MauiPunter

Bilge Pump Flow Rate

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I currently have a 2200 GPH Rule pump that goes into a 1" flexible pipe.  I know there is loss  because of being below the water line by around 3' plus the 15-20' run to the stern, but wondered if I upgraded to the 4000 GPH pump, would I see an increase flow rate or am I close to maxed out already for that line?  How do I calculate/estimate what max is?

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From a plumbing flow standpoint....2200gph in a 1” smooth pipe is wayyy too much for efficient flow. Maybe impossible for such a pump. Recommended smooth pipe size for 2200gph is at least 1.5”, best would be 2”. Diameter is hugely important for flow. Small increases in diameter greatly increases flow:

https://www.engineeringtoolbox.com/pvc-schedule-40-pipe-friction-loss-diagram-d_1147.html

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4 minutes ago, daddle said:

From a plumbing flow standpoint....2200gph in a 1” smooth pipe is wayyy too much for efficient flow. Maybe impossible for such a pump. Recommended smooth pipe size for 2200gph is at least 1.5”, best would be 2”. Diameter is hugely important for flow. Small increases in diameter greatly increases flow:

https://www.engineeringtoolbox.com/pvc-schedule-40-pipe-friction-loss-diagram-d_1147.html

Ok.  So there is nothing to be gained by going with a bigger pump.  What would you estimate my effective flow rate be in my current setup?

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This would also lead to upsizing the thru-hull to be effective.

 

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Well that depends on “things”. But those charts would imply that the flow becomes turbulent, inefficient, around 1000gph in a smooth pipe. In that corrugated crap marine stores sell probably more like 500gph.

I would guess that a Rule 2200gph bilge pump will not pump anywhere near that volume under any conditions found on Earth. Certainly not with a 1” hose. Barb fittings are about the worst technology possible as well...from a flow standpoint.

Your 4000gph pump should have a 2” pipe, minimum. Without smaller barb fittings, as fufkin points out above.

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Resistance to flow goes as the inverse square of the pipe diameter and is proportional to pipe length. With a 15-20' run, there would probably be a big benefit to upsizing the hose to 2", way more so if the existing stuff is corrugated on the inside and the new stuff is smooth. This would be true even if you've got narrow fittings, again because the length of the run likely makes it the dominant source of flow restriction.

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Along with what IStream is saying, an approach might be larger, smooth hoses, along with a "Y" valve (appropriately sized).  One side of the "Y" can go to the thru-hull for 'normal" bilge emptying functions.  The other side of the "Y" can go to a length of smooth hose that can be lead overboard for high flow-rate de-watering in an emergency.  

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Why not just measure it?

Fill the bilge to the brim, stick a bucket under the outlet, then turn on the pump. Report back.

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Here is a link that has the Rule Bilge Pump Curves.  http://www.windandsun.co.uk/media/326710/Rule-Bilge-Pumps-Performance-Curves.pdf

 Probably a bit of an approximation but pretty close.

Might take some effort to figure out the head losses in your system - but keep in mind the Rule 4000 only has a flow rate of 4000 gph (15,000 l/h) against zero head.

Area, Length, roughness, bends, height of discharge, and exit losses all factor into it.

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7 hours ago, IStream said:

Resistance to flow goes as the inverse square of the pipe diameter and is proportional to pipe length. With a 15-20' run, there would probably be a big benefit to upsizing the hose to 2", way more so if the existing stuff is corrugated on the inside and the new stuff is smooth. This would be true even if you've got narrow fittings, again because the length of the run likely makes it the dominant source of flow restriction.

For laminar flow, resistance is proportional to the 4th power of radius (dia), and proportional to length.  Even worse! 

Get as much large dia smooth pipe into your plumbing circuits as you can. 

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bobo_Page_11.jpg.51500533216d6f0ed0b20fcc9d753616.jpgbobo.thumb.jpg.7f9bb98718b8ef3437bbdc549575bae7.jpg

This is the actual graph from Rule. If you are lifting the water 3' you are already at only 1600 GPH. Without any hose... 3700 pump needs 1-1/2" hose at minimum, smooth wall strongly preferred. 

 

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Thanks for all the guidance.  It seems that for sure, I will be needing a 2" pipe, and as much smooth surface as possible.  It seems for now, I will stick with my 2200 GPH since there is nothing to be gained by stepping up the power.

When I redo the bilge lines I will upgrade the plumbing for sure.   The setup I wanted to do was to have a 2 x 4000 GPH,  one on port and stbd and put in a third run for maintenance water which I would put my low profile pump on. I have two runs already which have a 2200 GPH on one side and the low profile pump on the other,  If something major happens, I think having 8000 GPH total properly installed could possibly save the boat in an emergency.

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16 hours ago, Fleetwood said:

For laminar flow, resistance is proportional to the 4th power of radius (dia), and proportional to length.  Even worse! 

Get as much large dia smooth pipe into your plumbing circuits as you can. 

You're right, I was thinking velocity for a given flow rate. 

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If it is an open transom boat you can save significant resistance by plumbing the pumps to the cockpit (lot shorter run than to the transom).  Second advantage of this is that you immediately notice if the pump is on.

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Every bit helps but its the diameter which stuffs up the flow.

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What is a good source for the 2" through hull outlets and 2" smooth hose?  Most of the usual places all have plastic through hulls and corrugated small diameter hoses.

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Looks like going from 2000 to 4000 is a 75 to 100% increase and simpler than other options.  Not perfect, but why not?

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At risk of stating the obvious, the larger discharge line will also increase the volume of water within it that can flow back into the bilge.  This may create a short cycling problem if a float switch is present and could flatten the battery over time or cook the pump.  A check valve can help, but almost all check valves leak over time.  

Big hose for big dewatering.  

Getting the last gallon or two out may require a smaller pump and hose.  YMMV  

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3 hours ago, Pinching said:

At risk of stating the obvious, the larger discharge line will also increase the volume of water within it that can flow back into the bilge.  This may create a short cycling problem if a float switch is present and could flatten the battery over time or cook the pump.  A check valve can help, but almost all check valves leak over time.  

Big hose for big dewatering.  

Getting the last gallon or two out may require a smaller pump and hose.  YMMV  

I have a small slim profile pump with a built in check valve for the last bit of water through a seperate discharge hose.  This change would be for my other pump that is for the "big dewatering".  Its a 2200 and I am interested in moving to a 4000 but I need to upgrade the line to a 2" smooth hose for most effective use.

 

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Most corrugated hose is actually smooth wall inside. It's only the cheapest stuff that isn't.

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On 7/5/2018 at 4:43 PM, MauiPunter said:

What is a good source for the 2" through hull outlets and 2" smooth hose?  Most of the usual places all have plastic through hulls and corrugated small diameter hoses.

A few years ago I ended up finding 2" smooth wall PVC hose from a spa/pool supply company - it looked and handled very similar to marine head hose but was that little bit bigger that I needed for transom cockpit drains I installed.  2" through-hulls shouldn't be hard to find, but likely more expensive - may have to special order through West or Defender or your local chandlery to get the material/etc. that you want.

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On 7/5/2018 at 3:43 PM, MauiPunter said:

What is a good source for the 2" through hull outlets and 2" smooth hose?  Most of the usual places all have plastic through hulls and corrugated small diameter hoses.

 

fisheries supplies  has a couple in 2"..  that hot tub hose would work as well..

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