The following is a Reliability World Case Study from Noria mailed out today. A quick read of the following might make it sound like a sucess story.....

"Vibration Analysis Reveals Poor Motor Mounting Structure

"At the installation of a new 150 gal/min centrifugal pump driven by a 125 HP motor, high vibration levels were found immediately upon first start. After several days of alignment checks, pump teardowns, bearing inspections, motor uncoupled runs and vendor technical support, no solution was found. The plant maintenance manager asked the company's vibration analyst to take readings to troubleshoot the problem. After measuring extremely high vibration levels, resonance was suspected and was found at the motor support structure due to inadequate mounting design for the new installation.

"The company's vibration analyst took readings on the horizontal pump/motor unit and found extremely high vibration levels focused in the horizontal direction at the motor bearings on the order of 1 in/sec velocity with only 0.2 in/sec vertically. Directional resonance was immediately suspected.

"The analyst shutdown the unit immediately and performed a visual inspection of the motor support structure. Twin "C" channel beams were set to the foundation parallel, supporting one front and one back foot on the motor on either side. No cross support was present and the six inches under the motor was wide open.

"Seeing this, the analyst performed a simple bump test with the unit off and found a natural frequency at 3,600 cycles per minute in a horizontal direction on the motor. The running speed of the motor was 3,585 RPM fully loaded, which provides the driving force necessary to excite this natural frequency resulting in a resonant condition. The bump test involves placing a vibration sensor in the direction of interest and exciting the system with a three-pound rubber hammer to excite random vibration in the range of motor/pump driving frequencies. A good kick will do the same thing.

"The vendor-designed motor support was then temporarily stiffened through the use of a porta-power ram and the unit was started to confirm this resonance condition existed. The resulting vibration levels in the horizontal direction dropped to 0.085 in/sec, confirming an inadequate support design for the motor.

"The final fix involved welding bracing across the "C" channel at both the front and back feet areas of the motor, creating a box support. This resulted in an even further reduction in vibration levels and eliminated the resonant condition after only three hours of troubleshooting and repair, compared to the three days of repeated teardowns and reassembly upon initial installation due to high vibration.

"The lack of detailed specifications for installation of new machinery provided to a vendor permits poor installation designs which have repeatedly resulted in resonance situations and early failure of associated components in numerous installations.

"Acceptance testing of all new installations and repairs by vendors is an integral piece in the care of company assets which, though not in place at this time, was used by plant personnel who refused to accept the extreme vibration felt by hand and requested vendor action.

"Our company is now actively pursuing new installation guidelines (on the order of API Recommended Practice 686), repair guidelines with root cause analysis findings and acceptance criteria on new and rebuilt equipment and installations."



Sadly, I expect this is a typical situation. High vibration levels were noted on startup. Several days worth of testing and work was done, including alignment checks, pump teardowns, etc., etc. Finally, after 3 or more days of screwing around to solve a *vibration problem* the resident vibration analist was called in to have a look! After several days, after tearing down the pump, after removing the coupling! Several days of wasted time, effort and money before the vib guy was even called! I'm sure it took him less than an hour or two to determine what was causing unacceptable vibration levels and to determine that a resonance was involved.

To me, this is a major disconnect. Vibration problem ==> vibration guy. If the vib guy isn't near of top of the list of people called when there is a vibration problem, something just isn't right. Is the vib guy on an ivory tower and too important to disturb? Or is he stuck in a dark closet somewhere, all but forgotten?

It's our intention and goal that vibration analysis be part of the routine maintenance work, and taking vibration readings should be a no-brainer when the comments include high vibration.

How common are events like this?

Jon
Spintelligent Labs

I have to say, the mill I used to work at had an established program and the area maintenance foremen and project engineers came to rely on the vibe guys and a few of the trainers to sign off on contractors' work during outages, startups, etc. Not to say it was perfect or foolproof, but it was usually a success.

It's interesting to see the employees at the sites I'm working with now become more aware of their own capabilities and the different technologies. They've hired some good people and have support from upper management--it's a big hill to climb, but they're doing pretty good. Just last night, one of the sites asked me to hang around while they tested some refurbished fans. Tomorrow I'll probably spend most of the day with a cooling tower vendor to help solve a resonance (we hope) problem.

I'm an outside contractor who was called in this morning on a similar problem...

The customer I was working for was an 'in house analyst' who, unfortunately has most of his time occupied with other activities (you 'in house' guys probably know what I mean!).

The machine in question is a hoffmam blower with a gear coupling operating at 3600 RPM. The horizontal vibes were approx. 1.8 ips while the vertical and axial vibes were below .2 ips. No need for a bump test! It took about 5 minutes to identify the problem and about 90 minutes to convince the maintenance staff. The crowbar worked, but I guess the portapower test was more scientific. After better securing the base we reduced the vibes to .5 ips (not great, but better!). At this point our plans are to replace the coupling, realign (cold, then hot), and reevaluate the vibes. I still suspect resonance, but the time and $$$ doesn't seem to be available at this time to make permanent corrections.

When I returned from the jobsite I had a forwarded email from the guy I was working for. The same email Jon and many of you received. My customer's response... "Strange how I received this mail minutes after you left..."

Thanks for mentioning this Jon...

"It's amazing what can be accomplished when no one cares who gets the credit."
~Harry S. Truman

Jon,
I was recently called on a problem almost exactly like the one you posted the article about all the way down to the I beams under the motor. Like Michel, I do contract work and was called as a last resort. The customer had first turned to the pump repair people and the motor repair people who ended up pointing fingers at each other. It was pretty obvious what the problem was. Ten to one ratio horizontal to vertical amplitudes, bump test confirmed resonance. The fix was also the same as the article described. I don't know about the rest of you, but it seems like I have been seeing more resonance issues lately. Anyway, this particular job was kind of like a balance job, it was an easy fix and made such a difference that all parties were convinced that it was the right fix. Not all analysis jobs are so obvious or convincing.

Thanks,
Ronnie

Poor base design is perhaps the most common problem I see, be it pumps or fans. My question is, why don't the manufacturers just do it right before they send it out? Don't they have a vibration guy on staff?

I used to run into 10:1 ratio situations but bump test did not confirm a resonance, not even close.

Any similar experience?

Best regards,
David

Rusty,

Very simply put-money.

David - I have not run into it but one though is there may be several different modes of motion with different frequencies and you may not be exciting the same one during bump test as excited during machine operation. For horizontal vib on horizontal machine if you hit machine in the middle you would exite normal rocking motion. If you hit it on the end you might be able to excite a horizontal motion pivoting around the center of the machhine. Then again this may be a tough mode to excite with a bump test without being able to apply opposite bump at exact same time on opposite end of the machine. A way to check for this type of horizontal pivoting motion (difficult to create in bump test) would be looking for 180 phase shift with the machine operating

"don't they have a vibration guy on staff......?"

Puhleeze.........and right after they hire one, pigs will take up aviation......

Sorry, cynicism showing again.....

Jon - that case is very typical, it happens here all the times. When the enduser complains after all is paid and vendor wants to sell heavy crack repairs within a year, the buying process simply failed. All too often the project people are in full action with next cost black hole, I mean flashy project. It would be good to force project people to run the machines at least three years before they can go on to next.

I do not like the API mentioned, too vendor friendly. The swedish SSG3030 is more neutral covering standard machines for balancing, installation with resonances, levels and alignment. It is also available in english, unless you prefer swedish maybe :-)

David - on finding a resonance, I agree with Peter. If you do not find it right off, apply a bit fantasy kicking. Softer impact? Foot, fist or steel hammer gives very different frequency range of energy bump. Direction: on foundation a 45 degr angle can sometimes help to excite twisting modes. A hint is to hit and measure where the level in operation was high.

I had that situation on a fan motor.There was no support under the motor at all.It seemed they just slapped up a I beam square and laid the motor on it.
I was getting readings of 12-14mm/sec before I
added a I beam in the middle of the base.It cut
the readings down to 1.2-2.5 mm/sec.You could almost see the motor undulating before but now it's rock solid.

Strange how few plants use vibration testing as a sign off check prior to acceptance of equipment. If they did, some of the more dodgy vendors might get the message and do it right.
And the plant might find themselves not paying for installation cockups.
Also strange how hard it is to find someone on a greenfeild site to sell the idea to.
If it was my money I would hire my core maintenance crew before I spent one single cent on plant, and I am confident I would get my money back in the first week of operation.

quote:

You could almost see the motor undulating

.... take a strobe light and tune it about 100 rpm below the running speed, and you actually can see it undulating. With the strobe light on it, you can sometimes use a digital video camera an take a movie of it.

The Poor base design has created all these problems. I also have some problems in the existing base plate frame and i want to modify the design.Where can i get a good literature on th base plate design. I have seen some of the base frames are grouted and some are not. If grouting helps, why do all base frames are not grouted.

Hey, look at it on the other side of the coin. These guys had spent days and days of "getting nowhere" work and were certainly getting close to throwing up their hands. Then the bright idea to acquire the services of the resident vibration analyst was probably even brought about in the mid stages of giving up... Then the problem was apparently accurately diagnosed and the problem remedied. To the guys that had been hammering away at a pointless solution, the vibration guy just earned a Sams Club membership load of Kudo's...