2in $230 pump vs. 3in $250 pump ?

[bcarwell]

I'm buying a semi-trash pump for use in controlling some small limb burn piles and transfering river water between tanks from time to time.

There are two pumps at Tractor Supply. One (SKU 1023085) is rated 9480 GPH and is 2 inch in/out for $229. Another (SKU 1023108) is rated 15,840GPH and is 3 inch in/out for $250. Otherwise they appear identical, e.g. both 6HP, 1 year warranty, "Red Lion" brand.

Question is: why would I not want to spend only 20 bucks more for the 3 inch when it delivers almost 70% more pumping capacity ? I plan on sizing down the output to 1 1/2 inch hose/nozzle for higher pressure for fire supression and might have a little more one-time hassle/expense in getting a hose coupling, etc. And maybe the larger capacity pump requires more gas ? (but I only will use it occasionally so don't see this as an issue).

(BTW the closest other deal I've found is Northern Tool SKU 158230 on sale for $215 including shipping (no shipping needed for Tractor Supply down the road). But its only a 30 day warranty, with extended 1 yr warranty for $25, 5.5HP, although 13,200 GPH.

If I size down the nozzle I assume I'll still get more flow at higher pressure from the 3inch pump than the 2 inch pump, correct ?

Does the 3 inch pump sound like the best deal ? (NOTE: Its also easier to return the Tractor Supply pumps down the street than to ship back a Northern Tool pump).

And advice is appreciated.

Bob

 

[ustmd]

I am not an engineer, but...

The only apparent difference between the two pumps is the size of the pipe (2" vs 3") and the flow rates differences between them are directly proportional to the ratio of the area of a 2" pipe versus a 3" pipe--a 2" pipe has a cross section of ~12 sq in and a 3" pipe has a cross section of ~28 sq in.

Since you have the same size motor on both pumps, you have the same amount fo "energy" to draw and pump the water. The 2 in pump states that it has a 20' draw/prime. I didn't see a spec on thre 3 in pipe, but I bet it is a lot less. If you try to use the 3" pump with a long or high draw you may have problems--especially if you are not using a rigid supply line

Also, there are no pressure specs on the web site for the pumps. In your OP, you mention about wanting to use this for fire suppression and choking the line down to 1.5" or less. On the 3" pipe you are going to get a significant higher back pressure which may load down the motor or even stall it.

Not sure of that helps...normally I would say go bigger, but....

 

[bcarwell]

Thanks for responding USTMD. I know, no such thing as a free lunch. If both the 2" and 3" use the same motor something has to give with the 3" pump. I'm sure you're right that the 3" would provide less head/draw and in exchange for the increased flow probably less output pressure. I'm not so concerned about the intake side since the pump will almost always be right next to the tank its pumping from and a 10 foot hard suction line would be sufficient, e.g. I'm not drawing up 20 feet from a river or remote swimming pool, etc.

I was thinking I'd size the 2" down to a 1 1/2" for better pressure for fire control, and now that you mention it, I guess the 3" could only be sized down to 2 1/2 or 2 or, as you noted, the engine might stall from the back pressure or certainly it would not be good for it to run for long with that back pressure. (BTW for the fire use I was going to keep the pump pumping and just cycle it back into the tank with a diverter valve until I needed it at the nozzle to avoid harming the pump when its running but not pumping anything out the fire nozzle).

But I guess the problem is trying to get the pump to serve two purposes- fire supression where I want a 1 1/2 hose and higher pressure, and transfer where I don't care about pressure but would like to take advantage of the higher GPH of the 3" pump. I suppose if I don't mind a larger 2 inch discharge hose for fire use on the 3" pump, still it seems the extra $20 would be worth it for the increased 70% flow rate when using it merely as a transfer pump. One frustration with these pumps is they never show flow rate - pressure curves, I guess because they always assume you're just pumping unconstricted output continuously into a short hose into a ditch or whatever.

Bob

 

[lucky llama]

I had a 2" trash pump I used for jetting in dock pilings. Had to reduce the size of the outlet to 1" inorder to easily go around the pile. It did seem to increase the pressure but not to any great extent. When left at 2" it did put out alot of water. That being said I'd go with the 3" for the little bit more$.

 

[mmurphy]

Check to see if they have a head spec on both pumps.
Some pumps are made to pump large volumes with low head. (Head basicaly pressure) Then determine the amount of lift or elevation, gpm you need.

 

[BruceWard]

3" accessories are much more expensive than 2" .

 

[ctgoldwing]

Check to see if they have a head spec on both pumps.
Some pumps are made to pump large volumes with low head. (Head basicaly pressure) Then determine the amount of lift or elevation, gpm you need.

Bingo!

Without a head pressure vs flow graph I don't know how you could compare them. They may use the same motor but the impellers may be cut differently.

With a head (pressure) vs flow chart you could better decide which one meets your needs. You may need/want 500 gph (~8 gpm) at 75 psi. A pump that can do 15000 gph at 10 psi (1 psi ~ 2.3' of head) may not even be able to produce 75 psi.

A pump chart is your friend

 

[Brad_Blazer]

Are these the same pumps at HF?
3" 345 GPM Dirty Water Pump - Approved in All States Except California
Portable Clear Water Pump with 6.5 HP Gas Engine and 2" Inlet/Outlet

If the specs are right the 3" has better numbers all the way around. It might simply be that the 3" setup is that much more efficient. Friction is generally proportional to the inverse of diameter to the 4th power.

If you order from HF I recommend a phone order and verify they have several in stock. I just went around with them for shipping only a fraction of my order with most items sold out even though they all indicated "in stock" on the web site.

Northern will ship to the store for free. Looks like San Antonio might be closest? The pump you linked isn't listed as semi-trash. That might be why it seems more efficient.