Why Do the Last Two Sprinklers Have a Smaller Spray Pattern?

[roro36]

A question was posed to me with the following diagram. http://imgur.com/6ltYKd5

11 Sprinklers, in series with identical heads, but 10 and 11 displaying a much smaller spray pattern.
1. Is this possible, and under what circumstances.
2. What would a good solution be to prevent this?

My thoughts:
1. Not sure if it is actually possible, to occur. I understand it is something to do with pressure vs flow. I would think that the pipe would fill with water to system pressure and all of the heads should be reduced. My other thought is that as it might be completely theoretical and not based in practice, that the reduction in pressure after each sprinkler due to losses, results in the system pressure being reduced at the end (but surely the pipe would fill up and be at 1 pressure. So perhaps a flow problem, which I would think would affect them all equally?
2. 2 solutions I had were to extend the end to below the second valve so there is a sort of ring circuit. If that won't work, I would add another valve and split the sprinklers in the middle. 1 at time operation, i.e. 5 and then 6 or vice-versa.

Question has been bothering me since it was posed. Can't wrap my head around it. Thanks for any help.

 

[stockzahn]

Are the spray patterns of the 10th and the 11th of the same size (as indicated in the drawing) or is the 11th smaller than the 10th one? In the first case there could be a problem in the pipe between sprinkler #9 and sprinkler #10.

 

[Nidum]

(1) Independent feeders to individual or small groups of spray heads all taken from a common source point and ideally equal length .

(2) Flow control valves on each spray head .

 

[roro36]

So the question was based around solving the problem as cheaply as possible, more than anything else. In the image, the circles seemed equal to me. Assuming they were not, and that 11 was slightly smaller than 10?

 

[stockzahn]

In the image, the circles seemed equal to me. Assuming they were not, and that 11 was slightly smaller than 10?

I would try to "close" the first two or three heads and watch what happens. If at sprinkler #10 and #11 the water flow increases to the amount of the previous ones, maybe the pressure loss is to large / to much water is already used by the previous sprinklers. If possible a slightly decrease of water used by all the sprinklers (by closing them somehow) could solve the problem (at least all the sprinklers then have the same amount of water, all together it will be less). Otherwise a larger pipe (but that is not cheap) or decreasing a pressure loss before the sprinklers, like shortr supply hose/pipe).

EDIT:

Assuming they were not, and that 11 was slightly smaller than 10?

I read that one badly, my previous answer was for the case #11<#10. If #10<#11, there must be something wrong with head of sprinkler #10, which also affects the last one.

 

[JBA]

"stockzahn, post: 5245210, member: 567491"]Are the spray patterns of the 10th and the 11th of the same size (as indicated in the drawing) or is the 11th smaller than the 10th one? In the first case there could be a problem in the pipe between sprinkler #9 and sprinkler #10."

If your relative diameters are correct, I am inclined to agree with stockzahn. THe first thing I would look for is a problem with the feed pipe between #9 and # 10. If this were a general flowing pressure loss due to an insufficiently sized feed pipe then a progressive reduction of the spray diameters starting farther upstream should be observed, not a sudden reduction on the only #10 and #11. If the feed line is made of a flexible plastic then it may have gotten crimped, damaged or have some obstruction trapped between #9 and #10.

 

[dandoni]

Hey Mate

Eliminate the obvious first.

Have you checked the last two heads, they might be partially blocked. Switch heads , check main pipe. Pressure should be equal

 

[Tyler Worden]

Think of what counts in the flow. You want to supply 11 sprinklers at 1 time. Flow loss is resistance. Equalize the resistance. The feed pipe must be able to supply all the sprinklers at once. You can do this by starting with a small feed tube to the first sprinkler, in the direction of the flow, and gradually increase the diameter of the feed tubes up to the last sprinkler. You can also use a header tube with a greater diameter then the sprinkler feeds. eg. Use a .875” header tube (2.405 Sq in. area). Use.250” feeder tubes to the sprinklers (.196 Sq in. area) and you can put 12 sprinklers on the same header tube. Either way works, but the increasing feed tube system is usually used with gas systems like fire retardants. The easiest method is your case would be to use the header tube method. If your pipe system in the building is smaller than the 7/8” you can use a 2” or 3” header or a holding tank as an accumulator. You can work out the requirements needed to accommodate your local bylaws.

 

[GaryWilbourn]

A question was posed to me with the following diagram. http://imgur.com/6ltYKd5

11 Sprinklers, in series with identical heads, but 10 and 11 displaying a much smaller spray pattern.
1. Is this possible, and under what circumstances.
2. What would a good solution be to prevent this?

My thoughts:
1. Not sure if it is actually possible, to occur. I understand it is something to do with pressure vs flow. I would think that the pipe would fill with water to system pressure and all of the heads should be reduced. My other thought is that as it might be completely theoretical and not based in practice, that the reduction in pressure after each sprinkler due to losses, results in the system pressure being reduced at the end (but surely the pipe would fill up and be at 1 pressure. So perhaps a flow problem, which I would think would affect them all equally?
2. 2 solutions I had were to extend the end to below the second valve so there is a sort of ring circuit. If that won't work, I would add another valve and split the sprinklers in the middle. 1 at time operation, i.e. 5 and then 6 or vice-versa.

Question has been bothering me since it was posed. Can't wrap my head around it. Thanks for any help.

The system pressure is static because water is an incompressible fluid in this example. Spray pattern can be reduced by having smaller output orifice diameter.