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Thermal Profiling Of Long Distance Pipelines Carrying Chemicals (Gas And Liquid)




Thermal Profiling Of Long Distance Pipelines Carrying Chemicals (Gas And Liquid) Dear All,

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.

This is all for today folks. This has been a long blog entry but I hope you enjoy what I share with you and I look forward to your comments and observations.

Regards,
Ankur.




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