Pressure loss for head & swing joint?

sjensen,
We have talked with multiple fittings manufacturers, the maker of Lasco swing joints, contractors, etc, and we have not heard of anything other than very slight pressure loss by fittings. Our fittings slider should provide an acceptable range for this.

The head itself — what data are we supposed to use? I have not seen anything in the manufacturer chart to account for this. Are there performance charts you are using to compare against your results? Please let us know your findings and what you think is incorrect and we can go from there. Thanks!

Jake,

Thanks for the reply. I'm aware of the lack of printed data on the pressure loss of sprinkler heads but it seems that there should be some sort of allowance included in the calcs for the water passing through the swing joints and the heads?

The assumption we typically make comes from years of doing irrigation design and asking questions from product development people working with Rain Bird, Hunter, Toro and Thompson. The information we've gathered and use in our manual calcs is that the pressure loss for a typical 4" pop-up rotor is approximately 5.0 lbs. and for a 6" pop-up rotor is approximately 6.0 lbs. A spray head body has a much lower rated spring tension so the pressure loss we typically use for those is 2.0 lbs. for a 4" pop-up body, 3.0 lbs. for a 6" pop-up and 5.0 lbs. for a 12" pop-up body.

The swing joint manufacturers I think are not concerned about pressure loss, but lets think about this for a minute. We are shoving 8 to 12+ GPM through a 1" hole and a series of 90 degree elbows to get the water to the rotor itself. With a typical rotor operating at approximately 11.0 GPM for example, a 1" manufactured O-ring swing joint will have a pressure loss of approximately 2.5 lbs. (if the GPM of the rotor goes up, we add another 0.5 lbs. of loss for each 6-8 GPM). We use about the same losses for a full I.D. 3/4" rotor swing joint.

Flexible "Funny Pipe" swing joints are a whole different deal. Here we are now talking about putting from 0.5 to 4.0 GPM through a 5/16" diameter hole (and the barbed fittings are even smaller with a 1/4" hole) which is why we never use "Funny Pipe" with rotary sprinklers. When they are required by some clients on spray heads (only) we figure 2.0 lbs. of pressure loss for these "Funny Pipe" swing joints.

Since the Land F/X software does such a great job of running pressure loss calcs I figured I'd at least ask the question about head and swing joint losses since I couldn't be certain that those losses were accounted for when reviewing the Critical Analysis information generated by Land F/X. There is a definite pressure loss through everything from the mainline connecting valve tee at the water service line, the water meter, backflow assembly, mainline itself, mainline fittings, irrigation zone control valve, lateral lines, lateral fittings, etc. so it stands to reason that there should be pressure losses accounted for at both the swing joints and within the sprinkler heads themselves when running the pipe-sizing routines using Land F/X.

Hope the above gives you some information for consideration of future inclusion in the pipe sizing calculations performed by Land F/X.

~Shawn

Shawn,

The data that you're getting from the manufactures has to take a few things into account. For instance, A Rain Bird 1800 series has various configurations the standard, the one whit the seal-a-matic and the one with a pressure regulator. Then you can even specify a 1800-SAM-PRS. Any time that you see a PRS or Hunter's CV you have to know that it will take more pressure. For instance if the CV or SAM hold 7' of head then you know that you'll need 3 more lbs of pressure at the head (7X.333). if you're specifying PRS models, then you probably know that you have enough pressure or your lateral lines are really on a weird slope. I don't know where you're getting 5lbs per head. You must be assuming a CV or SAM is installed and giving a lot of leeway.

We will typically allow 10% for fittings. Maybe you'll need to adjust it for more.

BTW, I've been in the industry for 30+ years. I am aware of lots of variances by region, but I've never seen a loss for the sprinkler head included in the calculations. Now, saying that, I could imagine a calculation that includes "Head Loss". Beware. Here "Head" does not mean sprinkler head. It means the difference in elevations between the POC and the application device which could be a hole bib, drip line, drip emitter, etc. "Head Loss" is always included in calculations related to pump sizing.

Example.
Elevation of POC = 100
Elevation of highest application device = 150
Head Loss = The difference between the two x .43333333
Head Loss = (150-100) x .42222
Head Loss = 21.66 PSI

Notice that you can also have Head Gain. This is a better problem to have since it's a lot cheaper adding regulators to your system than pumps (and regulators depending on the variance of your project site).

One last thing. Remember that the pressure loss tables in the back of your catalogs are for 100' of pipe. How significant can a 1' length of pipe be? Lets use an example:

Assume the Hunter PGP Ultra. Let's just say that you are using the highest flow nozzle the high-flow green 13 which has a flow of 13.6 at 60 GPM.
Assume that the swing joint is a Hunter HSJ-3. It's a 1-1/4" swing joint.
Go to the catalog friction loss calcs in the back. in a 1-1/4" pipe at 14 GPM you will have a velocity of 3 fps and a loss of 2.5 PSI per 100' .025 for a foot). But the length of the swing joint is less than 2'. So the loss through the pipe in the joint is less than .05 and the rule of thumb is to use 10% of the loss for fittings. So add .005. So the pressure loss through the swing joint would be about .055 PSI. That's pretty insignificant.

We set our minimum residual to be about 10 to 15 PSI. Maybe an argument could be made to raise it. We all want to bee sure that our designs work as planned, so more residual is better. We'd all like that. Like I said, pressure regulators are sure less expensive and fail less often (if originally set properly) than a pump.

So I don't think that we would benefit in the least if LandFX added a calculation factor for swing joints since the .055 (approx.) psi loss is not accumulative, though they already account for "Head Loss".

Hope that this helps my friend.

Seaweed