TURBOMACHINERY
NABLA
PUMP
Food-grade twin screw pump
CLIENT
Multinational company producing progressive cavity pumps
CHALLENGE
Design a new leading product for the food industry, to be added to the product catalogue
SUCCESS
The industrialisation process has begun
01
CHALLENGE
A multinational company specialised in progressive cavity pumps intended to introduce a leading product for the food industry into its catalogue. The choice fell on a particular type of volumetric pump, the twin screw pump, which allows the pumping of food fluids even at high viscosity — chocolate, honey, yoghurt — minimizing the alterations of the organoleptic properties and respecting the highest hygienic standards.
The company had already made a prototype of a twin screw pump, but the tested product had not met the requirements on efficiency, functionality and production costs. Given the high number of design parameters involved and their high degree of interaction, a systematic approach was required to estimate the costs and performance characteristics of the pump in advance.
A single parameter can have a significant impact on the design: a change in the hydraulic part affects the mechanical part and vice versa. Nor can the external design constraints be overlooked — the obligatory choice of standard industrial components, for instance.
Of particular importance is the estimate of the shaft deflection, which depends on both hydraulic factors (operating pressure, speed of rotation) and structural ones (shaft section and length, bearing positioning), and directly affects efficiency and costs. Lower deflections reduce the distance between rotor and stator, and so the hydraulic recirculation losses — but they can only be obtained by increasing the bearings or the shaft diameter, which normally increases costs.
02
SOLUTION
Having previously developed software for the automatic design of various types of pump, we were able to implement a specific module for twin screw pumps quickly. It carries out:
01
the design of the pump hydraulics, with the characteristic flow-prevalence curve and the efficiency estimated as a function of size, the pitch of the screws and the tolerances between rotor and stator
02
the design of the gearbox, including gear sizing and the choice of the bearings and their lubrication system
03
the prediction of friction losses related to the mechanical seal
Having a method able to identify quickly the pros and cons of different configurations gave us the opportunity to select the optimal design for the target audience.
Once the basic configuration was chosen, we took care of the detailed design of the pump, in order to:
01
simplify the assembly, disassembly and shaft-gear coupling as much as possible
02
prepare the mechanical seal housing for flexible use of both single and double seals
03
design the internal ducts and all static seal housings in compliance with both European (EHEDG) and American (3-A) hygiene regulations
03
RESULTS
At the end of the detailed design we built the prototype of the twin screw pump, then defined the experimental test campaign. Bench tests made it possible to trace the characteristic head-flow curves and to detect the energy consumption of the various sub-components — hydraulics, mechanical seal, transmission — using fluids at different viscosity levels, different mixtures of water and food-gel.
After the tests we verified that the twin screw pump complied with the design requirements by a wide margin. Minimal changes were made to the lubrication system, to increase oil circulation in the bearings.
The industrialisation process has now begun, and further activities are planned shortly:
01
the development of larger twin-screw pumps, made easier by a fast design method that has already been successfully tested
02
the search for design solutions using innovative materials, to reduce weight and production costs
