Gas Turbine Power Plant

Abstract

The present invention relates to an arrangement of controlling a compressor station for a gaseous medium in which the compressor station comprising turbine driven compressors is interposed between a source of a gaseous medium and a consumer. The compressors have associated therewith controllable bypass conduit means so that the gaseous medium coming from the said source can selectively be passed through said compressors in succession or can bypass at least some of said compressors.

Parent Case Text

This is a continuation of application, 622,730 filed Mar. 13, 1967, now abandoned.

Description

The present invention concerns an arrangement of and a method for controlling compressor stations for gaseous media using gas turbines as prime movers which latter serve for compensating for the pressure drop in the conduits so as to maintain a minimum pressure at the consumer.

The object of the present invention consists in providing the arrangement and control of a plant of the above mentioned type, as used for instance for supplying a pipe line net for natural gas, in such a way that the driving gas turbines as well as the compressors during the time period during which the station is only under partial load, i.e. operates with reduced delivery, will work within the range of a favorable degree of efficiency and thus will operate economically.

This object has been realized according to the present invention by equipping each compressor station with a plurality of sets of turbocompressors which with regard to the direction of flow are arranged one behind the other and which can be bridged by bypass conduit means adapted to be closed. Said turbocompressors are variable as to speed. The turbocompressors are adapted, when the delivery drops and the bypass conduits are closed, to be controlled so that beginning with the last step they are one after the other reduced as to speed, then by opening the respective associated bypass conduit are turned off as far as flow is concerned and are stopped. In view of the arrangement of the sets of compressors in series, the conditions are created for economically operating the station when only partial quantities are delivered, i. e. when a lower output is consumed than at maximum flow, since the delivery drops rapidly with a reduction in the throughflow.

If, in conformity with the present invention, with dropping throughflow the speed of the respective last compressor set is reduced in conformity with the course of the delivery curve, and if said last mentioned compressor set is eventually bypassed, the respective still operating gas turbines will operate in the upper output range and thus within range of a satisfactory degree of efficiency. Furthermore, due to the fact that always only so many compressor sets are in operation as is necessary under the respective circumstances, running time is saved which in its turn results in longer periods of operation of the compressor sets between fundamental overhauls.

When designing the control, attention is to be paid to the fact that with a reduction in the throughflow through the station in view of the inherent reduction in the flow losses in the conduits, the pressure drop from the source of the medium to the station and from the latter to the consumer decreases. This means that with a partial delivery, the input pressure of the station is higher than at maximum throughflow and that the station output pressure can be less than with maximum throughflow. With decreasing throughflow, also the pressure build-up to be produced by the station will decrease. More specifically, with partial deliveries, the pressure build-up is a minimum when the control is so effected that the end pressure at the consumer maintains the value which it had at maximum throughflow, or is a maximum if the control is effected for a constant output pressure of the compressor station. In view of the buffer effect occurring with variations in the delivery, it is not advantageous to effect the control to obtain a minimum. On the other hand, it is disadvantageous to control the effect to a maximum because in such an instance higher compressor outputs are required. In practice, it is expedient to go beyond the minimum to such an extent that the end pressure corresponding to the maximum throughflow does not drop at the consumer which means that it is expedient to control for the minimum possible pressure build-up, a method suggested according to a further development of the present invention.

To approximate the minimum build-up of the pressure furthermore results in the advantage that from a certain partial delivery downward, the starting pressure at the source will suffice to assure at the consumer the required minimum pressure without additional pressure build-up at the station.

According to the present invention it is suggested with plants in which the source pressure is constant to base the control for the operation of the compressor station on the input pressure at the compressure station.

It is furthermore advantageous according to a further development of the invention, to provide identical compressor sets which must have a sufficiently wide characteristic field. This has the advantage that the compressor sets can be interexchanged and that the stocking of replacement parts is less expensive.

The present invention is illustrated by way of example in the accompanying drawings, in which:

FIG. 1 diagrammatically illustrates a natural gas supply line with a compressor station having three compressor sets. This figure also graphically shows the course of the pressure from the natural gas source to the consumer.

FIG. 2 is a graph illustrating the course of the obtainable fuel consumption with a compressor station arrangement according to the invention.

FIG. 3 shows a compressor characteristic field explaining the operation of the compressor station according to FIG. 1.

Referring now to the drawings in detail, the plant shown in the upper portion of FIG. 1 comprises a source 1 of natural gas, a supply line 2 leading to the compressor station, and an output line 4 leading to a storage container 5 to which the consumers are connected. In the lower portion of FIG. 1, over the flow path S of the natural gas from source 1 to the storage container 5 there has been plotted the course of the pressure p. The compressor station 3 has three identical compressor sets each of which comprises a gas turbine 6, 7 and 8, a turbo-compressor 9, 10 and 11. Flowwise, the compressors 9, 10 and 11 are arranged in series with regard to each other and are adapted to be bridged by bypass conduits 15, 16 and 17 respectively which are controlled by shut-off valves 12, 13 and 14. Moreover, all of the compressor sets are controllable individually as to their speed.

During a maximum throughflow, the source pressure p.sub.1 drops due to the flow losses in conduit 2 up to the compressor station 3 to the station input pressure p.sub.2. All compressor sets are in operation, and the valves 12, 13 and 14 are closed in bypass conduits 15, 16 and 17. At the said station, a pressure build-up is effected up to the pressure p.sub.1 which in its turn in the supply line 4 to the consumer storage container 5 drops to the desired end value p.sub.2. If now the throughflow drops for instance to such an extent that the pressure ahead of the station increases to p.sub.3, according to the present invention, first the compressor 11 is controlled downwardly as to its speed to such an extent that the end pressure in conduit 4 does not drop below p.sub.2. Depending on the reduction in the throughflow, the compressor 11 is bypassed flowwise by opening the shut-off valve 14, the compressor set is stopped and, if desired, the next compressor 10 may be controlled downwardly as to speed.

With regard to the extent to which the pressure build-up is to be effected in station 3 within the range of partial delivery, there exists the possibility on one hand always to compress to the station output pressure p.sub.1. The second possibility consists in always compressing only so high (in the illustrated example to p.sub.4) that, taking into consideration the decrease in the flow losses in conduit 4 as a result of a reduction in the throughflow, the pressure at the inlet to container 5 will not drop below p.sub.2. The last mentioned control method has the advantage that it is more economical than the other method. When controlling for constant station output pressure, it would be necessary with the selected example to have the first two compressors run at full speed and to have the third stage run at reduced speed. If, however, a pressure build-up is produced according to the second method, only the first compressor has to run at full speed while the second compressor is operated at reduced speed and while the third compressor is stopped completely and bypassed.

FIG. 2 illustrates the course of the fuel consumption. As will be seen, the fuel consumption B is plotted over the throughflow per weight. Curve I shows the course of the fuel consumption within the range of partial delivery and with a pressure build-up for uniform end pressure at the compressor station. The irregularity of the curve is due to the successive switching-off and switching-on of the individual compressor sets. Curve II analogously shows the course of the fuel consumption when the pressure is built up to a station output pressure which will assure the minimum admissible pressure at the consumer end.

FIG. 3 shows a compressor characteristic field in which the pressure ratio p/p is plotted over the throughflow per volume at the compressor inlet V.sub.e. The curve P represents the pumping limit of the compressor. n.sub.1 to n.sub.4 represent the lines of the same compressor speed, n.sub.1 corresponding to a compressor speed of 100 percent. Within the said characteristic field, there are entered the lines of operation of the three serially arranged compressors. The points a, b, c, d respectively refer to the operation with uniform starting pressure of the compressor stations, while the points A, B, C, D refer to operations with a minimum starting pressure at the station at which the end pressure at the consumer is still assured. The index 1 always refers only to compressor 9, while the index 2 refers to the compressor 10, and the index 3 refers to the compressor 11.

When full throughflow takes place, all three compressors respectively run within the operational points a.sub.1, A.sub.1 ; a.sub.2, A.sub.2 ; a.sub.3, A.sub.3. When the throughflow drops to such an extent that the third stage of the compressor has to be switched off, the points of operation of the first two compressor stages are displaced along a line of uniform speed toward the left to the corresponding points b.sub.1, B.sub.1 ; b.sub.2, B.sub.2, whereas the operational lines of the third stage drop to up to the pressure ratio p/p = 1 to which the points b.sub.3, B.sub.3 on the abscissa correspond. When the throughflow drops further to a level at which the delivery of a single compressor will suffice, the operational points of the first compressor stage will displace themselves accordingly to c.sub.1, C.sub.1, whereas the operational lines of the second stage drop from b.sub.2, B.sub.2 to a pressure ratio 1 in conformity with the points c.sub.2, C.sub.2 on the abscissa. With a further reduction in the throughflow, the operational lines of the first stage drop from points c.sub.1, C.sub.1 to the points d.sub.1, D.sub.1.

The illustrated lines of operation show that for the operation with uniform output pressure of the compressor station, the characteristic field may be wider than in other cases but will always be located within still admissible limits. The closer the approach to the control method at which only one station output pressure is produced which still suffices to assure the minimum pressure at the consumers, compressors with narrower and thus with such characteristic fields can be employed which have a somewhat higher optimum degree of efficiency. Thus, with the last mentioned control method, the operation will also on the compressor side take place in an area of a more favorable degree of efficiency.

It is, of course, to be understood that the present invention, is, by no means, limited to the particular arrangements shown in the drawings but also comprises any modifications within the scope of the appended claims.

Claims

I claim:

1. The method of controlling the pressure of the gaseous medium in a main conduit leading from a constant source of gaseous medium at a substantially constant pressure to a receiver station to maintain a constant minimum pressure at said receiver station, in which a compressor station is interposed in said conduit distant from said receiver station and said source and at which the input pressure varies with variations in the volume of flow of said gaseous medium, said compressor station comprising a plurality of individually speed controllable centrifugal compressors and a plurality of individually variable speed regulated gas turbines, each connected to one of said compressors, and individually controllable bypass circuits associated with said compressors for bypassing each of said compressors, said method including operating only the number of turbines and compressors necessary to provide an output pressure to maintain a substantially constant pressure at said receiver station while bypassing each inoperative compressor by a bypass circuit, and varying the output pressure at said compressor station by operating all but one of the operating compressors at full capacity and controlling the operation of only one turbine and its compressor to vary the output pressure in accordance with the input pressure at said compressor station to produce an output pressure variable with the volume of flow of said gaseous medium to maintain a substantially constant pressure at the receiver station with varying flow of said gaseous medium through said conduit.