Choosing a Slurry Pump - Features

Nnamdi Nwaokocha offers practical advice on pump selection

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Pumps constitute a critical component of the process industry. It is essential to transport materials within a process plant, and often this involves mechanical assistance to move fluids from areas of high pressure to low, adhering to thermodynamic principles. With a plethora of pump types available, selecting the appropriate one, especially for slurries, can pose challenges.

In this article, we will explore several factors to consider when defining a slurry and determining the most suitable pump for transporting it within a plant. While this overview does not encompass all aspects of slurry handling, it aims to offer valuable insights and a solid foundation for consideration.

In a spin: Slurry centrifugal pump with rubber lined casing and rubber impeller

Summary

The pumping of slurries can frequently lead to blockages or equipment malfunction. Designers must evaluate all influencing factors, including client needs and existing site configurations, to create a system and select a pump that is durable enough to minimize blockages while facilitating easy maintenance for operators and providing a safe working environment.

Understanding Slurry

A slurry is defined as a mixture of liquid and solid particles. The liquid, commonly referred to as the carrier fluid, is mostly water but can also be an acid solution (e.g., nitric acid) or a hydrocarbon (e.g., diesel).

This article will not explore the production of slurries or maintaining solid suspension but will focus on classifying slurries into two primary categories: settling and non-settling slurries. Non-settling slurries comprise fine particles that stay suspended without mixing energy, while settling slurries consist of solids that sink when mixing energy ceases. Understanding the type of slurry is crucial for designers; non-settling slurries flow under laminar conditions, whereas settling slurries require turbulent flow, particularly in horizontal sections.

A common guideline from Sinnot and Towler's Chemical Engineering Design indicates that solids with particle sizes smaller than 200 microns (0.2 mm) are typically non-settling, while larger particles correspond to settling slurries.

Before choosing the right pump, ascertain the pressure drop requirements based on the system characteristics, including:

The following equations are useful for determining the slurry's density:

For settling slurries, velocity within the piping is vital.

Perry's Chemical Engineers' Handbook

Figure 1: The relationship between pressure drop and slurry velocity compared to a pure liquid in horizontal pipework

Figure 1 from Perry's Chemical Engineers' Handbook illustrates the significant relationship between pressure drop and slurry velocity in comparison to pure liquids in horizontal pipes. The critical takeaway is that the horizontal pipe velocity must exceed Vm2 (minimum transport velocity), where solids remain fully suspended, determined using the Durand equation.

where:

After calculating the minimum transport velocity, it's common to apply a safety factor. Caution is essential, as overly high velocities significantly raise the needed pressure drop and the work required from the pump. For vertical flows, a good starting velocity is double the solids' settling velocity. Maintaining speeds within 1-3 m/s is a useful guideline.

To determine the pressure drop for settling slurries, use the calculated velocity with the slurry's density and the carrier fluid's viscosity within established pressure calculation frameworks, augmented by a correction factor. Perry's Chemical Engineers' Handbook suggests applying a 25% correction factor.

This advice operates under the assumption that the solid particles are denser than the carrier fluid. At certain concentrations, particle interaction may influence the slurry's viscosity, a topic elaborated on in various reference materials.

Pump Types

Various pump types exist, including rotary and positive-displacement pumps, to manage slurry transport effectively. Here are crucial elements to consider when determining pump type. Always check with specialized pump suppliers before finalizing any decision.

Centrifugal Pumps

Centrifugal pumps are the most commonly used pump type. Important specifications when selecting these pumps include:

• Impeller Type - Recessed impeller types are conducive for minimizing particle contact and wear. Open impellers are easier to maintain, while closed types offer excellent efficiency, but complicate cleaning. Ensure adequate wear allowance in thickness, and consider the impact of necessary impeller speed.
• Casing Type - Options include metal casings that may be rubber-lined for extra protection or to act as sacrificial wear parts. Split casings can be evaluated but may incur higher costs. Thickness should allow for suitable wear tolerance.
• Clearances - Due to solids presence, slurry centrifugal pumps require larger clearances than pure liquid pumps to facilitate solid passage and reduce pump velocity, thus minimizing wear.

These highlights serve as a primer for selecting a centrifugal pump for slurry applications. Close collaboration with a pump vendor will help designers choose the best options for their systems and address concerns regarding shaft and seal impacts, ensuring cavitation issues are avoided.

Centrifugal pumps operate based on differential head and fluid properties.

Warman Slurry Pumping Handbook

Figure 3: Example operating and efficiency curves for a solid-water mixture

Often, pump vendors provide operating and efficiency curves based on water. Translating these figures for slurry pump selections requires careful consideration, as depicted in one example. This also factors in the driver efficiency to aid in motor confirmation.

Positive Displacement Pumps

Multiple types of positive displacement pumps are available for slurry applications, including air-operated diaphragm pumps, peristaltic, rotary lobe, progressive cavity pumps, and piston diaphragm pumps. Evaluating each type can be extensive; thus, this section summarizes only essential considerations for handling slurries.

Positive displacement pumps effectively manage fluids displaying pseudo-plastic behavior, overcoming initial flow resistance with lower operating speeds, which is gentler on solid particles. However, some positive displacement pumps may induce acceleration losses that require calculation adjustments.

Air-Driven Diaphragm Pumps

Air-driven diaphragm pumps are particularly effective for slurry handling. Nevertheless, abrasion and erosion present concerns, especially regarding the balls and seats in the check valve. Choosing suitable materials is essential; if neglected, worn balls may fail to seal correctly, impairing pump efficiency.

Considerations regarding slurry include check valve assembly materials, diaphragm composition, and maximum particle size clearances.

Peristaltic Pumps

Peristaltic pumps can serve as alternatives to air-driven diaphragm pumps due to the absence of complex components. Only the motor and tube need maintenance, making upkeep straightforward. A key advantage is their capacity to handle slurries with up to 80% w/w solids, although this should be verified with your pump provider. Maximum discharge pressure, dictated by tube properties, is a limiting factor.

Gear, Lobe, and ECP Pumps

These pump types move fluid through interstitial spaces, typically suited for slurries with softer solids. ECP pumps particularly excel with settling slurries, allowing for scooping the solids when flow resumes. Hypothetically tight clearances in these pumps necessitate caution with abrasive solids to prevent excessive wear.

Factors to consider include the specific type of slurry and solid characteristics.

Progressive Cavity Pumps

Predominantly utilized in wastewater and process industries, progressive cavity pumps are reliable for slurry handling. Coating rotors improves wear resistance while pumping slurries. It’s advisable to run the pump slower for highly abrasive solids, though slower operation risks solid suspension loss, prompting blockages.

Key considerations involve solid characteristics (size and abrasiveness), slurry classification, and seal configuration.

Conclusion

Choosing an appropriate pump for slurry applications is complex, with numerous factors to consider. The primary goal is to keep solids suspended while minimizing wear and blockage. This guide serves as a discussion starter, prompting users to gather extensive information on carrier fluids and solids while collaborating with the relevant pump vendor.

References

1. Sinnot, R and Towler, G, Chemical Engineering Design, Fifth Edition, Elsevier.

2. Processing of Solid-Liquid Suspension, ed Ayazi Shamlou, P, Chapter 11 by Shook, CA, Chapter 12 Etchells, AW, Butterworth-Heinemann.

3. Green, DW and Perry, RH, Perry's Chemical Engineers' Handbook, Chapter 6, 8th Edition, McGraw-Hill.

4. Warman Slurry Pumping Handbook, Warman International.

5. https://bit.ly/2Ud76ls

6. Coulson, JM, Richardson, JF, Backhurst, JR, Harker, JH, Coulson and Richardson's Chemical Engineering Volume 1 - Fluid Flow, Heat Transfer and Mass Transfer, 6th Edition, Elsevier.

7. Jones, GM, Pumping Station Design, revised 3rd edition, Elsevier.