Page 162 - Pipeline Rules of Thumb Handbook

Pipeline Drying 149

being internally sand blasted by using air or nitrogen to propel clay, rust, and mill scale. Caution must be taken to ensure that
the sand. This procedure, if properly executed, will leave the short sections are cleaned each time so that high velocities
pipeline dry upon completion. may be maintained; that large quantities of dry air or gas are
After the pipeline has been dewatered, clay is blown into available to sustain the high velocities; that the pipeline is dust
short sections of the pipeline with a dry gas to absorb the dry; that all sand has been removed from the pipeline after
remaining water. The correct type of sand is then blown the sand blasting is complete; and, that the pipeline facilities
through the dust dry section at a high velocity to remove the will not be damaged by the high velocity sand.

Chemical cleaning

Cleaning with chemicals is typically used for smaller diam- Typically, chemical cleaning is more involved than other
eter lines. A properly planned chemical cleaning operation cleaning processes in that some special equipment will be
can produce a very clean pipeline. Chemical cleaning will required to safely handle the chemicals. Disposing of the
remove rust from any internal pits. Chemical cleaning is spent material will also present a problem. Care must be exer-
accomplished by pushing a batch of hydrochloric acid and cised in the selection of pigs to be sure they will not be con-
water for rust removal, a neutralizer, and a passivator, all sumed by the cleaning solution. Personnel safety must be
separated by pigs, through the pipeline. This method will carefully considered when choosing this method of pipeline
leave a bright metal finish on the interior of the pipeline. If cleaning.
a pipeline is being converted for a different service, such as
a crude oil line being converted to chemical service, a batch Purging with gas. This method is seldom used for
of detergent can be added behind the cleaning process to pipeline cleaning since the amount of debris that will be
remove any traces of the crude oil or other product. removed is small. A high gas velocity is required.

Pipeline drying

Pipelines used to transport petrochemicals such as pro- Air at +70°F with a humidity of 100% has a dew point of
pylene and ethylene must be dried in order for the delivered +70°F and holds approximately 1,253 pounds of water (150
product to meet moisture specifications. Natural gas pipelines gallons) per million standard cubic feet.
are usually dried to a lesser extent to prevent the formation The most common methods for drying pipelines are as
of hydrates. It is not unusual for a petrochemical line to be follows:
dried to a dew point of -80°F. A typical dew point for a pro-
pylene pipeline will be -70°F. A carbon dioxide pipeline • Drying with super dry air
might be typically dried to a -40°F dew point. The natural • Drying with methanol
gas industry specifies dryness in pounds of water per million • Drying with inert gas such as nitrogen
standard cubic feet of gas. The table at the end of this section • Internal sand blasting
will enable you to convert from one method to the other. • Drying with the medium to be transported
Dew point, by definition, is the temperature at which water • Vacuum drying
vapor begins to condense out of a gas at atmospheric pres-
sure. For example, at atmospheric pressure, water vapor All of these methods may be applied to pipeline drying
begins to condense out of a gas that has a moisture content depending on the particular line and amount of dryness
of seven pounds of water per million standard cubic feet at required. No single method can be considered ideal for
-39°F. Therefore, a gas that has a moisture content of seven all situations. Many times, a combination of two or more
pounds of water per million standard cubic feet has a dew methods will be used to achieve a dry pipeline at the least
point of -39°F. cost. The first three methods are probably the most eco-