Axial thrust balancing

The successful combination of high-quality slide bearings and comprehensive axial thrust balancing demonstrates the quest for absolute precision. This results in HERMETIC canned motor pumps running without contact and wear. Manufactured and tested to the highest quality standards in the factory, HERMETIC pumps guarantee long service life (MTBF) when used properly. Systems integrated into the pump, such as the MAP, monitor the position of the rotor and thus the axial thrust balancing. This ensures reliable operation of the entire system.

Axial thrust is a challenge that must be met in almost all centrifugal pumps. The pump shaft is drawn in the direction of the suction side due to the pressure difference between the pressure and suction side. For example, bearings must absorb the resulting forces. Slide bearings are used in hermetically sealed pumps, such as magnet-coupled or canned motor pumps. This leads to specific requirements for axial thrust balancing.

The specific properties of the media that requires conveying exclude the use of mechanical thrust bearings in hermetically sealed pumps. Axial thrust balancing can therefore only be achieved by the hydraulic balancing of the rotor.

In HERMETIC canned motor pumps, the axial position of the pump shaft is controlled automatically during operation. The applied technology automatically creates a powerless state of equilibrium. No axial forces act on the thrust bearing collar of the slide bearings. A thin liquid film with high load-bearing capacity similar to aquaplaning between the rotating and the static part of the slide bearing causes the entire rotor to float.

This state of equilibrium, the so-called HERMETIC ZART^® principle (Zero Axial and Radial Thrust), results from the fact that there is no contact between the rotating and stationary components. The non-contact running leads to wear-free normal operation with much longer service life (MTBF) compared to other designs.

The technical design of axial thrust balancing depends on the size, number of stages and the medium to be conveyed of each pump used.

Methods of axial thrust balancing

The hydraulic balancing system of canned motor pump series CN/CNF/CNK for the chemical industry, with hydraulic systems according to ISO 2858, consists of two main components: a fixed and a variable throttling device (Fig. 1).

The interaction of a fixed throttling device (labyrinth gap) on the outer diameter of the impeller and a variable throttle in the region of the impeller hub creates axial thrust balancing. When the rotor shifts axially from the equilibrium position, the pressure in the pressure compensation chamber changes due to the valve action of the variable throttle gap and counteracts the displacement of the rotor self-regulating.

The hydraulic balancing device of HERMETIC series CNP/CNPF/CNPK (according to API 685) and of multi-stage HERMETIC pumps (CAM series) is based on a variable throttling device on the cam disc. Depending on the axial position of the pump shaft, the pressure in the control chamber changes due to the valve action of the variable throttle gap and thus counteracts the axial thrust of the rotor (Fig. 2).

Use of magnetic support bearings

For particularly critical applications and especially for canned motor pumps in vertical installation, magnetic support bearings are used in addition to the proven axial thrust balancing systems. In normal operation, a cam disc will hydraulically balance the axial forces. When operating conditions are unstable or change, states may develop frequently that do not ensure complete axial thrust balancing. Examples are the operation of systems with frequent starts and stops, low speeds (such as operation with frequency converter) or extremely low viscosity of the conveying fluid (liquid gases, low boilers). If in these cases the hydraulic control system is too sluggish, or the restoring torque is not strong enough, magnetic support bearings will be used as well. The magnet systems balance the rotor weight, hold the pump shaft in position and thus the axial thrust is compensated by magnetic forces even at a standstill and in unstable modes.

These systems have been in use at HERMETIC since 2001 and more than 400 HERMETIC pumps have been equipped with magnetic support bearings.

Put to the acid test

Quality is our top priority. Therefore, we test every pump produced by HERMETIC in our new high-tech test bench before delivery. We check, record and log the following: characteristic curve, power requirement, NPSH3 value, HERMETIC ZART® principle and the hydrodynamic restoring forces as well by using a specially developed measuring device. When all quality standards guarantee the non-contact operation, the pump leaves our factory.

Monitoring the axial thrust

Each canned motor pump used is the heart of a highly sensitive conveying system. The highest priority is their reliability and safety. To ensure reliability and safety over many years of use, HERMETIC has developed a series of special monitoring systems. One of these monitoring systems, we would like to introduce in this issue.

MAP Monitor Axial Position:

Monitoring the position of the rotor and axial thrust balancing for early detection of possible sources of faults. MAP is a measuring device based on the LVDT principle (Linear Variable Differential Transformer) for non-contact monitoring of the axial shaft position in a pump. MAP consists of a displacement sensor with supply cable and evaluation electronics accommodated in a glass-fibre reinforced polyester housing with IP 65 protection. A measuring pin with ferromagnetic core is located at the shaft end of the pump that requires monitoring. A sensor assembly records the movement of the pump shaft with high accuracy. The supplied electronic system evaluates the sensor signal that can be forwarded then to the control room where warning and shut-off values can be programmed. This MAP system offers every plant operator / owner practical advantages: The rotor position can be determined reliably even when the pump is at a standstill and during operation with frequency converter.