Pump with seal cooling means

Abstract: A Pump for high temperature liquid includes duct means (56,60) interconnecting the suction inlet space (22), and the annular clearance space (58) between the shaft (4) and the pressure housing (2) at a point immediately adjacent to the seal (34) around the shaft. During hot stand-by uniform thermosyphon circulation of liquid occurs from the suction inlet space through the annular clearance space and back to the suction inlet via the duct means, which helps prevent thermal stresses being set up in the shaft.

Description: The present invention relates to a pump for high temperature liquid, particularly a boiler feed pump for pumping high temperature water to a boiler, which has a rotary seal having cooling means to keep the seal temperature within its recommended range. Boiler feed pumps for power stations typically pump water at elevated temperatures (150° centigrade and greater) and at elevated pressures. Such pumps are usually driven by turbines or electric motors. The pump has a number of impellers on a driven shaft which progressively pressurise the feed water up to pressures of typically 150 to 300 bar. The shaft is provided with rotary mechanical seals at either end, sealing the shaft into the pump housing. These seals, however, require cooling in order to operate satisfactorily and have a reasonable lifetime. In the past, the seals have been cooled by means of a cooling jacket located inboard of the seals through which coolant is passed from an external source. Additionally the seal is commonly cooled by means of a closed loop cooling circuit. By this means, boiler feed pump water within the space occupied by the mechanical seal is pumped through a cooler by means of a pumping ring mounted on the shaft. Thus the temperature of the seal is kept lower than the rest of the pump whilst the pump is running. When the pump is at rest, there is still a requirement to maintain the mechanical seals at a lower temperature than the rest of the pump. This is effected by continuing to circulate coolant through the aforesaid cooling jacket augmented by circulation of boiled feed pump water within the closed cooling loop by means of natural convection. The temperature differentials arising, during this hot standby condition, from the need to cool the mechanical seal result in vigorous convection currents being set up in the annular space between the shaft and the cooling jacket inboard of the seal. This condition leads to local thermal distortion of the stationary shaft, which tends to become bowed. Thermal bowing of the shaft has certain undesirable consequencies. Firstly, there may be premature wear of internal clearances when a pump on hot standby is started. Secondly, on a turbine driven pump being subjected to low speed barring operations, the friction torque, or even seizure, that develops when barring is discontinued for any reason can be sufficient to prevent reinstatement of barring. At the least, it will take several hours for the pump and its contents to cool sufficiently to permit barring reinstatement. Thirdly, the bowed shaft gives rise to mechanical imbalance and vibration on start up which will persist until the shaft temperature becomes uniform. Fourthly, the equalibrating forces between the thermally bowed shaft and constraining internal and journal bearing clearances give rise to orbital motion of the shaft journals within the bearings. This condition can result in the pump being tripped out if shaft displacement safety sensors are installed. For these reasons, it is desirable to reduce or eliminate the thermally induced bowing of the shaft during hot standby. Several attempts have been made to do this in the past. On turbine driven pumps, a barring mechanism is provided which continually rotates the shaft at a slow speed during hot standby. This is a requirement of the turbine itself. However, there are significant control problems if barring gear is fitted to electric motor driven pumps where motor start up is automatic. Then, special provision must be made to disengage the barring gear as part of the start up sequence and this involves the additional risk of wrecked barring gear should disengagement fail to take place. Another approach to this problem has been to inject hot boiler feed water from other operational pumps directly into the annular space between the shaft and housing of the pump on hot standby. This inhibits the convection currents which lead to bowing of the shaft. However, the effect of this is to maintain the seal area at an undesirably high temperature resulting in premature deterioration. Cooler water from another source could be injected to overcome this problem but the resulting additional equipment and complexity involved is undesirable. The solution proposed in patent specification GB 2106593 is to provide a flushing flow of liquid produced by an auxiliary pump, which is started when the main pump is stopped. However, the use of an auxiliary pump involves problems of control and reliability, and adds to the complexity of the apparatus. It is an object of the present invention to mitigate these problems in a simple and effective manner. Thus, the present invention provides a pump suitable for high temperature liquid, comprising: a pressure housing having a suction inlet and a discharge outlet for the liquid being pumped; a driven shaft supported by suitable bearings in the housing; impeller means mounted on the shaft for pumping liquid and generating a liquid head between the inlet and the outlet; a rotary seal mounted on at least one end of the shaft sealing the shaft with respect to the pressure housing and having a cooling jacket around it; a liquid reservoir space, within the pressure housing and located inboard of the mechanical seal, the pressure within the reservoir in use being substantially equal to the suction inlet pressure; an annular clearance space between the shaft and the pressure housing communicating at one end with the reservoir space and extending to the seal at its other end' duct means connecting the reservoir space and the annular clearance space immediately adjacent to the seal, such that when the pump is not running but contains high temperature liquid, a thermosyphon driven by the temperature difference between the high temperature liquid within the reservoir and the cooled seal is established, circulating liquid from the reservoir space via the annular clearance and the duct means to the reservoir again. Thus, the solution offered by the present invention is to provide duct means whereby the thermosyphon flowpath is via an external connection to the reservoir space rather than by local convection currents within the annular clearance space, this latter condition giving rise to undesired bowing of the shaft. The liquid reservoir space at one end of the pump will usually be constituted by the suction inlet. At the other end of the pump there is usually provided a balance drum whose function is twofold; namely to break down the high pressure within the pump generated by the impeller means to a pressure essentially equal to pump inlet pressure. The second function is to reduce the axial thrust generated in the shaft by hydraulic forces, to a net value capable of being handled by a thrust bearing. In order to maintain the pressure at the low pressure end of the drum to essentially that of the pump suction pressure, the balance drum discharge chamber is connected to the suction inlet by means of a suitable return pipe. The balance drum discharge chamber constitutes the liquid reservoir space corresponding to that at the other end of the pump formed by the suction inlet. The thermosyphon set up through the duct means ensures that water, at a more uniform temperature than would otherwise be the case, flows through the annular space from the liquid reservoir space, thereby minimising any thermal stratification around the shaft. This is achieved without the use of complex additional pumps and pipework. In the present invention, it is also preferred to minimise cooling of the shaft by arranging the cooling jacket to be restricted to only that part of the housing immediately around the seal itself-thereby restricting the length of shaft subjected to residual stratification to the section inside the seal. Moreover, it is preferred to provide an air gap within the housing to lessen thermal conduction from the remainder of the hot pump to the cooled seal. It may also be desirable to fit a close clearance restriction bush around the shaft between the seal and the point at which the duct means connects into the annular clearance space. This restricts passage of hot liquid to the seal. Moreover, an insulating sleeve may be provided between the seal and the shaft, so that any residual heat flow results in thermal distortion of the sleeve rather than the shaft. Although it should not be necessary in most cases, a small pump can be provided in the duct means to assist the thermosyphoning recirculation. An embodiment of the present invention will now be described by way of example only with reference to the drawings wherein; Figure 1 is a cross sectional view of a conventional high temperature boiler feed pump; and Figure 2 is a detailed elevation of a seal assembly modified according to the present invention. Figure 1 shows a conventional centrifugal multi-stage boiler feed pump having a pump housing 2 and shaft 4 rotatably mounted therein by drive end bearing 6 and non-drive end bearing 8. On the shaft are mounted impellers 10,12,14,16 of a centrifugal multi stage pump mechanism. Hot boiler feed water at an elevated temperature (e.g. 150-160° C) passes into suction inlet 20 and into inlet space 22, from which it is pumped through the impellers and passes out under pressure through discharge outlet 24. Feed water is progressively pressurised as it passes through the impellers, so that a net force is exerted on the shaft towards the drive end. To counteract this, a balance drum 26 and double acting tilting pad thrust bearing 28 are provided. The balance drum reduces the water pressure between the shaft and the housing such that in the balance drum space 30 the pressure is equal to the suction inlet pressure,and its proportions are determined so that there is minimal net axial thrust on the thrust bearing. To ensure equal pressure, a balance water return 32 connects the balance drum space 30 to the suction inlet 20. A drive end seal 34 is provided around the drive end of the shaft to prevent egress of water. A non-drive end seal 36 is provided at the non-drive end of the shaft 4. These seals are mechanical seals having rubbing faces of carbon or ceramic material which are spring biased into contact with each other. The leakage rate of such seals is very low. Figure 2 shows in more detail a seal arrangement according to the present invention. The same reference numerals are used for analogous parts. The drive end seal 34 comprises rubbing sealing surfaces 42 biased together by spring 44,and sealing O rings 46,48. A seal support sleeve 50 is mounted on the shaft 4. A screwed pumping ring 51 is provided and acts to circulate cooling water across the seal faces. In addition cold water is circulated through the enclosed cooling jacket 38 by a separate external pumped cooling system. An air space 52 is provided in the housing to assist thermal insulation of the drive end seal 34 from the remainder of the hot pump. A close clearance restriction bush 54 of the fixed or radially floating type is located inboard of the seal to offer a restricted passage between the hot water in the pump and that in the immediate vicinity of the seal. A duct 56 is provided in the housing just inboard of the seal and in communication with an annular clearance space 58 between the shaft and the housing. The duct 56 is connected via a relatively wide bore tube to duct 60 communicating with inlet space 22, so that thermosyphoning can occur around this circuit. Operation is as follows. Whilst the pump is operational shaft 4 is rotating so that cooling water is pumped by pumping ring 51 to cool the drive end seal 34. Further cooling is provided by circulation of cold water from an external pumped cooling system to and from the enclosed cooling jacket 38. When the pump is stationary, some cooling of the seal still occurs by circulation of cold water by a separate pumped cooling system within water jacket 38, so that a temperature difference exists between the seal and the inlet space 22 in hot standby mode. In a conventional pump, this would lead to strong convection currents being set up within the annular space 58 leading to a severe temperature difference (typically 60°C) between the top of the shaft and the bottom of the shaft, which leads to bowing of the shaft. However, according to the present invention, a duct 56,60 is provided which allows thermosyphoning of water from inlet space 22 through duct 56; before being returned to inlet space 22 via duct 60. This thermosypon arrangement ensures that hot water of substantially the same temperature is drawn from inlet space 22 evenly around annular space 58. Typically, duct 56,60 is of diameter 2 - 4 centimetres. The provision of this duct has been found to reduce thermal distortion by a factor of about 10. If desired, a local cutaway 62 may be provided at the inlet to duct 56 to assist water flow.

Claims: A pump suitable for high temperature liquid, comprising: a pressure housing (2) having a suction inlet (20) and a discharge outlet (24) for the liquid being pumped; a driven shaft (4) supported by suitable bearings (6,8) in the housing; impeller means (10,12,14,16) mounted on the shaft for pumping liquid and generating a liquid head between the inlet and the outlet; a rotary seal (34) mounted on at least one end of the shaft sealing the shaft with respect to the pressure housing and having a cooling jacket (38) around it; a liquid reservoir space (22), within the pressure housing and located inboard of the mechanical seal, the pressure within the reservoir in use being substantially equal to the suction inlet pressure; an annular clearance space (58) between the shaft and the pressure housing communicating at one end with the reservoir space and extending to the seal at its other end; characterised in further comprising duct means (56,60) connecting the reservoir space and the annular clearance space immediately adjacent to the seal, such that when the pump is not running but contains high temperature liquid, a thermosyphon driven by the temperature difference between the high temperature liquid within the reservoir and the cooled seal is established, circulating liquid from the reservoir space via the annular clearance and the duct means to the reservoir again; said duct means being separate from and additional to said annular clearance space. A pump according to claim 1 wherein the liquid reservoir space is the suction inlet (22) of the pump. A pump according to claim 1 or 2 wherein the cooling jacket is restricted to only that part of the housing immediately around the seal. A pump according to any preceding claim wherein an air gap (52) is provided within the housing between the seal and the liquid reservoir space. A pump according to any preceding claim which further comprises a restriction bush (54) around the shaft between the seal and the point at which the duct means connects into the annular clearance space. A pump according to any preceding claim which further comprises an insulating sleeve (50) between the seal and the shaft. A pump according to any preceding claim which further comprises pump means in the duct means.

Inventors: BROWN ROBERT WILLIAM, TAYLOR JOHN MCFARLANE, BROWN, ROBERT WILLIAM
Applicants: WEIR PUMPS LTD, WEIR PUMPS LIMITED
Application year: 1990
Main classification: F04D7
IPC codes: F04D7, F04D29