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

"random intermittent small amounts of water" is what hose bibs are all about, and any good pro will design for the physical reality of a system, and not what anyone will say about using the hose bibs only in a certain way. But just how a hose bib will be used is not the issue. The issue is that the pressure switch will not be able to reliably distinguish the difference between the water pressure with your pump feeding a hose feeding a sprinkler, and the water pressure with your pump deadheading. Check the performance curve for your pump, and you can read that the pump output pressure at 3 gpm (one hose-end sprinkler) is about one psi lower than the deadhead pressure. That difference is not enough for pressure switch operation, which is bound to vary a tiny bit more or less than its set cutoff pressure.

And we aren't even getting into hose operation with a trigger sprayer on the end, which has to be foreseen by any designer, regardless of what anyone might say to the contrary.

So, in order to 'bulletproof' your present pump install to work reliably with a pressure switch, you might approach it from two directions. First, you look for a way to get constant pump operation when water is being used, while making sure the pump gets a full minute of run time whenever it turns on. Second, you make any tank selection compatible with straightforward operation with some future jet pump as yet not needed. The second one is easy, if you work the jet-pump curves into a gallons-per-minute number. I won't bore you with the details, or what other jet pumps may be had with higher flow capacities, that go into a tank recommendation. Suffice it to say that a precharge tank with a 30 gallon volume would handle the smaller two jet pumps without your having to do any advanced pressure switch adjusting. (the 1 HP pump has enough flow that you would have to do some fiddling with the pressure switch to get that one minute of runtime)

That same 30 gallon tank would give you enough capacity to work in combination with the smaller size of Cycle Stop Valve, and give your current pump a one minute minimum run time, as long as you can maintain a pressure switch cutoff point a few (very few) psi below the deadheading pressure of 47 psi. The way that valve works is that once you reach its built-in 40 psi pressure limit, it will only pass a flow of one gallon per minute, and it will be that low flow that will take time to build up pressure, and it will be that low-flow time in combination with a large enough tank, that gives the pump its minimum runtime. Up until the CSV reduces flow, the tank will be filling at a much faster rate, as you can see from the CSV1 pressure-loss graph.



You will note that the CSV1 flows top out at 30 gpm, and a more practical limit is 20 gpm or less. You will have to abandon the idea of a sprinkler system that uses all the possible flow of your centrifugal pump, if you want all-encompassing pump protection at anything near a reasonable cost.

Note that the tank sizing calculator page gives you the scientific theory that the calculator uses. If you want to try it yourself, remember that a pressure switch comes standard out-of-the-box with a range of 20 psi between cut-in and cut-off. Also, since there is no 1 gpm flow, you can use 10 gpm, and adjust the answers accordingly.

That sounds good, except for the 30/50 psi range on the pressure switch will exceed your pumps performance, so you might as well start with a 20/40 psi model, and save some adjusting time. These switches all have a separate adjustment designed to increase only the cutoff pressure, so you can start with 20/40 and work it up towards 20/45, and you might even be able to shave a few psi from its cut-in point, and look towards achieving something like 15/45 psi. That 15 psi cut-in is somewhat theoretical, as I expect that switch has a minimum operating point not too far below 20 psi, but I mention it for the idea of gaining maximum pump operating time between cycles.

There is one brutally simple trick you can try for extending the runtime, at least for the moment, if the tank and switch don't get you there after making switch adjustments. That trick is to include a gate valve in the plumbing on the pump outlet, and closing it down so it limits flow. Gate valves are preferred for this, because you can always remove the handles from gate valves, to keep someone from messing up the adjustment. Of course that trick might just take away so much pressure that the pump won't support much of a sprinkler system. (You can work the tank-sizing calculator and see that it would take a 14 gpm flow to match your tank and a 20/45 pressure switch, to get that one minute of runtime)