Sub Synchronous vibration - ~30% of running speed
Sub Synchronous vibration - ~30% of running speed
(OP)
Ladies and gents,
We have a unit that gets a few mils of vibration at 52 Hz (3150 Hz). Multi-stage centrifugal compressor (between brg design). Line freq is 60 hz. Motor speed is 1800 RPM. Running speed = 11,500 RPM. Rotor dynamic study checks out with flying colors.
I'm really wondering what could cause this vibration. It doesn't seem to get any larger than a few mils, but the customer wants to know what we think.
I'm going to have the hanging rotor hung and see what frequency it rings at. Maybe ring the casing too? Do you think there is a natural frequency somewhere in the system being excited?
We have a unit that gets a few mils of vibration at 52 Hz (3150 Hz). Multi-stage centrifugal compressor (between brg design). Line freq is 60 hz. Motor speed is 1800 RPM. Running speed = 11,500 RPM. Rotor dynamic study checks out with flying colors.
I'm really wondering what could cause this vibration. It doesn't seem to get any larger than a few mils, but the customer wants to know what we think.
I'm going to have the hanging rotor hung and see what frequency it rings at. Maybe ring the casing too? Do you think there is a natural frequency somewhere in the system being excited?
RE: Sub Synchronous vibration - ~30% of running speed
what is the power rating? onshore/offshore? is this a high pressure service? type of compression service (hydrogen, refrigerant, etc) and is gas dirty subjecting machine to fouling, polymerization, etc.? any performance degradation noticed or changes recently by the process control? any insight on the history of the machine? is it drive through casing? could you post a snapshot of the vibration log report? any recent start up/shutdown? maintenance/overhaul? is this a heavy rotor (subject to rotor bow on cold stand by) ?
Just a few questions that came to mind...
By the way, compressor casing is normally stiff so it is normal practice to disregard its impact on rotor-dynamic behavior (i.e. infinite support stiffness).
RE: Sub Synchronous vibration - ~30% of running speed
The customer did add a weight to the balance line for some reason. No one (including customer) seems to know why. History of the machine is bit foggy and since this extra vibration never caused an alarm and we are in the middle of an overhaul, I'm trying to keep my research to our side only. I'm trying not include the customer at this point. We're too far along to make any real changes anyway.
I'm here to satiate my curiosity, if I can.
RE: Sub Synchronous vibration - ~30% of running speed
I think a thorough examination of the lateral analysis report would give an idea whether the machine has been pushed rotordynamically. Which does not mean it is not OK, but that will say where it initially stands and how much room do exist when performance deteriorates or when there is an upset somewhere. For instance you could examine damped response analysis (separation margins, the amplification factors) and the shape of the vibration amplitude nears the picks. If you see a double pick, may want to check also if there is no critically damped speed between the critical speeds and the operating speed. Again this is a good exercise to understand the machine design. Think the other key info is vibration log reports.
You indicated there was no trip/alarm so everything is withing the OEM design settings - why worry then?
Are the vibration levels still in the allowance / limits stated in the API617 - Ch 1 & 2?