ACHRNEWS

Thermal Applications: Pumps, Fans, Compressors

May 21, 2007
Monitor rotating equipment while it is in operation and under load. Be sure to scan the equipment’s drives (electric motors and gearboxes, if any). On pumps and fans, get thermal profiles of the housings as well as scans of shaft couplings or drive belts and sheaves.

Pumps, fans, compressors, and other motor-driven rotating equipment are essential to manufacturing, commercial, and institutional enterprises, from fluid-handling systems in petrochemical plants to large air comfort systems in shopping malls.

Many facilities monitor this type of equipment on a regular basis because often a simple problem like lubrication can be spotted and fixed inexpensively, before the entire unit burns out. Such strategies fall under the general heading of predictive maintenance (PdM).

Many impending failures are accompanied by overheating. That is why thermal imaging is especially useful for monitoring rotating equipment. In this predictive technique, the maintenance employee holds a thermal imager and uses it to capture two-dimensional images that represent the equipment’s apparent surface temperatures. (Note: Apparent temperatures can differ significantly from actual temperatures, due to the emissivity of a material’s surface - its ability to emit radiant energy.)

Thermography can also be used together with other predictive techniques, such as analysis, vibration monitoring, and ultrasound measurement.

WHAT TO CHECK

Monitor rotating equipment while it is in operation and under load. Make sure you monitor equipment that is critical to your operations (i.e., equipment whose failure would threaten people, property, or production).

Be sure to scan the equipment’s drives (electric motors and gearboxes, if any). Also, on pumps and fans, get thermal profiles of the housings - scans that are likely to reveal any problems with bearings or seals - as well as scans of shaft couplings or drive belts and sheaves.

For a compressor, use several images, if necessary, to get a thermal profile of the entire unit.

In general, look for hot spots. Pay special attention to differences in temperature between similar units operating under similar conditions. For example, if a bearing in one fan in a bank of similar fans is running hotter than the rest, the hotter one may be heading toward premature failure.

On a pump, a difference in temperature along a seal or gasket is the signature of a failure. A hot spot on the housing adjacent to a bearing may signal an impending bearing failure, although you probably won’t be able to determine the root cause from a thermal image alone. Perhaps there is a lubrication problem or maybe misalignment in the drive.

FANS AND COMPRESSORS

An overheating bearing on a fan also signals a problem, but a thermal image of it alone is not definitive. Again, the root cause could be lack of lubrication, the wrong lubrication, drive misalignment, or unbalance in the fan itself. You will need to investigate further.

Many industrial and building system fans are belt-driven, as are some pumps. According to one source (Snell Infrared, an IR training firm), a belt-and-sheave drive that is designed and installed correctly generates very little heat; as the belt moves through the air, it tends to be cooled to near ambient temperature. Overheating that is detected by thermography reflects a problem with the drive’s design or installation, perhaps mismatched belt and sheaves or misalignment.

Vibration analysis and/or an alignment check will confirm the latter condition.

Since a compressor is a “heat machine,” a thermal imager can quite literally see a compressor work (compression produces heat while expansion cools). To check the efficiency of a compressor, you should look for belt slippage on cooler fans, shaft misalignment, bearing problems, and blocked or leaking valves.

A good approach is to create a regular inspection route that includes all critical rotating equipment. Then, save a thermal image and associated data of each unit you’ve scanned on a computer; track the measurements over time. That way, you will have a baseline for comparisons with subsequent images. They can help you determine whether or not a hot spot is unusual, and following repairs can help you verify that they were successful.

RED ALERTS, COSTS

Equipment conditions that pose a safety risk should take the highest repair priority. However, the imminent failure of any critical pump, fan, or compressor represents a red alert.

Consider using key safety, maintenance, and operations personnel to quantify “warning” and “alarm” levels for these assets. Then you can set alarm levels for specific equipment on your thermal imager.

Whenever you use a thermal imager and find a problem, use the associated software to document your findings in a report that includes a digital photograph as well as a thermal image. That’s the best way to communicate the problems you find and to suggest repairs. If it looks like a catastrophic failure is imminent, the equipment must either be removed from service or repaired immediately.

Because pumps, fans, and compressors are key to productivity in so many industries, it is difficult to speak generally about the cost to a company from the failure of a critical unit. However, a failed pump at one automotive facility cost more than $15,000 to repair; lost labor costs totaled $600 per minute and lost production opportunities amounted to $30,000 per minute. Try developing similar figures for a critical equipment failure in your operations. It may help you justify thermal imaging to your managers.

SIDEBAR: TIP: WATCH THE WIND

Winds or indoor air currents that are in excess of even a few miles per hour will reduce the surface temperatures you are seeing with your imager, causing real problems to seem less significant or even making them invisible.

Inside plants, air currents are often 10-15 mph.

Buy a good-quality wind meter and record the wind speed when you record the apparent temperature. When you must inspect in high-convection situations, note all problems for a follow-up inspection. Even those pieces of equipment with seemingly small temperature increases may become critically hot when the airflow is reduced.

Publication date: 05/21/2007