Know Your Pumps

Due to the current influx of federal stimulus money, some contractors are finding themselves taking jobs outside of their normal expertise. In so doing, they occasionally need to rent equipment they are not very familiar with, and pumps are no exception.

Pumps are a common item in many rental inventories, and in a lot of cases, renting them to customers is a simple matter of determining the right type of pump and the correct size. But when your customer has a large or complex project, pump rentals get more complicated, which is why it's important to ensure your employees know and understand the science behind pumps and how to properly set them up for optimum effectiveness.

The start to any successful pump rental begins with asking your customer what they are trying to do. This will help determine the follow-up questions and, ultimately, the design of the pumping system.

Some questions a rental counter employee should ask initially include:

  • Where is the application/pumping taking place?
  • What are you pumping?
  • How much needs to be pumped?
  • How high does the liquid need to be pumped up/how much does the liquid need to be lifted/is the liquid in a large ditch (in other words, what is the lift pressure)?
  • How long will you be pumping?

    Small dewatering jobs can be handled by gasoline-powered wet-priming pumps with suction and discharge sizes of 2 to 3 inches in diameter. These pumps can run for several hours on a tank of gas and will move up to 250 gallons per minute. On the other hand, large dewatering or sewer bypass jobs are far more complex and can involve 24-hour pumping operation and flows up to 4,000 gallons per minute and more.

    Start with the basics

    To fully serve customers with large dewatering projects, you need to first understand the fundamentals of pumping, which can be broken down into flow, lift and distance.

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

    Lift - This is the vertical elevation difference from the water to the pump suction and from the pump discharge to the end of the hose or pipe. The more lift (or elevation difference), the stronger the pump required to overcome gravity resistance when moving water.

    Distance - This refers to the space between the pump and the discharge point. This determines the amount of friction resistance encountered when water travels through the hose or pipe.

    Lift, 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 (TDH) and is the vertical axis on a pump performance curve.

    Each pump is rated 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.

    Ask about the job

    To best serve customers with large pumping projects, rental personnel need to understand that all jobs are different. They must have a working knowledge of pumps, the different types of pumps and which is best for certain applications. They also need to know about various accessories and how to properly set the pump up on the jobsite.

    There might be contributing factors in the specific site that can lead to incorrect selections if not known. For example, a typical customer might ask for a diesel-driven pump capable of pumping 500 gpm. There are a multitude of pumps that will meet this requirement. If the customer is a little more specific and requests 500 gpm at 100' TDH, there is now enough information to size the pump, but there is still not nearly enough information to properly fit the application.

    If a typical end-suction centrifugal pump is rented by a rental company and the application is a sewage bypass, you would have a sewage spill occur, as a typical end-suction centrifugal pump is not meant to pump sewage. The proper pump needed would be a diesel-driven trash pump. If the rental company did catch that the customer needed a trash pump but then did not pay attention to the solids handling capacity, which for most bypasses is 3" or larger, then a sewage spill is likely to occur as some pumps of this capacity have smaller solids handling capacities.

    If all this information was gathered - 500 gpm at 100' TDH pumping sewage with 3" solids handling capability - but the suction lift was not given and it turned out to be 28', there is a still a problem. Under this condition, unless the rental employee is very familiar with Net Positive Suction Head (NPSH) both AVAILABLE (NPSHa) and REQUIRED (NPSHr) by the pump, very likely the wrong pump would have been selected as the NPSHr of the pump and suction lift are big factors in proper pump selection. Should the suction lift be any more than 28', then the rental center employee needs to know NOT to rent a diesel-driven pump and suggest a submersible pump, as suction lifts of greater than approximately 28' need to be submersible and not above-ground pumps, regardless of whether there is any type of priming device on the pump.

    Other items that must also be considered are the size of the suction and discharge lines, their pressure ratings, valves required and a host of other factors.

    Bottom line

    Highly critical projects are best tackled by specialty contractors who use all types of pumps in their everyday operations, but ultimately, your customer is reponsible for the job he or she does. When a complex pump rental is clearly outside your realm of experience, it might be in your best interst to refer the job to a specialized pump rental company.

    But in those cases where you have the equipment your customer needs and the situation is within the scope of your services, the best thing you can do to assist your customer is to know your stuff. Educating your staff on the science behind pump rentals will help them be the best solutions provider to your customers who need to move water.

    For a rental company, the process of renting pumps is more than just getting the customer out the door with products. With renting comes the responsibility for ensuring that the customer has the right pump for the job, that the pump is in top condition and that the customer understands how to operate the pump. ƒÞ

    Information provided by Godwin Pumps of America, The Gorman-Rupp Co., Griffin Pump & Equipment and Thompson Pump & Mfg. Co.

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