Page 166 - Pipeline Rules of Thumb Handbook

Pipeline Drying 153

Commissioning petrochemical pipelines

Pipelines carrying a variety of gaseous and volatile liquid The main problems posed by recommissioning are (1) the
products are extensively employed in today’s industry as a safe fast and economical purging of nitrogen so as to obtain uncon-
and efficient means of transportation. New pipelines must be taminated products in the line, and (2) possible damage to the
placed into service initially (i.e., commissioned), and older pipeline due to cold temperatures to which it may be sub-
lines occasionally are taken out of service in order to perform jected during the process. The latter can be a problem when
some desired maintenance and are then recommissioned. the pipeline is to be recommissioned with a product at a pres-
Among the reasons for taking a pipeline out of service (i.e., sure significantly higher than that of the nitrogen inerted line.
decommissioning) are: hydrostatic testing to recertify or up- The pressure drop of a petrochemical product entering a
grade the pipeline’s ability to be used at higher operating pipeline can cause large drops in temperature and, con-
pressures; performance of construction work on the pipeline; sequently, potential damage to the carbon steel pipeline.
a change in the product transported by the pipe- Due to the demands of the expanding petrochemical indus-
line. Pipelines, or sections of a pipeline may need to be relo- try, and the aging of the present pipeline system, the need for
cated because of highway work, the necessity to deepen a decommissioning, repairing, cleaning, drying and recommis-
canal, or because of increases in the population surrounding sioning pipelines is increasing. Given the value of today’s
the pipeline. It may also be necessary to replace valves, petrochemical products, a recommissioning procedure, which
fittings or a damaged section of the pipeline or add a new brings the product purity to acceptable levels as quickly as
connection to service a customer or supplier. possible, is desirable. Furthermore, processes and equipment
Typically, decommissioning and recommissioning a petro- which reduce the possibility for damage to the pipeline due
chemical pipeline will include the steps of decommissioning to cold temperature conditions which can occur during the
by removing the product from the pipeline, and flaring any recommissioning process are also highly desirable. (2)
remaining residual product. Any necessary construction, up- Due to the unusual properties of ethylene, decommission-
grading, or cleaning of the pipeline can then be performed. ing and recommissioning ethylene pipelines require special
Usually, the pipeline is then filled with water for hydrostatic consideration. Ethylene is a colorless flammable gaseous
testing. After pressure testing, the water is removed, the unsaturated hydrocarbon obtained by the pyrolysis of petro-
pipeline is cleaned and dried to a specific low dewpoint (to leum hydrocarbons. The relative density of ethylene is 0.9686
avoid the problems of water contamination of the product) (air = 1) and its molecular weight is 28.054. The molecular
and the pipeline is inerted with nitrogen for recommissioning. weight of air is 28.964 and the molecular weight of nitrogen
Refer to the section on Pipeline Drying for drying techniques. is 28.013.
In order to recommission the pipeline, the nitrogen must
be displaced by the desired petrochemical. Before returning
to service, product purity must be established and the line Ethylene decomposition
safely filled to operating pressure. The terms “commission-
ing” and “recommissioning” are used interchangeably to refer Ethylene is subject to thermal decomposition (self propa-
to a process whereby a first inerting gas, normally nitrogen in gating reaction zone, explosion or flame) under certain
a pipeline is replaced with the desired product at the desired circumstances. Thermal decomposition occurs when the
purity and pressure. temperature of a substance is raised above the value needed
Safety and economics are two primary concerns for any to cause it to begin to self-heat because of a change in mo-
proposed pipeline operation. Thus, a commissioning process lecular structure at a rate high enough to result in com-
which brings the purity and pressure of potentially explosive bustion. An external source of ignition is not required to
products such as ethylene or propylene up to specification precipitate this process nor is a source of oxygen needed.
quickly, but at the risk of damage to the pipeline or reduced Thermal decomposition is usually initiated by a source of heat
safety to operating personnel, is not acceptable. Similarly, a from events such as sudden compression, external source of
(4)
process which uses large quantities of product to push nitro- heat, or the thermal runaway of a heater. Heaters are some-
gen from a line results in wasted product/nitrogen mixtures times used in ethylene measurement stations to maintain the
that must be flared or otherwise disposed of would also be ethylene at an optimum temperature for measurement.
unacceptable. Improvement over known methods requires Ethylene is routinely handled under conditions where
consideration of its safety and economic benefits (considering decomposition may be initiated. The point at which decom-
both the cost of wasted product and the cost attributed to the position begins is dependent upon system parameters such as
time the line must remain out of service) as well as its ability line or vessel size, operating pressure, and operating tem-
to bring the line back into service with product at desired peratures. Generally, ethylene decomposition is initiated in
pressures and purities. (1) pipeline facilities by a rapid rise in temperature associated