Fuel Restriction, 3/8 pickup to 1/4 shuttoff?

[rothfm]

Hi All,

My ongoing fuel tank work brought up another question...Pics attached. Not sure if a prior owner did this or not. Original 155 gallon tank had a 3/8 Pickup Bung in tank, with a 3/8 pickup tube.

The topside port on the pickup is 1/4 npt. Attached to this was a manrine brass 1/4" shut off valve ....Then going to a 1/4-to-3/8 barb.

The whole fuel path from bottom of tank to motor is 3/8, EXCEPT for this 1/4" shut off valve. Looking into the shut off, the hole is pretty small. DOES this present a fuel restriction if the systems requires 3/8??

I am buying a true 3/8 Pickup, that has the topside port also at 3/8, and going to keep it all 3/8 all the way through--seems to make simple sense. Isolating the diff metals with a nylon coupler.

Unless someone in the know thinks that 1/4" shut in the sequence makes sense somehow (can figure why it would).

Will be 2 feeds fueling twin 150's.

Thanks for any thoughts. Almost ready to bring my replacement tank to the welder for Bung install and swapping fill/Vents.

 

[dingbat]

While the temporary reduction will restrict flow thru the 1/4” orifice, the reduced flow would have minimal effect on the overall flow capability of the system. Certainly not enough to worry about.

Bronze and “marine” aluminum (5xxx series) have minimal issues.

Do yourself a favor and back the nylon. More problems (future) than its worth. Ions would still transfer through the fuel

 

[rothfm]

Thanks Dingbat...To clearify, stay away from the Nylon? Should I complete these in Aluminum or remain with the Brass as it was? Dont want to have to do this job again

 

[rothfm]

sorry, just re-read your response. Got it. No nylon....I was trying to keep 3/8 along the path. If this is the case, I'm happy to use the marine grade 1/4 (quality) shuttoffs that I have. The stuff I bought doesnt appear good quality.

Sweet, Now I just need bungs and pickups 3/8 x 1/4

Thanks!

 

[achris]

Get that 1/4" fitting out of there! It WILL cause enough of a flow reduction to cause serious engine damage. It will cause the pumps to pull harder than they are designed to and lean the mixtures, or weaken/damage the pump.

Feeding twin 150s, I'd be looking at upgrading to 1/2" out of the tank, and then splitting into 2 lines of 3/8", one to each engine. With a 3/8" shutoff in each line.

All outboard manufacturer's manuals are VERY clear about fuel line sizes. Do not use 1/4" on anything more than about 60hp... And a single point in the line of 1/4" downgrades the entire system to 1/4"..

Even on my boat, I have 1/2" pick up then 3/8" all the way to the engine, and that's just a single 220hp.

Chris......

EDIT: Think about this... Cross-section of a 1/4" tube/fitting/shutoff valve is 0.05square inches. Cross section of 3/8" is 0.11 square inches. By going to 1/4" you are reducing the area for the fuel to flow through by more than half.... :facepalm:

 

[dingbat]

And a single point in the line of 1/4" downgrades the entire system to 1/4"..

While the fluid velocity will increase thru the length of the restriction, the velocity will return to normal once past the restriction. Thus, the restriction (friction) is limited to the area/distance of reduced cross section.

Pressure drop pushing 60 GPH thru 3/8" hose is roughly 0.07 psi per foot

Pushing 60 GPH (300 HP x 2) through a 4" long, 1/4" diameter restriction imposes an additional 0.16 psi pressure drop over and above the 7 psi per 100' noted above.

Pushing 30 GPH (150 HP x 2) in the same scenario as above imposes no additional drop in pressure over and above the the 7 psi per 100 figure.

Compare this to the typical 0.61 psi (from Racor Spec sheet) pressure drop through a "clean" separating filter

In this case, the loss, or increase depending on how you look look at it, associated with the 1/4" valve is insignificant in the scheme of things.

 

[Scott Danforth]

all, 1/4" NPT fittings and 3/8" hose barbs have the same ID. there literally is no impact.

ID hole in a standard 1/4" NPT fitting is 0.281" ID hole in a 3/8" hose barb is 0.281"

the ID of the 3/8" metal lines used on most boats is .312"

this is a non-issue.

you are good if you put two 250hp motors on there.

 

[Ned L]

I will also point out that unless the USCG has changed things, that "packless valve" (the old one you are taking out) is what the USCG wants to see for fuel shutoffs, not that new ball valve.

That packless valve is a proper fuel shutoff valve.

 

[rothfm]

Great input on this guys, Very much appreciated.

Agreed on the ID of the 3/8 Barbs, and the 1/4 valve fitting. By eye, not much difference.

Yes too, on the original 1/4 Marine brass valve. These are truely Marine grade NOT the ones I got at the box store.

My new 3/8 Pickups arrived, 3/8 Bungs...along with the original 1/4 Valves and all new 3/8 lines going to dual Filters, I think my fuel project is on track.

Appreciate all the response.

 

[Ned L]

Hmmm, ......... when I look I still see packless valves as the USCG approved type, not ball valves. We're not talking "marine grade" here, there is a difference. "Marine grade" is a label. USSCG approved is certified. A big difference.

.... It's your boat.

 

[rothfm]

Going to use the original Valves that were in the Tiara, and in the pic above. They are stamped and I looked them up on the Mfg Website: "meets ABYC H-24 and USCG approval for under deck fuel isolation"

keeping the originals, seems good to me....I just wanted to confirm they were ok with the 3/8 lines.

 

[Ned L]

Sounds good, ...... and maybe I misunderstood your previous post. My apologies if I did.