Not sure about the numbers for pump-please help me calculate

[amjohn]

I have Doughboy pump P/N0-1087-200, 1.5 hp, 12.4 amps, 3450 RPM. Also a 19" doughboy sand filter 2.0 sq ft and max 50psi. We are installing 160 sq ft of solar panel as I write this. We are still trying to decide if we will have enough pressure or WAY too much pressure on the panels. When the filter is clean, the gage normally reads 15 psi, but I do not really think the gage is accurate. It fills with water every time it rains, and it changes its mind from time to time. I do know that the gage goes to 0 when the pump is off, and it rises as the filter gets dirty, so it is at least giving us the relative info we need, even if it is not numerically correct.
We are using 1.5 inch PVC- mostly sched 40 rigid, with a little flexPVC for funky angles.
The solar panels are rated for a recommended ideal flow of 4gpm per panel with .87 psi head loss. We will have two 4x20 panels hooked in parallel. Their max operating pressure is 45 psi at 80degreesF and max recommended flow is 8 gpm.
So my questions: Do we have enough pump pressure or too much? How close to the ideal 4gpm will we be?
We are building some pressure reliefs into the system in case we are way over, but would like to have some idea how much relief we may need.
NOTE: the panels are not on a roof- they sit on racks on the ground below the water line of the pool.

 

[JasonLion]

That pump will be more than enough for two panels at ground level. With only two panels, you don't want to run all of the water through the panels. I expect you will have a bypass valve that lets you adjust how much flow goes to the panels and how much goes directly to the pool. You can use that to adjust the relative flow between the panels and the direct return.

Now would be a great time to replace the pressure gauge. Pressure gauges tend to break after not too long, fortunately they are inexpensive. A working pressure gauge will make it much simpler to adjust the flow between the panels and the direct return.

 

[amjohn]

Where do we get a decent gage? Everything I have seen has been very cheap, designed to break. Is there a particular brand or part number that will last longer and be more accurate?
Also, would like have a better idea of what our gpm from the pump actually is. I have read several threads on how to calculate that, but don't seem to be understanding, or perhaps I am missing some important factor in the equation. Can someone help me with my math?

 

[duraleigh]

Amjohn,

The manufacturer should publish a "pump curve" for your pump. If it's not printed on paperwork that came with the pump, check their website.

It's a graph that relates gpm to TDH (head) in your pool. Use 50 for the TDH and you'll get close on GPM.

 

[launboy]

I calculated my GPM one time, but I don't know if my method was accurate or not, anybody know?

What I did was to attach a 3ft. length of flex hose to the return in the pool and held the other end in a 5 gal. bucket. I turned on the pump and timed how long it took to fill the 5 gal. bucket. I then use the following simple equation to calculate the GPM: (5*60)/x=GPM 5=gallons in bucket, 60=sec. in a minute, X=time in sec. it took to fill the bucket. In my case is was 6 seconds, which ends up being 50GPM. This was using a 1HP pump, with solar on.

Not to get off topic, but for everybody with only 1 return, it seems like an easy, accurate way to figure GPM. This could work for multiple returns too, you'd just need more hoses and buckets and you'd have to modify the equation a bit.

P.S. I realized while typing this post that I originally calculated my GPM wrong. Before I came up with 30 GPM. That seemed low, but put me in the sweet spot for my solar panels. Now I know I've been trying to put too much water through them and that I'll have to crack my diverter valve a bit.

HTH,
Adam

 

[duraleigh]

Launboy's solution may be more accurate than the pump curve. It's a real life situation where the pump curve is theoretical (I think)

I did the same thing for my irrigation pump and found it easy to get very accurate.