Question
QUESTION: Jeff,

I am planning to install a sprinkler system which draws water from the lake at the edge of my property.  I plan to use a 1.5 HP centrifugal pump located on my boat dock which is 2 ?8 feet above the lake (depending on the lake level).  In addition to lifting the water, the pump will need to push the water up to the lawn which is about 8 ?10 feet higher.  There will be 12 sprinkler circuits, each with about a dozen sprinkler heads.  The sprinkler circuits will be controlled by a conventional sprinkler controller/timer (with an auxiliary 24 volt output which is active whenever any of the twelve circuits are turned on).

I am not experienced with such systems and would appreciate your advice on the following options for controlling the pump:

Option 1:  Employ an electrical relay which turns the pump on whenever the sprinkler controller/timer is active.  At all other times the pump would be turned off.

Option 2:  Employ a pressure switch and a pressure tank such that the pump is turned on whenever the pressure in the tank drops below a specified level (say 20 pounds), and turns the pump off when the pressure reaches a specified higher level (say 40 pounds).  With this option, the pump could be active even at times that the sprinkler systems are not active.

It seems to me that either option would in theory work.  Option 1 is perhaps the simplest design, but unless the pump power is well-matched to the required flow rate (not likely), the pump will likely be trying to push more water than needed through the sprinkler heads (this will produce an undesired load on the pump).

Option 2 offers the advantage of turning off the pump when the pressure is high, thus lowering the load on the pump.  In addition it makes water available at the dock at times the sprinkler system is not active.  However, since it keeps the system pressurized at all times, it might cause the pump to run more often than necessary.  In addition, while the sprinklers are running, the pump will cycle on/off quite frequently to keep the pressure in the specified range.

Thus it is not clear to me which approach is superior.  Also, am I correct in believing that both options are susceptible to pump burnout if the intake pipe fails to hold the prime?  This might happen if the pipe is hit by a boat on the dock or the foot valve fails.  Is pump burnout a serious issue?  If so is there some type of flow device that is commonly installed to prevent the pump from continuing to run without adequate prime?

Your advice on how these issues are normally addressed and what are the standard methods of building such systems would be greatly appreciated.

Thank you for your help,

Vince Mrstik


ANSWER: Wow, that's quite a question.  But here goes:

I think putting in a pressure tank would be a little superflous.  A lot of people use direct in-line pump systems with good success.  But you will need to make sure that the installer of the irrigaiton system gauges the output of the sprinklers to match the output of the pump.  I am not very experianced at this, but I would reason that the demand/flow rate of the system would need to be very close to the output of the pump to make everything work effectivly without a lot of added pressure on any of the components, pump, valves or sprinklers.  I would also suggest that you install a check valve on the mainline that would aid in keeping the system primed.  Also, a good filter would help keep contaminants from affecting the performance of your valves and sprinklers.

---------- FOLLOW-UP ----------

QUESTION: Jeff,
    You did not comment on the question about protecting the pump from no-flow situations due to (stuck value on the output) or loss of prime (due to break in the inlet line).  Can you offer any comments on the need for such protection, if such protection is sometimes included in the system, and what type of flow detector would be appropriate?

Thanks for your help,

Vince

Answer
If you are concerned about protecting the pump, then perhaps a pressure system with a pressure switch to turn the pump on or off according to the pressure in the system would be best for you.  I have never used a flow control device in the way that you are describing.  My experiance with these is in systems where there is existing line pressure, and then when the demand on the system increases, the flow switch turns on the pump as a booster.  It may be that there are flow control devices available that would shut off the system if there were no flow, but I have not used them nor do I have any experiance with them.  I have used pumps on pressure systems (with pressure tank and pressure switch) and these would keep the pump from running if there were a failure in the irrigation system.  As far a protection for loss of prime when there is a break in the inlet line, I dont know how it would be accomplished.  The inlet needs to be installed so that it is as protected as possible.