Pipeline capacity and economics
PIPELINE CAPACITY AND
ECONOMICS
INTRODUCTION
the construction of natural gas transmission pipelines a lot
of things should be considered. For example pipe sizes, the pressure required
to transport natural gas from one location to another and the pressure drop
that will proceed. To accurate this, compressors are required which need to be
sized for the compression requirements. In addition Pipe loops will be designed
that increase the capacity of the pipeline by adding theoretical diameter.are
planned to integrate into existing networks and are constructed to transport a
company’s own natural gas, to transport natural gas through an owned section of
pipeline for another company, or to transfer the ownership of natural gas to
another and into their assets (pipeline). For these 3 permutations in operation
investment in the infrastructure and assets will be made to a varying degree
while meeting all of the regulation requirements to ensure that the safe
operation and environment considerations have been made.meet the capital
requirement (CAPEX) for developing pipelines, companies have to provide strong
business cases in how the financing and remuneration through operation will be
managed. The budget for pipeline development will be determined by the capital
costs of the planned length of pipeline and operational term in service. Some
of the elements of the capital costs are as follows:
1. Pipeline
2. Compressor stations
. Mainline valve stations
. Meter stations
. Pressure regulator stations
. SCADA and telecommunication
. Environmental and permitting
. Right of way acquisitions
. Engineering and construction management
PIPELINE
CAPACITY AND ECONOMICS
In the design and planning of the physical pipeline systems
several variables are to be considered. The majority of these variables come
from the general flow equation which is determines the daily capacity of a gas
pipeline:
Using this equation produce derivative equations to
illustrate the relationship with some of the variables and how impact on the
overall capacity of the pipeline on a daily basis. This is shown as a capacity
factor or as volumetric capacity for daily deliverables. In conditions of
overall economical potential the pipeline capacity as earlier mentioned is a
important factor in determining the transportation price and hence the capital
recovery for investment in projects of this nature.of the capacity functions
are:
. Inlet pipe pressure
2. Pressure drop
. Pipeline diameter
. Pipeline Length
PRESSURE
Use the derivative equation below the illustration of
variation of inlet pressure in stages of 10 bar for a standard pipe diameter
with a constant pressure drop ∆P of 20 bar can be seen.this arrangement
we can use the formula:
Q ∝
This allows us to determine the capacity factor CF which is a
function of the inlet pressure.
Table 1
Effect of Inlet pressure on pipeline capacity
Q
|
∆P
|
P₁
|
∆P(2P₁ -∆P)
|
CF
|
|
20
|
0
|
-400
|
|
0
|
20
|
10
|
0
|
0
|
20
|
20
|
20
|
400
|
1
|
28.28427
|
20
|
30
|
800
|
1.06066
|
34.64102
|
20
|
40
|
1200
|
1.154701
|
40
|
20
|
50
|
1600
|
1.25
|
44.72136
|
20
|
60
|
2000
|
1.341641
|
48.98979
|
20
|
70
|
2400
|
1.428869
|
52.91503
|
20
|
2800
|
1.511858
|
56.56854
|
20
|
90
|
3200
|
1.59099
|
63.08724
|
20
|
100
|
3980
|
1.585107
|
Figure 1. Pipeline capacity increases with increased inlet
pressure
From figure 1 any increase in inlet pressure for a standard
pressure drop of 20 (bar) will increase the capacity of the pipeline. There is
a rapid increase in capacity between 10-20 (bar) and then a uniform or linear
increase between 30-90 (bar) for inlet pressure. Standard systems will use the
linear increase in capacity as they will require a pressure drop in the system
to be able to transmit efficiently. In terms of transmission pressures the
majority of pipelines in the UK will transmit around 65 (bar) which is in the
range of linear capacity building in pipelines.using an example for a pipeline
of fixed dimensions we can explore further the original equation by producing
the derivative equation.
Where:(inlet pressure) is varied in stages of 10 bar(outlet
pressure) is a function of the pressure drop known to be 20 bar for the
system.is the diameter of the pipeline (30’’ or equivalent to 750mm DN)is
length of a standard section of pipeline taken to be 48km
CALCULATION
Q ∝ x = 217.855 capacity (mill m3/
day)
Table
2
Inlet pressure varied in stages of 10 bar
p1
|
∆P
|
p2
|
d
|
L
|
Q ∝
|
10
|
20
|
-10
|
750
|
48
|
0
|
20
|
20
|
0
|
750
|
48
|
44469530
|
30
|
20
|
10
|
750
|
48
|
62889412
|
40
|
20
|
20
|
750
|
48
|
77023485
|
50
|
20
|
30
|
750
|
48
|
88939059
|
60
|
20
|
40
|
750
|
48
|
99436891
|
70
|
20
|
50
|
750
|
48
|
1.09E+08
|
20
|
60
|
750
|
48
|
1.18E+08
|
90
|
20
|
70
|
750
|
48
|
1.26E+08
|
100
|
20
|
80
|
750
|
48
|
1.33E+08
|
110
|
20
|
90
|
750
|
48
|
1.41E+08
|
120
|
20
|
100
|
750
|
48
|
1.47E+08
|
130
|
20
|
110
|
750
|
48
|
1.54E+08
|
140
|
20
|
120
|
750
|
48
|
1.6E+08
|
150
|
20
|
130
|
750
|
48
|
1.66E+08
|
160
|
20
|
140
|
750
|
48
|
1.72E+08
|
170
|
20
|
150
|
750
|
48
|
1.78E+08
|
180
|
20
|
160
|
750
|
48
|
1.83E+08
|
190
|
20
|
170
|
750
|
48
|
200
|
20
|
180
|
750
|
48
|
1.94E+08
|
210
|
20
|
190
|
750
|
48
|
1.99E+08
|
220
|
20
|
200
|
750
|
48
|
2.04E+08
|
230
|
20
|
210
|
750
|
48
|
2.09E+08
|
240
|
20
|
220
|
750
|
48
|
2.13E+08
|
250
|
20
|
230
|
750
|
48
|
2.18E+08
|
Figure 2. Potential capacity
increase for inlet pressure increase
DROP
The effect of pressure drop on pipeline capacity for a fixed
inlet pressure can also be calculated from the derivative of the General Flow
Equation., Q ∝ √ (P12-P22)
with a fixed inlet pressure and variation of pressure drop.
SAMPLE
CALCULATION
√ (702-502) = 48.99
Table
3
Uniform pressure drop in stages of 5 bar
Q
|
p1
|
p2
|
∆P
|
CF
|
0
|
70
|
70
|
0
|
0
|
25.98076
|
70
|
65
|
5
|
0.53033
|
36.05551
|
70
|
60
|
10
|
0.73598
|
43.30127
|
70
|
55
|
15
|
0.883883
|
48.98979
|
70
|
20
|
1
|
53.61903
|
70
|
45
|
25
|
1.094494
|
57.44563
|
70
|
40
|
30
|
1.172604
|
60.62178
|
70
|
35
|
35
|
1.237437
|
63.24555
|
70
|
30
|
40
|
1.290994
|
65.38348
|
70
|
25
|
45
|
1.334635
|
67.08204
|
70
|
20
|
50
|
1.369306
|
68.37397
|
70
|
15
|
55
|
1.395678
|
69.28203
|
70
|
10
|
60
|
1.414214
|
69.8212
|
70
|
5
|
65
|
1.425219
|
70
|
70
|
0
|
70
|
1.428869
|
4. Capacity reduction after 20
bargas, we can see by increasing the pressure drop there is a gradual decrease
in capacity. We base this prediction by looking at how the capacity behaves
before and after the standard pressure drop of 20 bar.before the standard
pressure drop of 20 bar, in the early region on the graph between 0 - 10 bar
there is an uniform increase in capacity. This steadily declines towards the
pressure drop of 20 bar and beyond it. As the pressure drop increases the
capacity steadily decreases to the point whereby at pressure drop of 70 bar
there is no apparent increase in capacity at all.
The affect the diameter has on the pipeline is quite
apparent. As the pipelines diameter increases so does the capacity of that
pipeline to hold potentially higher volume of gas or other hydrocarbons. This
is because the dynamic viscosity of the gas increases due to greater width of
pipeline boundary layers.gas is a compressible fluid it tends to be able to be
‘packed into’ pipelines of higher theoretical diameter in methods such as
re-enforcement for line packing technique.relationship is proven through the
derivative of the general flow equation:
Where Q ∝ d 2.5
For the Midcontinential pipeline example which is comprised
of 3 sections:
. 48km of 30 inch pipe (750mm)
2. 317km of 36 inch pipe (900mm)
. 442km of 42 inch pipe (1050mm)
This is evident that the planning is to allow an increase in
capacity along the run of the pipeline. Calculations show how the capacity gained
quite considerably along the run of the pipeline.
Table
4
Diameter driven capacity increase
CF(capacity factor)
|
Q(volume)
|
d (DN mm)
|
1
|
15404697
|
750
|
1.577441
|
24300000
|
2.319103
|
35725083
|
1050
|
Figure 5. Diameter driven capacity increase
From the graph of Midcontinential gas pipeline we can say
that will increase in capacity factor from pipeline section to section. This
corresponds to an increase of 1.5 from the 750mm section to 900mm section and
an increase in capacity of 2.3 times from the 750mm to 1050mm section. This
increase in capacity is used over ranges of 317km and 442km respectively and
highlights how the capacity of 48km would most probably be for a feed in for
the major transmission line and their potential to increase capacity.the higher
the diameter the more suitably cost effective it is to transport the gas from
region to region. This would generally be used for the interstate (zone)
relaying of gas on a national network. The pipelines would be designed to be
used in rural locations due to the design factor for safety involved. However
the increased costs of laying large diameter pipelines are generally offset by
the increased capacity of these lines in years to come. This returns to the
premise that most gas pipeline operators will design the network to efficiently
yield the maximum capacity possible over the lifetime of the pipeline with
planned increase designed in which will allow them to optimize the capacity of
the pipeline and gain the maximum transportation tariff for the network within
prescribed remuneration limits.
The capacity of the pipeline can be also fluctuate when the
length of the pipeline is varied. For example in the 3 section pipeline which
connects the Midcontinential pipeline with the Transcontinental pipeline in the
USA, the variation in length needs to be determined for each section for
optimization of capacity in operation.the total pipeline, which is planned,
there is a current span of pipeline which is accumulated from;1 (442km),
section 2 (317km), and section 3 (48km)gives an overall span of 807km of
pipeline of diameters varying from 30’’(750mm DN) to 42’’ (1040mm DN).look at
the varying economic trade off from this we can take one section of the
pipeline e.g. Section 3 (48km) and vary the length of this pipeline to
illustrate.
Table
5
Length as an inhibitor to capacity
Length (km)
|
CF
|
20
|
0.223607
|
24
|
0.204124
|
28
|
0.188982
|
32
|
0.176777
|
36
|
0.166667
|
40
|
0.158114
|
48
|
0.144338
|
52
|
0.138675
|
56
|
0.133631
|
60
|
0.129099
|
64
|
0.125
|
68
|
0.121268
|
72
|
0.117851
|
76
|
0.114708
|
80
|
0.111803
|
Figure 6.The effect of pipeline length on capacity
From this graph we can see that as with the drop in capacity
the curve provided has the appearance of a declining semi parabolic. The red
line indicates the minimum losses in capacity for an achievable span of 20km.
However in planning if this length of pipeline were to be undertaken additional
costs may be incur in permits and choice of pipeline to meet with design factor
needed by the location, if in proximity to population and planning of the
pipeline. There will always be an economic and capacity trade off.we take the
midpoint of this curve (as a point of reference to where the optimum distance
should occur knowing that pipeline cost increases and capacity factor decreases
less steadily beyond this point) we should find the optimum length of pipeline
length to capacity factor. gas pipeline financial
capital cost
From the graph this comes to a value of 44 km whereas the
actual section under investigation is the 48 km pipeline. The differences in
actual planning a pipeline of this length may be down to a number of factors
such as costs in negotiating terrain, environmental permits, location of
pipeline to dwellings, suburban limits etc. However effectively the most economical
length of pipeline would be achieved post the 44 km length as this is when the
rate of decline in capacity begins to tailor away. Additional to these factors,
as with the other variables, there is an amount of tolerance to the planned
length.
STATIONS
Compressor stations and compressors generally added to a
pipeline to increase capacity. By adding these units at strategically placed
intervals in the pipeline the pressure drop in the pipeline is rectified in
order to maintain a responsible pressure for further transmission. Pressure
drop occurs naturally through the frictional forces in the pipeline which are
ever occurring even though some major pipelines are fitted with an epoxy
coating to minimize friction. An additional, the major contributor to pressure
loss is when terrain inflicts a gradient in the pipelines design. In designing
a compressor station along a pipeline consideration must be given to achieve
efficiency in the design.a compression ratio of around 1.5 is acceptable. Thus
design factors are considered to try to reduce power input to maintain this
level of compression ratio. Effectively today, equipment such as centrifugal
compressors are seen as the most efficient in compressing gas however the cost
of running this sort of plant has high fuel costs. Therefore there is always a
trade off between the economic implications of designing compressor, which are
of a high capacity, to maintaining a high capacity in the pipeline. These
points will be investigated further.stations work as a function of the pressure
drop and as so will be designed and positioned as a corrective measure as shown
in the diagram below.the diagram above the intermediate compression station is
placed at the half way point between the source location and the end location.
However the pressure drop in pipelines is generally non-linear so this is only
a theoretical positioning. Further adjustments on the compression stations
position will be made by adjusting the compressor ratio in accordance with the
end location. Also to ensure that the compressor works efficiently and any
costs that are used up in compressing the gas back to a satisfactory
transmission pressure will be justified. If the compression station is placed
too close to the end location then two things will happen.
Figure7. Incremental compressor arrangement
In addition, adjust the discharge pressure of the compression
station so that the gas reaches the required pressure at the end location. Or
we will have to move the compression station further upstream so that the
pressure drop is able to decline the discharge pressure suitably to the end
pressure.to acquire a suitable location for compressor station are made on a
trial and error basis using the general flow equation.the taking a standard
value for source pressure P1 and receiving pressure P2 we can assume that the
compressor in use will be operating within its capacity:
Initial/Discharge Pressure P1 = 70 bar/Suction pressure P2 =
50 bar
∆P = 20 bar (standard Optimum)ratio = P1/P2 = 1.4:
1sizing 1.5 (centrifugal compressor)
CALCULATIONS
the general gas flow equation we can overview a system such
as the 48km pipeline in USA and how it could potentially use compressors to enhance
flow rate to a location. For this example we estimate that the flow rate to be
maintained beyond the end of the pipeline and hence the compression station
will be at the 48km point.
11. Practical example of compressor planning
Where, Ts = 288,= 1.013025,
70,
d = 750mm,= 0.7,= 48km= 288Z = 0.85= 0.000575 x x= 163.4 x x
= 1.36 mill m3/day
The daily capacity along the pipeline would be 1.36 mm3/day.
To maintain this capacity into the section of 327km pipeline a compression unit
could be placed at the end to rectify the pressure drop across the
pipeline.yearly capacity would therefore be 496.4 million m3/year.
ADIABATIC
COMPRESSION
In compression through a closed system, the compression can
be considered as adiabatic when it doesn’t give any heat off to the
surroundings. If the compression is heat efficient, which generally they are,
then the size of the compression unit can be calculated from the gas flow
(capacity) and the suction pressure and discharge pressure( pressure
differential) for the unit.the work done is calculated from the equation:
Wa = x T1 [ -1]
We can therefore use the 48km pipeline flow rate and the
optimum pressure differential to calculate the number of compressors needed to
maintain the flow rate prescribed for the next stage of the transmission line
I.e. 1.35 mill m3/day into the 327km section.simplified version of
the equation for work done discounts any heat lost due to adiabatic process.
Wa = x (288) Loge = 39,162 J/Kg
The volume of gas processed per second can be found by:
1.36 x 10 6 / (24 x 3600) = 15.74 cubic metres of
gas/second.
And the weight of the gas is given by 15.74/0.714 = 22.05 kg,
the total work done
22.05 x 39,162 = 863,336.4 J
The total work required to transfer 21.07 kg would be 863.33
KJ.more, we can size the compressor power unit that we need.
621.23 kW
The compressor driver may have mechanical efficiency of 98%
so
Driver power = = 633.91kW
Sizing the compressor units for Horse Power we can use the
equation
HP = 0.0857 x 48.85 ((288) ( = 470.66
HP.
the mechanical efficiency of the driver taken into
consideration as 0.98 we can calculate the break horse power of the unit.
BHP = = = 480.26 BHP
BHP required to drive the compression operation could be
effectively one 500 BHP unit.units vary in size from 50 BHP up to around 20,000
BHP. This is of the smaller capacity however in further planning it would be
recommended to oversize the unit.
CONCLUSION
From the results obtained it shows that due economic analysis
must be carried out before pipeline development:
. Detailed market survey to ascertain peak loads, average
and minimum demand of consumers in order to adequately size a pipeline
2. Detailed route survey to ascertain the nearest
distance for gas supply to a customer
3. Detailed energy audit of consumer equipment in order
for pressure distribution
4. Detailed engineering design to properly cost a
pipeline network
With the knowledge of how the above four parameters discussed
affects capacity one can properly cost and size adequately a pipeline for
either transmission or distribution purpose.
REFERENCE
1. Kadir
A., University of Salford, Distribution and Transmission Lecture Notes 2011/12.
2. E.
Shashi Menon, Pipeline Hydraulics, Taylor & Francis 2005.
3. Roberts,
John.,1996. Caspian Pipelines. The Royal Institute of International Affairs.