Maximum 90 deg bends before inlet suction guide

[davster01]

Hi,
I am currently working on my first chilled water plant design. For the condenser water circuit we are intending to use base-mounted centrifugal pumps. We are considering using suction guide on our pumps. I know that a centrifugal pump needs approx. 5 diameters of piping before pump inlet to ensure a smooth undisturbed pump inlet flow condition. Does this statement still hold with a suction guide at the inlet of the pump? My understanding of a suction guide is that it "straightens" the flow such that the 5 dia. requirement does not need to be met upstream of the suction guide. My boss seems to think otherwise...

Secondly, is there a maximum number of elbows to use before the suction guide inlet?

I would appreciate any reference/comments/remarks that anyone might offer.

Thank you
Davster01

 

[Artisi]

A correctly designed and installed flow-straightener (inlet guide) would / should allow a closer fitment of any bends before the inlet flange, bear in mind that on many pumps the impeller eye is usually a number of pipe diameters from the inlet flange anyway.

If you have the option at this stage of installing bends at least 5 diameters from the inlet flange, why would you want to use a flow-straightener?

As for the number of bends prior to any flow straightener - don't see any problem with any particular number - so long as the flow is straightened prior to the impeller eye.

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.)

 

[DubMac]

Suction diffuser is the term most commonly used in the HVAC industry for these "flow straighteners". As much as we would like the suction pipe to be straight for several diameters from the pump's suction flange, this is not always a real world solution in a cramped mechanical room.

These have been used for years and thankfully, chilled water service is not at all very demanding on a pump. As long as you have ample NPSH available to the pump, and pump is operating somewhere near the heart of the curve (say w/in 15-20% of BEP), you should be fine using a suction diffuser. Do your NPSHA calculations making sure to add friction losses for every twist and turn you build into the suction side piping.

One thing to check, since so many of these diffusers are now made in China, is the quality of the casting. If it has very rough internal passages, you could be defeating the purpose of the diffuser.

 

[bimr]

Because there is no governing body or general standard that identifies straight-run requirements, users rely primarily on manufacturer's recommendations.

"It is not recommended to place an elbow at the suction of any pump (Figure 16-2). This will cause a turbulent flow into the pump. If elbows are needed on both sides of the pump, you should use long radius elbows with flow straighteners. You should have 10 pipes diameters before the first elbow on the suction piping (Example: If the pump has a 4 inch suction nozzle, you should respect 40 inch of straight pipe before the first elbow). Short radius elbows cause vibrations and pressure imbalances that lead to wear and maintenance on the pump."

http://www.labourtaber.com/Pump Wear.pdf

http://www.irrigationcraft.com/flow_run.htm

The flow straightener provides the equivalent of seven to nine diameters straight run upstream. Flow straighteners generally require at least one pipe diameter upstream and 1/2 pipe diameter downstream.

http://www.mccrometer.com/products/product_FS100.asp

http://www.water.siemens.com/en/pro...rmutit_power_products_flow_straighteners.aspx

 

[bimr]

Here is an example of a long radius elbow:

http://www.pdhonline.org/courses/m134/m134content.pdf

"On most pumps, one usually gets away with five diameters of straight length next to the reducer. In the case of certain unspecified velocities and other interacting variables (e.g., viscosity, NPSH margin, pump style, etc.), it might be wise to install as many as 10 diameters of straight length next to the reducer inlet flange. The two different rules-of-thumb explain seeming inconsistencies in the literature, where both the 5 and 10-D rules can be found."

http://www.hydrocarbonprocessing.co...nd-straight-runs-of-pipe-at-pump-suction.html

Good piping layouts would include :

"Ten diameters of pipe between the pump suction and the first elbow."

http://www.mcnallyinstitute.com/01-html/1-3.html

 

[davster01]

Thank you all for the responses. From reading all these links it seems it's a matter of knowing the basics before making a judgement call on what is an acceptable/good piping/pump arrangement for a given situation.

As for suction guides (flow straighteners or suction diffusers), it does not seem to be as appealing as I originally taught for our specific application.

Cheers

 

[BigInch]

Good advice above. I'll simplify to something you can always kep in mind. It has always been min 10 diameters, and if you have more than 1 ell close by, be sure that they are not bent in the same plane, ie 90 deg up, 90 deg left, etc.

From "BigInch's Extremely simple theory of everything."