One
of the important tasks while doing a pipeline process study is to do a thermal
profile study of a pipeline for various operating scenarios and ambient
conditions. However, what are the practical aspects of doing a thermal profile
are mostly unknown to majority of new engineers who undertake this exercise. In
fact many young engineers are not even aware that a thermal profile for the
pipeline needs to be developed to understand the problems that can occur in
pipeline transport.
Let me first list down some problems related to
thermal effects in a long distance pipeline:
1. For a gas pipeline frictional pressure drop
in the pipeline can lead to Joule-thomson cooling in the pipeline which is an
isenthalpic process. It is possible that flowing gas temperatures over a
certain pipeline length may fall below the hydrate formation temperature of the
pipeline gas, which can lead to ice crystal formation in the pipeline, which in
turn can lead to pipeline flow disruption by partial or full pipeline blockage.
In a worst case scenario, this can lead to a pipeline rupture with loss of
property and human lives. Gas hydrate formation is favored by high pipeline
pressures, and already low transportation temperatures such as in deep sub-sea
pipelines. Thus a thermal study becomes absolutely essential to determine whether
such an event can occur and what mitigative measures need to be taken.
2. Depressurization of gas pipelines during
emergency or routine maintenance operations is another issue which needs
attention in terms of thermal study of the depressurization operation.
Adiabatic depressurization can lead to extremely low temperatures at the
depressurization source, and immediately downstream of the depressurizing
device (valve or orifice). Ordinary carbon steels can suffer catastrophic
brittle fracture at very low temperatures compromising pipeline safety. Normal
commercial carbon steel grades often used for fabricating pipe such as ASTM A
106 and ASTM A 53 are not designed for temperatures below -29 deg C. In process
engineering terms the pipe needs to be categorized for its "Minimum Design
Metal Temperature" abbreviated as MDMT. Among carbon steels ASTM A333 Gr.
1 and Gr.6 are suitable for low temperature service up to -45 deg C and are
impact tested for that temperature.
A thermal study of the depressurization process
can provide the lowest temperatures that a pipeline can see, and thus define
the MDMT of the pipe material. As an example, if a depressurization thermal
study indicates that the temperature of pipeline can fall below -29 deg C, the
material specialist is most likely to suggest to go for "Low Temperature
Carbon Steel" abbreviated as LTCS along with a brittle test for the pipe
material known as "Charpy V-Notch Impact Test" to determine the
brittleness of the material at low temperatures. If the depressurization
thermal study indicates temperatures falling below -46 deg C, and up to -80 deg
C, thus further lowering the MDMT of the pipe material, then the material
specialist may further require to upgrade the metallurgy to Duplex Stainless
Steel (DSS). Cryogenic temperatures either during normal operation or
maintenance tasks, may require selection of austenitic stainless steels. Again
this would be determined by thermal studies of the process system.
3. Pipelines transporting crude oils,
specifically highly viscous and with high asphaltene content, require a thermal
profiling to determine whether the pipeline temperatures fall below the pour
point or not along the pipeline length. Possible pipeline blockages due to
temperatures falling below pour point can lead to production disruption,
expensive cleaning, dechoking operations, and heavy economic losses. Based on
the thermal profiling and determination of lowest temperatures in the pipeline,
remedial measures can be taken to prevent such an occurrence.
4. Transportation of LPG (propane and
propane:butane mixtures) in pipelines is another area which requires particular
attentions in terms of thermal studies of the pipeline. Exposure to high
ambient temperatures for above ground pipelines or sections of above ground
pipelines of LPG require a careful thermal study, to ensure that the LPG does
not vaporize in the pipeline at the operating pressure,which can lead to vapor
locking and 2-phase flow at the determined temperature. At the destination end
of the pipeline, if there is a flow or pressure control valve, and due to
temperature rise of the LPG along the pipeline length there is partial
vaporization at the inlet of the control valve leading to inlet 2-phase flow,
it is very detrimental for the operation of the control valve in terms of valve
integrity. In a nutshell, it is absolutely undesirable to have 2-phase flow in
a LPG pipeline, and a thermal study would indicate, whether such a phenomena
occurs across the length of the pipeline from the source to the destination.
However, when doing such thermal studies for
above ground pipelines exposed to the ambient atmosphere it is absolutely
essential that realistic ambient conditions are used as inputs. To put this in
perspective, a good designer would consider average high or low ambient
temperature conditions and average high or low wind speeds over a time duration
(say summer / winter or 1-year average conditions) rather than one-off high or
low temperature and wind condition values which are highly unlikely to re-occur.
To put it somewhat crudely, freak weather values of temperature and wind
velocities should not be used for thermal profiling of pipelines exposed to the
ambient. Using such freak ambient conditions may have a huge economic cost,
which may not be justified for normal pipeline operating conditions. Abnormal
ambient conditions would actually dictate that pipeline operation be stopped or
curtailed.
5. There are many pipeline and process
simulation software that can perform thermal studies / profiling of pipelines.
Obviously, software dedicated for pipeline simulation such as PIPESIM,
PIPEPHASE, PIPELINE STUDIO, AFT, OLGA would be the best bet for performing such
thermal studies. But general process simulation software such as Aspen HYSYS,
UniSim, Aspen Plus are also capable of performing thermal profiling of
pipelines with certain limitations. Aspen HYSYS has a very nice
"Depressurization" utility which can provide thermal profiles of any
equipment or pipeline being depressurized. The key to using any such simulation
software is that you exactly know what you need from the thermal study and
provide the right inputs including checking the veracity of the default values
the software uses for your particular study, and if required, to change those
default values to suit your particular case study. In a nutshell, use the
software as a process or chemical engineer and not as a data entry operator.