For every time the question, “Why does my pump keep failing?” is asked, a different reply could be offered depending on the application and usage of the pump unit. Is the failure caused by what is being pumped? How it is being pumped? Or is it a question of the maintenance of the pump unit and the processes therein?
Follow-up questions consistently asked of the end user are: “What is causing the need to replace these parts so often? Is it a problem with the system?” The standard response is, “I don’t know, we just need them.” All too often these procurement processes are unnecessary and preventable. Instead of taking the time to question the root cause of failure, it seems easier to replace the bearings in the pump and wait for it to fail again than it is to go through the effort to identify the misalignment of the unit.
What do resulting secondary and tertiary expenses cost in time, money and effort?
3 most common component failures in process pumps
Sealing method — Shaft seal & packing
How the pump is sealed and against what chemicals is of utmost importance. Changes to the elastomers, seal faces and style for mechanical seals and the material and design of braiding for packing can change the ability of the sealing method to perform its job effectively.
Mechanical seals are becoming the norm but have challenges in installation to ensure proper fit and function. These seals must be clean upon installation; No oily residue from fingerprints or scratches on the sealing faces must be present and their spring depth must be set correctly. Using an environmentally friendly dish soap can ease installation, provide lubricity during installation, and wash away clean upon liquid testing or startup of the unit.
- The largest cause of failure in mechanical seals is allowing the pump unit to run dry. Mechanical seals survive by having the fluid go across the sealing face to keep it cool. When the seal is starved of this fluid, it overheats and causes pitting, chipping and cracking. This allows the fluid to escape the system, more often described as a failure. If the unit is not stopped in time, or if no control panel setup exists to react to the failure, the fluid being moved will be released to the outside of the piping system. At this point, permanent damage will be done to the seal in a matter of seconds.
Braided packing is not as common as it once was, but it still has a place in the industry for installation. Packing is supposed to leak and one common misconception is if it leaks, operators should wrench it down until it stops. The leaking is necessary because it provides the opportunity for the sealing surface to be cooled, like the fluid going across the seal face in a mechanical seal.
- Most setups for packing are for viscous fluids that will not leak out from the packing at a high rate. However, if the packing is tightened down too far it can create the opportunity for failure of the packing rings, failure of the shaft as well as the stuffing box. Attention needs to be paid to make up for the thermal expansion of the rings depending on the material used.
- Another failure is when the packing leaks heavily even if it is compressed down to the correct pressure. The two most common causes of this issue are pitting or scratches on the shaft below the packing rings, or the packing rings are not aligned at 90-degree axial offsets successively. The shaft should be checked prior to reassembly and the packing rings should always be installed as stated above.
Industrial seals — Lip & labyrinth
Industrial seals are used to ensure that the gearbox or bearing housing is clean of debris and safe from any chemicals in the atmosphere. Depending on the design, they can prevent washdown processes from allowing water in the lubrication housing, or prevent the lubrication from escaping. These components are integral parts of the longevity of equipment, process pump or otherwise. Lip seals have been used as cost-effective options for bearing protection, but when it comes to peace of mind, operators can switch to labyrinth-style seals. They are a little more expensive up front, but the results and repair quality are excellent. The average cost for a lip seal installation is almost three times as high over one year’s time because of the amount of failures and cost of repair and labor, including the cost of components. Many different styles can be considered, but the most important key to think about is what application it is going in.
If it is an application involving many different fluids, the potential for issues arising from chemical attack is much greater. The sealing faces and O-ring materials must be adjusted to perform in such an environment.
- The standard metallurgy for labyrinth seals is brass. While it is an inexpensive option, stainless steel is better for a larger amount of applications because of temperature and chemical resiliency.
- The elastomers change depending on the application. It is important to identify what has the best chemical resiliency for an application. The cost of an upgraded O-ring in a seal is significantly less than the downtime that can come with a failure.
If it is a material-handling application with a lot of dust floating in the air, a higher potential for failure occurs as the equipment cools down, creating a vacuum inside the gearbox and drawing air in from the outside. Dust particulates may still be suspended in the outside air that could be drawn into the rotating equipment.
- Specialized labyrinth seal designs have either an inboard or an outboard air purge. This allows the contaminants that have been working their way near or into the equipment to be pushed back out to the atmosphere and away from the rotating equipment.
- The purpose of the industrial seal is to make it more difficult for contaminants to enter into the lubrication cavity. Consulting an industrial sealing specialist can help reduce time and effort given to repair and procurement of repair parts upon failure.
Rotation support — Bearings & bushings
Bearings are the proverbial lifeblood of rotating equipment. These pieces function in numerous designs, specifications and levels of work. Described as a bearing or a bushing, their purpose remains the same: Allow the piece of machinery to turn repetitiously in a smooth and consistent manner.
The design characteristics of ball bearing versus roller bearing and carbon graphite versus bronze sintered bushing are important to consider. The manufacturer of the rotating equipment has specified their designs precisely and without their consent, so it is not recommended to switch technologies. Different styles of bearings (sealed, shielded and open) offer varying degrees of effectiveness in the manufacturers’ specific designs of bearing housings. Utilizing brands that are known and trusted in the industry is advised because they likely have the technology, history and position in the market to make a recommendation for performance, as well as the quality of product they will stand behind.
Rotating equipment is an integral part of the industry and should be given proper attention. Without performance at a high level, the pump is the component in the process that will be blamed. However, the question should be, “Is it a pump problem or a process problem?” Sending the pump back into the process without identifying and fixing the root cause of the issue will not create new results. It will only create more questions such as, “Why is it failing and how can I make it stop?” End users should take the time to ask this of the pumping system: “What is the real cost of ownership?”
Michael Aschenbrener is Motion Industries’ Process Pumps and Equipment branch manager. He can be reached at firstname.lastname@example.org. To find out more, visit motionindustries.com and Mi Process Pumps Specialist, Motion Industries’ newest Knowledge Site.