Pumps are a commonly rented item. In many cases, renting one is a simple matter of determining the right type of pump and the correct size. In other situations, however, it takes a deeper understanding of the science of pumping. Is it a small dewatering job at a construction site for a few hours or days, or is it a large sewer bypass requiring continuous pumping for weeks? The level of complexity dictates the type of pumping system required.
It’s important to come to a rental center armed with as much information about the particular dewatering project as possible. Thomas Aldridge, Jr., Griffin Dewatering, suggests starting with the basics: what is the application, how much water is being moved, where is it being moved to and where is it being moved from?
“Based on some of these pieces of information, a rental employee can guide the customer to the range of pumps that may suit their needs,” he says. “The application narrows the pump family, while the volume and pumping locations identify the performance range.”
The rental employee also needs to know the elevation from the water’s location to where it will be discharged, and how far away the discharge point is, says Pete Snow at Xylem. This will determine how much head needs to be overcome.
“Equipment rental centers need detailed, accurate information about flow, lift and pressure required by the application,” says Kirsten Petersen Stroud, Thompson Pump. “Without detailed information and a knowledgeable staff, a customer could end up with a pump that either cannot do the job or cannot do it efficiently.”
According to Mike Grant, Tsurumi America, the most commonly rented portable pumps are 2-in. electric (up to 82 gpm), 2-in. engine-driven centrifugal (up to 137 gpm) and 3-in. engine-driven trash (up to 360 gpm).
Small dewatering jobs can be handled by gasoline-powered wet priming pumps with suction and discharge sizes of 2 to 3 in. in diameter. These pumps can run for several hours on a tank of gas and will move up to 250 gpm. The system design usually includes 20 to 30 ft. of suction hose and 100 to 200 ft. of lay flat discharge hose. Large dewatering or sewer bypass jobs, on the other hand, can involve a 24-hour pumping operation and flows up to 4,000 gpm and more.
“Basically, the key factors for a pumping job are flow rate, head, suction lift and solids handling,” says Aldridge. “If any one of these is not properly identified as a requirement and used to select the pump, the equipment won’t meet the expectations.
“One of the most misunderstood factors is that the connection size determines how the pump will perform. In reality, not all 6-in. pumps are 6-in. pumps,” he continues. “By using the key factors mentioned and reviewing the pump performance curve, the renter can select the best equipment. The hardest part of pump selection is to determine the friction or resistance to the flow that can be caused by the piping or hose used. There are simplified charts that help to determine this. The critical thing is to be able to visualize the system and know the materials that are being used in the system.”
Snow agrees, noting the two primary factors in every application are gravity and friction. “Gravity depends on the vertical lift needed, and friction on the distance the water must travel and the pipe diameter,” he says. “More than specific formulas or calculations, one should consider the restraints of the job. The velocity in a pipe, and its friction resistance, will determine the amount of flow that can be reasonably achieved.”
A good guideline for centrifugal pumps is a maximum velocity of 12 ft. per second. From there, practical flows in a given diameter of hose can be estimated (see Table 1).
“A pump must create enough discharge head to overcome elevation at a jobsite,” Snow explains. “How well the pump performs is determined by the physical conditions of each application. Therefore, no two applications will be the same. A pump that worked great on one site might not be ideal for another.”