Pipe Stress 104 – Does Your Facility Need a Pipe Stress Audit?
This is the fourth article in a series about thermal expansion or contraction, pipe stress, and the art of pipe stress analysis. In a previous article, we discussed some of the different signs to look for out in your facility that might indicate the presence of a pipe stress problem. In this article, we will discuss some of the less obvious signs that your facility may have a thermal stress issue. By the end, I hope you will have the tools to help you decide if it’s time to call in an expert to help you identify and mitigate potential problems with a Pipe Stress Audit.
Sometimes thermal stress problems are easy to spot. In our previous article we showed you some of the signs that might catch your attention when you are out walking around in your facility. Sometimes, though, the clues are not as obvious. Here are just a few of the less obvious indications that have become red flags for the team here at Matrix Technologies over the years.
Pumps or other rotating equipment with a poor maintenance history could point to a pipe stress issue. Are you dealing with worn seals, worn bearings or alignment issues during routine maintenance? Is the base of the pump or compressor cracked? Is there a subtle, or not so subtle, vibration? Do these maintenance problems bring to mind a particular pump or centrifugal compressor at your facility? Over time we have come to realize that the nozzles on rotating equipment are some of the most sensitive and often the most overstressed components in a facility. (In a future article, we will go through an example and explain why this tends to be the case. The article will also explore how rotating equipment nozzles compare to other typical piping components and connections in terms of sensitivity to thermal growth. Keep an eye out for it!)
Management of Change
Vessels, heat exchangers or tanks that have changed service or operating conditions may also indicate a hidden thermal stress issue. It’s not uncommon for vessels in a plant to change service or for existing processes to be modified so that they are now operating at a new temperature or pressure. Even relatively small changes to the temperature could manifest as problems over long runs of attached pipe.
Similarly, when the vessel itself is operating at a new temperature it will impart new thermal movements to piping originally designed for a different operating point. Has the direction and magnitude of the equipment movement under new process conditions changed enough to create problems? Is there enough movement now to put the existing pipe into contact with support steel or with another adjacent pipe? Is there enough thermal movement to lift a pipe off its support and put additional load on the attached nozzle?
Clouded sections of the piping and instrumentation diagram (P&ID) often indicate a change in the process conditions or new piping interconnections. Does the clouded section indicate that a vessel or tank is operating at a higher or lower temperature than before? Has a new bypass line or new interconnect line between two runs of existing pipe recently been installed? If so, then you may have an unforeseen pipe stress issue waiting to reveal itself. Admittedly, not all P&ID changes will warrant your attention in this respect. Adding new instrumentation may not significantly change the pipe stress of an existing line, so a little engineering judgment is required.
Construction or Maintenance Actions
Not every change to the structure or piping of a facility is planned for; field conditions often require that adjustments be made. When this happens, it’s important to compare piping in the field to piping on the isometrics. In particular, be aware of recent construction or maintenance activities have caused changes to previously stress analyzed piping. Analyze any alterations to these pipes for new stresses. Isometrics for stress analyzed piping are often clearly marked. Many have a note, like the one shown in the figure below, that we use here at Matrix Technologies. These piping isometrics often show a wealth of additional information, like the predicted movements and direction of hot piping.
Changes to piping aren’t the only modifications that can occur during construction or routine maintenance. Be aware of changes to steel and surrounding structures; these can affect how a pipe is supported or restrained. This is especially true if the piping was previously stress analyzed. Sometimes even small changes to the support structure can affect the overall stress of the piping run.
You’ve spotted a shoe off of the support steel or a bent pipe, or my story about that pump with the maintenance problems reminded you of one in your own plant. Maybe it’s a dozen little maintenance items that suddenly came into focus after reading these articles. Now what? This is the point to contact a pipe stress professional and have them conduct a pipe stress audit.
Matrix has 40-plus years’ experience in dealing with pipe stress issues and strives for unmatched client service. That means we put the same care and attention into one single pump or a single run of pipe as we do into an entire facility. Add to that a track record of fixing problems that others can’t, and you will see why 90 percent of our customers are repeat clients.
But even if you don’t come to Matrix Technologies for your pipe stress problems, please remember that everyone is responsible for safety and asset protection. Please contact a professional when you notice indications like the ones we have listed here and in previous articles in your own facility.
In the next article we will look at a simple method for expansion loop sizing. This is something you can do in the field to establish safe expansion loop sizes or to quickly verify that an existing loop is adequately sized for the thermal growth in a line. Later we will talk more about the relative sensitivity of piping components to thermal stress and take a closer look at where thermally stressed lines typically fail.
Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about thermal expansion and pipe stress analysis, contact Chris Mach, PE, Senior Consultant or Brandon Grodi, Mechanical Department Manager.