how to calculate loads for selecting vibration spring mounts on pump inertia base

[nabilbasha]

For the purpose of selecting the correct vibration spring isolator weight ratings on a 75hp end suction centrifugal water pump, someone told me I should it the sum of:

concrete base + pump & motor assembly weight + valves & fittings weight up to first support/hanger + hydraulic weight.
he defined hydraulic weight = pump head x pipe x-section area !!!

See attached file.

I am not convinced it is correct and would appreciate comments, please.

To re-phrase my question: what are the weights/loads that I should take into consideration when selecting the vibration mounts springs other than the weight of the pump/motor assembly and inertia base?

 

[desertfox]

Try this

see steps 4,5,6

 

[nabilbasha]

Thanks desertfox but i can't see anything!!!
did you attach a file or posted a link ???
where I can see steps 4,5,6 ???

 

[desertfox]

Hi

Sorry about that, must be old age

http://www.civildesignhelp.info/pump.html

 

[nabilbasha]

I find this is great for pump foundation but what I am really looking for is regarding the spring vibration mounts size selection. What weights/loads that I should take into consideration when selecting the vibration mounts springs other than the weight of the pump/motor assembly plus the inertia base base weight?

 

[desertfox]

hi

If you look at the steps I told you, it gives the eccentricity of the masses of pump and motor from which you can calculate the dynamic loads and those loads are the ones you need for the dampers.

 

[nabilbasha]

Sorry to trouble you buddy, and I appreciate your support. The info you gave me is good to design the inertia base and determine its weight, etc plus the Natural frequency so I could also decide the deflection rating of the spring isolator under the base. However, it does not address the total weight/loads the spring would be subjected to.

Note, I am not designing the foundation from scratch but rather trouble shooting existing pump vibration problem.
I have 6 inch suction & discharge end suction centrifugal pump with 75hp motor 1750RPM controlled by variable speed drive. (photo attached). I intend of course to laser check alignment, coupling, bearings, etc but I am suspecting that wrong spring isolator is being used (over sized). I asked the contractor to show me his calculation and he gave me the sheet attached in my first message thread. I have not seen any reference anywhere to include what the contractor refer to as the Hydraulic Weight or the equation used! Or even if it should be considered as part of the weights/loads exerted on the spring. Also, if the weight of the elbow and valves should be considered, because naturally the pipe should be suspended so the pump is not carrying any weights. i.e. the spring should ONLY be sized based on the weight of pump/motor assembly and concrete base. nothing else. Am I over-looking something?

 

[desertfox]

Hi

Usually poor alignment of pump/motor shaft is one the major cause's of vibration and another is poor foundation coupled with not the correct pre-tension in the fasteners,not all installations have vibration mounts I have just overseen about 20 motor/pumps sets installed in a steel plant without vibration mounts.
How long have these pumps been in service? If these pumps have run awhile maybe some of the bolts have worked loose, although they look quite new.
I found this that might help you better:-

http://www.easyflex.in/pdff/latest/Intro to Anti Vibration Mounts.pdf

I'm curious as to why you think the wrong spring dampers are fitted and are causing a problem particularly if there oversized as you say.
Normally there should be no load transferred to the pump inlet or outlet flanges via pipework, so in my opinion the mass of the motor, pump and pump base should be considered. Another point just looking at the photo's of the pumps, on the discharge side, the pipe work support connects directly to the pump base which means that the two vibration mounts at the front see a greater than those at the rear, yet that sheet shows the load divided equal between the 4 mounts.
I wonder if that pipe work support is making the pump base to rigid and preventing the mounts from doing there job.

 

[nabilbasha]

The pumps are new and being commissioned.
I do not think the spring size is correct because I do not believe the contractor method of sizing is correct, hence I need to verify it and find a technical reference to it to support my claim to reject it.

I fully agree with you, normally there should be no load transferred to the pump inlet or outlet flanges via pipework, hence only the mass of the motor, pump and pump base should be considered.

This means the total weight would be only half of the contractor calculation. i.e. much smaller spring. If the spring is grossly over sized then it will not deflect .. it will simply act as it is solid support.

Finally, the pipework is OK except that the flexible connector fitted before the elbow should have been placed after it.

 

[desertfox]

Hi

Here is another site giving guidance on vibration mount selection, looking at the set up though I'm surprised you need anti- vibration mounts because both the motor and pump impeller should be well balanced as received from the manufacturer.
I still think that pipe work support should be moved off the pump base.

http://www.antivibrationmethods.com/product/enclosed-spring-mount

 

[desertfox]

Hi

Another site:-

http://www.fish4parts.co.uk/images/xj/ut/mounts/polybond_mounts_catalogue.pdf

If I can make a suggestion, why not contact the people who made the vibration mounts that are currently on those pumps and ask them how they would select and size them.

 

[nabilbasha]

Hi
The base size, vibration spring mount, etc all comply with almost all vibration isolators manufacturers recommendations as well as ASHRAE standard.

All pumps manufacturers as well as vibration isolators manufacturers suggest say that the inertia base must be extended to support the pipe elbow.

However, all of them talks about the type and deflection of the springs. But I failed to find a reference anywhere to indicate what weights should be considered; when selecting the spring; other than the pump/motor assembly weight.

Should all fluid-filled piping to the first supporting hanger be considered ?
Should what so called hydraulic load be considered ?

I strongly believe, the vibrations isolators manufacturer has never be consulted. The contractor made his calculations; as per attachment; then went and bought the springs from the hardware store. Possibly, cheap import, because he failed to date to provide me with the submittal/data sheet of those mounts.

I posted the same question in the pump activity section, but I guess the admin deleted because of duplication. Well, I did not know that I should not post same question in different forums.

 

[desertfox]

Hi

What I was meaning when I said contact the vibration isolator manufacturer, was that they might be able to provide guidance on how to size them for the given configuration similarly to the links I have provided.
Who is the pump manufacturer because I've worked with pump companies and I've never heard them extend a fabricated base to support pipe work or elbows, nor have I heard a vibration isolator manufacturer support such a claim unless there is something you know that I don't.
Back to the problem, fundamentally the contractor as assumed the weight of all the parts is divided equally on all four vibration mounts and just one look at the photograph you posted clearly shows that cannot be the case.
The only way to do this properly is obtain or estimate the centre's of gravity of the motor, pump etc and then take moments about the vibration mounts to get the loading on each mount, just like those calculations in the links.
If you must support the pipe work on the base then that as to be included in the calculation however your earlier comment about fitting the flexible connector after the elbow is valid because that ensures that only the elbow weight is taken by the pump base, from the picture as it stands at the moment that pump base could be carrying a lot more weight from the pipe work.

 

[rob768]

Some companies selling these mounts can even do the 6DOF calculation for you. Ask them?

 

[nabilbasha]

Kinetics Noise Control, Inc. and Mason Industries Inc. - a top manufacturers and pioneers in acoustical and vibrations specifically documents the fact that Pump bases shall be large enough to support suction and discharge elbows. These are also documented in other manufacturers specifications. ITT B+G, TACO, ARMSTRONG, PACO, PEERLESS, GRANDFOS, etc do all follow the same rule.

I agree with you about the weight distribution and it has to be re-distributed correctly.

I shall approach couple of vibration mounts/base manufacturer and see what would be their response.

If pump manufacturer did not provide center of gravity for the assembly. How can we figure this out?

 

[desertfox]

Hi
If you have the individual details of pump base,motor and pump etc you can calculate it.
The pump manufacturer might give the centre of gravity for the assembly on some documentation because it might have to be lifted as a complete assembly.