SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Quoted from "mud"

...Let's say you put one of those "high pressure nozzles" on the end of a water hose and it is now "high pressure"...

Yea, as pass1 explained, a "high pressure nozzle" is really a misnomer since the water doesn't have any pressure once it's out of the pipe.

These nozzles should really be called "high kenetic energy nozzle" because the nozzle basically converts the water pressure into kenetic energy, and that energy is then what can be used for various purposes.

But trying to explain it is like trying to explain a vacum cleaner. From our experience, we say a vacum cleaner "sucks" stuff up off the carpet. But the reality is that the vacum simply creates an area of low air pressure. The air around us then "blows" stuff into the vacum cleaner as the air pressure around us tries to fill that low pressure area.

The same could be said about a straw. A straw does not suck fluid up a straw. Instead, it creates a low pressure area in our mouth, and the air pressure around us pushes the fluid up the straw.

There are only two ways to increase water pressure. One being through a elevation difference and the other with the use of a pump. This will physically increase the available pressure. Manipulating pipe size, and flow will enable you to minimize Pressure Loss through the piping as the water begins to flow. What you are seeing as water exits one of those hose nozzles is very high velocity, not high pressure. Technically ,once the water leaves the confines of a enclosed pipe and reaches the atmosphere the pressure is 0 psig. Think of this, you have a garden hose and it is free flowing water out the end, maybe 6"-8". You then put your thumb over the end to reduce the water passageway. The water now goes 10-15 feet by way of velocity force.. You did'nt add a pump nor did you change the elevation, so there is no pressure increase. Hope this clears the picture a little. You may want to google "Bernoulli's Equation" if you really want to get deep into this.

You have a maximum amount of pressure which is called "Static pressure". This is the maximum amount of pressure you can achieve and is the pressure when the water is not flowing. As the water begins to flow, the pressure decreases due to the friction loss from the pipes it flows through. The nozzle limits the flow of the water through the pipes, and thus increases pressure. The flow you see when the nozzle is removed, is the maximum amount of water you can flow through that pipe and the least amount of pressure. Flow and pressure are inversely proportionate to one another. Increasing flow decreases pressure and vice versa. Increasing pipe size will decrease velocities and therefore increase "working pressure" when flow is a constant.



So in essence, yes, you can manipulate the factors to increase pressures. You will never increase your static pressure without the use of a pump. You can increase working pressure by manipulating pipe size and flow.

Thank you very much, very informative and a big help.

One other aspect of all this I am trying to get my mind around...

Let's say you put one of those "high pressure nozzles" on the end of a water hose and it is now "high pressure". Let's say the non-nozzled water at the end of the hose is at 30 psi.

So what you have done is dramatically reduced the "pipe" size and increased the velocity. But actually you have REDUCED the actual PSI even though it appears you have increased it?

This is what's troubling me. Does this scenario increase actual PSI? And also, is there a way to increase psi through manipulating these three forces of pipe size, flow, and velocity?

Big thanks!

For a given pipe size, as the flow (gpm) increases so will your pressure loss increase. Lower your flow, (gpm) and you will have less pressure loss. The available pressure is not changing, you are only increasing or decreasing presssure loss through the pipe as the water flows through it.

I simple answer would be that as pipe size halved, the pressure losses increase by a factor of 16. That's because as velocity doubles, the pressure loss quadruples. (Quadruple the velocity equates to quadruple * quatruple to get pressure loss).

But the real answer for determining flow rates and pressure losses is to either look at pressure loss tables (such as the ones located in this PDF booklet: http://www.hunterindustries.com/resources/pdfs/Technical/Domestic/LIT194w.pdf page 76) or run a program that can convert pipe size to flow rate and pressure loss.

Basically, pipe sizes are NOT exact. As a simple example, consider copper pipe. There is 3 types of copper pipe, Type "K", Type "L" and Type "M". A 1" copper pipe will have an outside diameter of 1.025" regardless of the type. That way, the same set of fittings can all be used on all three types of copper. But the primary difference between the types is the pipe wall thickness (K thickest, M thinnest). As the wall thickness increases, the inside diameter decreases. So there is a difference in the change of velocity (for a given flow rate) depending upon which type of copper you are using. However, the sizing of copper is such that the rules of thumb you are stating are rough values regardless of pipe size.

But PVC seems to be a whole different animal. I'm not sure where the sizing of PVC pipe comes from, but the difference between 1" PVC and 1/2" is a completely different factor than it is for copper, and the rules of thumb can throw things way off.

Example 1:
2" Sch40 PVC pipe carrying 34gpm will have a velocity of 3.5fps and a pressure loss of 1psi/100ft.
1" Sch40 PVC pipe carrying 34gpm will have a velocity of 13fps and a pressure loss of 25psi/100ft.
As expected, the velocity difference is 4 fold, but pressure difference is 25 fold.

Example 2:
1" Sch40 PVC pipe carrying 6pgm will have a velocity of 2.3fps and a pressure loss of 1psi/100ft.
1/2" Sch40 PVC pipe carrying 6gpm will have a velocity of 6.75fps and a pressure loss of 14psi/100ft.
The velocity difference is 3 fold (not 4), and the pressure difference is 14 fold.

Hello,

If you halve the pipe size the velocity of the water quadruples. If you halve the flow, you halve the velocity.

OK, but what does the pressure do? If you halve the pipe does the psi remain the same as it was or does it increase? Any information/formulas/ charts would be greatly appreciated.

Thanks!