Centrifugal pumps are considered quite robust, and account for up to 90% of the pumps used in the CPI. In contrast to displacement pumps, they are suitable for media containing solids, which are frequently encountered in CPI facilities. Among centrifugal pumps’ main benefits are their ability to function without closed pumping spaces and the fact that they can work without valves. Their functional principle is based on hydraulics, with an impeller, a casing and the seal and bearing unit. The task of the sealing system is to seal off the rotating shaft against the surrounding environment. The selection and combination of a suitable seal and bearing unit is highly important. The important factor here is the hazard potential that the pumped media represents for the surrounding environment.
For non-hazardous media, suitable seals include the following: stuffing-box packings made from various materials and lubricated with water, grease or graphite; hydrodynamic seals with downstream standstill seals; or labyrinth seals that are open to the environment and have pumping and blocking fittings.
When using shaft seal rings and simple mechanical seals, which are typically lubricated with the product to be sealed against, serious problems can result when there is contact with difficult media. Double mechanical seals can be used, but only if they are isolated and lubricated with a medium that is non-hazardous for the surrounding environment and compatible with the pumping medium. Canned motors and magnetic couplings are likewise in use, where the bearing of the shaft is designed as a plain bearing running in the medium, and where no shaft feedthrough is present.
For media with moderate to high hazard potential, the following sealing systems are suitable: double mechanical seal; magnetic coupling; and canned motor. In the case of mechanical seals, double mechanical seals are differentiated from liquid-sealed or gas-sealed versions. Despite major disadvantages, liquid-lubricated seals are used frequently. The disadvantages include the complex, but necessary, sealing systems, which incur higher maintenance costs. More difficult pumping tasks are performed by another variant, which has proven particularly effective with vertical pumps: the gas-lubricated mechanical seal. These seals are suitable because of the simple gas supply, and are characterized by low operating and maintenance costs. In terms of safety, and particularly where there is a risk of materials escaping into the atmosphere, magnetic couplings and canned motors are the preferred choice.
HERMETICALLY SEALED PUMPS
Pumps with canned motors and magnetic couplings are hermetically sealed pumps, with a comparatively reliable sealing. A rotating magnetic field is used to transfer the torque through the closed, thin, typically metallic wall to the pump shaft. This element is also described as the containment cup or can. In the canned motor pump, the pump and motor form a single unit. The rotor and impeller are mounted on a single shaft. In contrast to the double mechanical seal (which is sealed using a separate medium), the canned motor pump does not operate independently of the pumping medium. A further disadvantage of canned motor pumps is that in order to pump gas-laden or magnetizable media and solids, certain additional measures must be included, such as external flushing.
The aspects of energy efficiency and lifecycle costs highlight further severe disadvantages. From an energy-efficiency perspective, the canned motor pump is an obsolete model. Its overall efficiency is typically unsatisfactory by today’s more rigorous standards. Over 30% of the drive energy is lost through heat generation, or heats the pumping medium. One reason for these poor values is the larger gap between the stator and the rotor. The eddy-current losses in the can and friction losses of the rotor in the pumping liquid are additional factors contributing to these pumps’ inefficiency. Apart from the high energy losses from induced eddy currents (in the containment cup or can) and viscosity effects, the high maintenance and operating costs are an important factor when considering lifecycle costs. Compared to the procurement and installation costs (30% of the overall costs)
MAGNETIC COUPLING FUNCTIONS
The magnetic coupling is characterized by torque transmission without shaft feedthrough. Up to 15% of the drive energy is lost through eddy current losses in the metallic containment cup or through friction losses of the internal magnet rotor in the pumping liquid. Worth emphasizing, however, is the lower heat transmission into the pumping medium when compared to canned motor pumps. Particularly with media that have critical boiling points, the relatively high heat transmission from the motor to the pumping medium, which is typical of canned motor pumps, can lead to complications. If the efficiency of the drive motor of the magnetically coupled pump with a metallic containment cup is incorporated into the analysis, an efficiency advantage of approximately 5% is achieved for the magnetically coupled pump when compared to the canned motor pump.
Media with critical boiling points and a proportion of solids represent a further area of consideration. Solids impair the plain bearing and containment cup, and can even disconnect the can. The greatest hazard potential for magnetic couplings relates to the fracture of the containment cup. If fracture occurs, liquid under pumping pressure can then escape into the atmosphere unimpeded.
When the liquid is directed through the bearing arrangement in a magnetic coupling configuration, the flushing flow is directed either from the outer diameter of the impeller through the casing wall, or from the pressure port to the bearing arrangement. The prerequisite here is that the difference between the flushing pressure (PD) and the suction pressure (PS) must be sufficiently high to generate an adequate flushing flow to transport away the heat. The flushing pressure of the bearing is higher than the boiling pressure of the pumping medium, and should be higher than the suction pressure of the bearing.
The heat balance also must be considered. Eddy-current losses and hydraulic losses increase the discharge temperature of the pumping liquid. This heating of the medium must be transported away with the pumping flow; the minimum flowrate is calculated from this.
In order to secure against the risk of a containment cup fracture, double-walled containment cups with leakage sensors or sealing systems downstream of the containment cup are used. By contrast, the canned motor is a secure hermetic seal, because after a can fracture, the liquid is first caught by the can, and does not escape into the atmosphere.
Through the use of appropriate sealing or flushing liquids, which are fed into the bearing area, and through complete monitoring of the operating parameters, every hermetically sealed pump can also be equipped for pumping difficult pumping media, such as media containing solids
For non-hazardous media, suitable seals include the following: stuffing-box packings made from various materials and lubricated with water, grease or graphite; hydrodynamic seals with downstream standstill seals; or labyrinth seals that are open to the environment and have pumping and blocking fittings.
When using shaft seal rings and simple mechanical seals, which are typically lubricated with the product to be sealed against, serious problems can result when there is contact with difficult media. Double mechanical seals can be used, but only if they are isolated and lubricated with a medium that is non-hazardous for the surrounding environment and compatible with the pumping medium. Canned motors and magnetic couplings are likewise in use, where the bearing of the shaft is designed as a plain bearing running in the medium, and where no shaft feedthrough is present.
For media with moderate to high hazard potential, the following sealing systems are suitable: double mechanical seal; magnetic coupling; and canned motor. In the case of mechanical seals, double mechanical seals are differentiated from liquid-sealed or gas-sealed versions. Despite major disadvantages, liquid-lubricated seals are used frequently. The disadvantages include the complex, but necessary, sealing systems, which incur higher maintenance costs. More difficult pumping tasks are performed by another variant, which has proven particularly effective with vertical pumps: the gas-lubricated mechanical seal. These seals are suitable because of the simple gas supply, and are characterized by low operating and maintenance costs. In terms of safety, and particularly where there is a risk of materials escaping into the atmosphere, magnetic couplings and canned motors are the preferred choice.
HERMETICALLY SEALED PUMPS
Pumps with canned motors and magnetic couplings are hermetically sealed pumps, with a comparatively reliable sealing. A rotating magnetic field is used to transfer the torque through the closed, thin, typically metallic wall to the pump shaft. This element is also described as the containment cup or can. In the canned motor pump, the pump and motor form a single unit. The rotor and impeller are mounted on a single shaft. In contrast to the double mechanical seal (which is sealed using a separate medium), the canned motor pump does not operate independently of the pumping medium. A further disadvantage of canned motor pumps is that in order to pump gas-laden or magnetizable media and solids, certain additional measures must be included, such as external flushing.
The aspects of energy efficiency and lifecycle costs highlight further severe disadvantages. From an energy-efficiency perspective, the canned motor pump is an obsolete model. Its overall efficiency is typically unsatisfactory by today’s more rigorous standards. Over 30% of the drive energy is lost through heat generation, or heats the pumping medium. One reason for these poor values is the larger gap between the stator and the rotor. The eddy-current losses in the can and friction losses of the rotor in the pumping liquid are additional factors contributing to these pumps’ inefficiency. Apart from the high energy losses from induced eddy currents (in the containment cup or can) and viscosity effects, the high maintenance and operating costs are an important factor when considering lifecycle costs. Compared to the procurement and installation costs (30% of the overall costs)
MAGNETIC COUPLING FUNCTIONS
The magnetic coupling is characterized by torque transmission without shaft feedthrough. Up to 15% of the drive energy is lost through eddy current losses in the metallic containment cup or through friction losses of the internal magnet rotor in the pumping liquid. Worth emphasizing, however, is the lower heat transmission into the pumping medium when compared to canned motor pumps. Particularly with media that have critical boiling points, the relatively high heat transmission from the motor to the pumping medium, which is typical of canned motor pumps, can lead to complications. If the efficiency of the drive motor of the magnetically coupled pump with a metallic containment cup is incorporated into the analysis, an efficiency advantage of approximately 5% is achieved for the magnetically coupled pump when compared to the canned motor pump.
Media with critical boiling points and a proportion of solids represent a further area of consideration. Solids impair the plain bearing and containment cup, and can even disconnect the can. The greatest hazard potential for magnetic couplings relates to the fracture of the containment cup. If fracture occurs, liquid under pumping pressure can then escape into the atmosphere unimpeded.
When the liquid is directed through the bearing arrangement in a magnetic coupling configuration, the flushing flow is directed either from the outer diameter of the impeller through the casing wall, or from the pressure port to the bearing arrangement. The prerequisite here is that the difference between the flushing pressure (PD) and the suction pressure (PS) must be sufficiently high to generate an adequate flushing flow to transport away the heat. The flushing pressure of the bearing is higher than the boiling pressure of the pumping medium, and should be higher than the suction pressure of the bearing.
The heat balance also must be considered. Eddy-current losses and hydraulic losses increase the discharge temperature of the pumping liquid. This heating of the medium must be transported away with the pumping flow; the minimum flowrate is calculated from this.
In order to secure against the risk of a containment cup fracture, double-walled containment cups with leakage sensors or sealing systems downstream of the containment cup are used. By contrast, the canned motor is a secure hermetic seal, because after a can fracture, the liquid is first caught by the can, and does not escape into the atmosphere.
Through the use of appropriate sealing or flushing liquids, which are fed into the bearing area, and through complete monitoring of the operating parameters, every hermetically sealed pump can also be equipped for pumping difficult pumping media, such as media containing solids
