Have a customer who is purchasing some new baghouse fans and one of the vendors is proposing using a pair of prox probes at each bearing. While this is certainly desireable, there will no doubt be extra expense involved. And since none of the other 9 fans presently in service have them, there is some question as to whether or not they are needed. These fans will be about 1500 hp and run 700 rpm with Dodge Sleeve-Oil bearings (or equivalent).

We don't really have any problems with the current fans. We balance them to 0.5 mils (pk-pk) at the end of each shutdown (2x per year) and they are cleaned when they reach 4-5 mils. We have a casing mounted accelerometer mounted on each bearing. The balance spec quoted in the proposal calls for 0.1 in/sec, which is 2.7 mils @ 700 rpm.

I check these fans monthly (walk around PdM) and will be checking the new ones as well.

My question to you guys is, do you think the X-Y probes are needed? Is the extra expense justified? Personally, I'd be fine with them as they'd give me a nice platform for "research" (the only machines I monitor with probes is Centacs, and they won't let me 'play' with them). But I question if they will be worth it, and if they will just confuse my client. But what do you think?

This message has been edited. Last edited by: rustythevibeguy,

Rusty
I don't know if they can be cost justified but I like them on sleeve bearings. I like to know what the shaft is doing and how much clearance it is using. I always do a case reading and readings off the probes and rely a lot on the probe findings.

When sleeve bearings:
High speed machines (N >> 1000 rpm) usually have a low rotor mass/inertia to high casing mass/inertia ratio and therefore shaft vibration relative to casing can be much higher than casing vibration. Proximity probes are therefore recommended. Also, these machines may suffer oil film or critical speed instabilities that should be captured by proximity probes preferably.
For low speed machines (N << 1000 rpm) it is all the other way round: imo, accels on the casing, right?
Arie Mol, NL

I've seen shaft relative probes on hydroturbines, which are low speed. I don't know why low speed is an issue.

The transducer measures (when properly positioned and installed) relative vibration between the shaft and bearing. This will be at least one of the measurements desired.

Orthogonal probes look at the planar motion in the bearing.

Do you need the casing measurement? Maybe, you have some base cases to decide. Housing vibration is not the same as shaft relative vibration; a simple velocity to displacement does not usually suffice to give alarm and shutdown criteria.

I like to have them on fans. If you track shaft centerline position you will see a bearing wipe if you've lost lubrication.

I think in the cost of the fan, probes are a pretty minor expense.

As far as costs go, there's a big difference between having the hardware for periodic monitoring (walk around) compared to online/continous monitoring that includes a rack.

At a minimum have proximity probes mounted and route them to a local junction box where you can power up the proximiters with a portable 24 volt battery pack and collect routine data along with your case readings. This is a very cheap intermediate solution and will allow you to see relative vs absolute vibration. This will come in handy when you balance your machines.

I appreciate all the comments. Experience with these and similar fans has proven (to me at least) that you really don't need prox probes to monitor or diagnose these machines because they are just so simple. They are well aligned; have large, well-maintained and lubricated gear couplings; use proven Dodge Sleeve-Oil bearings; are checked regulary by a dedicated lube guy; are inspected, repaired, and balanced twice a year to a level 1/4 of that recommended by "the standard".... there's just not much that can go wrong.

I take a route reading on one cooling water line on each bearing which tells me when the plunger bolt needs to be torqued. They are cleaned as needed. If we're doing everything that we should be doing, it just doesn't take much.

Still, I will probably recommend they go with the prox probes. After all, it's not my money is it?

Rusty
Yes I agreed, especially they should buy probes from you.
I have 3 hot gas fans and 5 main air fans for baghouse, fan supported by sleeve bearing with velocity probes attached on bearing housing (original manufrature design).When probes fail or mis-reading I did not repalced but thrown it to Electric Arc Furnace, now have 10 probes from 16 probes.
still OK, collecting vibration data every month with portable and Acc.probe from bearing housing, I could detected unbalance (fan worn out) and sleeve bearing looseness (bearing worn out). Spectrum shown low amplitude but as long as problem growth numbers of hamonics was shown, trend was grown.

Regards.
Phoo

quote:

As far as costs go, there's a big difference between having the hardware for periodic monitoring (walk around) compared to online/continous monitoring that includes a rack.

Whether or not to monitor or what type of monitoring is an additional question.

Putting probes in while new is superior and cheaper than doing a retrofit.

I've heard arguments like this for not needing the right equipment since I've been working with vibration and rotating equipment. Nuclear plants would and still may argue that they "don't need no stinking probes; the shaft riders work well enough."

At least one large manufacturer still provides shaft riders as their standard option on large steam turbine-generators. Does it make it right? Of course not!

ISO standards for machinery protection cannot rule out protection equipment (methodology) that has 'proven to work.' This means if a manufacturer provides it it must protect the equipment. Here we are talking more than machinery protection, condition monitoring.

The equipment manufacturer that provides the shaft riders (and most or all will provide relpacements)had a well respected rotor dynamicist who is now a professor work for them for 25 years. I didn't gather that he believes that this is the way it should be (in an ideal world of course )

If you have little or no experience with measurements using shaft relative measurements this puts you in a poor place to decide on this. I have found many international 'experts' have reduced experience with shaft relative measurements compared to the US, in particular oil and gas and petrochemical with power gen people not far behind (although perhaps with less variety in equipment, but bigger stuff [Is bigger really better? In some instances, yes!]).

Well, this rant goes many places if any place.

Bill,
I am really torn on this one. Always been a firm believer in using anything and everything I have in the way of readings to diagnose a problem, and a set of prox probes through a permanent monitor would be really nice for protection. And, as long as Rusty isn't paying, and the customer wants to spend the money, hooray!
That being said, consider that this is a very low speed machine and the rotor-to-housing weight ratio is very low, there isn't much else to learn from the prox than from a housing mounted accel. With the money that this system will eventually cost, I would opt for accels on all of the fans brought into a monitor/alarm panel. One transducer per bearing, one power supply for all transducers, etc. Much more bang for the buck.
What do you think about those considerations?

Rotor to housing weight ratio, I take to be rotor weight/housing weight - Low ration seems like place to use shaft relative, perhaps without seismic in the case of journal bearings. Then one needs to decide what 'low' means (Some politicians probably could answer this.).

Heavy shaft and light casing: everything may be moving, but nothing in nature dictates that they must move in phase. Light can also imply flimsy support (very vague termonology expressed here). Both relative and casing transducers may be needed.

As for low speed, this is the real of the displacement measurement.

I have looked at installations that had probes but no power to them for periodic trending.

Well it is just a bag house fan. Spare capacity? If the fan is down, nothing is lost because of a spare? Or does permitting require it to run? These points go into deciding the need for a monitor.

Do you use bearing temperature measurments?

There are many questions and answers for the consultant to ask and answer to aid in decisions for monitoring and protection, not necessarily the same.

If it is cheaper to run to failure, maybe that is the desired strategy. This may eliminate the need for periodic trending if this is the case.

I have certainly seen the reaction numerous times that the machinery didn't have any vibration problems before installing shaft relative probes. All that was needed was routine maintenance - you know maybe a few bearing replacements. Properly designed and operated fluid film bearings can last the life of the machine; they need not be a 'wear item.'

Just to elaborate on Erik's comment of 24 Vdc supply - make sure you wire it for negative 24 Vdc. +24 Vdc will fry your proximitors. If AC nearby I would use a din rail mounted 24 Vdc supply in the junction box that operates on 115/230 vac. Can get small 0.2A supplies from Omron and others. Happy to supply the BNC j'box and PS.

As an option or service one could carry around pre-wired oscillator-demodulators on one's tool belt. This way only the probes and cables are needed (Eliminating the extension cable could lead to issues.).

I have carried around the wired oscilators before, but I didn't have the big tool belt that some have .

As far as "shaft relative" goes, when we balance these fans to 0.2 mils using a bearing casing accelerometer, I'm pretty sure that nothing is vibrating very much. Current fans run 900 rpm and the new ones will run 700 rpm.

So, the casing is moving 0.2 mils with no measurement on the shaft. If the supports are flexible, this may be the case; otherwise, how can you tell? Modal analysis or modelling may help, but a direct measurement is easiest and cheapest.

0.2 mils pp at 900 cpm is 0.002 g 0p. At 700 cpm this will be 0.001 g 0p. At 100 mV/g this is 0.001V. Not bad.

Other than this being in the noise this would be 40 mV (.04 V) for a standard 200 mV/mil shaft relative probe, and I would not [left out a crucial word here - probably >> 0.2 mil pp shaft relative vibration]be surprized to see greater shaft relative vibration than this.

This message has been edited. Last edited by: William_C._Foiles,

Certainly there'd be more than 0.2 mils movement of the shaft, but I suspect it still wouldn't be too much. If you get the bearing casing on a 10,000 lb. rotor down to levels this low, then for all practical purposes, it doesn't matter what the shaft is doing.

Of course Bill will disagree.....

I've carried board mounted drivers too! And took cap readings with very hi-res transducers w/sens @ 1 V/g. One must be out of the noise floor and your standard run-of-the-mill transducers doesn't cover all cases. Can you have low cap readings while the rotor is out of phase and what is the damp factor?

And of course as Bill eluded to: new bearings every 5 years could be cheaper than monitoring cost; or??????

quote:

If you get the bearing casing ... this low, then for all practical purposes, it doesn't matter what the shaft is doing.

Why? I am not sure that I understand this, or perphaps the point isn't well explained.

Does it matter how much the shaft relative vibration is? Or, do you just think that the shaft relative vibration will be low, because the casing measurement is low?

The way I look at it, if this large 10,000 pound rotor at 700rpm were far above it's critical, then since it is "mass-controlled" it does not rely on spring force to restrain motion and could move a lot within the bearing without transmitting any force to the bearing and without showing up on housign vibration.

If it were below first critical, then force is transmitted through the bearings. Since the frame doesn't sound excessively stiff, we would expect to see rotor movement show up in housing vibration pretty well.

Does that sound reasonable? If so, where would we expect this machine to be operating in relation to its rotor critical?

This message has been edited. Last edited by: electricpete,