GB2046848A - Preventing overspeed in a power recovery system - Google Patents

Abstract

A power recovery system includes an expander to receive and be driven by relatively high temperature fluid operatively connected to drive an electrical power generating device, an aerodynamic friction brake being connected to the expander for absorbing power generated thereby, the brake being rendered operable as a result of the predetermined operable condition of the expander, e.g. overspeed. The system may be used to provide fine speed control for example on synchronous generator. <IMAGE>

Description

SPECIFICATION Power recovery system and method This invention relates to a system for recover ing power from a pressurized or relatively high temperature waste heat fluid and in particular, to such a system utilizing a dynamic brake to control the speed of the expansion means employed in the power recovery system.

In many installations, such as refineries and steel mills, the process utilized in the manufacture of a product results in the production of pressurized or relatively high temperature waste heat fluids. As a result of the shortage of energy and the increased cost of fossil fuels, it is extremely desirable that the energy contained within the pressurized or high temperature waste heat fluid be employed to generate usable power.

For example, in refineries employing a catalytic cracking process, substantial quantities of waste heat fluids are generated at a relatively high pressure or temperature. The waste heat fluid in some instances has been employed to drive a turbomachine such as a fluid expander. The expander has heretofore been directly coupled to a compressor and operates as the prime mover therefore. The compressor is generally employed to provide substantial quantities of high pressure air required in the catalytic cracking process. As shall be more fully explained hereinafter, it has been necessary to directly connect the expander to the compressor to maintain a power absorbing load on the expander. It is understood that such direct connection might include a suitable speed changer where applicable.

However, many of the established refineries do not have the expander-compressor string affixed therein. In such refineries, it has been the general practice to emply an electric motor or steam turbine to drive the compressor, to provide an expander to utilize the energy of the pressurized or high temperature waste heat fluid and directly drive the compressor would be extremely costly if attempted on a "retrofit" basis.

For example, refinery floor space is generally at a premium. Thus, in established plants, there is seldom ample available space in the vicinity of the compressor to locate the expander. Furthermore, it would be necessary to interrupt the process during the time when the compressor and expander were being mechanically connected. Generally, a process shutdown results in a severe economic penalty and thus is avoided if at all possible.

Dor the above reasons, it has not been deemed advisable to add an expander on a "retrofit" basis to established plants, such as refineries, even though the same would be extrememly desirable from the standpoint of energy conservation and reduction of the operating costs of the process.

As previsouly noted, it has heretofore been necessary to have the expander directly connected to the compressor. The elimination of this requirement would permit the expander to be located wherever convenient. Additionally, the expander could be used to drive a generator rather than a compressor with the generator developing electrical power for uses in addition to driving the compressor's motor.

The reason for such requirement may be readily observed from the following discussion. The pressurized or relatively high temperature waste heat fluid furnished to the expander is a high mass flow, low density, high specific volume fluid. Generally, the fluid is furnished through relatively large conduits, as for example 48 inch diameter pipes. Thus, upon abrupt shutdown of the expander, whereby valves located upstream of the expander are closed to interrupt the flow of fluid thereto, a substantial volume of fluid is trapped between the shutoff valves and the inlet to the expander. It is highly imperative that the energy of the trapped fluid be dissipated without permitting the speed of the expander to exceed safe limits.

When the expander has been connected to a compressor, the compressor has operated as a power absorbing load which has enabled the expander to safely dissipate trapped energy. However, if the expander were employed to drive a generator and the electrical load on the generator were suddenly disconnected, the expander-generator string might accelerate rapidly and exceed safe operating speeds due to the absence of a power absorbing load. The generator inertia would not, in the absence of an electrical load, prevent the expander string from accelerating to unsafe speeds. Thus, expander-generator strings have always included compressors to insure that a power absorbing load is continuously attached to the expander.

In addition, in many process applications utilizing a power recovery expander, "afterburn" of the fluid may sometinme occur, thereby increasing the energy level of the fluid suppplied to the expander above design conditions. Due to the resulting excessive energy level of the waste heat fluid, it has been necessary to oversize the generator, of an expander-compressor-generator string, by as much as 20% to absorb the excess energy available in the fluid due to the occurence of an "afterburn". This excess available energy must be absorbed by the generator, as the compressor's power absorbing capabilities are exposed during "afterburn" conditions. By providing a 20% oversized generator, the generator normally operates inefficiently.As is obvious, it is desirable to reduce the generator's size to, not only reduce the manufacturing cost thereof, but also to increase the operating efficiency thereof.

Furthermore, it has heretofore been imprac ticable to utilize an expander to drive a synchronous generator. A synchronous generator is extremely speed sensitive. Due to the properties of the fluid delivered to the expander, acceleration of the expander and thus the speed thereof could not be accurately controlled to permit the synchronous generator to be connected to a power supply grid.

These and other prior art disadvantages are overcome, in accordance with the present invention, in a system for recovering power from a relatively high temperature waste heat fluid which includes expansion means to receive the relatively high temperature fluid.

The expansion means is operatively connected to an electrical power generating means, with the expansion means driving the generating means as a result of the expansion of the high temperature fluid therethrough. An aerodynamic friction brake means is connected by said expansion means. The brake means is rendered operable upon the occurrence of a predetermined operational condition of said machine.

This invention will now be described by way of example, with reference to the accompanying drawing in which there is schematically illustrated a power recovery system utilizing the aerodynamic friction brake means of the present invention.

In particular, power recovery system 10 includes an expander 12, coupled through a suitable speed reducing mechanism 16, to an electrical generator 14 which may either be an induction or synchronous generator. The expander and generator are axially aligned and include a shaft 20 coupled to an aerodynamic friction brake means 22.

Aerodynamic friction brake means 22 is preferably a windage generator having a rotor 23 comprising a wheel or disc 24 or a series thereof and blades 26 mounted within a chamber 27 defined by shell or casing 29.

Chamber 27 normally contains a relatively low density fluid or is evacuated to maintain the load generated by rotor 23 on expander 1 2 at a minimum. By maintaining a vacuum within chamber 27, rotor 23 essentially moves within a frictionless environment thereby limiting to a relatively insignificant level the work required to be performed by he expander in turning rotor 23. A valve 46 selectively communicates chamber 27 with a source of fluid 48 at a relatively high pressure or relatively high density.

Expander 12 is provide with a source of relatively high temperature waste heat fluid through inlet conduit or line 32. The waste heat fluid is expanded through device 1 2 and exits therefrom through conduit 40. Valve 34 is disposed in line 32 upstream from expander 12. Additionally, valves 36 and 38 are respectively disposed in conduits 28 and 42 for a reason to be more fully explained hereinafter. Conduit 28 communicates with a suitable waste heat recovery apparatus 30, as for example a boiler or similar device. Sensor means 44 is provided for sensing the rotational speed of shaft 20. Sensor 44 generates a signal for opening normally closed valve 46 which thereby communicates chamber 27 with the source of relatively high pressure.

There are many known processes which produce relatively high temperature waste heat fluid whch may be satisfactorily employed to generate electrical power as a result of the recovery of the energy contained within the fluid. One such process is involved in oil refinery installations and is known as a catalytic cracking process. In such a process, the waste heat fluid is developed in a regenerator (not shown) which communicates with line 32 via line 28.

The flow of waste heat fluid from the regenerator through lines 28 and 32 to expander 1 2 and the expansion of the fluid therethrough, results in operation of the expander and consequently, the generator connected thereto. The generator delivers electrical power through electrical conductors 50, 52 and 54 to an electrical load (not shown). The electrical load may include the electrical motor employed to drive the compressor and may also include other electrically operated components in the plant, or in some instances, may include the commercial grid served by a public utility.

As noted previously, it is extremely important, due to the fossil fuel shorgages and the increased cost of fossil fuel, that power be obtained from normally unused energy sources. In many refineries or other applications, high pressure gas, as for example air, is required to support the manufacturing process of the installation. Typicaklly, a compressor used to provide the high pressure gas, is driven by an electrical motor furnished with electrical energy from a power utility. In order to reduce the operating cost of the process and in addition, reduce the load requirements on the utility, it is extremely desirable that the installation generate as much of its own electrical power as is possible. The present invention permits an expander-generator string to be situated at a point remote from the compressor and furnish electrical power to the compressor motor. Thus, since the string may be positioned at a remote location, a minimum disruption of the actual process will occur when the string is retrofitted into an already exiting installation.

Heretofore it has been necessary to utilize the compressor as a power absorbing load on the expander for preventing the expander from operating above a safe speed. In essence, this has mandated that the compressor be directly coupled to the expander. Thus, the typical installation has included an expander, compressor and generator coupled together on a common shaft, with the expander driving both the compressor and generator. The generator has been limited to those of the induction type as synchronous generators are extremely speed sensitive. Heretofore, the inability to regulate the speed of the expander within a very small range has prevented users from obtaining a fine speed control that is a prerequisite for synchronizing the speed of a synchronous generator before the generator is connected to an electrical load such as the public power grid.

Furthermore, very often in processes producing a relatively high temperature waste heat fluid, a phenomenon called afterburn occurs which results in the temperature of the waste heat fluid rising substantially above normal operating conditions. The extra energy thus contained within the waste heat fluid has to be dissipated within the expander.Thus where a generator has heretofore been cdupled to the expander, it has been necessary to oversize the generator by as much as 20% so that the generator produces an adequate load on the expander to absorb the extra power produced during afterburn conditions. In the absence of the oversized generator, the generator must be disconnected from the grid otherwise an electrical overload of the generator will occur.As is obvious, in employing an oversized generator, the generator normally runs at less than its optimum designed conditions resulting in inefficient performance of the generator. In addition, the oversized generator increases the manufacturing costs of the expander-generator string.

In order to overcome the above-mentioned difficulties, the present invention includes an aerodynamic friction brake operable when the expander is operating at other than normal operating conditions. As used herein, "other than normal operating conditions" shall include starting conditions, sudden unloading of the generator and afterburning conditions all of which are transient, as opposed to steady state conditions.

During normal operation, the relatively high temperature waste heat fluid flows through line 32 and valve 34 to expander 1 2. Should it be necessary to suddenly disconnect the generator from the electrical load, it becomes necessary to immediately shut down expander 12; valve 34 is closed and valves 38 and 36 are opened. This results in the flow of fluid being directed through line 28, valve 36 and liiie 37 to waste heat recovery boiler 30. In addition, since valve 38 is opened, the relatively high temperature waste heat fluid captured between valve 34 and expander 1 2 will escape. However, since mechanical actions are required to move the various valves in the desired sequence, a delay of two or three or sometimes more seconds will occur before the flow of fluid to the expander is actually terminated.During this relatively short period of time, the expander string may accelerate beyond safe operating speeds. Such an abrupt increase in the speed of the expander results from the high mass flow, high specific volume properties of the fluid. In addition, there will be a relatively large trapped volume of the fluid between valve 34 and the expander due to the size of the conduit; for example 48" diameter is normally employed to deliver the fluid to the expander. Before valve 38 can fully open to exhaust the fluid, the fluid will enter the expander. Since the generator is "unloaded" the expander will accelerate very rapidly as the fluid continues to expand therethrough.

In order to prevent the uncontrolled acceleration of expander 1 2 when the electrical load on the generator 14 is removed, sensor means 44 immediately senses the occurrence of the abrupt acceleration increase. Sensor 44 generates a control signal which immediately opens valve 46 communicating chamber 27 with a source of relatively high pressure fluid 48. The fluid is delivered into chamber 27 which results in a load being placed on rotor 23 of aerodynamic friction brake means 22.

The rotation of the rotor through the relatively high pressure or relatively high density medium within chamber 27 places a power dissipating or absorbing load on the expander. By enabling the expander to safely dissipate the energy contained within the fluid trapped between valve 34 and the expander, the expander is prevented from accelerating to unsafe speeds. When sensor means 44 determines that shaft speed 20 has decreased to relatively safe limits, a signal will be generated to close valve 46. Additionally, a further signal will be generated to permit the evacuation of the fluid from chamber 27 to again form a vacuum within the chamber. Sensor means 44 enables valve 46 to open to permit the pressurization of chamber 27 anytime shaft speed increases above a predetermined level as for example when the load on the generator is removed of during the occurence of an afterburn.

If generator 1 4 is of the synchronous type, the aerodynamic friction brake means 22 is further operable to bring the expander and generator to synchronous speed for enabling an operator to connect the expander-generator string with the load. During startup of the expander-generator string, valve 46 will be opened to fine tune the speed of the expander. In effect, by utilizing the brake means 22 a variable brake torque may be placed on shaft 20. Thus, the speed of the generatorexpander string may be readily controlled.

As may be readily observed the aerodynamic friction brake means 22 of the present invention achieves a multiplicity of functions in that it provides a fine tune speed control when expander 1 2 is driving a synchronous generator. In addition, the brake means eliminates the need for the expander to be directly coupled to a compressor. The brake provides a readily available source to absorb power on an instantaneous basis should the normal load be disconnected from the expander-generator string. Furthermore the brake means eliminates the need to oversize the generator coupled to the expander as heretofore required for providing means to dissipate the additional energy contained within the motivating fluid upon the occurrence of an afterburn.

Aerodynamic friction brake means 22 effectively provides all the functions of the hereinabove described apparatus. It should be specifically understood that although system 10 has been specifically described relative to a catalytic cracking process, the present invention should not be limited thereto but may be satisfactorily employed in any other process wherein a pressurized or relatively high temperature waste heat fluid is used as a source of energy.

While a preferred embodiment of the present invention has been described and illustrated, the invention should not be limited thereto but may be otherwise embodied within the scope of the following claims.

Claims (10)

1. A system for recovering power from a pressurized, high temperature waste heat fluid comprising expansion means for receiving said waste fluid; and electrical power generating means connected to said expansion means and operated thereby as a result of the expansion of the fluid therethrough; including selectively operable aerodynamic friction brake means connected to said expansion means for absorbing power generated by said expansion means and sensor means responsive to a predetermined operational state of said expansion means for rendering said brake means operable.

2. A system for recovering power in accordance with claim 1, wherein said aerodynamic friction brake means includes a casing defining a normally evacuated chamber; a rotor mounted for rotation within said evacuated chamber; a rotor mounted for rotation within said evacuated chamber; and delivery means connected to said sensor means for supplying a fluid to said chamber upon the occurence of said predetermined operational state whereby rotation of said rotor through said fluid dissipates energy generated by said expansion means for preventing the speed of said expansion means from exceeding a predetermined magnitude.

3. A system in accordance with claim 2, wherein said delivery means includes a valve for communicating said chamber with the atmosphere for delivering air into said chamber, including means to open said valve upon the occurrence of said predetermined operational state.

4. A system in accordance with claims 1 or 2, wherein said sensing means is activated in response to an overspeed condition of said expansion means.

5. A method of recovering power from a high temperature waste heat fluid comprising the steps of expanding the waste heat fluid through a turbomachine; and generating electrical power as a result of the expansion of fluid; including aerodynamically frictionally braking the turbomachine when said machine is operating other than at steady state conditions.

6. A system for recovering power from a relatively high temperature, high mass flow, low density, high specific volume waste heat fluid formed directly from the combustion of a process gas comprising expansion means for receiving said relatively high temperature waste heat fluid; and electrical power generating means connected to said expansion means and operated thereby as a result of the expansion of the fluid therethrough; including selectively operable aerodynamic friction braking means connected to said expansion means for absorbing power generated by said expansion means; and sensor means responsive to a predetermined operational state of said expansion means for rendering said brake means operable.

7. A system in accordance with claim 6, wherein said aerodynamic frictio brake means includes a casing defining a normally evacuated chamber; a rotor mounted for rotation within said evacuated chamber; and delivery means connected to said sensor means for supplying a fluid to said chamber upon the occurrence of said predetermined operational state whereby rotation of said rotor through said fluid dissipates energy generated by said expansion means from preventing the speed of said expansion means for exceeding a predetermined magnitude.

8. A system in accordance with claim 7, wherein said delivery means includes a valve for communicating said chamber with the atmosphere for delivering air into said chamber, including means to open said valve upon the occurrence of said predetermined operational state.

9. A system in accordance with claims 6 or 7, wherein said sensing means is activated in response to an overspeed condition of said expansion means.

10. A system as claimed in claim 1 or 6, substantially as herein described, by way of example, with reference to the accompanying drawings.