Thrust Wear Detector

Abstract

An improved thrust wear detector for a turbomachine includes a hydraulically positioned follower piston which is testable against sticking to the detector casing. A calibrated orifice used to blanace the follower piston is accessible for maintenance without disassembly of the wear detector. Movement of the follower piston, indicating bearing wear, is measured directly from the piston and false tripouts due to pressure switch failure is averted by providing redundant pressure switches.

Description

BACKGROUND OF THE INVENTION

This invention relates, in general, to protection systems for turbomachines; and, in particular, this invention pertains to a hydraulically positioned device for following the axial movement of a turbomachine rotor.

U.S. Pat. No. 2,888,023 to Eggenberger, issued May 26, 1959 and assigned to the assignee of the present invention, discloses a hydraulic thrust bearing wear indicator including a hydraulically positioned follower piston and integral nozzle portion mounted closely adjacent to the turbomachine rotor. Balance of the follower piston is dependent upon differential piston surface areas, a calibrated orifice therebetween and a constant pressure hydraulic gap between the nozzle and the rotor. A pilot valve is positioned by the follower piston to pass or shut off fluid to a pair of pressure switches connected to an alarm and trip circuit. Movement of the rotor due to thrust bearing wear may either increase or decrease the hydraulic pressure gap causing movement of the follower piston to an equilibrium position or the steady state gap. The pilot valve is also moved and at a predetermined critical movement will cause the pressure switches to close due to a shutoff of fluid pressure causing the activation of the trip and alarm circuit. Bearing wear is measured by switching the trip and alarm circuit to a test circuit and then moving a slidable valve sleeve, surrounding the pilot valve, until one of the ports is shut off. This gives an indication of the remaining permissible thrust bearing wear from which the amount of bearing wear may be calculated.

While the foregoing device has been extremely successful, it may be desirable to make further improvements which would increase its reliability. For example, it is possible that the follower piston may become frozen within the detector casing due to oil contamination or breakdown and thus prevent operation of the device. Another possible problem might occur within the calibrated orifice which might become clogged, thereby causing follower piston unbalance and possible false tripout. A further improvement would be to measure thrust wear directly from the follower piston rather than indirectly as heretofore described. Finally, it would be desirable to decrease the probability of false tripout due to pressure switch failure.

One object of the present invention is to provide an improved thrust wear detector and trip system having increased reliability.

Another object of the invention is to provide an improved thrust wear detector in which the axial movement of the follower piston may be tested.

Another object of the invention is to provide an improved thrust wear detector wherein the calibrated orifice may be easily maintained.

Another object of the invention is to provide an improved thrust wear detector wherein thrust bearing wear is directly determined.

Still another object of the invention is to provide an improved thrust wear detector having improved reliability against false tripping due to pressure switch failure.

In accordance with one embodiment of the present invention, a thrust wear detector is provided with a hydraulically balanced follower piston having a piston nozzle slidable therein. The follower piston and piston nozzle may be moved as a unit in normal operation or they may be moved separately during test operation. A calibrated orifice is positioned externally of the wear detector casing in the fluid supply lines for maintenance and inspection. A linear variable differential transformer is used to directly measure movement of the follower piston, thus giving direct readings on bearing wear. Finally, redundant pressure switches are used for tripping to avert false tripouts due to the failure of any single switch.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood with reference to the following description, taken in connection with the appended drawing.

The drawing shows an elevation section of a thrust wear detector according to the present invention and further shows an electrical schematic diagram of an alarm, trip and test circuit. The drawing also shows external fluid supply piping to the detector.

A thrust wear detector 11 and mounting frame 13 is positioned adjacent a turbomachine rotor 15. The axis of the detector is generally perpendicular to the rotor axis as shown, although it is possible to mount the detector so that its axis is parallel to the rotor axis. The alarm, trip and test circuit 17 may be mounted elsewhere on the mounting frame.

The thrust wear detector includes a stationary casing 19 formed with an axial bore 21 therethrough. A follower piston 23 is slidably mounted within a portion of the casing axial bore and includes a piston collar 25 having opposite piston surfaces 25a and 25 b defining balance chambers 27a and 27b, respectively, within the detector casing. The balance chambers 27a and 27b are provided with fluid inlet ports 29a and 29b, respectively.

The follower piston includes an axial bore 31 which houses slidable piston nozzle 33 therein. The nozzle is positioned closely adjacent the rotor surface defining a gap therebetween and includes an open end 35 and an enlarged collar 37 slidable within an enlarged portion 39 of the piston axial bore. The enlarged portion 39 of the piston axial bore is divided into a spring housing 39a and an internal test chamber 39b. The internal test chamber is fed with fluid through port 41 in the follower piston and port 29c in the detector casing. The open end 35 of the nozzle is fed with fluid from balance chamber 27B through port 41a in the follower piston.

A spring 43 is positioned between the follower piston and detector casing to prevent contact between the nozzle and rotor when fluid to the detector is shut down. Another spring 45 in the spring housing 39a bears against the nozzle collar 37 and follower piston 23 so that the nozzle and follower piston may move as a unit in operating position when fluid is not supplied to chamber 39b.

A pilot valve 51 is attached at one end to the follower piston and is positioned axially thereby. The pilot valve is slidable within an axially slidable sleeve 53. The axially slidable sleeve is positioned by a micrometer screw 55. The slidable sleeve includes outlet ports 57a and 57b as well as inlet port 59 therebetween. The pilot valve includes lands 61a and 61b which are axially spaced apart at a greater distance than outlet ports 57a and 57b. Therefore, when fluid is delivered through inlet port 59 and pilot valve 51 is centered with respect to outlet ports 57a and 57b, the inlet fluid will flow through both outlet ports. The slidable sleeve allows setting of the alarm and trip points with respect to the normal operating position of the shaft.

Movement of the follower piston may be determined and recorded by use of a linear variable differential transformer (LVDT) 63 which is attached directly to the follower piston. The linear variable differential transformer translates the mechanical movement of a rod 63a through a coil into a voltage differential which then may be interpreted to give the amount of bearing wear which has occured. Movement of the follower piston is read directly on an instrument reflecting the position of the LVDT. This construction also allows continuous monitoring of shaft position during alarm, trip point setting using the slidable sleeve.

Ports 57a and 57b supply fluid through casing ports 65a and 65b, respectively, to redundant pressure switches 67 and 69. Failure of a single switch will not cause a false trip because both switches in either set must be closed to cause a trip. The electrical circuit 17 connected to the pressure switches has been shown in simplified form for completeness.

The fluid supply system 71 includes a main conduit 73 interconnected with a branch pipe 75, a branch pipe 77, a branch pipe 79 and a branch pipe 81. The branch pipe 75 supplies fluid to the pilot valve through casing inlet 83. The branch pipe 77 supplies fluid to first balance chamber 27a. The branch pipe 79 supplies fluid to internal chamber 39b and may be selectively operated for test by valve 85. Valve 85 may be a solenoid operated valve. The branch pipe 81 supplies fluid to balancing chamber 27b and may be selectively operated through valve 87 which may also be a solenoid valve.

Branch pipes 77 and 81 are interconnected by a conduit 89 including a calibrated orifice 91. The connection with branch pipe 81 is made downstream from valve 87 so that fluid may be selectively added to the flow from orifice 91.

The operation of the device may be described as follows. A turbomachine rotor may shift axially, in either direction due to thrust bearing wear. According to the present invention, the thrust bearing wear detector includes a hydraulically balanced follower piston (the general operation of which is described in U.S. Pat. No. 2,888,023) which includes a slidable piston nozzle as distinguished from an integral nozzle portion of the cited patent. Moreover, fluid is supplied during normal operation to balance chambers 27a and 27b through pipe branch 77, conduit 89 and ports 29a and 29b. Valve 87 is closed so the operation is similar to the device in the cited patent except for calibrated orifice 91 which is external to the detector casing for easy maintenance.

In order to test the operativeness of the follower piston and nozzle, valve 85 may be opened causing only the follower piston to move toward the rotor while the piston nozzle remains stationary. This causes the pilot valve to move covering port 57a, thereby closing pressure switches 69. Thereafter, valve 85 is closed and valve 87 is opened causing an increase in pressure in chamber 27b, thereby moving follower piston 23 and nozzle 33 away from the rotor. This moves pilot valve 51 to cover port 57b, closing pressure switches 67. In this manner both trip positions of the follower piston are fully tested.

Movement of the follower piston is detected directly by linear variable differential transformer 63 and thus it is possible to fully set the trip points by moving sleeve 53 while still monitoring the follower piston. Hence, during trip circuit setting, continuous monitoring of rotor axial movement is maintained.

Finally, the use of redundant pressure switches reduces the probability of false tripouts due to failure of a pressure switch.

While there has been described what is considered to be the preferred embodiment of the invention, other modifications will occur to those skilled in the art. It is intended to cover, in the appended claims, all such modifications as fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. A device for following the axial movement of a rotating shaft including a casing mounted adjacent the rotating shaft having a hydraulically positioned follower piston slidably mounted within the casing defining opposite first and second chambers; the device further comprising:

a piston nozzle slidably mounted within the follower piston, the piston nozzle having an open end closely adjacent the rotating shaft defining a hydraulic gap therebetween; the first chamber in fluid communication with the second chamber and the second chamber in fluid communication with the piston nozzle and hydraulic gap;

a collar formed on the piston nozzle defining an internal chamber within the follower piston;

means for selectively introducing hydraulic fluid into the internal chamber; and,

means biasing the piston nozzle against the follower piston.

2. The device recited in claim 1 further comprising a calibrated orifice interconnecting the first and second opposite chambers.

3. The device recited in claim 2 wherein the calibrated orifice is outside the casing.

4. The device recited in claim 3 further including:

a first fluid supply connected to the first chamber;

a second fluid supply connected to the second chamber including a valve for selectively introducing the second fluid supply into the second chamber; and,

a pipe interconnecting the first and second fluid supplies at a point downstream from the valve, the pipe including the calibrated orifice.

5. The device recited in claim 4 further including:

a third fluid supply connected to the internal chamber; and,

means for selectively introducing fluid into the internal chamber.

6. The device recited in claim 1 further comprising:

a pilot valve positioned by the follower piston and hydraulically connected to first and second pairs of pressure switches; and,

an electrical alarm and shutdown circuit connected to two pairs of pressure switches, whereby loss of hydraulic pressure in either pair of pressure switches causes activation of the electric circuit.

7. The device recited in claim 1 further including a linear variable differential transformer having a rod slidable therein positioned by the follower piston.