevented.
As pipeline shutdown is not required, the customers do not require prior interruption of gas.
Hot tapping saves a lot of time for the operation managers.
10 Summary of Lessons Learned During the Use of Hot Taps
The maintenance of pipelines is a compulsion in gas transmission. While performing these operations, several lessons are learned that make the process safer and more efficient. Following is the summary of the lessons learned that are extracted from the above-given discussion (Wahab, Sabapathy, and Painter 2005):
Hot tapping has evolved as an efficient method to perform activities in the transmission and distribution companies. Hot tapping has tremendously reduced the gas leakage and resulted in gas savings associated with this practice. Hot tapping can also be used in situations where it is not conventionally used.
While performing the hot tapping procedure, the area for the branch welding should be clear from any corrosion, cracking and laminations.
In immediately upstream situations, a hot tap should not be performed on rotating equipment and automatic control valves. In such situations, it should be protected by using filters and traps.
When hot tapping is used for steel pipes, welded branch connections are included in the fittings. In the case of cast iron, cemented or concrete pipes and asbestos, welding cannot be performed. A split cast iron compressions sleeve is an alternative fitting attachment technique for this purpose.
When the piping system consists of plastic, the workers should make sure that hot tapping fittings comply with the type of plastic used in the construction of the pipes. It is the responsibility of the vendor to provide suitable fittings and tools for the system.
As hot tapping is a new procedure, its procedure should be comprehensively developed and operators should be trained before initiating such a program.
For safety purposes, all equipment must comply with the minimum industry standards including pressure, temperature and other such parameters.
The hot tapping procedure should be in line with the industrial standards.
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