LNG storage tanks

LNG Terminal:

Safe Isolation of LNG tanks and shipping transfer lines.

The Problem

LNG tanks must be isolated quickly in an emergency. If the large isolation valves close too slowly then isolation is delayed, if they close too fast, this can cause surge pressures in the offloading lines, which can cause pipe failure. Trial closing of the valves to determine surge pressures could cause these excessive surge pressures that we want to avoid. The client needed to predict maximum surge pressures without closing the valves during live shipping unloading operations.

The Solution

  • A simulation model was required that predicts the surge pressure against valve closing time, at various expected plant conditions.

  • Detailed model of the piping network with valve trim details, holdup and closing dynamics of valves.

  • Evaluation of two or three stage closing times to be compatible with valve trim.

  • Prediction of cavitation and vapour pockets. Use the model to develop recommended valve closure times and acceptable tolerances.

The Benefits

  • Surge pressures in the line were accurately simulated enabling prediction of acceptable valve closure times. Valve operation was then tested during non-transfer periods. Potentially hazardous plant trials and shipping delays were avoided, testing was done with no impact on operations.

  • Engineering time and costs were reduced through “right first time” design, that had been validated and de-risked from the outset and had been sensitivity checked to avoid unexpected surprises.

Services Provided

  • Develop an accurate representation of the plant geometry.

  • Assist the client with scenario development.

  • Develop dynamic models using correct thermodynamic methods and surge analysis.

  • Expert evaluation of the modelling results and determination of required valve closure times.

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