Designing for Mechanical Integrity: Piping

Transcription

1 Designing for Mechanical Integrity: Piping Daniel Dettmers Associate Researcher, IRC More information at:

2 Key Considerations Prior to Design Documentation from designer/fabricator Materials certification (mill reports) Welding certifications (welders and welds) Piping drawings (isometrics, weld maps, ) Inspection (visual/pressure) report of pipe Other Documentation Criteria for acceptable corrosion PSM documents (PHA s, ) Results and analysis of independent inspection Develop method to store information

3 Key Considerations Prior to Design Longevity of piping Proper specification of pipe materials Proper specification of weld procedures Proper installation of piping & components Preparation and treatment of base pipe Ongoing maintenance of insulation system Insulation system specification & installation Design for MI inspection Visual (Routine & Annual) NDT testing (Independent)

4 Designing Piping for MI 1. Minimize High Risk areas Protect piping Reduce potential trouble spots 2. Make inspection easy as possible Accessible inspection points How? Must know potential failure mechanisms

5 Potential Failure Mechanics There are many potential piping failures, but which are important? Corrosion? Erosion? Weld Failure? Pitting? Stress Corrosion Cracking? Primary concern: Corrosion Under Insulation (CUI)

6 Corrosion The deterioration of a material, usually a metal, by reaction with its environment. - National Association of Corrosion Engineers (NACE) An anodic-cathodic reaction Positive ions detach at the anode and travel through a fluid (i.e. water, ice, etc.) which acts as a carrier (electrolyte) to the cathode. Can create a pitted surface Can reduce over all wall thickness

7 General Corrosion Gradual corrosion of wall material Looks like effects of sandpaper on wood May be small pits present Danger is from wall thinning sufficient to allow cracking

8 Pitting Corrosion Deep pits form in surface Danger from pit extending through wall

9 Corrosion Under Insulation (CUI) CUI is general corrosion that occurs undetected under piping insulation Water enters through a break in the jacket/vapor barrier Insulation aids rusting process by holding the water tight to the pipe Pipe must also be without or have a failure in the surface protection (i.e. pipe paint)

10 Corrosion Under Insulation (CUI) Vapor Barrier Pipe Rust/Insulation Inside Insulation Outside Insulation

11 Corrosion Under Insulation (CUI) Paint/coat the base pipe! SSPC-PS 13.01, and others Specify & install proper insulation system Install vapor barrier and jacket outside insulation but assume it will fail due to weather, damage or eventual degradation 1/6 th original thickness

12 Corrosion Low Risk Areas Uninsulated, painted pipe Easy to see any rust forming High temperature pipe If pipe is on the high side of the system and insulated (i.e. HPL) there aren t large driving forces pulling moisture Always Frozen The water needs to flow to cause corrosion Ice locks down moisture and severely retards corrosion process Pitting General Deposit Always Frozen

13 Always Frozen Be careful of always frozen pipe Wet suction returns often frozen on bottom only where liquid sits Periodically used processing equipment (i.e. spiral freezers) may warm up Exposed valves may help local pipe to thaw Wet Suction Return With Compromised Insulation

14 Corrosion High Risk Areas Piping that is near to or above the freezing point Piping that cycles temp (defrost condensate) Piping that has circumferential temp distribution (wet suction) Low points in the piping system Dips in line or end/corner of horizontal run Bottom of vertical run Bottom edge of a horizontal piece of pipe

15 Erosion The destruction of materials by the abrasive action of moving fluids, usually accelerated by the presence of solid particles or matter in suspension Externally, not typically a problem Internally, usually found in piping where a denser particle is suspended in a fast moving medium Dirt & debris in liquid lines Liquid droplets in wet suction return piping

16 Internal Erosion

17 Erosion High Risk Areas Wet suction returns Entrained liquid droplets erodes pipe internally Worst is on the outside radius of elbows or valves that cause direction change High pressure liquid piping far from high pressure receiver/high elevations where flashing may create bubbles Opposite injection points, where a pipe T s into a second pipe Dirty systems

18 Other High Risk Areas Insulated piping susceptible to damage (or that has been damaged) because it is located in area of heavy traffic, outside in an area heavily exposed to the elements or in an area subject to frequent clean-up wash downs At welds and screwed connection joints (particularly those that experience vibration)

19 Develop an Inspection Program Hire a certified inspection contractor Help develop inspection program and plan piping design for easy inspection Determine documentation to be provided by designer/installation contractor Establish the inspection plan Develop isometric drawings Include pipe slope and elevation, tagged component and weld location Document everything

20 Develop an Inspection Program Baseline Thickness Measurement Locations (TML) After fabrication but prior to insulation, easy to take with UT Establish excess TML s Measurement is quick Use RBI to determine needed TML TML s should include: High Risk locations Periodic spots on long piping runs Every 40 feet at the butt weld Every 100 feet Mark location Mark, removable plug, different insulation, etc.

21 What About Other Materials? Stainless steel SCC can occur in stainless steel when exposed to chloride solutions Usually in excess of 150 F, but still a risk at room temperature Salt on a pipe in high moisture environment, seacoast locations, rainwater leaching chlorides from insulation and Perspiration dripping on warm stainless steel can initiate cracking. 1 1 Stress Corrosion Cracking: Austenitic Stainless Steels, Vol. 11,ASM Handbooks Online

22 Conclusion Proper design of piping for MI will cost more up front Require much more documentation Requires more engineering to the pipe layout Requires more inspection to verify proper pipe is used and installed correctly Proper design of piping for MI will save money, downtime and headaches More reliable system Easier to inspect Less down time for plant