1. ## More pump questions

Okay, I've been reading about pipe size, dynamic head, head loss, flow rate, etc. it is often confusing and very daunting, but I'm trying to understand. One thing is puzzling me: looking at flow curves for a two speed Whisperflo, specifically the low speed curve, none of the shown dynamic head values are above 22 or so. Does dynamic head drop this much when flow rates decrease? If it doesnt, how do these speeds work against higher dynamic head values?

http://www.pentairpool.com/images/produ ... ve_new.gif

2. ## Re: More pump questions

Originally Posted by bpricedo
Does dynamic head drop this much when flow rates decrease?
Yes, it is dynamic and therefore changes with water velocity.

3. ## Re: More pump questions

So, you are saying a system with a pump that flows 90 gpm at 50 ft of dynamic head will only have 15 ft of dynamic head at 40 gpm (using curves B and G, which is for the 1 1/2 hp up rated pump, I believe)?

4. ## Re: More pump questions

Your systems actual dynamic head would need to be figured out for each flow rate to be exact, but in general, the drop in head from high to low will be in that range.

The pump curves you see are what the pump can pump at different head pressures...but in most cases if you know your system/piping head loss at high speed, and if the pump curve shows the pump can operate there with an appropriate flow rate for the equipment, then usually the low flow curve will also work.

One caveat that I can think of is at pump startup. For example, I have solar on my roof and I have to prime the panels on high since they won't prime on low. Once the panels are primed then I can run on low. The same might be true for priming a pump that is above the level of the pool.

5. ## Re: More pump questions

According to the pump affinity laws, flow rate is proportional to the motor RPM and dynamic head is proportional to the square of the flow rate/RPM, so if the full speed operating point is 90 GPM @ 50 ft of head, the half speed operating point will be 45 GPM @ 12.5 ft of head.

I have a write-up on this in the Hydraulics sticky in my signature.

6. ## Re: More pump questions

This came up because the PB agreed to give me a 2 speed upgrade for no charge but he wasn't sure how much benefit I would get. I wanted to be able to show him on the flow curve that it was still possible to get usable flow on low speed.

I just want a valid argument to take to the pool builder about pump sizing, I am thinking the 2 hp up rated 2 speed Whisperflo would be more than enough. Seem okay?

7. ## Re: More pump questions

In my mind the 2 HP is way too much pump unless you need it fro something else than the pool. I don't have all the details about your setup but from what you have given me, here is a comparison between the 2 HP Up rated and 1 HP Uprated pumps:

Note that the WFDS-28 is about 20% less efficient than the WFDS-24 and it also comes very close to the filter maximum flow rate. You really don't need much pump to run just the pool. For example, I am using a 1/2 HP two speed pump to run my pool with both solar and a suction side cleaner at the same time.

8. ## Re: More pump questions

The PB actually spoke to the Pentair distributor about this (not sure if being a distributor makes him an authority, but who knows). They felt like it would be okay, but said I would need to pay special attention to filter backwashing and keeping skimmer baskets clean as the effect on low speed circulation would be more pronounced. Also that skimming action may not be as good on low vs high speed. I am actually impressed that they are discussing this seriously and not just blowing me off.

9. ## Re: More pump questions

Just wanted to come back and thank you, Mark, for putting that chart together for me. Sorry it took awhile but I was consumed by getting these bids' equipment lists narrowed down so I could really see the difference in cost between PBs. You really helped me and I appreciate it.