While a pump’s function is simple--moving liquid from one place to another--selecting a pump may not be so simple.

Contractors who choose the right pump for the job will ensure success every time, so learning the differences in pump models will go a long way. Purchasing a good-quality pump will also prevent downtime for repairs and maintenance, making it more efficient and less costly to operate. Here you’ll find guidelines on how to choose the right high-quality pump for your job.

Considering Water Type and Location

While all pumps operate in the same general way--by transferring liquid from a low-pressure location to a high-pressure location through the use of force--pump capabilities vary from specialized to highly versatile and from average output rates to high-pressure discharges. Some work well for moving debris-filled water while others will clog under the same conditions. Some can pump water uphill, which may not be effectively done with another model. The key to years of success from a pump purchase lies in determining how the pump will be utilized and where. Questions to ask include: “Can the same type of pump be used for emptying a backyard pool as well as clearing muddy water from a jobsite? Should it? What if the liquid needs to be moved quickly?” Knowing the answers to these questions will help when shopping for a pump.

First, consider the type of water that needs to be moved. Relatively clean water will flow well through any type of pump but water with debris will not. A pump that is not designed to move solids may cost less, but it will likely clog and break if any type of debris-filled water is channeled through it.

Second, consider the locations the water will be moved to and from. When moving water uphill, prospective pump buyers should calculate the total dynamic head to know if a particular pump can handle the job. The total head takes into account the height that the liquid needs to be moved, along with the resistance the liquid will encounter while traveling through the pipe or hose.

To calculate total head, begin with measuring the static lift, which is the height from the lowest level of the water to the ground level of the pump. Next, measure static height--the distance from the pump level to the highest point to which the liquid will need to be moved. Finally, the amount of friction loss is calculated in, taking into account the amount of resistance there will be through a particular diameter and length of pipe between the pump and the discharge outlet. Pipe friction tables list various flow rates for pipes of different size and quality to help determine friction head. For instance, a pump discharging 350 gallons per minute through a 2-inch diameter hose might have a friction loss of 0.75 psi per linear foot.

Although determining the total dynamic head is anything but a simple calculation, knowing the numbers that need to be plugged into the equation will provide a guide for choosing the type and size of pump needed for a particular situation. The third factor that needs to be considered when determining the best pump for the job involves the deepness of the area from which the water is being pumped. Water can be pumped from no more than 26 feet down when at sea level due to the effects of atmospheric pressure. This depth lessens as altitude increases. For every 1,000 feet above sea level at which a centrifugal pump might be operated, subtract about 2 feet from the depth of 26 feet.

Pump Choices

The entire assembly of a submersible pump can be submerged into liquid. Mechanical seals prevent liquid from entering the motor.

All pumps use force to move liquid. This level of force, as well as the source of the force--compression or physical lift--sets each pump model apart from the rest. Having a clear idea of the types of applications for which a pump will be used will enable you to narrow your choices for a best-fit solution.

Centrifugal pumps offer a basic, inexpensive way for moving clean water, such as emptying a swimming pool. While other pumps work for these applications, the centrifugal pump does it least expensively.

Centrifugal pumps operate by using centrifugal force, the principle of which is force moving objects away from the center of a system in a circular motion with pressure increasing as it rotates. An impeller causes this centrifugal force within the pump as the vanes on the circular impeller disk sling water around. Each rotation of the impeller increases velocity, which causes the liquid moving into the volute (the component that houses the impeller) to be collected. Within the volute, velocity is reduced and its energy converted to pressure energy, which causes the fluid to be forced quickly from the pump.

As an analogy, think of swinging a rock from the end of a string in a circular motion. The velocity increases with each swing, and the rock moves out quickly from the center once the string is released. Centrifugal pumps operate in much the same way.

Two types of centrifugal pumps offer varying outputs. A high-pressure pump moves water with twice as much pressure as a standard centrifugal pump. High-pressure pumps also move water a considerable distance or uphill, but like a standard centrifugal pump, they should not move water containing debris unless an appropriately sized strainer has been installed.

When considering the difference between the two centrifugal pumps, compare running water through a garden hose to water discharging from a standard centrifugal pump. Holding a thumb over the end of the hose causes the water to shoot out with greater force, much like the high-pressure centrifugal pump does. This difference makes high-pressure pumps particularly useful as a watering pump, rather than a dewatering pump, and especially practical to use for spraying applications or cleaning with a higher pressure.

Semi-trash pumps or trash pumps should be used when water containing debris needs to be moved. The impeller in a semi-trash pump has thicker vanes than those in a standard centrifugal pump, which enables it to accommodate larger debris. Built like a semi-trash pump, trash pumps offer a stronger model capable of handling larger solids. The impeller vanes run deeper yet, allowing for the largest solid-handling capacity. This makes trash pumps especially suited for pumping muddy or sandy water or water with debris up to several inches in diameter. Contractors frequently use trash pumps to move water that might contain abrasive materials. The heavy-duty pump housing makes a trash pump a versatile piece for almost any situation.

Diaphragm pumps should be considered for jobs that require more versatility with varying applications. These pumps can handle almost any pump application with ease but come with a price tag to match their capabilities. For this reason, pump users typically look at diaphragm pumps as an option only when it is the only tool that can effectively handle a particular job.

Rather than using a rotating impeller within the pump to move water, diaphragm pumps have two chambers that operate much like an internal combustion engine. As the volume increases in one chamber, the pressure decreases in the other, drawing in fluid. When the pressure in the second chamber increases, the fluid is forced out and the process repeats. Diaphragm pumps take on virtually anything--sludge, slurries, abrasives and any trash or solid material that will fit through the pump.

In sharp contrast to the diaphragm pump’s versatility, the submersible pump handles very specialized applications. The pump’s hermetically sealed motor is closely coupled with the pump body, and the entire assembly can be submerged in liquid up to about 25 feet. Mechanical seals prevent the liquid from entering the motor as it is being pumped up through a connected pipe or flexible hose. Applications well suited for submersible pump use include pumping slurry, draining areas and pumping sewage, as well as general industrial pumping.

Recognizing Quality

Semi-trash pumps’ impellers are made with thicker vanes than those in standard centrifugal pumps, which enables them to move muddy water and larger debris.

Once options have been narrowed to the best pump for its planned usage, the next challenge lies in determining the pump’s quality. A quality pump starts with quality materials and is powered by a well-made engine.

A high-quality engine will prolong the pump’s life and lessen maintenance issues. In addition to a good engine, components should be evaluated for high quality. The mechanical seals and impeller especially should be well made since these two parts are intrinsic to pump operation.

Self-lubricating carbon ceramic mechanical seals in centrifugal pumps and self-lubricating silicone carbide seals in semi-trash and trash pumps are valuable components. A worn seal can cause unwanted air to enter the pump and keep it from operating. Mechanical self-lubricating seals help prevent these problems.

Cast-iron impellers are a sign of quality on any style of pump but are especially important in pumps that frequently transport solid material since that is one of the main causes of pump wear. Additionally, pumps that frequently handle debris will last longer when they’re built with a wear plate--and it is less expensive to get a new wear plate than it is to replace the impeller and volute it protects.

For a long-lasting unit, look for durable die-cast aluminum housing, a cast-iron volute, a metal strainer to keep oversized debris from entering the intake hose, heavy-duty knobs and a full-perimeter frame. A durable pump also should offer low-oil shutdown protection and come with a warranty to further ensure the extended life of the product.

In the end, spending a little extra money for a wear plate, heavy-duty parts and a protective frame will more than pay for itself when the pump outlasts lesser-quality models. Looking for a quality pump also means ensuring that valuable time will not be lost waiting on maintenance work or ordering repair parts. Reducing downtime results in increased productivity, which translates into profits for professional contractors and rental centers alike.

When flooded with options for pumps--from centrifugal to trash to diaphragm--it’s good to know which will get the job done best. Making the wrong choice and using a pump in a situation that it was not designed to handle can cause undue wear and possibly break the pump’s components. Understanding pump types, features and options makes choosing the right pump for the job a much simpler task.


To read about pump maintenance, click to “Keep It Pumping.” www.siteprepmag.com/CDA/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000000335271