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3 minutes reading time (642 words)

Restoring High Energy Piping System Performance by Reducing Friction

pipe_friction2 Damaged/Destroyed Low Friction Sliding Surfaces

"When You're Stuck, You're Stuck"

Over years of normal operation - like cycling and wear and tear - High Energy Pipe support friction can increase causing distress in these critical systems. Friction is a common phenomenon which is widely understood and considered in both piping analysis and support design.  Generally, a piping analyst models a piping system with a friction coefficient on all base type supports of 0.3 for steel on steel or lower if a low friction sliding surface will be used, and hits “run” on his design software program.  The stresses are evaluated, loads on supports determined, and the analysis is off to support design.  The support designer takes the loads (which include friction) and designs the support.  No errors were made, the calculations are correct, and everything should work.

pipe friction1But what happens in the field after years of operation and cycling? That depends on several variables, including the type of line or lines, location of equipment, sliding surfaces used, etc.  However, there is a good chance that if there is equipment close by, the friction is affecting it.  In some cases, it could work in your favor, but sometimes it can have a negative effect on the connected load-sensitive equipment. 

A good example of the adverse effects friction can have on equipment is what occurred to a steam turbine at a 2x1 combined cycle power plant.  The overall support design had many base type supports with friction located in a pipe rack in front of the steam turbine.  During an outage for the steam turbine, the turbine keys between the HP/IP and LP casings were removed, a standard maintenance protocol.  After a major effort to remove the keys and to everyone’s surprise, the lower HP/IP casing proceeded to slide away over ½”.  The plant suddenly found themselves in a large, costly predicament they were not prepared for.

What had happened is over years of operation and cycling, the supports in front of the turbine were “walking”.  The piping would expand away from the turbine upon warming, and when cooled would almost return to its original installed position.  This difference could’ve been as small as 0.01”, but after a number of cycles, that 0.01” added up to a larger number.  Over the years of operation, the low friction sliding surfaces, which are not easy to maintain or correct, deteriorated.  This deterioration results in much larger friction forces than anticipated.  There was enough friction in the supports and strain built up in the piping, that when the keys were released, the supports held their position, and the deformed piping pulled the turbine casing.  To restore the turbine, the steam piping had to be cut and refit (see below), supports redesigned, etc.  This large amount of unintended maintenance greatly extended the outage to nobody’s liking.

pipe friction3For existing plants, this example illustrates the importance of knowing how the supports are behaving.  Becht offers an extensive High Energy Piping Condition Assessment Program.  Part of the program is to review how the base supports/sliding surfaces in the field are behaving compared to what is specified on the design documentation.  By reviewing these items during the inspections, problems can be identified early and remedied, avoiding unintended major consequences.

For new plant and piping design, friction can’t be eliminated completely, but with proper consideration and support design, it can be controlled, and the equipment protected.  An important concept is that when designing a supporting system with friction, don’t let the computer program tell you what is happening.  As the analyst, you must tell the computer what is happening, and let the computer tell you the consequences.

pipe friction5
Pipe Cutting/Refit to fix Turbine

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Wednesday, 20 March 2019

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