Will there be water coming out of the last hole?

[sendhelp]

Hello everyone!

I am planning to make an irrigation system for our new hedge by drilling small holes (1mm diameter) in a 1/2 inch hose. The hose will have a length of approximately 28 meters and there will be one hole every 30cm, so about 93 holes in total. The water will be coming out of the tap with a pressure of 3 bar. The water is coming out of the tap at a rate of about 30 litres per minute. The hose will run mostly in a straight line. Will there be enough water coming out of the last hole in order to have the furthest plant properly irrigated? Could I drill the holes even wider or is it already above the maximum diameter for a hose of that length?

I hope you guys can help me, I don't know how to calculate it but someone mentioned this forum to me when I had that question! Thanks in advance! :)

 

[DaveC426913]

Hello everyone!

I am planning to make an irrigation system for our new hedge by drilling small holes (1mm diameter) in a 1/2 inch hose. The hose will have a length of approximately 28 meters and there will be one hole every 30cm, so about 93 holes in total. The water will be coming out of the tap with a pressure of 3 bar. The water is coming out of the tap at a rate of about 30 litres per minute. The hose will run mostly in a straight line. Will there be enough water coming out of the last hole in order to have the furthest plant properly irrigated? Could I drill the holes even wider or is it already above the maximum diameter for a hose of that length?

I hope you guys can help me, I don't know how to calculate it but someone mentioned this forum to me when I had that question! Thanks in advance! :)

Distance from spout isn't only thing that affects pressure. So does slope for example.

Rather than trying to calculate your settings beforehand, why not make them adjustable so you can tune them?

You could change the size of the holes, but something that might be easier is to place constrictions on the hose at regular intervals
- either washers inside the hose*
- or some sort of pseudo-clamp around the hose.

Make them adjustable, so that you can tune the pressure that gets to each segment.

* This is what some pro systems use. For lengths longer than 50ft, they insert a restricting washer in the connector to keep the pressure up.

Also, is there som reweon you donlt just buy a 30m length of soaker hose?

 

[sendhelp]

Distance from spout isn't only thing that affects pressure. So does slope for example.

Rather than trying to calculate your settings beforehand, why not make them adjustable so you can tune them?

You could change the size of the holes, but something that might be easier is to place constrictions on the hose at regular intervals
- either washers inside the hose*
- or some sort of pseudo-clamp around the hose.

Make them adjustable, so that you can tune the pressure that gets to each segment.

* This is what some pro systems use. For lengths longer than 50ft, they insert a restricting washer in the connector to keep the pressure up.

Also, is there som reweon you donlt just buy a 30m length of soaker hose?

Hi!

Thanks for the quick reply.

Adding adjustable clamps around the hose to finetune the pressure seems like a good idea, I will certainly keep that in mind.

All I could find around here so far were MicroDrip irrigation systems, where a tiny amount of water constantly leaks out of the hose, basically running all day long. I want to irrigate larger quantities of water in a short amount of time, twice or so per day. I don't want to run the drip system, because my garden pump would basically turn on and off during the whole day then when pressure is lost. I didn't know that there were "sprinkler" hose solutions, could you link me some of them?

 

[DaveC426913]

Hi!

Thanks for the quick reply.

Adding adjustable clamps around the hose to finetune the pressure seems like a good idea, I will certainly keep that in mind.

All I could find around here so far were MicroDrip irrigation systems, where a tiny amount of water constantly leaks out of the hose, basically running all day long. I want to irrigate larger quantities of water in a short amount of time, twice or so per day. I don't want to run the drip system, because my garden pump would basically turn on and off during the whole day then when pressure is lost.

You can get watering timers. I had one to water my yard during the cooler parts of the day, when it wasn't prone to evaporation.

You can get a single zone timer or a multi-zone timer.

The advantage of a multi-zone timer - which may or may not prove useful to you - is you can parcel out your watering over time to maximize pressure. There's an upper limit to the length of a soaker hose system, when the pressure drop so low that it stops becoming effective.

My yard was too big to do all at once. So I ran more than one hose to different parts of my property, and then set the timer to run one for 30 minute sthen the other for 30 minutes. That way, each gets the benefit of full pressure.

Multi-zone timers can vary from two to four 'zones' and the prices range accordingly.

If you are concerned about the cost, consider the financial cost of running too much water or too little or the wrong time of day when it will just evaporate. It will never forget, or get ill or go on vacation. And on hot, dry days, you just hit a button to "add 10 minutes" for an extra soak. Or skip watering on rainy days. Factoring in these things makes the timer pay for itself.

I didn't know that there were "sprinkler" hose solutions, could you link me some of them?

Look up "soaker hose" or "weeping hose" on any garden supply place.

Some micro waters system are very complex, but a soaker hose is just a simple hose made of a porous skin that just leaks water along its whole length, like this:

It would be ideal for your hedge.

The advantage to a soaker hose, BTW, is that it doesn't spray all your precious water into the wind where it will be carried away or evaporate. It drips it straight onto the ground. This also means you need to water less, as a larger percentage of the water is actually getting directly to your hedge's roots.

You can cut or clamp it to the exact length you need (which sounds like you'll need two 50' rolls), and you can even bend it around a between your hedge trunks to hold it in place (or use the U-shaped spikes).

I would link to some products but I assume, by default, that you are in the US, and - nothing personal, but - we here in the Great White North are doing everything on our power to not buy American goods. So, my advice to you is come north of the border and buy from us! Shop at any Canadian-owned Sheridan Nurseries or Rona.

2 x $25 soaker hose = $50
1 single zone timer = $25.
Total: $75.
Up-front Labour: 1h
Maintenance Labour: 0h

 

[russ_watters]

Here's a calculator for flow vs pressure and orifice size:
https://toolbox.tlv.com/global/US/calculator/water-flow-rate-through-orifice.html

I get 0.21gpm per orifice or 20gpm total. However, if your rated pressure is with the tap closed, you'll get a much lower pressure and flow, but all of the orifices should still have some flow.

 

[256bits]

The water is coming out of the tap at a rate of about 30 litres per minute.

That doesn't mean all that much if it is the flow from the tap with no hose connection.

The hose will have a length of approximately 28 meters

Test the flow coming out of the of the end of the hose. before you drill holes.
The flow will be less than the 30 L/min

 

[256bits]

Could I drill the holes even wider or is it already above the maximum diameter for a hose of that length?

No. Don't drill them too large.

 

[erobz]

but all of the orifices should still have some flow.

I have some doubts about that in this long and small diameter hose.

 

[Baluncore]

I would consider some form of dripper flow regulation. There will be a flow regulator available that is less sensitive to pressure than a simple hole in a wall, but with sufficient flow for short periods of irrigation.

Some systems regulate flow at each outlet by using a meandering path molded into the line. Another solution is a short length of capillary tube that is stabbed into the feeder line at each point that needs water.

 

[russ_watters]

I have some doubts about that in this long and small diameter hose.

Yeah, agreed - It occurred to me but I didn't bother checking the friction loss. if OP wants uniform flow he might want a bigger hose.

 

[jrmichler]

I did not do the calculations, but some hand waving arguments should be adequate here. The total flow area of the holes is equivalent to a single orifice 9.6 mm diameter, or somewhat less if you include the orifice coefficient. The hose is 1/2 inch (12.7 mm) ID. Since the flow area of the holes is significantly less than the flow area of the hose, there will be flow out the last hole. The last hole will flow less than the first hole, possibly about 1/2 as much as the first hole.

 

[erobz]

I solved the following system with 3 holes analytically:

Where

$Q_i$ are a particular segment volumetric flowrate.
$k$ is the hole loss coefficient in each branch (currently the same across all holes)
$a$ hole cross sectional area

$f$ is the mains friction factor from Moody Diagram (approximated as constant for the time) .
$A$ is the mains cross sectional area
$D$ the mains diameter

I mostly just wanted to try this Desmos link:



So when there are three 1mm holes across the 30 m length the flow rates are anticipated to be quite even. As for generalizing to $n$ outlets, I'll maybe have a go at 4 outlets analytically - the way it shook out for I could be looking for a pattern to emerge.

The other approach for getting a numerical result for your 93 hole manifold is linearization and iteration. The issue with that is I (a computer program) have to invert a $185 \times 185$ coefficient matrix. its predominantly zeros being entered, but it would be ridiculously laborious putting in the entries even if anything accepted something that large. What I will do is do some manageable number of holes (it becomes kind of large very quickly) and try to extrapolate based on a few points for kicks.

 

[marcusl]

Think about using a commercial drip system like that from RainBird. Running 90 taps at 30 L/min suggests a max rate of 1/3 L/min or 20L/hr, but that would run your pressure to zero so you want to back off by 1/2 or more (I.e., 10L/hr or less). A 2 gallon per hour (2GPH) dripper would be ideal and is a popular flow rate. RainBird’s drippers are pressure-compensated, so you get the advertised flow rate everywhere along the hose length. You just poke a hole through the tube wall with the punching tool and insert the dripper. Several other brands have comparable systems. I’ve used drip systems in my yards for over 40 years, and they work great.

 

[jrmichler]

I had some free time, so spent about two hours creating a spreadsheet to calculate this problem. I used an iterative method to solve.

The method is as follows:
1) Start with a supply pressure and an estimated total flow.
2) Calculate the pressure drop for the first 30 cm of pipe.
3) Subtract from the initial pressure to get the pressure at the first orifice.
4) Use Bernoulli to calculate velocity from the first orifice.
5) Calculate orifice flow from orifice diameter, orifice coefficient, and velocity.
6) Subtract orifice flow from from entering pipe flow to get pipe flow after the orifice.
7) From pipe flow, calculate pipe velocity and pipe Reynolds number.
8) Look up friction factor from Moody chart.
9) Calculate pipe pressure drop for next length of pipe.
10) Repeat until reach the end of the pipe.
11) Iterate the initial flow until the pipe velocity after the last orifice is zero.

A screenshot of the spreadsheet is shown below. Somebody taking a close look will notice room for improvement on the friction factors. The solution is not sensitive to friction factors at lower pipe flow rates, so I cut some corners there. This was a quick effort, and not fully cross checked, so I might be off on the exact number of orifices. Or some other error.

 

[DaveC426913]

Or just get 100 feet of soaker hose for 50 bucks...

 

[jrmichler]

Or just get 100 feet of soaker hose for 50 bucks.

Good grief, where's the fun in that?

The above calculation assumes that the water supply will deliver 9.2 GPM at 30 PSI. Since the OP mentions 30 LPM (8 GPM), apparently at free flow, the actual pressure and flow rate will be less. So I dropped the inlet pressure to 10 PSI. The resulting total flow was 5.3 GPM. In both cases, the flow from the last orifice is 52% of the flow from the first orifice.

The water velocity from the orifices is enough to cause erosion. The pipe should be oriented so the water jets point straight down into the ground without hitting the plant roots.

 

[erobz]

Good grief, where's the fun in that?

I was going to say the same thing! I need something to toil with next week. Plus, I've always had much curiosity about finding solution to this problem type.

Since the spread sheet is set up, would it be too much trouble to check it against the analytical result for 3 holes I shared?

 

[DaveC426913]

I am confident this is the first time in recorded history that a layperson has asked for advice from a bunch of engineers and the engineers took the idea and ran with it.