Total Dynamic Head TDH

[gordonhyde]

Where can I find a chart that will give me the amount of resistance on diffrent pipe and fittings?

We are building our pool (digging starts in about 3 weeks) and I am trying to figure out TDH. I know all the big parts like the filter, heater and valves have the TDH already figured out, but I haven't been able to find the values anywhere.

Any idea where I can get these numbers?

Pool size about 38Kgals pool/spa combo
Piping: 2"
Skimmers: 2
Main Drain: 2
Filter: Hayward Pro-Grid DE6020 60 Sq. Ft. D.E.
SWG: AquaRite Electronic Saltwater Chlorination System, 115/230 volt
Heater: StaRite Max-E-Therm, 400,000 BTU, Natural Gas
Valve: Hayward 2.0" Selecta-flo Multiport Valve
Booster Pump: Hayward Booster Pump for Poolsweeps, 3/4 HP, 115/230v
Pump: ????

P.S. I don't IE Spell installed here, pleae forgive any spelling errors.

[mas985]

What you are trying to do is not easy. I have tried it myself and found that there is a lot of error involved in calculating head. First, not all PVC fittings are created equal. Friction coefficients can be quite different for different manufactures. There are sites that can give you some information but there is still quite a bit of uncertainty. Skimmers, three port valves, main drains etc. are not characterised well.

Do you plan on using Darcy-Weisbach or Hazen-Williams equations? Even that choice will give you different answers.

If you don't recognize either of those, then you may want to start here:

http://www.poolplaza.com/pool-pump-sizing-2.shtml

This is pretty much rule of thumb but if you don't want to get into the details, then it may be your best choice.

Also, for 8 hour turnover, you don't need more than 80 GPM. A good guess for your plumbing would be around 50 feet of head.


More detailed information:

http://www.haestad.com/library/books...tml/wwhelp.htm

http://www.engineeringtoolbox.com/da...ion-d_646.html
http://www.engineeringtoolbox.com/ha...ter-d_797.html
http://www.mcnallyinstitute.com/Char...on_valves.html
http://www.mcnallyinstitute.com/Char..._fittings.html

[eljefe281]

It's worth noting that a pool's plumbing system itself doesn't have a TDH.

TDH is a function of the flow you put through the system.

The pool's plumbing system has a "total system resistance". This is what you want to calculate.

Then, TDH is related directly to the square of the flow rate.

[gordonhyde]

The resistance is measured in "feet of head". The best way is to approximate the resistance is as follows:

1. Measure the vacuum pressure at the pump and multiply by 1.13.

A reading of 12 inches mercury times 1.13 equals 13.5 feet of head.

2. Measure the clean filter pressure and multiply by 2.31

A reading of 22 times 2.31 equals 51 feet of head.

3. Add the two together to get total friction loss in feet of head.

13.5 plus 51 equals roughly 64 feet of head

I don't see how using this info can help me in pre build planning.

Mark:
Is the 50feet an average number? How can you tell that is close?

[Waterworks]

The method listed on poolplaza is the only way to accurately figure your TDH. You can only do it once everything is up and running though. Trying to do it before hand based on the number of elbows, feet of pipe etc is too difficult and will probably not provide the right answer anyway. Most in ground pools have a TDH of 50-60'. Just use that number and you will be close enough.

Brad
Waterworks Pools

[mas985]

I don't see how using this info can help me in pre build planning.

Mark:
Is the 50feet an average number? How can you tell that is close?

First, the quote above was for existing pools so you cannot use that to determine head if your pool has not been built.

The pool plaza link was to familiarize you with head loss and method of determining the optimum pump size. At the bottom of the pool plaze page is a link to calculate head:

http://www.poolplaza.com/pump-head.shtml

Again, those are only approximations but they may get you closer. The proper way is very involved and requires some knowledge of hydraulics engineering so I generally don't recommend that.

Most swimming pools will have a head loss of between 45 and 65 feet of head based on experience so given your 2" plumbing it is likely that you will be well below 65 feet of head if you choose a low HP pump. Head loss depends on a lot of things, pipe diameter, number of bends, equipment, pump size and few other things. So for a given set of plumbing, a higher HP pump will see higher head loss than a lower HP pump.

A 1 HP Northstar will supply 80 GPM for an 8 hr turnover with 60 feet of head so that is a safe bet. You will likely have less than 60 ft so your flow will be higher which is OK.

To compare, I have a 1 HP Northstar 2 1/2" suction pipe, 2 skimmers, 2 mains, 2" return pipe, 4 returns and about 50 feet to the pool. My head loss is about 55 ft with a flow of 90 GPM. So you can use that as another data point.

With 2" on the suction side as well, I did a quick calculation and my head loss would bump up to 60 feet with a flow of 86 GPM which would still be ok for you.

So I would recommend not going above a 1 HP pump unless you plan on using the same pump for a Spa or additional water features. Also, I run my vacuum on the 1 HP so I don't think that would be a problem.