Sizing a water system pressure pump

sailman

Super Anarchist
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Portsmouth, RI
Not happy with the current setup, low pressure and wide range on the start stop of the pump.  The system does not have an accumulator.  There is the galley, head (w/shower) and deck shower.  The chances of two of those running at the same time are remote to nil.  Is this as simple as moving up to a pump with high pressure?  Current pump is a Jabsco with a 25 psi max I believe.

 

slug zitski

Super Anarchist
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Don’t know 

But , the bigger the accumulator tank the better 

your pump will cycle less and last longer 

On the plumbing  side avoid joints, tee’s, elbows, rough interior surface hose  or anything that restricts water flow rate 

 

slug zitski

Super Anarchist
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Don’t know 

But , the bigger the accumulator tank the better 

your pump will cycle less and last longer 

On the plumbing  side,use large inside diameter hose ,  avoid joints, tee’s, elbows, rough interior surface hose  or anything that restricts water flow rate 

high psi on a small diameter hose , with many friction points delivers reduced flow rates

 
Check out the company in Florida with the composite gear pumps, Marine or Mariner mate something.  I helped a friend spec two to replace a huge johnson pump setup on a big sailboat.  I think it's a variable speed pump with a simple processor.  Accumulator setup will fail period.  The diaphragm pumps are nice but this gear pump setup seems like it might be the next jump to a better product.  Have used these with metal parts for years as utility pumps on ships they are bullet proof.

 
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I speced two of the 12 gpm pumps to replace a 25gpm Johnson, I think it was a repackaged ind pump, huge 24v dc.  They have been able to just run one of the marco pumps with great performance.  If we need to replace our Johnson diaphragm I will go to the Marco.

 

IStream

Super Anarchist
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My UP6/e freshwater pump is spec'd at 9A/12V (4.5/24) and routinely pulls 20A for a few seconds if I let the accumulator leak down. Built like a tank. Not cheap, but you get what you pay for with these.

 

climenuts

Anarchist
726
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PNW
I don't have pressurized water on my boat but I design plumbing systems and will give my $0.02.

A constant speed pump without an accumulator is a horrible solution. Even with an accumulator it's a really old fashioned way of doing it (sorry, Slug).

You want a variable speed pump like Sassafrass mentioned. The control is so fast you won't need an accumulator and you can correctly size the pump for your peak flow without excessive cycling at partial flow. It'll be longer lasting and prices of variable speed motors are getting very competitive.

The real key is making sure your pump is selected not only based on the flowrate but the pressure it produces at a given flowrate. The documentation for marine stuff is generally completely useless because a 4.5GPM rating tells you absolutely nothing unless you know what pressure it's providing it at. Absent of well defined numbers I'd recommend assuming the a flow rating is at ZERO pressure gain and you're at 10% of that flow at the pressure cutoff.

To figure out the pressure you need you can look up pressure drop tables for piping. For a 1/2" plastic pipe you're looking at 1.4ft/100ft for 1GPM, 4.5ft/100ft for 2gpm, 9ft/100ft for 3gpm plus 3ft of length per elbow, 1ft of length for a flow through tee, 4ft of length for a hard tee, plus whatever pressure you want at your fixture and whatever elevation gain you're looking at. 1ft == 0.44 psi.

Anything more than 25psi at your faucets, showers, or hoses is a waste. If you have electric flushing it may specify what's needed and I'd assume it's the most stringent requirement.

 

IStream

Super Anarchist
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Clime,

My boat came with a fixed speed pump + accumulator and I left the accumulator in place when I retrofitted with the variable speed pump, figuring it would do no harm. However, it's a big fucker in a tight compartment below the sink and I'd prefer to use the space for a water filter. Other than smoothing out pump response to transient demands, can you see any reason not to pull it?

 

El Borracho

Verified User
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Pacific Rim
Some minimal accumulator is a requirement. Some pumps have a small one built in. Without gravity, like in a land-side tanked system, there must be give somewhere or when the pump stops the system pressure will go to zero. That's physics....water is incompressible. Maybe stretchy tubing is enough, or a springy pump diaphragm?

 

IStream

Super Anarchist
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I would imagine there's plenty of compliance in the ~100'+ of fiber reinforced clear poly water tubing running around the boat. 

I thought about running copper but only just for a second  ;)

 

climenuts

Anarchist
726
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PNW
Clime,

My boat came with a fixed speed pump + accumulator and I left the accumulator in place when I retrofitted with the variable speed pump, figuring it would do no harm. However, it's a big fucker in a tight compartment below the sink and I'd prefer to use the space for a water filter. Other than smoothing out pump response to transient demands, can you see any reason not to pull it?
The installation manual for the pump should be clear about it. I did a quick google and a loop of flexible hose at the outlet of the pump as a shock absorber was all an RV Remco pump required. The reaction time of decent variable speed control is 0.05s ish so no accumulator shouldn't be a problem.

To draw a parallel; on buildings with booster pumps and rigid steel/copper piping we run no accumulator at all. Doesn't matter if it's 1hp or 100hp, 1 sink or a 1000 bed hospital. Flush valve toilets have massive spikes in demand over short periods of time and they cope just fine. ECM motors that you would find on small DC pumps would be just as fast or faster because the pump has next to no inertia.

There's no harm in having an accumulator if you've already got one and it's typical to leave them in during retrofits but they're not needed.

 

climenuts

Anarchist
726
292
PNW
To figure out the pressure you need you can look up pressure drop tables for piping. For a 1/2" plastic pipe you're looking at 1.4ft/100ft for 1GPM, 4.5ft/100ft for 2gpm, 9ft/100ft for 3gpm plus 3ft of length per elbow, 1ft of length for a flow through tee, 4ft of length for a hard tee, plus whatever pressure you want at your fixture and whatever elevation gain you're looking at. 1ft == 0.44 psi.
Want to clarify that you only count your longest run. e.g. you count only the pipe and fittings on your longest run with the flows you can expect on each section. Shorter branches off of a main pipe, parallel hot water runs, or after a tee don't need to be counted.

It's a bit in the weeds for the size of a boat system but quick arithmetic will at least get you in the ballpark so the system works.

 
The marco pumps can run at fairly high pressure.  With a decent PID built into the logic and some sort of hysterisis you don't need a accumulator. It's going to come on spool up slow to meet pressure and have a differential to eliminate short cycling.  This is super common in industrial process controls and with reasonable priced components more available like the Marco pump. If anything a accumulator will complicate the logic of a setup like this and make less efficient. They are old technology and were needed for simpler relay logic operation but not so now.

 

See Level

Working to overcome my inner peace
What model is your pump? Some jabsco pumps you can get a higher pressure switch.

Having a accumulator tank is nice at night so the pump doesn't come on when it's quite but also quells water pulsation at the faucet.

 

Max Rockatansky

holy fuckfarts!
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981
I’m running a system on my 40’ cat with two heads, aft shower, galley up. No accumulator. Vinyl lines. Pump is a Jabsco demand pump about 4gal/min, 40psi IIRC. No issues.

I turn the switch off when not in use; I have this fear I’ll develop a leak and the damn thing will pump every drop out my tank. The vinyl lines do indeed act as accumulator.

 
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