Question about Constrictions in fittings and equipment

[mguzzy]

So I've been searching around and I am sure this has been addressed somewhere on the forum already and if so I'm not finding it. I'm trying find out what the effect of the constrictions in fittings and equipment have on flow rates. I imagine a short narrow run is offset by the Venturi effect if the constriction isn't too dramatic. What got me thinking about it is I got a used replacement pump for my waterfall and it had these kind of fittings on it.

so a 1.5 inch pipe was constricted down to the size of a 1.25 inch pipe for about 4 inches. And again on the out flow from the pump. And a basic 2" ball valve I was looking at constricted down to the same diameter as a 1.5" pipe. It made me wonder how narrow and how long a constriction it takes to affect the flow rate and by how much when going through those types of fittings, Ball valves, check valves, etc.

And as long as we are at it what effect does right angle turns in the piping have compared to two 45 deg turns or sweeps.

Things that make you go hmm.

 

[ajw22]

@mas985 thoughts?

 

[mas985]

The types of fittings you are describing will have an insignificant impact on performance. The amount of head loss in those components is a small part of the total head loss.

Also, one thing to keep in mind is that when you reduce head loss in plumbing, it will increase flow rates but it will also increase energy use. Since run time requirements are usually independent of flow rate, reducing head loss can have negative impact on cost unless you have a VS pump where you can compensate with lower rpm.

 

[mguzzy]

@mas985 Hey, Mark, thanks for your response.. I understand there effects on energy usage of pumps and flow rates on the overall design of the plumbing in an actual pool.. My question is more basic.. You said the impact on performance (and I imagine you are looking at performance from the standpoint of head loss) is insignificant. I was wondering exactly how insignificant in a real world pool system. What is the total impact of the constrictions and turns various fittings of a pool system. Is it 1%, 5%, 10%.? As I read through the piping construction some people have done.. some have lots of 90's, various constricting valves and some people make great pains to avoid it by making every 90=2x 45's or oversizing the valves to avoid constrictions. I am wondering if it really makes that much difference in the range of pressures and flow rates that are typical in most pools. You say no, and I believe you but by how much..do you have a feel for that?

 

[mas985]

The exact impact of these fitting changes depends on your specific setup. Having a a lot of 90s is going to have a larger impact than having only a few. Also, the other equipment plays a big part. Heaters, valves, eyeballs, skimmers all add head loss so the more "other" head loss you have, the less impact the 90s have since they will be a smaller percentage of the total head loss. But even with the simplest of setups with little "extra" head-loss, the impact is still quite low, assuming a reasonable number of fittings:

1.5" Plumbing, 50' runs, single suction line to a skimmer, 2x1" return eyeballs, cartridge filter, 15x90s, 1.5 HP Uprated Single Speed Pump, no extra valves or equipment:

Operating points:

Sharp 90s: 74 GPM, 50' of head, 1762 Watts
Sweep 90s: 77 GPM, 47' of head, 1781 Watts

All 15 90s were changed to sweeps and it only improve flow rate by 4%. However, if you have any other equipment/fittings, this will dilute the flow rate improvements. Parallel runs from another skimmer or MD will also dilute the flow rate improvement.

As for your pipe constriction, 4" of 1.25" pipe is equivalent to about 8" of 1.5" pipe. Again, not much of a change unless you are using a lot of the smaller pipe.

One other factor is that sweep 90s do take up more room so if you have to add more 90s or use longer pipe to accommodate them, then that would need to be factored in as well. I can almost guarantee that the plumbing shown below is worse off than if the PB had used street ELLs.

 

[mguzzy]

Ok.. so the individual effect of one valve constriction or one 90 is less than 1% probably closer to 0.1%. And the cumulative effect of multiple constrictions that may become significant depending on the number of them and the system design. That makes sense. So from a ballpark sense the over all effect of a bunch of constrictions and El's in a system might approach 10% or something in that range in some heinous design.

I recently had a conversation with a "dude" at HomeDepot.. he was trying to talk me into upsizing the ball valve I was looking at.. and I was trying to tell him that I didn't think it would make any difference considering the pipe size and what I was using it for... but he insisted. I decided to nod nicely and walk away. You've kind of confirmed my gut feeling on this. thanks

BTW... I love how this pic gets used so frequently here as an example!

 

[mas985]

It is a great example of what not to do.

 

[ajw22]

I don’t want to hijack this thread. Is there another thread explaining what all that plumbing is doing? First time I saw that pic.

 

[mguzzy]

Hijack away!

I saw an install where everywhere a two way valve was needed a three way valve was put in and the other leg was caped off with a one foot extension.. It made the piping look horrendous.. I could only figure the PB only had three way valves and was too lazy to order the right ones... or he was planning for some unforeseen addition that didn't make sense.

 

[mguzzy]

Well I did a very unscientific test... My physics teacher always said good science is accompanied with informative graphs, illustrations, an introduction, description of the testing process and conclusions reached. So I made a graph but my dog ate the analysis.

I took a 5 gal bucket and timed the flow to fill up the bucket with a variety of things in the line.. a 1.5 inch ball valve, a 2 inch spring check valve and variety of constrictions and couplers from 1.5 to 2 inches...honestly there was no significant difference that I could tell. To facilitate quick exchange of the test pieces everything was dry fitted together. The biggest difference was how much my kids laughed when a pipe wiggled loose and blew off soaking me with water. sometimes the pipe at the equipment pad blew off which evoked a greater response since I had to bound to the switch. I submit that graph for you consideration and peer review.