Planning and Pumping the Unknown: What You Need to Anticipate

By Dave Meister, P.E.

Staged. High head. High flow. High efficiency. Solids handling. Non-clogging. What's the best way to begin the process of cutting through the maze of claims and the lists of pump characteristics when developing a new application or revamping an old one? The answer begins by looking at the past and getting a good feel for what has been successful for similar situations. The next step is collecting data on current conditions so that you can plan for the future.

Whether developing a completely new pumping system or updating an old one, reviewing the history of similar systems will go a long way in ensuring success when facing unknowns. In many cases, it's surprising to learn that what is expected from a system is not what actually happens. The pump must be able to function properly under the new requirements — whether caused by the pipe friction coefficient affecting actual verses calculated head, the recovery head or lack of expected recovery head, or any other reasons. Obviously, the more experience you have at your fingertips to draw on, the fewer surprises you will encounter.

Garbage In, Garbage Out

(Click image for larger version) Speed, Friction, and Horsepower In the pumping world, friction is bad. Using speed to overcome large friction forces should be avoided as much as possible to attain the greatest economy over the life of the pump. This figure illustrates that a relatively shallow system curve will allow significant flow changes with relatively little speed change. Compare Curves A and B. Curve A shows a capacity increase of 250 gpm with a 300 rpm increase in speed and about 4.5 additional horsepower required. Now look at Curve B where the same 300 rpm increase yields only 100 additional gpm and requires 5 additional horsepower.

Collecting data for a new system is probably the most important step of the process. Generally, the length of pipe and number of fittings is fairly easy to determine. When pumping sewage out of a wet well, the flow may be determined by the expected inflow — or by how often the pump needs to cycle to keep from going septic. Or, a minimum velocity may be required to keep solids moving through the discharge piping. As the old adage goes: You may not be able to choose your parents, but you can do the best with what you've got. In pump terms, there is usually little that can be done about changing the flow range since it is most likely established by other factors. Likewise, the designer of the pump site often cannot avoid the ups and downs of the terrain that the force main must traverse. How well placed and how well the siphon breakers and air release valves function — and remain functioning — can greatly affect the head requirements over the life of the pumping equipment.

A well-designed pump can have a life expectancy of 50 years or more. To take advantage of that feature, however, it is important to look as far into the future as possible. While initial installation is costly, prudent designers will think about the long-term future to come up with a pumping system that can easily be upgraded as demands grow, avoiding even costlier revamping. In fact, some progressive solutions have included the burying of a second parallel main that can be tied into an existing line when additional flow is needed. This preplanning effort will avoid the ongoing future costs of friction energy loss and the problem of too large of a pipe to keep solids in suspension.

But even the best laid plans can fail. That's why it is so important to work with a consultant, pump distributor, or company that can add to the experience circle. These partners can direct your team to the most flexible pump available to accommodate the range of conditions that the actual system might experience.

Power of Experience

Traditionally, solids-handling, self-priming pumps have been designed to operate at relatively low heads (around 100 feet or less). Therefore, for every 100 feet of elevation increase or for long force mains with high friction, a new lift station is required. While most pumps can be run at higher speeds to get additional head, they become increasingly noisy and eventually run out of practical mechanical capacity.

To meet growing demands, a series of pumps were developed called the Ultra V. They can operate at higher speeds without the associated noise, maintaining many of the traditional features that users have come to expect from their high-performance pumps. At the same time, this new line of high-performance pumps have a broadened performance envelope, allowing for the surprises mentioned earlier in this article.

In conjunction with this new pump series, a more convenient method of staging two pumps was developed for applications that are beyond the capability of a single pump. Previously, conventional staging consisted of setting two pumps side by side and running plumbing from the discharge of the first stage into the suction of the second stage. While some friction loss does exist when pumping from stage one to stage two, the real problem revolves around the necessary floor space required, and the challenges involved in installing piping. To solve these problems, an innovative transition casting is now available that allows for a straight centrifugal pump (the UltraMate) to be mounted directly on top of the Ultra V's lower module self-priming pump discharge.

With the exception of the transition piece and the volute scroll, all major components are common to both pump units. However, since plumbing convenience is also a major consideration, the UltraMate volute is designed to rotate to different positions, thereby accommodating a variety of discharge header configurations where multiple pairs are going to a common header. When one pump will satisfy the system for a reasonable length of time, a section of pipe can be plumbed into the system in place of the second stage pump until that additional investment must be made. With this alternative staged configuration, end-users can have two-and-one-half to three times the head that has previously been available with conventional single-stage pumps. Also, the broader operating range allows for more "wiggle room" when actual performance does not meet the design.

Regardless of the configuration or pump innovation being enlisted for any given challenge, it's important to stress the attention to detail that must be exercised in designing a total pump system. Keep in mind that friction is the most expensive element in a design. It not only requires larger motors and controls, but it also robs the user of electrical power each and every time it runs. When developing for the future, careful projections and planning will pay dividends in the end.

Dave Meister, P.E., is director of engineering at the Gorman-Rupp Co., a manufacturer of pumps and pumping systems for the municipal, water, wastewater, sewage, industrial, construction, petroleum, fire, and OEM markets. The company's pumps include self-priming centrifugal, centrifugal, submersible, priming assist, rotary gear, and air-driven diaphragm pumps. It also manufactures packaged lift stations and booster stations, which include pumps, motors, controls, piping, accessories, and enclosures. More information is available at www.grpumps.com.