Anyone who has ever picked up (or tried to pick up) a five gallon container full of water will know water is heavy. In fact, it weighs about 8.5 pounds per gallon. If you were to lift this five gallon container of water from the floor to the top of your tank, it would require a fair amount of work on your part. So, it should not be too surprising that the same is true for a water pump trying to pump water from a low elevation, such as a sump, to a higher level such as a tank. Taking the example of a sump and a tank and separating the sump and the tank further apart by raising the tank to a higher level, the tank would eventually reach a level where the pump would no longer be able to lift the water any higher. The height of this stationary water column is referred to as its maximum head, or height, of the water column that the pump can support. If the tank is lowered below this maximum water height, the water will start to flow into the tank again. As the tank is lowered even further until it is at the same level as the sump, the flow rate will increase and will be at its maximum. The maximum flow rate for a pump occurs when it is discharging its contents at or near the same level as its intake. Elevating the pump's discharge relative to its intake will cause its output to decrease, and eventually stop altogether when the elevation difference is equal to the pump's maximum head. Figure #1 shows a graphical representation of this relationship between an example pump's flow rate and its head.

Different pumps have their own flow rate versus head curves. These curves are normally included with a pump's documentation or are readily available from the manufacturer. For those of you who know only the maximum head and flow rate for your pump, you can assume that the flow rate changes in a linear relationship with the head and get results that will be nearly accurate. If the relationship between flow rate and head or pumping height is known, all that is necessary is to select a pump that meets our flow rate requirements at the necessary height. Unfortunately, it is not that simple. Other factors need to be taken into account, and the concept of pumping height or head must be refined.

To better understand what factors can affect the pump's flow rate, I first need to introduce the concept of pressure. Pressure is force divided by the area that it is applied to, often quoted in pounds per square inch, or psi, for short. As an example, a tray filled with a gallon of water an inch deep but 10" wide by 23" long would have a pressure on its bottom of about 0.037 psi (8.5 lbs / (area of tray bottom or 230 square inches). The same amount of water, but in a tube whose cross-sectional area is one square inch but 230" tall, would have a pressure of 8.5 psi on the bottom of the tube (8.5 lbs / 1 square inch). The reason for the different pressures, even though the weight of the water is the same in both cases, is that the weight is supported by a large area in the first example and so the force on any small section is low. But, in the second example, all the water's weight is supported by a small area and so its effective pressure is correspondingly higher. Consequently, the maximum head or water column height that a pump can support is really a measure of the maximum water pressure that a pump can produce. As the pressure on the output of a pump is reduced, its flow rate will increase. This is why the maximum flow rate for a pump occurs when pumping at zero head or no elevation change; the pressure it has to pump against is minimal.

Not only can elevation changes cause pressure losses on a pump's output, but losses due to the resistance to water flow in the plumbing can also have an effect. As an analogy, if you blow forcefully but slowly into a straw, it requires little effort. But if you try blowing into it very quickly, it is much more difficult because there is more resistance to the flow of air in the straw. The same phenomena occurs when you try to pump water quickly through pipe or tubing; the faster the flow, the more resistance or pressure on the pump which will reduce the pump's flow rate. Other factors such as the roughness of the pipe or tubing's interior, and whether the water is traveling in a straight line or a curved path, will also affect this pressure loss (the more severe the curvature the larger the loss). The easiest way to limit the reduction in flow rate is to reduce the velocity of the water flowing through it, and the simplest way of doing this is to increase the effective diameter of the plumbing. Pressure loss with higher flow velocity explains why higher flow rate pumps typically have larger diameter inputs and outlets; the pumps are designed to keep the velocity of the water lower at higher flow rates. Remember that flow rate refers to how fast a given volume of water is moving. If the diameter of the pipe or tubing is larger, then it can move a greater volume of water at a lower velocity and maintain the same effective flow rate as it would with smaller diameter plumbing and higher water velocities. There is a positive aspect to this loss of flow rate and that is that control is easily achieved with the convenient use of valves to adjust the flow rate of a pump.

A valve reduces the effective diameter for the water flowing through it, and thus increases the effective flow resistance or pressure on the pump, and correspondingly reducing its output flow rate.

Example:

A pump is needed that will provide a flow rate of at least 500 gallons per hour (gph) between a sump and a tank that has its water level located 4 feet above the sump's water level (see figure #2). Note in figure #2 that the effective pumping height as shown is not measured from the inlet of the pump to the level of the outlet in the tank, but rather from the water level in the sump to the water level in the tank. The reason is that the water level in the sump is actually causing a pressure increase at the pump's input which will aid the pump's flow rate and make it perform as though the pump and its inlet were really level with the water surface of the sump. On the output side of the pump the water level in the tank is actually causing a higher pressure on the pump's outlet, making it seem as though the pump were actually discharging near the surface of the tank. In general, the actual head of a pump is determined by the difference between the highest water level open to the atmosphere on the inlet side of the pump and that similar water level associated with its outlet (figure #3 shows additional examples). Assuming that figure #1's chart represents the flow rate versus head of a given pump, then at a head of 4 feet the flow rate is 850 GPH (which is above what was required).

If the max flow rate and head were used, and assuming a linear relationship, the estimated flow rate would be 700 GPH, which, while lower, is not too far off from the manufacturer's published documentation. Trying to estimate specific head or pressure losses in the plumbing itself can be done, but is very difficult and probably not necessary under most circumstances. As long as the correct diameter plumbing is matched to a given pump's inlet and outlet, the number of elbows or tight turns is limited and long runs of pipe or tubing are avoided, the plumbing losses can generally be kept to less than 2-3 feet of equivalent head. This means that in the example above that the actual effective head or elevation loss would be closer to 7 feet rather than 4 feet after accounting for the additional flow losses from plumbing. Examining the curve in diagram #1, at a 7 foot head there is a flow rate of 600 gph; a level still above the desired 500 gph. Always try to select a pump with a somewhat higher flow rate than needed, since a valve can always be used to reduce it to the desired level. This also gives some leeway in case the flow rate should drop over time due to fouling inside the plumbing or reduced pump performance from wear. It might be advisable to just use a very big pump with a valve to adjust to the desired requirements. If flow rate were the only selection criteria, it would be possible to do just that. However, other factors will also likely influence your pump selection.

First, consider some of the other factors that could affect the choice of pumps: cost, power usage (also affects operating costs), safety for aquarium use (especially saltwater), reliability, operating noise level, heat transfer to the tank, size and installation restrictions. This list is by no means complete, but it gives some idea as to other things to think about when selecting a pump. In general, larger pumps will cost more to buy and operate, make more noise, transfer more heat to the tank and be harder to install. The factor listed as "safe for aquarium use" was discussed in part 1 of this series, and primarily deals with making sure the pump is constructed using aquarium safe materials. Reliability is also important when selecting a pump since you do not want it failing and possibly causing your whole system to crash. When selecting a pump(s), ask around and see what experiences other aquarists have had with similar units and check to see if the particular pump in question has been around for a while and recommended for aquarium use.

While having a good pump helps to improve your system's reliability, the best way to reduce the likelihood of a pump failure harming your system is to have redundancy. While one large pump may do the job, two smaller pumps may be able to accomplish the same task, and the probability of both smaller pumps failing at the same time is normally much less. Installation of multiple pumps is more difficult, and possibly more costly, but the peace of mind it gives to know your critters are safer may be worthwhile.

Another significant selection criteria not yet mentioned is whether to choose a submersible or non-submersible (external) pump.

In Part 1 of this series, I noted that the principal advantages to submersible pumps were the ease of installation and a generally more compact size. They may be best suited for internal circulation pumps, and are called powerheads. Powerheads are normally small (allowing placement within tanks in inconspicuous locations), offer low to moderate flow rates, but usually provide little pressure capability. I will consider more about installation and system design tradeoffs in the final part of this series, but there is one personal comment I'd like to make now. In all but the smallest aquarium systems I prefer to have both external pumps as well as internal circulation pumps to move water from sumps or other external filter systems. This dual approach allows me to use smaller external pumps since not all of my circulation requirements need be provided by them, and it gives me yet another form of pump redundancy to improve overall system reliability. As mentioned in Part 1, if you are not concerned with heat transfer to the tank, a non-submersible pump is probably the preferred choice.

In regard to reliability and safety, most centrifugal water pumps are not designed to run dry (i.e., no water in them), and will be damaged if this occurs. Water in the pump is required to act both as a lubricant as well as a coolant. The use of float switches in sumps or tanks that turn pumps off if there is no water, is a good way of making sure you do not mistakenly run a pump dry. The use of pump controllers or wave makers also deserves mention. Pumps are normally operated by electrical motors, tricky electrical loads to turn off and on safely. If either the controller or the pump is not designed properly and unable to reliably turn motors on and off repeatedly, one or both of these units could be damaged by switching. If you plan to use pumps and a wave maker in this way, I recommend that you make sure they operate safely together by either contacting the manufacturer of the units or by finding someone else who already uses a similar configuration. Lastly, I highly recommended that you use Ground Fault Interrupter (GFI) equipped electrical circuits when using any electrical equipment around water to reduce the danger of electrical shocks to both yourself and your tank.

The next and final installment in this series on aquarium water pumps will deal with installation procedures for pumps, as well as discussing some ideas on to how to use pumps for various aquarium related applications.