Pipe Split-tees are inevitable components required for hot tapping or line-stopping operations. These are full-encirclement fittings with two broad areas of application as follows:
- Clamp-on Bolted Split-tees (mostly used in marine / offshore applications and can be used onshore)
- Butt-welded Split-tees (used in onshore applications)
This document discusses split-tees which are intended for onshore hot tapping or line stopping operations that involve welding of two halves of a split-tee. Their manufacturing processes are broadly classified as extrusion and fabrication. Split-tees manufactured via both processes are in use in the industry and are claimed to meet international and local regulatory requirements related to strength, durability and performance. This paper identifies in detail the engineering and cost consideration of both types of manufacturing processes.
1.1 Extrusion Production
Extruded Split-tees are manufactured by hot working and forming to make steel flow into a die from a rectangular steel sheet to achieve the tee shape. This process does not involve welding of branch connection with main pipe run; however welding of flange to the Split-tee is still required. In some instances, there are benefits to manufacture Split-tees by extrusion, such as:
- Method eliminates welding at Split-tee neck and (apparently) provides more confidence to owner
- Easier dimensional control for mass production
- Does not involve special procedures for weld inspection as there is no saddle weld (between branch and main run)
- Production time is short when strict temperature controls of Split-tee material and process are ensured
- Quenching and tempering can provide more strength for smaller wall thickness of branch connection (provided work hardening and material heterogeneity is avoided)
1.2 Fabrication Production
Fabricated split-tees are manufactured by welding a branch outlet and weld neck flange on main pipe run. Although this process involves welding of branch with main pipe run to create the Tee-shape, which is always an important concern for weld quality and strength, it is more flexible with regards to following considerations:
- Selection of Split-tee material
- Manufacturing process does not involve cyclic heating and cooling
- Options for main-pipe run to branch connection sizes
- Strength of branch and weldability of Split-tee with flange material
- Post-weld heat treatment provides control on metallurgy and compliance to codes
2. Production of Hot Tapping and Line Stopping Fittings
The term wrought steel as used in industry is a broad term that refers to steel that is rolled, drawn, forged, extruded, heat treated and so on. In this broad classification of wrought steel those that come under forged steel and extruded steel are generally accepted as tougher and more durable. With the increasing utilization of flanged fittings like branch connections in factory made Split-tees it is safe to state that such Split-tees are treated by industry as fittings under ASME B16.9 or MSS SP 75. Actually they are not strictly wrought steel factory made butt-welded fittings; rather they are fabricated in factory as full-encirclement fittings made from wrought steel and their manufacturing may or may not involve extrusion.
Split-tees can be produced using any of the following methodologies:
Fabrication of Split-tee
The process starts with two rectangular rolled plates or pieces of pipe which form upper and lower halves of Split-tee. Upper half which forms the saddle involves fabrication welding of branch connection on rolled half pipe and weld-neck flange to complete the fabricated Split-tee. Use of internal braces during welding and post-weld heat treatment may be required for strict dimensional control. Finishing, inspection and testing follows.
Extrusion Pulling of Split-tee
The process starts with two rectangular rolled plates or pieces of pipe which form upper and lower halves of Split-tee. A hole is drilled in the center of upper half for insertion of a forming punch. Upper half is then placed in a die and forming punch is made to pull the steel outwards into the die hole to form branch connection. Necessary heating and cooling cycles are involved which require strict control for material temperature, pulling speed and material ductility. Quenching and tempering are sometimes used to improve material toughness. Weld neck flange is welded to the saddle to complete the Split-tee. Finishing, inspection and testing follows.
Extrusion Pressing of Split-tee
The process starts with two rectangular rolled plates or pieces of pipe which form upper and lower halves of Split-tee. A hole is drilled in the center of upper half and placed in a die having a female recess. A forming punch is made to push through the hole into the female die thus forcing the steel to form a branch outlet. Necessary heating and cooling cycles are involved which require strict control for material temperature, pressing speed and material ductility. To preserve dimensional accuracies quenching and tempering are not used in this process and strength of Split-tee is limited by the strength of base material. Weld neck flange is welded to the saddle to complete the Split-tee. Finishing, inspection and testing follows.
2.2 Material Strength
Through virtual design validations carried out using Finite Element Analysis it is well established that greatest stress concentrations occur at the Split-tee neck i.e. the point where branch connection and main pipe run meets. It is therefore required during physical validation of Split-tee design to establish that a Split-tee does not fail at the neck irrespective of the fact that the neck is extruded or fabricated. However, concern about weld quality is often a reason why some owners may have reservations with regards to Split-tees that are produced by welding branch connection to main run.
To make an informed decision with regards to material strength or more specifically strength at Split-tee neck it needs to be remembered that strength at Split-tee neck is a function of different variables. These include:
- Selection of base material and weldability with flange and pipe run
- Wall thickness at Split-tee neck
- Ratio of branch size to main run size
- Work hardening during extrusion
- Material heterogeneity during hot and cold cycles of extrusion process
- Heat treatment processes involved
As such it can be safely said that both the processes of welding or extrusion are prone to material inconsistencies depending on experience and control measures implemented during each production process. Ensuring strict temperature and process controls for material strength followed by physical validation of Split-tee design can definitely lead to production of Split-tees that will perform as per design during field operation.
With regards to materials and strength requirements in the field, there are numerous applications that require use of specific / special base materials and fabrication process is often flexible with regards to ensuring compliance with codes. Some of the field material requirements are provided below:
- Carbon Steel Split-tees:
- Including ASTM A234 Gr WP-Gr B, WPB-W / WPHY 42, 46, 52, 60, 65, 70
- ASTM A537 Class 1 steel for elevated temperature applications
- ASTM A672 Gr C55 CL 12, ASTM A516 Gr 70, ASTM A105 B 860 WPHY 70, A860 WPHY52 / WPHY70, A420 WPL6, API 5L X52 for Oil pipeline applications
- Stainless steel split-tees:
- ASTM A403 WP 316 / 316 L, ASTM A403 WP 304 / 304L, ASTM A182 F316L 304L
- Alloy Steel Split-tees:
- ASTM A234. ASME SA 234 WP1 / WP5 / WP9 / WP 22 / WP 91
- Duplex Steel Split-tees:
- ASTM A815. ASME SA815 UNS No S31803 – S32205
- Super Duplex Split-tees:
- ASTM A815. ASME SA815 UNS No S32750 – S32950
- Nickel Alloy Split-tees:
- ASTM B336. ASME SB336 Nickel 200 / 201, Monel 400 / 500, Inconel 800 / 825 / 600 / 625 / 601, Hastelloy C276
Note: MSS SP 75 provides for 7 pressure classes which are meant for factory-made butt-welded pipe fittings meant for welding to a pipe cross-section. These are:
“WPHY” stands for “Wrought Pipe High Yield” or “Wrought Plate High Yield”. For factory made flanged Split-tees these class specifications may be considered as a reference to pressure containment requirements (only) as and when specified.
2.3 Grain Structure and Stress Concentration
Grain structure of metal is important for consideration of a fitting’s capability to sustain stresses generated by contained fluid. Foremost consideration for manufacture of Split-tees is the requirement to use fully killed steel – a term used to describe de-oxidized steel. Silicon or Aluminium or both are used to de-oxidize steel and resultantly fine grain structure is obtained. This improvement in microstructure of steel increases its strength and makes the steel fit for manufacture of pressure containing vessels.
Various studies have shown that hot extrusion improves mechanical properties of material as compared to as-cast alloy. One such study indicated that after hot extrusion tensile strength of the alloy increased from 245 MPa to 327 MPa, yield strength increased from 135 MPa to 322 MPa and elongation increased from 14.4% to 24.9%. The reason for the significant improvement of material properties is mainly due to the dynamic recrystallization during thermal processing, which greatly fines the grains of as-cast alloy.
Thus it is correct to state that extrusion definitely improves mechanical properties of steel. However, this cannot be considered as a deciding factor for selection of extruded Split-tees or discarding fabricated Split-tees. There are options available in fabricated Split-tees to counter the increased stress concentration at the neck e.g. by increasing the wall thickness of branch connection.
3. Engineering Considerations
Pipeline fittings are engineered to facilitate and safeguard sustainable development of pipelines and to improve the quality of repair and maintenance activities. For pipelines in North America as well as internationally, acceptable codes are ASME B31.4 (Pipeline Transport Systems for Liquids and Slurries) and ASME B31.8 (Gas Transmission and Distribution Piping Systems) with (additional) significant applications of ASME B31.3 (Process Piping) and occasional considerations under ASME B31.1 (Power Piping). Due do some differences like application of design factors; Canadian Standards Association CSA Z662 is followed in Canada as a regulatory requirement for pipelines.
Normally for inline welded fittings ASME B16.9 (Factory Made Wrought Steel Butt-welding Fittings) and MSS SP 75 (Specifications for High Test Wrought Butt-welding Fittings) are considered to be standards for selection of fittings. However, ASME B16.9 and MSS SP 75 do not specifically cover Split-tees as Spit-tees are full-encirclement fittings (rather than inline fittings which are welded to an open pipe-end) which are not classified as inline fittings. This aspect and use of Split-tees under ASME B31.4 is further discussed under Section 3.2 Design Parameters.
It is pertinent to state here that nothing contained in the DOT regulatory requirements under “e-CFR Title 49 – Subtitle B – Chapter I – Subchapter D – Part 192: Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards” prevents or suggests the use of extruded and / or fabricated split-tees for hot tap operations on pipelines.
3.1 Site Requirements
There may be special site requirements which bring into consideration factors other than those discussed under the scope of this paper. Such may include:
- Permanent installation or temporary use in a repair operation
- Internal corrosion considerations for corrosive products and extremely high temperatures
- Vents requirements for hydro testing, relieving product or water or to introduce sealant
- Placement of Lifting lugs in case hot tapping or line stopping operation is to be carried out on a large diameter overhead pipeline crossing or piping structure
- Requirements to attach Anodes
- Dealing with pipelines used for Category M fluid service. Although ASME B31.3 does not specifically identify Category M fluids, however a broad definition is provided which points to all fluid service which is judged to have potential for personnel exposure and where a single exposure to a very small quantity of a toxic fluid can produce serious irreversible harm to persons on breathing or bodily contact, even after prompt restorative measures.
3.2 Design Parameters
The design parameters that come under the scope of this paper are those design criteria and requirements that relates to welded branch outlets (only for fabricated Split-tees), flanged branch connections and pressure containing full-encirclement fittings as described in ASME B 31.4.
For code conformance of pressure containing performance, Split-tee design is required to comply with relevant sections of ASME B 31.4 which include:
- Relevant design parameters of Section 404.3.3
- Relevant design parameters of Sections 404.3.4 and 404.3.5
- Relevant design parameters of Section 404.4
- Relevant design parameters of Section 4220.127.116.11 (d) (2)
As long as stated requirements are fulfilled there need not be any additional requirement for Split-tee design to be compliant with ASME B 16.9 or MSS SP 75 as both these standards relate to inline welded fittings rather than full-encirclement fabricated fittings.
Above discussion is also true for compliance of extruded or fabricated Split-tees with ASME B31.1, ASME B31.3, ASME B31.8 as well as DOT requirements.
3.3 Regulatory & Code Compliance
Regulatory requirements in various international jurisdictions normally refer to accepted industrial best practices and standards / codes that are widely accepted. Following most widely used standards / codes and recommended practices are listed as a ready reference of the reader:
- API 1104: Welding of Pipelines and Related Facilities
- API RP 2201: Safe Hot Tapping Practices in the Petroleum and Petrochemical Industries
- ASME B31.1: Power Piping
- ASME B31.3: Process Piping
- ASME B31.4: Pipeline Transport Systems for Liquids and Slurries
- ASME B31.8: Gas Transmission and Distribution Piping Systems
- ASME B16.5: Pipe Flanges and Flanged Fittings
- ASME B16.9: Factory Made Wrought Steel Butt-welding Fittings (Compliance scope limited for parameters like matching pressure class, welding of branch-end with weld-neck flange, etc. Refer Section 3.2 of this paper for details)
- ASME B 16.25: Butt-welding Ends
- ASME BPVC Sec VIII: Rules for Construction of Pressure Vessels
- ASME BPVC Sec IX: Welding, Brazing and Fusing Qualifications
- ASTM A370 – 19e1: Standard Test Methods and Definitions for Mechanical Testing of Steel Products
- MSS SP 25: Standard Marking Systems for Valves, Fittings, Flanges and Unions
- MSS SP 44: Specifications for Steel Pipeline Flanges
- MSS SP 53: Specifications for Steel Castings and Forgings for Valves, Flanges, Fittings and Other Piping Components
- MSS SP 75: Specifications for High Test Wrought Butt-welding Fittings (Compliance scope limited for parameters like matching pressure class, welding of branch-end with weld-neck flange, etc. Refer Section 3.2 of this paper for details)
- SSPC VIS-1: Pictorial Surface Standard Dry Blast Cleaning (By: Steel Structures Painting Council)
For Pipeline Operators working in US, relevant DOT regulatory compliance requirements are provided in following documents:
- e-CFR Title 49 – Subtitle B – Chapter I – Subchapter D – Part 192: Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards
- e-CFR Title 49 – Subtitle B – Chapter I – Subchapter D – Part 195: Transportation of Hazardous Liquids Through Pipelines
3.4 Inspection & Testing Requirements
There are differences in inspection (NDT), documentation and material certification requirements of fabricated and extruded Split-tees owing to the variations in production processes. Additionally, there are variations in heat treatment tests as per process requirements to control mechanical properties of finished Split-tee.
Dimensional control for extruded Split-tees is strictly dependent on tight tolerances of die and pulling or pressing punch, without the option of ID machining. Dimensional control of fabricated tee is achieved through use of braces and final tweaking can be done through machining.
Pressure testing requirements for both types of Split-tees are similar and provided for in the relevant codes. There is no difference as such because both types are intended to fulfill same pressure containment requirements over their useful life.
3.5 Life Expectancy & Durability
Durability and life expectancy of Split-tee is largely dependent on selection of right type of steel as base material, field application and type of fluid service the fitting is subjected to. In certain instances extruded Split-tees, when compared to Split-tees fabricated from similar specification steel, may prove to be more durable over time because of aligning of grains during extrusion. For example, where the Split-tees are subjected to cyclic loads (downstream of a compressor station) combined with higher temperature fluids (in the vicinity of well head), extruded Split-tees provide better resistance to stress corrosion cracking.
4. Cost Considerations
When compared to the purpose for which split-tees are intended, cost of manufacturing cannot be regarded as a deciding parameter for selection of extruded or fabricated split-tees. Of course the cost saving always matter, but ensuring a successful hot tapping or line stopping operation is a far bigger consideration.
4.1 Material Costs
There is not much difference in material costs for extruded and fabricated Split-tees other than the cost of welding material.
4.2 Cost of Production
Cost of production of extruded Split-tee is higher as compared to fabricated Split-tees due to involvement of strict temperature controls and extrusion dies. Cost of production can be a significant consideration in case of unusual specification of branch diameter on large diameter Split-tees.
For standard Split-tees produced in bulk quantities, extruded Split-tees can provide significant time saving which can be an important consideration at times.
4.3 Cost of Heat Treatment
Heat treatment costs are higher for fabricated Split-tees as compared to extruded Split-tees (if at all there is a requirement of heat treatment for extruded Split-tees).
Split-tees may not be seen as requiring general compliance with ASME B16.9 or MSS SP 75 because these codes refer to inline fittings to be welded on an open cross-section of pipe. Rather Split-tees, which are full-encirclement fittings, are to be checked for compliance under ASME B31.4, ASME B31.8, ASME B31.3, ASME B31.1 and DOT as general minimum requirements.
There may arise very unique service conditions where cyclic loading of pipeline / piping stress is severe thus calling for enhanced consideration of fine and aligned metal grain structure for ensuring required and useful service life of Split-tee. Other than that there is no particular advantage or disadvantage of using extruded Split-tees over fabricated Split-tees.