Page 141 - Handbook Of Multiphase Flow Assurance

Hydrate of natural gas 137

The plug moved driven by a pressure differential of 35 bar with high velocity to the closed
valve near to the drum.
The impact created additional forces to the bolts, reduction of surface pressure to the gas-
ket and finally a leak.
No foreign body was found but ice or hydrate can form solid bodies. The line including the
two valves is a local low point.
If free water is present it will accumulate there. Water has been found once in the past.
Temperatures in February were low and the line toward the two valves is not trace heated.
September 1998 Australia onshore process plant
Hydrate inhibition was originally designed for methanol, but eventually switched to
glycol.
A 3 °F safety margin was built into the glycol calculation per operator practice.
In June (winter) hydrate formed in the slug catchers receiving a mixture of gas, condensate,
water and glycol at gas production and processing facilities.
Liquids from the slugcatchers carried over into gas plants GP2 and GP3, again causing
shutdowns.
About 10 days before the June hydrate incident, substantial amount of aquifer water en-
tered the pipeline from the operation of some high-GOR wells.
Significant increase in individual pipeline flowrates feeding into the slugcatchers due to
gas demand increase during a holiday, caused liquids lying in the pipelines to be swept from
the pipelines into the slugcatchers.
Significant amounts of methanol were used to clear hydrate in slugcatchers.
The TRC3B valve had been giving trouble for some time before the accident.
TRC3B valve was a control system designed to regulate the temperature of condensate in
the bottom of Absorber B.
TRC3B valve repair was done in March. On that occasion the TRC3B valve was not closing
fully. One of the block valves was also not closing fully.
Repairs were effected by injecting methanol into the process, apparently in the belief that
a hydrate was causing the problem.
Whatever the cause, the injection of methanol appeared to solve the problem.
In September, explosion and fire with loss of 2 lives occurred as GP905 heat exchanger
ruptured.
A physical connection was drawn between the two events concerned the presence of mo-
lecular sieve dust found in GP903B heat exchanger when it was opened for inspection after
the September incident.
The compound used in the molecular sieves, or dehydrators, is damaged by contact with
methanol, large quantities of which were injected into the slugcatchers during the hydrate
incident in June.
Methanol would have entered the dehydrators as methanol vapor leading to a possible
breakdown of the molecular sieve particles and an accumulation of molecular sieve dust in
GP903 heat exchanger. Investigation deemed that hydrate did not contribute to the incident.
January 2009 US refinery flare line
A freeze-related restriction developed in the gas flare line in US between flare knock out
drum and flare stack which caused a high purge gas pressure alarm. Operator observed that
the high pressure alarm for the flare knock out drum was also active, which confirmed that