Designing a New Chemical Pump for the Global Market

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The wet shaft design provides low shaft deflection, i.e. higher shaft stiffness. In the pump industry, the shaft stiffness is characterized by L3/D4. The parameter ‘L’ is the distance between the impeller outlet and the nearest radial bearing; ‘D’ stands for the diameter of the shaft in the area of the shaft seal. The average value of the Mega CPK type series is 2.23 mm-1, which is in the range of API 610 pumps (heavy-duty centrifugal pumps for the petroleum, petrochemical and natural gas industries). In the dry shaft design, a sleeve protects the shaft from the fluid handled.

This means that even carbon steel shafts can be used for highly corrosive applications, resulting in lower costs than for a shaft made of a duplex material. Both options described above exceed the requirements of ISO 5199 with regard to shaft deflection. Since shaft deflection has a major impact on the lifetime of mechanical seals, this generates more operational reliability. Shaft deflection also has an impact on the leakage rate of mechanical seals.

How to Achieve Reliable Pump Performance

The reliability of a pump is influenced by many parameters, which should be taken into consideration. The relevant features and their positive impact are given below:

Optimized stiffness of bearing bracket: Vibration is less than stipulated by ISO 5199. Bearing and mechanical seal life is enhanced since higher vibrations increase the likelihood of fatigue failure.

Fully confined gasket: Prevents wear in cases where solids are pumped by eliminating erosion of the gasket surface exposed to the fluid. In addition, assembly is easier as the gasket does not need to be aligned and extrusion is avoided.

Single-piece bearing bracket: Avoids misalignment by reducing interfaces between different parts.

Optimized axial thrust balancing: Generally speaking, two possibilities exist to reduce axial thrust caused by different pressure profiles at the front and the rear impeller shroud. One option is back vanes; the other is the use of balancing holes in conjunction with a throttling device. Investigations have shown that back vanes can produce high pressure pulsations with relatively high pressure peaks. These peaks have a direct impact on the mechanical seal. Also, pressure peaks are equivalent to axial thrust peaks, which could damage the axial thrust bearings.

Larger conical seal chamber: Experience gathered in slurry applications has shown that a large conical seal chamber decreases wear from solids-laden fluids and improves venting.

Forces and moments on flanges/ branches: ISO 5199 requirements are substantially exceeded due to stiff design. This reduces coupling misalignment (safety, lifetime) and mechanical bearing losses, which could result in reduced efficiency.

A view of the shop floor of KSB Irrigation and Process Division in Pimpri (Picture: KSB Pumps Ltd. India)

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