We’ve discussed the distinctions, but how well do you know the difference between centrifugal and positive displacement pumps? Let’s look at positive displacement pumps and what you should know about them.
What is a Positive Displacement Slurry Pump?
Positive displacement pumps, or PD pumps, are vacuum pumps that provide a consistent flow of fluids that transports liquids through the system at a constant speed. Slurry and sludge, mud, oils, chemicals, pulps, pastes, wastes, foods, and other drinks can all be moved using PD pumps. The positive displacement slurry pump creates a suction or vacuum that sucks in and captures the fluid in its chamber before passing it through. A positive displacement slurry pump helps transport higher viscous liquids at a lower flow rate but higher pressure.
How Do Positive Displacement Pumps Work?
Positive displacement pumps come in more than ten varieties. To put it simply, PD pumps work by drawing fluid into a chamber or cavity through the creation of a vacuum.
When the section is complete, the pump cycles, increasing the fluid pressure within the pump head. This allows it to be discharged into the pipes via the discharge port.
Positive Displacement Slurry Pumps fit into two main classifications:
To create suction, rotary pumps employ a revolving mechanism, whereas reciprocating pumps use a back-and-forth motion. Rotary pumps utilize gears, lobes, vanes, and screws to move fluids. Diaphragms, pistons, and plungers are used in reciprocating positive displacement pumps. The various types of PD pumps all have applications where they excel, and they also give great pumping in a range of other cross-applications.
Reciprocating power pumps are built with excellent adaptability for more accessible product selection and faster turnaround—electric motors or diesel engines power these units via a V-belt or gear. Electric motors are more popular since they are more efficient and have less environmental impact. The diesel engine is more powerful and is commonly utilized in industrial applications.
Reciprocating Pumps Usage
- The pump may transfer many types of oil, including crude oil, diesel oil, and lubricating oil.
- The bronze impeller pump can carry low flash point liquids such as gasoline and benzene. etc.;
- A stainless steel pump may convey both beverages and corrosive liquids.
- The temperature of the medium cannot be higher than 70 degrees Celsius.
- The anti-high temperature gear oil pump can transport liquids with a maximum temperature of 300 degrees Celsius and a viscosity of 5*10-5—1.5*10-3m2/s.
- The pump is incompatible with hard solids or fiber.
Other Classifications are:
Diaphragm Pump Air Operated
An air-operated diaphragm pump is a positive displacement slurry pump that is powered by compressed air (also known as a membrane pump). A linked shaft transports compressed air from one chamber to the next, allowing the chambers to move simultaneously.
Diaphragm Pump Electric
An electric-driven diaphragm pump is more energy efficient and requires less maintenance than air-operated diaphragm pumps. They are perfect for applications requiring low pulsation and a smooth flow. Positive displacement pumps are classified as diaphragm pumps.
To transport fluids, a gear pump uses a gear meshing system. Gear pumps are positive displacement pumps due to the mechanism involved. The gears in a gear pump move the same quantity of fluid with each revolution. Gear pumps are frequently used for transferring high-viscosity liquids due to their design, particularly in the chemical industry.
Helical Rotor Pump
The helical rotor pump is a positive displacement slurry pump that rotates a winding rotor and pushes discrete amounts of liquids through the pump. A corkscrew-like movement offers pulse-free flow, with the rotor speed determining a precise flow rate, allowing the helical rotor pump to be used as a dosing pump.
Oil & Grease Pump
Oil & Grease Pumps are a type of positive displacement slurry pump used in industrial settings to pump automotive and equipment fluids such as oil and grease.
They do not have any seals, valves, or glands and are inexpensive to maintain.
In a piston pump, a form of positive displacement pump, the high-pressure seal reciprocates with the piston. Piston pumps may transport liquids or compress gases and pump viscous and solid-particle-containing media.
Pump that self-primes
Self-Priming pumps can lift fluid from below their suction without using a foot valve or a hydraulic column of fluid in the suction line. The liquid in the pump volute powers the pump.
Characteristics of Positive Displacement Pumps
Most pumps are classified into centrifugal and positive displacement pumps. Centrifugal pumps are more commonly used due to their simplicity. Positive displacement pumps are more versatile and diversified in their applications. They can generally endure harsher circumstances than centrifugal pumps and provide a consistent flow regardless of pressure. Positive displacement pumps are suitable for applications requiring high pressure output, low flow rate, or precision dosing. Furthermore, they can operate at any position along the pump curve.
Applications for Positive Displacement Pumps
Positive displacement pumps are utilized in manufacturing, food processing, water treatment, and petrochemicals. Because of their delicate operation, PD Pumps (such as peristaltic or diaphragm pumps) are also appropriate for fragile materials such as cell cultures or shear-sensitive polymers.
Electric motors or diesel engines operate reciprocating pumps through V-belts or gear reduction. Stuffing boxes are intended for applications requiring long packing life and minimal maintenance. Equipment can be packaged to satisfy even the most exacting specifications.
- Abrasive liquids & slurries
- Alcohol, gasoline, fuel oil, lube oil
- Amine & glycol circulation
- Beverages, other viscous foods; peanut butter/corn syrup
- Blow out preventers
- Boiler feed
- Desalination high-pressure feed
- Injection (chemical, saltwater, lubrication)
- Hydrostatic testing
- Hydraulic systems
- General industry services
- Product transfer
- Subsea controls
- Tars and asphalts
Advantages of Positive Displacement Pumps
Such pump designs have a variety of application benefits. The following are six advantages of positive displacement pumps:
Accurate Predictable Flow
Because the chambers in such pump designs have a set volume, flow is proportional to speed. The flow produced by each rotation may be calculated with a great degree of precision. This allows for estimating the pump’s predicted value or stream throughout time.
Wide Viscosity Handling
These designs are not impacted by viscosity, which means that if the density increases with temperature, the equipment used with various fluids or the liquid being pumped will behave as a non-Newtonian fluid.
This occurs when viscosity increases with shear, a phenomenon known as a shear thickening, while the unit’s flow or pressure capabilities remain unaffected.
PD pumps favor thicker fluids, with higher viscosity typically generating higher pressures and flows.
These units maintain a steady pressure at pressures up to several bars over the specified duty point. In contrast to centrifugal pumps, which create a specific pressure and flow at a single duty point.
This can be advantageous in applications with varying discharge pressures. For example, tanker loading, spray applications, and dosing and metering.
Lower Wear and Tear
When opposed to centrifugal pumps, PD pumps run at slower rates. This slower speed allows for handling abrasive or solid-laden liquids that would otherwise wear out parts in a centrifugal pump.
Preservation of Liquid Characteristics
Reduced pump speed will enable pumps to be enlarged, which means that a unit running slowly can meet an application’s flow or pressure requirements.
This decreases wear and guarantees that the liquid is not influenced by the pumping motion, hence keeping its properties. This is especially significant with shear-thickening liquids such as milk, creams, polymers, and gels.
Every pump has a minimum and maximum viscosity level, the viscosity limit for which it is developed and designed. If the viscosity of the fluids is reduced to a level less than the intended limit, the pump will be unable to handle it, and a slide will occur.
A pump slide appears when fluid recirculates within the pump head, resulting in a drop in pump flow and pressure. This implies that fluid can continue to recirculate within the pump head and, if unnoticed, can cause the mechanical seal to fail.
Positive displacement pumps with non-slip designs are available. Pump slip will not occur regardless of fluid viscosity, as shown in this case study.
Disadvantages of Positive Displacement Pumps
Positive displacement pumps have several drawbacks. Here are the specifics for the most prevalent 7:
Flow is restricted on some pumps due to internal design, which means flow will be less than a centrifugal pump for low-viscosity fluids. Some positive displacement pump designs are absolutely out of the question for specific applications. This is because the flow does not fulfill the requirements for the intended application.
Difficult to Maintain
They can be tough to maintain due to their internal design. This can be due to the amount and size of parts, how they fit together, and the liquids they are used with being thick, corrosive, toxic, and encrusting/coating components within the unit, resulting in more time-consuming maintenance.
Positive displacement pump maintenance typically requires two or more workers, implying that additional labor is necessary to service such pump designs.
Because of the narrow clearances, application demands, contact of internal parts, and abrasive liquids utilized, service intervals are more frequent than in centrifugal designs.
Because flow might pulsate, pulsation dampeners are required to reduce pulsations. This can impact the dependability of flowmeters, the smooth constant coating of liquids on surfaces, and the reliable metering of fluids into processes or containers in container-filling applications.
Cost Effectiveness with Low Viscosity Fluids
Such units are typically inefficient for low-viscosity fluids or fluids requiring a transfer at low to medium flows when a centrifugal pump could be employed.
Not the cheapest solution
Positive displacement pumps are typically specified to provide consistent liquid metering, transfer, and flow.
They are never the cheapest option but frequently the lowest lifetime cost solution. They are built differently from centrifugal systems and designed for fluids much thicker than water. This is true for abrasive fluids, including particles or viscosity changes.
Required accessories for System or Operator Protection
PD pumps are excellent for increasing pressure within discharge piping, but they can be overly successful, leading to other problems.
Positive displacement pumps will continue to create pressure in the outlet piping if left unfettered until something gives, alleviating the pressure. This can be a valve, the pump itself, or pipes, which is why many systems come with an inbuilt relief valve.
Some pump designs will stall or pause when the outlet pressure hits its maximum output. Other methods, such as pistons, require relief valves to lower pressure or a pressure regulating valve to maintain a constant pressure.
Why do we need Positive Displacement Pumps?
How do they function? A Positive Displacement Pump is created specifically to pump thick and dense materials. The pump uses spinning cavities to move liquid from one pumping side to the other. As the fluid flows, It is expelled from the other side of the pump after passing through the pump. Positive Displacement Pumps generally work on a similar principle but with a different design, and most have reversible flow directions. The two most prevalent methods are seen here. In addition, we will talk about how others work.
A positive displacement pump moves fluid by pulling a predetermined amount into an intake valve and distributing it through an output valve. They do not rely on impellers like centrifugal pumps. Instead, they employ rotating or reciprocating elements to guide fluid into an enclosed space. When enough pressure is built up, the liquid enters the discharge system. As a result, the fluid velocity of a positive displacement pump is substantially lower than that of a centrifugal pump.
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