We have a large amount of pumps that we monitor at our facilities. I am new to the plant and do not have a lot of experience with pumps. Could anyone give me some pointers on what to look for and how to element or reduce cavitation and high VPF? Any help would be greatly appreciated.
I work at a large papermill with hundreds of pumps. I chased the vane pass issue and cavitation for a while when I first started doing vibration analysis. This is what I have learned..every pump is different, the product the pump is moving has an effect on vpf and cavitation, The design of the piping in and out of the pump effect this as well as temperature. I have learned to go by the history of the pump rather than the reading. If vpf normaly runs at .2 in\sec and jumps to .5, I know I have a problem. Normaly it's caused by pluggage, a valve not completely open, or another pump pushing more into a pipe than the pipe can handle.(this I see too often). In some cases I have found the the design of the pump system was incorrect from day one. If vpf slowly over time increases, you can assume the impeller, suction plate, or casing is wearing out. Cavitation usualy occurs when the pump is starving. ex: tank level low causing air to get in, viscocity of liquid changes or is aireated, impeller to big or incorrect for process, and of course valve position effects this also. Flashing in a pump is caused by a hot liquid breaking its boiling point when exposed to the vacuum side of the pump. I see this alot in my condensate pumps. It sounds like the pump has bolts going thru it. The impeller design itself will effect your readings. I have seen vpf jump way up from normal because an impeller from a different vendor was used. It would be great if we could eliminate this "process" type vibration, but it seems to me that the only way to reduce this would be to let a computer manage each pump and adjust itself (speed, valve possition)for demand. Remember....go by the history of the pump. If your history tells you your vpf is always high, and this vibration is shorting the pumps life, you may have to re-engineer that system. Hope this helps.
I use almost exclusively the trending capabilities of my Vibration Analyzer Software. I use to a lesser degree the parameter profiles( the frequency bands where I expect trouble) For a list of industry standards for alerts and warnings (if you use that stuff) go to a search engine and type in ISO 10816-3 Good Luck
Thanks for the replies to this post they have been very helpful. After I post this I did a little more research and found a very informative site McNally Institute (http://www.mcnallyinstitute.com/).
We had a problem with cavitation reciently . We inspected the pump and found that the line from the discarge leading to the packing housing was disconnected. The discharge was used as cooling medium and to create a possitive presssure in the packing . Packing was fitted in the slot where the cooling water had to go in.This caused air to be sucked in through the packing causing the pump to cavitate.The process controller could actually control the cavitation be adjusting the valve(throtteling). However the cavitation then became recircultion with high vane pass frequencies present in spectrum (high Net positive head)and was felt in the condenser tubing.
Hope this would help in your search.
All pumps have some cavitation or vane pass vibration. The amount depends on where on the curve they are operating. If you want to know if you have too much you must first take pressure readings on the pump suction & discharge. This will help you tell if the pump is operating on the designed curve or not. If it is not, you must find out why it is not. If the pump vibration has increased from normal then something has changed. Locate the change and start there. If the pump is a new installation you may want to get engineering involved to verify that the complete pumping system (pump & piping) was properly designed, built and installed. In a over simplified explanation, vibration due to pump cavitation is usually caused by starving the pump suction. Pump vane pass vibration is usually caused by excessive pump internal recirculation caused by a restriction in the pump discharge piping. We had a maintenance manager who thought it would be a great idea to put the maximum diameter impellers in pumps as they were rebuilt. Needless to say we had all sorts of cavitation & vane pass vibration problems. We finally convinced him that we needed to return to the designed impeller diameters. Once the impellers were replaced we no longer are having vibration problems caused by cavitation or vane pass. Beware of pumps being used on multiple systems. They usually operate fine under one operating condition and vibrate heavily at vane pass under the other because they are usually designed for the high side system and experience severe vane pass vibration on the low side unlessthey have a recirculation line to bleed the excessive flow/pressure off.
|Powered by Social Strata|