What hole size in pump test tank perforated baffle?

[Scott03]

I am in charge of rehabing an existing submersible pump test tank. The tank is 10'L x 8'W x 4.5'H with a working volume of about 2500 gallons, while the maximum allowable system flow is 2000 gpm. I want to separate the tank's inlet and discharge halves with a perforated baffle to 1)help control pump inlet turbulence and 2) help alleviate entrained air at the pump inlet.

Per ANSI/HI standards, I would like to keep my flow through the baffle holes at less than 1.5 ft/sec. I can calculate gpm and velocity per a given hole size, but how can I determine the optimum hole size to counteract turbulence and entrained air?

 

[Artisi]

Seems that ANSI has already set the hole size at 1.5ft/sec.

However, I would think the tank is too small as you have insufficient retention time for any entrained air to escape,a tank volume of something like 3 or 4 times the fow rate is probably more like what is needed, plus you will have a further problem of temp. increase if the testing time is overlong.

 

[Scott03]

Artisi,

I'm not quite sure I understand your reference to the hole size. ANSI/HI specifies that the inlet flow rate is to not exceed 1.5ft/sec. I can achieve that by many combinations of the following variables:
1) flow (gpm)
2) # of perforated holes
3) perforated hole size

The absolute maximum we may produce is 2000gpm, but most of our pump tests do not exceed 800gpm. Temperature rise is not issue as our tests rarely exceed 5 minutes.

I'm simply looking for a preferred hole size in baffles that facilitates the ?release? of entrained air. Once I know a preferred hole size, I can then calculate the actual number of holes I need to keep inlet velocity below 1.5ft/sec at a maximum flow rate of 2000gpm.

 

[Artisi]

The first thing you need to come to grips with is that the perforated holes do not release air, retention time of the water in the tank releases air. The idea of a perforated baffle is to make sure the flow from the inlet to the tank to the outlet is steady and uniform and that flow containing entrained air doesn't go directly to the pump inlet.

"I can achieve that by many combinations of the following variables:
1) flow (gpm)
2) # of perforated holes
3) perforated hole size "

Correct - Your choice on hole sizes and number.

I have seen tanks with a series of baffles directing flow under and over alternate bafffles, I have also seem tanks with a wire basket filled with rocks - in both systems the idea to to ensure there is no direct path from inlet to tank outlet of water containing entrained air and flow is uniform to the pump inlet.

A tank that is too small will always have problem with entrained air what ever you do.

Of course this is based on the premise that you have entrained air to start with, which is usual for a test tank.

 

[Scott03]

I completely understand your explanation that the perforated holes do not in themselves release air, rather the holes facilitate the release of entrained air by keeping the turbulent flow on the discharge half of the test tank long enough for the air to be released to atmosphere.

Also, the tank is not too small for the majority of our pump tests. Most of our tests range from 0-800gpm, which is within the capabilities of our tank. We have never tested a pump at the maximum flow of 2000gpm I mentioned earlier, so don't let that number throw you off.

All that being said, does anyone know a preferred hole size in a perforated baffle for this application? 1/8", 1/4", 3/8", 1/2", etc.

 

[vpl]

Scott

Do you have to worry about any particulates in the test fluid? The first two sizes you have listed will act as a pretty good filter that will clog rapidly.



Patricia Lougheed

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[Scott03]

Patricia,

Thanks for mentioning a valid pint. Most of the pumps are new or thoroughly cleaned before a test, so clogging is not a major concern. When the tank is not in use, we chlorinate and filter the water to alleviate sediment, algae and other unmentionables.

Scott

 

[Scott03]

Patricia,

Thanks for mentioning a valid point. Most of the pumps are new or thoroughly cleaned before a test, so clogging is not a major concern. When the tank is not in use, we chlorinate and filter the water to alleviate sediment, algae and other unmentionables.

Scott