Does your facility need a pipe stress audit?
Learn to identify potential pipe stress or thermal growth problems using an explanation of pipe stress and thermal expansion
- Understand how thermal expansion of piping affects connected equipment.
- Learn how to spot common indications of high thermal stress in the field.
- Review how changing process conditions or pipe routings can cause thermal movement issues.
Pipe stress audit insights
- By knowing what to look for, a pipe stress audit can help a facility increase savings and production down the road.
- Unidentified safety and operational issues can be rectified after identification.
Heat up a run of pipe, and it gets longer. That’s simple physics and of itself isn’t necessarily a problem. But piping rarely runs from point A to point B without being connected to something, usually a piece of equipment.
Consider a straight run of pipe that connects two pressure vessels. How much force can a straight pipe heated to 300°F, fixed at both ends, generate? With perfectly stiff anchors, a 6-inch diameter pipe can generate nearly 250,000 pounds of force, limited only by the structural buckling of the pipe itself.
Where thermal growth meets a fixed object, like a piece of equipment, force is generated. Forces acting over an area result in stress. Pipe stress analysis calculates thermal growth and resulting forces and stresses. Good pipe stress design adds flexibility to the system and keeps forces and stresses manageable.
Flexibility isn’t just about routing but also supports and restraints designed to control the movement of expanding pipe. When this step in the design process is skipped or not given adequate attention, issues can result in the field. Below are some common indications that a thermal growth problem may exist at a plant.
Here are several piping and support issues in the field that may indicate the need for a pipe stress audit:
Bent or “squirming” pipe runs: This happens when a long run of pipe has grown thermally but is bound in the axial direction, the resulting force buckles the pipe. In long runs of pipe or piping runs with an expansion joint, you may see this as “squirming” of the pipe or lateral bends outside of the centerline of the pipe.
Bent elbows or tee connections: Another common indicator is excessive bending at a piping elbow or tee connection. Pay attention to elbows that seem extended too far (bent to an angle greater than 90 degrees) or compressed too far (bent to an angle less than 90 degrees). Look for piping connections that are no longer square.
Leaking flanges: Leaking flanges can have many causes, including improper bolt tightening, over pressure, corrosion and gasket failure. While leaking flanges may not be directly related to thermal growth issues, a poorly designed piping system can put excessive lateral or bending forces on a flange, resulting in leaks.
Pipe support issues: Pipe supports are meant to support the weight of the pipe and, in some cases, direct the movement of the pipe or restrict excessive movements. Pipe shoes that have lifted or slid off support structures may indicate thermal growth issues. Pipe shoes, guides, line stop lugs or supporting steel that has been bent or broken are also good indicators.
Bent support steel or rod hangers: While it’s not uncommon to see rod hangers that are somewhat tilted, a rod that’s obviously bent or with a large rotation could point to thermal growth problems that need to be addressed. Bowed rods can also point to piping that has grown in unexpected directions.
Most spring cans and hangers have an indicator showing the relative position of the spring in relation to maximum allowed travel for the hanger. This same indicator typically shows both hot and cold positions. Indicators outside of the cold-to-hot range or completely topped- or bottomed-out can indicate thermal growth issues.
Spring hangers should always hang vertically. Most manufacturers specify that spring hangers be installed within 4 to 5 degrees of vertical. Significant lateral tilt or rotation of the hanger indicates unplanned thermal movement. Significant tilt to the spring shaft can indicate the same issue in base cans.
Rotating or reciprocating equipment: Pumps or other rotating equipment with a poor maintenance history, worn seals, worn bearings or alignment issues during routine maintenance can indicate high nozzle loads, which could be due to thermal loading. Rotating and reciprocating equipment nozzles are typically sensitive to high loads and are often found to be overstressed in the field.
Fixed equipment nozzles: Bent or cracked nozzles are sometimes seen on fixed equipment; if the bend is slight it may be seen as a distortion in the shell around the nozzle. More rarely, cracks around the nozzle or nozzle repad can form after long periods of high nozzle load or thermal cycling.
Management of change indications
Some pipe stress problems are easier to spot from the office; plant documentation can sometimes show issues more clearly.
Process modifications: Equipment that has changed service or operating conditions can hide thermal stress issues. Changes to process fluid temperatures often manifest as thermal growth problems over long runs of attached pipe. Vessels operating at modified temperatures may impart new thermal movements to attached piping, lifting pipes off support steel or changing spring hanger loadings.
Piping and instrumentation diagram (P&ID) revisions: Clouded sections of P&IDs often indicate a change in the process conditions or piping interconnects. Does the change indicate equipment operating at new temperatures? Has a new bypass line or piping interconnect recently been installed? Even small changes in piping connections or process conditions can result in thermal growth issues.
Construction or maintenance actions: Not every change to the structure or piping is planned; field conditions often require that adjustments be made. When this happens, it’s important to compare field piping to piping isometrics. Be particularly aware of construction or maintenance activities that cause changes to stress analyzed piping or adjacent support steel. Isometrics for stress analyzed piping are often clearly marked and should not be modified without additional analysis.
Next steps in a pipe stress audit
So, you’ve spotted a shoe that’s off the support steel, a bent section of pipe or bottomed-out spring can. Maybe that pump with maintenance problems sounds all too familiar. Maybe a dozen little maintenance items suddenly came into focus after reading this article. Now what? It is time to contact a pipe stress professional and have them conduct a pipe stress audit.