Tips for Selecting the Proper Pump
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Step
1: Determining Desired Flow Rate
There are several things to consider in order
to make a good decision for a pump purchase. The first thing is to determine
your required flow rate. You will want your minimum flow rate to be at
least half of your pond volume (Pond
Volume Calculator <http://www.WaterGarden.com/calculate/Flex/FlexVolumeCalculator.html>).
For example if you have a pond that contains 2,000 gallons of water then
you will want to pump at least 1,000 gallons per hour (gph). This is a
starting point. You may find that 1,000 gph is not enough if you plan
to have a waterfall that is more than 10 inches wide. Generally for a
waterfall you will want 100 - 150 gph per inch of width of the weir (the
portion that the water spills over). This amount will vary depending on
how smooth or rough the weir is. If you want less than a full flow then
less flow is acceptable. You will also need to consider your filter and
ultraviolet sterilizer (UV) to make sure that you are moving the right
amount of water for these devices.
If your filter or UV has a maximum flow that is less than your required flow rate for good circulation of your pond water, then you will need to divert some of the water around these devices and back into the pond. This is accomplished by adding a tee fitting in the pipe coming from the pump and before your filter and/or UV. A ball valve is added between the tee and the filter. Another ball valve is installed on the other leg of the tee. Add more pipe to this leg of the tee and extend the pipe to the waterfall or to the pond. Adjust the ball valves to get the appropriate amount of water flowing to your filter.
Step 2: Calculating
Head Pressure
The next thing that you want to consider is the amount
of head that your system will have so that the correct pump size can be
determined. The three main sources of head are:
- STATIC HEAD- this is the vertical distance you raise the water above the pond surface;
- FRICTION HEAD- this is the resistance from pipe and fittings as the water flows through;
- PRESSURE HEAD- the additional pressure required by some filters, venturis, and other devices.
Most pond applications can ignore PRESSURE HEAD. First determine the vertical distance in feet above the water surface, this is the STATIC HEAD. Next, use our Friction Loss Chart <watergarden.com/catalog/_pumps/friction.html>to determine your FRICTION HEAD. Add this to your PRESSURE HEAD and you can use this figure to size the pump. We highly recommend using flexible PVC pipe that eliminates elbows and provides higher flow rates by reducing friction loss. Even though cost may be higher than rigid PVC pipe, it is much easier to install. There is no problem in using pipe larger than required but smaller will cut your flow rate.
Step
3: Submersible vs. External
Before you choose your pump however, there are other things
to consider. Should you use a submersible or an external pump?
Submersible pumps are usually less expensive and easier to install. However, some may cost more to operate and usually don't last as long as external pumps. There are a lot of variables in determining the longevity of a submersible pump. How dirty the water, how often the pump is cleaned, and if it is operated continuous or intermittently will all influence the lifespan of a pump.
External pumps are generally very energy efficient and long lasting, but involve a little more plumbing and you will generally want to find a way to disguise the pump from view.
It is often good to compare the warranty of the pumps you are considering as this can give an idea of which pump will last longer. We have found that on average a pump will last 2-4 times the stated warranty period. A pump that costs twice as much as another but lasts 3 times as long will be a better value.
Step
4: Calculating Operation Expenses
You can determine the operating cost of any pump by using
this formula: amps x volts divided by 1000 x KWH cost x 24 hours-a-day
x 30.4 days-per-month = cost per month.
If the pump is rated in watts instead of amps use this formula: watts divided by 1000 x kWh x 24 hours-a-day x 30.4 days-per-month.
KWH is the kilowatt-hour cost, which you can get from an electric bill or by calling your local electric company. (Pump Opration Cost Calculator <watergarden.com/catalog/_pumps/cost_index.html>)
Step
5: Which Pump?
The pumps in our online catalog have a performance chart
associated with them that gives the flow rate for various head pressures.
For example: you have a pond that is 1500 gallons and your waterfall weir
is 10 inches wide. You will need ½ of 1500 gallons or 750 gph for
good circulation of your pond but your waterfall requires 1000 to 1500
gph (based on 100 to 150 gph per inch of width of weir). You will want
a pump that will provide somewhere around the 1000 gph or 1500 gph that
the waterfall requires.
The next step is to determine your head pressure. Your waterfall is 4 feet above the pond water level and the waterfall will require 25 feet of plumbing from the pump to the waterfall. If you consult the Friction Loss Chart, you will see that 1 ½" pipe will be a good choice for 1500 gph.
You will have a FRICTION HEAD of approximately 1 foot if you use 1 ½" pipe. Add to this the 4 feet of STATIC HEAD and your total head pressure will be 5 feet.
The next step is to look at the flow charts for the various pumps. For instance the Savio SAP1450 delivers about 1200 gph @ 5 feet of head. Pondmaster PMP1800 delivers 1200 gph @ 5 feet of head. If an external pump is desired, you will find the Sequence MDM3600 delivers about 2700 gph @ 5 feet of head. While this is more than necessary, the flow can be adjusted by installing a ball valve on the discharge line. Any of these pumps may work for this application.
Find two or three pumps that will deliver the required flow rate and calculate the operating cost for each pump. Compare this information with the purchase price of each pump and use this information to make an educated decision about which pump is best for your needs.
Should you have questions or wish to discuss your pump options, please contact us at: consult@watergarden.com or by phone at the numbers listed at the bottom of this page.