Determining the load on a pump vibration isolator

[mrev23]

It seems to me that pump vibration isolators need to support:

1. The weight of the pump
2. The weight of the water in the pump
3. The weight of the water (only) in the drop to the pump​

The supplier of vibration isolators on my project does not seem to share this view.

I understand that pump manufactures want none of the pipe weight to bear on their connections. The pipe hangers can be adjusted to prevent that.

A braided flex connector can prevent the thrust force due to pressure in the pipe from being an issue at the vibration isolators.

But the water weight in the drop to the pump seems to have no support other than what is provided by bearing on the bottom of the pump’s volute. This weight is transferred to the vibration isolators.

In the diagram below (considering only on the discharge side of the pump to explore the concept), the vibration isolators need to support 8 feet of water in the 14” diameter drop.

The water weight is 59.75 lb/ft in the chart here:

https://www.engineersedge.com/calculators/std_pipe_weights/std_pipe_weights.htm

So the water weight (due to only the discharge pipe) on the vibration isolators seems to be:

(59.75 lb/ft) * (8 feet) = 478 lbs​

Is this correct? Or are the vibration isolators not subjected to the water-weight in the drop for some reason?

 

[GT-EGR]

I think you are technically correct about including that additional weight, although ive never seen the water weight included when reviewing submitted isolator from manufacturers or contractors.

How does it compare relative to the pump and piping weight? You might be caught up in a technicality argument where the end result is the same isolator or maybe the next spring up which hopefully shouldn’t change much for anybody.

 

[Artisi]

If you have an unrestrained isolator on the discharge you will have a thrust equal to the discharge pressure X the area of the isolator which will far exceed the weight of the pipe and water.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[mrev23]

Gregory:
The supplier wants to include the weight of the pipe. I think that is a mistake. The pump supplier wants no pipe weight on their connections. No pipe weight can be transmitted to the connections if the flex connectors are doing their jobs.

Artisi:
The thrust force was my concern in this post:

https://www.eng-tips.com/viewthread.cfm?qid=436920

As you suggest, it seems to be addressed by using tie rods on a neoprene flex connector or by using a braided hose flex connector.

I would expect the contractor to adjust the tension in the pipe hanger above the pump so the operating heights of the spring isolators match the submittal from the supplier of the spring isolators.

In this case, the supplier is assuming a length for the piping whose weight they intended to include in their calculations (where they should include only the weight of the water, not the pipe). They do not seem to be working in tandem with the contractor to produce the delegated design that is owed the project.

And the supplier's submittal is "assuming hydraulic forces are insignificant, supported by other means, or constrained by the piping geometry."

If the supplier of the isolators isn't accounting for hydraulic forces on their isolators, who is?

I am disappointed in the supplier's disclaimers, uninformed assumptions (apparently without inquiry to the contractor or doing their own measurements from the drawings), and apparent lack of attention to detail (or possible ignorance ... or active discounting ... of basic engineering statics).

This is not rocket science. Doing it right has never been easier. The supplier should have software that accounts for all the items discussed in this thread. This should be a 21st century engineering effort, not a 19th century "let's make some uninformed assumptions, apply rules of thumb, hope for the best, then change it later if someone notices a problem and makes us fix it" effort.