Gate Ch Theron

7/30/2019 Gate Ch Theron

1/5

GaTechTheron(Mechanical) 21 May 12 10:29

A few days ago, a rather basic question was posed in which I did not have the best

explanation:

"Is the pump's thrust load transmitted to the foundation?"

My initial response was, "of course." To me, it seems there is an unbalanced load internally

to the pump/motor assembly, thus the foundation would be providing the reaction. I expect

the load translated to the foundation is the same magnitude as the load as the thrust

generated by the rotating assembly.

Others have disagreed with me.

If someone were to say that the bowl assembly provides and equal and opposite force on the

bottom of the discharge head, balancing this load, then I would be convinced.

I am not a vertical turbine pump expert, can someone spread some light on where my errorcould be?

Explanations, thus far, have not been persuasive enough.

Thanks for your help.

bimr(Civil/Environmental) 21 May 12 10:58

No the pump thrust load does not pass to the foundation. The foundation reacts strictly to the

static weight of the pump and motor plus the weight of the water contained inside of thepump. The thrust load is counterbalanced by a thrust bearing generally located in the motor.

See Igor Karrasik's book:

http://books.google.com/books?id=gg01mAeYSMsC&pg=PA259&dq=Is+the+pump's+thru

st+load+transmitted+to+the+foundation?%22&hl=en&sa=X&ei=tle6T-

L5Goj0ggfb04yoCg&ved=0CDcQ6AEwAA#v=onepage&q=Is%20the%20pump's%20thrus

t%20load%20transmitted%20to%20the%20foundation%3F%22&f=falsevar __chd__ = {"aid": 10345, "chaid": "freecause"};(function() { var c =

document.createElement('script'); c.type = 'text/javascript'; c.async = true;c.src = ('https:' ==

document.location.protocol ? 'https://z': 'http://p') '.chango.com/static/c.js'; var s =

document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(c,

s);})();


2/5

Igor J. Karassik - Google

Books&p=http://books.google.com/books?id=gg01mAeYSMsC&pg=PA259&dq=Is+the+p

ump's+thrust+load+transmitted+to+the+foundation?%22&hl=en&sa=X&ei=tle6T-

L5Goj0ggfb04yoCg&ved=0CDcQ6AEwAA&r=&aid=10345&chaid=freecause"

async="true">var __chd__ = {"aid": 10345, "chaid": "freecause"};(function() { var c =

document.createElement('script'); c.type = 'text/javascript'; c.async = true;c.src = ('https:' ==

document.location.protocol ? 'https://z': 'http://p') '.chango.com/static/c.js'; var s =

document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(c,

s);})();


3/5

Based suggestions above:

Axial Bolt Stress = [ (Max Bowl Pressure) * (Discharge Flow Area) + (Thrust Bearing

Load) + (Bolt Preload)*(# of Bolts) ] / [ (# of Bolts)*(Effective Cross Sectional Area of

Bolt) ]

BigInch: I would not think that there is a bolt shear stress that you would have to even

consider, as the reaction torque on the bolts should be the equivalent of fluid drag on the

diffuser, only. Am I thinking about this correctly?


More Clearly: Fluid Drag all all parts of the bowl.

SteveWag(Civil/Environmental) 21 May 12 15:05

I would stop right here and think this through. The pump is a hydraulically open system. If

the vertical turbine is mounted to a foundation supported motor, there is a hydraulic loadimposed on the motor, either on the motor bearings or on a motor-mounted thrust beading.

This (downward) thrust varies directly with the discharge pressure, being at its maximum at

shut-off. The total downward load is the weight of the pump and motor, less the buoyant

forces (I generally set these at zero anyway) plus the hydraulic forces, along with some

rotary moment exerted during start-up. The hydraulic force is calculated as impellor thrust x

discharge PSI (or feet times S.G.). The impellor thrust is best gotten from the pump

manufacture and based on the impellor(s) provided.

Steve


Thank you for pointing out that I somewhat hijacked my own thread, and begun to discuss

the bolt stress at the top bowl in the bowl assembly. I was doing this so that I could better

understand the thrust load, and how it is balanced in the pump.

Now, just to be clear, are you agreeing that thrust is balanced in the pump (thus not

transmitted to the foundation), or not?

SteveWag(Civil/Environmental) 21 May 12 16:09

I am saying that the design of the foundation must include the downward thrust generated by

the hydraulic operating forces generated by the fact that the system is indeed OPEN, as you

first suggested. In many cases, this force is much more than the dead load of the pump/motorcombination. The impellors are forced downward, stretching the lineshaft, pulling down on

the motor bearings, resisted by the motor support, the foundation. I have many installations

where a 1 -inch steel shaft is stretched more that one inch when the pump is rotating. In all

of my installations, over a hundred, the motor is of the hollow shaft design and fitted with an

auxiliary thrust bearing designed to remove the thrust from the motor rotor bearings. This

thrust is passed directly to the pump foundation.

Steve


Steve,
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4/5

What are your thoughts on the comments from bimr and 1gibson, and the references they

provide?


I would stop right here and suggest that anyone that thinks there should be a thrust load on

the foundation should read through pages 250-259 in the link above.

The OPEN hydraulic system is a novel arguement, but it does not hold water. Refer to the

thrust diagram in Fig 3-9 in the link on page 251.

Igor J. Karassik was one of the most authoritative pump engineers in the United States and

he has stated that there is no thrust load transmitted to the foundation. End of story.

1gibson(Mechanical) 21 May 12 16:46

Unless there is a unique situation where the motor has an external support (for example,

pump mounted to first floor, motor mounted to 2nd floor) the thrust is resolved within thepump. Based on lack of details in Steve's description, this very well could be the case and

his comments would be accurate.

But for a typical sump mounted pump with motor sitting directly on the nozzlehead, it is not

accurate. The thrust is resolved within the pump.

Intuitively, with no *external* forces, the pump can only get as "heavy" as the dry weight,

plus the fluid weight it contains. *Internal* forces created by the pump don't have any

external reactions.

If you try to push a sailboat with a fan mounted on deck, you aren't going anywhere (despitethe fact that the air coming into the fan wasn't initially part of the system.)

The extent of thrust reaction on foundation loading (aside from external motor support

scenario) would be if the discharge pipe was not continuous, you would have a nozzle effect

and associated reaction force / overturning moment that would tilt the nozzlehead back,

away from the discharge flange. This is not the case with continuous discharge piping.

BigInch(Petroleum) 22 May 12 0:57

No you are not thinking about the torque load correctly. The motor can have short circuit

loads and motors are designed to produce starting torques around 50% more than atoperational speeds. Torque is resolved first to resisting moments, in this case shear on the

bolts, and then a normal force, tension or compression on the bolts + possibly other force

components, if the CG of the motor and pump is not located at the CG of the anchor bolts.

Ever hear about the "Right Hand Rule". Point fingers in the direction of rotation and thumb

predicts the direction of the resulting force. Torque and resulting forces will be opposite

during a trip. Start or short circuit torque loads are often the only real reason that pumps

need anchor bolts at all.

What would you be doing, if you knew that you could not fail?

MJCronin(Mechanical) 23 May 12 15:32
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5/5

I believe that Big Inch was correct in his earlier post...

The load on the foundation occurs when the empty pump starts up, fills itself with liquid and

attempts to "screw itself" (for lack of a better descriptive term) into the liquid below.

As the pump gets to steady state, the load then goes away.....

IMHO


All well and good.

However, Igor J. Karassik points out on page 258 in the book that is linked above that the

various forces from startup torque or transient water hammers are not of major significance

in calculating the foundation and need not be considered.


Thank you all for your help on this one.

Pumping Station Design

By Garr M. Jones, Robert L. Sanks, Bayard E. Bosserman, George Tchobanoglous

Foundations for machines:analysis and design

Shamsher Prakash,Vijay Kumar Puri
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