Head

[jordangregory]

Can someone help me to calculate the amount of head I have for my pool. I have looked all around the internet and am still confused as to how to figure this out.
I am not even sure at this point what variables to take into account.
Thanks in advance

 

[JasonLion]

Are you sure you really want to do this? Knowing your dynamic head isn't all that useful. Assuming you want to continue, you should tell us the basic inventory of your plumbing, what size pipes, what size/model pump, distance between the pump and the simmer & returns, what your filter pressure runs when the filter is clean, do you have an in floor cleaning system or other water features, are there several parallel runs of pipe or only a singe pipe in each direction, etc.

 

[jordangregory]

2" pipes. The one pipe that shoots water into the pool and it is 45 feet from the pump. I have one scimmer that is about 30 feet from the pump and a second return that is about 15 feet from the pump.
The pump is a Hayward 0.75kwh, 1hp, ~3400 RPM superpump. The sand filter is 300lbs of sand, 60gpm flow rate.
I occationally hook up a kreepy krawly to my skimmer to clean the pool, but it really doesn't work well. Anytime the pressure in my sand filter goes up, the pump looses its prime. Pressure starts out at 10psi and the pump works well. It will stay at 10psi for weeks, until I add DE, then it does up to about 20psi within about 8-12 hours and sometime after that, loses it prime. I just clean the pool manually with a brush hooked into the skimmer.

 

[mas985]

A pump that loses it prime will stop pumping (i.e. no flow rate). Is that really what is happening or you just getting air in the pump basket?

Do you ever see air in the pump basket without the cleaner attached?

Also, why do you need to know your head loss?

The most accurate way to measure head loss is with the filter gauge and a vacuum gauge.

 

[jordangregory]

I really am losing the prime. The water stops moving.
It is very depressing. I was soooooooooooooo ready to fill my pool in with dirt last year.
I want to know the head because it seems that everything I look at as far as finding the correct pump for a pool has to do with the amount of head.

 

[mas985]

First, you don't really need to know head to choose a pump. Unless, you need high flow rates, the smallest pump possible is usually your best bet. I use a 1/2 HP on a 20k pool just to prove the point. Your 1 HP Superpump is about the same as my 1/2 HP.

Second, your problem may not be with the pump. A few more questions:

Do you lose prime only when the cleaner is attached?

Do you only have one skimmer and no main drain?

Attaching the cleaner may be starving the pump of water unless you have a bypass valve in-line with the cleaner. This is the most likely cause of your problem.

But guessing at the answers, I get about 70 GPM @ 47' with a filter pressure of about 11 PSI. But keep in mind that when you change pumps, unless the head curves are identical, the head loss and flow rates change as well.

 

[jordangregory]

I lose prime in a number of situations.
Situation #1 - I have two pipes that bring water into the filter. One is the skimmer, the other is a pipe on a side wall, about 2 feet below the surface. I do not have a main drain. I have a valve that allows me to use both pipes, or just one at a time. If I go to using just one, then tons of air gets involved and I lose my prime

Situation #2 - when the pressure moves from 10 psi to 20-25 psi, I lose prime.

My assumption, in both cases, it that the pump is starving for water. I feel like the only solution is to get a pump that has "less of a need for water".

I am thinking about getting a whisperflow 2 speed filter, thinking that the low speed will not only save money, but will stop me from losing the prime due to "lesser needs for water".

When looking at both of those pumps, it seems that there is a "full rated" and an "up rated". No idea what the difference is here.

 

[zea3]

So if you use just the skimmer then air gets in pump? Is the water level at least 1/2 way up the skimmer opening? Does the weir door on the skimmer move freely or does it stick?

 

[Bama Rambler]

It sounds like you have a suction side leak and anytime you increase the suction head (i.e. shutting off one of the suctions) you introduce air and lose prime. Your first order of business is going to be to find that leak. Have you cleaned and lubed the pump strainer basket lid?

 

[jordangregory]

I spent a whole year looking for the suction side air leak. I replaced everything. I cleaned and lubed the pmp strainer basket lid millions of times. I replaced all above ground pipe, unions and valves. I totally took apart the sand filter, took out all the sand, inspected the unit. No problems there. Sand was all great. Filter looked wonderful. I dug under ground to look at every union for the pipes underground. None of it helped. I am still having the same problems with air in the skimmer basket before the pump. The only think that aleaviated the problem is backwashing the filter and keeping all the pipes wide open.
The water is plenty high enough in the skimmer. I checked for the vortex thing. It was not the problem. Even if I use the pipe that returns water to the filter that is 2 feet under the water, I still lose my prime when using that pipe alone. The problem gets worse when I shut off one of the returns and have all the water coming in through just one pipe.

 

[JasonLion]

One good way to search for air leaks is to run a garden hose slowly running water over one plumbing joint at a time while watching the air in the pump strainer basket. If the air disappears you have found the joint with the air leak.

 

[Bama Rambler]

The problem gets worse when I shut off one of the returns and have all the water coming in through just one pipe.

That is very weird. Shutting off a return should increase positive head and decrease suction head. That should actually make your problem better. Unless of course the speed of the water through the suction is partially closing off the leak and by slowing down the water flowing through the suction piping it leaks worse.


In case you didn't recognize it: that was a SWAG!

 

[sukhbir]

Garden hose trick worked for me. I had two joint that were leaking. Used some clear silicon around the joint, bubbles went right away.

 

[jasonknox]

Can someone help me to calculate the amount of head I have for my pool. I have looked all around the internet and am still confused as to how to figure this out.
I am not even sure at this point what variables to take into account.
Thanks in advance

jordangregory I am right there with you. I am not sure the company I purchased my pump from sized it correctly so I did an inventory of my pipe lenghts and fittings to perform head loss calculations. It is not an easy task but wanted to be well informed and since I built my own pool the inventory of the fittings and pipe lengths was not a problem. I used the friction loss flow chart and friction loss in PVC fittings from http://www.PlumbingSupply.com to calculate the head loss. I plugged quantities and equivalent lenghts into a spread sheet and used the appropriate GPM and pipe size to get the correct Head Pressure loss of my pipe. I do think that putting in a vacuum side pressure guage would be easier but I never do things the easy way! There are some other sites that will do the calculations for you once you have the equivalent lenghts established, pipe size & type, and GPM desired. If I remember the names to those sites I will let you know. Good luck

www.freecalc.com has a friction loss calculator.

 

[Just-a-PB]

Pressure starts out at 10psi and the pump works well. It will stay at 10psi for weeks, until I add DE, then it does up to about 20psi within about 8-12 hours and sometime after that, loses it prime.

Maybe I am missing something, but this sounds like your problem. If you add DE to the sand filter, (which I know is promoted here), and your pressure doubles, the amount of water that can get through the filter is less then half. It is like plugging your return pipe and wondering why it wont prime.

 

[jordangregory]

jasonknox,
It sounds like our pools are pretty similar. How much head did you calculate for your pool?

 

[mas985]

I lose prime in a number of situations.
Situation #1 - I have two pipes that bring water into the filter. One is the skimmer, the other is a pipe on a side wall, about 2 feet below the surface. I do not have a main drain. I have a valve that allows me to use both pipes, or just one at a time. If I go to using just one, then tons of air gets involved and I lose my prime

Situation #2 - when the pressure moves from 10 psi to 20-25 psi, I lose prime.

My assumption, in both cases, it that the pump is starving for water. I feel like the only solution is to get a pump that has "less of a need for water".

I am thinking about getting a whisperflow 2 speed filter, thinking that the low speed will not only save money, but will stop me from losing the prime due to "lesser needs for water".

When looking at both of those pumps, it seems that there is a "full rated" and an "up rated". No idea what the difference is here.

Again, is situation #1 and/or #2 with or without the cleaner. It makes a difference.

It still sounds like an air leak of some sort. In situation #2, the higher pressure would result in lower suction and if the leak is in the pump lid, then more air could actually enter the system because the seal is not as good with low suction.

However, the flip side can also be true for situation #1. If the suction gets too high, then the seal is only so good and will again start to leak air. Also, in situation #1, do you shut off one of the pipes when you use the cleaner? If so, try running both pipes wide open with the cleaner attached and see if you still are losing prime.

In general, the reason the pump loses prime is because you are getting too much air into the system so you need to find out why. At this point changing pumps would probably not solve the problem unless the leak is in the pump itself.

I cover the difference between up rated and full rated pumps in this pool school article.

Again, in case you missed it, your head loss is about 47' @ 70 GPM.

 

[jasonknox]

jasonknox,
It sounds like our pools are pretty similar. How much head did you calculate for your pool?

By my spreadsheet calculations I came up with 20 feet of head loss at 65 GPM. Free Calc. figured it at 13.7. Let me disclose that in my calculations if I had a run that was at between the GPM numbers on the chart I used the higher GPM number which may account for the difference between my numbers @ 65 GPM and Free Calc. numbers. Consequently my numbers should be higher than the Free Calc program as they are. Free Calc let me put in the actual flow in each run. Had I interpolated between GPM numbers, my calculations probably would have been closer to Free Calc, however I do not believe the head loss table is a straight line interpolation.

Now the guess factor: I can not find anywhere what the Head loss is for my heater or filter. I threw in a 5 ft. head loss for each as well as a 2 ft. elevation difference from pump height to water height.

Totals
my numbers 20+5+5+2=32
Free Calc 13.7+5+5+2=25.7

 

[jordangregory]

Thanks yo Jansonknox. That was very helpful.
Thanks also to Mark. To answer your question, situation two is without the cleaner attached. In situation one, it occurs with the cleaner attached.
regareless, if my head loss is about 47 feet and I am at 70GPM, that is more than the 60GPM max my sand filter is supposed to deal with.

 

[mas985]

It is more than the recommended flow rate but it doesn't mean that your filter will be damaged only that they don't recommend exceeding the flow rate. It could cause channeling in the sand but sand filters in general are pretty tough and there are many examples of people on this forum with the same conditions. Also, the pump you have is actually a fairly low flow rate pump and nearly any replacement pump is going increase the flow rate and may the situation worse unless you change the filter as well.

So in situation #1 with the cleaner attached, do you open both suction ports or close one?

In situation #2, you may be adding too much DE to the sand filter. I know that some people have recommending do this but I think it is a bad idea and obviously causing problems.