In addition to my monthly column “Common Pumping Mistakes” the Pumps & Systems editors have asked me to write an article on the common ways that the systems can also reach out and bite you.
If you own a home, a car or for that matter any machine—from a lawn mower to a dishwasher—you already know that all things left to themselves never get better. It is a law of our universe captured eloquently by the word entropy (for the engineers I am using a loose second definition).
For those of you that already own, operate or maintain a mechanical, pneumatic, steam, hydraulic or electrical system, you carry all the battle scars from these real life lessons. As a review for those that already know and as a primer for those that are new to this area, I offer in no particular order, some discourse on the subject.
Your system responsibility—whether a building with HVAC systems, a power plant, a chemical processing or manufacturing facility, a paper mill, a brewery/distillery, wastewater treatment or a manufacturing center—is comprised of machinery: electrical, mechanical, steam, pneumatic and hydraulic. Every system and component has a finite life.
All machinery, operating or idle, will eventually fail. Machinery fails for the following basic reasons: corrosion, erosion, stress or impact. Your job is to prevent it from failing or failing at the wrong time. When the machine does fail, we hope to manage it in a predictable and safe manner.
Ask yourself if you and your staff have the proper training and equipment for flash protection, confined entry, general electrical safety, chemical handling, radiation exposure, noise, noxious/carcinogenic or poisonous gases, safety chains, ladders, rigging/lifting equipment, signage, rotating machinery and OSHA guards.
Education/training in all of these areas is very important. Additionally, consider whether the current training is adequate for new personnel or is simply a review of procedures for the seasoned employee. Note that most injuries occur during non-routine or emergency evolutions.
Manage by Walking Around
If you do not manage the system, it will manage you. Choose a new system every day and grab the system logic process diagram for review. Do you really know how the system operates? If you do not know how the system operates, you will not know if anything is wrong until it is too late. So, the prudent thing to do is learn the system. With a flashlight, rag, clipboard and a camera, walk and trace the system “hand over hand” to look for issues and changes.
Look for leaks, wear, erosion, corrosion, abrasion and discoloration. Look for unauthorized additions and alterations. Use all of your senses including smell and hearing. Look for what has changed in the system operation, starting with different pressures, flows or levels. Check and review system logs for long-term trends.
Most plant issues occur when there is a change in the status quo.
The biggest “corrupter” of heat exchangers is fouling and corrosion. (From a dictionary aspect, corrupter is not the correct word choice to describe the issue, but I insist in this case it is appropriate). Both fouling and corrosion lead to a reduction in performance, and in some cases to erosion with consequential and undesired leaks. Determine the exchanger performance or heat balance by measuring the flows, differential pressures and temperatures on both sides of the unit. Compare readings and thermal balance results to prerecorded, new or clean parameters. Heat exchanger maintenance or replacement is time consuming and costly, so do not wait for catastrophic failures when a few simple checks can help you predict the need for repair or replacement.
Because my heat exchanger experience is with nasty fluids at high pressures and temperatures, I have a prejudice for shell and tube exchangers. I am also aware that plate and frame exchangers offer many advantages and that the newer designs have improved capabilities and reliability.
On shell and tube type exchangers, the tubes can plug from the process fouling, corrosion or debris. From past tube failures, there may also be tubes that are manually plugged and taken out of service. An accepted common industry maximum level of tube plugging is 10 percent, but I caution users to always consult the manufacturer since some designs will demand a lower percentage. According to Tubular Exchanger Manufacturer Association (TEMA), the acceptance level for “U” tube designs is only 1 percent. Whatever the acceptable range is, at some point plugged tubes will need to be replaced or the heat exchanger performance derated. You can also clean the tubes and shell side as a maintenance procedure.
Plate and frame heat exchangers are less susceptible to fouling due to designs that yield higher fluid velocities and turbulence in the channels. Plate and frames are also easier to clean, maintain and change performance dynamics. My caution to personnel adding, changing or cleaning plates is to be very careful of the plate orientation and design. The most common issue I witness is how easy it is for plates to be installed incorrectly—in the wrong order or the wrong plate altogether. The other issue is with gasket leaks due to inadequate torque or mishandling.
Control and throttling valves have exposed trim, stems and seats that will wear and corrode, which not only changes the pressure drop across the valve, but pieces of the valve/trim will lodge in downstream machinery. Another issue is with zealous mechanics that tighten the packing so tight the valve actuator cannot develop sufficient torque to operate the valve. The end result is a system not operating properly and the expensive actuator destroyed.