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Submersible and semi-submersible pumps

Submersible pumps

General

Advanced developments in pumping equipment resulted in the emerging of different types of pumping units designed for different tasks. One of such challenges is pumping of liquids from deep wells or inaccessible areas. As a rule liquid enters the pump via a pipeline, but sometimes the pump may be submerged into the handled medium which makes the suction line redundant.

Depending on the mutual arrangement of fluid and pump parts there are the following types of pumps: surface, semi-submersible and submersible pumps.






In the first case the pumping unit is arranged above the level of fluid which is lifted via a submerged pipe. Such pumps do not require additional sealing and are simpler in manufacture and design, however, the presence of a suction line may involve some issues limiting the application of surface pumps. Since to move large volumes it is reasonable to use centrifugal pumps, it is necessary to provide for some measures against cavitation – the process of bubbling inside the flow – which may result in a pump failure. There is no risk of cavitational wear in case of shallow well pumping, however, the larger the distance from liquid surface to the impeller the more critical is the cavitation issue, which requires using another type of pump or some additional components which lead to a higher cost of the pumping unit. Besides, in some cases the pump needs to be mobile for a quick re-dislocation. A simple surface pump does not always meet such requirements.

Within this classification the opposite of surface pumps are submersible (submerged) pumps. The operating element of such a pump is installed inside a sealed casing which may be placed directly into the fluid to be pumped. Thus, the pump is actually operating inside the fluid with a zero length of a suction line. So, in a submersible pump there is no need to take additional measures against cavitation and dry run. The result is that the depth of the well is only limited by the available pump head. Due to this feature submersible pumps may be used when surface pumps are unsuitable or inefficient.

Classification and principle of operation

Submersible pumps are most fully classified by their scope of application. As a rule, each application sets strict requirements in terms of design and capabilities of pumping equipment, which makes the pumps of the same group similar in their features. There are the following types of submersible pumps:

  • well pumps
  • drainage pumps
  • sewage (sanitary) pumps
  • deep-well pumps

Submersible well pumps, as a rule, have the simplest design of all submersible pumps. This is because they are designed for handling clean water only, without any coarse solids. For this reason they are not submerged to the bottom but are left suspended to avoid gathering bottom sludge. They are used when water level is about 8 meters below the surface and surface pumps are no longer applicable. The maximum depth for using well pumps is about 20 meters.

Drainage pumps unlike well pumps can handle dirty fluids with solid inclusions such as sludge, sand, etc. This is enabled by installing special shredders and strainers to prevent entry of coarse particles.

Sewage pumps are more heavy duty as compared to the drainage pumps and are suitable for viscous fluids with large solid particles. They may also be equipped with shredders to handle heavily contaminated fluids.

Deep-well pumps as is evident from their name are designed for pumping water from deep wells or boreholes. That is they are used to lift water from significant depth. For better efficiency boreholes are made deep but have a small diameter. For this reason deep-well pumps have an elongated cylindrical shape and are selected according to the downhole, so that the outer diameter of the pump is 1 or 2 cm smaller than the downhole diameter to avoid the risk of sticking. Since level of fluid may be significantly below the land surface, b head is required, which is why these pumps have a multi-stage design.

There are two subtypes of deep-well pumps: sucker rod and rodless pumps. Rodless pumps are typical submersible pumps with a motor and impeller installed together in a single sealed casing. In case of sucker rod pumps the motor is arranged outside the downhole, and the movement is transferred to the pump by a mechanical link - a rod. Sucker rod pumps are usually used in oil production and are remarkable for their appearance due to the walking beam or “horse head” transferring the movement to the rod.

According to the principle of operation submersible pumps are divided into centrifugal, screw-type, vibration (diaphragm) and vortex pumps. The principle of operation is determined by several factors depending on the task to be performed by the pump.  Submersible pumps have the same advantages and disadvantages as regular pumps with the same pumping mechanism. Thus, vibration pumps are quite reliable, but have lower flow rate and head. Screw-type submersible pumps are suitable for viscous and delicate fluids. When high values of head and flow rate are required, the pump should be centrifugal and multistage.

Design

The key design feature of all submersible pumps is that the operating element and motor are installed together inside a single casing. Since during operation the pump is completely immersed into the fluid, the casing is sealed to prevent penetration of liquid inside the casing. Power is supplied via cable connected to the mains on the surface. The discharge line of a submersible pump may be either connected to a rigid pipe via a flange or to a hose if the well is not horizontal or the immersion depth is changing.






Besides a supporting rope may be fixed to the outside of the pump holding it in its operating position at the required depth. This rope is used in case of heavy weight of the pumping unit, when the power cable strength is insufficient. Besides a submersible pump may be equipped with a float - a hollow plastic chamber connected to the pump with a special rope. The float indicates the level and may trigger the pump into operation when water level is rising and the float is going up straining the rope. Thus, a submersible pump may be set to maintain a targeted water level by evacuating excess water.

The suction point of submersible pumps may be additionally protected with a screen protecting the operating element from excessively coarse particles which could lead to pump jamming or failure.

Internal design of a submersible pump depends on its type. As an example we can take the most popular centrifugal submersible pump. Rotor and impeller are installed on the common shaft supported by bearings. The shaft is sealed between the impeller and motor to avoid short circuiting from fluid penetration into electric motor. The principle of operation of a submersible pump is the same as that of a regular pump of the same type.

Advantages and disadvantages

Submersible pumps are noted for the following advantages:

  • Deep well pumping
  • Pumping from inaccessible locations
  • Low noise
  • Additional cooling by surrounding liquid

The first two items are the most important b points of submersible pumps as compared to others. Specific nature of the challenging tasks performed by submersible pumps makes other pumps unsuitable for these applications. Deep-well pumping by surface pumps is either impossible due to insufficient self-priming capacity and cavitation effect or requires an overly complicated and expensive surface pump. A submersible pump has a key advantage in this case since it is easier to push the liquid up than to pull it.

It is worth noting that these pumps are highly efficient for evacuating fluids from pits, wells, cellars and the like. The design of submersible pumps makes them mobile and suitable for any depths which may only be limited by the head developed by the pump and power cable length. Their capability to handle even heavily contaminated media makes them convenient for pumping liquid from the near bottom layer of different reservoirs and tanks where sludge and other deposits may accumulate.

Submerged operation provides for additional advantages improving some characteristics of pumps. Thus, vibration generated by the pump and respective noise are partially absorbed by the fluid reducing the overall noise level of the pump. It is a simple pattern that the deeper the pump is submerged and the greater the fluid volume, the higher the noise absorption. Another benefit of submerged operation is additional cooling of the pump. This is explained by the fact that the heat exchange rate is much higher in pump/water system than in pump/air system.

However adaptation of pumping equipment for such special applications inevitably results in some disadvantages. Some of them may be remedied by using new technologies and materials, while others may just be reduced to an acceptable level.

Application

Submersible pumps cover quite a significant scope of application from household water supply to mining industry. Submersible well pumps are usually used for water supply in the locations without centralized water supply for water intake from wells. If there are no water-bearing strata at shallow depths, but there is a possibility to lift artesian water from a downhole, more sophisticated deep-well pumps are used. This is complicated because water has to be lifted from significant depths (over 100 m).

Well and downhole pumps are also used in mining industry. They are good for evacuating underground water from wells and mines. Besides, deep-well pumps may be suitable for extracting rare earth elements by leaching: special leach solutions are used to liquefy valuable ore components, which are then pumped out and target metal is recovered.

Drainage and sewage pumps capable of pumping liquid from the bottom layers where there is a high concentration of various kinds of solids are also suitable for a wide range of applications. In household application they are used to evacuate water from swimming pools or flooded cellars, while cutter pumps are good for waste pits and water reservoirs. Drainage pumps are used for similar tasks during natural disasters and accidents when dewatering of submerged premises is required.

In wastewater disposal and sanitation sewage pumps are also widely used. They are well-suited for both the sewerage system of a country house and the sewage pumping stations, where the flow rate is much higher. The advantage of the drainage and sewage pumps in this case is their ability to evacuate heavily contaminated media from deep waters.

Semi-submersible pumps

General description and classification

A semi-submersible pump is a very broad concept encompassing a host of types of pumping units of different designs. In terms of spacial arrangement semi-submersible pumps are vertical pumps due to the vertical position of impeller shaft (in the case of centrifugal pumps). According to the principle of operation they may be split into positive displacement and dynamic pumps depending on availability of different operating elements, such as: centrifugal wheels, screws, etc.

A particularly broad classification of semi-submersible pumps is by application as the tasks of liquid evacuation or tank emptying are in high demand everywhere from food industry to oil production. Semi-submersible pumps using the principle of operation of regular surface pumps are gaining the same popularity. Thus, there are oil semi-submersible pumps, drainage, chemical pumps, etc.

Design

Semi-submersible pumps have some elements inherent in all of their types. Such elements include: an electric motor, an operating element and a vertical cylindrical casing (submersible tube). The motor and operating element are installed on different ends of the cylindrical casing, which is partially submerged in liquid. With such arrangement the operating element is immersed into the pumped fluid and the electric motor is above its surface. The casing houses the shaft transmitting rotational motion from the motor to the operating element. The shaft is connected to the output spindle through a flexible coupling.






A centrifugal or vortex wheel, an eccentric screw or a piston may be used as the operating element. Thus, the principle of operation of a semi-submersible pump may be both positive displacement and dynamic. Such diversity allows these pumps to handle  a wide variety of liquids of various degree of contamination and different flow rates. The functionality of a semi-submersible pump is determined by the type of the operating element in use.

Semi-submersible pumps are usually fastened using a mounting plate bolted to the cover of the tank with the pumped liquid. The plate is rigidly connected to the pump casing. In case of small semi-submersible pumps the mounting plate may be dispensed with.

Advantages and disadvantages

As an intermediate type between submersible pumps and surface pumps, a semi-submersible pump combines the advantages of both types, but in most cases it is unable to perform specific tasks for which the first two types of pumps have been developed. At the same time such an intermediate position makes semi-submersible pumps capable to deal more effectively with a number of other tasks.

As compared to the surface pumps the following benefits may be named:

  • efficient arrangement for evacuating fluids from tanks
  • absence of a long suction tube
  • always operate on flooded suction

When using a semi-submersible pump there is no need to install a long suction tube to lift the liquid from tank bottom. Thus, there is no problem of cavitation and significant self-priming force. Besides, in case of semi-submersible pumps with centrifugal wheels additional vacuum pumps for the primary filling of the working chamber may be dispensed with since the operating element is already immersed into the liquid.

A semi-submersible pump may be considered a regular surface pump of a vertical design with an outboard operating element. With such arrangement major part of the pump is located inside the fluid, while only the motor is installed vertically above the surface occupying significantly less space. In some cases this allows you to save space and simplifies the installation.

These advantages are offset by metal consumption and complexity of design of semi-submersible pumps versus surface pumps.

As compared to submersible pumps the following advantages are worth noting:

  • Significantly simpler design
  • No need for complete sealing of the pump
  • Simpler maintenance and repair

Since the most vulnerable part of the pump (electric motor) is located above the liquid, there is no need for complete sealing of the pump or tight arrangement of parts inside the casing. This reduces greatly the manufacturing complexity and the price of the pump. The above-surface arrangement of the motor engine also facilitates its repair and maintenance for there is no need not to extract the entire pump. However this still has to be done to repair the operating element.

Application

The design and distinctive features of semi-submersible pumps determine the area of their effective application. Most often they are used for the evacuation of fluids from tanks and reservoirs of a fixed depth, as well as from drums and cisterns. When working on relatively shallow depths all the benefits of submersible pumps are available without significant complication of design and maintenance.

Fluids are pumped from large tanks and reservoirs by semi-submersible pumps which are rigidly secured using a mounting plate, while for the evacuation of fluids from small tanks more compact drum pumps are used with a simpler fastening principle or no fastening at all.

Semi-submersible pumps are used everywhere as in almost every industry there is a need to empty storage tanks or move liquids or viscous substances. Just a few examples are oil, chemical, pharmaceutical and food industries, water supply and disposal, as well as cleaning and neutralization of hazardous substances and pollutants. Besides, this type of pumps is used for handling liquid and viscous media. For example, to empty barrels and railway tanks.