Larger suction pipe to pump - better?

[Almot]

All inside tubes, inlets on the pump, and suction tube from fresh tank are 1/2". 3GPM pump, 40 PSI on the push side, self-priming up to 10ft (I think).

Now, imagine fresh tank 6ft away, sitting on the ground on seasonal camp (no city water there). If I run 3/4" suction pipe to the pump, leaving the output side unchanged 1/2" - will it be better, worse, or the same? Plumbing theory suggests suction/input 1 size bigger than output, but I have a feeling that it will not matter much on this short distance.

There is 0.5 micron sediment filter before the pump, with 3/4" outlets and 1/2" adapters, so I can have input tube either 1/2 or 3/4. Filters have been known to prefer larger input, too.

[mapguy]

Almot wrote:
Ok, so 3/4 will be the same or not much better than 1/2.

Another question: will it make any difference to the pump if the intake tube is 1) up-lift tube inside from the bottom of the aux tank, through the opening near top of the tank, then 3ft down to the ground, then 5ft up to the faucet (let's ignore shower for now),
or 2) from another opening near bottom of the aux tank, few inches to the ground - and then again 5ft up to the faucet.

#1 adds one more 90-bend, this should add more friction - but I have a feeling that pump won't even notice this.

Your pump has a specification for height (head) & tube diameter of the suction line. Generally speaking these pumps have a limited amount of head they will pull fluid reliably. Adding one-way check valves can extend the suction abilities somewhat but add failure points. On the discharge side -you are not as limited in regards to size and distance.

[Almot]

Ok, so 3/4 will be the same or not much better than 1/2.

Another question: will it make any difference to the pump if the intake tube is 1) up-lift tube inside from the bottom of the aux tank, through the opening near top of the tank, then 3ft down to the ground, then 5ft up to the faucet (let's ignore shower for now),
or 2) from another opening near bottom of the aux tank, few inches to the ground - and then again 5ft up to the faucet.

#1 adds one more 90-bend, this should add more friction - but I have a feeling that pump won't even notice this.

[DrewE]

mobeewan wrote:
With the tank sitting on the ground part of the water in the tank is going to have to be lifted higher than the water that would normally be in the onboard tank. The pump will have to work harder then it normally would to lift it that distance. The Shurflo pumps are designed to lift water about 12 feet using 1/2 inch ID tubing so your tank on the ground is well within the operating parameters. However, keep in mind when switching to three quarter inch pipe (1.066 OD/.824 ID for standard schedule 40) your internal diameter will be significantly larger and your volume and weight of water in that pipe that the pump is lifting will more than double and the pump will be working harder especially while self-priming to lift that amount of water.

The weight of the water in the pipe is larger, but the pressure at any given height is the same because the weight is spread out over a larger area (ignoring the frictional resistance from flowing along the pipe wall). The pump doesn't "see" how big the feed pipe is, only what the pressures are, be they positive or negative (with respect to air pressure).

You can demonstrate this by comparing how hard it is to drink a beverage through a skinny straw and a fat straw. You don't need to suck more strongly with the larger straw. If you try to slurp rapidly, the skinny straw is harder to use because of the frictional losses.

[joshuajim]

I don’t think the “weight” of the water has any effect. Pumps don’t actually “suck”, they create a low pressure which allows atmospheric pressure to push the water to the low pressure area. A larger pipe hs a larger cross section for the 14.7 PSI pressure to act against.

That is the exact reason that most pumps cannot lift water more than approx 33 feet.

[mobeewan]

With the tank sitting on the ground part of the water in the tank is going to have to be lifted higher than the water that would normally be in the onboard tank. The pump will have to work harder then it normally would to lift it that distance. The Shurflo pumps are designed to lift water about 12 feet using 1/2 inch ID tubing so your tank on the ground is well within the operating parameters. However, keep in mind when switching to three quarter inch pipe (1.066 OD/.824 ID for standard schedule 40) your internal diameter will be significantly larger and your volume and weight of water in that pipe that the pump is lifting will more than double and the pump will be working harder especially while self-priming to lift that amount of water.

[Almot]

westend wrote:
If you have a loss of pressure, elevate the aux. storage tank. The small diaphragm pumps used in most RV's do not have a big pump head lift.

I didn't notice a loss of pressure with 1/2 tube on aux tank. Little ugly noise-maker is pushing same sub-par flow (compared to average shore home) with same disgusting noise as it was with the built-in RV tank :).

The aux tank is sitting on the ground, its bottom is ~1ft lower than the trailer tank, but it's taller, so half the time the water level is at least same high as it would be in RV tank.

[westend]

No, it won't change delivery rate on the pump output. If you have a loss of pressure, elevate the aux. storage tank. The small diaphragm pumps used in most RV's do not have a big pump head lift. You could also put a bigger pump on the aux tank and use that through the utility fitting input.

[joshuajim]

GordonThree wrote:
When I plumbed my aux water tank to a 4.5 gpm 65 psi marine pump, I originally used a 1/2" right angle hose bib. It didn't supply enough water to the pump and there was considerable cavitation on the impeller. I upgraded to a straight 3/4" boiler drain valve and that solved the issue and improved the discharge.

A budget hose Bibb might have had an orafice of 1/4”. A full bore ball valve would have been a better choice.

[GordonThree]

When I plumbed my aux water tank to a 4.5 gpm 65 psi marine pump, I originally used a 1/2" right angle hose bib. It didn't supply enough water to the pump and there was considerable cavitation on the impeller. I upgraded to a straight 3/4" boiler drain valve and that solved the issue and improved the discharge.

[STBRetired]

X2 ^^^^

[DrewE]

In theory the larger pipe would have very very slightly less internal resistance to flowing water.

In practice it makes not one bit of difference given the low flow rate and fairly short distance you're dealing with. Reducing the suction head would likely have a greater benefit (if that is possible).

A larger filter may possibly be of some value in terms of frequency of servicing it and its tendency to choke off the flow of water. That's hard to say without investigating the specifics of your setup and water clarity and so forth. Even with that, I doubt I know enough myself to make any useful recommendations.

[Almot]

"Better" - I meant easier on pump or better for a filter? Or - less cr-ap deposited in the tube before it gets to the filter (not really "less" cr-ap, but less notable in larger tube).

[Old-Biscuit]

No change.......

Pump inlet/outlet still 1/2"
Pump 3 gpm/45 psi regardless

[ScottG]

I had a 5.7 GPM 65 psi pump that drew from a 1/2" line in my last TT. It worked great and could provide flow to more than one faucet at a time.
So I honesly dont think a bigger line would help especially with a smaller pump