What is a Self-Priming Pump, and How Does it Work?
Self-priming pumps are a specific type of liquid pump designed to have the required liquid inside the cavity or pump body necessary to start the pumping process. This offers the potential for increased operating efficiencies in process plants where pumps are used for a variety of repeated yet intermittent operations.
In the sanitary and hygienic processing industries, self-priming pumps are incorporated into process lines to perform several different tasks.
Pumps are used to:
- Move products through process lines
- Pump CIP (clean-in-place) fluids to clean and sanitize pipes and components
- Transfer liquids from one storage vessel to another
- Empty tanks and drums
Some pumps are designed to always have the requisite start-up liquid in the pump’s chamber or body. These are called “self-priming” pumps. Other pumps require the start-up liquid be added to the pump before initiating operation. This process, known as manual priming, involves external methods and can be labor-intensive. No one type of pump is intrinsically superior to another: it simply depends on what the pump is used for and how often.
Types of Self-Priming Pumps
There are several types of self-priming pumps available, each designed to handle specific fluids and operational conditions:
- Positive Displacement Pumps: Utilizing a piston or diaphragm, these pumps move fluid through the pump with precision. Self-priming positive displacement pumps are particularly effective for viscous fluids or those containing solids.
- Diaphragm Pumps: These pumps employ a flexible diaphragm to move fluid, making them suitable for handling corrosive fluids or those with solids.
Each type of self-priming pump offers unique advantages, making them suitable for a wide range of industrial and commercial applications.
How a Self-Priming Pump Works
Some types of pumps are engineered to be automatically self-priming. Pumps of this nature feature close tolerance working parts that trap fluids in the pump’s body, preventing them from returning from the discharge side of the pump to the suction side when the pump is not in operation.
In these types of pumps, the constant presence of fluid in the pump’s body allows the pump to better handle what are called “air pockets”. Air pockets are an accumulation of air bubbles in the pump’s working mechanism, which can impair proper pump operation.
Most liquid pumps used in the sanitary processing industries are centrifugal-type pumps. Centrifugal pumps are:
- Simple
- Sturdy
- Relatively inexpensive
- Ideal for pumping water and other low viscosity fluids such as CIP solutions under circumstances where suction conditions are difficult
For these types of low viscosity fluids, a V-type centrifugal pump design is often preferred, featuring a vacuum pump and positive seal that is located close to the discharge port. This allows a V-type centrifugal pump to readily pull a vacuum until it is full of fluid.
However, standard centrifugal pumps are not designed to be self-priming. Unlike the pump types already described, standard centrifugal pumps do not trap liquids when not in use — although engineered modifications to the pump’s housing, such as surrounding the pump with a tank and ensuring it remains immersed in liquid, may allow some of these types of pumps to be self-priming.
With centrifugal pumps, the pumping action occurs when an impeller is rotated in a liquid within a cavity or chamber of the pump, displacing the liquid and forcing it to flow into the discharge port of the pump via centrifugal force.
Fluids typically enter the pump at the center of the impeller and are discharged via a port on the outer perimeter of the pump’s body. The impeller is often directly coupled to the pump’s motor, in what is called a monoblock design.
Air is the primary enemy of this type of pump. The pump casing plays a crucial role in maintaining pressure and ensuring proper operation. When a standard centrifugal pump encounters air pockets, it can become “air bound”, and refuse to operate. Air intrusion into standard centrifugal pumps is typical, as these types of pumps don’t feature tightly coupled pumping mechanisms such as gears or screws and have no seals between the suction and discharge sides of the pump. The pumped fluid is essential for lubrication and cooling of the pump's bearings, and air presence can lead to air-binding.
Self-priming centrifugal pumps overcome air binding by initially mixing any residual air in the pump’s working mechanism with fluid during the priming process.
This blending allows the resulting mix to move readily through the pump’s body on initial start-up, removing the air and aiding product flow on the suction side of the pump. The process liquids and any entrained air move toward the impeller, and normal pump operations commence.
In comparison to a standard centrifugal pump, what’s notable in the design of a self-priming centrifugal pump is that it features a liquid reservoir built into the body of the pump, typically either above or in front of the impeller.
This reservoir allows the pump to rid the pump’s body and suction line of air during the priming cycle, replacing it with liquid from the reservoir that is blended into the residual air. The “self-priming” capability of this type of centrifugal pump comes from the ability of the reservoir to retain liquid after its very first prime.
A Guide to Choosing the Right Pump for Hygienic Applications
This guide is intended for engineers, production managers, or anyone concerned with proper pump selection for pharmaceutical, biotechnology, and other ultra-clean applications.
Advantages of Self-Priming Centrifugal Pumps
- Can handle a variety of liquids
- Work well with slurries, corrosive liquids, and suspended solids
- When compared to submersible-type pumps, self-priming centrifugal pumps will continue to pump liquids even after the pump is no longer submerged in a liquid tank or vessel
- Ideal for frequent and intermittent pumping operations, as the steps involving pump priming on start-up are eliminated
Disadvantages of Self-Priming Centrifugal Pumps
Self-priming pumps have disadvantages, including lower efficiency, higher initial costs, and greater energy consumption.
- Can’t operate without the presence of the initial priming liquid in the pump’s reservoir
- Due to the need for a liquid reservoir, this type of centrifugal pump can be larger than a standard model, which may cause issues in applications where space is limited
- Need to be as close as possible to production lines to avoid depletion of the pump’s liquid reservoir during self-priming operations
An Overview of CSI Self-Priming Pumps
LKH Prime
The LKH Prime pump from Alfa Laval is a centrifugal-type self-priming pump that is ideal for CIP operations. The LKH Prime also easily pumps products with entrained air. Once the liquid no longer contains entrained air, you are left with a highly efficient and quiet LKH Pump, one of the most capable centrifugal pumps produced.
For product applications, this pump features an air screw coupled with a highly efficient centrifugal impeller to effectively handle a variety of process media.
The sanitary design incorporates wetted parts made from acid-resistant AISI (American Iron and Steel Institute) 316L stainless steel, with other components made from AISI 304 stainless steel. The LKH Prime is EHEDG and 3-A approved for sanitary applications.
SP Series
The SP Series is a liquid ring pump from Ampco that uses compression chambers to pump, and only uses centrifugal force to seal the chambers with a liquid. It is excellent for CIP return operations and pumping products with entrained air.
The SP Series pump is efficient at emptying tanks and drums, and can handle shear-sensitive products due to its tight manufacturing tolerances and unique impeller shape.
These pumps feature a 304 stainless steel adaptor and are in conformance with 3-A sanitary standards.
SLH Twin Screw Pump
Bornemann's SLH Twin Screw Pump is a self-priming positive displacement pump best for aseptic processes and applications involving high viscosities, high pressures, or sensitive materials. The variable speed feature of this pump is suited for varying operating conditions.
The Bornemann SLH Twin Screw Pump is EHEDG and 3-A approved for sanitary applications.
C-Series
The C-Series is an eccentric disc pump featuring Mouvex technology. This positive displacement pump uses its strong suction power and ability to run dry to gently and effectively move both liquid and dry products.
The C-Series is available in stainless steel models that carry 3-A approval and are designed to meet EHEDG specifications. This pump can be flushed and cleaned using CIP methods without disassembly and can pump both high and low viscosity products with outputs not affected by variations in liquid viscosity.
Next Steps
For hygienic processing applications that require the frequent yet intermittent use of liquid pumps, choosing a self-priming pump may be the best solution for ensuring efficient pump operations. We're here to help!
CSI offers a number of self-priming pumps based on centrifugal, twin screw, and eccentric disc technologies. Contact CSI at (417) 831-1411 to discuss your requirements.
ABOUT CSI
Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.