pump curve selection for STA-RITE

[little_trader]

Hi,

I am having a new house built and have a well driller who based on the area expects to dig 200' down into the ground and produce 12-15GPM.  The people they use to install the pump and stuff want nothing to do with a cycle stop valve.  Thus I will have to add it after we move in.  They use STA-RITE pump motors.

Based on past conversations with you, and reading your website, you need to get a pump with the proper performance curve.  As far as I can tell, they plan on using a 1HP, 10GPM 4" submersible pump for the water load of the house based on the plumber's (a 3rd party not associated with the driller or well pump vendor) calculations.  If that ends up being the case, what would be your recommended STA-RITE submersible pump to be used with a CSV1Z?  I will have a tankless water heater and read that they work best with constant pressure as opposed to the 40-60psi cycling.

Thanks

[Cary Austin]

I am sorry you are having to deal with an idiot pump installer.  I would suggest you find another one, but in my opinion there are very few pump installers left who actually know what they are doing.  The best thing a consumer can do these days is to educate yourself about pump systems, then tell the “so called” pump installer how you want it done.  Otherwise you will only get what the pump installer thinks you need, and we already know this one is an idiot, because they do not understand the importance of a Cycle Stop Valve.  Or even worse, they do understand the importance of a Cycle Stop Valve and don’t want you to have a dependable system that will last a long time, because that cuts into their repeat business. 

A 10 GPM Sta-Rite pump, like the model 10PE402H is an OK pump.  The new Pentek motors haven’t been around long enough for me to have any faith in them yet.  I would prefer something with a Franklin or Grundfos motor.  And a Grundfos pump, or one of the other copies of this Stainless Steel pump will have a better brake horsepower than the plastic Sta-Rite or some other pumps.

The tankless water heater is only one reason you need a CSV.  The best reason for a CSV is so you don’t have pump problems and won’t have to call the idiot pump installer ever again.

[little_trader]

I have no control over the pump installer.  I may have some control over the pump selection.

The pump installer stated that the normal pumps weren't meant to run at real low flow rates at the higher pressure that a cycle stop valve would cause as it regulated the outlet pressure to 50psi.  They recommend a VFD for the low flow application.  (While the max output might be 10GPM, I might be trickle watering (so the ground has time to accept the water flow) a tree in the yard at .5GPM for 1/2 hour (with many trees meaning about 6 hours continuous @ .5GPM)

I work for a company that sells VFD for a living (I use them in all kinds of industrial applications) so know all about their advantages, but from an initial cost point of view, and return on investment, it seems like the cycle stop valve would be a much better solution for me at this point in time.

As far as a Franklin or Grundfos motor, which models of those would you recommend?  I know only enough about pump curves to compare what motor they suggest vs what you recommend, so can hopefully get them to supply a pump with a curve that you feel will work well with the cycle stop valve.

By the way, do you see any issue with using a cycle stop valve for 6 hours at .5GPM on a standard pump or does this actually put me into a VFD situation?  They stated that there is a sweet spot they design their pump selection to.

Appreciate your help by the way...

[Cary Austin]

A pump controlled by a CSV can safely operate at much lower flow rate than a pump controlled by a VFD.  A CSV de-rates the motor enough that it can safely pump hot water, so it takes very little cool water to prevent overheating.  A VFD creates a smaller motor from a larger one, so the motor still needs the same flow for cooling as a fully loaded motor.  

When controlled by a VFD, a motor in 4” casing needs a minimum of 1.2 GPM, and in 5” casing needs a minimum of 7 GPM to prevent overheating.  The same pump controlled by a CSV only needs about 2/10’s of a GPM for proper cooling.

With a 10 GPM pump, that “sweet spot” they are designing the pump to fit, must be between 1.2 GPM and 10 GPM.  I’ll bet they haven’t even thought of what happens when you try to run .5 GPM continuously with a VFD controlled pump.  But it will burn up the motor if you do that.  

Since you deal with VFD’s, you know that they will need to use a fairly sophisticated drive with a sleep mode.  Then they will need to know the exact frequency that will deliver a minimum of 1.2 GPM at the total head required.  After shutting down at the minimum frequency, a big pressure tank can deliver the .5 GPM while the pressure drops in sleep mode.  Then the pump must start and complete the cycle over and over again.  How often this is repeated, depends on how large a pressure tank you have.

The regular domestic VFD’s sold by Franklin, Goulds, Grundfos, and Pentair do not have sleep mode, and do not have an adjustable minimum frequency.  You certainly won’t be able to run .5 GPM with these units.  Your pump guys should know these things, but I can already tell they don’t.  Saying that a pump wasn’t meant for low flows and high pressure from a CSV, and that you can run a pump at .5 GPM with a VFD shows a lack of understanding.

Even though a pump can safely operate at 2/10’s of a GPM with a CSV, we have a 1 GPM minimum built in.  So the pump would still cycle with a CSV, unless you use two of those .5 GPM emitters at the same time.  With only .5 GPM, a CSV working with a 20 gallon pressure tank will only cycle 90 times in 24 hours.  With a 44 gallon tank it would only cycle 40 times per day.  Either way it would be much better for the pump that being controlled by a VFD.

Now don’t get me wrong.  I think the VFD is a wonderful invention.  There are many, many good applications for VFD’s.  Just that pumping cool water at a constant head or pressure is not one of them.

[little_trader]

Hi Cary,

I tried searching for the performance curve for the model 10PE402H and could find nothing.  The only thing remotely close was a PE series centrifugal pump.  I found HS series and J series submersibles???

http://www.sta-rite.com/ResidentialSearchResults.aspx#ProductType=4%22+Submersible

Am I looking at the right thing? Do you maybe have a link to something for me?  Sorry for not knowing much about this yet...

[Cary Austin]

You were almost there.  Here is the link to the curve.  It is the 10 GPM, 1 HP.  

http://www.sta-rite.com/ResidentialProduct_sr_ws_4s_s10p4jp05121.aspx
Just click on the "performance" curve at the right.

I still use my old pump books which I have a library full of.  A lot of pump companies have confusing web pages that make it hard to find what you need.