NOTES ON POOL / SPA PUMP SIZING
Hyrdaulics is the science used to determine the optimal flow rate
when transfering fluids. Knowing hydraulic principles enables us to
match our requirements with a given pump that suits our needs best for a
particular application.
This is meant to be a general discussion and primer on pump sizing
or replacement and is nowhere near exhaustive. It is intended only to make you aware
of factors you need to consider when replacing your existing pump, or
determining your requirements for a new installation. We have also provided links
at the bottom of the page that are of related interest which we hope you find useful.
REPLACING AN EXISTING POOL PUMP / SPA PUMP
First, we normally make the assumption that the company that
installed your pool/spa is/was reputable and is/was qualified to spec
out and install a suitable pump to begin with. Unless you have
problems early on after the installation, you can generally be assured
that your pump was sized properly. Often, by the time a pump is at or
near the end of its useful life, it is not operating at its maximum
rating, and there is often the temptation to increase the horsepower when replacing it.
This may work, then again it may not. If the system as installed
worked fine, we generally advise a comparable replacement.
In the mid 1980's, pump design underwent a radical design
improvement. The newer designs are far more efficient, and horsepower
for horsepower, run circles around the old style pumps due to closed
faced impeller designs and other changes. The old bronze style with
open-faced brass impellers simply cannot compete with the newer design.
If you have one of these old style pumps, you will not need as much
horsepower if you replace it with a newer, higher efficiency pump. It is
not uncommon to go from a 2 HP old style to a 3/4 or 1 HP new style
pump and get similar flow rates. This results in up to 60% savings on energy costs.
We find many customers attempting to replace their old style pump with a new style
higher efficiency pump at the same horsepower rating - THIS IS NOT ADVISABLE!!!!
Fortunately, there are not many of these old style pumps in service any longer.
A common misconception is that more horsepower is better, and this
is not always the case. It is as easy to oversize a pump as it is to
undersize it. Oversizing a pump can result in premature wear on the
pump itself, poor filtration and/or damage to the filter media,
lower efficiency, excessive noise, and excessive power
consumption/higher energy costs. The goal is for a hydraulically balanced
installation that will minimize energy expenditure and maximize
flow rates, subject to the limitations of the pipe sizing and
overall configuration.
While on the subject of horsepower, please note that there are
FULL-rated and UP-rated pumps/motors. Without going into a lot of
detail, it is important to know which one you have so you are not
comparing apples to oranges so to speak.
Here are couple of examples:
|
|
| UP-RATED Model# |
Marketed as |
Brake H.P. |
FULL-RATED Model# |
Marketed as |
| SP3007x10 |
1 H.P. |
equivalent to |
SP3007 |
3/4 H.P. |
| SP3010x15 |
1.5 H.P. |
equivalent to |
SP3010 |
1 H.P. |
The reason this situation exists is because in
the past, certain manufacturers decided to offer both types for marketing purposes. You are getting
what you pay for and are not being scammed, (note that prices are
nearly equal if not identical) but it can seem a little
misleading. The motor's horsepower rating is only one factor in
determining it's output. Service Factor (SF) needs to be considered
here as well. If you take the rated horsepower and multiply it by the
service factor, as in the above examples, the full-rated 1 horsepower has the
equivalent brake horsepower as the up-rated 1 1/2 horsepower.
Be sure that you take this into consideration before deciding on a new
pump. Until the industry uses meaningful terms like brake-horsepower,
then this practice will continue. We have noticed that Sta-Rite has
voiced their concern over this practice and they began to publish
their brake-horsepower specifications beginning with the
2005 catalog. We applaud them for this and hope others will follow suit.
If you have one of the newer designs in need of replacement, it is
recommended that you stick with the same model, especially if you were
satisfied with it, it meets your needs, and is still available. This
is the easiest and most practical solution.
If you want or need to replace your pump with a different
model/manufacturer, you will need to find a pump which has output
performance characteristics similar to the one you currently own. A
pump curve shows the pump output in GPM (gallons per minute), based on
total dynamic head (resistance to flow).
Understandably, most people have no clue of what the resistance in their
system is. This can tend to complicate the replacement of an existing
pump if it is not going to be replaced by the same model. No two pumps
are alike, except the same model. Each pump model has unique design
characteristics in its horsepower and ability to overcome resistance.
As long as you know the model and horsepower of your current pump, it
should be easy to compare pump curves. If you do not have the original
owners manual, most manufacturers have their literature on-line and will
typically provide this information.
Most manufacturers have different lines of pump models, and fortunately,
there are similarities enough between these lines between different
manufacturers that you should be able to find something reasonably
comparable, if not ideal. If you stick with the same manufacturer, they
can easily advise you on a comparable replacement. If you decide to go
with a different manufacturer, they usually can help you with a
recommended replacement from their product line as well.
Bear in mind that pump output is determined not only by a pump's design
characteristics, but also by other equipment, diameter and length of
pipe, number, sizes, and types of fittings, etc. - everything that contributes to the
resistance within the system. It will also be limited by the maximum flow rate allowed
by the filter. Exceeding the flow rate of the filter can result in poor
filtration as well as possible damage to the filter and/or filter media.
Our objective is to determine the most
efficient pump for a given situation. Factors that must be considered
when sizing a pool pump or spa pump are:
1. Number of gallons to turnover
2. Amount of time required to turnover
This will enable us to determine our desired/required GPM (gallons per
minute), and is our starting point.
For example, lets say we have 30,000 gallons, and need/desire a turnover
in 8 hours.
30,000 / (8*60) = 62.5 GPM Gallons / (hrs turnover * minutes per hour) = GPM
There are generally accepted guidelines which you should follow when
determining the amount of time required for the water to turnover. For
residential pools, which are not always regulated by health codes in
this regard, an 8 - 12 hr turnover period is considered adequate. For
commercial pools which are strictly regulated, 6 hours is usually
mandated. Commercial spas are usually 15 - 30 minutes, and wading pools
are usually 60 minutes. Consult your local health department for exact
requirements.
There are certain velocities which should not be exceeded for optimal
efficiency. It is generally accepted that you should not exceed 7 FPS
(feet per second). Note that as velocity in fps increases, so does
resistance, and it does so geometrically. Ideally we want to keep this
resistance to a minimum.
To achieve the turnover we are seeking, we must determine the pipe size
which will efficiently handle 62.5 GPM at ~ 7fps. To do this, we simply
consult a chart which has built in these calculations already.
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