Innovations
in sealless pumps
Dieter
Hohenstein discusses how to maintain a balance between operating cost
and overall efficiencies
Sealless
pumps are the most commonly used pumps in the chemical and process industry,
besides the conventional chemical pumps with a seal system. The use
of sealless pumps enables industries to improve the safety of the process.
This trend has been enforced by a recently introduced environmental
legislation.
Today, the live cycle cost has also become an important additional decision
criterion. This write-up provides some information on how the operating
cost of magnetic drive and canned motor pumps can be reduced while increasing
the reliability and improving maintenance at the same time. We have
seen a continuous improvement in the design and quality in the pump
industry in the past years to that accommodate the users requirement.
Sleeve bearings
In sealless pumps, like in any other rotating equipment,
the bearings are one of the main design components, since they influence
the reliability as well as the maintenance of pumps.
The bearings have to fix the rotor in the axial and radial position.
They also have to carry hydraulic and mechanical loads, under the condition
that unallowable corner pressure is avoided.
The sleeve bearings have to exhibit corrosion resistance against all
types of liquids in the chemical industry. The wear caused due to dirt
particles should be kept to a minimum. By reduced liquid lubrication,
as by liquefied gases due to low viscosity, the bearings have a sufficient
safety margin on their bearing surface to avoid damage. According to
recent technology, silicon carbide (SiC) is the most common bearing
material in sealless pumps.
Due to the extreme resistance in the full pH range and very high hardness,
silicon carbide, in its pure sintered condition, seems to be the ideal
material. But is has some disadvantages in comparison to metal. As seen
in (Table 1) it has a low tensile strength and a lower linear thermoexpansion
than metal. In an ideal situation, the pump would need to have a silicon
carbide sleeve on its shaft as well. But in this case, the low temperature
expansion and the low tensile strength in SiC has to be solved in order
to avoid cracking of the sleeve under various harsh operating conditions.
The solution is a patented bearing design concept. The SiC bushings
are not directly mounted on the shaft. They are mounted on special clamp
rings of metal, which are fixed onto the shaft. With this solution,
the shaft is able to expand under temperature four times that of the
silicon carbide sleeve without causing any damage. This alone is not
sufficient— the sleeve has to be fixed in axial direction as well.
Therefore, the metal clamps have a spring function in axial direction.
This means that under all operating conditions, including maximum expansion
of the shaft under hot conditions, the bearing parts are always secured
in the axial direction.
The special shape of the bearing sleeves, together with the metal clamping
elements allows for accurate adjustment. This results in the ceramic
sleeve never reaching the critical stress values. The advantage of this
design has been proven in practical operation and shows a long life
in the temperatures ranging from -50 to +300oC.
This simple arrangement offers easy maintenance. At the ceramic bushing,
no grooves or key ways will be found, as these could be the starting
point for cracks.
Of further importance for the function of the bearing is the proper
venting before and during the operation to avoid the critical dry running
conditions.
A positive defined flow of liquid, which also functions as a lubricant,
has to be established. There are two patterns of flow for lubrication,
venting and cooling of the magnets. To guide the venting of gas, which
normally is concentrated close to axis, a central hole leads to the
shaft centre to the suction of the impeller. This concept is a proven
success and has set new standards in MTBF(Mean Time Between Failures)
values.
....CONTD
