Myth on increasing pipe diameter?

[BeanAnimal]

This is why even 100' across water towers pump water up and over the top with 3-6ish inch pipes instead of hooking the same pumps, that can support at least 100' of head up to the bottom of the tank.

No...that is not even close. The water is pumped over the top through a pipe because if it were plumbed directly into the tank, the tank would drain back through the pump.

Secondly, the fixed head allows the pump to be properly sized for the required discharge rate (and resulting suction heard), regardless of the water level in the tank. Discharge rate stay constant and there is no chance of cavitation at low ta no levels.

AGAIN... The weight of the water is not a factor here.

 

[SGT_York]

+1 water is pumped to the top of a water tower so you can turn the pump off. Newer systems keep the pump on to maintain the pressure without the water tower.

Aqualund, If your theory of bigger is always better, why does it not show up during actual tests? You have made no rational response as to why three of us have stated we have conducted controlled tests where the same pump performed better at height with a smaller diameter pipe, all other things remained constant on my test (technically it was someone elses but I was there).

I also cannot grasp why 1.25 should be used in lieu of 1.5" pipe the larger the pipe difference the more quantifiable the results should be 4" pipe would be preferred, but I'm too cheap to get it.

Why would bigger pipe be a hinderance. Larger pipe has lower friction loss? By your statement it should always have higher flow so larger is better when practical???

 

[Aqualund]

I feel like you people are not even reading what I write.:headwally:

Aqualund, If your theory of bigger is always better, why does it not show up during actual tests? You have made no rational response as to why three of us have stated we have conducted controlled tests where the same pump performed better at height with a smaller diameter pipe, all other things remained constant on my test (technically it was someone elses but I was there).

I have repeatedly told you with regards to this that I can already identify one different variable: different material between pvc and garden hose, so different coefficient of friction. Do you understand what this means? Please, tell me if not and I can explain it to you.

for 1200gph, 1.25 is optimal balance between friction loss and cavitation. Above 1.25" cavitation begins to counteract the gains made by reduced friction loss.

so yes even with this example...1.25" is always better than .75" for 1200gph.

you would only want 4" pipe if you were pushing like 15000gph or more.

 

[SGT_York]

You are obviously missing what I have written, I used no garden hose just PVC. One person mentioned tn briefly and that has consumed your through process and rational reading abilities.

I do like your clarification, of 1.25 and should be able to add it to the test with no issues.

 

[Aqualund]

Oh, you're the one with the mag 7 with regular pipe. jda was the garden hose. sorry.

For the mag 7 at around 600gph you should be using 1" pipe to see an improvement. above 1" for 700gph starts to impact the gains made by larger diameter in the same way I mentioned earlier about the 1.25 to 1.5 difference for 1200gph.

 

[SGT_York]

That's what I've been saying the whole time but more focused on the height. Always is certianly not the case and you should never use it with exceptions.

It's a mission then hopefully I can get some data in a few weeks. (I'm still trying to run my horizontal siphon with a john gusset valve to purge the air test) Outdoor test will have to wait for warmer weather.

 

[Aqualund]

Always is certianly not the case and you should never use it with exceptions.

But it is. 1" will always be better than .75" are you literally saying that because 3" pipe isnt better for a mag 7, that you should never use bigger pipe on a mag 7? that's just silly.

Just a reminder, mag drives like 9 and below are notoriously weak in counteracting the reverse flow of water in the pipe above them. so the more height of pipe you have above them, the less effective they will be. add larger diameter above their rated flow (in this case...anything above 1") and you will also be adding cavitation and turbulence in a manner that will also reduce output...so of course a .5" pipe will outperform 1.5" pipe for this pump because it's only getting friction loss and less turbulence-cavitation loss.

But it's important to remember, you can get a larger diameter, like 1" and always outperform .5", .75", and above 1". always. so larger is better.

 

[SGT_York]

I'm certainly NOT suggesting NOT using a larger diameter pipe, read my posts at the thread beginning for evidence.
The Manufacturer calls for 1.5" pipe to meet the flowcharts on their MAG Drives. I'm sure they ran the tests and 1.5" and it was the max flow on those pumps. In order to get MAX GPH on their advertizing.

I can say for surety that a MAG 7 will outperform with a 1.5" pipe than a 1/2" pipe, to near 7 feet of head. It's after that point in height where the "cavitation" (still not sure this is the correct term for the "weight of the water" discussion) starts to degrade GPH. I'm still sticking to my "impeller variance" as the cause for the flow differences.

 

[BeanAnimal]

I'm certainly NOT suggesting NOT using a larger diameter pipe, read my posts at the thread beginning for evidence.
The Manufacturer calls for 1.5" pipe to meet the flowcharts on their MAG Drives. I'm sure they ran the tests and 1.5" and it was the max flow on those pumps. In order to get MAX GPH on their advertizing.

It is not that larger pipe would begin to hinder the flow, it is that given the pump curve, moving past 1.5" discharge pipe enters the zone of diminished returns (size and cost increase for little or no gain in performance).

It's after that point in height where the "cavitation" (still not sure this is the correct term for the "weight of the water" discussion) starts to degrade GPH.

The only time "cavitation" comes into play is due to inlet restrictions.

Lets take an extreme example to illustrate the issue:

A pump advertised at 1000 GPH with 10' of head is plumbed with a 1" intake and a 1" discharge. At those plumbing sizes, the pump may produce 800 GPH, as it sees the 10' of vertical head as 12' of total head due to the added head created by the higher velocity and smaller diameter (friction) in the discharge.

Increasing the diameter to 1.5" (reducing the total head loss) of the discharge pipe allows the pump to do more work and it tries to pump 1000 GPH. However the 1" suction plumbing can not handle the increased flow and the impeller imparts a higher negative pressure in the suction pipe than it can deliver. The result is that the surface tension (cohesion) between the water in the volute starts to break down. Vacuum bubbles appear and the impeller does not function as it is designed. This is cavitation, and the discharge flow will drop, possibly even below the original 1000 as the pressure in the volute oscillates back and forth between the suction and discharge trying to equalize.

This the larger discharge pipe only reduced the flow because the suction side of the pump was not setup to handle it. The suction side of the pump must ALWAYS have less total head loss than the discharge. Using pipe that is larger on the discharge side than it is on the suction side is a sure fire way to degrade centrifugal pump performance. This not only goes for pipe "size" but total head loss due to bends, turbulence, changes of direction, etc.

The physics are sound, it is simply a matter of the pressure differential that the impeller sees.... period. The "weight" of the water on either side has no bearing.

 

[SGT_York]

"It is not that larger pipe would begin to hinder the flow, it is that given the pump curve, moving past 1.5" discharge pipe enters the zone of diminished returns (size and cost increase for little or no gain in performance)."

What I have seen is that the smaller pipe has more GPH at higher head, that is what I can't quite explain using engineering. Diminished returns I understand but why is there less flow with larger diameter???

"The only time "cavitation" comes into play is due to inlet restrictions." I agree hence why I added the remark about I don't think it's the correct word.



Not sure why increasing the diameter would reduce the headloss, HL should remain constant friction loss would decrease by increasing the diameter for sure.

Don't think it would be possible to initiate cavitation by changing the outlet alone. they work better with nothing attached to the outlet at all. Cavitation would need to be caused by the inlet as you mentioned without regard to the outlet. Increasing pressure will reduce flow but zero pressure on the outlet won't reduce flow. (certainly would if you adjusted the pump's outlet housing but not after the initial restriction) In other words, not by adjusting the plumbing.

 

[BeanAnimal]

Not sure why increasing the diameter would reduce the headloss, HL should remain constant friction loss would decrease by increasing the diameter for sure.

Head loss is being used here as an aggregate term, a combination of vertical head, friction head, turbulence, etc.

Don't think it would be possible to initiate cavitation by changing the outlet alone. they work better with nothing attached to the outlet at all.

Sure.. Lets say you have a Dart Gold plumbed through a 1.5" pipe 2 feet long with (2) 90's, one of them right at the intake of the volute. You have the pump hooked to 1" discharge plumbing and all is well. You decide to tweak the setup because somebody told you that larger discharge pipe is less restrictive.... You setup the new discharge pipe and BAM cavitation because the suction side can no longer supply the pump with its new (less restrictive) discharge plumbing.

 

[Lavoisier]

Sure.. Lets say you have a Dart Gold plumbed through a 1.5" pipe 2 feet long with (2) 90's, one of them right at the intake of the volute. You have the pump hooked to 1" discharge plumbing and all is well. You decide to tweak the setup because somebody told you that larger discharge pipe is less restrictive.... You setup the new discharge pipe and BAM cavitation because the suction side can no longer supply the pump with its new (less restrictive) discharge plumbing.

This example is quite helpful. Thank you.

 

[SGT_York]

Well that would certainly be the case in that situation, but I think you'd have some cavitation regardless of the outlet due to the 90 at the intake. (hopefully it was a swept 90) should be at least 18" to provide space for laminar flow. (water only flows nice and streamlined with low flows, or high pressure) Most pumps wouldn't be plumbed so ignorantly. (it wasn't your setup was it? )

 

[BeanAnimal]

Lets also note that these are not closed loop systems:

If the pump draws from a pool and does not return the water to that pool, then the level of the pool decreases over time. As the level of the suction pool draws down, the suction head decreases or, in other words (by definition) the total vertical head loss increases.

Again, if we are talking about an experiment where water is drawn from a container, but not returned, then the total "head" of the system changes as the water is removed from the pool. When the experiment is AT or near the shut-off head, then things can get strange as the more efficient system will draw the pool down faster, but also reach shut-off head faster. The less efficient (more restricted discharge) will flow at a lower rate for a longer period of time before reaching shut-off, but the output will look much more uniform.

Moreover, observing the discharge pool can be confusing. The efficient pump with large discharge plumbing will hold twice the volume that the less efficient (smaller) discharge plumbing holds.

Likewise, observing the discharge stream may also be counterintuitive, as the large pipe will have more surface area, a lower velocity and what appears to be a less aggressive (low pressure stream) flow. The smaller plumbing will discharge at a higher velocity and look more aggressive (with a higher pressure stream).

So again, when we are talking about observations of the systems in question at or near "shut off head", what we see is a complex interaction of variables, but the physics don't lie Given the same physical properties, larger discharge plumbing presents less total head than its smaller counterpart. Less total head means more total flow

 

[BeanAnimal]

Well that may certainly be the case but I think you'd have some cavitation regardless due to the 90 at the intake.

You only have cavitation when the intake can not keep up with the discharge, that is portions (or all) of the volute move into a negative pressure situation

It is (as mentioned) a simple case of pressure differential. In fact, many of us have to increase the effective head with a valve on the discharge to prevent cavitation due to poorly thought out suction plumbing. HOw many times have you see a ReeFlo pump necked down to 1" or 1.5" on the intake, with a 90 at the volute so that the pump sites at right angles to the sump? "Hey I got a new pump, but I have tons of micro-bubbles in the tank and it sounds like rocks are tumbling around in it".


No, I (when possible) follow best practices with regard to pump plumbing

 

[SGT_York]

yes I've corrected a few I've seen like that.

FYI,

I drilled two holes in my sump first goes to a 1" 90 then to a 1.5" T to a 2" union ball valve. another 1.5" 90 then 14" to the 1" opening. the second hole really assists in the external pump arrangement. (lost my first tank while TDY when the inlet got clogged and the flow stopped) Highly recommend to all. The 2" ball valve is about a 1.5" internal diameter.

 

[SGT_York]

Sorry, TDY is temporary duty meaning time away from home.

 

[atreis]

You missed my experiement

Still no one has been able to explain why when you take a Mag 7 it will pump more at 15 feet with 3/4" pipe than with 1.5" pipe. Same pump same PVC style. Pump impeller is the variable I think is being left out of the equations to show if increased diameter will increase flow at all height levels. Nothing was changed just used a 3/4" to 1" thread to slip then 1" to 1.5" slip to 1.5" pipe. on the 3/4" just used a threaded to slip coupling.

See the bit about turbulence introduced by a rapid change in pipe diameter from my post. (This applies whether you go from larger to smaller or vice versa.)