A suction in the gear pump is required
One of the most important parameters of a pump is the suction lift. After the pump is stopped, the liquid usually flows out of the suction pipe. In this case, after restarting, the pump also has to convey the air plug that has formed in the pipeline. Self-priming pumps, which are able to let air portions through and continue to work, are used for such purposes.
There are numerous types of self-priming pumps, which differ both in the number of impellers and in their type and material. The self-priming pumps can be used for pumping not only clear water, but also petroleum, waste water and other liquids.
The self-priming pumps are used when one or more of the following conditions are met:
- the self-priming ability;
- the pumping of fluids with solid inclusions;
- the pumping of highly abrasive fluids;
- pumping viscous liquids.
Self-priming pumps are usually one of the following types:
- Volume pumps;
- Vortex pumps;
- Liquid ring pumps;
- Centrifugal pumps;
- Ejector pumps (with internal or external ejector).
The suction capability allows the pump to be arranged above the level of the liquid to be pumped (surface pump) without the need to accommodate the working element, as is the case with a semi-submersible pump, or the entire pump unit together with the motor in the volume of the working medium, as is the case with a submersible pump.
The self-priming vortex pumps are usually used for pumping clean media without solid inclusions and with a low viscosity. The simple construction and user-friendliness compensate for their relatively low delivery rate and efficiency.
The self-priming liquid ring pumps are used for pumping foaming liquids as well as water, heating oil and diesel fuel. When such pumps are operated, part of the working fluid is inevitably lost because it is carried away together with the pumped medium.
Vortex pumps and water ring pumps with a side channel are also used as self-priming pumps. Such pumps occupy the niche between the volume pumps and the centrifugal pumps and are intended for pumping gas-liquid mixtures that do not contain any abrasive particles. The self-priming pumps with a side channel have the following properties:
- self-priming ability;
- the ability to move gas without disrupting the flow of liquid;
- no damage when running dry,
- low NPSH.
The self-priming piston pumps have a very limited suction height, require constant cleaning of the valves during suction and delivery and the replacement of piston rings when pumping a contaminated medium. They are mainly used on ships.
Self-priming centrifugal pumps are often found in the area of individual water supply. In relation to centrifugal pumps, the term "self-priming" means that they are able to circulate a gas-liquid mixture (including gas bubbles) and to generate low pressure in the suction pipe so that the liquid is lifted and then fills the internal volume. Some special features of this type of pump are described below.
Special design features of the self-priming centrifugal pumps
- Simple design and a single moving part - the impeller;
- theoretical possibility to grind the impeller;
- easy access to the impeller for visual inspection;
- the applicability to media with fairly large solid inclusions;
- the possibility of arranging the self-priming pump outside the volume of the pumped medium; this makes the design less complicated and makes technical inspection and maintenance much easier. The possibility of continuous monitoring of the pump greatly reduces the likelihood of severe damage.
- Fairly long lifespan. The expected operating time of the self-priming pump, which is regularly and properly maintained, is at least 20 years.
- Self-priming pumps are available with a free shaft. The scope of delivery includes an electric, diesel or gasoline engine or a water turbine. They can also be supplied in an explosion-proof version.
Areas of application for self-priming pumps
Self-priming pumps are used everywhere in very different areas: from the chemical industry to the water supply for private houses. They are used to pump liquid from drains, tanks and underground storage facilities.
Self-priming pumps are used in heating and cooling technology, in water pipes and sewers, etc. Under certain conditions, a self-priming pump can also be used to pump air out of the suction line. The maximum suction capacity of the pump is decisive for the permissible dimensions and configuration of the suction pipeline.
In contrast to household pumps, the self-priming industrial pumps usually have a higher performance and reliability. They are used in various production facilities and oil storage facilities, to clean up chemical spills in chemical plants, as well as in mining and metallurgy, in the pulp and paper industry, oil and gas processing, water supply and sanitation.
Pumping of various media:
- clean or slightly contaminated, neutral, acidic or alkaline liquids;
- Liquids that contain sand, rust, mud or solid particles in suspension;
- clean or contaminated oil products with low viscosity (gasoline, heating oil, diesel fuel without additives, paraffin, etc.);
- Milk of lime;
- Aerosols and foam;
- Caustic soda etc.
The self-priming industrial pumps are used to flush and cool the machine tools and slip rings, to circulate liquids during cutting and hardening, to separate smoke, to wash out fuel tanks, etc.
Pumping out floods; pumping sewage; Fire extinguishing; Separation of hazardous liquids; Dredging for pumping water out of pits, canals or bodies of water; Drainage or lowering of the groundwater level (for efficient foundation laying during construction work); Water supply from bores or channels; Washing of concrete surfaces. The self-priming pumps are also used in the manufacture of building materials and in the repair and maintenance of supply and disposal lines.
Working at sea
Loading and unloading of liquid media; Pumping and drainage works; Pumping salt or sea water; Fire extinguishing; Water supply for sanitary services etc.
Pumping of polluted, warm or corrosive wastewater; Pumping water that contains sand, mud or solid particles in suspension; Dosing of equalizing fluids; Pumping out mud.
Surface irrigation and sprinkling; Pumping and atomizing manure or liquid fertilizers; Distribution of liquid feed for animals; Pumping press cakes, young wines and fruit juices etc.
Construction of self-priming pumps
The usual structure of self-priming pumps:
In the usual arrangement, the working element of the pump is placed together with the electric motor outside the volume of the pumped liquid over its surface. Only the suction line is immersed directly into the liquid. It ends with a non-return valve, which is necessary to prevent the liquid from flowing back into the natural or artificial reservoir. The pressure line in turn transports the liquid from the pump to the customer and can also be equipped with a backflow preventer.
The housing of the self-priming pump can be made of cast iron, bronze, stainless steel and other alloys. The self-priming household pumps for water are also made of stainless steel, copper alloys and composite plastics.
The self-priming pump makes it possible to do without an additional external suction device. Since the pump is above the level of the pumped liquid, it is necessary to raise the pumped medium to a certain height before switching to the operating mode, which must not exceed the maximum suction height. Usually you do not need to fill the suction pipe to start the self-priming pump, but you may need to fill the working chamber beforehand. The presence of a return valve on the suction line means that it is no longer necessary to fill the pump repeatedly before starting it. “Dry” starting of the pump is not desirable as this can lead to failure of individual components or the pump as a whole.
Removal of air from the system
Self-priming pumps do not require the installation of a number of complicated valves for air evacuation. As soon as the gas-liquid mixture gets into the working volume of the pump, the phase separation begins, in which the air is directed upwards and out through the outlet nozzle, while the water continues to circulate in the pump. This takes so long until the air is completely removed. After that, the priming pump continues to function as an ordinary pump. In addition, an air distribution chamber can be used to remove gas inclusions from the system, as well as a filling chamber in which the supply of liquid is stored, which is to be fed into the pump to start the suction process.
The starting of the self-priming pump with inlet and distribution chamber is detailed as follows. The design of the pump makes it possible to always fill the working area of the pump in such a way that the impeller is completely flooded by the pumped medium. After filling, it is impossible for the liquid to leave its assigned volume on its own, with the air pressure in front of and behind the pump remaining constant.
When the pump is started, the volume of the previously filled liquid is directed into the distribution chamber under the action of the centrifugal forces of the impeller. This creates negative pressure in the suction pipe, which is why the suction of air begins. The sucked in air is mixed with the liquid previously filled in. That is why the wheel starts to deliver the gas-air mixture. As soon as the mixture reaches the distribution chamber, the separation process begins: the air escapes from the pressure pipe, while the liquid flows back onto the impeller. This means that gas is displaced from the suction pipe without any real displacement of the liquid previously filled in.
As the proportion of air in the suction pipe decreases, the pumped liquid begins to move in the direction of the pump until it arrives at the impeller. At this point all the air has already been removed from the suction pipe, so that the pump now functions as a normal centrifugal pump. According to the same principle, isolated air inclusions are conveyed that can arise during the pump operation after the normal operating mode has been reached.
Characteristic features of self-priming pumps
The self-priming ability can occur in different types of pumps, each type having certain peculiarities that are unique to it. Below is a list of the advantages and disadvantages of self-priming centrifugal pumps as they are particularly widespread.
- the possibility of conveying contaminated media as well as media with gas inclusions,
- High efficiency,
- Self-filling of the suction pipe when starting the pump,
- seamless transition of processes when changing operating modes,
- easy operation and maintenance.
- limited suction height,
- the need for the pump to be flooded beforehand and to be checked accordingly in order to avoid starting and operating “dry”.
Suction line of self-priming pumps
The maximum suction head is usually 7-8 m (hs). It must be taken into account that this value does not only include the immediate height to which the liquids are lifted from the surface of the conveyed medium to the pump, but also the hydraulic resistance that is generated by the suction pipe (friction losses + losses at the local resistances).
The nominal diameter of the pipeline in the suction line must not be smaller than the nominal diameter of the inlet connection of the pump. The total length of the suction line is kept as small as possible in order to reduce the total resistance, the increase of which has a negative effect on the suction performance of the pump.
The hermetic sealing of the suction line is also a critical factor: the leaks can cause the self-priming pump to malfunction if air gets into the suction pipe and ultimately lead to the failure of the pump.
The configuration of the suction line also plays an important role. It must be constantly ascending in the direction of the pump and must not contain any areas where air pockets can form. This is because such air ashes not only worsen the suction conditions, but also make it difficult to remove them from the pipeline.
Often additional equipment is installed on the suction pipe (especially at the inlet): e.g. a non-return valve and a filter. The return valve prevents the pumped medium from flowing back out of the suction pipe. This means that it is not necessary to flood the pump when it is re-let in or to lift the liquid from the extraction reservoir to the pump. Instead of a return valve, a simple non-return valve can also be used. A filter is used if the pumped medium is heavily soiled by various admixtures - for example pieces of dirt, leaves, paper, wood chips or fine stone chippings.
An ordinary pump cannot possibly replace a self-priming pump. But if there is no alternative, a simple pump can be used as a replacement. In this case, however, it will be necessary to fill both the pump itself and the entire suction line with liquid beforehand, which is often associated with difficulties. If air gets into the suction pipe, a non-self-priming pump could break; in addition, the entire procedure for filling the suction line will have to be repeated each time before the pump can be started again. In practice, such emergency solutions only lead to unnecessary costs and damage to the equipment. It is therefore recommended to use only self-priming pumps for lifting water from the depths, which are specially designed for this purpose, and to select their parameters according to the calculated suction conditions.
Important information for the operation of self-priming pumps
- Before starting the self-priming pump, carry out the previous filling procedure in order to avoid a “dry start” which can lead to the pump failing.
- The back pressure created in the discharge pipe due to the accumulation of air bubbles prevents air from passing through the suction pipe. In order to avoid such situations on the pressure pipeline, an air separation valve must be installed between the pump and the return valve, with the aid of which the gas inclusions will be removed from the system.
- For self-priming pumps, a face seal is preferable. Air can get into the stuffing box seal: this could make filling more difficult and worsen the operating properties.
- When selecting a self-priming pump, the required delivery head of liquids must always be taken into account: for this one would have to evaluate the NPSH curves (holding pressure head of the pump).
- It is extremely important to keep the possibility of leaks under control, as doing so may decrease or completely lose the self-priming properties of the pump.
- The smallest possible suction height and length of the suction pipe and thus a shorter filling time should be aimed for in the calculations. An excessively long filling time can result in evaporation of the liquid in the filling section before filling is completed.
- If the pump is used in dirty sections, it is recommended to install a sieve filter to prevent solid impurities from accumulating in the filling section.
- To avoid back pressure in the pressure pipeline, which can hinder the filling of the self-priming pump, it is essential to install a filling bypass line.
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