Vertical Turbine Pump Foundation 1

[CrabbyT]

I'm working on a job that involves the installation of a 150 HP - 200 HP pump for a new well and I'm trying to understand how the foundation should be designed. For centrifugal pumps, the rule of thumb is that the base should weigh at least 3x the equipment it supports.

Does anyone know if the rule of thumb applies for foundations that support vertical turbine pumps (i.e. well pumps)?

Can someone tell me what weights to consider? For example, I have the following:

[ul]
[li]Bowl weight = 1200 lb[/li]
[li]140' column pipe (10" i.d.) and shaft = 7000 lb[/li]
[li]140' of water inside the column pipe = 6800 lb[/li]
[li]Discharge head = 500 lb[/li]
[li]200 HP motor = 2500 lb[/li]
[li]TOTAL WT (not considering water) = 11200 LB[/li]
[/ul]

3x the total weight is 33600 lb, and now I'm looking at an 8'×8'×4' block of concrete. The problem is, this doesn't come close to reflecting any existing details I've come across. The existing details are more like 3'×3'×6'.

This leads me to believe that maybe I don't need to consider the weight of the column pipe and the shaft. Maybe those two things are heavy enough and long enough that they help to alleviate vibration. Problem is, I can't find any literature to support this.

Thoughts? In terms of vibration, am I only trying to restrain the 200 HP motor (2500 lb)? Or do I need to consider the total weight?

For soil bearing strength purposes, I'm including all the weights listed above (including the weight of the water).

 

[cvg]

total weight as the line shaft extends clear down into the well and the pump is at the end of the shaft
but you should not have any appreciable vibration

 

[GC_Hopi]

Here is a snip from Newcomb. He uses the weight of the reciprocating parts. I would say that is the motor. Also there is ACI 351.3R for "Foundations for Dynamic Equipment" if you need a more info.

 

[JedClampett]

We've had this same conversation at our company many times. I've examined wells designed by our company and many others. No one uses the 3X weight for the base and the pumps don't vibrate. I don't have a good explanation, but if you add in the weight of the well casing and the grout, they're pretty much integral with the base and it adds a lot of weight.

 

[r13]

Does the drawing lists operation speed and range of frequency? If so, instead of using the rule of thumb, I think you should perform a dynamic analysis on the system to optimize the foundation size.

 

[cvg]

rotating equipment is just the motor, shaft and bowls
the rest is dead weight

column pipe and well casing pipe are not included and the discharge header shouldnt be either.

dynamic analysis is a waste of money for a well pump

 

[r13]

Dynamic analysis is a big word carrying too much weight. How about figuring out the fundamental period using the total weight, and sizing the block foundation by comparing with operation frequency.

 

[LittleInch]

What does the pump vendor recommend?

but you don't need such a big block for a vertical turbine.

What's 3 x 3 x 6. W x D x H?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[canwesteng]

How are you going to figure out the fundamental period without knowledge of soil properties? IMO the only reason the 3 times weight works as a rule of thumb is because it must clearly be quite conservative as it isn't grounded in any kind of sound principle, so I usually err on the side of keeping the pads bigger rather than smaller.

 

[r13]

Since the pump is less than 10,000 lbs, analysis can be performed using "Equivalent Static Loading Method". See ACI351.3R for more information.

 

[CrabbyT]

Does the drawing lists operation speed and range of frequency? If so, instead of using the rule of thumb, I think you should perform a dynamic analysis on the system to optimize the foundation size.

Right now, I'm working off an example cut sheet for a pump that they might buy someday. Even worse, the client doesn't know what pump they're actually going to buy, so the calcs I've done are purely theoretical.

I figured that if I size the block for a 200 HP pump with a known pressure head, and if I base the design on the "3x mass is good enough" principle, it should work well enough for whatever 125 HP pump that they intend to buy. I understand the 3x mass rule wouldn't always pan out depending on the sensitivity or the type of the machinery.

All I got from the vendor was an anecdotal observation, something like "the engineer usually just does a 3' sonotube."

How are you going to figure out the fundamental period without knowledge of soil properties? IMO the only reason the 3 times weight works as a rule of thumb is because it must clearly be quite conservative as it isn't grounded in any kind of sound principle, so I usually err on the side of keeping the pads bigger rather than smaller.

I do have some information on the existing soils, but it's not very detailed.
I'd like to see if anyone knows where the 3x rule came from. It seems to work out OK in most cases (ACI 351 points towards it being valid in certain cases) , but I wonder who determined that was a good idea in the first place. Seems like it revolves around the "bigger is better" or maybe the "I dunno, build it bigger" principles.

 

[r13]

I think the 3X rule of thumb is an industrywide open secrete that nobody really knows where it came from, but has been upheld through years of experiences for most (not so complicate) cases. For a single degree of freedom system, I would say the analysis is worth the trouble, provides you have known soil properties on hand (poison ratio, shear modulus...etc). I am opposing the idea that using the parts that produce the vibration only to size the foundation, as the other static parts, although themselves do not vibrate, are the integral part of the machine (system stiffness), that could alter the vibration characteristics. Simply put, we don't know the consequence of ignoring the weight other than the moving/rotating parts, unless it is proven by literatures, which I would like to know/learn too.

 

[WARose]

Actually the rule as I have heard is it is 5X the mass of the machine's complete weight for reciprocating machines and 3X for centrifugal machines. Obviously that gets higher if we start talking components. (10X is typically the number I've heard for the moving parts.)

But based on what you posted, a lot of this stuff doesn't sound like it should be considered as part of the equipment. (Just the wet weight of the motor.)

The 3X/5X rule is just a rule of thumb. Most people don't even do a dynamic analysis when we are talking a 200 HP motor (that is that light).

Also important is the other rules of thumb as far as thickness and dimensions go. (See Arya's text on this subject.)

 

[GC_Hopi]

My understanding is that the 3x/5x rule is based on a FOS of ~ 2 for a single DOF system. This is very rough...

 

[CrabbyT]

We've had this same conversation at our company many times. I've examined wells designed by our company and many others. No one uses the 3X weight for the base and the pumps don't vibrate. I don't have a good explanation, but if you add in the weight of the well casing and the grout, they're pretty much integral with the base and it adds a lot of weight.

I've thought about this post a lot, and based on other posts in this thread, I'm glad to hear that this problem has some ambiguity around it. I have a hunch that the reason smaller bases work for vertical turbine pumps has a lot to do with the well casing.

My hunch is that if I had a very big, rigid cheater bar, it would be easier to overturn the foundation and the pump than it would be to twist it.

I haven't done the math to prove it, but here's my hypothesis. Vertical pumps don't rock or pitch. They just twist. The twisting would translate to shear in the anchor bolts, which would transfer to the foundation and the casing. For a 150' long column pipe, there should be a lot of surface area/skin friction between the casing and the soil. Based on some gut check calcs, my hunch is that the lateral resistance from skin friction would be substantially larger than weight of the base and should be able to resist whatever vibrations that would occur from twisting (assuming the anchor bolts are sufficiently large to transfer the forces). Hence,

the pumps don't vibrate.

My hope is that I'll be able to take some time to read more of Suresh's book.

 

[cvg]

dont forget the sanitary seal. grouted casing which extends from the surface, down to 5 meters below ground. this is intended to prevent surface water contamination and also provides a significant amount of vibration resistance...