
Why
can’t I use just any pump for my pond? © 2001 By
David A. Dec
Pump head or total dynamic head is the most misunderstood topic among pond owners. Most pond owners think their pump head is between 5 and 7 feet. They are shocked when they find out that it is actually between 20 and 40 feet. Matching a pump to the pump head is vital, but seldom done. If you buy a pump that is way
too small it may only move a trickle of water, or possibly none at all. One that
is a bit larger can still be too small to give good aeration, filtration and
surface skimming. Overloading a pump that is too small can result in a shorter
pump life, and more repairs. Often people who buy too small a pump will buy 1 or
more of the same pump, so they wind up running several pumps with higher
operating costs than 1 properly sized pump. On the other hand, choosing a
pump that is too large will not only waste a lot of money to run it, but can
actually result in damage to the plumbing and equipment. In order to pick out the correct pump there are 5 steps you need to go through:
I. Determining the volume of your pondThe first thing you need to do is determine the volume of you pond. If you have not done that yet it is the length (ft) x width (ft) x depth (ft) x 7.48 gallons / cubic foot = U.S. Gallons.
If your pond has a very irregular shape you may want to backcalculate its volume by measuring changes in the salinity when you add salt. There are some very good salinity test kits that are extremely accurate. The procedure is to:
II. Determining the flow you wantIf you have a pond that is under
a few thousand gallons you may want to turn it over 2 to 3 times per hours. This
is similar to large marine aquarium owners who have learned to turn over their
aquariums' water a minimum of 3 times per hour. If
it is a larger pond you may want to turn it over only once every 2 hours. Peter Waddington, in his book
“Koi Kichi”, says the real volume of water a fish lives in is determined by
multiplying the pump's flow per hour times 24 hours per day. For instance, 3,333
gallons/hour yields a "real volume" of 80,000 gallons that the Koi
actually live in, regardless of the actual size of the pond. This says that the
water pump’s output is
more important than the size of the pond. So people with smaller
ponds will want to turn them over more often than those with larger ponds. So let’s say you have a 5,000gallon pond, and you want to turn it over every 1½ hours. We simply divide the size of your pond by the number of hours you want for a complete turnover to get your flow rate. So for our example the flow needs to be 5,000 / 1.5 = 3,333 gallons per hour (GPH) or 3,333 / 60 = 55.5 gallons per minute (GPM). The flow rate is very important and determines the size of you piping and pump. III. Determining the correct pipe size for your pondThe Plastic Pipe and Fittings
Association (PPFA) says PVC pipe should be designed for a maximum flowrate
velocity of 5 to 8 feet per second (fps) through the pipe. They say 8 fps is ok
for pipe sizes less than 1” in diameter, but it should be less than 5 fps for
pipe sizes of 1 ¼ “ or larger. Higher velocities can actually cause pipe
failure and rupture, as well as astronomically large resistance to water flow,
which necessitates higher horsepower requirements, and higher operating costs. How do you determine the
velocity of the flow rate in feet per second? The equation is: Velocity in fps =
.4085 x GPM / d^{2} Where GPM = gallons per minute, and d =
inside diameter of the pipe in inches. The following table shows the
results of these fps calculations for various pipe diameters (d) and flow rates
in GPH and GPM: Table One
So we need to pick a velocity that is less than 5 fps from the above table. So looking at the above table for our example, we want to look down the 3,600 GPH column (since we want a flow of 3,333) until we find an fps that is less than 5. When we do that we see 4.10 fps corresponds to a 2½ “ pipe. One 2” pipe would be pushing
the envelope, but we could use 22” pipes; like one 2” pipe from the bottom
drain, and another 2” pipe from the skimmer. Both pipes could terminate in the
ends of a Tee fitting, with valves for each, with the center branch feeding the
pump. By the way, 22” pipes have about the same area as 13” pipe. IV. Determining the water pressure needed from the Total Dynamic Head (TDH), or the sum of all the sources of pump head Ph, for your designHead is best defined as
“resistance to flow”. A higher head means you need more pressure to overcome
it. The term “head” is further modified by whether the
resistance is encountered on the suction side of the pump (suction head (H_{S})
from the pond to the pump) or the discharge side (discharge head (H_{D})
from the pump to the pond); whether it is caused by the standing height of the
water (static head h_{sh} = height of the waterfall or fountain above
the water’s surface) or by the movement of water through the system (dynamic
head = h_{d}); whether the resistance is caused by simple friction due
to fittings and pipe sizing (friction head = h_{f }) or by the equipment
resistance (h_{e}). TDH = H_{S}
+ H_{D} = (h_{sh} + h_{d} + h_{f} + h_{e})_{S}
+ (h_{sh} + h_{d} + h_{f} + h_{e})_{D
} In order to determine the total
dynamic head (TDH) we need to consider all of these sources:
This TDH or Ph is the most
difficult calculation for everyone, because it is very complicated. Here is a
table of the resistance in feet of pump head for every 10foot length of pipe as
a function of water flow: Table Two
Where do these values come from? The PPFA says to use the HazenWilliams Equation. The equation is: Ph = 104.4 / C^{1.852} x (GPM)^{1.852 }/ d^{4.8655 } where Ph is the pump head in
feet per 10 feet of pipe, GPM is gallons per minute, d is the inside diameter of
the pipe in inches, C is a pipe smoothness coefficient that is 150 for new PVC;
140 for smooth walled copper, brass, etc.; 100 for ordinary iron pipe; and 80
for old iron pipe. Lasco’s PVC fittings website
also uses this equation to show the friction losses. However, they convert their
results to Pounds per square inch (PSI) per 100 feet of pipe length. So according to the above table,
if we have 30 feet of pipe, and a flow of 3,333 GPH, the pump head due to the
pipe alone, without any fittings, would be 4.22 * 3 = 12.66 feet of pump head
for 1 ¼ “ pipe; 1.99 * 3 = 6 feet for 1 ½ “ pipe; 0.58 * 3 = 1.7
feet for 2” pipe, etc. The next consideration is the number and type of fittings
we plan to use. Following is a table of the resistance per fitting, expressed in
length of equivalent pipe in feet, not in feet of pump head. This is a very
important distinction and is a source of much confusion. Table Three
Assuming
we are using a total length of 30 feet of 1 ½ “ pipe, with six 90º elbows,
two 45º elbows, four Tee’s, 1 check valve, and 10 gate valves. We use Table
Three to construct the following Table Four: Table Four
Using these values we get a
total equivalent pipe length of 125.2 feet. Now we go to Table Two and find the
pump head for a flow rate of 3,333 GPH, for a pipe diameter of 1 ½ “, which
is 1.99 feet of pump head for every 10 feet of equivalent pipe length. Using
these numbers to calculate the total pump head: 125.2 * 1.99 / 10 = 24.9 feet of pump head, which is due to the friction losses through the 1 ½ “ pipe and fittings. When we perform this same calculation for all the pipe diameters at a flowrate of 3,333 GPH, we get the following results: Table Five
As you can see the pipe diameter
has a huge effect on the pump head requirement. If we choose the correct pipe
diameter, based on the 5 feet per second velocity restriction, as is shown in
Table One, we would use a 2 ½ “ pipe for a flow of 3,333 GPH. This would give
us a pump head of 3.1 as seen in Table Five, and the guideline of “1 foot of
pump head for every 10 feet of pipe” holds true for our 30 feet of pipe. However, if we choose any other
size of pipe, then this guideline does not hold true, and can be way off the
mark. So choosing the correct pipe size for our pond is absolutely critical. As
seen in Table Five, if we used the 1 ½ “ pipe the pump head would be over 8
feet of pump head for every 10 feet of pipe. Unfortunately this is an easy
mistake for a beginner to make, and becomes very difficult to correct after the
pond is constructed. There are too many ponds with the wrong size pipe buried
deep under the liner, or concrete, or the waterfall with its many tons of rock
and boulders. The next step is to add the pipe
and fittings pump head to the other equipment pump head losses (not equivalent
pipe lengths) to get the Total Dynamic Head (TDH): Table Six
All these calculations have been based on ideal “new” pipe, fittings, and equipment. Older systems may have “algae, hard mineral scale, or muck buildup" on the piping walls, filters, strainers, valves, elbows, heat exchangers, etc., making the published numbers way too low; and that is assuming there are no rocks, gravel, or tree roots in the pipe. If any of these things are present, then the smoothness coefficient is no longer valid, and neither is the inside diameter of the pipe. In other words, the TDH pump head can in reality be much higher than calculated for new pipe. Another way of determining the TDH is to measure it, if your existing pump is working. You can install a flow meter on each of the suction lines to help you balance the system, plus a vacuum gauge on the suction side of the pump, and a pressure gauge and another flow meter on the discharge side of the pump. Every inch of mercury on the vacuum gauge is multiplied by 1.13 feet of head to get the suction head. Every PSI on the pressure gauge is multiplied by 2.31 feet of head to get the discharge head. Then you add those two head numbers together to get the Total Dynamic Head (TDH), at your existing flow rate. One thing that is important to remember is that the TDH changes dramatically with the flow rate. You system's head can be one value at one flow rate, and dramatically different at another one. V. Determining the best pump that will give you the proper flow and pressureNow we know the flow rate we want is 3,333 GPH or 55.5 GPM, and the Total Dynamic Head (TDH) for our pond design is 58.1 feet of pump head. Our next step is to check out the various performance curves for available pumps. Graph One is a typical performance curve. We find 55.5 GPM on the X or bottom axis, and draw a line up. Then we find 58.1 feet of pump head on the Y or side axis, and draw a line to the right. Where they meet is the pump that we want. In this case we want a pump that is a little less than a 1 horsepower pump. Our next step is to find the most efficient pump for our conditions. The most efficient pump will be the one with the highest Creech Pump Index = (GPM x TDH / watts), and if they have the same CPI, then the lowest amps. If we can find a variable speed pump, like the Energy Saving
Pump®, we can dial the horsepower and amps down to exactly where we need it to be,
and save money. This is especially true when our GPM and TDH point falls well
below the horsepower we need. We are not using a pump that is too large because
now we can dial it down to the proper size. It is like having a pump with a gas
pedal, which does not need to be pushed to the floor all of the time. When we
let up on the gas pedal, we save money. One thing to avoid is picking a pump with no “head” room. We want to make sure that we are not too close to the maximum pump head, in case we have not allowed for “dirty” pipes, fittings, etc., which eventually could result in no flow at all. The gas pedal allows for real world changes, and system additions and expansions. Graph One

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