There are a lot of pump selection criteria that go into maximizing centrifugal pump efficiency and ensuring that you have the proper setup for your site. The size of the pump itself. The sort of liquid you need to move. That liquid’s viscosity, acidity, and percentage of solids. The impeller dimensions and its type of connection to the pump. How far the material needs to move. Indeed, these are all important factors. But there’s something else you can’t forget: the piping itself.
Indeed, pipe can prove just as important as a pump itself — if not more so. Elements as seemingly innocuous as the angle of the pipe and even its diameter can mean the difference between the successful movement of liquid and an ignominious failure. This article will discuss pumps requirements for various kinds of applications, including how pipe length and diameter can come into play.
How to Determine the Pressure Requirements of a Pump
At their heart, pumps and their associated systems are easy to understand. According to PumpFundamentals.com, “A pump system consists of a pump, usually some sort of tank for storing or supplying liquid, and pipes or tubes to transfer the liquid from one place to another. The start of the system is at the free surface of the suction tank and the end is at the outlet of a pipe or the free surface of a discharge or storage tank.” Yet that seeming simplicity can quickly turn complex in real-life applications. Just consider what goes into determining the pump discharge pressure for a specific system.
Industry periodical National Driller explains that “there are three factors that go into the determination of the pressure required from a pump in a residential water system.” Those are lift pressure (“pressure required to get the water from down in the well to the pressure tank”), household pressure (“pressure required in the house to properly feed the fixtures”), and friction loss (“pressure required to overcome the friction in the pipes and fittings”). Again, this sounds relatively simple. Yet installers must consider multiple factors when making these calculations.
Let’s start with lift pressure. As you may have guessed, the positioning of the pump relative to its end destination will significantly impact the amount of energy required to successfully move a liquid. We all know that moving an object from a higher elevation to a lower one allows us to transform gravity into kinetic energy, requiring less overall energy. However, when users need to pump liquids from a lower elevation to a higher one, the system will have to supply additional energy — and sometimes it’s substantial.
In order to determine the required lift pressure, you must first determine the following:
- Static Head (a height measure that’s taken by determining the distance between the centerline of a pump’s impeller and the highest point in the discharge line)
- Friction Loss (the amount of resistance caused by the liquid coming into contact with the pipe’s internal surface area, which is rendered as pounds per square inch or PSI)
- Total Dynamic Head (the sum of the static head and the friction loss)
We’ll go into more detail about the calculations themselves, but for the moment, it’s enough to understand that total dynamic head only provides a measurement at a single particular point. However, that point can be extrapolated into a linear relationship between static head in feet / total meterage and gallons per minute (GPM) / liters per minute (LPM). This is called a pump performance curve, and we’ve written extensively about it here. Basically, it charts a pump’s performance range, and manufacturers provide them as part of a pump’s documentation.
With all of that in mind, let’s consider friction loss and how it interacts with a pipe’s diameter and length.
Pipe Diameters for Different Pump Systems
Friction loss rises or falls based on a number of factors. For instance, let’s say that you have three sections of pipe that all share the same diameter, but they all have different lengths. As you might expect, the required PSI rises as the length of the pipe grows. That’s because the friction increases as the liquid comes into contact with more surface area over time.
That makes sense to a lot of clients. However, sometimes the rules that govern pipe diameter surprise them. Many seem to think that a smaller pipe requires less energy when it comes to moving a fluid. The opposite, though, is true. Pipes with larger diameters lead to lower PSIs, whereas smaller pipes generate far greater PSIs.
Let’s illustrate this with an example. Suppose you have two different pipe systems, both seeking to move 15 GPM through 200 feet of pipe. One system uses 1.5” PVC and requires a PSI of 2.2 to achieve its goals. But the second system that employs a pipe with a half-inch diameter? Its PSI would be a whopping 610.
It’s not hard to see why this is the case once you reconsider the idea of friction loss. In a pipe with a smaller diameter, more of the liquid comes into contact with the pipe’s interior surface area, slowing it down. That requires more output from the pump to make up the difference. In a larger-bore pipe, less of the fluid rubs up against the pipe and thus drops in velocity, requiring less energy.
Of course, you can alter the pipe diameter flow rate, and trying to calculate pipe size from flow rate always makes sense when attempting to construct the most efficient pump system for your end use. In fact, there are several rough rules of thumb that you ought to consider when determining pipe diameter, length, and arrangement. These include:
On Suction Side:
- Keeping the suction side of your piping as short as you can. Additionally, you should have straight pipe going into the inlet, preferably with a length of ten times the inlet size of the pump. For example, a 1-inch inlet should have 10 inches of straight pipe entering the inlet. This will prevent turbulence.
- Minimizing elbows close to the pump’s inlet. This also aids in maintaining a steady flow into your pump.
- Ensuring that your pump’s casing doesn’t have to support your piping. Pumps that have extra strain placed onto their casings are pumps with a shortened operational life.
On Discharge Side:
- Keeping the same pipe size or larger as the pump outlet. If larger pipe is used, start using the larger pipe a distance of ten times away equal to the size of the outlet. For example, if the pump has a 1-inch outlet, start using the larger pipe ten inches away from the outlet of the pump. This will provide the pump with backpressure, which will help the pump to start up.
- Minimize Elbows.
If you find yourself struggling to construct a workable, efficient pump system, reach out to us here at March Pumps by calling (847) 725-0580. Not only are we the originator of the centrifugal sealless magnetic drive pump, we have extensive experience with creating pump systems for numerous types of industrial applications.