Purpose of this mechanical work procedure is to provide guide lines to carry out underground piping fabrication and laying and to ensure that proper quality control and documentation are maintained at project site.
The scope of this procedure covers fabrication, installation and relevant inspection for all underground piping works including relevant civil works at the project.
This procedure is to describe various activities such as surveying, excavation, installation of manholes and catch basins, fabrication, NDT, lowering, hydro test, joint coating and back filling.
Reference documents:
- Piping Fabrication Procedure
- General Welding Method
- External Painting and Protective Coating
- Underground Services Construction Methodology
- Relevant Project Specifications
- Project Quality Plan
- Project HSE Plan
List of Personnel:
- Construction Manager
- Project QA/QC Manager
- QC Engineer / Inspector
- Site Engineer
- HSE Engineer
- Foreman
List of necessary Equipment’s
- Welding Machine and its Accessories
- Fitter Toolbox
- Crane of Suitable Capacity
- Proper Rigging Tools i.e. lifting belts etc.
- Inspection and Testing Tools
Surveying
Survey word shall be done as per approved procedure and as per the drawings.
Coordinates shall be established, for the routing of the underground piping, location of manholes and catch basins. Any fouling or interruption in the routing shall be noticed to the consultant, any change in the pipe routing shall be done after consultant approvals.
Excavation
Before starting the excavation for trench, the location and elevation factors should be verified so that the trench for underground piping can be excavated to correct location and elevation. Trench has to be excavated in slopes to avoid the collapse.
For catch basins/ manhole, the excavation shall be done after marking all the 4 corners of cash basin / manhole on ground by taking allowances for working clearance and slopes.
Excavations shall be done manually or mechanically as per suitability to the site.
Bottom of the pit, shall be dressed to the required level and compacted manually.
Trench shall be excavated so as to provide a cover as shown in the approved drawing, required depth of cover shall be excavated under the road.
When excavating near adjoining, buildings or other existing works or structures special precautions against slips, slides or settlement of the existing work or infrastructure shall be considered.
No excavated material shall be deposited within 1.5 meter from the excavated trench.
Suitable signs and barricades shall be provided to prevent accidents.
After excavation the bottom of the trench shall be properly graded to make the trench ready for underground piping.
Trench shall be rock free in accordance with the trenching and underground services procedure.
After trenching the elevation shall be checked for the correct slope if required as per drawings.
Trenches shall be of sufficient width allow satisfactory joining of pipes and tempering off backfill under and around the pipe.
The trench bottom shall be firm and fully capable of supporting the pipe to be installed.
Where unstable material such as muck or peat is encountered, the trench bottom shall be over excavated to a depth at which stable material is encountered and the over excavation shall be backfilled with the same excavated material or other suitable material.
The stability of all excavated faces is to be maintained in compliance with specified excavation safety standards through final acceptance of the work.
Safety barricades shall be provided around all excavated areas, until the work is complete.
Installation of manholes and catch basins
Blinding concrete
After the dressing and compaction the shuttering for blinding concrete shall be done using timber and plywood.
If the delay between compaction test and laying of concrete shall be more than 10 hours, the area shall be watered properly before laying concrete.
The blinding concrete of appropriate grade As per drawing shall be laid and compacted, using vibrators.
Concrete top shall be finished using wooden float.
Top level of concrete shall be checked.
Concrete shall be cured for minimum 3 days.
Construction of manholes and catch basins
Construction of RCC manholes and catch basin shall be precast and will be done as per approved procedure for reinforcement formwork and concreting work.
All valve pits shall be constructed on the site.
All masonry work shall be done as per approved procedures.
Erection of precast manholes and catch basin
Manholes and catch basins shall be erected after 24 hours of PCC work.
The centre lines shall be marked on the PCC top in such a way that they are visible even after the erection of manholes and catch basins is done.
The manholes can catch basins shall be transported to site by suitable means, with proper tools and tackles.
Capacity of lifting tools shall be at least 2 times greater than the expected load on it.
The centre of manhole / catch basin shall be marked on all its four sides before erection, so as to place it in proper alignment.
Manholes shall be installed and wherever the openings, are there, the pipe shall be inserted and secured. Manhole top shall be covered by suitable means.
Fabrication
Prior to start of work all documents related to piping, such as welding procedure specification, procedure qualification test reports and method statement shall be established and approved by consultant.
Pipes shall be stretched near the trench area so as to facilitate fabrication.
Pipe fabrication shall be done as per approved drawings.
Material shall be marked as per size requirement, marking will be inspected and recorded by the inspector and released for cutting/
After cutting the pipe pieces, edge preparation will be done as per drawings and WPS requirements.
fit up as per drawing shall be offered to inspection and once cleared shall be released for welding.
Welders shall be qualified as per approved WPS/PQR.
Before fitting up the well joint, the profile and dimension of the weld end preparation shall be checked.
If the specified tolerances are exceeded this shall be corrected by grinding.
Fit up shall be done by using bridge cleats of same material for alignment of pipe joints.
If alternatively tack welds are made, then it shall be incorporated into the final weld.
If temporary welded attachments are used then these shall be of the same nominal chemistry as the base materials.
Temporary attachments shall be removed by flame cutting, arc gouging or grinding. Hammering off shall not be done.
Location of removed temporary attachments shall be examined visually.
Welders shall be qualified as per the approved welder qualification procedure.
Consumable control and storage shall be as per the procedure for electrode and filler wire storage control.
Unless otherwise specified all but welds shall be full penetration welds.
The external surface of the welds shall be free from undercuts, overlaps and abrupt ridges or valleys.
To avoid distortion during welding temporary supports shall be provided with spools.
All branch connections shall be of stub-on type.
Applicable NDT will be done as per approved documentations and applicable procedures.
NDT requirements shall be as per project specifications and applicable drawings, codes and standards.
Hydro Test Fill Test (or) Service Test
Hydro pressure test shall be done as per specifications and piping class.
Hydro pressure test shall be done for fire water pipes and fill test or service test shall be done for open drain and close drain system.
There are two options to conduct the hydrotest.
- Hydro test shall be carried above ground on the identified sections of the piping and upon completion coating of field joints, holiday test and lowering shall be done.
- Hydro test shall be conducted below ground after lowering of identified sections of pipes and by keeping all welded joints exposed. After completion of hydrotest coating, holiday test of joints and backfilling shall be done.
In some cases like road crossing, where the line has to be buried before hydro testing, 100% radiography shall be done for field joints. These joints should be approved.
Joints between two hydro tested sections shall be considered as tie in joints.
The pressure and holding time are generally 1.5 times the design pressure and for minimum one hour time.
Water must be used from approved sources as testing medium for hydro pressure testing of the piping system.
All the pressure gauges shall be having valid calibration certificates.
Service Test / Fill Test
This shall be done for open drain and closed drain systems. Test shall be carried out by using clean water as the testing medium, this shall be done upon completion of all welding and NDT activities. Testing shall be completed before starting the commissioning of piping system.
Field Joint Coating: After acceptance of hydrotest the field joints shall be released for coating.
Surface Preparation
Steel surface shall be free of mud, oil, grease or any other foreign materials. Surface preparation can be obtained by using power wire brush to achieve clean surface as per Swedish Standard Association ISO-8501 ST 2 1/2.
All burrs and weld slags shall be removed from the pipe surface. The pipe surface shall be dry and free of any dust particulate prior to the joint coating system. Mill applied primary coating should be beveled at the cutback areas.
Application of Field Joint Coating
Immediately after the surface preparation primer shall be applied to pipe steel surface using brush.
The primer should cover entire exposed steel surface including the raised circumferential weld bead and overlap onto the mill applied primary coating system by minimum four inches.
The primer shall be dry to “tacky to touch” prior to the application of weld joint inner wrap.
The corrosion protection inner wrap shall be applied to the primed field joint in spiral.
The wrapping process starts at minimum four inches beyond the cutback edge.
The inner wrap shall be applied under taught tension that will result in a smooth wrinkle free coating.
Minimum spiral overlap shall be 50%. The mechanical protection outer wrap shall be applied to the top of first layer.
Outer wrap shall be applied under taught tension that will result in a smooth wrinkle free coating.
The minimum spiral overlap shall be 50% at the end of the wrap, which shall be cut on the down side at 3 or 9 O clock position.
Field Coating Inspection
Careful inspection of the coating shall include visual observations. Coating shall be checked of any holidays using 10 KV holiday detector. All holidays and all damaged coating shall be clearly marked and repaired immediately.
Repair shall be carried out by filling the damaged area with patch and then wrapping around the circumference to extend 50mm beyond the damage.
Peel test shall be carried out using hand peel test gauge. The frequency of peel test will be one in fifty welded production.
It shall be done after 24 hours of application.
Lowering of Pipe Sections
Before lowering 100% holiday test of pipe sections shall be carried out.
Pipe shall be protected during handling against impact shock and free fall.
Proper facilities shall be provided for lowering the sections of the pipe to prevent disturbances of the bed and sides of the trench.
The skid marks and other places of damage shall be thoroughly examined to ensure proper patching wherever necessary before the pipe is lowered in the trench.
The barrel of pipe shall be in contact with the quadrant shaped bedding throughout its full length.
To prevent shear failure the branches of all fittings shall be supported by granular material backfilled in the trench to provide support equal to that furnished the barrel of the pipe.
Ends of the pipe shall be kept securely closed to prevent the entry of any foreign material or moisture after lowering into the trench.
For spacing pipes of different diameters, the spacing requirements for the larger pipe shall be used.
Wherever manholes and catch basins are provided a single pipe shall be inserted and laid.
The inserted line shall be sealed using approved sealant inside the manhole and catch basin.
Back Filling
Backfilling and compaction shall be done as per contract specifications. Proper compaction shall be carried out after backfilling as per the acceptable method statement approved by consultant.
After inspection and approval of pipe lines (after holiday test of coating or other tests), the trench shall be backfilled with the excavated material.
Backfilling using the excavated material shall be done in layers, back filling material shall be free from rocks, well watered and rammed to avoid settling afterwards.
Stage wise inspections shall be done and completed as per the approved inspection and testing plan ITP.
All documentation shall be completed as per the project quality documentation requirements.
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