Sump and submersible pumps

So what exactly is the difference between a sump pump and a submersible pump?

It can be a little confusing, but is quite simple really.

In essence a submersible or immersible pump is one which allows the entire assembly (including the electric motor) to be placed under water, whereas a sump or pedestal pump keeps the electric motor above water level.

Both are essentially used for the same purpose: to raise fluid to a higher level without the risk of cavitation (see our earlier post on cavitation) caused by differing pressures.

Single stage pumps are used for drainage, sewage pumping, general industrial pumping, slurry pumping and are even popular with aquarium filters.

Multiple stage submersible pumps are typically lowered down a borehole or well for water extraction.

A sump pump (or pedestal pump) is used to remove water that has accumulated in a sump pit; a hole used to collect water which can enter via the drainage network. The motor is not submerged and remains above the liquid.

In some cases, a sump pump is used when a lower floor is below the sewer lines, to pump greywater or blackwater waste from that floor to the sewer lines.

For a copy of the ecex leaflet please click Pumps, valves & pipework

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The Booster Pump

Booster Pumps are centrifugal pumps used to increase the pressure of the liquid already flowing from one place to another in a pipeline.

Most booster pumps are centrifugal pumps. Relying upon one or more impellers to draw the pumped fluid through the intake of the pump, the water is then boosted to increase the pressure as it flows through the impeller, then the volute or diffuser casing.

Some are single-stage booster pumps. In other words, they have a single impeller and are generally used in applications where the amount of additional pressure (head) required is not significant.

Multi-stage booster pumps with more than one impeller are used to deliver higher heads. An example of this increased requirement is when water needs to be transferred to substantially higher points within a building, or through much longer pipelines.

A booster pump should never be connected directly to the mains incoming line, but will instead draw water from the “break tank” which is filled under normal mains pressure (1 or 2 bar.

This pump is not designed to hold the pressure of the rising main which increases by 1 bar per floor. It is usually combined with a pressure vessel or hydraulic accumulator.

The pressure vessel consists of an inflatable diaphragm (imagine bladder or inner tube) within a sealed tank  which is set to operate at the required system pressure.

Any failure of this bladder will cause the pump to run continuously and will usually result in a loss of performance at higher levels.

For a copy of the ecex leaflet please click Pumps, valves & pipework

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What is cavitation in pumps?

Cavitation occurs when cavities or bubbles form in the liquid being pumped; a process which will directly affect capacity, head pressure and efficiency.

The cavities (bubbles) will also collapse within the higher pressure areas and cause noise, vibration and mechanical damage to components. 

This damage was caused by imploding air bubbles

Pump cavitation is an audible implosion of the gas bubbles due to suction pressure differences within the pump.

The five basic reasons for cavitation are:

Vaporisation – fluid vaporises when its pressure becomes too low, or its temperature is too high.

Air ingestion – a centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous. Air must be removed from the system using AAV’s etc.

Internal recirculation – as the name implies, the fluid recirculates increasing its velocity until it vaporises and then collapses in the surrounding higher pressure.

Flow turbulence – corrosion or obstructions can change the velocity of this liquid, and any time you change the velocity of a liquid, you change its pressure.

Vane passing syndrome – this type of cavitation damage is caused when the OD of the impeller passes too close to the pump cutwater.

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Causes of pump failures

For a little while now we’ve been discussing different types of pumps.

Recently, we’ve had a lot of requests to discuss the most common pump failures and the reasons behind them. We thought we’d take a little break from our ‘types of pumps’ posts to expand a little.

Please click for a schematic cause and effect – Anatomy of pump failure

Pressure – a restriction of the pump’s suction can result in cavitation of the pump. Root causes can include a plugged suction strainer or a possible valve issue.

Cavitation – as liquid pressure falls below its vapour pressure, bubbles form and implode on impellers and interior surfaces, damaging pump internals,  disrupting flow and leading to seal failure.

Temperature – a blocked discharge can result in fluid stagnation causing a spike in temperature.  This can also result in cavitation, damaging pump internals and seal failure. Inadequate monitoring of standby pump temperature can also result in damage when a cold pump is put into hot service.

Level – inadequate monitoring of auxiliary seal flush levels can result in:

• Missed low level conditions, indicating a loss of flush.
• High level conditions, indicating mechanical seal leakage and eventual flaring.

Leakage – leaks caused by mechanical failures can be catastrophic. Early detection of abnormal conditions such as cavitation, pressure imbalance or excess vibration can help avoid leaks and their consequences.

Vibration – there are several causes of vibration which can damage seals or internals and cause pump failure. These can be as simple as incorrect bolt down or more complex system failures.

Installation – improper installation can lead to shaft misalignment, excessive vibration leading to pump damage and possible failure.

For a copy of the ecex leaflet please click Pumps, valves & pipework

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Circulator Pumps

As the name suggests, circulator pumps are used to move water around a system.

They can be used within a building’s HVAC systems (i.e. chilled water circulation, hot water circulation, potable water circulation). Another important use for these kinds of pumps is to  circulate cooling water in process plant.

Because they only circulate liquid within a closed circuit, they only need to overcome the friction of a piping system and are usually electrically powered centrifugal pumps.

Small to medium sized circulator pumps are usually supported entirely by the pipe flanges that join them to the rest of the system; large pumps are usually pad-mounted.

Bronze circulator pump

Pumps that are used in closed systems can be made with cast iron components as the water in the loop will either become de-oxygenated or be treated with chemicals to inhibit corrosion. Where there is a steady stream of oxygenated, potable water flowing through then more expensive materials such as bronze will be used.

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Axial Flow Pumps

An axial flow pump is a type of centrifugal pump that uses an impeller with vanes to direct the flow axially.

Axial: adj – relating to, forming, or characteristic of an axis, situated in, on, or along an axis

In this way, they differ from most other centrifugal pumps, which direct water flow radially.

Radial: adj – emanating from a common central point; arranged like the radii of a circle; of, like, or relating to a radius or ray; spreading out or developing uniformly on all sides.

In general, axial flow pumps create less pressure (head) than radial flow centrifugal pumps, but they can produce much higher flow rates.

Axial flow movement through a pump

This type of pump usually consists of an impeller with a smaller number of vanes, typically only three or four which are arranged in such a way that the pumped fluid exits axially (i.e. in the same direction as the shaft), rather than radially (90 degrees from the shaft). The impeller is normally driven by an electric motor and the orientation of the vanes produces very low head as the liquid is pumped.

They are useful as circulating water pumps

For a copy of the ecex leaflet please click Pumps, valves & pipework

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Centrifugal Pumps

A pump? Literally “A machine or device for raising, compressing, or transferring fluids”.

Centrifugal pumps are the most common type of pump. A centrifugal pump uses one or more impellers attached to and rotating with the shaft providing the energy that moves liquid through the pump.

This movement also pressurises the liquid to move it through the piping system.

A centrifugal pump works by directing the liquid in the system into the suction port of the centrifugal pump and from there into the inlet of the impeller.

The rotating impeller moves the liquid along the spinning vanes, increasing its velocity energy into the pump volute or diffuser casing.

This is where the high velocity of the fluid is converted into high pressure through a diffusion process to be guided into the outlet port.

From there it is moves into the system, or on to the next stage in the case of a multi-stage centrifugal pump.

For a copy of the ecex leaflet please click Pumps, valves & pipework

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