Page 167 - Pipeline Rules of Thumb Handbook

154 Pipeline Rules of Thumb Handbook

with sudden compression in the presence of a diatomic gas interface during commissioning or trapping pockets of the
such as nitrogen or oxygen, exposure of the pipe to flame, pos- less dense nitrogen at each high place in the pipeline.
sibly hot tapping operations. In this case, the amount of ethylene burned to produce
The autoignition temperature (AIT) for ethylene is 914°F product purity may be expensive, time consuming, and create
(3)
at atmospheric pressure. Experience of others suggests that environmental problems in addition to subjecting the steel in
at 1,000 to 1,500psig, the AIT could drop to as low as 400° the pipeline to sub-design temperatures.
to 700°F. (3,4) As the concentration of oxygen increases, the If the pressure of the ethylene source used for commis-
(4)
decomposition initiation temperature decreases. From this, sioning is high, and the nitrogen pressure in the pipeline
it may be concluded that a mixture of low oxygen concentra- being commissioned is also high, cold temperatures will not
tion and ethylene could be very hazardous. For this reason, be encountered. However, even at warm temperatures, high-
ethylene pipelines, which have been evacuated for mainte- pressure ethylene is still much more dense than nitrogen at
nance or repairs, are usually filled with nitrogen to displace the same pressure. Therefore, the extensive product interface
oxygen from the pipe. Compressing nitrogen with ethylene and the tapped nitrogen at the high points will still occur,
may result in high nitrogen temperatures—especially if the resulting in an expensive and time consuming purification
compression is rapid. Also, when the pressure of the ethylene process plus the cost of the large volume of nitrogen. Also,
source is high, refrigeration across the throttling valve used there is a large inventory of ethylene in a pipeline at high pres-
to control the ethylene may result in extremely low tempera- sure. If a pipeline blow-down is required because of a leak,
tures and could cause the valve to fail due to excessively low lack of purity or for whatever reason, it will be a major product
temperature. loss and expense.
Commissioning an ethylene pipeline with warm ethylene The Pipeline Dehydrators system utilizes a tube and shell
(+/-70°F) at a low pressure (+/-50psig) is quick, easy, inex- heat exchanger and a water-glycol mix that is used as the heat
pensive and very safe when the ethylene is heated before it transfer fluid. A pump forces the heat transfer fluid through
is admitted to the pipeline being commissioned. the fired heater and then through the shell side of the
A patented process used by Pipeline Dehydrators can be
used to safely commission ethylene pipelines. The process
uses a shell and tube heat exchanger to heat the high pres-
sure ethylene before the pressure is reduced to the low pres-
sure desired for line commissioning.
The methods currently used to commission ethylene
pipelines are potentially hazardous, expensive and time con-
suming. The two most severe problems are potential damage
to the carbon steel in the pipeline due to the cold tem-
perature of the expanded ethylene and contamination of
the ethylene from the nitrogen used to inert the pipeline
during commissioning.
An ethylene pipeline that is being commissioned has
usually been dried to a -70°F dew point or more and inerted
with nitrogen. The nitrogen pressure left on the line for com-
missioning varies from 10 to 1,000psig depending on the exchanger. The ethylene passes through the tubes of the heat
pressure of the ethylene source and the commissioning exchanger and is safely and properly warmed by the heat
procedure used and the pipeline owner’s preference. transfer fluid without entering the direct fired heater, thereby
The source of ethylene used for commissioning an eliminating the possibility of an ethylene decomposition due
ethylene pipeline is usually over 900psig and may be up to to heater thermal runaway.
2,200psig. The heater will produce ethylene at ground temperature
If the pressure of the ethylene source used for commis- (+/-70°F) at +/-50psig corresponding to the conditions of the
sioning is high and the nitrogen pressure in the pipeline being nitrogen in the pipeline. Under these conditions, ethylene
commissioned is low, the resulting temperature of the ethyl- and nitrogen have basically the same density; therefore, the
ene, due to the pressure drop, may be well below the -20°F product interface during commissioning will be insignificant,
design temperature of most carbon steel pipelines. no nitrogen will be trapped at the high points in the pipeline,
The carbon steel in some pipelines is subject to becoming the steel in the pipeline will not be subjected to cold tem-
brittle at temperatures below -20°F and may fail at a very low peratures, purification will be simple, short and inexpensive
internal pressure because of the low temperature. and the ethylene inventory in the pipeline will be minimal in
At low pressures, the very cold ethylene will be much more case a pipeline blow-down is required. The savings on nitro-
dense than nitrogen, thereby producing an extensive product gen will also be significant.