Suction diffusers. Are there hard and fast rules about when to use or not to bother?

JohnNY

I am led to believe that there is a minimum number of pipe diameters required before a suction diffuser becomes desired or necessary. Can anyone share with me what those parameters might be?
Thank you!

hvacfreak2

I would say that the minimum inlet distance of straight pipe as required by the pump manufacturer would be the main factor for selecting these. I can't recall a generic rule for these , I wan't to say 2.5 times diameter but then again I see that straight ones are available so I'm doubting that one , lol.

EBEBRATT-Ed

In most cases the piping layout in mechanical rooms is such that you usually (but not always) can't get the room you need.

Suction diffusers do a bunch of things:
Vanes straighten flow going into pump
Strainer (and usually a start up strainer as well) inside
Most suction diffusers are reducing fittings as well you may have a 6" inlet and a 4" into the pump

So it saves on not having a separate strainer and a reducing fitting.

Most of the pump mfgs like to quote there pumps with a suction diffuser and a triple duty valve on the discharge

JohnNY

EBEBRATT-Ed said:

In most cases the piping layout in mechanical rooms is such that you usually (but not always) can't get the room you need.

Exactly my problem. The job I'm on will require an additional $3,000 worth of diffusers unless I can figure out a way to eliminate/reduce turbulent flow.
It's annoying.

hot_rod

12 pipe diameters is a common number to use for piping on the suction side.

JohnNY

hot_rod said:

12 pipe diameters is a common number to use for piping on the suction side.

Thank you VERY much, @hot_rod .

Gary Jansen_4

Going back through my notes from years ago attending classes by both B&G and Taco. both recommended a straight length of pipe equal to a minimum of 10 times the inside pipe diameter of the intended suction line. So a 2" ID suction would need a minimum of 20"of straight pipe before entering the pump.
Additionally, if you are using an eccentric fitting before the pump, to transition from pipe to a pump flange, the "flat" side of the eccentric should be at the top side. If the "slope" of the eccentric is on top, you risk having air trap in that offset area.
Again, we are talking minimums, so depending on your velocity, more length may be better.
Thanks, Gary

Gary Jansen_4

I can remember any number of projects that I have gone on to do boiler or pump startups over the years. No suction diffusers used. What may have seemed a minor technicality at the time, but I now realize the importance of it. The engineered and mechanical drawings always showed a long sweep elbow, then the straight pipe where the suction line dropped down to the return of the pump. Now I know it was to eliminate turbulence and provide laminar flow, as much as possible, into the pump inlet.

pecmsg

Its been a long time since I opened this but I know its in here

EBEBRATT-Ed

Who ever the pump MFG is I would follow their direction. I have tried to save money and not use them. Sometimes it doesn't work out.

The suction diffuser takes the place of:

The long turn suction elbow (because ) you don't run pipe on the floor

a strainer

a reducing fitting because suction diffusers can be ordered reducing

When you look at it this way the $$$$$ don't hurt as much

dopey27177

Everything depends on how much room you have for the long straight piping and long turn ell.

In some cases the suction diffuser can be a more economicall install. Larger pumps with large piping have a labor cost attributed to the job.

Unfortunately, some times you need to do the dance in the office
to determine what will fit into the budget.

How about when a vibration pad is spec. into the job. Do you buy the rails and pour the concrete or do you farm it out?

Jake

Jamie Hall

I might add -- probably quite unnecessarily --to the various above comments that provision of a diffuser -- or the suggested long elbows and straight pipe -- is more critical if the net positive suction head at the pump is relatively low. Relative being an impossible to define term. The objective there being to ensure that the turbulence does not result in a low pressure area and localized cavitation. The other aspect of it is that turbulence at the inlet an result in unbalanced loading on the impeller, and resulting vibration or accelerated seal or bearing wear.