David Gomez, Alberto Ramos, and Daniel Rivera, SacyrFluor, Spain, detail a case study of an LNG invasion phenomenon in carbon steel pipes, using dynamic simulation to study the kinetics.
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fter open rack vaporisers (ORV), an increase in fluid temperature allows for the pipe material to be changed from stainless steel (SS) to carbon steel (CS). Industry standards recommend performing a transient analysis to check the risk of cold propagation on piping downstream from the vaporiser. In the LNG terminal case of the study, the pipe specification break was very close to the equipment outlet. In order to verify the existing system, SacyrFluor (part of the Sacyr Group) studied the kinetics of an LNG invasion phenomenon in case of seawater failure in the ORVs by using a dynamic simulation. The dynamic simulation results were used to check the validity of the existing LNG terminal protections and, in case of inadequacy, to recommend the required modifications.
Introduction The EN1473 standard (Installation and equipment for LNG design of onshore installations) states in section 8.1.2 Materials of Vaporisation of LNG, that “a transient analysis shall be performed in order to check the risk of cold propagation on piping downstream the vaporiser.” Additionally, in section E.2.6. Control/Safety of Specific Requirements for LNG Vaporisers, it states that “in case of low gas temperature” “the vaporiser shall be automatically
isolated.” “Valve closure time should be set to prevent cold temperature extending over limits defined by thermal transient analysis.” SacyrFluor was requested to perform this thermal transient analysis at an existing European LNG terminal. By using a dynamic process simulator which evaluates the kinetics of an LNG invasion phenomenon in ORVs in case of seawater failure, SacyrFluor checked the validity of the existing interlocks and recommended modifications if the existing interlocks were deemed to be inadequate.
Methodology Dynamic simulation was performed with Aspen Hysys Dynamics v11. The thermodynamic package Peng Robinson with Lee-Kesler for enthalpy has been selected because, based on SacyrFluor’s experience in previous LNG projects, this was the option that best matched the results with field data. ORV inlet conditions corresponded to high-pressure LNG header conditions. No change over time was considered in these conditions. Even when there was no high-pressure LNG flow as a result of the presence of protection, the pressure was assumed to remain constant in the pipes. Outlet conditions corresponded to the NG header. Pressure was
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