Does water velocity depend on pump HP?

[repairman]

Does water velocity depend on pump HP? I imagine the smaller the pipe the more velocity for the same pump. Is there a figure to look up in the specs for specific pumps to show how much water they move per time? Does a 1 1/2 HP pump necessarily move more water than a 1 Hp?
Thanks,
Repairman

 

[mas985]

There is only a very loose relationship between HP and flow rate. Within the same pump line there is a much tighter relationship.

But flow rate is only important for things like spa jets and waterfalls. For pool filtering and circulation, flow rate is much less important.

Why are you asking?

 

[Griswald]

It takes pow er to make power. Power to run a pump = flow.

So, yes, there is a relationship, but there are many more variables to consider.
Fluid power and fluid flow are 2 full semester classes in engineering school.

 

[JamesW]

Most pumps have a published head curve which specifies the flow rate at different levels of resistance.

The right pump choice depends on multiple factors including gpm range needed, resistance etc.

 

[repairman]

Thanks Mark,
I have a spa and pool on separate systems and I'm thinking of combining them. I'm redoing the plumbing in my 60's era 22,000 gal gunite pool and moving the octagon spa to the shallow end. The spa now has 1 1/2' pipe and the pool will be 2". I have an American Products 1 1/2 HP on the spa and a 1 HP Sta-Rite on the pool, and an old Hayward dual speed MaxFlo with control box and blower that a friend pulled off a spa and gave me years ago. It's just been sitting in my backyard. Don't know if it works but if it does the dual speed I could use because I want to run low speed 24/7 on the pool. Problem is I just called Hayward and they don't have info on it anymore so I don't know the HP or flow rate.
I don't want to lose flow rate to my spa when I combine systems, if I do.
With my limited knowledge, if I combine systems, the following seem to be my options:
1. See if the old 2 speed Hayward works, run a comparison test on that and the existing spa pump to see if it has enough flow, and use if it does.
Just got off the phone w the Pentair/Sta-Rite tech who says both the American Products and Spa-rite push about 100 gal/min
2. Buy a new variable speed pump. Pentair tech recommends the 3 HP Intelliflo which I could set for any flow I want.

- - - Updated - - -

and thanks James and Griswald

 

[mas985]

By separate systems, do you mean separate pump and filter? If the spa has it's own filter and the filter is smaller or equal to the pool filter size and type and the two pumps are similar, then the performance should be fairly similar. But if the spa had 1 1/2" plumbing, then the performance probably suffered some anyway.

Does the spa have a single loop for the jets and how many jets do you have?

The MaxFlo is a low head pump so probably not suitable for spa jets unless you have 4 or less jets.

I take it the goal is to reuse these pumps if possible?

If so, another option is to share the filter but have two separate pumps. A circulation/filter pump for both the spa and pool plus a pump for just the jets. But this would require either a separate return line for the spa or more complex plumbing.

 

[repairman]

Thanks Mark,
Now I have a separate pump and filter for both the spa and pool (2 pumps and filters). The spa filter is a Hayward 50 cartridge, and the Pool is a Hayward 4800 DE. Spa plumbing is 2" to the T at the spa where it splits to 2 jets on one side and 2 on the other. The 2 jets closer to the T get better flow. It's an old octagon spa but I like it, but it would be an improvement to get those end jets working better.
Yes, the idea was to see if I could reuse the pump or pumps I have, but I have the money to buy a new one if it would do a better job. Apparently, I could dial in the power I want for the pool or spa with a variable speed pump, and set it's schedule.
I was planning on plumbing a separate loop for the pool and one for the spa joined by valves to set incoming and outgoing flow to the same pump and filter.
That MaxFlo pump was off an old spa but I never saw it and don't know how it performed or how big the spa was.
Thanks,
Repairman

 

[mas985]

So are all the labels gone with the MaxFlo on either the pump or the motor?

But since you have only 4 jets, the MaxFlo is probably more than sufficient.

 

[Hayward Pool]

If the Max Flo came off an old spa pack it most likely came of what was called a Max Pak, over 25 years old. The HP should be on the motor label or a black and silver label on the side of the pump. If not, giving me the molded impeller number will help to determine. Most Max Paks had 1 or 1.5hp pumps. Unlike me most of the techs on line will not even remember this pak. Did I just date myself.

 

[Pool Clown]

There is flow rate, which is commonly tied to GPM.
Then there is velocity or feet per second (fps) which is simply speed, and is not tied to any particular volume or mass of water, just the speed thru a given conduit (pipe).

don't want to lose flow rate to my spa when I combine systems, if I do.

If you keep the same pipe size, the only loss you will get will be from any increase in distance, referred to as "feet of head". (This is assuming that your existing pool plumbing has been sized right for correct flow rates.) Remember, count both suction, return runs, to the spa, AND all the fittings including the ones throughout the system! All your components, filter, heater, etc. should have service plates listing max flow rates on them. You should also be able to find head loss through fitting schedules on the internet somewhere. FYI, this is for you if you wish to do the calcs. I've been in the industry for... well, lets just say for a while, and i don't know anyone that sits down and does a "complete" head loss calculation. There are always unknowns, like unforeseen fittings underground, etc. Usually, it's a good guess, and maybe add a little.

Buy a new variable speed pump. Pentair tech recommends the 3 HP Intelliflo which I could set for any flow I want.

True, but dont forget, pipe size will limit flow (GPM) after a "certain pump speed". YOU do the calcs on that to find out what your "sweet speed" is. I'm too old and tired...

To answer (what i think is) your main question, use at least 2" inch pipe to preserve flow rate for the spa, and you should be ok as long as your run isn't too long to and from the equipment.

 

[mas985]

I've been in the industry for... well, lets just say for a while, and i don't know anyone that sits down and does a "complete" head loss calculation. There are always unknowns, like unforeseen fittings underground, etc. Usually, it's a good guess, and maybe add a little.

I have done full head loss calcs for many a member on this forum. I even tools for others to do it as well (see sig).


But I do have to correct one thing.

True, but dont forget, pipe size will limit flow (GPM) after a "certain pump speed"

Not true.

There is no physical flow rate limit in plumbing. If you have a large enough pump, you can push as much water through a pipe as the pressure will allow (i.e. PVC burst rates). For a VS pump, flow rate will always increase with an increase in RPM. The flow rate limits that you often see on the internet are only recommendations. Even the flow rate limits on equipment aren't really limits but recommendations to prevent damage.

 

[Pool Clown]

Agreed, but (i don't think) you will never achieve burst rates with pool pumps. Do you? The "service plate flow rate" crack i made was to be sure that if your, rather Repairmans' rate was going to be greater, he would have to take that appropriate head loss into consideration while figuring total loss, that's all.

If i had a gun to my head, i could probably squeak out an accurate head loss. (I guess) The point i was trying to make was, if you can make a rough calc, and add a little, you will, for the most part be ok. With the exception of course to commercial applications, which have mandates with regard to turnover rates.

If your calc forces an unwanted increase in pipe size, you could decrease hp and increase run time. Or select a shorter run location. There are ways to get around having to use larger pipe sizes which as everyone knows equates to increased costs, especially when it comes to having to dig larger trenches!!!

 

[Pool Clown]

I have done full head loss calcs for many a member on this forum.

You are to be commended. And i'm not trying to make fun.

I'll be honest, i don't like to do them. They make my head hurt!

 

[mas985]

Agreed, but (i don't think) you will never achieve burst rates with pool pumps. Do you?

Of course not. You made the comment though that the flow rate is limited above a certain RPM and that is simply not true. Flow rate is proportional to RPM (per pump affinity laws) and there is no fundamental limit.

There are ways to get around having to use larger pipe sizes which as everyone knows equates to increased costs, especially when it comes to having to dig larger trenches!!!

If by this you mean you can use multiple smaller pipes, I would agree. But sometimes it is easier and lower cost to use a single larger pipe. But in general, the larger effective pipe cross sectional area (single or multiple pipes), the better performance and more efficient the plumbing will be which can then offset the cost of the pipe. This can mean cost savings when running a pump, especially a VS. However, there are diminishing benefits and pipe above 3" doesn't have much gain.

 

[Pool Clown]

Before we go any further, i for one see this as a healthy, and a learning discussion.

I DO NOT see this as an argument...

you can push as much water through a pipe as the pressure will allow (i.e. PVC burst rates). For a VS pump, flow rate will always increase with an increase in RPM.

I would agree with this too IF you could supply the pump with all the water it wanted. Sometimes even i forget about the suction side. I think that is a considerable limitation to the pumps ability to move a certain amount of water.

Something that i repeatedly forget about when discussing the performance vs. pipe size of variable speed pumps is that the big reason for the development of the VS pump is to: "Run the pump as slow as you can get away with, for as long as you can."

And usually, with that, the pipe size discussion goes right out the window...

 

[mas985]

I would agree with this too IF you could supply the pump with all the water it wanted. Sometimes even i forget about the suction side. I think that is a considerable limitation to the pumps ability to move a certain amount of water.

Granted, if the plumbing was not designed properly and the pump goes into cavitation, then this is an issue. But in most cases, this is not an issue so the relationship of RPM to GPM should always hold.

"Run the pump as slow as you can get away with, for as long as you can."

I agree with the first part but not the second. Why run a pump as long as you can when you may not need to? I would reword that as "Run the pump as low a RPM as you can, for as short a run time as you can get away with".

 

[repairman]

Harward,
Thank you, and all for replying to my post. Hayward tech, I hope you're still looking at this thread. You're the only one in the world who knows about Max Paks! I'm trying to hook mine up. Is it 110 or 220? It has a black wire, a white wire, and a green wire going into the control box. The writing on the black and silver tag is not legible. Do I take off the pump to find the molded impeller number? I have the whole Max Pak.
Repairman

 

[mas985]

It has a black wire, a white wire, and a green wire going into the control box.

Unless wired incorrectly, that usually means 115v. Another way to confirm is if wires go to a single breaker (115v) or a dual breaker (230v).

 

[peterl1365]

Just throwing in my two cents. If you own or are building a pool in California, a VS/VF pump is a no brainer. It will likely pay for itself in a few years, and the flexibility it buys you is enormous.

When I had my pool built in 2007, I put in the Pentair VS. Best decision I ever made. I also paid an extra hundred or two and specified 2.5" pipes. I considered 3" pipes, but figured that would be overkill for my small pool. My filter pressure never rises above 12 PSI even when I run at "high" (2200 rpm) speed for surface skimming. For normal filtering, it's at around 2-3 psi (maybe even less, but I can't tell because pressure gauges are ridiculously inaccurate below ~5% of full scale).

I thought about calculating out the flow rates at the time, but I realized that the VS pump made that all unnecessary.

 

[repairman]

Thanks, Mark. I didn't see the circuit breakers for the Max Pak. I'll assume it's 115 and try it. Anything I should know on that? I'm cleaning the connections and about to unscrew the motor from the impeller case to turn it and oil the bearing first. Is the MaxFlow low speed pretty much the same as setting a new VS pump on low and running it 24/7, or would the MaxFlow Dual Speed run faster on Low than the variable?
Peter, That's the reason I thought of using that old Hayward 2 speed pump, because it has a low speed. Am I wrong there? I'm thinking if it's strong enough to run my spa I'm in. Also what does VF mean, variable flow? I was planning on running 2" pipes. Do your 2.5" pipes come down to 1 1/2" through the pool wall, and then through 1 " directional eye inlets? This is a curiosity I am running into. Do you run your VS all the time or part of the day?
Repairman