Monitoring Vibration On Ammonia Screw Compressors

[bobhousejr1]

I have been told that the only way to monitor vibration on an ammonia screw compressor was to collect data at 100% and 0% loads. What do you thing about this? Also, what is the best fmax to for collecting data at thes loads?

 

[VibraSense]

I would think that any load is a good point to measure the vibration as long as this load is repeated each time that the vibration is collected. However if this is not possible the "random" collected data trend will ultimately establish a Dead band (high and low overall vibration) levels boundary. It will take time but this will point towards the "definite change" where the closer look (analysis) will be necessary. A good approach, if not sure, is to ask the local operator something like: "How does it sound to you"? I found that these inputs are most valuable. A published standards and severity charts should not be neglected.

 

[bobhousejr1]

Thanks for your response. I do have one more question. We monitor these ammonia screw compressore 4 times a year for a certain company. While it is pretty easy to determine the lobe pass frequency in these units, we have no access to the bearing model or manufacturer. Our customers do not know the bearing numbers and the compressor manufactures will not give them out (they only give part numbers) so we cannot identify the possible fault frequencies. One month we collected spectral data out to about five times inner race frequency of some of most common bearings that may be in the unit. There was quite a bit of nonsynchronous energy on the spectrum in between the lobe pass frequencies -(we covered three times this frequency). We told the customer that due to this much nonsynchronous energy on the spectrum, there may be a problem with a bearing. Three months later we checked the unit again and all that appeared on the spectrum were the lobe pass frequencies 1, 2 and 3 times. All nonsychronous energy was gone! The customer had done nothing to the compressor since last check. It was the same unit. Can you explain what might be going on? Is there something we can watch for to be able to tell our customer if there is truly a problem? We have also had compressors fail that showed no indication of a problem. Thanks for your help.