SummaryRecommendation to identify modifications and related work associated with Fractionation/Distillation Column or Tower at a refinery.
The work should be performed during the refinery turnaround. The work including demobilization should be completed during specified time. All welding, testing, post weld heat treatment, inspection and testing should be in accordance with applicable standards and specifications pertinence to project specific.
Drawings and DataDrawings issued for this engineering work package.
Specifications and StandardsAll specifications and standards should be referenced in this section.
Prefabrication and Assembly RequirementsReference all manufacturers/builders drawings and specifications in this section.
Quality Surveillance RequirementsQA/QC plan should be developed as part of the construction planning for this work package.
Contractor-Supplied and Drawing/Data Requirements
- All installation labor, materials, tools, equipment, machinery and external scaffolding necessary for complete modification, testing and turnover of the column/tower submitted to Company
- All rigging equipment including spreader bars, shackles and cables as necessary to ensure stability of tower during Post Weld Heat Treatment (PWHT)
- Shipping, unloading, storage, hauling, and installation of all material furnished
- All documentation should in accordance with Company Vendor Document Requirement Form
Itemized Work Scope
- Remove insulation (if applicable)
- Provide temporary support for piping in post weld heat treatment (PWHT) area
- Perform wet flourescent magnetic particle test (WFMT) on all internal areas to be welded. Tower interior where welding is to be performed should be inspected per WFMT. Any areas showing defects should be subject to hydrogen bakeout using electric resistance coils. Contractor should have electric resistance heating coils, temperature elements, insulation and all equipment necessary for hydrogen bakeout on standby in the event this is necessary
- Perform internal tower modifications (welding, cutting, grinding)
- Nozzle installation (If applicable)
- Internal welded attachments and distribution pipes
- Removal of tower internals
- Perform internal tower modifications (non - welding)
- Tray installation and flanged distribution pipes
- Perform external work requiring welding to tower shell
- Perform external platform modifications
Perform tower post weld heat treatment per applicable specifications
Contractor should submit all documentation to completely describe method of supporting and stabilizing tower during PWHT. Documentation should include, but not be limited to all tower temporary support structures, crane placement as applicable, stabilizing methods, methods of attachment of support structure to tower, and calculations. Contractor should ensure that constant crane support load is maintained during expansion of tower during PWHT. Attachment of temporary support equipment should utilize lifting lugs at top of tower. Portions of top platform should be removed as required to provide access to support lugs. All documentation should be submitted for review and approval prior to starting any activity.
Tower stabilization should consider the followings. This is not an all encompassing list. Contractor should be ultimately responsible for providing comprehensive plan:
- Combined stresses due to wind (forecast) and self weight should not exceed allowable tensile and compressive stresses at maximum stress relieving temperature.
- Calculations should be based on the actual minimum thickness of the column if it is less than the nominal thickness indicated on the drawing. Thickness survey should be carried out to determine minimum actual thickness.
- In the event of local heat treatment, it is expected that entire circumference will be heat treated and not just a rectangular patch of the shell around the nozzle. This will eliminate longitudinal thermal differential expansion stress.
- In case of local heat treatment, circumferential thermal differential expansion stresses should be calculated and evaluated against the allowable stresses.
- Lower holding temperatures are acceptable as per the table UCS-56.1 at the cost of longer holding time for P1 Gr.1 and 2 materials. Lower holding temperatures will reduce the risk of buckling. However, extended holding time may increase the risk of weather uncertainty. The concurrence may be obtained by the Process Licensor for lower holding temperature.
- For local heat treatment the contractor should have enough experience in calculating and monitoring heating and cooling rates in consideration of the heat sink effect from rest of the column shell along with expected ambient temperatures and wind velocities. A longer portion of the shell may be insulated to reduce the risk.
- All external attachments like ladders, platforms, piping etc. should be dismantled and removed as required to facilitate PWHT. This should be an inspection/hold point prior to heat treatment.
- Temperature should be recorded at several locations during heating, holding and cooling of the column to ensure that there is no significant temperature gradient along the circumference of the shell. In case of the local heat treatment, longitudinal temperature gradient must be less than the one considered in the calculation of circumferential thermal differential stresses.
- After heat treatment visual inspection should be carried out to ensure that there is no local deformation outside the code limits (out of roundness, flat spot etc.)
- PWHT temperature tolerances should be tightly controlled for all new pressure retaining welds and tower attachment welds. Temperature tolerances for locations not in the immediate vicinity of these welds (including locations with attachments pins) may be relaxed however temperature gradients should be less than the values considered in the calculations which determine thermal differential stresses.
- Support of PWHT equipment including insulation, heaters thermocouples, etc. may be accomplished by use of support pins attached to the tower exterior by spot or tack welding. All locations utilizing attachment pins should receive PWHT. All areas using pins should be restored to the original condition by surface grinding or buffing to a base finish. Any grinding or buffing performed after PWHT should ensure metal temperatures are kept below 500F per Standard 0601 Section 220.127.116.11. All areas should be subjected to PT test to indicate any possible cracking prior to returning the vessel to service. All applicable procedures should be submitted for review and approval prior to commencement of work.
Perform non-destructive examination in accordance with applicable specifications and drawings
- Support lug attachment welds at top of vessel should be subjected to MT testing prior to applying load to tower.
- UT Thickness measurement tests should be conducted in the vicinity of the support lugs prior to applying load to tower.
- NDE should be conducted after PWHT.
- All external welds attaching support pads to the tower shell should be subjected to PT testing. NDE should be conducted after PWHT.
- As identified in Section 3 above, all locations which utilized support pins for PWHT equipment should be subjected to PT testing. NDE should be conducted after PWHT.
Perform vessel hydrostatic tests per applicable specifications
- Install Hydrotest Fixtures as applicable.
- All hydrostatic testing should be performed at a pre-determined pressure.
Perform final Tray installation inspection
- Contractor should perform final inspection of tray installation and associated internals in accordance with applicable specifications and drawings.
Re-install platforms, ladders, supports as required
Replace tower insulation applicable specifications
Bills of Materials
- If applicable
- If applicable
Company-Supplied Equipment and Materials
- Tower Passivation (flushing of tower with alkaline water solution to control polythionic acid stress corrosion cracking)
- Preparation of Tower for internal access including ventilation, lighting, internal and external scaffolding, tray removal and “tunneling” of tower as required
- Temporary Power Supply for Contractor’s use
- Potable water supply for Contractor’s use
- Passivated water for hydrostatic testing
- Crane for support of tower during Post Weld Heat Treatment