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Thread: Irrigation system needs more pressure....

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    DIY Junior Member TXTom's Avatar
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    Default Irrigation system needs more pressure....

    We recently expanded the irrigation system on our 5 acre property. We are now watering just over 1 acre. Prior to the expansion the system had good pressure and perfromed as expected. With the additional coverage we are having pressure problems on all stations. I have made many adjustments and currently have a programmed delay of 5 minutes between each station. This gives our well system some additional receovery time, but still doesn't solve the problem. I think we need to install an atmospheric storage tank and jet pump between our well pump and current 80 gal pressure tank. How do I size the jet pump and storage tank? This complete water system services a 3500ft house with 5 people and the 1+ acre sprinkler system. We are already softening and chlorinating the house water due to poor smell. The sprinkler and house systems are isolated properly and our concerns are only related to low pressure on the sprinkler side. However, we would expect an increase in house pressure with a storage tank and jet pump.

    Thanks,
    Tom

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    Plumber Cass's Avatar
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    I have never seen a 3500' house in my life.

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    DIY Junior Member TXTom's Avatar
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    It is a 3500 square foot house. I need a square foot symbol on my keyboard.

    Thanks,

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    DIY Senior Member SteveW's Avatar
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    Quote Originally Posted by TXTom
    We recently expanded the irrigation system on our 5 acre property. We are now watering just over 1 acre. Prior to the expansion the system had good pressure and perfromed as expected. With the additional coverage we are having pressure problems on all stations.
    ...
    Thanks,
    Tom
    If it worked before the expansion, but now doesn't perform as expected, sounds like it's fairly obvious that something you did during the expansion has caused the current problem.

    What did you do to expand the "old" sprinkler system? Did you add additional zones and/or add heads to existing zones? Did you properly calculate GPM rates based on the nozzles you used?

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    DIY Senior Member Phil H2's Avatar
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    Does the irrigation system come on strong and then dwindle. If so that may mean your well isn't producing enough water. I am guessing that this is not your problem.

    You need to start by calculating how much water you need and at what pressure. Since the irrigation system is the biggest demand, you can start there. For each zone you need to the elevation above the pump, the flowrate (gallons per minute), the pressure required for the sprinklers or ?, and the length and size of piping to the zone and to the furthest sprinkler. The sprinkler manufacture websites should have the gpm and pressure required for the heads.

    With this information it should be possible to find why your system is not performing correctly. It could be your pump's capacity or it could be too much flow which causes the pressure to drop in undersized pipes. Or, it could be something else. But, I am curious. Were your existing zones changed when you increased the area being irrigated? They should still work if they weren't changed.

    Typically when you add an atmospheric tank, you need better water treatment. If your chlorinator can do the job, it may not be a problem. Or, you just use the atmospheric tank only for irrigation. Usually tanks are required when your well can not produce enough water in the time needed. A bigger pump or a booster pump can be used to increase pressure. Or, larger pipes can be used to reduce pressure drop.

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    DIY Junior Member TXTom's Avatar
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    Phil

    Yes the irrigation system comes on strong and then dwindles as time passes. I was able to reduce the dwindle by spacing the stations 5 minutes apart, but it is still a problem. The stations are sized properly, however we are in a drought and our well may not be performing at its best. I am prepared to deal with the problems associated with an atmospheric tank. How do I size the tank so that I have enough water for the entire sprinkler cycle? How do I size the jet pump so that I have pressure between 60-80psi? I would like to put the tank/jet pump between my well pump and 80 gal. pressure tank. I just need a float switch for the well pump and I can continue to use all of the current controls to operate the jet pump. If the well pump is producing less gpm then I am using during the irrigation system cycle, how do I keep the well pump from running continuously while I am draining the storage tank during the irrigation system cycle?

    Thanks,
    Tom

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    In the Trades Bob NH's Avatar
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    Before you buy pumps and controls, you should determine your demand.

    For the house, how many gallons in the peak 5 minutes, peak 10 minutes, peak 30 minutes, peak hour?

    The peak 5 minute demand will determine size of your pump and pressure tank. The peak 10 minute demand will determine the capacity of your pump. The peak 30 and 60 minute demands will determine the minimum storage you need in your atmospheric tank.

    There are multistage pumps that will provide more flow and pressure than a jet pump, and they are more efficient than a jet pump. You should select the pump after you determine the demand.

    Then you need to determine your irrigation flow. That will be limited by your well capacity. There is no point in having a pump that pumps a lot more than your well can produce.

    You need a controller that will sense low water in the well and protect the pump. They are usually based on the fact that when a pump runs out of water, it draws less current.

    You also need a low-water float switch in the tank to shut off the irrigation, and maybe a lower one to protect the jet pump, or whatever pump you select.

    You may want to develop some logic about when you are going to irrigate based on how much water is in the atmospheric tank. You irrigation controller probably must be more than just a timer.

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    DIY Senior Member Phil H2's Avatar
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    Tom,
    I will give you a very general and basic idea. You need to determine how much water is required for irrigation and your household use during irrigation. Then you need to determine you wells output and subtract that from the need.

    So you need to find the flowrate and total volume for each zone of your irrigation. Example: 10 sprinklers that use 1.5 GPM for 20 minutes = 15GPM for 20 minutes = 300 gallons. Then add up all of you zones for total. If you have 6 zones that are the same your irrigation system will need 1800 gallons in 120 minutes or 2 hours. Add up the maximum amount of water you will use in the house while running your irrigation (ie running washing machine, and taking a shower etc) I will make up a number for this example and say 120 gallons per hour in household use or 240 gallons in 2 hours. Adding the irrigation to the house hold and you will need 2040 gallons.

    Now you need to determine what your well will dependably produce. Letís say 10 GPM for this example. Therefore your well will produce 1200 gallons in 2 hours while you are watering. For a starting point on determining the tank size you can subtract the make-up water from the well to the demand or 2040 Ė 1200 = 760 gallons. This is the minimum amount and that is when the tank is at the lowest level. This does not have any margin for error or little water leaks etc.

    I can be pedantic. So I would make a time table to see how much water that I can expect the tank to have in the tank at the end of each zoneís cycle, because it is unlikely that they will have the same flowrate. This should give you an idea of how to size the tank but not the pump.

    For the pump, you need to know two things, the maximum flow rate and the pressure required. The irrigation system you already know from above. But the instantaneous flowrate for the house will be different. You need to get an idea of the flowrate when everything is running (ie 2 showers at 1.5 GPM + sink at 1GPM etc.) Then you can determine the pumps flowrate.

    The pressure or head can be a little trickier to determine. It can be expressed in psi or feet of head: 1 ft = 0.43 psi or 1 psi = 2.31 ft. First you need to determine the pressure required at the irrigation system and your house. Maybe, 40 psi for each sprinkler head and 40 psi for the house, or about 92 feet. Then you need to know the elevation of the the sprinklers above and house above the tank bottom. Letís say 30 feet elevation (fairly flat property). Next is the tricky part, it is determining the friction losses and minor losses in the pipe, valves and fittings.

    Probably the easiest way for you is to use one of the many online calculators. I just put ďfriction loss in pipeĒ into google and a number of calculators came up where you enter the inside diameter of the pipe, the length, and the flowrate. Here is one link http://www.freecalc.com/fricdia.htm (for this one: use PVC if you are using any type of plastic, they should be close; viscosity is 1; specific gravity is 1). You need to find the pressure loss in each section of pipe at its maximum flowrate.

    Minor losses are in fittings and valves. Donít worry about the house, it should be covered in the pressure required at the house. If you donít have a bunch of elbows and tees, you can probably neglect the loss or add a couple psi to your final number for saftey. But your sprinkler valves and backflow preventers can add up. You can usually find the expected loss for these on the manufactures website.

    You want to find the maximum required pressure, so for each sprinkler head (and your house) you need to add the pressure required, the elevation above the tank, the friction loss of each section of pipe going to the device, and any minor losses from valves. The highest number will determine the required head for the pump. Note: you should talk to a pump supplier for recommendation on a pump.

    This is very basic information. It does not account for many possibilities and senarios. After typing this, I remembered a good website with irrigation design information. http://www.irrigationtutorials.com/

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