Self-priming pumps

One of the key parameters of a pump is suction head. After a pump stops fluid tends to flow out of its suction line. In this case after the pump is restarted it also has to pump the air plug, which is formed in the inlet pipe. For this purpose self-priming pumps are used capable to remove the air and continue operation.

There are many types of self-priming pumps, which differ in the number of impellers, their type and material. Self-priming pumps may be used for pumping pure water as well as oil, effluents and other liquids.

Self-priming pumps are used when one or several of the following conditions have to be met:

• self-priming capacity
• movement of fluids with solid inclusions
• pumping of highly-abrasive media
• handling of viscous fluids

Self-priming pumps are mainly divided into the following types:

• positive displacement pumps,
• vortex pumps,
• liquid ring,
• centrifugal,
• ejector pumps (with an internal or external ejector).

Due to its self-priming capacity such a pump may be located above the fluid level (surface pump) without submerging the operating element (semi-submerged pump) or the entire pump unit with motor into the working media (submerged pump).

Vortex self-priming pumps are usually used for pure low-viscosity liquids that do not contain any solids. Comparatively low capacity and efficiency are offset by a simple design and easy operation.

Liquid ring self-priming pumps are used successfully for foaming liquids. They are also good for water, black oil and diesel fuel. These pumps inevitably lose part of the working fluid which is carried out of the pump with the pumped media.  

Vortex and liquid ring side channel pumps are also used as self-priming pumps. These pumps are a niche product between positive displacement and centrifugal pumps and are designed for liquid/gas mixtures without any abrasive particles. Self-priming side channel pumps feature the following characteristics:

• self-priming capability,
• gas movement without disrupting the liquid flow,
• zero damage in case of dry run,
• low NPSH value.

Piston self-priming pumps are strongly limited in suction height, require regular cleaning of inlet and outlet valves and replacement of piston rings if used for pumping dirty fluids They are mostly used on ships.

In individual water supply area self-priming centrifugal pumps are quite popular. For centrifugal pumps “self-priming” means that they are capable of moving liquid/gas mixture (including gas bubbles) and generating vacuum in the suction line to lift the fluid and fill the internal chamber. Some of their features are listed below.

Design features of self-priming centrifugal pumps

• Simple design and only one moving element - an impeller;
• Theoretical possibility of impeller grinding;
• Easy access to impeller for inspection;
• Fairly large solid inclusions are admissible;
• Self-priming pump arrangement outside the pumped fluid keeps the design simple and facilitates technical inspection and repair procedures. Due to the possibility of continuous monitoring the risk of extensive damage is reduced significantly.
• Reasonably long service life. Expected life of a self-priming pump undergoing regular maintenance is 20 years minimum.
• Self-priming pumps are available in a bare shaft design, complete with an electric motor, diesel motor, gasoline engine, hydraulic turbine and in explosion-proof design.

Scope of application of self-priming pumps

Self-priming pumps are widely used in different areas from chemical industry to private water supply. They are good for handling effluents from drainage wells, tanks and underground reservoirs.

Self-priming pumps are also used in HVAC, water and sewage pipelines, etc.  Under certain conditions they may also me utilised for evacuating air from the suction line. Maximum suction capacity of the pump determines the size limits and configuration of the suction line.

Industry:

Self-priming industrial pumps unlike household units usually feature higher capacity and reliability. They are used at various production sites, in oil storage tanks, to remove spills at chemical plants, in mining, metallurgical, pulp and paper industries, oil and gas processing, water supply and drainage.

Pumped fluids:

• clean or lightly contaminated, neutral, acid or alkali liquids;
• fluids containing sand, rust, sludge or suspended solids;
• low-viscosity clean or dirty oil products, such as gasolene, black oil, pure diesel fuel, paraffin, and others
• solvents;
• lime milk;
• aerosols and foams;
• caustic soda solutions, etc.

Industrial self-priming pumps are used for washing and cooling  (machine tools, grinding wheels); circulation of liquids for cutting and quenching; fume capture; cleaning of fuel tanks, etc.

Construction:

Flood pumping; conveying of waste waters; fire-fighting; hazardous liquid collection; water evacuation from pits, channels or basins during excavation works; groundwater drying and lowering (to improve foundation laying efficiency during construction works); water supply from wells or channels; washing of concrete surfaces. Self-priming pumps are also used in production of construction materials and repair and maintenance of utility systems and networks.

Sea:

Loading and unloading of liquids; drainage; pumping of salt or sea water; fire-fighting; water supply for sanitary services, etc.

Waste removal:

Handling of dirty, hot or corrosive waste waters; pumping of sand-containing water, sludge or suspended solids; dosing of make-up liquids, sludge pumping.

Agriculture:

Surface watering and irrigation; pumping and spraying of liquid manure or fertilizers; distribution of liquid feed for animals; handling of oil cake, new wines, fruit juices, etc.

Self-priming pump design

Standard self-priming pump design:

In standard configuration the pump (operating element of the pump) and the motor are arranged outside the handled fluid above its surface. Only suction pipe is submerged directly into the liquid with a check valve preventing outflow back into the reservoir (natural or artificial). Discharge line feeds the liquid from pump to the consumer and may also be equipped with a check valve.

The casing of a self-priming pump may be manufactured from iron, bronze, stainless steel and other alloys. Household self-priming water pumps are also made of stainless steel, copper alloys and polymer composites.

Principle of operation

Self-priming pumps do not require any additional suction device. Since it is arranged above the level of fluid, before ramp-up to the operating conditions it is necessary to lift the fluid to a certain height, which cannot exceed the maximum suction height. Usually it is not necessary to pre-fill the suction line for starting up a self-priming pump, however it may be necessary to pre-fill the pump chamber. A check valve on the suction line saves the trouble of re-filling the pump before starting. Dry start of a pump is undesirable, since it may result in failure of individual components or the entire pump.

Air evacuation from the system

Self-priming pumps do not require any complex valves for air evacuation. When liquid/gas mixture enters the pump, phase separation starts whereby air is directed up and is discharged via outlet pipe, while water continues circulating inside the pump. The process continues until all the air is removed, after which the self-priming pump operates as a regular pump. In addition an air separation chamber for evacuating entrapped gas from the system and a priming chamber with liquid reserve for starting the self-priming process may be used.

In more detail the starting of a self-priming pump with inlet and separation chambers is as follows. Due to the pump design the working space of a pump is filled so that the impeller is completely covered with handled fluid. After filling self-discharging of a fluid is impossible, while air pressure before and after the pump is equal.

When the pump is started, pre-filled liquid is fed into the separation chamber under the impact of centrifugal force created by the impeller. This results in negative pressure in the suction line, which causes air intake. This air is mixed with pre-filled liquid making the impeller move this liquid/gas mixture. The mixture enters the separation chamber where the separation process starts pushing air into the discharge line, while the liquid flows back to the impeller. Thus gas is evacuated from the suction line without actual movement of pre-filled liquid.

While air share in the suction line is decreased, the liquid starts moving towards the pump until it reaches the impeller. By that moment all the air is evacuated from the suction line, and the pump starts operating as a usual centrifugal pump. The same principle is used for moving individual air pockets during normal run.

Key features of self-priming pumps

Different types of pumps may have self-priming function, each with its own specific features. Advantages and disadvantages of centrifugal self-priming pumps as the most common type of this family are listed below.

Advantages:

• suitable for dirty gas-containing fluids,
• high efficiency,
• self-priming of the suction line at pump starting,
• smooth mode-to-mode transition,
• simple maintenance and operation.

Disadvantages:

• limited suction head,
• necessity of pre-filling the pump and control to avoid dry start and dry run.

Suction line of self-priming pumps

Usually maximum suction head is 7-8 m (h_s). It should be noted that this value includes not only direct height of lifting from the surface of handled fluid to the pump, but also hydraulic resistance generated by the suction line (friction losses + local resistance losses).

The suction line shall have a nominal diameter not less than that of the pump inlet pipe. The total length of the suction line should be made as short as possible to reduce the total load which, if high, can have a negative effect on the suction ability of the pump.

Besides, sealing of the suction line is of critical importance – leakages may disrupt self-priming pump operation because of air intake into the suction line, which in its turn will cause the pump to fail.

Another key factor is suction line configuration. It should be continuously uprising towards the pump and have no air pockets. Such air pockets not only deteriorate suction conditions, but are also difficult to remove.

Quite often additional equipment is installed at the entry to the suction line such as a check valve and a strainer. Check valve prevents the fluid from leaking back from the suction line, so that pump priming and lifting of liquid from reservoir to the pump at restarting is not required. A simple non-return flap may be used instead of a check valve. A strainer is used if the handled media is bly contaminated with dirt, leaves, paper, wood or stone chips.

A self-priming pump cannot be replaced with a regular one. Nevertheless, if there is no other option a conventional pump may be used. However this will require preliminary filling of both the pump itself and the suction line with fluid which often involves some difficulties. Besides, air ingress into the suction line may easily result in failure of a regular pump and will require re-filling of suction line before resuming operation. In practice such extreme measures involve unnecessary costs and damaging of equipment, that is why for upwelling it is recommended to use self-priming pumps designed specifically for this purpose with characteristics selected according to the suction conditions.

Key information for self-priming pump operation

• Before the start of a self-priming pump it is necessary to carry out priming procedure to avoid dry start which may result in pump breakdown.
• Backpressure generated in the discharge line as a result of accumulation of air bubbles blocks air passage through the suction line. To avoid such situations it is necessary to install an air release valve on the discharge line between the pump and the check valve. This valve will evacuate entrapped gas from the system.
• For self-priming pumps it is preferable to use end seals. In case of a stuffing box seal it may trap some air which will complicate the filling and deteriorate functional quality.
• When selecting a self-priming pump it is always necessary to take into account the lifting height for which pump NPSH diagram is used.
• It is of critical importance to control for leakages since they can reduce significantly or even destroy self-priming capability of a pump.
• In your calculations it is necessary to try to minimize suction height, suction line length and priming process time. Overlong priming time may result in evaporation of liquid from the filling compartment before the priming process is finished.
• When a pump is used in dirty areas it is recommended to install a strainer to prevent accumulation of solids in the filling compartment.
• To avoid backpressure in the discharge line which may impede the priming process, it is necessary to pay special attention to the installation of a bypass filling line.