pumps and pumping systems  
 
 

2018-06-11

Pump drives for wastewater working in difficult conditions.

Sewage pumps may be equipped with electric drives with various degrees of protection, the classification of which is regulated in European countries by EN 60529 standard. The choice of the pump drive results mainly from the installation in which the pump works. It concerns also pumps with similar design as sewage pumps but designated for pumping other media for example: raw water, rain water etc. All the above mentioned pumps are produced and delivered to Vietnamese market and for over 44 other markets all over the word by Polish company Hydro-Vacuum S.A.
The basic design ways of above mentioned pumps can be classified as follows:

  • Dry vertical or horizontal– pumping aggregate (pump with electric motor) work outside the pumped medium, air cooled drives with protection class IP55 or lower are applied
  • wet – pumping unit (pump and el. motor) work submerged in the pumped medium to an appropriate level, el. motors with protection class IP68 are applied.
  • mixed allows for pumping unit operation both submerged in the pumped medium, emerged or in dry installation, electric motors with protection class IP68 with internally built in cooling system. Cooling system can use both pumped medium or coolant in the internal closed circuit (the best solution from the point of view of optimal operation).

Air cooled el. motors IP 55 can be used quite narrowly, only in dry pumping stations, eg with a separate chamber for pumps or in dry modern pumping stations, such as pumping stations with solids separation, where there is no risk of wetting or flooding of electric motor.

IP68 electric motors they are the most common drives in sewage pumps. The condition of proper operation of such engines is their total or at least partial immersion in the pumped liquid. The heat is transferred to the liquid as a result of free convection through the walls of the pump and the electric motor casing, which are in contact with the pumped and surrounding pumping unit medium.

Drives with a jacket and an autonomous IP68 cooling system with an external cooling system are relatively rarely used in pumps designed for dry pumping stations. In this solution cooling jackets are applied. Coolant, usually oil or a mixture of water and glycol, is supplied to the jacket from the outside. The circulation of liquid in the system is enforced by an additional circulating pump that pumps liquid along the electric motor casing. The heated liquid is directed to the fan cooler to cool down. The disadvantage of this solution is the need to build an external cooling system consisting of: pump, radiator, fan, wires, control system.

Drives with a jacket with an internal cooling system, cooled by pumped liquid, IP68 are used for work in dry or dry pumping stations that can be flooded. The drive is enclosed with a jacket, and the cooling system is built into a pump unit. The engine is cooled by forcing circulation of the pumped medium inside the jacket. The pressure generated in the pump is used to create the pressure that drives the liquid in the cooling system. The cool liquid is pumped into the jacket by pressure lines and then, after heating, it flows into the impeller`s hub, where the pressure is low. The cooling of the liquid takes place by mixing heated and pumped medium. The solution causes operational problems if the pumped medium is not clean enough and causes contamination of the liquid supply lines to the jacket of electric motor. Pumps equipped with this type of drive require careful monitoring and periodic cleaning of the cooling system.

Drives with a jacket with an internal cooling system, circulating liquid in a closed circuit built into the pump unit IP68 - "Cool-me-in" system are used in pump units designated for dry pumping stations, especially in conditions when there is a possibility of flooding the pumping station, as well as in wet pumping stations with significant fluctuations of the liquid level and its lowering to a level that does not allow for proper cooling of electric motor mounted in the pump aggregate. It is the most innovative and universal pump drive. It eliminates the disadvantages of previously presented drive solutions with internal and external cooling system. The movement of liquid in the internal circuit of the electric motor body is forced by an additional impeller with high energy efficiency mounted on the pump shaft. The coolant that receives heat from the electric motor body is a mixture of glycol with water or Vaseline oil. The coolant is not in contact with the pumped medium, it is clean, therefore the channels in which it circulates are not contaminated. Heated liquid is pumped along the wall separating the internal system from the pump's working chamber. The liquid gives the heat to the walls, through which it penetrates into the medium pumped by the pump, and then it is directed back to the jacket. An innovative drive developed at Hydro-Vacuum S.A. using CFD numerical calculation methods, allowed to optimize shapes and flows in the internal cooling system. This led to an intensification of heat exchange while maintaining very low energy losses for liquid pumping stations in the internal circuit An innovative cooling system was developed in which the required cooling conditions of electric motors are maintained with the minimum energy expenditure. For example: for a 55 kW el. motor, it was developed a system that provides internal cooling using an impeller with power consumption not exceeding 150 W. Thus, the final efficiency of the pump unit with this cooling system decreased only by 0.3%, which is not important value, especially from the point of view of operation simplicity. Implemented for the production of Hydro-Vacuum S.A. drives were tested using a thermal imaging camera and PT100 sensors. The tests were carried out at full load for the temperature the pumped liquid 40 ° C during continuous operation. The thermal condition of electric motor stabilized after about 6 hours of continuous operation. The temperature of the external walls of the el. motor jacket was higher than the temperature of the pumped liquid by ~ 25 ° C and was approximately 55 ° C. The highest temperature was recorded in the el. motor windings and it was 93 ° C (permissible temperature 140 ° C). Such temperatures give the electric motor safety in all environmental conditions.



Summary
The choice of pump drive for wastewater depends on the type of pumping station. For wet pumping stations, drives without additional cooling systems are applied, which results in the necessity to keep the right amount of liquid in the tank. A much more flexible alternative to this solution are drives with an internal, preferably closed cooling system, adapted to work in both dry and wet conditions. Among the available solutions, they are the safest, comfortable in operation and do not require building any additional

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