Come Prepared to Rent the Right Pump

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.”

Pumping Fundamentals

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.”

Following are short explanations of some of the scientific concepts involved in any pumping job:

Flow: The amount of liquid (usually measured in gallons per minute) dictates the size of the pump and hose. Flow is the horizontal axis on a pump performance chart.

Static head: This refers to the vertical elevation difference from the water surface to the pump suction and from the pump discharge to the end of the hose or pipe. The more head, or elevation difference, the stronger the pump required to overcome gravity resistance when moving water.

Distance: This is the space between the pump and the discharge point. This contributes to the amount of friction resistance encountered when water travels through the hose or pipe. Static head (or gravity resistance) and distance (or friction resistance) are combined to determine the total amount of resistance the pump will see during the application. This is referred to as total dynamic head and is the vertical axis on a pump performance curve.

Each pump is rated as capable of achieving a certain amount of flow and overcoming a certain amount of resistance. Ideally, the pump system will be designed so that the pump operates in the middle area of flow and head, known as the “sweet spot” of the curve.

The Science Behind Pumping Fluid

While a bit academic, some of the principles governing the behavior of water can help in understanding a pump’s capabilities.

To start with, atmospheric pressure on Earth at sea level is 14.7 psi (1 bar, or 1 barometric unit). The pressure becomes lower as altitude increases and higher in areas lower than sea level. With (distilled) water weighing 8.33 lbs. per gallon, atmospheric pressure will force that water to rise 33.9 ft. in a cylinder at sea level if all the air in the cylinder is evacuated. Likewise, mercury (Hg) weighs 112 lbs. per gallon and atmospheric pressure will force it to rise to 29.9 in. in a cylinder at sea level if all the air is removed.

Need a translation? Pumps often have an Hg rating and what this means, for example, is 1-in. Hg equals the ability to move 1.13 ft. of water (basic rule of thumb is 1 in. = 1 ft.). Therefore, a 20-in. Hg rating means the pump will lift approximately 20 ft. of water.

Pumps are designed to do two things: move liquid in and move it out. This is achieved by creating a difference in pressure between the suction and discharge sides of the pump. The pump must create low pressure on the suction side of the system. The combination of low pressure in the pump and atmospheric pressure results in positive suction head. Conversely, the pump must create high pressure on the discharge side of the system. This results in water being thrown out of the pump in the case of centrifugal models, or being pushed out in the case of positive displacement pumps.

Checklist for Renting a Pump

As your pumping application and pumping needs become more complex, it pays to take advantage of available expertise in this area. Some rental centers will have individuals specifically trained in hydraulics and pump technology who can guide you to the appropriate dewatering solution.

Following is a checklist to go over before renting a pump for a basic dewatering job: