Page 163 - Pipeline Rules of Thumb Handbook

150 Pipeline Rules of Thumb Handbook

nomical and technically feasible for most pipeline drying accelerate the evaporation of remaining methanol/water
applications. solution.
Before any type of drying operation commences, it will be Also of great concern with the methanol method of drying
necessary to clean the line using one of the previously is the fact that explosive mixtures can easily be formed,
described processes. If the rust and mill scale are not removed whether gas or air is used to displace the methanol batches.
from the pipe wall, moisture will remain trapped and will It may be desirable to use an inert gas such as nitrogen to
bleed out over a long period of time. A pipeline can be dried buffer the methanol batch from the air or gas used to displace
without cleaning; however, the cost and time required will the methanol batch. However, many pipelines have been
be great. dried without the use of nitrogen buffers with no adverse
In drying with super dry air, soft foam pigs pushed by dry results. The air/methanol mixture is also highly poisonous and
air are used to absorb any free water remaining in the pipeline corrosive. 1
after dewatering. After the line is dust dry, wire brush pigs Plans will need to be made for proper disposal of the spent
are run to remove any water bearing debris from the pipe methanol.
wall. The wire brush pigs are then followed by soft foam pigs If internal corrosion protection is desired, then another
to absorb the loosened debris. Near the end of this phase, the drying method should be considered. If the pipeline is to be
pigs may be weighed prior to insertion and after removal to used to transport sour gas, the methanol drying method
monitor the amount of debris that is being removed. The first should be carefully evaluated before it is used.
pigs run through the line will naturally weigh much more than Drying with nitrogen is accomplished in much the same
their clean weight. As the cleaning and drying progress, the manner as when using super dry air. Nitrogen drying will
pig weights will approach their original weight. Pig color will cost more than super dry air by a factor of approximately 50%.
also give some indication of how the debris removal is Also, super dry air is plentiful and non-polluting.
proceeding. Internal sand blasting will leave the pipeline clean and dry.
Dew-point readings will need to be made to determine The procedure for internal sand blasting is described earlier
when the line has been dried to the specified dew in this section.
point. Drying with natural gas requires large volumes of gas. This
It will be necessary to give special consideration to method is slow and not very effective unless the line is
laterals, by-passes, and valve body cavities as any free thoroughly cleaned by one of the cleaning processes previ-
water trapped here could affect the final dew point ously described. If, however, the gas being used to dry
readings. with can be blended with another dry gas stream and sold
Drying with super dry air provides internal corrosion pro- or used, then this is an economical method for pipeline
tection if the line is to remain out of service for some period drying. The cost of the gas that will be used to purge the
of time before it is placed in service. line during the drying process should be weighed against
Methanol drying relies on the hygroscopic effect of the the cost of using super dry air. The Btu value of a volume of
methanol. Any remaining moisture in the line will be natural gas is approximately fifteen times greater than that
absorbed by batches of methanol pushed through the line of the fuel required to produce an equal volume of super
with either gas or dry air. Pigs are used to separate the dry air.
1
methanol batches from the displacing medium. Methanol Vacuum drying is a slow process and all free water should
drying usually requires fewer pig runs and, consequently, less be removed from the pipeline before drying begins. This
line cleaning is accomplished. Pure methanol is expensive and method appears to be used infrequently, and perhaps only
sometimes a 96% methanol/water mix is used. Since the offshore.
methanol mix contains water, some water will be left in the If the pipeline has been properly cleaned by the water slug
pipeline. Some of the methanol will vaporize in the pipeline method using brush pigs run with liquid, drying can be
and will be absorbed by the displacing medium. Towards the accomplished by running soft foam pigs with dry air or gas to
end of the line, the moisture content of the methanol will remove any free water left in the pipeline. This will usually
increase, which in turn reduces the amount of water that it produce a pipeline dry enough for natural gas operations. If
can absorb. If the pressure used in the drying operation is too additional drying is desired, it can be accomplished by using
high, hydrate formation can occur, usually at the far end of methanol or super dry air.
the line. If natural gas is being used to push the methanol
batches, it will probably be necessary to flare some of the line
fill volume to be sure that no methanol impurities are con-
tained in the gas. 1 Reference
Methanol run with a dry gas will absorb most of the
water and facilitate the vaporization of the remaining 1. Kopp, Pipe Line Industry, October 1981, January
water. Soft swabs run through a line with a dry purge gas will 1986.