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Pipe Stress 103 – How to Spot Pipe Stress Problems in Your Facility
This is the third article in a series of articles about thermal stress, pipe stress and the art of pipe stress analysis. The first article provided a look at what is thermal expansion and contraction, and an idea of the magnitude of thermal growth you could expect to see from a hot pipe. In the second article we took a closer look at the importance of proper stress analysis and good engineering practice to offset the risks that thermal expansion poses to piping and equipment.
Unmitigated thermal expansion and pipe stress issues can result in pipe or equipment damage, maintenance issues or even pipe ruptures. This article covers signs of potential thermal growth or stress issues you can spot while walking around your facility. A future article will focus on less obvious indications or signs that might require you to consult the piping and instrumentation diagram (P&IDs) and isometrics of your system.
While out in your facility pay attention to the following items. Each of these could be a sign that pipe stress issues due to thermal expansion or contraction are wreaking havoc in your piping system. While these signs are not absolute proof, they can be a good indication that a particular piping system may need a closer look.
Bent or “Squirming” Straight Pipe
This most often happens when a relatively long run of pipe has grown thermally but is bound in the axial direction at both ends; the resulting force bends the pipe. In very long runs of pipe, or runs of piping with an expansion joint, you may see this as “squirming” of the pipe.
Another common place to see an excessive bending is at a piping elbow. 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). If the bare pipe is visible, further indication that the pipe may have bent from a 90-degree angle (as opposed to a piping elbow trimmed to a different angle) and is out-of-roundness in the elbow curve is known as ovalization. For elbows that have been compressed, the ovalization will typically be perpendicular to the direction of compression. For elbows that have been extended, the ovalization will typically be in the same plane as the extension.
Bent or Cracked Nozzles
Bent nozzles on equipment are another sure sign that thermal expansion and pipe stress may be causing problems in your system. Typically, these signs will be seen on tanks and pressure vessels with thinner walls. This might also show up as a distortion in the shell around the nozzle, in these cases. It’s rare to see a bent nozzle on a piece of rotating equipment, but if this is seen, it could indicate a severe overloading of the nozzle.
Leaking flanges can have many causes, including improper bolt tightening, over pressure and gasket failure. While leaking flanges may not be directly related to thermal growth or pipe stress issues, a poorly designed piping system can allow excessive lateral or bending forces to negatively impact a flange, resulting in leaks. If you see a leaking flange take note and look for other potential signs of excessive thermal stress, especially if this particular flange is a repeat offender.
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 movement. Pipe shoes that have lifted or slid off support structures may indicate thermal growth and stress issues.
Pipe shoes, guides and line stop lugs or supporting steel that have been bent, broken or otherwise damaged are prime indicators that an unexpected force has affected the pipe. Keep in mind the damage doesn’t have to be limited to the support and could be noted as a bend or dent in the pipe at the point of attachment. These damages could result from water hammer, excessive thermal growth, high expansion joint loads or some other external force like high winds or a seismic event. All of these indicate that further investigation may be warranted.
Bent Support Steel or Rod Hangers
These could indicate a large thermal movement or thermal stress that was not adequately designed for. While it’s not uncommon to see rod hangers that are tilted (as opposed to obviously bent) due to small axial or lateral movements of the pipe, bent rods or significant movement can indicate problems you need to address.
Piping up or downstream from rotating or reciprocating equipment that has visible or excessive vibration can indicate either excessive pipe stress or insufficient piping support. Remember, not all pipe stress issues are caused by thermal growth. Piping that is not adequately supported and results in excessive loading on an equipment nozzle (as potentially indicated by excessive vibration) is also a pipe stress issue.
Variable spring hangers and constant support hangers are designed specifically to support pipes that move during operating. In most cases this movement will be from thermal expansion or the thermal expansion of an attached piece of equipment (or a combination of the two). There are a couple of easy-to-spot things with a spring hanger that might indicate an issue with the thermal growth of the system.
First, the spring hanger or hanger assembly should remain nearly vertical. The rod hanger attached to a spring (or the whole assembly if the spring is hung in line with the rod) should be no more than 4 degrees from vertical in the cold or hot position.
The second indication of a problem is the spring hanger that is bottomed or topped out. Most spring hangers will have an indicator showing the relative position of the spring in relation to the maximum travel allowed for the specific hanger. Usually both the hot and cold positions are clearly indicated. If the indicator is outside of the cold-to-hot range, this can indicate trouble; seeing a spring hanger completely at the top or bottom of its range is a much stronger indication of problems.
Keeping an eye out for these kinds of indications can alert you to the possibility of excessive or unplanned for thermal growth, resulting in pipe stress problems. Remember that not all pipe stress issues are thermal in nature, so keep an eye out for excessive vibration or unexplained pipe movement as indicated by bent or off steel shoes.
In the next article, we will dig into some of the more subtle signs that a facility may have unmitigated pipe stress problems to address, or perhaps your plant might benefit from a more comprehensive Pipe Stress Audit. Check back with us to see what the signs are.
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, Senior Consultant or Brandon Grodi, Mechanical Department Manager.
William Helm, PMP, Department Manager at Matrix Technologies talks about the Matrix approach to A Better Process for Success. We place a high priority on the work we do, and its value for our clients, but the only way that really happens is if we place the highest priority on our people. Learn more about our People first, Mission Always philosophy.
Cost and Safety are Among the Pros and Cons of PLC IO Fusing
Opinions vary on whether or not to fuse individual inputs and outputs when wiring a programmable logic controller (PLC) module in an industrial control panel. Different opinions stated by many end users include fabrication panel cost and size to accommodate the additional large number of fuses versus the time to troubleshoot problems in the field if an input shorts and damages the module. As always, personnel safety takes top priority.
The first place to start with any design engineering project is to follow end-user client standards on fusing. After client standards are followed, design firms may also have specific standards that apply in this case. Each needs to be followed starting with the client standards.
Other considerations must include following the manufacturers’ PLC input/output (I/O) module requirements to ensure a safe, functional and warrantied installation. Fusing I/O should also take into account the voltage of the system; 120VAC inputs — and potential faults — represent a higher risk to personnel and components than 24VDC systems. After all the above steps are considered and followed, some clients and design engineering consultants may not have set standards for PLC I/O fusing. Both sides of the argument are considered here.
Consider the Pros
When installing individual I/O protection, fuses and circuit breakers may both be used. However, fuses are the preferred choice as they interrupt faults by opening faster than breakers. They are also less costly and can be installed using blown indicator fuse blocks for easy fault identification and fuse replacement.
Fusing individual points versus grouping several together allows for a single I/O point interruption from a fault, whether from a field wire short, device failure or panel wire problem. In the case of a single main panel circuit breaker fault, the entire I/O module or modules would lose power and could cause the untimely and potentially costly down time loss of a plant production line.
Individual I/O fuses can save a lot of maintenance fieldwork and plant downtime versus if you have a short with multiple field devices sharing a single field power source. You can also have a power wire fall off a device due to vibration and short against a metal case or conduit. One short in the insulation of a single wire could cause an entire module to lose power. Trying to find a single short among multiple field devices could be time consuming and costly in both idle plant personnel and loss of product production.
Some industries, such as food or pharmaceutical plants, may not allow glass fuses in panels. This restriction is somewhat rare, and an exception can usually be allowed through the use of proper PLC enclosure styles and panel Installation locations away from the product lines.
Care should also be taken to research the manufacturers’ recommendations on external fusing if the I/O module selected has internal fuse protection. Even with internal module fuse protection, additional input fusing may be required or preferred by client standards.
And the Cons
Fusing every PLC input can significantly increase the cost of a PLC panel. Depending on the number of I/O points, you could have a large number of fuse terminal blocks and glass fuses. This also results in a larger panel enclosure. In locations where stainless steel enclosures are required, the larger enclosure cost could be significant. Only installing a main branch feeder or single fuse or circuit breaker per input module can greatly reduce the cost associated with the panel design.
Two field wires per discrete field device are required if fusing every PLC I/O point. Depending on the number of instruments, this would add significant costs for wire and conduit, plus the installation time to mount the conduit and terminate the wires.
Fusing every PLC I/O point may not be necessary in all applications. If the inputs are strictly used for monitoring and not for line control, having an entire module lose power due to a short may not affect the plant operations. The short could be fixed as time allows. However, this could still damage the PLC module.
Situations where a non-critical fault occurs and blows an input fuse can occur when fusing every PLC I/O point. Since the input does not interrupt plant operations, maintenance may leave the fault until they have more time to investigate the trouble. This could lead to a dangerous short somewhere in the panel or field for an undetermined amount of time.
Conclusion: Fuse each PLC input
This article in not intended to serve as a definitive answer to the question of how or when to fuse PLC inputs. It is intended to raise questions during the design engineering phase to ensure a safe installation that meets client and integrator standards.
Taking into account the pros and cons listed above, the safest option for plant personnel is to fuse each individual PLC input. This is not a one-size-fits-all answer but tries to balance the different requirements between cost and safety. As in any design engineering application, the number one priority must always be safety.
As a design engineer with over 30 years of experience, my personal recommendation is to always fuse each individual PLC input, regardless of voltage. With many years in the field working with plant maintenance and electrical installation contractors, the time saved troubleshooting field or panel faults is well worth the extra panel costs to add the fuses.
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 our manufacturing operations management capabilities and manufacturing process control solutions, contact Dan Wood Senior Consultant in the Process & Electrical Design Department.