The initiating point of the pipeline system, into which petroleum products are lifted, must have a pump station. Also, a long pipeline may require multiple pump stations along the mainline. The pumping requirements should be considered in terms of the number and locations of the stations. The number of pump stations is dictated by the installation and operating costs as well as the flow velocity and controllability of the pipeline system and pump station. If the number of stations increases, the costs and flow velocity increase while making the system control difficult due to large surge pressure and its fast response. Refer to Section 5.1.3 for controlling surge.
The key criteria of initially locating mainline pump stations are that the MAOP should not be violated downstream of each pump station and each station has the same differential pressure or head. Here, the differential pressure includes all minor pressure losses due to station piping, bends, fittings, and various valves including control valve. The second criterion offers the following advantages:
The total energy or power consumption is reduced by adding the same amount ·
of energy to the liquid at each pump station.
The pump maintenance and spare part inventory costs can be minimized, be- ·
cause the equipment can be identical.
However, these advantages should be compared against potential extra costs to design a pipeline system as such. For example, the power line may be too far from an optimum location to satisfy the above criterion.
This criterion is applicable to the design of all new pipelines in locating pump sta- tions. However, the procedure of locating stations can be different for different terrains or pipeline configuration:
Relatively flat terrain, ·
Complicated terrain in terms of elevation profile, ·
Simple pipeline system with one injection and one delivery, ·
Complex pipeline system with multiple injection and delivery points. ·
For a simple pipeline system with relatively flat terrain, the criteria for locating stations results in almost equal station spacing along the pipeline, and the number of pump stations can be determined by dividing the total required pressure by the differ- ence between the MAOP and the minimum pressure;
No. of stations = Total required pressure / (MAOP – minimum suction pressure) The procedure of locating pump stations is to start from the delivery pressure, drawing the pressure gradient upstream to the intersection of the maximum design pressure, which is superimposed on the elevation profile. If the discharge pressure of the initiating station is smaller than the design pressure, then reduce the design pressure and move the initial locations to further downstream locations. The same differential pressure can be calculated by dividing the total pressure requirements by the number of pump stations.
Example 1: Simple Pump Station Location
Refer to the design example described in Section 3.3.1. The total required pressure is 15,389 kPag, maximum design pressure 9765 kPag, and minimum delivery pres- sure 350 kPag. It is assumed that the minimum suction pressure is 350 kPag. Since the elevation profile is flat, the number of pump stations is obtained from the above formula:
15,389 / (9765 – 350) = 1.6
Therefore, the total number of pump stations required is 2; one at the initiating sta- tion and the other at an intermediate location. Applying the station location criteria, the intermediate station is located at the mid-point of the pipeline as shown in Figure 3-20.
For a simple pipeline system with severe elevation changes, the station locations can be determined by applying these criteria through trial and error on a graph. The procedure of locating intermediate pump stations is as follows:
Step 1. Using the maximum design pressure as the discharge pressure at the ini- tiating station, the first intermediate station is found at a location where the pressure reaches the minimum suction pressure by drawing the pressure gradient on the eleva- tion profile. In practice, a pressure allowance of 200 kPa to 300 kPa at the intake of the pump station is required to account for the losses due to station piping, valves, fittings, and other equipments.
Step 2. Progressing downstream from the maximum design pressure at the inter- mediate station, the next intermediate station is located in the same way as above. Repeat these steps until the minimum suction pressure of the last section is greater than or equal to the delivery pressure.
Step 3. If the suction pressure is much greater than the delivery pressure, reduce the discharge pressure equally at each pump station and then repeat the second step to move the initial locations to upstream locations.
Step 4. If the discharge pressure has to be reduced significantly, the maximum design pressure can be lowered by selecting lower grade pipe or thinner pipe wall thickness.
Figure 3-21 shows that the total pressure requirement is greater than the design pressure. This pressure requirement can be met by installing an intermediate pump sta- tion or choosing a thicker pipe in the upstream segment where the required pressure is
Design Pressure = 9,765 kPag Pressure (kPag) PD = 7,780 PS = 350 200 km 100 km 0 Distance Main Line Pump
Booster Pump
Figure 3-20. Locating intermediate pump stations for flat elevation
Head (m) Pressure ( kPag) 1,000 500 8,600 4,300 Distance (km) 0 200 PD Design Pressure PD PS PB Ps
violated. Assuming an intermediate pump station is installed, it can be located in such a way that the differential pressure, PD – PB, at the initiating station is the same as the differential pressure, PD – PS, of the intermediate station. In this case, the station loca- tion is shifted toward the high elevation side. The shift depends on the elevation profile and site conditions.
Example 2: Pump Station Location in Changing Elevation Profile
A pipeline company plans to build and operate a crude oil pipeline, delivering to a tank farm. The pressure rating of the tank equipment is designed at 700 kPag. The average flow rate is 1175 m3/hr and the pipeline system is expected to operate at least 345 days a year. The average operating temperature is 4°C. The density and the viscosity of the crude at the operating temperature is 870 kg/m3 and 40 cSt, respectively. The vapor pressure is 80 kPa or –21 kPag, and a slack flow condition has to be avoided. Analyze the pressure profile for the minimum design flow rate of 500 m3/hr. Assume the suction pressure at each pump station is 350 kPag and the pump pressure differential should be less than 8000 kPa. The pipe specifications are as follows:
Pipe sizes – NPS · = 20² Wall thickness – 0.281 · ² Pipe roughness – 0.0018 · ²
Pipe grade – API X60 ·
Design factor – 0.72 ·
The pipeline length is 350 km and the elevation profile is given below.
KMP 0 50 150 190 230 290 320 350
Elevation 10 m 250 m 250 m 250 m 250 m 310 m 460 m 10 m
Solution:
Step 1. Determine the design flow rate and the maximum design pressure.
Since the number of yearly operating days is 345 days, the load factor is ·
345/365 = 94.5%, and thus the design flow rate is 1175/0.945 = 1243 m3/hr, or rounding up to 1250 m3/hr.
The design pressure is obtained by applying the Barlow formula and the design ·
factor for the X60 pipe grade; 8370 kPag.
Step 2. Calculate the pressure gradient.
Flow velocity · = 1.82 m/s Reynolds number · = 1.82 ´ 0.494/0.00004 = 22,500 Relative roughness · = 0.00009 Friction factor · = 0.0377 Pressure gradient · = 0.0377 ´ 870 ´ 1.822/(2 ´ 0.494) = 110.0 Pa/m = 110.0 kPa/km
Step 3. Determine initial station locations and calculate the pressures at the locations.
Assuming that the discharge pressure at the initiating station is 8230 kPag, the ·
first pump station will be located approximately 53 km downstream with a suc- tion pressure of 354 kPag.
Since the elevation difference is zero for about 180 km downstream of the ·
differential pressure to the initiating station; 7920/110 = 72 km. Therefore, the first and second pump stations are located at 125 km and 197 km, where the discharge pressures are 8274 kPag and 8270 kPag, respectively.
The fourth station is located at 269 km if the elevation were 250 m. Since it ·
is higher than 250 m at 269 km point, the spacing will be shorter than 72 km and the elevation is determined by prorating elevations between two adjacent known locations. At the 266 km post, the elevation is prorated at 286 m and the suction pressure becomes 374 kPag.
If the discharge pressure of the fourth station is set at 8270 kPag, then the pres- ·
sure at KMP = 290 is 5596 kPag and the pressure at KMP = 320 is 1021 kPag. Note that the KMP = 320 is the peak point in this pipeline system. Since the peak point pressure is higher than the vapor pressure by 1042 kPa (1021– (–21) = 1042), theoretically the discharge pressure can be reduced by about 1000 kPa. However, considering the transient effect on the pressure, an extra allowance of about 300 kPa has to be added to the vapor pressure.
Step 4. Since the discharge pressure of the last station can be reduced by (1000– 300) kPa, that is to 700 kPa, the initial station locations can be adjusted.
First, locate the first intermediate station at 52 km, where the suction pressure is ·
set at 350 kPag, the discharge pressure and differential pressure at the initiating station are 8116 kPag and 7766 kPa.
Using the similar differential pressure, the station spacing of the next two sta- ·
tions is 70.5 km and the second and third station locations are 122.5 km and 193 km, respectively. Then, setting the suction pressures of the second and third stations at 350 kPag, the discharge pressures of the first and second stations are 8105 kPag and the differential pressures are 7755 Pa. This differential pressure is very close to the differential pressure at the initiating station.
The fourth station was located initially at 266 km. By taking into account the ·
elevation difference and locations due to the location shift, the new location is determined at KMP = 263 km. Then, the discharge pressure at the third station is 8160 kPag. If the discharge pressure at the fourth station is set at 8079 kPag, the peak point pressure is calculated at 300 kPag.
If a surge analysis shows that the peak point pressure is too low, the pump sta- ·
tions need to be moved slightly toward downstream locations.
Step 5. Determine the delivery pressure when the station locations are finalized.
The hydraulic pressure gain from the peak point to the delivery point is 0.87
· ´
9.8 ´ 450 = 3837 kPa, but the friction pressure loss is 3300 kPa. Thus the de- livery pressure is 887 kPag, which is greater than the tank equipment pressure rating. Therefore, a pressure control valve (PCV) is needed upstream of the tank farm.
Since the MAOP is much greater than the delivery pressure, a pressure-reducing ·
station (PRS) is not required as long as the peak point pressure is maintained above the vapor pressure.
Step 6. Analyze the pressure profile for the minimum design flow rate.
Flow velocity · = 0.727 m/s Reynolds number · = 0.727 ´ 0.494/0.00004 = 8980 Relative roughness · = 0.00009
Friction factor
· = 0.0505
Pressure gradient
· = 0.0505 ´ 870 ´ 0.7272/(2 ´ 0.494) = 23.5 Pa/m = 23.5 kPa/ km
Since the pressure gradient is low, the first and second pump stations can be ·
bypassed. Assuming the suction pressure is set at 350 kPag at the third pump station, the discharge pressure required at the initiating station is 6952 kPag. If the discharge pressure at the third station is 4844 kPag, then the pressure at the peak is 350 kPag and the delivery pressure becomes 3482 kPag.
Since this pipe pressure is much higher than the tank equipment pressure rat- ·
ing, the PCV must have the capacity to reduce pressure by 3482 – 700 kPa = 2782 kPa.
Figure 3-22 shows the pump station locations with elevation and pressure profiles for the design and minimum flows. Note that the pressures at the delivery gate for low flows are higher than those for high flows in order to keep the minimum pressure required at the peak point.
In general, the same criteria are applied to more complex pipeline systems for lo- cating intermediate pump stations by a trial and error method. Through this hydraulic analysis, the approximate pump station locations are determined that would meet the design and operating parameters. However, the same differential pressure at all pump stations cannot always be achieved.
Example 3: Pump Station Location with a Branch Line
The pipeline from CE to QU is 214 km long and is 20² in nominal diameter, with a 0.281² wall thickness. It is constructed of API X-60 grade steel. At CE, diesel enters the pipeline at the design flow rate of 1800 m3/hr. The booster pumps at CE discharge into the main line pump at 350 kPag, and the minimum delivery pressure required at QU is 350 kPag.
The diesel is taken off at TO, 176 km downstream of CE, where up to 600 m3/ hr is stripped off the pipeline, and the rest is delivered to the final destination, QU. Occasionally, the full flow has to be delivered to QU. At TO, a 50-km branch line is connected to a third party pipeline, which requires the delivery pressure of 3000 kPag.
Head (m) Pressure (kPag) 1,000 500 8,600 4,300 Distance (km) 0 200 PD PS 50 100 150 250 300 350 350 350 350 350 300 8,116 8.105 8,105 8,160 8,079 887kPag
The pipeline is constructed with X52 grade pipe, and the pipe diameter is 16² with a 0.25² wall thickness.
Locate the pump stations along the main pipeline, using the following data: Average operating temperature: 15
· °C
Density: 850.0 kg/m
· 3 at 15°C at the operating temperature Viscosities at 15
· °C: 10.0 cSt
Pipe roughness: 0.0018
· ²
Delivery pressure at QU: 350 kPag ·
Assume that the design factor of 0.72 is applicable and that the elevation profile is flat and flow is isothermal.
Solution:
Step 1. Calculate the design pressure (MAOP) of the main and branch lines.
MAOP of the main line
· = (2 ´ 60,000 ´ 0.281 ´ 0.72/20) ´ 6.895 = 8470 kPag MAOP of the branch line
· = (2 ´ 52,000 ´ 0.250 ´ 0.72/16) ´ 6.895 = 8067 kPag
Step 2. Calculate the pressure required at TO on the branch line side.
Flow velocity · = 1.37 m/s Reynolds number · = 1.37 ´ 0.394/0.00001 = 54,000 Relative roughness · = 0.0001125 Friction factor · = 0.0208 Pressure gradient · = 0.0208 ´ 850 ´ 1.372/(2 ´ 0.394) = 42.3 Pa/m = 42.3 kPa/km The pressure required at TO
· = 3000 + 42.3 ´ 50 = 5115 kPag, which is the
minimum pressure required at the take-off point.
Step 3. Calculate the pressure at CE.
Flow velocity upstream of TO
· = 2.62 m/s Reynolds number · = 2.62 ´ 0.494/0.00001 = 129,400 Relative roughness · = 0.00009 Friction factor · = 0.0175 Pressure gradient · = 0.0175 ´ 850 ´ 2.622/(2 ´ 0.494) = 103.3 Pa/m = 103.3 kPa/ km
The pressure required at CE
· = 5115 + 103.3 ´ 176 = 23,296 kPag
Since this pressure is much higher than the main line design pressure, pump ·
stations should be installed along the main line.
Step 4. Find the minimum number of pump stations and locate the required pump stations along the main line.
Making a small allowance of 370 kPa in the discharge pressure, the discharge ·
pressure is set at 8100 kPag.
Since equal pumping head reduces overall cost, the equal spacing in flat ter- ·
rain can achieve the equal pumping head. Also, too short a spacing should be avoided to minimize capital and operating costs. It can then be safely assumed that the suction and discharge pressures at each pump station are 350 kPag and 8100 kPag, respectively.
The station spacing is determined by (8100 – 350)/103.3
· = 75 km, which is the
required along the main line. In order to maintain equal pump head for each station, the spacing is 214 km/3 = 71.3 km or 72 km, if the pressure require- ment of 5115 kPag at TO is satisfied. Therefore, the mainline pump stations are temporarily located at 72 km and 144 km.
Step 5. In order to justify the selection, we need to prove that the pressure require- ments at TO and QU are satisfied with the pump stations. Since the full flow can be delivered to QU, we need to study the hydraulic behaviors of both operations.
TO is located at 32 km downstream of the third mainline pump station, and ·
the pressure required at TO is 5115 kPag. When the pump station discharges at 8100 kPag, the pressure at TO is 8100 – 103.3 ´ 32 = 4794 kPag. This pressure does not satisfy the pressure required on the mainline side of TO. The upstream pump has to be located at (8100 – 5115)/103.3 = 28.9 km from the TO or 176 – 28 = 148 kmp.
Dividing this distance in two stations, the station spacing is 148 km/2
· = 74 km,
which is still less than 75 km. Therefore, the new station locations become KMP = 74 and KMP = 148. When the second pump is located at 148 km and discharges at 8100 kPag, the pressure at TO is 5208 kPag, which is higher than the required pressure there.
If the pressure is maintained and other pressure losses are less than 93 kPa ·
(5208 – 5115), no pumping is required along the branch line. Instead, a pressure control valve is required at TO on the branch line side to regulate the delivery pressure for low flow rate.
The delivery pressure at QU for full flow delivery – The distance between the ·
third station and QU is 66 km. When the pump station discharges at 8100 kPag, the full flow delivery pressure is 8100 – 103.3 ´ 66 = 1282 kPag. This pressure falls outside the acceptable delivery pressure range, and thus a pressure regula- tor is required at the delivery point.
The delivery pressure at QU for partial flow delivery – The partial flow rate is ·
1800 m3/hr – 600 m3/hr = 1200 m3/hr. Flow velocity between TO and QU
· = 1.75 m/s Reynolds number · = 1.75 ´ 0.494/0.00001 = 86,450 Relative roughness · = 0.00009 Friction factor · = 0.0189 Pressure gradient · = 0.0189 ´ 850 ´ 1.752/(2 ´ 0.494) = 49.8 Pa/m = 49.8 kPa/ km
The delivery pressure at QU
· = 8100 – 103.3 ´ 28 – 49.8 ´ 38 = 3315 kPag.
This pressure is much higher than the required delivery pressure range, and thus a pressure control valve has to be installed at or upstream of the deliv- ery location.
It should be noted that the differential pressure at the third pump station is different from the pressure at the other stations.
As a final step of locating the pump stations, the best pump station locations are ad- justed on the basis of the following criteria at the time of detail design and construction:
Site terrain conditions ·
Availability of power infrastructure ·
Availability of access roads ·
Potential impact to environment and habitat ·
Potential impact to the local land owners due to noise, etc. ·