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Key Differences Between Positive And Non Positive Displacement Pump
Table of Contents
Definition
In fluid management, pumps are essential for moving liquids and gases, ensuring many industrial processes run efficiently. Pumps are mainly divided into two types based on how they operate: positive and non positive displacement pumps.
Positive displacement pumps move a fixed amount of fluid with every cycle. They work by creating a temporary space to trap the fluid and then pushing it into the discharge pipe, resulting in a pulsing flow. These pumps provide consistent flow regardless of pressure changes, making them ideal for high-viscosity fluids and precise flow control.
Non-positive displacement pumps, on the other hand, increase the fluid’s velocity and pressure by adding kinetic energy. These pumps use centrifugal or axial force to propel the fluid, maintaining a continuous flow rate. They are typically used for applications where a steady, smooth flow is more important than precise volume control, such as in water transfer and low-pressure systems.
Positive Displacement Pumps
A positive displacement pump works by taking in a set amount of fluid into a space and then making that space smaller to push the fluid out. This method keeps a steady flow rate, even when the system pressure changes. That is why they are great for high-pressure uses. Their design makes positive displacement pumps ideal for working with thick fluids.
The ability to provide a constant amount of fluid is why these pumps are widely used in hydraulic systems. In these systems, a set amount of hydraulic fluid is used to transfer power. This means a pump needs to give the same fluid amount no matter how the pressure changes. This feature shows how important they are in situations where precise fluid control is needed.
Non-Positive Displacement Pumps
A centrifugal pump is a type of pump that works quite differently from positive displacement pumps. It uses centrifugal force to move fluid. This force comes from the turning of the impeller, which sends the fluid outward. As this happens, a low-pressure area forms at the center, pulling in more fluid. When the fluid moves from the center of the impeller to its outer edge, its speed increases. This change in speed changes kinetic energy into pressure.
The rate of flow in centrifugal pumps isn’t steady like in positive displacement pumps. It can change based on things like discharge pressure and the size of the pipes. When flow is restricted or pressure is high, the efficiency of these pumps usually goes down. This makes them good for moving large amounts of low-viscosity fluids in low-pressure situations.
Operation Principle
The main difference between positive and non-positive displacement pumps is how they move liquids.
- Positive displacement pumps use a mechanism that is often either back-and-forth or rotary. They catch a set amount of fluid and push it into the discharge pipe. The pressure created depends on the pump’s size and the system’s resistance.
- Non-positive displacement pumps, on the other hand, work differently. They use fluid dynamics to give the fluid kinetic energy. This makes the fluid move faster and increases the pressure. However, the flow rate changes and is affected by the discharge pressure and system resistance.
Positive Displacement Pumps
In positive displacement pumps, the flow rate mainly does not depend on the system pressure. Instead, it is directly linked to the speed of the prime mover. This means if you increase the pump’s speed, more liquid is moved in a given time. One important feature of these pumps is their ability to create high pressure. This comes from how they work, pushing a fixed amount of liquid every time.
The highest pressure you can reach depends on the pump’s design and how much resistance is in the system. Remember, positive displacement pumps are not meant to work with a closed discharge valve. Running them this way can quickly build up pressure and may harm the pump or the system.
Non-Positive Displacement Pumps
Non-positive displacement pumps use centrifugal force to speed up the fluid. When the impeller spins, it gives speed to the fluid and pushes it toward the discharge pipe. The design of the impeller and the pump casing is very important. They help turn this speed into pressure energy. The flow of the pump can change if the discharge pressure changes.
If the discharge pipe is fully closed, a non-positive displacement pump will not create pressure, unlike a positive displacement pump. The impeller will keep spinning, making the fluid move around in the pump casing. This situation does not cause pressure to build up, but it can lead to higher frictional losses and heat. This can hurt the pump’s efficiency and lifespan.
Pump Types
Positive displacement pumps and non-positive displacement pumps include many types of pumps. Each type has its own features and uses. It is important to understand these differences. This knowledge helps in choosing the best pump for different industrial tasks.
Now, let’s look at some common types in each category:
Positive Displacement Pumps
Positive displacement pumps are essential in industries that require precise fluid handling under varying pressures and viscosities. These pumps operate by trapping a fixed amount of fluid and forcing it through the system, offering reliable and consistent performance.
- Gear Pumps: Gear pumps use interlocking gears to move fluid. Internal gear pumps are ideal for high-viscosity fluids, while external gear pumps are commonly used in hydraulic systems and chemical transfer.
- Diaphragm Pumps: These pumps use a flexible diaphragm to create suction and move fluid. They’re ideal for aggressive chemicals, slurries, and viscous fluids, and can be powered pneumatically, hydraulically, or electrically.
- Piston Pumps: Piston pumps use reciprocating pistons to move fluid. They are perfect for high-pressure applications like water injection and hydraulic systems, providing precise flow control.
- Screw Pumps: Screw pumps use rotating screws to move fluid smoothly and continuously, making them ideal for transferring viscous liquids and slurries, commonly used in oil, gas, and food processing.
- Vane Pumps: Vane pumps feature a rotor with sliding vanes to trap and move fluid. They provide consistent flow under pressure and are widely used in lubrication and hydraulic systems.
Non-Positive Displacement Pumps
Non-positive displacement pumps are used in situations where you need high flow rates but low pressures. Here are some common types:
- Centrifugal Pumps:These are the most common type of non-positive displacement pump. They use a spinning impeller to create centrifugal force, which speeds up the fluid, increasing its velocity and pressure. Their design is simple and cheap, which makes them good for moving a lot of fluid. However, their efficiency depends on things like how fast they spin and the thickness of the fluid.
- Propeller Pumps:These pumps are great for moving large amounts of fluid against low pressures. They have a propeller-like impeller to create mostly axial flow. Their straightforward design and strong build make them perfect for uses like irrigation and cleaning wastewater.
- Diffuser Pumps:These pumps use a diffuser around the impeller to change the mechanical energy from the spinning fluid into pressure. This design makes them more efficient and allows for higher pressures than standard centrifugal pumps. However, their complicated design and the risk of cavitation mean they are used in specific industrial applications only.
Application
The decision to use positive displacement pumps or non-positive displacement pumps depends mainly on the need of the application. Many things matter, like pressure needs, how thick the fluid is, how steady the flow rate should be, and the price.
Here are some common uses where each pump type works well:
Positive Displacement Pumps
Positive displacement pumps are the best choice for jobs that need high pressure, accurate flow, and the ability to move thick liquids. They are great for situations where it’s important to keep a steady flow, even when pressure changes. Here are some examples:
- Chemical Processing: Positive displacement pumps ensure precise and consistent fluid transfer for chemicals, solvents, and reactive substances in processing systems.
- Oil & Gas: These pumps handle high-viscosity oils, crude, and additives, maintaining reliable flow under high pressure in drilling and transportation.
- Pharmaceutical: Positive displacement pumps are used for transferring sensitive liquids, ensuring accurate dosing and contamination-free fluid movement in pharmaceutical production.
- Food Processing: These pumps transfer oils, syrups, and other food-grade liquids without altering product integrity, making them essential for consistent food production.
- Water & Wastewater Treatment: Used for chemical dosing, sludge transfer, and filtration, positive displacement pumps help maintain water quality and treatment efficiency.
- Industrial Applications: From hydraulic systems to lubrication, positive displacement pumps provide reliable fluid transfer in a wide range of industrial machinery and processes.
Non-Positive Displacement Pumps
Non-positive displacement pumps are great for moving large amounts of thin liquids at low pressures. They work best in situations where high flow rates are important. Here are some common uses:
- Irrigation:Centrifugal pumps are key in irrigation. They draw water from rivers and wells to supply crops. They can handle water with small debris and provide a high flow rate.
- Water Supply:Large centrifugal pumps are often used in city water supply systems. They move treated water to homes and businesses. They can boost water pressure in tall buildings too.
- HVAC Systems:In heating and cooling systems, centrifugal pumps move water or glycol solutions. Their ability to maintain low pressure while providing high flow rates helps keep temperatures steady in large buildings.
Efficiency and Maintenance
Understanding how well positive displacement pumps and non-positive displacement pumps work is important. This helps to keep them performing well for a long time. Each pump type has its own benefits. However, their efficiency can change based on things like the type of fluid, the pressure they operate under, and how well they are maintained.
Let’s take a closer look at how these two types compare in terms of efficiency and maintenance.
Positive Displacement Pumps
Positive displacement pumps are very efficient. They work well with thick fluids and under high pressure. The small spaces inside reduce slippage. This means that most of the mechanical energy goes into the fluid. Efficiency is highest at a certain pressure and drops when the pressure is lower.
A big benefit of positive displacement pumps is that they perform consistently over time when they are taken care of. But, because they have many moving parts, especially in the reciprocating type, they need regular maintenance. This means checking seals, valves, and bearings for damage. It’s also important to keep them lubricated to reduce friction.
Non-Positive Displacement Pumps
The efficiency of non-positive displacement pumps depends on their operating point. This point is where the flow rate meets the head pressure. They work best at what is called the best efficiency point (BEP), but their efficiency goes down when they move away from it.
Compared to positive displacement pumps, non-positive displacement pumps usually need less maintenance. This is because they have a simpler design and fewer moving parts.
Still, things like cavitation, wear on the impellers, and oil for the bearings need attention. Regular checks, monitoring vibrations, and replacing worn parts on time can help these pumps last longer. Good maintenance keeps them running well and avoids costly downtime in important tasks.
Cost
When you think about the cost of pumps, you need to look at both the buying price and the long-term costs to operate them. Positive displacement pumps usually have a higher initial cost. This is because they are built more carefully and need precision.
But these pumps are often more efficient and last longer. They also need less maintenance, which can lower your costs in the long run. Choosing a positive displacement pump that is well-made and has a good history can save you money later, even if you have to spend more now.
Positive Displacement Pumps
While positive displacement pumps may cost more at first, they can save you money in the long run. This is especially true for jobs that need high pressure or deal with thick liquids. These pumps keep a steady flow rate, even when the pressure changes. This helps keep the process stable and cuts down on waste.
Also, they usually need less maintenance than NPDPs under tough conditions. This helps make them a more cost-effective choice overall.
Non-Positive Displacement Pumps
Centrifugal pumps are the most common type of non-positive displacement pump. They often cost less at the beginning. This lower price is due to their simple design and parts that are easy to find. There are also many choices available in the market.
However, it’s important to think about the long-term costs of using them. Non-positive displacement pumps usually need less maintenance. But, if they don’t run near their best efficiency point (BEP), they can lose efficiency faster. This can lead to higher energy use, which raises the overall costs to operate them.
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Conclusion
In summary, knowing the main differences between positive and non-positive displacement pumps is important. This knowledge helps you choose the right pump for your industrial needs. Positive displacement pumps give you accurate flow control. They work best for high-pressure situations. Non-positive displacement pumps are better for handling thick or viscous fluids. When making your choice, think about the way each type operates, how suitable they are for your application, their efficiency, maintenance needs, and costs. If you want high-efficiency options for industrial tasks, check out Vacculex’s Screw Vacuum Pumps. If you have questions or need help choosing a pump, reach out to our experts for support and guidance.