Hydrocarbon Processing - May 2021 - 79
Process Controls, Instrumentation and Automation
Plant test. A ramp horizon control was deployed on a feed drum
to a reactor in an industrial plant. The feed to the drum varies and
the existing PI gap controller was tuned to keep the OP as steady
as possible while maintaining the level within limits of 30%-70%.
If the low level is exceeded, the reactor feed pumps will cavitate;
if the high level is exceeded, a high-level protection controller
will send reactor feed material to another process where the feed
material would be converted to a lower value product.
The ramp horizon controller results were compared to the
original PI gap controller and analyzed using the VOD and
AAM, as well as visual inspection of the PV and OP trajectories.
Data of the feed to the drum were collected and used as disturbance data to a simulated level controlled by a ramp horizon
controller. Tuning was then refined to minimize the standard
deviation of the OP.
The controller was commissioned, and data was collected
for a period of 1 mos. This was compared to data collected during the same month of the previous year to eliminate seasonal
influences, as far as possible. The data sets show that the ramp
horizon controller managed to maintain the level between 30%
and 70%, while the PI gap controller allowed the level to vary
between 36.7% and 84.7%.
Data where the controllers were switched to MAN were excluded. This accounted for 11.9% of the data for the ramp horizon controller and 19.7% for the PI gap controller.
FIG. 8 shows one day's data while the ramp horizon controller
was used. When comparing this with one day's data from the PI
gap controller in FIG. 9, the ramp horizon controller moved the
OP less than the PI gap controller, but it had to move it faster
when it was required to remain between limits.
All metrics shown in TABLE 6 indicate that the ramp horizon
controller outperformed the PI gap controller. The simulation results show how the non-linear PI controller will perform similarly
to the ramp horizon and SOALC when a disturbance of the exact
size that was used to tune the non-linear PI controller occurs.
When a disturbance occurs that is larger than was assumed to
tune the controllers, the non-linear PI controller will allow the
level to move outside the limits. As the ramp horizon and SOALC calculate a response that is not based on an assumed maximum disturbance, they will maintain the level within limits.
When a smaller disturbance occurs, the ramp horizon may
make no changes initially and the SOALC will make small
changes throughout. They both allow the level to reach the limit. In this case, the non-linear PI controller will prevent the level
from going to the limit, using only part of the available buffer
capacity offered by the drum. Consequently, the non-linear PI
controller will move the OP more aggressively than the ramp
horizon and SOALC controllers.
The plant test shows how the ramp horizon controller outperformed the non-linear PI controller by:
* Keeping the level within limits
* Implementing smaller and fewer moves.
50
49
48
47
OP, %
Positive step disturbance of 3.75 m3/hr. If the magnitude
of the disturbance is decreased by 25%, FIG. 6 shows how the
non-linear controller did not use the full range allowed for level
buffering. FIG. 7 shows how the ramp horizon controller initially
did not move its OP because a limit violation was not predicted
within the defined time horizon. Later, the controller then had
to move the OP faster to prevent a limit violation, which increased its VOD (TABLE 5).
TABLE 3. Performance metrics for a step disturbance of 5 m /hr
3
Non-linear
Ramp horizon
SOALC
VOD
0.073
0.074
0.067
AAM
0.041
0.041
0.041
46
45
44
43
42
Non-linear OP
TABLE 4. Performance metrics for a step disturbance of 6.25 m3/hr
Non-linear
Ramp horizon
SOALC
VOD
0.098
0.115
0.097
AAM
0.051
0.051
0.051
Ramp horizon OP
SOALC OP
FIG. 5. OP response to a step disturbance of 6.25 m3/hr.
75
70
TABLE 5. Performance metrics for a step disturbance of 3.75 m3/hr
Ramp horizon
SOALC
0.05
0.056
0.041
AAM
0.031
0.031
0.031
TABLE 6. Performance metrics comparing ramp horizon
controller and PI gap controller
Ramp horizon
PI gap
VOD
0.09
0.11
AAM
0.013
0.018
65
Level, %
Non-linear
VOD
60
55
50
Non-linear PV
Ramp horizon PV
SOALC PV
FIG. 6. Level response to a step disturbance of 3.75 m3/hr.
Hydrocarbon Processing | MAY 2021 79
Hydrocarbon Processing - May 2021
Table of Contents for the Digital Edition of Hydrocarbon Processing - May 2021
Contents
Hydrocarbon Processing - May 2021 - Intro
Hydrocarbon Processing - May 2021 - Cover1
Hydrocarbon Processing - May 2021 - Cover2
Hydrocarbon Processing - May 2021 - Contents
Hydrocarbon Processing - May 2021 - 4
Hydrocarbon Processing - May 2021 - 5
Hydrocarbon Processing - May 2021 - 6
Hydrocarbon Processing - May 2021 - 7
Hydrocarbon Processing - May 2021 - 8
Hydrocarbon Processing - May 2021 - 9
Hydrocarbon Processing - May 2021 - 10
Hydrocarbon Processing - May 2021 - 11
Hydrocarbon Processing - May 2021 - 12
Hydrocarbon Processing - May 2021 - 13
Hydrocarbon Processing - May 2021 - 14
Hydrocarbon Processing - May 2021 - 15
Hydrocarbon Processing - May 2021 - 16
Hydrocarbon Processing - May 2021 - 17
Hydrocarbon Processing - May 2021 - 18
Hydrocarbon Processing - May 2021 - 19
Hydrocarbon Processing - May 2021 - 20
Hydrocarbon Processing - May 2021 - 21
Hydrocarbon Processing - May 2021 - 22
Hydrocarbon Processing - May 2021 - 23
Hydrocarbon Processing - May 2021 - 24
Hydrocarbon Processing - May 2021 - 25
Hydrocarbon Processing - May 2021 - 26
Hydrocarbon Processing - May 2021 - 27
Hydrocarbon Processing - May 2021 - 28
Hydrocarbon Processing - May 2021 - 29
Hydrocarbon Processing - May 2021 - 30
Hydrocarbon Processing - May 2021 - 31
Hydrocarbon Processing - May 2021 - 32
Hydrocarbon Processing - May 2021 - 33
Hydrocarbon Processing - May 2021 - 34
Hydrocarbon Processing - May 2021 - 35
Hydrocarbon Processing - May 2021 - 36
Hydrocarbon Processing - May 2021 - 37
Hydrocarbon Processing - May 2021 - 38
Hydrocarbon Processing - May 2021 - 39
Hydrocarbon Processing - May 2021 - 40
Hydrocarbon Processing - May 2021 - 41
Hydrocarbon Processing - May 2021 - 42
Hydrocarbon Processing - May 2021 - 43
Hydrocarbon Processing - May 2021 - 44
Hydrocarbon Processing - May 2021 - 45
Hydrocarbon Processing - May 2021 - 46
Hydrocarbon Processing - May 2021 - 47
Hydrocarbon Processing - May 2021 - 48
Hydrocarbon Processing - May 2021 - 49
Hydrocarbon Processing - May 2021 - 50
Hydrocarbon Processing - May 2021 - 51
Hydrocarbon Processing - May 2021 - 52
Hydrocarbon Processing - May 2021 - 53
Hydrocarbon Processing - May 2021 - 54
Hydrocarbon Processing - May 2021 - 55
Hydrocarbon Processing - May 2021 - 56
Hydrocarbon Processing - May 2021 - 57
Hydrocarbon Processing - May 2021 - 58
Hydrocarbon Processing - May 2021 - 59
Hydrocarbon Processing - May 2021 - 60
Hydrocarbon Processing - May 2021 - 61
Hydrocarbon Processing - May 2021 - 62
Hydrocarbon Processing - May 2021 - 63
Hydrocarbon Processing - May 2021 - 64
Hydrocarbon Processing - May 2021 - 65
Hydrocarbon Processing - May 2021 - 66
Hydrocarbon Processing - May 2021 - 67
Hydrocarbon Processing - May 2021 - 68
Hydrocarbon Processing - May 2021 - 69
Hydrocarbon Processing - May 2021 - 70
Hydrocarbon Processing - May 2021 - 71
Hydrocarbon Processing - May 2021 - 72
Hydrocarbon Processing - May 2021 - 73
Hydrocarbon Processing - May 2021 - 74
Hydrocarbon Processing - May 2021 - 75
Hydrocarbon Processing - May 2021 - 76
Hydrocarbon Processing - May 2021 - 77
Hydrocarbon Processing - May 2021 - 78
Hydrocarbon Processing - May 2021 - 79
Hydrocarbon Processing - May 2021 - 80
Hydrocarbon Processing - May 2021 - 81
Hydrocarbon Processing - May 2021 - 82
Hydrocarbon Processing - May 2021 - 83
Hydrocarbon Processing - May 2021 - 84
Hydrocarbon Processing - May 2021 - 85
Hydrocarbon Processing - May 2021 - 86
Hydrocarbon Processing - May 2021 - 87
Hydrocarbon Processing - May 2021 - 88
Hydrocarbon Processing - May 2021 - 89
Hydrocarbon Processing - May 2021 - 90
Hydrocarbon Processing - May 2021 - Cover3
Hydrocarbon Processing - May 2021 - Cover4
https://www.nxtbook.com/gulfenergyinfo/gulfpub/HPI-Market-Data-2023-v3
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2024-v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2024
https://www.nxtbook.com/gulfenergyinfo/gulfpub/gas-processes-handbook-2022_v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/gas-processes-handbook-2022
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hp_202212
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2023-v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2023
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hp_202211
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202210
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202209
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202208
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202207_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202207
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202206_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202206
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202205_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202205
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202204_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202204
https://www.nxtbook.com/nxtbooks/gulfpub/hp_032022
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202203
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202202
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202201
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202112
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202111
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202110
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2022_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2022
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202109
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202108
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_processes_handbook_2021_v2
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_processes_handbook_2021
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202107
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202106
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202105
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202104
https://www.nxtbook.com/nxtbooks/gulfpub/hp_ebooks
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202103
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202102
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202101
https://www.nxtbook.com/nxtbooks/gulfpub/catalyst_handbook_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/catalyst_handbook_2020
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202011
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202010
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2021
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202009
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202008
https://www.nxtbook.com/nxtbooks/gulfpub/refining_processes_handbook_2020
https://www.nxtbook.com/nxtbooks/gulfpub/refining_processes_handbook_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202007
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202006
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202005
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202004
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202003
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202002
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202001
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201912
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201911
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2020
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201910
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201909
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201908
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201907
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201906
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201905
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201904
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201903
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201902
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201901
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_2018_v2
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_2018
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201812
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2019_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2019
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201811
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201810
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201809
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201808
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201807
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201806
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201805
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201804
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201803
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201802
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201801
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201712
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201711
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201710
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201709
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201708
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201707
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201706
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201705
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201704
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201703
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201702
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201701
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2017mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201612
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201611
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201610
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201609
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201608
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201607
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201606
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201605
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201604
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201603
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201602
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201601
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201512
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201511
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201510
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2016mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201509
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201508
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201507
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2015tmsd
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201506
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201505
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201504
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201503
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201502
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201501
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201412
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201411
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2015mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201410
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201409
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201408
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201407
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014petrochemicalprocesses_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014petrochemicalprocesses
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201406
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201405
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201404
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201403
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014calendar
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201402
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201401
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201312
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201311
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201310
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201309
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201308
https://www.nxtbook.com/nxtbooks/gulfpub/hp_srsguide2013
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201307
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201306
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201305
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201304
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201303
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201302
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201301
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201212
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201211
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201210
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201209
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201208
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201207
https://www.nxtbook.com/nxtbooks/gulfpub/hp_srsguide2012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201206
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201205
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201204
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201203
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201202
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201201
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201112
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201111
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201110
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201109
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201108
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201107_srsguide
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201107
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201106
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201105
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201104
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201103
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201102
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201101
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201011
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201010
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201009
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201008
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201007
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201006
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201005
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201004
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201003
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201002
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201001
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200912
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200911
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200910
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200909
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200908
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200907
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200906
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200905
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200904
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200903
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200902
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200901
https://www.nxtbookmedia.com