CONTROL OF LUBRICATION AND TEMPERATURE OF THE BEARINGS OF A PUMP

This invention relates to the transport of a solution of cellulose in a solvent, particularly a tertiary amine N-oxide solvent, through a gear pump.

US Patent 4,416,698 describes a method of producing cellulose filaments by dissolving the cellulose in a suitable solvent such as a tertiary amine oxide mixed with water. One feature of the solution, commonly referred to as a dope, is that it is both hot and, if it contains a significant quantity of cellulose, viscous requiring the use of exceedingly high pressures of up to 200 bar in order to pass the solution through filters and spinnerette jets. The cellulose dope is prepared by mixing shredded cellulose material with a solution of amine oxide and water, for example in the manner described in detail in WO 94/28215.

It is a problem when producing cellulose filaments that fibrous matter and various particles, in particular silica particles, are carried into the cellulose solution with the cellulose material and remain suspended in solution until filtered out before the spinning stage. Typically cellulose material will contain 40-200 ppm silica by weight, and the silica particles will have a size distribution from less than 1 micron diameter up to 250 microns in diameter, the larger particles, say in excess of 30 microns diameter, accounting for about 60% by weight of the silica present.

The silica particles are carried by the cellulose dope through the process. The cellulose dope when it enters the final phases of the process has a high viscosity in the order of 500-1500 Pa and generally in the order of 800-1000 Pa at a shear rate of between 1-10 s^-1 and a temperature of 100-120°C. It has been found that gear pumps are particularly suitable for moving materials having these viscosities, and that these pumps can operate at very high pressures, e.g. up to 200 bar. A known gear pump is described in US-A-4,725,211 in which the viscous material is pumped by intermeshing gears carried on gear shafts which are rotatably mounted in bearings in the pump body. The pumped material is fed to the annular space between the shaft and the bearing to lubricate the bearing.

In some prior art gear pumps the bearing surfaces of each bearing are provided with at least one lubrication channel extending from the high pressure side of the pump into the bearing. These channels may be at a bias angle to the axis of rotation of the shafts so that rotation of the shaft helps draw material into the channel. The channels are typically blind channels with the pumped material being forced from the channel into the annular gap between the shaft and the bearing. Larger silica particles present in the cellulose dope may become trapped in the lubrication channel leading eventually to blocking of the channel and a loss of lubrication and failure of the bearing.

This problem is further complicated by the fact that cellulose dope in an amine N-oxide solvent is susceptible to exothermic reaction, and it is therefore necessary to maintain the dope temperature in the range of from 100°C to 120°C. The blocking of the channels in the bearings may therefore lead to an exothermic reaction due to the lack of lubrication, and the build up of heat in the bearing caused by heat and the build up of heat in the bearing generated by friction due to the lack of lubrication and by the reduced flow of the dope failing to remove heat from the bearing.

Surprisingly we have found that the onset of an exothermic reaction in cellulose solution passing through a gear pump can be predicted by monitoring the temperature of the bearings.

According to the invention there is provided a method of preventing an exothermic reaction within a gear pump used for the transport of a solution of cellulose in an amine oxide solution and which comprises a pump body with a pair of meshed gear wheels mounted on shafts which rotate in bearings fixed relative to the body, wherein the temperature of at least one of the bearings is monitored and when said temperature exceeds a first predetermined limit the volume throughput of the pump is reduced.

The invention also includes a method of transporting a solution of cellulose by pumping the solution through a gear pump having a pump body and a pair of meshing gear wheels mounted on shafts rotatable in bearings fixed relative to the pump body, wherein the method comprises monitoring the temperature of at least one of the said bearings and controlling the volume throughput of the pump in dependence on the monitored temperature, and preferably reducing the volume throughput when the monitored temperature exceeds a first predetermined limit.

Preferably, when the temperature exceeds a second higher predetermined temperature limit the pump is stopped. The first and second limits may be about 112°C and about 115°C, respectively.

Such a method may, but would not necessarily, also prevent damage to the bearings. However the main object of the invention is to prevent an exothermic reaction in the cellulose system being initiated by a build up of excessive heat in the gear pump, which reaction could spread to other parts of the system.

A gear pump for the transport of a solution of cellulose in connection with the invention comprises a pump body having a pair of meshed gear wheels mounted on shafts which rotate in bearings secured to the pump body, and a temperature sensing device for sensing the temperature of at least one of the bearings, suitably adjacent its bearing surface and preferably a separate temperature sensing device for each bearing. Each temperature sensing device can comprise a thermocouple assembly housed in a stainless steel tube. Each bearing may have a lubrication groove on its bearing surface through which the cellulose solution can pass in use to lubricate the bearing, and each temperature sensing device can be open to the or an associated lubrication groove. A control system for the above pump operates to reduce the throughput of the pump when the sensed temperature reaches a first predetermined level, e.g. about 112°C, and to stop the pump when the sensed temperature reaches a second predetermined level, e.g. about 115°C.

A control system for controlling the transport of a solution of cellulose by means of a gear pump in a method according to the invention comprises an electric motor for driving the gear pump and control means for comparing the temperatures monitored at any of the bearing surfaces with preset parameters and, in the event that any monitored temperature exceeds a set parameter, causing the pump to slow down and, if the temperature returns to below said set parameter, causing the pump slow-down to be cancelled. Advantageously, when the sensed temperature exceeds a second set parameter the control means causes the pump to shut down, the pump being restartable only through a reset control.

A gear pump for pumping a solution of cellulose in an amine oxide/water solvent for use in connection with the invention comprises a pair of meshed gear wheels rotatably mounted on shafts located in bearings fixed in a housing, the shafts passing through said bearing being lubricated by the cellulose solution which enters each bearing through at least one lubrication channel in the bearing surface of the bearing which extends inwardly from an inlet at the inner end of the respective bearing, at a bias angle to the axis of rotation of the shaft, characterised in that the lubrication channel is connected to the outer end of the respective bearing, preferably through an outlet having a smaller cross-sectional area than the inlet.

The connection of the lubrication channel to the outer end of each respective bearing allows the silica particles to be washed out of the bearing. However, if the outlet from the lubricated channel is for example the same cross-sectional area as the inlet then the efficiency of the pump will be severely impaired, and furthermore the dope that provides the lubrication will pass preferentially down the lubrication channel rather than into the bearing, again leading to bearing failure.

It therefore follows that there is a balance between the desired lubrication requirements, the acceptable efficiency of the pump, and the prevention of the build up of silica particles leading to bearing damage.

Preferably the ratio of the cross-sectional area of the outlet to the cross-sectional area of the inlet is between 1:30 and 1:60, and more preferably between 1:40 and 1:50.

The outlet is considerably shallower than the inlet. Preferably the outlet has a maximum depth of about 0.5mm.

The blockage of a lubrication channel in the bearings of a gear pump can be monitored and an increase in pump efficiency is taken as an indication of a build-up of matter within the lubrication channel.

An embodiment of the invention will now be described, by way of example only, and with particular reference to the accompanying drawings, in which:-

• Figure 1 is a cross-sectional view through a gear pump according to an aspect of the present invention;
• Figure 2 is an enlarged view of a portion of Figure 1 showing a thermocouple in place in a bearing of the gear pump;
• Figure 3 is an end view of a bearing as viewed from the line III-III of Figure 1;
• Figure 4 is a side elevation of a bearing; and
• Figure 5 is a flow chart for a pump control system.

With reference to Figures 1 to 4 there is shown a gear pump 11 of a type used for the transport of cellulose dope preferably containing about 15% cellulose, 76% by weight amine N-oxide, and 9% by weight of water. The dope at 100°C-110°C has a viscosity of between 500-1000 Pa at a shear rate 1-10 s^-1. Such material requires extremely high transport pressures in order for the dope to be forced through filters prior to spinning. The pump 11 is a high pressure pump capable of operating up to 200 bar and comprises a housing 12 which has cooling channels 13 running through the walls thereof. A pump chamber 14 is formed within the housing and has two intermeshed hardened tool steel gear wheels 15 and 16 mounted therein. The gear wheel 15 is driven by an electric motor (not shown) connected to its shaft 17. The other gear wheel 16 is driven by the gear wheel 15 with which it is intermeshed and rotates on its shaft 18. In an alternative embodiment (not shown) both gear wheels may have synchronised independent drives.

The two gear wheels 15 and 16 are mounted for rotation in the housing 12 by cylindrical bushes or bearings 21. Each bearing 21 is securely fixed in the housing 12 with each bearing having a flat surface 22 on its outer cylindrical surface. Pairs of adjacent bearings are held rotationally fast relative to each other by engagement of the respective flat surfaces 22 one against the other. For each such pair of bearings, a key 23 engages in aligned axial slots 24 in the two flat surfaces 22 to ensure that the bearings 21 do not turn relative to each other.

The meshing contra-rotating gear wheels 15, 16 are supplied with cellulose dope delivered under a pressure of between 5-10 bar to the feed side of the pump. The rotation of the gear wheels 15 and 16 transports the dope to the high pressure side of the pump and the dope exits the pump at pressures of up to 200 bar.

The bearings 21 are suitably made from hardened tool steel or a ceramic material. Each bearing 21 has special lubrication channels 26 formed radially in the axially inner face 27 of the bearing. The channels 26 connect with second lubrication grooves or channels 28 formed in the inner cylindrical bearing surface 30 of the bearing and which extend across the bearing at a bias to the rotational axis of the bearing, typically from 40° to 70°. For the purposes of example only a single lubrication channel 28 is shown in Figure 4. Each second lubrication channel 28 completely traverses the axial length of the bearing, having an inlet 31 at the axially inner end of the bearing on the high pressure side of the pump, and a smaller cross-sectional outlet 32 at the axially outer end of the bearing. The outlet 32 discharges into a return channel (not shown) in the housing for feeding back into the low pressure side of the pump in a known manner.

The pumped cellulose dope, as it passes through the pump chamber 14, is caused to flow into the lubrication channels 26 and 28 by the high pressure on the high pressure side of the pump. Cellulose dope will pass from the lubrication channels 28 into the clearance between each shaft and its associated bearing surface 30. Some dope will also be caused to flow directly from the pump chamber 14 into any running clearance around the shafts 17 and 18. The pumped medium will pass along the clearances around the shaft to the axially outer end of each bearing 21, and any material passing through the bearings will be returned to the feed side of the pump as previously described.

As discussed in the preamble the lubrication channels 28 may become blocked by silica particles or other foreign bodies which in turn may lead to the cellulose dope undergoing an exothermic reaction in the pump.

Each second lubrication channel 28 comprises a first part 33 which extends outwardly from the inner face 27 of the bearing, and a second part 34 of a much smaller cross-sectional area which connects the first part 33 to the outlet 32 for bleeding of dope and silica particles. The ratio of the cross-sectional areas of the two parts 33, 34 is determined by a balance between pump efficiency, lubrication and prevention of a build-up of silica particles.

For a good balance between different conflicting requirements it has been found that a cross-sectional ratio of inlet to outlet should be between 30:1 and 60:1 and preferably between 40:1 and 50:1.

The first part 33 of the second lubrication channel 28 is of a substantially semi-circular cross-section, and the second part 34 of the lubrication channel 28 will also be of semi-circular cross-section. A typical bleed groove may be between 1 and 2 mm in diameter.

Under typical operating conditions the volumetric pump efficiency will be in the order of about 80% with a 150-200 bar (1.5 x 10⁷ - 2 x 10⁷ Pa) operating pressure on the discharge side of the pump, preferably 150-170 bar (1.5 x 10⁷ - 1.7 x 10⁷ Pa).

If the second part 34 of the lubrication channel 28 were increased in size so that the ratio of the inlet to outlet cross-section exceeds 10:1 then the efficiency of the pump would drop below acceptable limits.

For different sizes of the pump, the exact ratio of the inlet cross-sectional area to outlet cross-sectional area will have to be determined.

Although the channel 28 extends completely across the bearing allowing silica particles to pass through the bearing this does not totally prevent the build up of silica in the lubrication channel 28 in the bearing 21, but does increase the time period between any necessary maintenance. It has been found that the build up of silica in the groove over a prolonged period of time can be monitored by monitoring the pump efficiency. When the pump efficiency starts to increase this indicates that there is a silica build-up in the lubrication channels. The pump efficiency is given by monitoring the dope throughput over a prolonged time period, say one shift (8 hours), and the efficiency is given as$$\frac{\text{dope throughput/rev}}{\text{Theoretical dope throughput/rev.}}\text{x 100\%}$$

The blockage of the lubrication chemical may also be detected by monitoring the temperature of the bearings 21.

The temperature of one or more of the bearings 21 is monitored by suitable temperature sensing devices, such as resistance thermometers or, preferably, thermocouples. As shown in Figures 2 and 3, for the or each bearing monitored, a thermocouple assembly 34 extends through aligned bores 35 and 36 in the housing 12 and bearing 21, respectively. The particular type of thermocouple selected is a reflection of the required accuracy over the temperature range to be monitored. For the present application, the thermocouple assembly 34 is preferably an iron-copper/nickel type thermocouple housed in a stainless steel tube available, for example from Degussa.

The bore 36, arranged on a chord of the respective cylindrical bearing 21, may open into one of the lubrication grooves 28 in the bearing surface 30, or preferably may terminate within the bearing 21 at a distance of from 2-4 mm from the bearing surface 30.

The thermocouple assembly 34 will be a close fit in the bore 36 with typically a clearance of from about 0.2-0.5 mm. Good thermal contact is assured by the use of thermally conductive lubricants within the bore.

If the temperature of only one bearing is monitored then in the case of a pump with a driven gear 15 and non-driven gear 16, the bearing on the non-driven gear 16 on the side away from the motor is preferably selected for temperature monitoring.

The or each thermocouple assembly 34, depending on the number of bearings monitored, is connected to a control system 50 (see Figure 2) operated by software and which is connected to control the motor of the pump. If any thermocouple senses a bearing temperature above a first predetermined limit, for example of about 235°F (112-113°C) in the case of cellulose dope described previously, the volumetric throughput of the pump is reduced by reducing the speed of rotation of the gear wheels. If a bearing temperature above a second predetermined limit, for example of about 240°F (115-116°C), is sensed, the pump is stopped. After the temperature sensors have been triggered it may be necessary to dismantle the pump for maintenance.

If the control system responds so that the monitored temperature returns to normal the pump may continue to run, on the other hand if the temperature remains high then the pump will require maintenance.

The control system may also verify that the temperature sensor devices are operating correctly to avoid the shut down or slow down of plant due to a faulty sensor.

The control system 50 is operated under computer control and is described with reference to Figure 5. The signals from the temperature sensors (34) are read at stage 1 and a resultant signal is analysed at stage 2 to determine if it is a "healthy" signal, e.g. if the sensors are operating correctly. If the signal is "healthy" a signal is passed to stage 3 to determine if the temperature is above the second predetermined limit or parameter, set for example at 240°F (115-116°C). If the temperature is above the set parameter a signal is passed to a shutdown control at stage 6 to shutdown the pump. If the temperature is below the second predetermined limit, a signal is passed to a stage 4 to determine if the temperature is above the first predetermined limit or parameter, set for example at 230°F (110°C). If the sensed temperature is below the first predetermined level a signal is fed back to stage 1. If the signal exceeds the first level set, then at stage 4 a signal is passed to the pump speed control to initiate a slow down, see stage 5. The system waits for a set time period and then confirms that the speed of the pump has slowed. This is stage 7. If there is no slow down, a signal is fed to stage 6. If there is slow down and the sensed temperature, monitored at stage 8, falls back within the accepted range (see stages 2 and 3), the slow down or shut down is cancelled (stage 9) and the control system returns to normal operating mode.

If the monitored temperature exceeds the higher second level set, then the system is shutdown (see stage 6). The system must then be reset, at stage 10, to recommence pumping, otherwise the signal loops around the shutdown control loop.