Some good responses already, I'll try not to repeat already good answers around startup errors, maint repair errors, infant mortality, etc. Here are a few additional ones.
1. Startup stresses - Stress is force per unit area on part. Force is mass times acceleration. Mass or acceleration may be constant or could change. Keeping the Newton's law of motion, an object at rest tends to stay at rest and an object in motion tends to stay in motion outside of friction. The acceleration of a mass (rotating equipment) is the highest stress time on machine components. The inertia creates higher stress on components than when normally running on many machines. There are different reasons on different types of equipment and too much to elaborate on here but this is generally what is happening.
How to mitigate? Startup stresses are normal and the machine should be designed for such but proper training on startup SOP and having the machine in good condition are necessary. Starting up a machine not loaded up is a typical SOP for most systems. In some cases on a VFD, a soft start or extending acceleration times on a drive can lower startup stresses.
2. Thermal growth - Starting up, the machine will heat up. The heating and cooling means all the machine parts thermally expand and contract. This changes the relationships with all the parts which induces more stress on machine components.
How to mitigate? Make sure the machine is designed and setup to handle the thermal growth and startup procedures do not create a condition to exceed machine design limits. Warm up procedures to slowly warm up equipment is a must.
3. Condensation - Humid air on cold surfaces causes condensation. Condensation inside our equipment leads to lube deterioration. While this effect typically does not cause failure instantly, it will eventually.
How to mitigate? Try to keep heat in areas. Also keep any circulating oil systems running as much as possible. Run dehydrators on larger circulating systems.
4. Static failure modes - There are some failure modes which occur while the machine is not running (static). Static corrosion on a rolling element bearing can only occur while the machine is down not running. Condensation occurs (see #3) and water can collect locally in machine components causing significant damage. Some low stiffness shafting may suffer from deflection (sag) on long shutdowns so startup may need to consider that.
How to mitigate? Similar to #3 on keeping lube systems on. Some idle equipment may benefit from a slow crawl condition while down. Must be careful that a slow roll does not run equipment below minimum speed and cause equipment damage.
5. Startup vibration effects - Due to all of the above, vibration can be affected upon startup. Could be a mechanical change or solids buildup on a rotor from shutdown. A fan rotor could sling a buildup mass off on startup (see that often) that increases imbalance. Increased vibration will decrease component life and similar to condensation or static failure modes, it may not be instant but delayed failures.
How to mitigate? Clean and inspect key rotors upon shutdown if possible. Vibration monitoring will not prevent but will allow intervention before serious failure modes are initiated.
Good question and reveals the hidden cost of shutting down equipment. Any floor operations or maintenance mgr will tell you if running, keep it running. Infant mortality is a very real thing.
------------------------------
Randy Riddell, CMRP, PSAP, CLS
Reliability Manager
Essity
Cherokee AL
------------------------------
Original Message:
Sent: 02-26-2021 01:47 PM
From: Thaddeus Lightner
Subject: Component failures after a shutdown period
Hello everyone,
I am a new member to SMRP but have been working in the reliability field for several years starting out as a technician in the Navy, then a technician for Bridgestone, and now I am working as a reliability specialist.
A discussion that always seems to come up after an extended shutdown period is why do the machines always seem to experience failures when we go to fire them back up. The closest thing that I have been able to come up with that seems to help explain it is a term called stress relaxation or a change in material components caused from the normal stress of operating being removed and then subsequently reapplied without proper stress conditioning. I know there are other factors that contribute to the failures such as boundary lubrication conditions before forming of an oil wedge for mechanical components, in-rush surges for electrical components, etc, but on the material component level I feel like there would be documentation associated with how the change in stress caused from operating to non-operating to operating again contributes to failures.
Does anyone know of any documentation associated with the concept I am talking about or what the terminology for the changes in component material is actually called under these circumstances? Stress relaxation is a term that I found that seems to be close but doesn't seem to line up perfectly.
Also, from everyone's experience, what actions do you take to help mitigate these types of failures? One thought that I have brought up to my manager is after a shutdown period, rather than just turning the machine over to production and letting them go straight to full speed, what if we started up a day earlier to just start rolling the machines without material being produced at a controlled ramp up speed/rate.
Thoughts and ideas are appreciated.
------------------------------
Thaddeus Lightner
Bridgestone Americas Tire Organization
Trenton
------------------------------