Specific Gravity vs. Viscosity — How Do They Affect Centrifugal Pumps?

Several factors, including cavity, affinity laws, air entrainment, hydraulic gradients, specific gravity (SG), and viscosity, affect the performance of centrifugal pumps in industrial applications. Therefore, centrifugal pump operators must efficiently account for these factors to ensure the high performance and reliability of the equipment in their facilities. This article overviews viscosity and specific gravity and identifies some effects of these factors on the overall performance of industrial pumps.

What’s the Difference between Specific Gravity vs. Viscosity?

Specific gravity and viscosity are fluid properties that engineers leverage to evaluate fluid flow dynamics within the industrial equipment, such as pumps. The next couple of sections overview these properties, highlighting their fundamental differences.

Specific Gravity Explained

Specific gravity is the ratio of the density of a fluid to the density of a reference fluid. Water and air are the two primary reference fluids ideal for SG evaluation, depending on whether the fluid is in a liquid or gaseous state. SG is crucial to centrifugal pump sizing since it is equivalent to how much the fluid weighs when compared to water. Consequently, the SG of liquids directly influences the pump motor capacity. It is noteworthy that if the SG of a sample fluid is less than 1, the fluid is less dense than the reference fluid. Conversely, a sample fluid with an SG higher than 1 is usually heavier than the reference fluid.

Viscosity Explained

On the other hand, viscosity is the internal friction of a liquid when it comes in contact with another material. This fluid property is ideal for describing how sticky a fluid can be when pouring (i.e., fluid resistance to flow or, in this case, its resistance to pumping). For instance, when comparing water and motor oil, it is evident that the oil is more viscous than the water since it will take more time to flow from one point to another. The unit for this fluid property is newton-second per square meter (N·s/m2), pascal-second (Pa·s), or centipoise (cP).

The Effects of Specific Gravity and Viscosity on Pumps

Although fluid viscosity and SG directly affect pump performance and, by extension, determine the pump type, power, and other features, these properties exhibit different effects.

Effect of Specific Gravity on Pumps

Operators can opt for a magnetic drive or directly-coupled centrifugal pump, depending on the SG of a fluid. Since the magnetic drive centrifugal pump comprises a magnetic field-driven impeller that lacks physical fastening to the end of the drive motor shaft, operators must correctly size the impeller to generate sufficient drive for high SG fluids. If improperly sized, the pump may experience magnetic decoupling, resulting in down times in facilities. Similarly, operators require different motor sizes in direct-coupled centrifugal pumps to efficiently transfer fluids from one point to another, depending on their SG.

Effect of Viscosity on Pumps

Viscosity significantly impacts the head and flow of a pump. For instance, if the fluid’s viscosity is close to or equal to water’s, the pump will exhibit a similar head and flow when transferring water. However, the pump will demonstrate a lower efficiency when moving a more viscous fluid from one point to another. Consequently, operators must make several viscosity considerations to identify the ideal pump for their applications.

Newtonian, Thixotropic, Dialant, and Rheopectic fluids exhibit distinct behaviors when they receive different energy levels. For example, while Newtonian fluids, such as motor oil and glycerin, exhibit decreased viscosity with temperature increases, Thixotropic fluids, such as paint and ketchup, become more viscous with increased temperature. Thus, armed with the knowledge of the fluid type, operators can determine the ideal pump by:

• Evaluating the flow rate and total dynamic head requirements at the operating temperature
• Applying appropriate correction chart and factors
• Using the corrected values to identify the ideal pump in light of the manufacturer’s supplied water performance curves

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