Calculating head and proper filter pressure

[Water_man]

I’m trying to use this post
http://www.poolforum.com/pf2/showthread.php?p=25620&highlight=calculate+head#post25620
in order to calculate my flow rate from calculating head.
The reason I need it is because of changes made in the system both in plumbing and equipment.

Originally the pool was designed for the previous owner with two ¾” return pipes, one 1½” auxiliary (Polaris) return and a 1 ½” suction line. The original filter was 24 SF DE and the original pump was ¾ HP. I “inherited” the system three years ago from the previous owner who also upgraded the filter to 38 SF and the pump to 1 HP.

I tried to attach the poolskimmer to the ¾” return last year but the flow wasn’t strong enough to create the needed vortex for skimming leaves and other floaters.
Since the aux pump for the Polaris was dead I T-ed the return from the heater into the 1½” return and the two ¾ “ returns and connected the skimmer to the 1 ½” return in the pool. Since doing the plumbing change I now have an option of choosing the return outlets: either have all the three open, or plug from the pool end either one or two of the ¾” returns.

The pump was approaching its death so I just switched to 1 HP Hayward Super Pump.
I noticed a dramatic increase of my flow rate, and also a rise in the filter pressure.
Now I wonder how to get the ideal flow rate, because I can control the return flow as explained above.

According to the above mentioned post , the head is about 3.2 X Filter PSI, namely, a 15.6 psi
pressure is expected for an assumed head of 50.

I’m having a problem with this number.
With my previous, sick pump, the filter pressure was 6 psi when only the 1 ½” return was open and 4 psi when all the three were open. This low number is understandable because the pump was dying.

Now with the brand new pump, I have 8 psi filter pressure when all returns are open and 14 PSI
when only the 11/2” return is open. This means heads of 25.6 and 44.8, respectively.
Does it make sense? Is it possible that the filter pressure gauge is way off?
The flow at the return is visibly strong.
If I knew how to connect a vacuum gauge at the pump end without re-plumbing I would have done it.

[mas985]

Hopefully I'm not breaking any rules here but some time ago I wrote this (http://troublefreepools.com/viewtopic.php?t=915) sticky which explains some of the principles of head loss, pumps and plumbing.

Anyway, 3.21 * Filter PSI is just the return head. Depending on your plumbing system, total head can range from 3.5x all the way up to 5x of the filter pressure.

I have done quite a few head calcs for various pools and I can get pretty close but I need some more details about your pool.

It would be surprising to have 3/4" pipe from the pad to the pool so is that just the eyeball opening or is the whole pipe that diameter from the pad to pool.

So basically, I need the following.

# of return pipe runs, pad to pool, their length and diameter.
# suction pipe runs, pad to pool, their length and diameter.

I guess at the rest and can get fairly close since I have the filter pressure.

However, since you have only one suction line and it is 1 1/2" pipe, my guess is that your suction head is quite high and probably not much less than the return head but it depends some on the return piping configuration.

So without seeing the details of your plumbing, my guess would be 4.5x filter PSI for the 8 PSI and or 32 feet and 4x for the 14 PSI or 56 feet. But that is just a guess. I don't use the same multiplier since the second case is adding head loss to the return but not the suction.

[Water_man]

I have the original blue print that was designed for a ¾ HP pump and a 26 SF filter.
As mentioned before, the previous owner upgraded to 38 SF filter and 1 HP pump.

On the blue print, the specs say in writing two ¾” return lines. The outlets are definitely ¾”. However, above ground (see pic) only one 1 ½” return is visible (the other one is the auxiliary Polaris line which I use now as an additional return line). On the blue print diagram you see one return line branches into two return outlets. This means there are two possibilities that can be verified only by digging:

Either there’s one main return line of 11/2” which is parallel to the pool edge, from which two
¾” returns T –off to the pool, or, the 11/2” main return line splits in the ground near the pad to two
¾” lines

The vacuum line is definitely 1½” and so is the Polaris line (which now serves as my additional return.)

Distances from pad (in feet) :

Skimmer : 28

¾” returns:

Near : 32
Far: 44

1 ½” return : 35

Filter pressure (PSI)

All returns open: 8
Only 1 ½” open : 14
11/2” open and either one of ¾” open : 10.

The flow rate is visibly higher than what I had before with my “sick” 1 HP Anthony pump.
Since I’m not accustomed to such a vigorous flow, I wonder if it’s within the acceptable limits.

Above ground plumbing pic:

http://www.mediafire.com/imageview.php?quickkey=1oxgihdnup2&thumb=4

[mas985]

Ok, based upon that information here are a few estimates and I say estimates because we really don't know what is happening on the suction side but from the filter pressure I can get close. We can nail it spot on if you can actually measure the suction at the pump via the drain plug but you would need a vacuum guage for that.

Anyway, here is what I have:

With the single 1 1/2" line: ~40 GPM @ 50' of head
All lines open: ~48 gpm @ 44' of head

The filter can probably handle much more than that so I wouldn't worry too much about flow rates.

Let me know if you need anything else.

[Water_man]

Thank you, Mark, for the calculations.
Without a flowmeter or a vacuum gauge I can't verify these numbers. However, the currents produced at the return outlets
indicate a strong flow. Can one have a "feel" for the flow rate from the intenisty of these currents? I can post a pic if this can help.

Suppose I want to get a vacuum gauge, I have two questions:
1. Where can you get it, and would it fit the hole in the skimmer?
2. Is there a way to comensate for the suction pressure drop between the skimmer and the pump? I undewrstand that what you need is the actual pump suction vacuum.

[mas985]

Thank you, Mark, for the calculations.
Without a flowmeter or a vacuum gauge I can't verify these numbers. However, the currents produced at the return outlets
indicate a strong flow. Can one have a "feel" for the flow rate from the intenisty of these currents? I can post a pic if this can help.

Suppose I want to get a vacuum gauge, I have two questions:
1. Where can you get it, and would it fit the hole in the skimmer?
2. Is there a way to comensate for the suction pressure drop between the skimmer and the pump? I undewrstand that what you need is the actual pump suction vacuum.

I think it would be really hard to judge flow rates by feel.

Any auto store or Sears would have vacuum guages for autos which work fine for this application. They come with tubes and if you get the correct adapter, will connect to the pump drain plug on the suction side which is the best place to connect it (no corrections needed). I think the adapter is 1/4" hose barb 1/4" male thread. Use teflon tape so that air does not leak into the pump.

However, the flow rate approximations are good enough for turnover calcs and filter limits. The error is probably less than 10% for flow rate. The vacuum measurement will get you that last 10% but at some cost and trouble.

[Water_man]

The drain plug! of course. Silly of me - I didn't see it.
Off to Sears tomorrow to get a vac gauge. Hopefully unscrewing the plug wouldn't be a big deal.

[Water_man]

Ok, based upon that information here are a few estimates and I say estimates because we really don't know what is happening on the suction side but from the filter pressure I can get close. We can nail it spot on if you can actually measure the suction at the pump via the drain plug but you would need a vacuum guage for that.

Anyway, here is what I have:

With the single 1 1/2" line: ~40 GPM @ 50' of head
All lines open: ~48 gpm @ 44' of head

The filter can probably handle much more than that so I wouldn't worry too much about flow rates.

Let me know if you need anything else.


OK, I finally got my vacuum gauge from amazon on line (Sears and others don't carry it) and I also replaced my
older filter pressure gauge.
The filter pressure when all returns are open is 11 (old reading was 9) and the pump vaccum is 14 inch Hg, which means head of 41. You came up with 44. Good job, thanks!