Hydrocarbon Processing - August 2021 - GP-31
EQUIPMENT
Are cylinder liners necessary for high-speed
reciprocating gas compressors?
S. ZARDYNEZHAD, TurboTech Consulting Corp., Calgary, Alberta, Canada
Wear is common in reciprocating gas
compressors (RGCs) due to sliding and
rotational motion of parts. In the design of
RGCs, wear study and calculation by empirical
formulas are conducted to estimate
and forecast the life of the parts. However,
wear during real working conditions is different
from theoretical estimations and,
sometimes, catastrophic failure is experienced
during operation due to considerable
mechanical or chemical wear.
Wear in the cylinder wall due to reciprocating
motion of the piston is a good example
of such circumstance and the main
subject of this article. Low- and moderate-speed
RGCs are normally furnished
with a cylinder liner; however, high-speed
RGCs may or may not be furnished with
a cylinder liner. In this article, the main
differences between low-, medium- and
high-speed RGCs are discussed; the wear
mechanism in the cylinder is explained;
the importance of cylinder liners to RGCs
is addressed; and the author's recommendations
for decision-making when considering
liners are explained.
Standards for RGC operation. The
American Petroleum Institute (API) produced
API Standard 618 (1964)1
Standard 11P (1976).2
and API
These standards
are frequently used for the design and
manufacture of RGCs. API 618 (Edition
1) was published in 1964 based on the operating
experience of 1950s compressor
models. During that time, manufacturing
and materials technology were not advanced
and the speed was limited to 100
rpm, which is why the 1st edition of API
618 covers only low-speed compressors.
Manufacturers encouraged coupling
RGCs to gas and diesel engines with
speeds of 800 rpm and higher, which increased
their size and cost. This led to the
publication of API 11P in February 1976,
TABLE 1. Comparison of rotational speed range of RGCs
Speed, rpm
Low
API
Author's experience
Some manufactures
Not specified
100-350
100-600
approximately 12 yr after the publication
of the first revision of API 618. Moderatespeed
units were derived from high-speed
units and added to API 618 in 1995.
RGCs are classified into three types
in terms of rotational speed, as shown
in TABLE 1. High-speed, packaged RGCs
for oil and gas service are covered under
API 11P, which has been withdrawn by
API and replaced by ISO 13631 (2002),
" Petroleum and natural gas industries-
Packaged reciprocating compressors. " 3
The driver can be either an engine or an
electric motor. Sometimes they are coupled
via a gearbox. Their operating speed
is typically 800 rpm-1,800 rpm, based
on the author's experience, with engine
drivers above 1,000 rpm and motor drivers
of 800 rpm-1,000 rpm.
High-speed RGCs are skid-mounted
and packaged. They are easy to install,
easily transportable to different sites due
to their modularized and compact design,
and have a relatively small CAPEX. Highspeed
RGCs have higher maintenance
costs than low- and medium-speed compressors.
Low- and medium-speed compressors
run at speeds of 100 rpm-799
rpm. These compressors have lower speed,
higher stroke and higher efficiency over a
wide range of operating conditions and
require less maintenance than the highspeed
units because of their lower speed.
However, low- and medium-speed compressors
usually must be field-erected and
require heavy foundations. They have high
degrees of vibration and pulsation suppresMedium
Not
specified
350-799
600-1,200
High
Not specified
800-1,800
1,200-1,800
FIG. 1. API 618 compressor cylinders are
frequently supplied with centrifugally cast,
removable liners to reduce reconditioning costs.
sion compared to low- and medium-speed
compressors. Large low-speed compressors
are shipped to the site as loose items
(e.g., throws, frame, driver, etc.).4
API 618 compressor cylinders are frequently
supplied with liners (FIG. 1) to
reduce reconditioning costs. RGC cylinder
liners are removable in case of accidental
scoring or wear over a long period
of service. In general, liners are not used
in high-speed machines (API 11P/ISO
13631) where the rotating speed is higher
than 800 rpm. Liners are almost always
used in API 618 compressors, especially
in critical services where the gas being
handled is corrosive, toxic, low-temperature
or high-pressure, and in special designs
like free-floating piston.
However, API 618 does not define
the meaning of " critical services. " Liners
have become a standard design for many
Gas Processing & LNG | JULY/AUGUST 2021 31
Hydrocarbon Processing - August 2021
Table of Contents for the Digital Edition of Hydrocarbon Processing - August 2021
Contents
Hydrocarbon Processing - August 2021 - Intro
Hydrocarbon Processing - August 2021 - Cover1
Hydrocarbon Processing - August 2021 - Cover2
Hydrocarbon Processing - August 2021 - Contents
Hydrocarbon Processing - August 2021 - 4
Hydrocarbon Processing - August 2021 - 5
Hydrocarbon Processing - August 2021 - 6
Hydrocarbon Processing - August 2021 - 7
Hydrocarbon Processing - August 2021 - 8
Hydrocarbon Processing - August 2021 - 9
Hydrocarbon Processing - August 2021 - 10
Hydrocarbon Processing - August 2021 - 11
Hydrocarbon Processing - August 2021 - 12
Hydrocarbon Processing - August 2021 - 13
Hydrocarbon Processing - August 2021 - 14
Hydrocarbon Processing - August 2021 - 15
Hydrocarbon Processing - August 2021 - 16
Hydrocarbon Processing - August 2021 - 17
Hydrocarbon Processing - August 2021 - 18
Hydrocarbon Processing - August 2021 - 19
Hydrocarbon Processing - August 2021 - 20
Hydrocarbon Processing - August 2021 - 21
Hydrocarbon Processing - August 2021 - 22
Hydrocarbon Processing - August 2021 - 23
Hydrocarbon Processing - August 2021 - 24
Hydrocarbon Processing - August 2021 - 25
Hydrocarbon Processing - August 2021 - 26
Hydrocarbon Processing - August 2021 - 27
Hydrocarbon Processing - August 2021 - 28
Hydrocarbon Processing - August 2021 - 29
Hydrocarbon Processing - August 2021 - 30
Hydrocarbon Processing - August 2021 - 31
Hydrocarbon Processing - August 2021 - 32
Hydrocarbon Processing - August 2021 - 33
Hydrocarbon Processing - August 2021 - 34
Hydrocarbon Processing - August 2021 - 35
Hydrocarbon Processing - August 2021 - 36
Hydrocarbon Processing - August 2021 - 37
Hydrocarbon Processing - August 2021 - 38
Hydrocarbon Processing - August 2021 - 39
Hydrocarbon Processing - August 2021 - 40
Hydrocarbon Processing - August 2021 - 41
Hydrocarbon Processing - August 2021 - 42
Hydrocarbon Processing - August 2021 - 43
Hydrocarbon Processing - August 2021 - 44
Hydrocarbon Processing - August 2021 - 45
Hydrocarbon Processing - August 2021 - 46
Hydrocarbon Processing - August 2021 - 47
Hydrocarbon Processing - August 2021 - 48
Hydrocarbon Processing - August 2021 - 49
Hydrocarbon Processing - August 2021 - 50
Hydrocarbon Processing - August 2021 - 51
Hydrocarbon Processing - August 2021 - 52
Hydrocarbon Processing - August 2021 - 53
Hydrocarbon Processing - August 2021 - 54
Hydrocarbon Processing - August 2021 - 55
Hydrocarbon Processing - August 2021 - 56
Hydrocarbon Processing - August 2021 - 57
Hydrocarbon Processing - August 2021 - 58
Hydrocarbon Processing - August 2021 - 59
Hydrocarbon Processing - August 2021 - 60
Hydrocarbon Processing - August 2021 - 61
Hydrocarbon Processing - August 2021 - 62
Hydrocarbon Processing - August 2021 - 63
Hydrocarbon Processing - August 2021 - 64
Hydrocarbon Processing - August 2021 - 65
Hydrocarbon Processing - August 2021 - 66
Hydrocarbon Processing - August 2021 - 67
Hydrocarbon Processing - August 2021 - 68
Hydrocarbon Processing - August 2021 - 69
Hydrocarbon Processing - August 2021 - 70
Hydrocarbon Processing - August 2021 - 71
Hydrocarbon Processing - August 2021 - 72
Hydrocarbon Processing - August 2021 - 73
Hydrocarbon Processing - August 2021 - 74
Hydrocarbon Processing - August 2021 - 75
Hydrocarbon Processing - August 2021 - 76
Hydrocarbon Processing - August 2021 - 77
Hydrocarbon Processing - August 2021 - 78
Hydrocarbon Processing - August 2021 - 79
Hydrocarbon Processing - August 2021 - 80
Hydrocarbon Processing - August 2021 - 81
Hydrocarbon Processing - August 2021 - 82
Hydrocarbon Processing - August 2021 - Cover3
Hydrocarbon Processing - August 2021 - Cover4
Hydrocarbon Processing - August 2021 - GP-1
Hydrocarbon Processing - August 2021 - GP-2
Hydrocarbon Processing - August 2021 - GP-3
Hydrocarbon Processing - August 2021 - GP-4
Hydrocarbon Processing - August 2021 - GP-5
Hydrocarbon Processing - August 2021 - GP-6
Hydrocarbon Processing - August 2021 - GP-7
Hydrocarbon Processing - August 2021 - GP-8
Hydrocarbon Processing - August 2021 - GP-9
Hydrocarbon Processing - August 2021 - GP-10
Hydrocarbon Processing - August 2021 - GP-11
Hydrocarbon Processing - August 2021 - GP-12
Hydrocarbon Processing - August 2021 - GP-13
Hydrocarbon Processing - August 2021 - GP-14
Hydrocarbon Processing - August 2021 - GP-15
Hydrocarbon Processing - August 2021 - GP-16
Hydrocarbon Processing - August 2021 - GP-17
Hydrocarbon Processing - August 2021 - GP-18
Hydrocarbon Processing - August 2021 - GP-19
Hydrocarbon Processing - August 2021 - GP-20
Hydrocarbon Processing - August 2021 - GP-21
Hydrocarbon Processing - August 2021 - GP-22
Hydrocarbon Processing - August 2021 - GP-23
Hydrocarbon Processing - August 2021 - GP-24
Hydrocarbon Processing - August 2021 - GP-25
Hydrocarbon Processing - August 2021 - GP-26
Hydrocarbon Processing - August 2021 - GP-27
Hydrocarbon Processing - August 2021 - GP-28
Hydrocarbon Processing - August 2021 - GP-29
Hydrocarbon Processing - August 2021 - GP-30
Hydrocarbon Processing - August 2021 - GP-31
Hydrocarbon Processing - August 2021 - GP-32
Hydrocarbon Processing - August 2021 - GP-33
Hydrocarbon Processing - August 2021 - GP-34
Hydrocarbon Processing - August 2021 - GP-35
Hydrocarbon Processing - August 2021 - GP-36
Hydrocarbon Processing - August 2021 - GP-37
Hydrocarbon Processing - August 2021 - GP-38
Hydrocarbon Processing - August 2021 - GP-39
Hydrocarbon Processing - August 2021 - GP-40
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