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

The script has a point in that absolute pump protection from too-rapid cycling is a combination of flow rate and tank size, which, along with pressure switch settings, determines the minimum possible interval between pump starts. All that said, for sprinkler zone operation, all the pump flow goes to the sprinkler heads, and there is no cycling, and a small tank will serve the purpose of keeping upstream pressure in the mainline.

Where the script comes back into play is when you use a hose connected to the pressurized mainline. Since you will never get a hose to use the full pump flow, when the pump is operating in the pressure range set by a pressure switch, you are going to get cycling, and too small a tank will make it very rapid cycling. Any current hose operation from that pump is at the very edge of its operating curve, and you can't safely use pressure switch settings at that edge, because you will be very close to the switch not shutting off the pump at all, and then you are deadheading. A larger tank will increase the period of cycling with hose operation, and that provides pump protection.

Based on a quick scan of tank prices, you might look at a pre-charged tank in the 32-40 gallon range as a starting point, in order to have hose bib operations without worries over pump life. Local tank prices might favor a 20 gallon tank, which I consider a bare minimum. You can visit pump/tank tutorials and work out the numbers yourself. When you called tech support, they work from an understandable assumption that your pump flow is a constant number, which is what you generally get with deep-well submersible pumps. On a centrifugal pump, the flow rate decreases as the pressure rises, so you can pick an arbitrary operating point at least 10 psi away from its deadheading pressure, and use a flow that is far less than 60 gpm.

Every device you put in the system will subtract pressure, and you do not have pressure to spare with a centrifugal pump. Since you chose a pump already and are sticking with it, you accommodate the system to it, and invest in a tank.

Besides, doing the math for your pump and proposed tank and pressure switch settings will build character.

Or you can employ a calculator - then your job involves reading the performance chart for your particular pump (and understanding it) and using a point on its flow/pressure curve to work from.

The "I Told You So Department" would like to take a moment to remind everyone that it was recommended that the centrifugal pump get traded in on a jet pump, which could become a stable water supply with the addition of a Cycle Stop Valve, a pressure switch, and a baby pressure tank, and all for less cost than was spent on the centrifugal and the flow sensor. The performance curve on that pump makes it a very touchy thing indeed, to have it operating at a pressure that would allow as little as a 10 gpm flow, since that pressure would be very close to where it would be deadheading. Maybe you will have to supplement the larger tank with some interface with the flow sensor you mentioned (make and model number, please)

You have to remember that these threads are often read by others trying to learn about equipment choices, and it's for the now and future benefit of a wider audience that I stress some of these points, because every day, someone looks at a cast-iron "lawn sprinkler pump" and thinks that's just the item needed for a sprinkler system installation, not knowing the designation is either outdated or downright deceptive. Today's common rotor sprinklers need much more pressure than the simple brass spray heads of yesteryear, and a system greatly benefits from having a water source with more pressure than a centrifugal pump can manage.

So trust me, no one's beating up on you. It just happens that you walked into a teachable moment. Smile!

Meanwhile, your pump is going to be performing as its curve describes, and to make the combination of pressure switch and pressure tank work for you, you will be fine-tuning the cut-off pressure adjustment, so that you get a full minute of pump operation every time it starts, on the way to the tank pressure going back up to the cutoff point. This is where we get back to the script that was being read to you, and what the larger tanks make possible.

The larger the tank, the lower your cutoff pressure can be, so long as you are having the same cycle interval. You want an interval of at least one minute as a safe minimum, and a pro setting up a deep-well submersible can be thinking in terms of two or three minutes between pump starts. So this is where you have a tough choice to make, because this is an application where bigger is better. The only downside to a bigger tank is that it costs more money. The downside to a tank that is too small, is that it won't be able to give you a full minute of runtime for your pump.

The pressure switch itself is a fairly simple mechanical item (I recommend a Square D Pumptrol FSG2) and they have their limitations, in that if you set a cutoff point of 43 psi, it will most of the time cut off at 43 psi and at other times may cut off one or two psi higher or lower. This detail is important because the absolute maximum cutoff pressure you might set for your pump is 47 psi, the pressure that it would be deadheading at. (see the chart)

So the bigger the tank you buy, the more wiggle room you give to your pressure switch adjusting. And you need that wiggle room, because the difference in output pressure with you running a hose, and the pump simply deadheading is going to be a very small difference. What works in your favor, is that a cycling pump with you running a hose will take a bit longer to reach cutoff pressure (but not so much longer that it makes up for an undersized tank)

Here's a simpler calculator to play around with, but no online calculator will take into effect that your pump flow rate is going to start out big and end up small. The selection of a tank is going to come down to your budget and willingness to invest in a reliable level of pump protection. See what tanks are available in your area, and compare prices and capacities and reviews of quality. I think you should start the search with a 32-36 gpm precharged tank.

You are both complicating and oversimplifying the situation. The over-simplification comes from your statement that you are pumping 60 gpm. What you are actually pumping is a flow rate that can be found on the performance curve. That curve starts at zero gpm and goes all the way to 88 gpm. In terms of your pump operation and pressure switch settings, your pump will begin operating at near to 80 gpm when it cuts in, and by the time it cuts out, you can hopefully have it fine-tuned to be pumping at 10 gpm or less. If you really wanted to mathematically work out precise cycling times from the performance curve, you would have to use calculus.

The complicating comes from assuming the tank is in place to deliver water. Not the case. It's just there to both provide some constant upstream pressure for the sprinkler zone valves, and to keep the pump from cycling too rapidly during operations other than lawn sprinkling. If you only cared about the constant upstream pressure, you could have a pressure switch and baby tank mounted right on the pump itself. The sprinkler system would be happy with only that.

The problem for you is that you also want to have the ability to draw random intermittent small amounts of water from hose bibs, fed by a pump that would cycle a pressure switch very rapidly, unless sufficiently sized pressure tanks are a part of the system. To have an install of your particular centrifugal pump that was proof against any and all possible operating situations might involve so much pressure tank capacity that it would actually be cheaper to replace the pump with a small jet pump, and work out the tank and pressure switch details from there.

It may work out that your best, and simplest, solution will be to employ a Cycle Stop Valve, and have it, the pressure switch, and the baby pressure tank on the pump itself or nearby. That will subtract some additional pressure, but you'll live with the loss, rather than have a tank farm.

{we interrupt this post for another teaching moment}

A jet pump is what this application really calls for, and we only need to see some jet pump performance curves to know why. Instead of unused overcapacity, you have a useful range of pressure and flow that graphs as nearly a straight line, for your calculating pleasure. The 1/2 HP jet pump would do fine in your application with a 20 gallon size precharged pressure tank, and there wouldn't be any agonizing over tweaking the pressure switch adjustment just right. And no Cycle Stop Valve needed.



{end of teaching moment}

And as for your own make and model of centrifugal pump, it possesses a key characteristic of all reliable electric pumps, which is a motor with a high service factor. More than anything else, that "service factor" is why some pumps endure and other burn out, assuming everything else is equal. The cast-iron jet pumps selected by conscientious professionals will also possess high service factors.