Self Priming Pumps
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Self-priming centrifugal pumps incorporate all standard centrifugal pump design features and add the following internal modifications:
- A casing design that surrounds the volute and impeller and enables the pump to retain liquid in a built-in reservoir, or priming chamber. This reservoir is filled during the initial prime of the pump, and when the pump completes a pumping cycle and shuts down, the reservoir retains liquid for the next priming cycle.
- An internal recirculation channel or port. This channel connects the pump’s discharge cavity back to the suction reservoir internally, allowing the continuous recirculation of liquid from discharge back to suction during the priming, usually to the peripheral portion of the impeller.
These two internal design features, the priming chamber and internal recirculation channel, are what distinguishes a self-priming centrifugal pump from a standard centrifugal pump.
How It Works
Self-priming centrifugal pumps can be placed above the liquid level of the source. Only the suction pipe enters the liquid being pumped. The pump is initially primed by adding liquid to the pump casing through a priming port, normally located near the discharge. The liquid fills the discharge reservoir, traveling into the eye of the impeller through the pump’s recirculation channel. The suction line, itself, is not filled. A check valve is usually located just inside the suction reservoir.
All connections must be airtight. During initial start-up, the impeller rotation causes the liquid in the pump reservoir to be directed to the discharge cavity via centrifugal force. Simultaneously, a lower pressure is formed in the suction reservoir. This draws the liquid from the discharge cavity back into the suction reservoir through the pump’s internal recirculation channel. This is a continuing action during the priming cycle.
While this is occurring, the air in the suction line is drawn by the lower pressure into the eye of the impeller with the priming liquid and travels through the volute into the discharge cavity. At this point velocities decrease, allowing the air and liquid mixture to separate. The air flows up and is ejected, and the priming liquid recirculates back into the impeller. This process continues to draw all the air from the submerged suction line.
In applications where the liquid level is at atmospheric pressure, that pressure on the liquid surface, coupled with the lower pressure in the suction pipe due to the evacuation of air, serves to push the liquid in the sump into the pump.
When all air is evacuated liquid pumping automatically begins. This priming action is actually a very easy task for a correctly installed self-priming centrifugal pump, and it happens automatically in a relatively short time (20 - 30 seconds for a normal 15 foot suction lift). It’s this feature that differentiates self-priming centrifugal pumps from standard centrifugal models.
Self-priming centrifugal pumps are usually classified into two groups: basic self-priming pumps and trash-handling self-priming pumps.
Basic self-priming pumps usually come with different impeller configurations, including fully enclosed and semi-open. Like all centrifugal pumps, the pressure developed is dependent on the impeller diameter and rpm.
- Fully enclosed impellers allow self-priming pumps to develop medium to medium-high discharge pressures, up to about 110 psi or 254 ft total dynamic head (TDH). Normal pump sizes range from 1 in. through 6 in. suction and discharge. Pumps with a fully enclosed impeller have a very limited solids handling capability, with sizes from 1 1/32 in. through 5/8 in. in diameter, depending on the size of the pump. This configuration is excellent for handling clear liquids, including processed hydrocarbons, along with general wash-down pressure applications.
- Semi-open multi-vane impellers are usually designed for slightly lower head conditions than fully enclosed impellers, but they have greater solids handling capabilities. Pump sizes usually range from 3/4 in. through 12 in. suction and discharge, with capacities to more than 5,500 gpm. Spherical solid sizes range from 3/4 in. through 3 in. in diameter, depending on the size of the pump. Basic self-priming pumps with semi-open impellers are sometimes referred to as general- purpose self-priming pumps. They are excellent for handling dirty, contaminated liquids. Applications include extensive use in industrial filtration operations and a wide range of engine driven models that serve the construction market.
Trash handling self-priming pumps generally uses a trash-type, semi-open, two-vane impeller that allows the pump to pass larger spherical solids.
- Trash handling self-priming pumps generate medium discharge pressures in the area of 62 psi or 145 ft TDH on electric motor drives and discharge pressures upwards of 75 psi or 173 ft TDH on engine-driven configurations, with capacities upwards of 3,400 gpm. Normal pump sizes range from 1-1/2 in. through 10 in. suction and discharge. The impeller design allows for excellent solids handling capability, ranging from l in. to 3 in. spherical solids diameter, depending on the pump size. Trash handling self-priming pumps are often referred to as the workhorse of centrifugal pumps due to their rugged design and large solids handling capabilities. These pumps can be found on some of the most severe pumping applications within plants or on construction sites.
- A desirable design feature of a trash handling self-priming pump is a removable cover plate, located directly in front of the impeller on the suction side of the pump. Trash handling self-priming pumps may be applied in waste sump applications where they are exposed to various size solids. Any pump may clog trying to pump larger solids than it was designed to pass. The removable cover plate allows quick access to the suction side of the pump, expediting the removal of a blockage. Some designs allow removal of the cover plate without disturbing the suction and/or discharge line.
Self-priming centrifugal pumps have a broad design range that allows them to serve a wide variety of applications. Many metallurgical choices and shaft seal configurations are available to best serve particular services. Mechanical shaft seals can be single, double, or tandem. They are available as double grease-lubricated for general purpose applications, oil- lubricated with silicon carbide faces for industrial applications with abrasives, carbon against Ni-Resist faces for clean water or refined hydrocarbon applications, or Teflon fitted with carbon/ ceramic faces for chemical applications.
Alloys available for pump construction also offer the same diversity. Cast iron and ductile iron are used for general purpose and refined hydrocarbons, hardened ductile iron (ADI) is employed for abrasive applications, CD4MCu SS serves in corrosive and abrasive applications, and 316 SS, Alloy 20 SS, Hastalloy B, and Hi-Resin Epoxy Plastic are used for other special chemical applications.
The principal application area for self-priming pumps is where their ability to self-prime is a cost effective solution; and when it is more convenient and desirable to locate a pump ”high and dry“ above the liquid.
Some general guidelines are:
- The liquid being pumped should be of low viscosity (550SSU or less).
- Horsepower and efficiency corrections are needed for liquid viscosity above 550 SSU.
- If subjected to liquid freezing temperatures, the pump must be protected against freezing to avoid damage.
- The vapor pressure of the liquid and the presence of high levels of entrained air are serious considerations in suction lift application.
- The NPSHA (net positive suction head available) must exceed the manufacturer’s published NPSHR (net positive suction head required) by a margin that accounts for the liquid properties.
- Repriming time increases with suction lift.
- Suction lifts with water as the liquid at normal ambient temperature should be limited to 15 to 18 ft. best efficiency range, although maximum practical lifts are obtainable to 25 feet.
- Reducing the speed of operation (rpm) significantly reduces the NPSHR.
- Suction line piping should be sized to velocities in the 5 to 7 ft. range at design flow. For self-priming pumps it is recommended that the suction piping should be the same size as the pump’s suction inlet.
The self-priming centrifugal pump offers a unique solution to many pumping applications.
A cut-away view of a self-priming centrifugal pump designed to handle solids-laden liquids and slurries (below)