Flow control

Control of a pumping station is accomplished in such a manner so as to maximize the energy efficiency of the pumping station by utilizing a variable drive pump to maintain the pipeline suction pressure seen at the input to the pumping station at an acceptable level. If the pipeline suction pressure decreases below a level which can be controlled by the variable drive pump, then a control valve located at the output of the pumping station is closed to the extent necessary to maintain the pipeline suction pressure at an acceptable level. If the pipeline suction pressure is greater than can be controlled by the variable drive pump then additional pumps are brought online to prevent overloading of the variable drive pump.

This invention relates to flow control. In a specific aspect this invention 
relates to method and apparatus for maximizing the energy efficiency of a 
pumping station. 
The transportation of liquid and liquefied products in the petroleum 
industry by long distance pipelines is widespread. Because of the pressure 
drop along the pipeline due to friction between the liquid and the pipe, 
it is necessary to utilize pumping stations positioned along the line in 
order to boost the pressure from station to station along the line. In the 
operation of such stations, particularly where large pumping units are 
installed, it is desirable to maximize the energy efficiency of the 
pumping station. 
In the past there has not been a concerted effort to maximize the energy 
efficiency of pumping stations, primarily because the fuel utilized was 
relatively cheap in comparison to the cost of installing the control 
systems and variable drive pumps required to maximize the energy 
efficiency of a pumping station. In the past it was common to maintain the 
pipeline suction pressure into the pumping station at a desired level 
simply by closing a control valve at the output of the pumping station, 
even though the closing of the control valve resulted in a lower pumping 
efficiency. It was also common to keep a sufficient number of pumps online 
to handle any predictable load even though all of the pumps utilized were 
not required a majority of the time. Obviously, the use of unneeded pumps 
would result in large, unrequired fuel expenditures which would result in 
a decreased energy efficiency of the pumping station. 
Accordingly, it is an object of this invention to provide method and 
apparatus for increasing the energy efficiency of a pumping station. 
In accordance with the present invention, method and apparatus are provided 
whereby a variable drive pumping means is utilized as the primary pump in 
the pumping station. The variable drive pumping means is controlled in 
response to a measurement of the pipeline suction pressure seen at the 
input to the pumping station. Thus, if the pipeline suction pressure 
begins to decrease the input-output speed ratio of the variable drive 
pumping means is increased to maintain the pipeline suction pressure at 
some desired level. If the input-output speed ratio of the variable drive 
pumping means approaches the highest possible value and the pipeline 
suction pressure continues to decrease, then the control valve located at 
the output of the pumping station is utilized to restrict the output flow 
and thus maintain the pipeline suction pressure input to the pumping 
station at a desired level. As suction pressure begins to rise, the 
input-output speed ratio of the variable drive pumping means is decreased. 
If suction pressure continues to rise after the variable drive pumping 
means has reached the maximum desirable input-output speed ratio, then a 
warning is sounded and additional direct drive pumps are put online. As 
the additional standby direct drive pumps are put online the pipeline 
suction pressure will begin to drop, thus allowing the input-output speed 
ratio of the variable drive pumping means to be increased. In this manner 
the variable drive pumping means is controlled so that it is not operating 
at maximum power unless such power levels are required, thus optimizing 
the energy efficiency of the pumping station when high pipeline suction 
pressures are seen. 
Other objects and advantages of the invention will be apparent from the 
description of the invention and the appended claims as well as from the 
detailed description of the drawing which is a schematic diagram of a 
pumping station with an associated control system. 
For the sake of simplicity the invention is illustrated and described in 
terms of a single pipeline and a pumping station which utilizes a single 
primary variable drive pumping means with two direct drive pumping means 
as standby units. The invention, however, is applicable to multiple 
pipeline configurations and is also applicable to pumping stations which 
employ different numbers of pumping means and different configurations of 
pumping means. 
Although the invention is illustrated and described in terms of a specific 
pumping station, the applicability of the invention described herein 
extends to other pumping station configurations and also extends to 
different types of control system configurations which accomplish the 
purpose of the invention. Lines designated as signal lines in the drawings 
are electrical in this preferred embodiment. However, the invention is 
also applicable to pneumatic, mechanical, hydraulic, or other signals 
means for transmitting information. In almost all control systems some 
combination of these types of signals will be used. However, use of any 
other type of signal transmission, compatible with the process and 
equipment in use, is within the scope of the invention. 
Controllers shown may utilize the various modes of control such as 
proportional, proportional-integral, proportional-derivative, or 
proportional-integral-derivative. In this preferred embodiment 
proportional-integral controllers are utilized. The operation of these 
types of controllers is well known in the art. The output control signal 
of a proportional-integral controller may be represented as 
EQU S=K.sub.1 E+K.sub.2 .intg.Edt 
where 
S=output control signal; 
E=difference between two input signals; and 
K.sub.1 and K.sub.2 =constants. 
As used in the following description of an exemplary embodiment of this 
invention, an input-output speed ratio of 1:1 for a variable drive pumping 
means indicates that the pumping means is operating as its maximum speed. 
An input-output speed ratio of 2.5:1 indicates that the pumping means is 
operating at its lowest speed. An increase in the input-output speed ratio 
indicates that the variable drive is changing from an input-output speed 
ratio of 1:1 to 2.5:1. These speed ratios are used solely as examples in 
the preferred embodiment of the invention. Any applicable speed ratios 
could be used.

Referring now to the drawing, a product, suitable for transport in a 
pipeline, is transported from a source 11 of the product through a 
pipeline 12 to a destination 22 for the product. Because the source 11 of 
the product and the destination 22 for the product are separated by long 
distance, a pumping station 13 is utilized to boost the pressure along the 
line. The product being transported is fed sequentially from the input of 
the pumping station 13 through check valve means 14, 16 and 18 and a 
pneumatically operated control valve 21, all of which are located in 
pipeline 12, to the output of the pumping station 13. The direct drive 
pumping means 27 is placed online by manually or automatically opening 
shutoff valve means 26 and 28. Check valve means 14 will automatically 
close. The direct drive pumping means 32 is placed online by manually or 
automatically opening shutoff valve means 31 and 33. Check valve means 16 
will automatically close. The variable drive pumping means 45, which is 
made up of the pumping means 42 with its associated variable drive 
transmission 43, is placed online by manually or automatically opening 
shutoff valve means 41 and 44. Check valve means 18 will automatically 
close. In this preferred embodiment the variable drive transmission 43 is 
a variable speed synchrodrive manufactured by Philadelphia Gear 
Corporation. 
Pressure transducer 51 measures the pipeline suction pressure at the input 
of the pumping station 13 and transmits a signal 52, representative of the 
pipeline suction pressure at the input of pumping station 13, to pressure 
controller 54. Pressure controller 54 is also supplied with a set point 
signal 55 representative of the minimum acceptable pipeline suction 
pressure. Pressure controller 54 compares signals 52 and 55 and transmits 
a signal 58, representative of a function of the difference between 
signals 52 and 55, to low select means 59. 
Signal 61, representative of the current being drawn by direct drive 
pumping means 27, is provided from direct drive pumping means 27 to the 
current controller 63. Current controllers may also be referred to as 
amperage controllers and are labeled AC for amperage controller in the 
drawing. The current being drawn by any of the pumping means 27, 32, 45 
illustrated in FIG. 1 is a function of the load on the pumping means 27, 
32, 45. Current controller 63 is also provided with a set point signal 65 
which is representative of the maxmimum desirable current which can be 
drawn by direct drive pumping means 27. Signal 66, representative of a 
function of the difference between signals 61 and 65, is transmitted from 
current controller 63 to low select means 59. 
Signal 71, representative of the current being drawn by direct drive 
pumping means 32, is supplied from direct drive pumping means 32 to the 
current controller 73. The current controller 73 is also provided with a 
set point signal 75 which is representative of the maximum current which 
can be drawn by direct drive pumping means 32. Signal 76, representative 
of a function of the difference between signals 71 and 75, is transmitted 
from current controller 73 to low select means 59. 
Signal 81, representative of the current being drawn by variable drive 
pumping means 45, is transmitted from variable drive pumping means 45 to 
current controller 83. Current controller 83 is also provided with a set 
point signal 85 which is representative of the maximum current which can 
be drawn by variable drive pumping means 45. Signal 86, representative of 
a function of the difference between signals 81 and 85, is transmitted 
from current controller 83 to low select means 59. 
Low select means 59 compares signals 58, 66, 76, and 86 and selects the 
input signal having the lowest value to be output as control signal 91. 
Control signal 91 is provided to both the variable drive pumping means 45 
and the pneumatically operated control valve 21. 
In this preferred embodiment pressure controller 54 is a direct acting 
controller. Direct acting means that as the pipeline suction pressure 
increases the magnitude of signal 58 will increase, and as the pipeline 
suction pressure decreases, the magnitude of signal 58 will decrease. 
Current controllers 63, 73 and 83 are reverse acting controllers in this 
preferred embodiment. Reverse acting means that as the current drawn by 
the pumping means increases, the output signals 66, 76, and 86 from the 
current controllers will decrease, and as the current drawn by the pumping 
means decreases, the output signals from the current controllers will 
increase. 
In this preferred embodiment the pneumatically operated control valve 21 
closes in response to a decrease in the magnitude of control signal 91. 
The input-output speed ratio of the variable drive transmission 43 
decreases in response to an increase in the magnitude of control signal 
91. 
As has been previously stated, it is an object of this invention to 
maximize the energy efficiency of the pumping station 13. In general, this 
object is accomplished by maintaining the pneumatically operated control 
valve 21 in a fully opened condition and allowing the variable drive 
pumping means 45 to control the pipeline suction pressure seen at the 
input to the pumping station 13. The pneumatically operated control valve 
21 is partially closed only when the variable drive pumping means 45 
approaches a 2.5:1 speed ratio and the pipeline suction pressure continues 
to drop. Control of the pumping station 13 in this manner is accomplished 
by using a portion of the range of control signal 91 to control the 
variable drive pumping means 45 and using a second portion of the range of 
the control signal 91 to control the pneumatically operated control valve 
21. In this preferred embodiment the control signal 91 has a range of from 
4-20 milliamps. The 10-20 milliamp range of control signal 91 is utilized 
to control the variable drive pumping means in such a manner that the 
variable drive pumping means will operate at an input-output speed ratio 
of 1:1 when control signal 91 has a value of 20 milliamps and will operate 
at a speed ratio of 2.5:1 when control signal 91 has a value of 10 
milliamps or lower. The 4-14 milliamp range of control signal 91 is 
utilized to control the pneumatically operated control valve 21 by means 
of a current to pressure transducer, contained with pneumatically operated 
control valve 21, in such a manner that the pneumatically operated control 
valve 21 will be fully opened when control signal 91 has a valve of 14 
milliamps or higher and will be fully closed when control signal 91 has a 
value of 4 milliamps. The overlap in the ranges of the control signal 91 
utilized by the variable drive pumping means and the pneumatically 
operated control valve 21 is utilized to provide a smooth transition 
between the control of the pipeline suction pressure by the variable drive 
pumping means 45 and the control of the pipeline suction pressure by the 
pneumatically operated control valve 21. 
In this preferred embodiment of the invention the variable drive pumping 
means 45 is a 1000 horsepower electric motor driven pump and the direct 
drive pumping means 27 and 32 are 500 horsepower electric motor driven 
pumps. Initially, only the variable drive pumping means 45 is placed 
online by opening shutoff valves 41 and 44. If the suction pressure seen 
at the input to the pumping station 13 begins to drop, then the output 
signal 58 from the pressure controller 54 will begin to decrease in 
magnitude. The output signal 58 will be selected by low select means 59 
and will be provided as control signal 91 to the variable drive pumping 
means 45. In response to the control signal 91, which is decreasing in 
value in response to the decrease in the pipeline suction pressure, the 
input-output speed ratio of the variable drive pumping means 45 will begin 
to increase. If the pipeline suction pressure should continue to decrease 
to the point where the control signal 91 drops below 14 milliamps, then 
the variable drive pumping means will be approaching a 2.5:1 speed ratio 
and the pneumatically operated control valve 21 will begin to close. In 
this manner the minimum acceptable pipeline suction pressure is 
maintained. 
If the pipeline suction pressure begins to increase, then the output signal 
58 from pressure controller 54 will begin to increase. If the output 
signal 58 is lower than the output signal 86 from current controller 83, 
then the output signal 58 will be selected by the low select means 59 and 
will be provided as signal 91 to the variable drive pumping means 45. The 
input-output speed ratio of the variable drive pumping means 45 will be 
decreased in response to an increase in control signal 91 in such a manner 
that the pipeline suction pressure will be maintained at an acceptable 
level. However, as the input-output speed ratio of the variable drive 
pumping means 45 decreases, the current drawn by the variable drive 
pumping means 45, represented by signal 81, will increase. The increase in 
the magnitude of signal 81 will cause a corresponding decrease in the 
magnitude of the output signal 86 from the current controller 83. As the 
magnitude of the signal 81 approaches the magnitude of the set point 
signal 85, the value of the output signal 86 from the current controller 
83 will be such that the output signal 86 will be selected by the low 
select means 59 and will be provided as control signal 91 to the variable 
drive pumping means 45. This prevents the variable drive pumping means 45 
from drawing excessive current (being overdriven) in response to 
increasing pipeline suction pressure. When this condition occurs, the 
direct drive pumping means 32 is placed online by opening shutoff valve 
means 31 and 33. The direct drive pumping means 32 may be put online 
automatically or by an operator. When direct drive pumping means 32 comes 
online, the pipeline suction pressure should show a dramatic decrease. The 
output signal 58 from the pressure controller 54 will decrease in response 
to the decrease in the pipeline suction pressure and will again be 
selected by low select means 59 and will be provided as control signal 91 
to the variable drive pumping means 45. The input-output speed ratio of 
the variable drive pumping means 45 will be increased in response to the 
decrease in the magnitude of control signal 91. This allows control of the 
pipeline suction pressure to be maintained by the variable drive pumping 
means 45, thus maximizing the energy efficiency of the pumping station 13 
when pipeline suction pressure is increasing. 
If direct drive pumping means 32 is online and the pipeline suction 
pressure again increases to a point where the variable drive pumping means 
45 is operating at a 1:1 input-output speed ratio, then control signal 86 
will once more be selected by the low select means 59 to protect the 
variable drive pumping means 45. The direct drive pumping means 27 will 
then be put online in a manner similar to that described in connection 
with direct drive pumping means 32 to again cause a decrease in pipeline 
suction pressure which allows the input-output speed ratio of the variable 
drive pumping means 45 to be increased, which will have the effect of once 
again allowing the variable drive pumping means 45 to control the pipeline 
suction pressure seen at the input of the pumping station 13. 
The invention has been described in terms of its presently preferred 
embodiment as shown in FIG. 1. Pressure transducer 51; pressure controller 
54; low select means 59; check valve means 14, 16 and 18; shutoff valve 
means 26, 28, 31, 33, 41 and 44; current controller 63, 73 and 83; and 
pneumatically operated control valve 21 are each well known commercially 
available control components such as are described at length in Perry's 
Chemical Engineer's Handbook, 4th Edition, Chapter 22, McGraw-Hill. 
While the invention has been described in terms of the presently preferred 
embodiments, reasonable variations and modifications are possible by those 
skilled in the art, within the scope of the described invention and the 
appended claims.