Sprinkler System

[97Magna]

If I have a 2" PVC water line coming from a 2" water meter, with about 78 gpm flow and about 50 PSI static pressure, what is the best way to determine the number of impact sprinkler heads I can operate at once? At that PSI, the 3/4" heads I was looking at can output 5.0 gpm, but I was planning on restricting the flow and taking them down to about 4.5 gpm. (I don't want to put so much demand on the system that I can't turn on the sink or flush the toilet.) I can split into zones, but I want as few as possible.

Also, I can add a booster pump if necessary. But, I don't need to overcome significant head losses of elevation or length, and I think the booster pump would only help with pressure and not total flow demand, right?

This system will help control dusty roads at an industrial site. So, the 2" line is available, because it is occasionally used for filling a large wash basin, but other than that, the only demand is one office restroom.

Thanks.

 

[coloeng]

Many sprinkler system suppliers will design the system for you. You usually have to pay them for their time, but that payment can usually be used as a credit if you purchase the material from them.

 

[bimr]

The spray heads do not all operate at the same time, sprinklers operate with zones.

Get the flow information from a supplier as to the capacity of each sprinkler.

Operate the sprinklers in zones.

A booster pump will add head but not flow.

 

[97Magna]

@bimr

Thank you for the reply. I didn't state this clearly, but I am aware that operating by zone is best. I was trying to figure out how many zones I would need for estimating material & installation costs. This will be controlled manually by opening / closing valves, not with a traditional timer / control panel. I was just trying to confirm if some of my assumptions were correct - like, if I only have 78 gpm available, there is enough of a linear relationship that at 78 gpm / 4.5 gpm, I certainly couldn't run more than 17 heads at once, even if I added a booster pump.

 

[Artisi]

It's a two pronged review, you need to establish what flow rate you have at various pressures versus how many sprinklers you can run at that flow rate and pressure, this needs to be established at each zone, not at the meter.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[97Magna]

@Artisi So I need to install the system first before I can determine what to install? That seems easy enough.

 

[coloeng]

97Magna, if you want to figure this out yourself you will need to model the system. It appears to be a fairly simple system so you could probably do it in Excel or some other spreadsheet. The following steps should help.
1) Develop a system curve just downstream from the meter by flowing the line at various flowrates and meauring the residual pressure at the downstream side of the meter for those flowrates. This will determine the headlosses upstream for the various flowrates.
2) Determine the configuration of your system, i.e. lengths and diameters of pipe, where the furthest sprinkler zone from the meter will tee into your service line, etc.
3) Calculate the headlosses and resulting residual pressures for various flowrates based on the length, diameter of your system.
4) Use an iterative process until you find the optimum flow to the sprinkler system that still allows you to use the sink or flush a toilet.

Remember that the water system that your meter is connected to operates at always changing conditions. Just because the pressure at your meter was 50 psi one day doesn't mean it won't be higher or lower than that at some different time. Leave a margin of error in your calculations.

You may find that you'll need a booster pump for your system.

 

[97Magna]

@coloeng Thank you for the help.

 

[Artisi]

97Magna, it's fairly simple, means applying some intelligence or asking for assistance, sarcasm won't do it for you.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[97Magna]

@Artisi Ok. I thought I had asked for assistance by posting in this thread. Your response indicated that I needed to determine exactly the information that I said I don't know how to determine. I am not that familiar with how this forum works, but I don't think you're required to post a response if you don't know how to solve my problem.

I don't work in hydraulics. I thought there was a way to model the system, on paper, with some kind of formulas, to determine what the system could be. For instance, giving my starting conditions, if I had 1000 feet of PVC pipe, let's say 2", and there was only minor head loss due to elevation change, let's say 4 psi. And, each sprinkler was capable of 3.4 gpm at 25 psi, then how many sprinklers could I operate at once. 5? 25? You wrote that I "need to establish how many sprinklers you can run at that flow rate and pressure." Maybe you didn't intend it that way, but that sounds like EXACTLY the question I was asking for help with. Hence, the reason for my sarcasm.

The sprinkler manufacture provides a chart of PSI v GPM (25/3.4 30/3.8 40/4.4 50/5.0). That's all well and good, but if I lay 1000 feet of 2" PVC pipe, and put 50 of those sprinklers on there, I don't believe it's not going to operate at ANY of those conditions.

 

[hydrae]

download the free program EPAnet from EPA.gov

you can use the program to model the entire system
each sprinkler head is an emitter instead of a fixed demand
The supply is modeled as a reservoir, use a general purpose valve to model the supply losses
you will need the residual pressure at the 78 gpm not just the static at 0 gpm

use the help section to understand about the various items in the model

PS this is doing what Artisi was suggesting but the model will do the hard number crunching
remember garbage in = garbage out

Hydrae

 

[97Magna]

@hydrae Thank you very much.