I'm not quite sure I understand your information ...

Are you saying that you have two zones each designed with a flow requirement of 40 and 45 gallons per minute and the design requires a starting "static" pressure of 50 PSI ?

"Static pressure" is the pressure at the hose bib when no water is running. "Dynamic pressure" is the pressure when water is flowing.

An irrigation design should start with your static pressure and flow rate .. sounds like you are working backwards and getting into trouble by overtaxing the system.

The flow requirements you state will need a flow velocity about 4 to 5 times faster then the safe limit. This could cause serious water hammer. The pressure loss will also be quite large .. about 60 psi loss so a dynamic pressure of 30 psi at the hose bib and you still have losses along the way to calculate.

Keep in mind your gauge could be off ...

In any case .. it looks like you designed a system way to large for your house. You need more zones. Ten to Fifteen GPM should be the maximum flow for a 3/4 inch supply line.

Good Day ...

Sorry that should be 90 PSI is the static pressure when nothing is flowing. The system isn't in or anything, just in my head right now!

When I opened the spigot wide and filled a bucket, I got the 19 GPM reading, which I assume is at 90 PSI. I can't believe that it's really that much pressure still when the water is flowing, but I don't know what kind of gauge to get that would tell me the GPM at 40 PSI.

I'm also wondering about the last two quotes I got for putting in lawn sprinklers for this little ~850 sq ft 25'x33' area. Both guys wanted to put in a zillion heads. Nine or twelve!

It's like they never heard of low pressure rotor sprinklers. I know they want to put in more heads and more valves because 1)it's more labor and so money for them and, 2)it's what they are used ot.

But when I try the more upscale landscape people, they charge so much it's ridiculous. Trying to reduce costs (water usage in the long run as part of this) isn't getting me anywhere so I wanted to have the system designed and laid out so I can say, bid on this system:

6 head
one valve
low pressure rotor sprinklers

CJ

The two numbers give you two points on a graph. Draw a graph with the horizontal axis labeled pressure/ psi and the vertical axis (y) labeled volume/gallons per minute.

The static pressure is 90 psi out on the horizontal axis with NO flow.

The flow rate is 19 gal/min with NO pressure. To first approximation you can connect the two points, and as long as the flow/pressure you need is below that curve you can get what you want with the right combination of flow restrictors/pressure regulators.

Reality and efficiency make life a bit more complicated. The line connected the no pressure full volume and the no volume full pressure is not a straight line, but is a curve that depends on the drag in the system, and the design of the pump. When you get to the point where your electrical bill pumping water starts to get measured in hundreds of dollars you want to call in a pro to advise on design. Sometimes something as simple as changing the impeller on your pump can drastically reduce your water pumping bill.

You have to be moving a LOT of water before this sort of consideration is worth doing.

Do those 6 heads meet head to head spacing requirements?

You need to go read http://www.irrigationtutorials.com/, and then get one of these http://www.toro.com/sprinklers/irrmisc/pressuregauges/53351.html

Most people who are going to charge you to install a sprinkler system are not going to put in a poorly designed piece of junk, because they know it will cost more to make it right later, than to spend 200 more when they put it in the first time.

Although you have already had your system installed, I think it would be informative to have this question answered.

The reason a 90 psi at 19 gal/min is important to know is because it it going to be too high of a velocity for the common schedules of PVC which are used. 19 gal/min is too high for 3/4 inch pipe which is rated for near 10 gal/min at the most. You will cause damage to you pipe.

If you bumped your pipe up in size, a 90 psi will make your sprinklers mist instead of spray. You will have a fog bank in your yard.

There are a number of factors that go into a design. A system depending on it size could potentially take care of the PSI problems due to friction loss. But normally a homeowner doesn't have large altitude changes or a long run from a meter.

Once you design your system and chose your sprinklers and pipe sizes, you will probably need to put on a regulator. You have enough PSI and Gal/min to irrigate any landscape, but it will probably need to be regulated.