Return Plumbing Questions

[algaeguy]

Hi everyone:

I'm working out the plumbing for my new 152 gallon system. My tank was drilled (through an external horizontal overflow)for a 1- 1/2 inch return bulkhead fitting. However, I plan on using an Eheim submersible pump for my return flow, which has a 3/4 outlet.

My question is this: If I use 3/4" fittings and pipe for the return from the pump and then use a 3/4"x1-1/2" bushing to increase diameter near the return outlet, will this dramatically reduce the return pressure, or create any problems for my pump? Is there are better solution?

Two more simple questions if I may:

1) Is SpaFlex available in 3/4" size? If so, where is a good online vendor to get it from?

2) When using Spa Flex, do you glue the the SpaFlex right into your fittings (ie; ball valves), or is it best to glue it to hard piping before attaching the ball valves?

Thanks in advance for your help!

Scott

 

[fio1022]

http://www.savko.com/index.asp

 

[algaeguy]

Thanks for the link, fio!

Scott

 

[MJAnderson]

Glue it directly on (at least that's what I was told)

Not sure about the pipe question. On my Mag 7 going from 3/4" to 1.0" increased the flow but I know at some point it becomes a negative.

 

[algaeguy]

Thanks MJAnderson!

I'm also wondering if it's not adviseable to glue spaflex right into slip bulkeads. or if it's best to use a rigid pipe first?

Scott

 

[merseyman]

... going from a smaller diameter pipe to a larger outlet will not reduce overall "flow" (nor increase it) but will disperse it. It's like letting a garden hose flow unrestricted versus partially capping it with a thumb. By capping it you may actually reduce overall flow, but the increased pressure equals increased force for the water spray. This may be more/less effective, depending on your goal.

 

[ChemE]

Going with a larger diameter pipe will always increase flow though you can move into diminishing returns quickly; larger diameter pipe will never reduce flow as MJAnderson stated. This occurs because as the pipe diameter increases, the fluid velocity decreases substantially. This in turn reduces turbulence which reduces drag in the pipe which reduces back pressure on the pump. Aquarium pumps are all centrifugal pumps (except for peristaltic dosing pumps which are positive displacement pumps) and centrifugal pumps produce increased flow with reduced back pressure.

 

[merseyman]

... it's not going to increase flow much when you're just talking about the outlet that is a larger diameter, rather than an extended section of the pipe that the water runs through.

 

[ChemE]

It very well could depending on the flow rate and length of pipe. Do you honesty think I don't understand fluid dynamics well enough to comment on this?

 

[MJAnderson]

ChemE,

Does the weight of the water in the pipe at some point offset the benefit of the reduced turbulence?

Sorry if I gave bad advice!

 

[brackishdude]

No the weight of the water is not important. A pump's "max head" is independant of the pipe size (Pressure is what matters, and the wight of the water is proportional to the increased crossectional area of the pipe).

I would recommend increasing the size of the return pipe at the pump to match that of the bulkhead up at the tank, rather than a narrower pipe enlarged only at the bulkhead. It's a short run, so the price shouldn't be much more. The increased diameter should result in some degree of increased flow. The difference between 3/4 and 1 1/2 is substantial.

Find a source (here?) that allows you to create a virtual return with pipe dameter, length, height, and fittings (elbows, etc. . .) to get a sense of the effect each has on the flow. Note that for any given height, the proportional change due to pipe diameter is the same, and vice versa.

 

[ChemE]

<a href=showthread.php?s=&postid=7943024#post7943024 target=_blank>Originally posted</a> by MJAnderson
ChemE,

Does the weight of the water in the pipe at some point offset the benefit of the reduced turbulence?

Sorry if I gave bad advice!

Nope. I've posted to a few threads now where this idea of water weight was brought up and discussed. Weight it totally unimportant all that matters is pressure which is weight (force more correctly) per area. The static pressure in the return plumbing is independent of the return pipe diameter.

No need to apologize though, don't sweat it. My last post wasn't directed at you.

 

[Qwiv]

When you glue spa flex, make sure you use the glue for spa flex. The glue used for spa flax stays somewhat pliable. The regular stuff cures hard. The spa flex can pull out of fittings when you use regular glue. The gray heavy duty stuff works as well IME.

 

[algaeguy]

Thanks again, everyone. This stuff is often confusing to me and I want to get it right!

So if I am understanding brackishdude's and ChemE's advice, it seems like I should increase the diameter right at the pump, since the static pressure on the pump will be the same regardless of pipe diameter, right?

Qwiv- thanks for the advice on the glue for SpaFlex!

Scott

 

[ChemE]

<a href=showthread.php?s=&postid=7949523#post7949523 target=_blank>Originally posted</a> by algaeguy
...So if I am understanding brackishdude's and ChemE's advice, it seems like I should increase the diameter right at the pump, since the static pressure on the pump will be the same regardless of pipe diameter, right?

You got it.

 

[algaeguy]

And they say miracles don't happen every day!

Scott

 

[MayoBoy]

What pump are you using? What's your projected flow rate?

Pipe sizing must take into account flow and friction. The suction and discharge size of the pump is usually NOT an indicator or pipe size.

Look at one of the calculators like this http://www.aquaart.com/pondFLC.html For swimming pools, Health Dept. regs generally recommend not exceeding 6FPS for suction, 8FPS for pressure and 4FPS for gravity. Stick with those guidelines and you won't go wrong.

Note that all those numbers are GPM not GPH so don't forget to convert if necessary.