Distance from inlet to suction -wet well 1

[ras717]

I'm designing a wet well (first time)and am seeking information on the distance from the inlet pipe to the wet well and suction pipe to the pump. To avoid influent vortexing from creating air pockets and thus caviation for the suction pipe, there should be a minimum distance between the two but I haven't found a discussion anywhere about what this distance should be. Anyone have a resource?

Thanks for the help,
Renee

 

[cvg]

try posting this in the Pump engineering forum

 

[bimr]

The inlet should not impinge against the pump, jet directly into the pump inlet, or enter the pit in such a way as to cause rotation of the liquid in the pit. The sump inlet should be as far away from the pump as the geometry will permit. A distribution nozzle can be used to prevent jetting.

Pump centerlines should be 1 1/2 to 2 diameters (of suction bell) apart. Keep flow in pit below 1 ft/second.

This information is available from the Hydraulic Institute and is also reprinted in the back of the Gould’s Pump catalog.

Where specific pit dimensions are required, the pump manufacturer should be consulted.

 

[ras717]

bimr. It's nice that I'm doing it right. I've taken of everything you mentioned. I have 8 ft between inlet and pump suction bell. My boss would like to see documentation that says this is o.k. I will check the resources you mentioned.

I will also repost in on the pump forum as you suggested cvg.

 

[stanier]

Pumping Station Design by Sanks covers the design of station wet wells. In addition there is a Hydraulic Institute Standard/ANSI standard that covers this matter

 

[JimCasey]

Just to split hairs:
When a pump sucks air, that's aeration. The flow leaving the pump will be laden with bubbles that will persist until there is a place in the flow path where they can separate.
Aeration can be harsh because the pump can overspeed when it loses its prime, and this can damage the drive coupling. Also, developed head goes 'way down while there is entrained air because of the lower density of the charge in the impeller. This is a prime justification for a check valve on the outlet of the pump. If the pump sucks air, loses prime, and the developed pressure falls off, then the pressurized fluid downstream will probably rush backwards into the pump. A big slug of liquid flowing into an overspeeding impeller can be really shocky to the drive coupling, shaft, keys, etc.

WHen a pump is flowing single-phase liquid and the inlet pressure decreases to below the NPSH number, the water in the impeller flashes to steam. Then the steam bubbles collapse on the front side of the next impeller blade. That is Cavitation. Cavitation causes surface damage to the impeller and housing, but the fluid leaving the pump is all liquid (the steam bubbles collapse and all the fluid returns to liquid phase). It sounds like the pump is pumping gravel.

 

[stanier]

Download a copy of ANSI /HI 9.8 American National Standard for Pump Intake Design. This will give you the documentation yuor boss requires. it will also teach you how to design a wet well pump suction.

The cost of the standard is far less than the cost of your time researching this matter on this forum.

If your boss wont let you invest in the standard then he is at fault.

 

[ras717]

stainer:

I got from the library the Sanks book and found the reference yesterday. I have to buy any and all references I use myself. Getting supplies is difficult too. Researching how to do all of this is on my time so the company is getting saving a lot.

Thanks for the input. I appreciate it.

Renee

 

[stanier]

Hi Renee,

Sorry to hear you have to buy all the tools to do you job yourself.

ITT Flygt have some great publications on the design of wet wells. In addition their Secad software allows the design of wet wells and covers all you need. In addition ABS pumps have their PSD software that aids the design of wet wells. If you were in Sydney I could help further with resources.

You will find that the US Army Corps of Engineers did a lot of the research that appears in the ANSI/HI standard and they may have on their website documentation to assist.

My contact details are on

http://www.waterhammer.bigblog.com.au

 

[Mattysdad]

Do not confuse air entrainment with cavitation - cavitation is hydraulic phenomenon - not related to air.

Air entrainment is a function of several items - most important that suction meets minimum submergence in all conditions - usually worst case is maximum flow rate at pump off liquid level.

Air from the sewage can get into the inlet to some extent from bubbles from splashing, etc. Provide a baffle or good horizontal separation distance if the inlet is dropping significantly to the water surface.

A vortex forms - a little water tornado - and the center can admit air into the suction if you do not maintain greater than min. submergence.

Min. submergence is only a function of inlet velocity. Increasing size of inlet will greatly recuce min. submergence required and provide for more effective use of wet well volume.

Is this a constant speed system ?How many pumps operate simultaneously ? Orientation of suction inlet is vertical or horizontal ? Pumps are submersible, dry pit, self-primers ?

Do you have a provision for air release ?

Are you aware that pumps can become air bound if air is admitted via vortexing and how to ensure that this is not an issue ? That is a centrifugal pump full of air cannot make pressure to open the discharge check valve and release the air to the force main. This is a common occurrence when operators pump a tank manually and forget to reprime the pump afterwards.

Advise if you need sources or this is unclear ....

 

[ras717]

Thanks Mattysdad,

I do believe I have an air release valve but I will check. I've gotten the Sanks book recommended on this forum and it's pretty good. I will be getting more since my boss is leaving the firm this week and that's leaves guess who? to finish this design and any future ones without someone to go to. Well, I can go to the President when he's around which isn't much and he's very busy. You will probably be seeing me post a lot. I really appreciate everyone's advice and help.

Thanks
RAS717