In some ways, pumps are very simple devices, which explains their regular and repeated use throughout history. In other ways, though, pumps can be extraordinarily complex. Affinity laws. Hydraulic gradients. Suction specific speed. Air entrainment. Cavitation. Those are only a few, randomly selected elements that you need to consider when selecting a pump and designing a system. And another element that you’ll need to think about is the dynamic viscosity and specific gravity — particularly when installing a centrifugal pump.
In this post, we will discuss the particulars of specific gravity vs. viscosity, how they impact pump performance, the practical limits of which you should remain aware, and various applications to the pump curve.
What Is Specific Gravity?
Before we begin our discussion, we need to take the time to define specific gravity. Defined simply, specific gravity is the comparison of a certain substance (e.g., a gas, a liquid) to a particular related substance. When measuring gases, the substance used is room-temperature air. ThoughtCo.com explains that “for fluids, the reference substance is usually the [sic] water, with a density of 1.00 x 103 kg/m3 at 4 degrees Celsius (water’s densest temperature), used to determine whether or not the fluid will sink or float in water. … [T]his is usually assumed to be the reference substance when working with liquids.”
When it comes to the mathematical side of things, specific gravity is easy to calculate. It’s the ratio of the weight of the liquid of interest (Wi) and the weight of water, which is the reference liquid (Wr). Written out, it looks like this:
SG = Wi / Wr
Pump curves are based on the specific gravity of water, which is 1 by definition. An increased specific gravity (i.e., above 1) will cause a corresponding increase in pressure throughout the system, while a decrease in specific gravity (i.e., below 1) will lead to the opposite.
Specific Gravity and Viscosity Differences
The first thing we will do is define specific gravity and viscosity. While both concepts share some similarities, their differences can trip up end users and could potentially lead to catastrophic results.
The simplest way to think about specific gravity is to start with the idea of density. Every kind of matter has mass that occupies a particular amount of volume, including liquids. But rather than explicitly listing the specific units associated with a particular density (e.g., grams per cubic centimeter, pounds per cubic inch), specific gravity allows us to simplify by dividing by the specific gravity of water, which is the density of water at 4 degrees Celsius. By definition, the specific gravity of water is 1.0. The specific gravity formula for other fluids is their density (mass divided by volume) divided by the specific gravity of water. To oversimply further, specific gravity is how heavy the liquid is compared to water. Viscosity is how sticky the liquid is. 1 gallon of water weighs 8.34 pounds, while 1 gallon of a liquid with a 2.0 specific gravity will be 16.68 pounds.
As Pumps & Systems notes, “Specific gravity is important when sizing a centrifugal pump because it is indicative of the weight of the fluid, and its weight will have a direct effect on the amount of work performed by the pump. One of the beauties of direct-coupled centrifugal pumps is that the head (in feet) and flow they produce have nothing to do with the weight of the liquid. It is all about the velocity that is added by the impeller.” In other words, liquids with different specific gravities will require different motors, but the flow and head (i.e., the maximum volume and height to which the pump can move a liquid) should remain essentially the same.
With magnetic-drive centrifugal pumps however, specific gravity is a critical part of selecting the right pump and features. Fluids with different specific gravities offer different resistances to both the motor driving the pump and the impeller spinning inside the fluid. The higher the specific gravity, the more resistance the fluid poses against the motor and spinning impeller. This is because a magnetic-drive centrifugal pump, unlike a direct-coupled centrifugal pump, uses a magnetic field between a drive magnet outside of the pump turning a magnetic impeller inside the pump. Unlike a direct-coupled centrifugal pump, whose impeller is physically fastened to the end of the drive motor shaft, there is no direct mechanical connection between the two components. For magnetic-drive centrifugals, that means the impeller must be sized correctly to ensure the magnetic coupling strength isn’t overcome by the resistance offered by the fluid.
Take the March TE-7P-MD as an example. When pumping fluids with a specific gravity of 1.0 (like water), the pump is capable of heads up to 60’ or flows up to 51 gallons per minute while using its full-sized, 3.750” diameter impeller. However, against a fluid with a specific gravity of 1.9, that same pump with its full-sized impeller will “magnetically decouple” – that is, the interlocking magnetic fields of the drive magnet and impeller magnet will get out of sync, resulting in the impeller not spinning at all while the motor spins full-speed. However, when we make a 0.625” reduction to the diameter of the impeller (3.125”) to accommodate the 1.9 specific gravity of the fluid, the pump is now capable of pumping the liquid again. Naturally though, there is a trade-off between impeller diameter and pump performance. As a result of this impeller diameter reduction, the TE-7P-MD with a 3.125” diameter impeller is only capable of heads up to 31 feet or flows to 37 gallons per minute. Thus, specific gravity is critical to sizing magnetic-drive centrifugal pumps. A March magnetic drive pump can only handle up to 300 SSU’s or 60CPS if the smallest impeller diameter is used.
Though we tend to associate viscosity with specific gravity, it is an entirely separate characteristic. One way to think about viscosity is to compare it with friction. However, viscosity is focused on the internal friction of a liquid rather than the friction produced when it meets another kind of material. Writing for Pumps & Systems, Jim Elsey defines viscosity as “a measure of a fluid’s resistance to flow at a given temperature. You can also think of it as fluid friction. A more technical definition would explain viscosity as a force required to move a liquid plane (think plate) of some unit area, over some distance above another plane of equal area in a defined time period. In training classes, I simply define viscosity as a fluid’s resistance to pour but, more importantly, a resistance to be pumped.”
As you can see, a fluid’s viscosity will have a dramatic impact on how a centrifugal pump functions. In the next section, we will provide some viscosity examples and describe how they may react with a pump.
How Viscosity Affects Centrifugal Pumps
When discussing viscosity and centrifugal pumps, understand that a liquid with a specific gravity that’s close or equal to that of water won’t significantly change the head or flow. However, if the liquid has a viscosity that varies significantly from the viscosity of water, you will see head and flow take a major hit. (Remember that viscosity doesn’t have any necessary connection with a liquid’s specific gravity, and some liquids with low specific gravities can still be very viscous.) Why? Because high viscosities impact centrifugal pumps at every area of operation. For instance, kinematic viscosity refers to a liquid’s inherent resistance to flowing, and pumping highly viscous materials successfully also requires a dramatic increase of break horsepower (i.e., a measure of an engine’s power absent any friction losses).
Selecting the Right Pump for Viscosity
Part of selecting the right pump for a viscous material lies in understanding that not all liquids behave in the same way. In fact, the viscosity of many liquids can behave differently depending on whether or not energy is applied to them in certain amounts or ways. Consider the following viscosity types and viscosity examples:
- Newtonian Fluids. These fluids decrease in viscosity as temperature increases. Examples include motor oil, alcohol, and glycerin.
- Thixotropic Fluids. When energy is added over time, thixotropic fluids exhibit decreased viscosity. Often this energy transfer involves shaking or agitation. Thixotropic substances include certain kinds of paint, mayonnaise, and ketchup. (Think of smacking the bottom of a ketchup bottle to start the savory and salty substance flowing.)
- Dialant Fluids. These function in the exact opposite manner from thixotropic fluids, becoming thicker when energy gets added (e.g., candy compounds, pseudoplastics).
- Rheopectic Fluids. Rheopectic fluids initially behave like dialant fluids, but their rate of viscosity continues to increase as energy is continually applied. Gypsum paste, lubricants, synovial fluid, cream, and some inks are rheopectic.
Once you know the viscosity characteristics of the liquid you plan to pump, you can apply several steps to determine the ideal pump for you. They involve:
- Calculating the required flow rate and total dynamic head at the operating temperature
- Apply an appropriate correction chart (see below)
- Apply the correction factors
- Use these adjusted values in conjunction with the manufacturer’s supplied water performance curves
Maximum Viscosity Levels for Centrifugal Pumps
Though centrifugal pumps are the most commonly used type of pump, common wisdom doesn’t recommend using them when a fluid’s viscosity exceeds 300 centistrokes. (A centistroke is a standard engineering unit of viscosity, one that compares the fluid’s absolute viscosity to its specific gravity.) This 300 centistokes limit is particularly applicable to magnetic-drive centrifugals. Some experts say that the effective range of viscosities for direct-coupled centrifugal pumps may range from 1,400 to 3,300 centistrokes.
When trying to determine the attributes of your liquid, it helps to take different measures of viscosity. A few useful devices for taking such measurements include:
- Viscosity cups
- Glass capillary viscometers
- Tuning fork vibration viscometers
- Rotational viscometers
How specific gravity affects centrifugal pumps and, by extension, pump curve
Pump manufacturers publish so-called pump curves, detailed charts that provide information about a pump’s flow, head, and power usage. However, that curve only describes the flow, head, and power usage when pumping water. So how does one use the manufacturer’s published performance curves to understand how their pump will handle more viscous fluids?
The answer is to calculate correction factors for capacity, head, and hydraulic efficiency. When applied, these will shift the manufacturer-provided curves to show the pump’s effective capacity with the viscous liquid. PDH Online has a detailed article that offers calculations and examples about how to determine the various correction factors.
If you need a pump that can function well under a variety of conditions, contact us here at March Pumps. We’ve been designing pumps since 1955, and we have the expertise and dedication to find one that works for you.