(183a) Application of Dynamic Simulation for Relief System Design in LNG Facility

Emergency relief in the process industries aims to protect equipment, the environment and operating personnel from abnormal conditions. Appropriate estimation of relief loads is important for the correct sizing of relief valves and flare headers, and for the selection of disposal media. In addition, during debottlenecking or revamping of process units, adding a new relief valve and modifying the relief system can be very costly and, in terms of construction, difficult to implement.

Application of dynamic modeling for relief system design can reduce the capital expenditure (CAPEX) without compromising on the safety. Dynamic simulation utilizing rigorous mathematical models has become an influential tool in process design, design validation, relief system design, control system verification, startup support and troubleshooting. Many recent developments in the simulation software and technology have led to the development of large scale dynamic models.

In the Inlet Facility of LNG or Gas Plant, the feed gas or mixture of gas and condensate are received by means of high pressure pipelines. The condensate or liquid slugs, if any, are separated in the Inlet Slug Catcher / Inlet Separator and feed gas is split into multiple trains for processing. The Condensate Feed Drum is also part of inlet facility for an LNG or Gas Plant that receives and separates front end liquids from the Inlet Slug Catcher / Inlet Separator. The operating pressures of Inlet Slug Catcher / Inlet Separator are significantly higher than the design pressure of Condensate Feed Drum thereby creating HP/LP interfaces, and a risk of overpressuring the Condensate Feed Drum.

There are many HP/LP interfaces in an LNG facility. Typically, the governing overpressure scenario for the vessel operating at low pressure occurs when the level control valve of the HP upstream vessel fails in the open position. During this scenario, the different stages of relief may occur as follows:

1. Initial liquid / two-phase relief: Liquid from the HP vessel flows through the control valve, and may flash at the lower pressure.
2. Liquid displacement: Once the liquid in the upstream vessel has emptied, vapor begins flowing through the line and upon reaching the control valve will expand, resulting in acceleration of the liquid in the downstream piping to the LP vessel.
3. Gas blowby: Vapor from the upstream HP vessel flows through the control valve to the downstream LP vessel.

Conventional methodology for determining the relief load does not consider any hydraulic losses in the system, resulting in overestimation of the required relief rate. In general, if the liquid volume in the upstream vessel exceeds the available vapor/liquid disengagement volume in the downstream vessel, then the liquid displacement portion of the relief results in the greatest required relief due to the expansion of vapor across the control valve. If the liquid volume in the upstream vessel does not exceed the available vapor/liquid disengagement space in the downstream vessel, then the greatest required relief occurs during gas blowby.

Applying dynamic simulation, the relief load is limited based on the resistance in the system and also the transitional change in composition from condensate to feed gas will provide a realistic determination of relief load.

This paper considers different methods for estimating relief load for a HP/LP interface system, compares the results from both steady state and dynamic methods relief rate determination, and discusses the strengths and weaknesses of each method. Furthermore, it describes how dynamic simulation was applied to an existing LNG facility in reducing the capital expenditure (CAPEX) and reducing the turnaround time for the modification.