When to Use a Positive Displacement Pump Instead of a Centrifugal Pump

When choosing between a positive displacement pump and a centrifugal pump, one of the primary factors to consider is the viscosity of the fluid being pumped. Viscosity refers to a fluid’s thickness or resistance to flow. Fluids with higher viscosities require pumps that can provide a consistent flow rate even when facing resistance.

Positive displacement pumps are specifically designed to handle fluids across a wide range of viscosities, from thin liquids like water to highly viscous substances such as oils or syrups. Unlike centrifugal pumps, which operate best with low-viscosity fluids and can struggle or become inefficient with thicker fluids, positive displacement pumps maintain an efficient and consistent flow rate regardless of viscosity.

Here is an outline of how different viscosities correspond to pump selection:

Centrifugal pumps typically exhibit a decrease in flow rate as the viscosity increases beyond 100 cP, leading to efficiency losses and potential damage due to increased heat generation and mechanical strain. On the other hand, positive displacement pumps are less affected by changes in viscosity, thanks to their operating mechanism. They trap a fixed amount of fluid and then force (displace) that trapped volume into the discharge pipe, which makes them highly effective for thick and viscous fluids.

• Rotary Lobe Pump: Ideal for high viscosity fluids, able to handle large particles robustly.
• Gear Pump: Works well with medium to high-viscosity fluids, offering a smooth, pulse-free flow ideal for precise applications.
• Screw Pump: Used for high-viscosity applications, providing steady flow and capable of handling high pressures.
• Peristaltic Pump: Extremely versatile with a gentle pumping action, good for shear-sensitive and high-viscosity substances.

Choosing the right type of positive displacement pump depends largely on the specific nature of the fluid — its viscosity, its potential abrasiveness, and how sensitive it is to shear forces. The correct pump selection ensures not only efficient operation but also minimal wear and tear on pump components, translating to lower maintenance costs and longer service life.

Handling high differential pressure scenarios

In applications where there are high differential pressures, positive displacement pumps often outperform centrifugal pumps. Differential pressure, or the pressure difference between the pump inlet and outlet, can heavily influence the operational efficiency and safety of the pump system. Positive displacement pumps have the inherent ability to sustain high differential pressures, making them particularly well-suited for such conditions.

Positive displacement pumps accomplish this by mechanically trapping a fixed volume of fluid and then displacing that volume towards the outlet. This mechanism allows these pumps to generate significant pressure, irrespective of the flow rate, which is ideal under high pressure differentials. Their robust construction and relatively low speed also enhance their ability to operate under substantial pressure without risking mechanical failure or excessive wear.

Centrifugal pumps, conversely, might falter under high differential pressure conditions due to their reliance on fluid momentum and speed to create pressure. At very high pressures, centrifugal pumps require specially designed components and materials, and even then, they might not reach the efficiency offered by positive displacement pumps. Additionally, with increasing pressure, the risk of cavitation—where air bubbles form and collapse in the pump—also increases in centrifugal pumps, potentially leading to damage and operational issues.

Here’s a breakdown of common scenarios where high differential pressure is a key consideration and the appropriate pump choice:

• Hydraulic Systems: These systems often operate under pressures that can exceed 1000 psi, where gear or piston pumps are typically employed.
• Injection Tasks: Whether injecting chemicals in a process stream or additives in fuel systems, precise control and the ability to handle high pressure are critical, tasks well-suited for diaphragm or piston pumps.
• High-rise Building Water Supply: Water supply systems in skyscrapers require pumps capable of lifting water to significant heights, demanding high differential pressures that are well-handled by multistage positive displacement pumps.

Given the importance of choosing the correct pump type for applications involving high differential pressures, it is essential to evaluate the pump’s capability to handle such pressures without compromising on operational life or efficiency. It’s advisable to consult detailed product specifications, and performance curves, and if possible, engage with manufacturers or experts to ensure the pump selected will perform adequately under the expected conditions.

Considerations for accurate flow rate control

When dealing with applications that require precise flow rate control, certain factors must be considered to ensure that the selected pump meets the operational demands effectively. Flow rate control is crucial in many industries such as pharmaceuticals, food processing, and chemical manufacturing, where even minor discrepancies in flow can lead to product inconsistency or non-compliance with regulations.

Positive displacement pumps are often preferred over centrifugal pumps in scenarios needing accurate flow control. This preference stems from their inherent design, which allows for a constant flow regardless of changes in pressure. Since positive displacement pumps displace a fixed amount of fluid per cycle, they provide a steady and predictable flow, making them ideal for applications requiring metering, dosing, or precise transfer of fluids.

Here is a comparison of how positive displacement and centrifugal pumps handle flow rate control:

• Positive Displacement Pumps: Deliver a fixed amount of fluid for each rotation, offering easy control over the flow rate through adjustments in the motor speed or by using a flow controller.
• Centrifugal Pumps: Flow rate depends on the total dynamic head and the system’s changing resistance, which can lead to variable flow rates and make precise control more complex.

For applications requiring highly consistent flow rates, especially under varying pressure conditions, the precise control offered by positive displacement pumps is more suitable. Additionally, these pumps can handle multi-viscosity fluid applications with minimal impact on flow rate, unlike centrifugal pumps, which can experience a decrease in flow rate with increased viscosity.

To further illustrate the advantages of using positive displacement pumps for precise flow control, consider the following application-centric views:

1. Pharmaceutical Production: Involves strict regulations requiring precise dosing of ingredients. Diaphragm and peristaltic pumps are favored for their ability to ensure accurate flow rates and prevent contamination.
2. Chemical Dosing: Essential for water treatment processes where chemical volumes must be meticulously controlled to ensure proper treatment. Piston or gear pumps are typically used due to their precision in handling small, exact fluid volumes.
3. Food and Beverage: Requires consistent replication of recipes, where even minor deviations can alter the product’s flavor or texture. Rotary lobe or peristaltic pumps are often selected for their reliability in delivering consistent flow rates.

Controlling the flow rate in a positive displacement pump can be achieved through various methods such as adjusting the motor speed, utilizing a variable frequency drive (VFD), or incorporating advanced control systems with feedback loops that monitor and adjust the flow rate in real-time to match the set parameters.

Ultimately, when precise flow control is a critical component of the pumping application, opting for a positive displacement pump offers significant advantages in terms of accuracy, reliability, and efficiency, making it a superior choice over centrifugal pumps in such scenarios. To ensure optimal performance, it is crucial to select a pump with the appropriate specifications and to consider employing advanced control systems for enhanced accuracy in flow management.