Compression machinery method and apparatus

Compression machinery including a low pressure stage, a high pressure stage and a single aftercooler, further includes a bypass path through which fluid discharged from the low pressure stage is directed about the high pressure stage. The bypass path includes the aftercooler. Upon activation of the high pressure stage, a portion of the fluid flowing through the bypass path is diverted to the suction side of the high pressure stage. As a result of continued operation of the high pressure stage, the flow of fluid through the bypass path is decreased and the flow of fluid through the high pressure stage is increased.

BACKGROUND OF THE INVENTION 
This invention relates to compression machinery, and in particular, to a 
method and associated apparatus to accomplish stable starting and shutdown 
of multi-stage compression equipment employing a single aftercooler. 
In multiple stage compression machinery of the type wherein each 
compression stage is sequentially placed in operation only after the next 
lower stage has been operating at design conditions for a predetermined 
period of time, discharge gas coolers or aftercoolers are required to 
reduce the temperature of fluid discharged from each stage. As is well 
recognized, a substantial reduction or elimination of the heat developed 
by compression of the fluid is particularly important when the fluid is 
recirculated from the discharge side to the suction side of the 
compression stage. Heretofore, it has commonly been the practice to have 
separate gas coolers for each separate compression stage. 
In starting multi-stage compression machinery, the lowest pressure stage is 
initially activated. The fluid discharged therefrom is directed through a 
bypass or recirculation path including the aftercooler. The low pressure 
stage is operated in this manner for a predetermined time interval to 
insure that all mechanical parts of the equipment are functioning properly 
and to further permit the unit to thermally expand at minimal load 
conditions. 
When the next higher pressure stage is started, the fluid discharged 
therefrom is directed through a bypass or recirculation path also 
including the gas cooler. A portion of the fluid directed through the low 
pressure stage bypass path is now directed to the suction side of the 
operating high pressure stage. As flow requirements of the high pressure 
stage are increased, an increased proportion of the fluid discharged from 
the low pressure stage is diverted to the suction side of the high 
pressure stage. The increased flow to the high pressure stage and 
concurrent decreased flow through the low pressure stage bypass path 
should be accomplished in an efficient manner to avoid a loss of operating 
efficiency and to prevent the creation of operating problems, as for 
example surge conditions. Additionally, it is important that in 
installations having a number of multi-stage machines operating 
concurrently to handle a single load, that either of the stages of a 
single machine can be independently stopped without interfering with the 
operation of the remaining stages. 
SUMMARY OF THE INVENTION 
It is an object of this invention to employ a single aftercooler during 
starting conditions for at least two stages of multi-stage fluid 
compression machinery. 
It is a further object of this invention to automatically divert a portion 
of the flow of fluid passing through a first bypass path to the suction 
side of a high pressure stage. 
It is yet another object of this invention to divert an ever increasing 
proportion of the flow of fluid to the high pressure stage as the flow 
requirements of the high pressure stage increase. 
It is a further object of this invention to terminate operation of one 
stage without adversely affecting the performance of the stages still 
maintained in service. 
These and other objects of the present invention are attained in 
multi-stage fluid compression machinery having at least a low pressure 
stage, a high pressure stage, and a single aftercooler comprising first 
conduit means defining a bypass flow path to deliver fluid discharged from 
the low pressure stage through the cooler and then to the suction side of 
the low pressure stage. When the high pressure stage is activated, a 
portion of the fluid discharged from the low pressure stage is diverted to 
the suction side of the high pressure stage. The remaining portion of the 
fluid continues to pass through the bypass flow path. As flow requirements 
of the high pressure stage increase, the quantity of fluid passing through 
the bypass flow path is reduced concurrently with an increase in the 
quantity of fluid directed to the suction side of the high pressure stage. 
If it is desired to terminate operation of a low pressure stage while 
maintaining the high pressure stage in service, a hog gas bypass line is 
opened to provide a flow from the discharge from the low pressure stage 
back to the suction manifold. This gas is delivered to the suction side of 
the remaining low pressure stages. By raising the temperature of the gas 
in the suction manifold in this manner, the specific volume of the gas is 
similarly increased. Assuming the mass flow rate remains constant, the 
quantity of gas, in cubic feet per minute (cfm), delivered to the suction 
side of the remaining low pressure compressor stages will increase thereby 
lowering the discharge pressure therefrom. The decreased discharge 
pressure will be sensed and a signal generated to increase the speed of 
the remaining compressors to effectively handle the increased load thereon 
.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the single FIGURE of the drawing, there is schematically 
illustrated compression machinery embodying the present invention. The 
present invention is particularly suitable for use in applications 
wherein, during startup and shutdown of the machinery it is desirable to 
recirculate the fluid being compressed. 
The fluid to be compressed, for example a gas, is supplied via main conduit 
12 from a suitable source thereof, for example a well (not shown). Conduit 
12 delivers the fluid under pressure to the compression machinery string 
represented in general by reference numeral 10. In the arrangement 
depicted in the drawing, the system includes a plurality of multi-stage 
compression machinery strings represented in general by reference numerals 
10, 10', 10", etc. As each individual string of the system is identical, 
only string 10 will be described in detail. Also, although only two stages 
are illustrated for each string, the invention contemplates the addition 
of further stages. 
A valve 14 is provided to throttle the flow of fluid passing from conduit 
12 through line 20, to the first or low pressure stage 16 of compression 
machinery string 10. Stage 16 is operably connected to its own prime 
mover, represented by reference numeral 18. The compressed fluid leaves 
stage 16 via line 22. Line 22 delivers the compressed fluid to the 
junction 25 of lines 24 and 27. 
A one-way flow control or check valve 26 and a flow regulating valve 28 are 
disposed in line 27. When valve 28 is in a closed position, the compressed 
fluid flows through line 24 and thence through valves 30 and 32 to line 
34. Valve 30 is a one-way flow control or check valve similar to valve 26, 
and valve 32 is a flow regulating valve similar in design to valve 28. It 
is assumed that valves 30 and 32 are in an open state when valve 28 is 
closed. The fluid passing from line 34 flows to suction 36 of a discharge 
or gas aftercooler 38. Aftercooler 38 is provided with a heat transfer 
medium which flows in heat transfer relation with the compressed fluid. 
The compressed fluid transfers a substantial portion of the heat generated 
during the compression stage to the heat transfer fluid. The reduction in 
temperature of the compressed fluid is particularly required when the 
fluid is being recirculated during startup or shutdown operations. 
During initial startup of the compression string, the fluid discharged from 
cooler 38 is directed through line 40 in communication therewith. Valve 64 
disposed in line 68 is closed. Flow control valve 44 and flow regulating 
valve 42 are placed in the flow path defined by line 40. Line 78 having 
flow control valve 76 disposed therein defines a bypass path about valves 
42 and 44. A portion of the fluid passing through line 40 is returned, via 
valves 42 and 44, to line 20 for recirculation through compressor 16. 
Thus, lines 24, 34, 40, and 78, aftercooler 38, and valves 30, 32, 42, 44, 
and 76 define a bypass flow path about the second or high pressure stage 
50 for the fluid discharged from first stage 16 of the compression string. 
The remaining portion of the fluid flowing through line 40 is directed via 
valve 76 to conduit 12 "upstream" of throttle valve 14. 
After a predetermined time interval, to insure that first stage 16 is 
functioning without any mechanical problems, flow control valve 28 is 
opened to permit flow of fluid from line 22 through line 27 and thence 
into manifold 46. From manifold 46, the fluid passes through a line 51 
having a throttle valve 52 disposed therein, through line 54, and thence 
into the suction side of a second or high pressure stage 50. Compressor 
stage 50 is independently connected to its own prime mover 48. 
The compressed fluid discharged from stage 50 exits via conduit 56 having 
flow control valve 58 and flow regulating valve 60 disposed therein. The 
fluid passing through valve 60 is delivered via line 62 to suction 36 of 
aftercooler 38. The cooler functions to substantially eliminate the heat 
of compression developed in stage 50. 
For a predetermined time interval, it is desirable to maintain stage 50 in 
an unloaded state to assure there are no mechanical problems. Valve 72 is 
retained in its closed position and valve 64 in its open position whereby 
the fluid discharged from aftercooler 38 is directed through line 68 to 
line 54 for recirculation through high pressure stage 50. The compression 
machinery further includes line 80 which communicates with line 24. Line 
80 has valve 82 disposed therein to control the flow of fluid 
therethrough. The function of line 80 and valve 82 will be explained in 
detail hereinafter. 
Flow regulating valves 14, 28, 32, 42, 52, 60, 64, 76, and 82 may be 
manually controlled; however, these valves are preferably automatically 
sequenced to function in the described manner via pneumatic or electrical 
signals generated as a result of sensed operating conditions. Automatic 
operation of the valves in response to sensed operating conditions is 
considered to be within the skill of the art and a complete explanation 
thereof is not deemed necessary. 
OPERATION 
For a better understanding of the compression system heretofore described, 
the manner in which string 10 is started shall now be explained in detail. 
For initial startup, gas flowing through main supply conduit 12 is 
throttled by means of throttle valve 14 to a minimum predetermined 
pressure. During the initial startup procedure, flow regulating valve 28 
is in a closed position and valves 32 and 42 are in an open position. 
Valves 64 and 72 are also in closed positions. 
The fluid compressed by operation of low pressure stage 16 passes from line 
22 to line 24. The fluid is thence directed through cooler 38 whereat the 
heat of compression is removed from the compressed fluid. As valve 72 is 
closed and valves 42 and 76 are open, the cooled fluid is directed through 
line 40 back to the suction side of low pressure stage 16, and through 
line 78 to conduit 12 "upstream" of valve 14. 
Stage 16 will continue to operate in the above-described manner for a 
predetermined time interval. After the predetermined time interval has 
elapsed, valve 14 is slowly opened to increase the suction pressure to 
design conditions. Valve 28 then opens and valve 32 is slightly closed to 
reduce the quantity of fluid being recirculated through line 34. By 
opening valve 28, a portion of the fluid heretofore directed through line 
24 is diverted to pressurize manifold 46, from whence the fluid passes 
into line 51. 
Valve 60 is partially opened to permit fluid discharged from compressor 50 
to pass to suction 36 of cooler 38. Valve 60 maintains the pressure 
"downstream" thereof at the same magnitude as the pressure "downstream" of 
valve 32. This permits continued flow through lines 24 and 34. Valve 58 
prevents any reverse flow through lines 56 and 62. Fluid is delivered from 
manifold 46 via line 51. Valve 52 throttles the flow of fluid to the 
suction side of high pressure stage 50 to a predesigned pressure. Valve 64 
is opened and valve 72 remains closed to thereby direct the fluid through 
recirculation line 68 to the suction side of stage 50. At this time, 
cooler 38 is receiving compressed fluid from both low pressure stage 16, 
via lines 24 and 34, and high pressure stage 50, via line 62. Thus, only a 
single aftercooler is required to remove the heat of compression developed 
in each stage of the multi-stage string 10. 
As the discharge pressure of the high pressure stage is increased as a 
result of increased suction pressure, additional flow of fluid is directed 
from first stage 16 to manifold 46 to maintain pressure conditions 
therein. This requires a further closing of valve 32. As valve 60 opens 
further to increase the pressure downstream thereof, this downstream 
pressure will exceed the pressure downstream of valve 32, terminating flow 
through line 34 to cooler 38. Valve 30 will prevent any reverse flow 
through lines 24 and 34. Thus, as flow requirements of the high pressure 
stage increase, the flow through bypass path 24 and 34 is automatically 
terminated, thereby delivering all the fluid discharged from stage 16 to 
stage 50. 
The recycle flow from high pressure unit 50 proceeds through cooler 38 and 
is throttled back to the suction of the high pressure stage through 
control valve 64. Additional flow will pass through line 40 back to the 
suction side of low pressure stage 16 or via line 78, to conduit 14. After 
a predetermined period has elapsed to insure that the high pressure stage 
is properly functioning, valve 72 is gradually opened and valves 64 and 76 
are closed to permit passage of the compressed fluid through discharge 
line 74. Valves 42 and 64 may be maintained slightly open; however, the 
flow therethrough will be reduced to meet discharge requirements as 
determined by the demand placed on line 74. Each of the remaining stages, 
10' etc., will be started in an identical manner. 
As an additional feature, due to the use of manifold 46 and the utilization 
of separate prime movers for each stage of each compressor string, the 
operation of any one of the high pressure stages or low pressure stages of 
any one string may be separately discontinued without requiring the 
stoppage of the other stage of the particular string. For example, stage 
16 may be shutdown independently from stage 50. The reverse is also true. 
If stage 16 is stopped, a pressure sensor in manifold 46 transmits a 
signal to the prime movers for the remaining low pressure stages 16' etc., 
to increase the speed thereof which increases the flow therefrom. If this 
satisfies the flow requirements of the four high pressure stages 50, 50', 
etc., they will remain at their same operating speed. However, if required 
the speed thereof may be reduced to obtain stable operation. Similarly, if 
any high pressure stage is removed, the three remaining high pressure 
stages will accept flow from all four low pressure stages. If required, 
the speed of the three remaining high pressure stages may be increased for 
stable operation. Assuming it is desired to remove stage 16 from 
operation, valve 28 is closed, as are valves 32, 42, and 78. Valve 14 
remains open. Valve 82 in line 80 is opened. Thus, the discharge of 
relatively hot fluid from compression stage 16 will be directed, via line 
80 and valve 82 to inlet line 14. Thus, the temperature of the fluid 
flowing to the remaining low pressure stages 16', etc. will be increased. 
By raising the temperature of the fluid in suction line 12, the specific 
volume of the fluid is similarly increased. 
If the mass flow rate remains constant, the quantity of fluid in cfm 
delivered to the inlet to the remaining low pressure stages 16', etc. will 
increase, thereby lowering the discharge pressure therefrom. The reduction 
in discharge pressure from the remaining low pressure stages will be 
sensed and a signal generated to increase the speed of the remaining 
stages to efficiently and effectively handle the increased load thereon. 
After stable operation has been attained, low pressure stage 16 may be 
stopped. 
The foregoing arrangement permits a single aftercooler to accept the flow 
from more than one stage of a multi-stage compression machine. In 
addition, the flow of compressed fluid from the low pressure stage through 
a bypass circuit is automatically terminated as the flow requirements of 
the high pressure stage increase. This provides for efficient and stable 
operation of the compression machinery. Further, the termination of 
operation of one or more stages may be effectively accomplished without 
necessitating the stoppage of the entire compression string. 
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.