Sizing Pressure Tank to Irrigation Pump
#1
05-22-12, 11:16 AM
Sizing Pressure Tank to Irrigation Pump
Good afternoon all! My first time posting! To begin, here's my existing system:
- 3/4 HP Dayton Jet Pump with Pressure Switch 40/60 psi. set at 40/60
- Pro Source Pressure Tank with 5 gallon drawdown at 40/60 psi. set at 37 psi.
- I can flow well at 10 gpm / 55 psi no problem.
I am starting the irrigation from scratch with the system above and I'd like it to be as efficient as possible. I know there's many options out there to consider and i mention some below. Perhaps I'm looking for the best and most cost effective scenario:
1) combine pressure zones to match 10 gpm flow from pump. what happens if I get less than 10 gpm? right now if I lessen flow, pressure builds to shut off pressure at 60 psi. I think my low flow drip lines can be fed by tank since they're gph and not gpm. Do you want your pump to run continuously the entire time you're irrigating?
2) What is a recommended cycle for a jet pump? 1 minute, 2 minutes? i can design my system to run at 5 gpm but then my pump is cycling every minute. To get two minutes I need a larger tank correct, perhaps matching the drawdown to the pump output at 10 gallons? Suggestions on size?
3) No tank at all? This is being used only for irrigation. Is it critical to have a tank? i will have low flow zones which i would assume a tank would be beneficial so the pump wouldn't have to run. I believe this assumes my pump is self priming which I think it is not. correct me if I'm wrong.
Let's start with these questions and see where this goes! Thanks for the feedback!
- 3/4 HP Dayton Jet Pump with Pressure Switch 40/60 psi. set at 40/60
- Pro Source Pressure Tank with 5 gallon drawdown at 40/60 psi. set at 37 psi.
- I can flow well at 10 gpm / 55 psi no problem.
I am starting the irrigation from scratch with the system above and I'd like it to be as efficient as possible. I know there's many options out there to consider and i mention some below. Perhaps I'm looking for the best and most cost effective scenario:
1) combine pressure zones to match 10 gpm flow from pump. what happens if I get less than 10 gpm? right now if I lessen flow, pressure builds to shut off pressure at 60 psi. I think my low flow drip lines can be fed by tank since they're gph and not gpm. Do you want your pump to run continuously the entire time you're irrigating?
2) What is a recommended cycle for a jet pump? 1 minute, 2 minutes? i can design my system to run at 5 gpm but then my pump is cycling every minute. To get two minutes I need a larger tank correct, perhaps matching the drawdown to the pump output at 10 gallons? Suggestions on size?
3) No tank at all? This is being used only for irrigation. Is it critical to have a tank? i will have low flow zones which i would assume a tank would be beneficial so the pump wouldn't have to run. I believe this assumes my pump is self priming which I think it is not. correct me if I'm wrong.
Let's start with these questions and see where this goes! Thanks for the feedback!
#2
05-22-12, 12:27 PM
Super Moderator
Do you want your pump to run continuously the entire time you're irrigating?
Yes. Ideally 50 or better psi constant run is best.
What is a recommended cycle for a jet pump? 1 minute, 2 minutes?
Depends on GPM your pumping and how long it takes to fill the tank.
To get two minutes I need a larger tank correct, perhaps matching the drawdown to the pump output at 10 gallons? Suggestions on size?
The pump will cycle less with a bigger tank.
( I have a 36 gallon bladder tank. But its for my home also.)
No tank at all? This is being used only for irrigation. Is it critical to have a tank?
For irrigation its not critical, but if you do not use a tank then a pump start relay will be needed. The tank will be beneficial to the low GPM zones
I would zone it at your 10 gpm. Thats how I did mine. I ran 3/4 pipe to each zone. I only pump 5 gpm at 55psi and have one head for each zone. ( Except a strip and the garden) I have rotors at 4 gpm that spray 35 ft or so.
I dont know what heads you want to use or how far you need to throw, but this helps you determine what you need to use. Toro, hunter, rainbird offer different heads and nozzles. They are all somewhat different.
Here is my budget install. Cost me $200 in parts and my labor.
[IMG]
[/IMG][IMG]
[/IMG][IMG]
[/IMG]Mike NJ
#3
05-22-12, 01:32 PM
Thanks for the response Lawrosa. I know pump cycling is dependent on tank size but I am trying to also figure out typically how often a pump should cycle, in other words, what they're built for. Given the information I gave, my pump will cycle once per minute if I size my zones to 5 gpm. Is that too often and bad for the pump?
Makes sense to run the pump continuously but it will only do it if I match the zones. I think the logical thing is to design for my zones slightly above 10 gpm because the pump still can put out more gpm but at a slightly lower pressure? Or I get as close to the 10 gpm as I can and adjust the pressure switch so the shutoff is just above the maximum demand. Is my logic correct?
Makes sense to run the pump continuously but it will only do it if I match the zones. I think the logical thing is to design for my zones slightly above 10 gpm because the pump still can put out more gpm but at a slightly lower pressure? Or I get as close to the 10 gpm as I can and adjust the pressure switch so the shutoff is just above the maximum demand. Is my logic correct?
#4
05-22-12, 07:02 PM
Super Moderator
Is that too often and bad for the pump?
As the pressure increases you get less gpm.
Example: I can run two heads at 40 psi/about 8 gpm. They do not shoot as far. I can run one head at 55psi/about 4 gpm and they shoot the full distance. ( The nozzles are 4 gpm )
If you have 10gpm at 55 psi size you heads for 3 gpm each. 3 heads on a zone. Say using rotors for a far throw.
On one of my strip zones as in the pic I used sprays. I run 6 on that zone at .6 gpm. .6x6=3.6 gpm total. This keeps me at my 55 psi@ 5 gpm without the pump shutting off.
In conclusion I sized all my zones for 5 gpm@55psi and the pump never shut off.
You should size your zone to the 10 gpm you get at 55psi. You can use more heads them me and water your area in less time. This will save you electric and time.
To water my lawn each zone I run 10-20 minutes. Thats about 4 hours it takes to water using 11 zones.
My therory and thinking with the ones head each zone that I have is I use less heads. If I used 40 psi to run two heads I would need additional heads. ( Remember I hand trenched all of this) I was looking for maximum throw and coverage, plus maximum gpm I could get on the lawn.
Also to reduce any friction and pressure loss I ran 3/4" home runs to each head. I have no 1/2" at all.
Hope this makes sense.
Mike NJ
#5
05-23-12, 07:13 AM
Thanks Mike. You are confirming my suspicions. So, to answer one of your previous questions, the 5 gpm was what my brother designed for but I'm not sure he knew exactly how my pump worked. So that can be changed.
The largest throw I need is 18 feet and that's only on a couple heads. Most will be in the 8-12 foot range. I will not be using any impact heads which put out the most gpm. Probably be using MP Rotators or spray heads. I can easily combine zones or add a sprinkler head here or there to achieve 10 gpm. So that brings up a question, say I can't get exactly 10 gpm and it's more near 8 gpm, am I right in assuming I need to increase my shutoff pressure higher? that's why i was thinking to design slightly over 10 gpm for my large zones so it will stay at or below 55 psi and continue to run.
Now, for the low flow pressure zones, I'm thinking my tank will take care of those no problem, especially if I have drip irrigation. In that case a 5 gallon tank would prevent the pump for cycling. Does that make sense. I guess I don't really want to add a pump start relay if I don't have too which would eliminate the tanks.
BTW - I'm starting the irrigation from scratch so I can really put whatever I need to in the ground to get what would be optimal for the pump.
Is there a general recommendation on how many heads per zone?
You also say you run 3.6 gallons on one zone and your pump stays on. Does it actually run at 55 psi or higher? i would think since you're not pumping 5 gpm, the pressure would be more near 60-65?
Brandon
The largest throw I need is 18 feet and that's only on a couple heads. Most will be in the 8-12 foot range. I will not be using any impact heads which put out the most gpm. Probably be using MP Rotators or spray heads. I can easily combine zones or add a sprinkler head here or there to achieve 10 gpm. So that brings up a question, say I can't get exactly 10 gpm and it's more near 8 gpm, am I right in assuming I need to increase my shutoff pressure higher? that's why i was thinking to design slightly over 10 gpm for my large zones so it will stay at or below 55 psi and continue to run.
Now, for the low flow pressure zones, I'm thinking my tank will take care of those no problem, especially if I have drip irrigation. In that case a 5 gallon tank would prevent the pump for cycling. Does that make sense. I guess I don't really want to add a pump start relay if I don't have too which would eliminate the tanks.
BTW - I'm starting the irrigation from scratch so I can really put whatever I need to in the ground to get what would be optimal for the pump.
Is there a general recommendation on how many heads per zone?
You also say you run 3.6 gallons on one zone and your pump stays on. Does it actually run at 55 psi or higher? i would think since you're not pumping 5 gpm, the pressure would be more near 60-65?
Brandon
#6
05-23-12, 07:51 AM
Super Moderator
i was thinking to design slightly over 10 gpm for my large zones so it will stay at or below 55 psi and continue to run.
You dont want to lose headway with the pump and then lose the distance of the throw. If you oversize your psi will constantly drop until it settles at the gpm.
I rather have it climb. If you size it right it will climb to the right pressure and hang there.
Example: My well, once on, takes a long time to climb back up. Think like. I have 5 gpm and I am pumping 4 gpm to the sprinkler. I have 1 gpm left trying to fill the 36 gallon tank. It should take 36 minutes, but it will never fill it. It reaches a equalibrium of gpm and psi.
I'm thinking my tank will take care of those
Yes tank will take care of low gpm zones.
Is there a general recommendation on how many heads per zone?
You also say you run 3.6 gallons on one zone and your pump stays on. Does it actually run at 55 psi or higher? i would think since you're not pumping 5 gpm, the pressure would be more near 60-65?
As I explained above even though I am at 3.6 it will take 1 hour or so before the pump raises to 60 psi, or maybe even turn off. Because as the pressure raises you get less gpm. Possible at 65 psi I am only getting 3 gpm and at 40 psi 8 gpm. Make sense?
I am not a hydronics expert and there are other smarter people on here then me. possibly others will chime in and offer some knowledge.
Oh this is where I read and learned most of this. Plus trial and error.
Its complicated. I e mail the guy several times. He was a great help.
Irrigation Tutorials Homepage and Main Index
Mike nJ
