If you look up the definition for the word “pump” in Merriam-Webster, you could be forgiven for thinking that pumps are simple machines. According to the venerable dictionary, it’s “a device that raises, transfers, delivers, or compresses fluids.” And while that definition is true, it fails to capture the complexity of the devices that have been used since at least 200 B.C. From allowable pipe stress and the Reynolds number to energy gradients and specific speed, pumps can get very complicated very quickly. One of the areas that often confuses end users is liquid velocity.
In this post, we will explain why liquid velocity is so important for centrifugal pump systems, the different kinds of flow, how pipe diameter can impact your system, and how one calculates pump discharge velocity.
Like the overall function of a pump, understanding fluid velocity appears easy at first glance. It’s essentially how many feet or meters per second that a liquid moves throughout a pump system. However, that’s where the simplicity ends. PumpFundamentals.com explains, “Velocity can change throughout the system because of pipe reductions, partially closed valves or other. But one thing that does not change is the flow rate. This is constant because of the law of conservation of mass, what goes in must come out. This is why the first thing that must be established for a new system is the required flow rate.”
Flow rate can be calculated a number of different ways, such as mass flow rate (a measure of how much mass passes through a pump system over a particular period) or volumetric flow rate (a measure of how much volume is processed). Both of these metrics, though, have two things in common: Their final answers are averages, and both of their results can be impacted by variable velocities. We’ll explain how in the next section.
Any fluid can flow in one of two ways. The first type of flow is laminar flow. Imagine the fluid that you’d like to pump as being comprised by many thin layers. Up until a certain velocity, those layers all flow smoothly in the same direction, maintaining a more-or-less parallel trajectory and not mixing vertically. However, once the liquid reaches a certain velocity (called critical velocity), it begins to behave differently: It starts to become turbulent.
This is called (appropriately enough) turbulent flow. Call to mind those thin layers we mentioned in the above paragraph. But instead of all flowing in the same direction, they split up, some circling up, others folding down, and still others folding back in the opposite direction. The point at which a liquid reaches critical velocity depends on its viscosity, and a turbulently flowing liquid can lead to serious problems for certain pumps, which we will discuss in more detail below.
If keeping a liquid moving in a laminar flow throughout a system matters for a particular pump, then it makes sense to determine how pipe diameter impacts flow. Fluid velocity can be calculated as follows:
Velocity = 1.273 * (Volume Flow / Pipe Inside Diameter)
You don’t need a degree in mathematics to understand that when the diameter of the inside of a pipe decreases, the liquid’s velocity will increase. Additionally, if you have pipes of many different sizes, your fluid’s velocity with likewise vary throughout the system — and that can lead to problems.
Why does variable velocity matter in a centrifugal pump? Well, centrifugal pumps work by transforming kinetic energy into fluid velocity by means on an impeller. At its most basic, an impeller is a rotating implement connected to a shaft — or, in the case of our pumps, spun by use of magnets — that uses an outward-pushing movement to move fluid through the system.
When pressures start to vary throughout a centrifugal-pump system, its efficiency starts to decline. Fluid flow becomes turbulent throughout various sections, and more of the fluid crashes up against the piping. This leads to something called head loss, which is a measure of total pressure loss in the system. Additionally, because the impeller is constantly rotating at a fixed rate, it continues to operate even when fluid doesn’t steadily enter the chamber. This causes decreased efficiency, increased wear, increased pressure throughout the system, and increased stress on components throughout the system.
To ensure the longest life of the pump, pipe and fittings, the suction velocity should not exceed 6.5 feet per second. Similarly, the discharge velocity should not exceed 15 feet per second.
The exact pump flow velocity that you should use for your system will depend on the characteristics of the fluid you need to pump. The first thing that you should consider is the fluid’s viscosity and the maximum velocity at which you can pump it before it begins to become turbulent. The second element involves sediment. If you are pumping a liquid that contains significant amounts of sediment, you might want to pump it at a slightly higher velocity in order to avoid sediment accumulation on your pipes.
So-called official guidelines for laminar velocities for commonly pumped industrial fluids may vary slightly from source to source. Still here are some general rules of thumb (in meters per second) that you can use for different substances (all of which are liquids unless otherwise stated):
As we’ve discussed above, a centrifugal pump’s impeller continues to spin even when turbulent flow occurs, which leads to increased pressure and decreased efficiency throughout the system. This manifests itself in an increased discharge pressure and a decreased flow. You can easily calculate discharge pressure if you divide the volumetric flow rate by the cross-sectional area of your pipe.
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