Changing to 2-Speed - Sizing Question - Need help

[lews]

I have a 21000 gal in-ground with a Sta-Rite System 3 cartridge system with 2 inch piping. I have a pool cleaner but it has a separate booster pump. I've got a flood system but only by 5 feet. No spa or fountain, just the pool (gunite). I have a SWG.

Currently I have Sta rite max e pro single speed 1.5hp pump. It has a SF of 1.47 so a SFHP of 2.2 (220v). P6E6F-207L.

I'm thinking about upgrading to a 2-speed. Sta Rite makes 2

P6RA6YG-206L - 1 1/2 HP but SFHP of only 1.65

P6RA6YG-207L- 2hp with a SFHP of 2.2

So to match my existing SFHP, I'd need to go to the 2hp it would seem. Is this right?

But the other question is whether the existing is properly sized. Right now, I keep the pump on 8 hrs during season and I'd planned on changing to 12 hrs using the low speed of the 2-speed system.

In thinking further, since I have a cartridge system and a booster pump, why do I need the higher speed of the 2 speed pump? Why not just get the lower speed as a single speed pump? Maybe I need to increase to the high speed with the pool cleaner, even with the booster pump.

Thanks for the help

 

[mas985]

You probably won't find a single speed motor at 1725 RPM but without a spa or water feature you could downsize to a much lower HP 2 speed and get muchbetter efficiency. Your pool is about the same size as mine and I have a 1/2 HP full rated two speed pump and gives me enough flow for solar on high speed and low speed is fine for just circulation.

A couple suggestions would be the Whisperflo WFDS-3 (3/4 HP full rated two speed) and the WFDS-24 (1 HP up rated two speed). Both would have very good efficiency and performance on a 2" plumbing system.

 

[lews]

Thanks. How many hours a day do you run it? And what is your GPM target? I have a 24,000 gal pool so I think I need about 33gpm miniumum for a 12 hour cycle. 24K*/12hr/60min

 

[mas985]

My pump is a 1/2 HP Northstar pump with a two speed motor replacement so unless you plan on putting together your own, it would be hard to replicate. I get about 65 GPM on high speed and 33 GPM on low speed. The Whisperflo pumps I suggested will deliver approximately, 78 GPM on high speed and 39 GPM on low speed on the same plumbing. If you want something closer to what I have, you can look at the following pumps:

SuperFlo 3/4 HP up rated: SF-N2-3/4A (59 GPM high, 30 GPM low)
SuperPump 1 HP up rated: (64 GPM high, 32 GPM low)
SuperFlo 1 HP up rated: SF-N2-1A (71 GPM high, 36 GPM low)

But even though these are smaller pumps, their efficiency is about the same as the Whisperflos. So the Whisperflo gives you more flow rate but has about the same gallons/watt-hr. If you want to run longer and have lower flow rates, then the SuperFlo may be a good choice. But just keep in mind that high speed is going to be much less flow rate than what your are seeing now.

 

[lews]

I don't get this. If I look at the specs for the pump for the Max-e-Pro, they show a performance curve for each of the two speeds and for the 1 1/2hp 2 speed. But when I go to the specs for the lower speed with a total dynamic head of 40, it won't even pump, either for the 2hp or the 1.5 hp.

http://www.pentairpool.com/pool-pro/pro ... mps-96.htm

http://www.pentairpool.com/images/produ ... oCurve.jpg

I calculated the TDH from this site. http://www.mypool.com/swimming-pool-pumps.htm, which in the end estimates 40 feet of TDH.

This makes no sense because if this is true, according to the performance graph for the low speed (lines a (1.5hp 2-speed) line b (2hp)), it can't pump if there's more than 20 ft of head. http://www.pentairpool.com/images/produ ... oCurve.jpg

help

 

[mas985]

First, did you actually measure suction and pressure at the pump? The vacuum estimate they use isn't going to be very accurate but if you measure suction it can be nearly exact. Also if you use the filter pressure, you must take into account the gauge height relative to the water height which they left out of the calculcation.

Second, the head loss of swimming pool plumbing is not constant, it is dynamic and is dependent on the flow rate going through the plumbing. So changing the pump or switching a pump to low speed will change the head loss. The low speed head loss will be about 1/4th the high speed head loss. So if your current head loss is 40', which I serious doubt, then the low speed head loss will be 10'.

The head loss of plumbing is also a curve which intersects the head curve of the pump. If you read through my Hydraulics 101 thread, I give a couple of examples. But every plumbing system curve will cross every pump's head curve at some point unless there is a significant change in elevation between the pump and pool.

If you want to know your head loss, I can help with that. Just answer the following questions:

Pump Make/Model
Height of pump relative to the pool water level
Diameter and number of individual suction pipes from the equipment to the pool
Diameter and number of individual return pipes from the equipment to the pool
Diameter and number of return eyeballs
Filter type and backwash valve model if known
Filter PSI reading at the desired valve settings and pump's highest speed setting.
Optional: Pump suction reading (in-hg) - not necessary but improves accuracy
Heater - yes or no
Pool/Spa Valve - yes or no (If there is a spa, the number of jets and size)
Check Valves - Number and locations

A very rough estimate to head loss is the following:

1.5" Return Line with 1 x 1.5" Suction Line:
Plumbing Head (ft)= 0.0167 * GPM^2 - CEC Curve A

1.5" Return Line with 2 x 1.5" Suction Lines or 1 x 2" Suction Line
Plumbing Head (ft)= 0.0140 * GPM^2

2" Return Line with 1 x 2" Suction Line
Plumbing Head (ft)= 0.0093 * GPM^2

2" Return Line with 2 x 2" Suction Lines or 1 x 2.5" Suction Line
Plumbing Head (ft)= 0.0083 * GPM^2 - CEC Curve C

2.5" Return Line with 1 x 2.5" Suction Line
Plumbing Head (ft)= 0.0074 * GPM^2

2.5" Return Line with 2 x 2.5" Suction Lines
Plumbing Head (ft)= 0.0069 * GPM^2

 

[lews]

Thanks but i can't check the vacuum because the pump is broken. Therefore I need to estimate. The distance from the pump to the main drain is about 30 ft and the piping is 2". The filter is a big Sta-Rite Cartridge - Sys 3. Here's the info where I got the 40 ft TDH, but your comment about the low speed cutting back the TDH makes sense and that would make it work.

Sizing Your Swimming Pool Pump
The pump consists of two principal parts: A motor and the wet end. The motor provides the power to move the components located in the wet end. The wet end consists of a trap, and impeller, which is responsible for moving the water.

The pump is probably the most important single piece of equipment in the circulation system. Selecting a pool pump is determined by a long list of factors. It is best to review this list even if you are replacing an existent pump with the same size. If you are tempted to increase the HP of the pump, you must read and understand this.

Total dynamic head (TDH) is the sum of the resistance experienced by water flowing through the pipes and filter system. Each elbow, foot of pipe, fittings and anything else that is in the circulation system increases the Total Dynamic Head. The TDH will affect the flow rate. For a new pool this is somewhat more easy to calculate, but for an existing pool, with piping below the ground it is much more difficult.

Typical Calculation:

1. Friction Loss (vacuum Side)
Common Head loss factors
Vacuum (at pump) -
Assumes 2" suction pipe not to exceed 40 feet.
Minimal fittings - One 2" valve and full rated pump.

Total Dynamic Head

Horsepower of Pump
Feet of water
3/4
4.5 - 5.5 ft of water
1
7.0 - 9.0 ft of water
1-1/2
10.0 - 12.5 ft of water
2
13.5 - 16 ft of water
Let's use a 1 HP pump to perform a calculation, Reading the table above we have 9 feet of head.

2. Friction loss (pressure side): _____(clean filter pressure) psig. x 2.31 ft of head / psig = ______ft of head (pressure side). Typically filter pressure should be 10-15 lbs.

Using 10 feet of pressure we will have 23 ft of head on the pressure side.

3. The Total Dynamic Head: __9_(Vacuum)__ + _23(Pressure)_ = ____32__

Practically speaking 30-40 feet of head is a good estimate to size a pump. which can be adjusted upward or downward depending on the circumstances.

 

[mas985]

Practically speaking 30-40 feet of head is a good estimate to size a pump.

That is way too low for nearly any IG plumbing on a single speed pump. Typical for 2" plumbing and your current pump is going to be around 50' of head. Was you filter pressure around 14-16 PSI?

 

[lews]

Yes - that's correct.

 

[lews]

Here's the graph. The 2-speed 1.5hp is "A" and "E" curves and the 2hp is "B" and "F" representing 1750 rpm and 3500rpm for each of the speeds. The max TDH isn't much different for either size, so if one works, they both work since the 1.5Hp pumps at least the 33 gpm I need. Well... actually I could be wrong if I look at it. TDH has to be below 10ft for the low speed to hit that gpm.

 

[mas985]

I am not sure those head curves are correct. I know Pentair posts them but they have screwed up on published head curves before so it doesn't surprise me. The CEC Curve-C measurements which represent close to 2" plumbing shows the following test results for the two pumps you are looking at plus the Whisperflo:

P6RA6YF-206LM
Full Speed: 76 GPM @ 47' of head & 1806 watts
Low Speed: 38 GPM @ 12' of head & 395 watts

P6RA6YG-207L- 2hp with a SFHP of 2.2
Full Speed: 83 GPM @ 56' of head & 2146 watts
Low Speed: 45 GPM @ 17' of head & 443 watts (42 GPM corrected)

Whisperflo WFDS-3/24
Full Speed: 78 GPM @ 50' of head & 1592 watts
Low Speed: 42 GPM @ 14' of head & 380 watts (39 GPM corrected)

There are some errors in the measurements (+- 3 GPM) since low speed should be exactly half of high speed but they all show higher flow rates than 33 GPM. So it is pretty safe bet that you could go with any one of the pumps although the Whisperflo is clearly the winner in efficiency.