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Dipl.-Ing. Georg Stausberg

DISCHARGE, BOOSTER AND PRE-POLYMER PUMPS FOR STATE-OF-THEART DIRECT SPINNING PLANTS 1. Introduction Today, modern spinning plants for manufacturing PET manmade fibers are predominantly designed and installed as direct spinning plants. To this end, the polymer is fed directly from the polycondensation reactor to the spinning process. The process steps, granulating the polymer as well as re-melting-up using an extruder, are no longer required for a direct spinning plant.

pumps are of elementary importance for the operating safety and reliability of a direct spinning plant.

Gear pumps are used for discharging the polymer from the reactor, as well as the feeding and distributing of the melt to the individual spinning positions. To this end, we generally distinguish between three types of pumps: pre-polymer pumps, for discharging a pre-polymer from the pre-polycondensation reactor, discharge pumps, for feeding the polymer melt from the finisher, and booster pumps, for feeding the polymer melt to the spinning plant or the granulator (Fig. 1). Furthermore, gear pumps can be used as oligomer pumps within the esterification process. While, in the past, two discharge and pre-polymer pumps have frequently been used in parallel, efforts today focus on feeding the entire throughput of a reactor with only one of each pump, in order to keep investment costs down. As, in parallel to this development, the throughput performances of polycondensation systems have constantly increased, an unplanned malfunction of a pump can have severe consequences. For this reason, the structure and design of the above-mentioned gear

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Figure 1: Gear Pumps in a Direct Spinning Plant 2. Application conditions Exactly where in the direct spinning plant the various gear pumps are used was described in the introduction. In addition to this, the following table shows typical application data and pump sizes for the respective pump type within the context of a PET direct spinning plant:

Pump type

Pre-polymer pump

Discharge pump

Booster pump

Typical throughputs

150 - 900 t/d

150 - 900 t/d

100 – 450 t/d

Typical pump size

1500 – 5000 cm3/rev

2500 – 20000 cm3/rev

1500 – 6600 cm3/rev

Typical speeds

100 – 180 rpm

30 – 40 rpm

40 – 60 rpm

Polymer viscosity

1 – 5 (20) Pas

200 – 300 Pas

200 – 300 Pas

Supply pressure

Vacuum

Vacuum

10 – 150 bar

Back pressure

< 20 bar

Max. 300 bar

Max. 300 bar

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Discharge, booster and pre-polymer pumps for state-of-the-art direct spinning plants

The various viscosities and pressures in the respective process stages have a considerable impact on the structure and design of the individual pumps, which we will look at in greater detail in the following. 3. Pre-polymer pumps Due to the low viscosities of the pre-polymers and the resulting low process pressures, efforts focus on deploying pre-polymer pumps with as low as possible a feed volume/revolution and to operate these pumps at a correspondingly higher speed level. To this end, the following criteria, in particular, must be taken into account for the pump structure and design:

teristics, it should be verified in each case whether the selected pump is being sufficiently filled. As – in comparison to polymer pumps – pre-polymer pumps are subjected to a lower pressure load, the inlet cross-section can be adapted to the process requirements by, among other things, increasing the gear width. 3.2 Pressure buildup Due to the low viscosity of the pre-polymer, the internal leak flow of the pump increases significantly with rising back pressures. Particularly in the case of plants where a filter has been installed between the pre-polycondensation reactor and the finisher, back pressures can occur that the pump can not build up without special modifications.

3.1 Filling the pump As the pre-polymer is discharged from the reactor under vacuum, there are – for given pipe and pump inlet cross-sections – limits for the pump speed. If these limits are exceeded, the pump will not be filled sufficiently. In turn, these speed limits depend on the viscosity and the filling level of the prepolymers in the reactor (Fig. 2). Using such charac-

Decisive for the internal leak flow and hence the pump’s volumetric degree of efficiency are, principally, the design of the bearings, the axial clearance between the gears and the bearings as well as the radial clearance between the gears and the housing. By volumetric degree of efficiency we mean the ratio between effective feed volume and theoretical feed volume.

Figure 2: Feed Behaviour of Pre-Polymer-Pumps 2

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Discharge, booster and pre-polymer pumps for state-of-the-art direct spinning plants

Figure 3: Influence of Pump Layout on Volumetric Efficiency

Advantageous within this context is the design of the Barmag pre-polymer pump, with its straight gears. As a result, there are no axial forces on the gears, which are hence able to center themselves freely in the pump chamber. This enables a pump design with low axial clearance, which in turn considerably increases the degree of efficiency and the pressure buildup capacity (Fig. 3). The widening of the gears already mentioned in Section 3.1., improves the degree of efficiency, as – with increasing gear width – the proportionate leak flow is reduced.

In contrast to the pre-polymer pumps, it is not the filling that limits the applications for the discharge pumps, but the temperature increase in the bearing.

4. Polymer pumps 4.1. Bearing design As already described in the introduction, discharge pumps have the task of discharging the polymer melt under vacuum from the finisher and to build up the necessary pressure for supplying the downstream units (melt pipe, filter, granulator). Here, sufficient filling of the pump is secured by a considerably enlarged inlet cross-section (6 – 12 times larger than the outlet) as well as special inlet grooves in the gear area (Fig. 4). 3

Figure 4: Polymer Discharge Pump Developments • Trends • Technologies Corporate Communications

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Discharge, booster and pre-polymer pumps for state-of-the-art direct spinning plants

4.1.1. Bearing design The bearings of the polymer pumps described here (and also those of the pre-polymer pumps) are designed as slide bearings, which are lubricated by the conveying medium (Fig. 5). Bearing diameter and length are determined by the mechanical design of the pump, particularly by the maximum back pressure and the resulting load on pinion and pinion shaft.

the pump speed must be increased, which in turn results in an increase of the bearing temperature. Barmag developed a calculation program for designing the lubricating system, which unites the individual channels and gaps in a single resistance model. This also permits the simulation and evaluation of interactions within the lubricating system when changing individual geometric variables. Particularly when calculating the temperature increases within the bearing, this program achieves good correspondence with the practical values. Hence, this permits targeted optimization of the bearing design for defined application conditions. 4.1.2 Energy consumption

Figure 5: Discharge Pump - Lubrication of Bearings

A sufficient through-flow of conveying medium is required to ensure lubrication of the bearing points. For this, a system comprising various lubrication channels is integrated into the bearing bushes and, in part, the pump housing. Furthermore, the radial clearance between the shaft and the bearing bushing must be designed in such a way to ensure that sufficient conveying medium can enter the bearing area from the lubricating channels. As the design of the lubrication system also has a direct influence on the internal leak flow and hence the pump’s volumetric degree of efficiency, there is a different optimal design for the lubrication system for each respective application: if the lubrication channels and the bearing clearance are too small, the lubrication flow will be too low and the temperature within the bearing will rise to an impermissible level. In the case that the bearing clearance is too large, then a simultaneous enlargement of the radial clearance between the gears and the housing must be carried out, to avoid the gears tarnishing. This measure has an extremely negative impact on the volumetric degree of efficiency. To maintain the desired conveying performance, 4

The energy consumption of a pump is predominantly determined by two factors: the hydraulic output and the work consumed by friction. By hydraulic output we mean the product of volume flow and differential pressure, i.e. the output required to convey a certain quantity against a certain pressure. The work consumed by friction is the energy that is dissipated in the individual gaps of the pump as a result of the shearing of the conveying medium. It is predominantly determined by the pump geometry, by the viscosity of the conveying mediums and by the pump speed. The work consumed by friction can be higher than the hydraulic output particularly for applications with low back pressures. As described in Chapter 4.1.1., the design of the pump bearings has a considerable influence on a pump’s volumetric degree of efficiency. As this in turn influences the effective speed, a low volumetric degree of efficiency always results in increased work consumed by fricition. Also in the case of polymer pumps, the axial forcefree straight gears can be designed with narrower axial clearance, which also increases the volumetric degree of efficiency and hence reduces energetic losses. Depending on the design and construction of the polymer pumps, the volumetric degree of efficiency can lie in the 80 - 98 % range, whereby values considerably in excess of 90% are achieved for almost all applications using Barmag pumps. Developments • Trends • Technologies Corporate Communications

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Discharge, booster and pre-polymer pumps for state-of-the-art direct spinning plants

Dependent on the application conditions, a 10% higher volumetric degree of efficiency results in a 5 - 10% lower energy consumption for a polymer pump, which produces considerable savings, particularly for large-scale systems. 4.2. Booster pumps As the case of discharge pumps, booster pumps (Fig. 6) are also used for conveying polymer melts. However, as the polymer is fed into the booster pump with a supply pressure, this series does not require the enlarged inlet cross-section.

As, for reasons of quality, the focus is to process the polycondensation plant as stationary as possible without sizeable changes in throughput, any change in throughput within the spinning plant must be compensated with a simultaneous change in throughput in the granulation. To this end, this may result in the standstill of individual consumers and hence the corresponding booster pump. Additional changes of the application conditions also result from the different degrees of dirt accumulation in the polymer filters, which can be integrated between the pump and consumer in the melt pipe. The above-mentioned general conditions result in frequently changing operating states for booster pumps. Here, maximum and minimum throughput can be varied in a 10:1 ratio, which in turn also results in changing differential pressures. For low throughputs, these can have a negative gradient (i.e. the supply pressure is greater than the back pressure), for high throughputs, a positive gradient. The design of the bearing lubrication, in particular, must take these changing application conditions into account. Even for a negative pressure gradient, the bearing must be provided with a sufficient polymer flow.

Figure 6: Booster Pump GCK40H In terms of bearing design and energy consumption, the same statements made in Chapters 4.1.1. and 4.1.2. apply. As – in comparison to discharge pumps – booster pumps are frequently used for a wider range of applications, additional criteria must be taken into account: 4.2.1. Changing application conditions Booster pumps are used in a direct spinning plant if several consumers (spinning plant, granulation) are fed from a single polycondensation system. The booster pump controls the quantity feed for the individual consumers. 5

Barmag has solved this problem exclusively with rheological design and the geometry of the lubricating system. Valves and throttle screws have consciously not been used, as the interactions on the flow of the individual lubrication channels, caused by readjusting a unit of this kind, are not clear to the user. In addition to the criteria described in the Chapters above, the shaft seal design has a considerable influence on the operating behavior and hence stability of the overall process and the quality of the polymer. Particularly for discharge and pre-polymer pumps, the seal must fulfill several requirements: - During start-up from the vacuum, no ambient air must enter the pump via the shaft seal; - Even during long-term operation, no air must be taken in, to avoid the oxidative degrading of the polymer; - Degraded polymer must not reenter the process from the seal area.

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Discharge, booster and pre-polymer pumps for state-of-the-art direct spinning plants

Figure 7: Combined Shaft Seal POLYVAC®

The above requirements are best fulfilled either using a stuffing box seal with buffer fluid or using the Barmag Polyvac® seal. The Polyvac® seal (Fig. 7) is a temperature-controlled thread seal in conjunction with a chamber for a buffer medium (glycol, silicon oil). This buffer medium enables the start-up from the vacuum. Following start-up of the pump, the leakage of the polymer is set via the temperature control of the sealing bushing in the area of the sealing thread.

6. Mechanical design 6.1. Materials

To ensure that no degraded polymer flows back from the seal area into the pump’s product chamber, the temperature control of the sealing bushing should be carried out to ensure that there is a small, but constant, polymer discharge at the shaft seal. This setting also prevents the inflow of ambient air into the product chamber of the running pump.

6.2. Heating Pre-polymer and polymer pumps in a direct spinning plant are either liquid-heated or steam-heated using suitable heat carrier oils. In rare cases, electrically-heated pumps are used. As a result of the compact construction of the Barmag pumps and the optimized design of the heating channels, heating the side plates (pump covers) is no longer required, even for pumps with larger feed volumes. This simplifies the installation of the pumps into the spinning plant as well as reducing time and costs for maintenance work.

As booster pumps require a supply pressure on the indraft, a vacuum seal is not required here. To this end, all that is required is a temperature-controlled thread seal to adjust the polymer discharge. 6

All components of the types of pumps presented here are manufactured from various stainless steels. For those parts that are mechanically heavily challenged (pinion, pinion shaft, bearing bushes), wear resistant tool steels are used, and housing, pump cover and seal components are manufactured from non-corrosion chrome steels.

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Discharge, booster and pre-polymer pumps for state-of-the-art direct spinning plants

6.3 Pump drives

7. Summary

In addition to the output requirement, the design of the pump drives must also take the installation situation and the ambient conditions into account. Pump and drive must be installed in the system so that heat expansion in the melt pipe can be compensated (Fig. 8).

In hardly any other plastic processing procedure are process stability and product quality so heavily influenced by the polymer as in the case of manmade fiber production. As the leading manufacturer of manmade fiber systems, Barmag has acquired comprehensive knowledge in the processing of spinning polymers, particularly with regard to how the design of the individual system components influences the quality of the polymer and hence the spinning plant process. This knowledge, coupled with 30 years of experience in the construction of discharge, booster and pre-polymer pumps, has resulted in Barmag continually further developing gear pumps and also fulfilling the highest requirements demanded by modern direct spinning plants.

Figure 8: Booster Pump with drive unit For booster pumps, in particular, it may be necessary to integrate a mechanical brake into the drive system, as the pumps must be stopped even with a supply pressure present, for the purpose, for example, of maintaining a granulator. A further option is the explosion-protected design of drive and pump if the ambient conditions require this.

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