RESPECT YOUR SEALS!

It’s your first job as a rotating equipment engineer in a big production facility. They left you a gift: a huge noisy machine that keeps you awake at night. It has been running for years. But recently you noticed that the humming noise is turning into something more scary, you can actually feel it now. Everybody’s concerned, you cannot speed it up and you cannot shut it down. Something’s seriously wrong with this champ.

A team meeting is called. Senior engineers suggest continuous condition monitoring and a couple of measurements. They discuss the bearings, gears, loads and all the usual culprits. You go out to the machine with the team and nothing turns up. One of the team says “It’s out of balance”, but you know it’s not the usual balance problem. It just doesn’t fit into the picture. Despite all the efforts, the unit has to be shut down. Data is collected and passed on to the black belt machinery guru. After a few days, he says “It’s the seal!”. You head down to the machine and disassemble it, and here it is: the seal clearances show signs of heavy wear, as if the seal clearance has opened up. You change the seal with a new one, and start up the machine, ramp the speed up and it runs smoothly.

After all the effort, you think to yourself, all these problems just for a seal! More experienced engineers know that the costs associated with a seal are way more than the price tag of the seal. The majority of failures in pumps and compressors are associated with seals. When you start thinking about all the extra costs: leakage, loss of efficiency, maintenance, replacement, downtime, condition monitoring, secondary damages to expensive parts such as rotors; it’s saying seals demand respect, way more than what is paid to them.

It all boils down to understanding your seals. It is so easy to miss the impact of that high-pressure flow inside the tight clearance and the amazing amount of force it can produce. When you have a vibration problem, do you forget about your bearings, of course not. Then you should not forget about your seals either. A new seal with a tight clearance provides you with ample damping to cross through critical speeds, but a worn-out seal isn’t so kind to your machine.

• Review your process data and hand calculations
• Monitor pressure drop, temperature and leakage rates
• Measure vibration and evaluate the spectrum
• Do a ring test and monitor the mode shape close to the seal
• Change flow rate, speed and load and monitor the trends
• Build a rotordynamic model, use a bulk-flow approximation
• Investigate what is happening inside using CFD
• Run an FEA on seal connections
• Perform material testing on seal parts
• Set up acoustic measurements and models

And the list goes on. It is amazing to see the amount of effort put into seal failure modes and analysis. This explains the real cost of a sealing system. Remember a seal failure is more than lost profit; leaking seals expose hazards to anyone nearby with potentially toxic, high pressure, and flammable material. Whenever you have such failure bombs lingering to explode, you should have a Professional Engineer experienced with seals and their failure modes on your support team. Also, don’t forget the famous saying: “Be wary of worn out seals!”