Flexible Pipes for Centrifugal Pump Vibration 3

[BillKapri]

On-site here nearly all centrifugal pumps are installed with flexibles on the suction and discharge piping. This seems to be a generally accepted practice which is backed up when you search for e.g.'flexibles for pump vibration' (this is usally on the websites of flexible hose suppliers). However, I have seen on eng-tips previously, experienced engineers saying flexibles do not reduce vibration and that centrifugal pumps shouldn't vibrate anyway.

My question really is: is there any sources I can reference to show that flexibles aren't required on a new pump installation, as long as the pipework is aligned correctly? Even though some of the accounts I have seen posting on this matter I trust a lot, I don't have much of a case if all I can go by is that I saw it on a forum.

 

[shvet]

Why do you consider these flexibles are related to the vibration? What is a negative factor(s) those are intended to avoid/compensate?
Why didn't you start with the question what reason those have appeared in your design?

 

[LittleInch]

It's very difficult to prove a negative.

However a relatively quick google of pump connection pipework or plant pipework will show hundreds of thousands of plants where pumps are hard piped.

You get two types of vibration really - mechanical vibration from e.g. a slightly unbalanced motor or pump and pressure pulsations which translate into vibrations further down the line.

Flexibles should stop most mechanical ones, but fluid pressure pulses or blade pass pulses they won't. Now for a centrifugal pump they are relatively small, certainly compared to a piston type pump. A flexible though might take some of the pulse out as it flexes, depends on the make up of the "flexible" which is itself a rather vague term.

The only real way is to do it like the vast majority of plants and designs do it and see if it works for you.

Having flexibles can be a good thing if you get a lot of thermal movement or stress on the pipe and can make installation a lot easier. But they tend to break/ leak and need replacing on regular basis ( hence why the flexible pipe suppliers push them for this uses.... /cynical off).

All you can do is scour some plant design manuals, guides etc for pump connection and show most talk about how to align pumps and restrict the forces going into the pump nozzles.

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

 

[1503-44]

Given that a properly working and maintained centrifugal pump should have only very small vibrations, if any, how would flexible connections ever solve vibration problems, unless a rigid pipe was transmitting them to the pump?

Stop trying to treat the symptoms and find out the source of the vibration and fix that.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Heaviside1925]

OP,

My question really is: is there any sources I can reference to show that flexibles aren't required on a new pump installation, as long as the pipework is aligned correctly?

Yes, the pump manufacture and their recommended installation practices.

Even if you are correct for a certain pump and you can prove it does not need a flexible connector, consider the reason you are seeing so many flexible connections on site may be driven by site or corporate directive/standard. It also may be maintenance driven, consider the flexible connection, a flexible removable pipe spool, where pumps may be removed or swapped out with less work. Also consider that slightly different pumps could be substituted with minimal piping rework or analysis.

 

[r6155]

@ BillKapri
Any photo?
Regards

 

[BillKapri]

@LittleInch @Heaviside1925 Thank you for your responses, very helpful. We have some batch processes that alternate between processing hot fluid and being left at ambient - Do you think this thermal expansion could justify some of the flexibles on hot duties (as a sort of expansion joint)? (for 2-6" lined steel or PP pipelines @75-90C).

@1503-44 It is not so much that we are getting vibration in the pipes, but that using flexible connections to the pump is common practice with this idea about vibrations. I suspect this may have started so as to make aligning the pumps to pipework easier and for maintenance to remove the pumps, as @Heaviside1925 suggests, but then maybe this idea about vibration developed over time.

 

[1503-44]

I'd Sort that out first. Otherwise you'll only get a black eye.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[bimr]

The pump piping should be designed so there is no flexible joint. Flexible joints are not reliable, prone to fatigue failure and thus cause loss of production and possible injury to operators and maintenance workers. Flexible joints are a weak point in the piping system.

There is no standard regarding the proximity of flexible joints to pumps.

There are many things that must be considered when you employ flexible joints. Pressure thrust is one of the most important considerations. You have to follow EJMA standards regarding the placement of guides and other restraints. Every piping system is different (geometry, pipe size and schedule, etc.) so there are no "rules of thumb" to be applied. If you have room to use expansion loops, guides and line stops that should be your first choice. You need to install piping anchors where the piping connects to the pump flanges to avoid putting excess structural loads on the pumps. Pumps aren't designed to handle loads from the piping.

Linke to horror stories: Metraflex

I have seen a lot of engineers add these joints to the piping, but if you asked why they were included, they will have no answer why they used them.

 

[EdStainless]

We like to think of centrifugal pumps producing a smooth output, but it has pulses in it.
Think of the vane pass frequency.
I have seen cases where flex lines resulted in vibration that would have easily been dampened by using solid piping.

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P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

Flexibles can be used as a simple and lazy way to solve issues like thermal expansion, pump loads and mechanical pump vibration into poorly supported pipework.

But like every "solution" it poses new problems as Bimr has pointed out, mainly regular leaks / failures and replacement of them on a regular basis, proactively to prevent to such failure could be as low as every 6 months. They are your weak point.

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