Standards for machining pump motor mount
I have to machine a centrifugal pump base that is bolted and grouted in the plant. The base is too large for me to cut with one setup, and I'm just cutting the part that the 50 horse motor sets on. I'm planning on using a #199 level to verify the taper per foot of the finished pad.
System engineer will not give me a tolerance. Period. "By the way, how fast can you do it? That pump needs to be in service." #$%^&*((*&^%$%^&!!!!!
Is there some kind of national standard for this?
I'll mill the two pads on the left of the picture.
Each hold down bolt on the motor will get a set of 3"x4" shims.
Not sure who you are working for or what the pump is for but the first reference I could find was API 610 "Centrifugal Pumps for Petroleum, Heavy Duty Chemical, and Gas Industry Services"
"3.3.3 Mounting pads shall be provided for the pump and
all drive train components. The pads shall be larger than the
foot of the mounted equipmentot allow levelingo f the baseplate
without removal of the equipment. The pads shall be
fully machined flat and parallel. Corresponding surfaces
shall be in the same plane within 150 p d m (0.002 in./ft) of
distance between the padsT. his requirement shall be met by
supporting and clamping the baseplate at the foundation bolt
3.3.4 All pads for drive train components shall be machined
to allow for the installation of shims at least 3 mm
(0.12in.) thick under each component. When the vendor
mounts the componentsa, set of stainless steel shimast least
3 mm (O. 12 in.) thick shall be furnished. When the vendor
does not mount the components, the pads shall not be drilled,
and shims shall not be provided."
Thanks Gas Ax,
Any standard is better than walking into that meeting tomorrow with my %^&* in my hand.
Do you know more about the motor and pump combo you will be using? A pump with packing can tolerate a bit more misalignment then a sealed design. If you know the model of the pump and driver, the manufacturer generally has an install manual that would point you in the right direction (tell the damn engineer to get you this )
Generally you level the pump, then set the motor to height by shimming, so the motor base has to allow the motor to sit lower then the pump CL without shims.
I'm used to working with a lot bigger pumps that don't have a shared base with the motor, so I cant provide you a good frame of reference. It wouldn't surprise me if something that small was leveled with a plain bubble level like a Starrett 135.
Edit: Gas axe beat me...
Last edited by DanP_85; 07-20-2011 at 02:43 AM.
Generally, the pump and the pump base goes in first and grouted or chocked into final position. Then the pipe fitters connect the piping to it ensuring the weight and strain of the piping imposes no significant load on the pump's connections. Pumps may look very sturdy but piping made up without regard for the alignments of the final connections can impose loads measured in tens of thousands of pounds on the pump's flamges. Thus piping loads have destroyed pumps in the past. For that reason many outfits make the last two piping connections the responsibility of the millwright or the machinist performing the motor to pump alignment. Once the pump is installed the motor is installed, aligned, and coupled to the pump.
Generally the motor pads on the pump base are machined in plane parallel to the pump axis but 1/16 below the "D" dimension on the NEMA motor frame chart. This 1/16" is shim allowance. NEMA Motor Size Chart If the installation is metric the motor may be made to the IEC stardard http://www.electricmotorservice.net/iechart.pdf
The motor is rough shimmed and the pads marked for drilling if that hasn't already been done. After the final shiming and alignment and the bolts fully tightened, drill and ream two diagonal motor feet and pump feet for taper pins. A 50 HP motor motor/pump combo should be located with #5 taper pins.
The final alignment is verified after the piping is connected and the strainers bagged for FO mitigation (you can't believe the damage a 3/16" Allen wrench or gasket punching can do to a $30,000 double suction pump.) Only then is the motor bumped for rotation, the system filled and primed, and the pump tested.
This set of pumps has been running for 20 years. Upped the motor size and put in different impellers for more flow. Now the vibration is greater than specs, so this engineer thinks that if we machine the base PERFECTLY flat, the vibration will go away.
He will not discuss pipe binding, because fixing that would take the pump out of the system for too long. ALTHOUGH, we did cut and re-weld both intake and outlet on the first pump in the series. THAT pump is running to spec.
We mount 50HP - 100HP motors all the time and I can tell you that the cast feet on the motor are NEVER parallel with any degree of consistency or accuracy relative to one another or to the motor shaft.
Shim stock and a flex coupling is all you need to solve that problem.
I highly recommend Quick-Flex couplings for this kind of duty.
Quick-Flex Elastomeric Couplings - QM Bearings - Built to Last
Soon after I retired I free lanced for a couple of engineering outfits doing plant and systems field work. One local water utility had a line of pumps in their treatment facility and they were suffering early case an wear ring failure.
I spotted their maintence man as knowledgeable and their engineering manager as a dolt. I looked at what they had, listened to their separate stories and diagnosed the problem as piping strains. These were 24" inlet 20" outlet 200 HP pumps - not big but not small either) I had the maintence guy break a suction connection the enginering manager seemed to think sacred and as the bolts came loose the flange opened up. These were 1 1/4" bolts but the nuts turned like they had significant load on them for some distance until the piping strain finally relaxed. The flanges were an inch or more out of alighment this was on big pipe with short runs and little flex. Even 1" thick iron castings will flex under this kind of load.
I went to lunch and wrote my reccommendations and an invoice for $580 (time and travel). I said (paraphrasing here I can't find my copy) at successive planned outages to loosen three connections starting from each pump suction and discharge, re-gasket and re-connect snugging up the joints collectively to minimize strains on the pump castings, then check pump to motor alignment and correct if need be. I delivered this to the PUD commissiner who looked at it. "Isn't this what my maintenece man wanted to do?" Yes, he knows his stuff. "How about my Engineer, he forbid what you want us to do." I looked at him. The commissioner said, "Should I fire him?" I said nothing. He smiled and said: "Well thank you" and he shook my hand and escorted me cordially out to my ride. I got a check and a thank you note.
By separate cover I got a nice note from the maintence guy thanking me for the support. What support? This aint politics. It's technical colutions for technical problems. Politics is for when there are for people problems when no clear solutions just dubious remedies.
Machining the base flat and doing a careful realignment isn't a terrible idea, and will help rule that out as a cause (if it fixes it, great!), but the fact that the impeller was changed leads me to believe that there are other factors at play here.
Originally Posted by i_r_machinist
Did this same engineer source the new impeller? My guess is that the pump is now running significantly below or above its best efficiency point causing excess vibrations. Other things to look for could be an unbalanced or out of tolerance impeller, hydraulic forces (were the volutes changed to match the new impeller?),or piping issues as you suggest.
BTW I like the Quick-Flex couplings too. For bigger stuff Shaft Couplings at Coupling Corporation of America the Flexxor's and UltraFlexx's kick ass.
What Forrest said is (as usual) right on, especially the part about plumbing strain. You have to relieve all stress on the pump for it to operate properly. The pump itself is nearly intolerant of ANY misalignment, packing or mechanical seals. That is what the coupling is for. Even when working with couplings having wide tolerances, we shoot for as close to no runout as possible. Under .001 on the faces and a couple of thou axial is about all we will settle for. Anything more is asking for trouble.
We build skid mounts like this from scratch ranging from a 10hp or so up to over 600hp, diesel and electric drives. If the motor and coupling is properly leveled and aligned, the relative flatness of the actual skid rails is not super critical, as long as it is within reason. If you have a bad vibration after putting on a new motor and/or impeller, I'd be looking at the new stuff instead of trying to flatten the skid.
Was the pump completely rebuilt when the new impeller and motor were installed? How close is the coupling runout? How does the motor sound when uncoupled and run alone? Do you have a chart for the condition the pump is now running in with the new impeller and motor? Can those conditions be verified as actual and not just what somebody is hoping is going on? Is this pump running on a VFD?
State of the art Rota-line lasers. Soft-foot indication. The two shafts are in alignment to within .001".
Flexible coupling. Don't have the name.
I put the brand new motor from Teco-Westinghouse on our granite surface table. You could see the gaps. .004" in 4" was typical of the 3 motors I checked. Engineer was shocked. He went back to Teco and bought a higher grade of motor that is supposed to come with documented vibration numbers in a free running state. I'll check that motor when it gets in.
Current uncoupled motor is "just inside" of specs running uncoupled. I have no way of knowing how the internal pump changes are effecting vibration.
I have a letter here where the engineer is calling for .0005" taper per foot on the base. I guess it will be an interesting meeting today.
I'll let ya'll know what happens.
Originally Posted by Mike C.
Lets look at that!
We had a special guest at the meeting. Rep from Gould Pumps nuclear division. He gave a number of .005" taper per foot was considered good on the base, and that pipe strain was the most probable culprit.
I was a heart beat away from jumping out of my seat and shouting "F*^k YOU! at the engineer.
+1. The flatness of the motor base is insignificant. It will be on 4 shim stacks anyway. A flex coupling should not be necessary beyond a grid segment.
Originally Posted by i_r_machinist
You might machine it and warn beforehand that this is not the problem. Something is up in the pump and I would contact the manufacturer. If the new impeller is not from the OE; then put the problem on the impeller manufacturer.
If it ran smoothly before the impeller change then the base is certainly not the problem. Unless your laser has a problem, it's not alignment. There is a very slim chance of a motor bearing that only shows up under load. Check with vibration analysis or a stethoscope.
That's why I asked what all was replaced when the new impeller went on. Pump bearings could have gotten beat up during a ham fisted impeller swap. Ditto the seals. Could be cavitation, if the NSPH is too low with the new impeller, which is why I asked about the chart. Could be turbulence issues if the bigger impeller is being fed from an elbow, instead of the required straight run into the inlet (seen WAY too many elbows right on the inlet). Good thing the pump rep put it on the plumbing stress (and they always do... lol. "Not our pump, it's your plumbing."), at least he got you off the hook having to machine flat within .0005, in situ. Gotta love engineers.
I usually machine pump bases a planer. Try for .002 tolerance on machining the base. The pumps here are mounted on the bases with at least .125 shims and motors are shimmed to height. Pipe strain is limited to .002 movement in the horizontal and vertical after bolts are loosened. I have seen elbows located to close to inlets and discharges. Cheap bearings and faulty installations from alignments to base problems and people using aligned machines as come along tie off points to pull other equipment into position.
My pump engineering manual recommends at least eight ( 8 ) diameters of length before a change of direction on piping. Given different companies have different standards. Rpm tightens the alignment tolerance. Instead of fighting the motors faults you can shim for them or machine the feet.
You can realign with a laser or dial indicators (rim face or reverse methods)
I have seen lasers do weird things the CSI or Rotalign Ultra will give interesting results.
Sounds like an identical job I did a while back.
‪Mattison Grind Pump Base Weldment‬‏ - YouTube
This base was grouted in and after about $600,000 of pump rebuilds the power plant builder decided maybe the base was off. In place machining was too expensive so they busted it out of the grouting and sent it to me to fix. I first ground the bottom to get a good bearing surface then the top. The shorter surface was milled.
From what I remember they wanted it as close as possible. I think it was about 10' long and 24" wide. Overall the 2 top surfaces had to be withing about .020".
There was as well a specific dimension for the step between the 2 surfaces. But they did not know that so they told me just average it out and do what you can.
Which for coming off a grinder then a mill was no issue.
Since doing this I have heard of no problems when they got everything back together.
I'd love to find a big planer to do exactly as you say. Hard to come by around here.