PTQ Q4 2023 Issue

Assessing LNG feed gas depressurisation

Methods and models for estimating time-dependent profiles of mass discharge rates, pressures, and temperatures for LNG feed gas non-adiabatic depressurisation cases

Tek Sutikno Fluor Enterprises

D epressuring systems are generally installed in LNG facilities and process plants operating at high pres- sures to timely reduce pressures of isolated segments when necessary. These systems can be used to reduce the risks of vapour cloud explosion or jet fire in an emergency, such as fire, seal failure, flange leakage, or other contain - ment losses. API Standard 521 discusses depressurisation time and pressure targets. In LNG facilities, the transient discharges from depres - suring are routed to a (wet or dry) flare system and typically result in major flare loads, which need to be considered in the flare design philosophy and flare system operating capacity. The system can reach much lower temperatures while depressurising, and these temperatures need to be evaluated for selecting the materials of construction. Typically, commercial simulators/programs are used to calculate the peak flow rate and minimum (or maximum in case of fire or non-adiabatic) temperature from depressur - isation. However, the calculation methods in the simulators may remain unclear to some users as algorithm details are not directly illustrated. Against this backdrop, simple cal - culation methods or models are described, which can be solved in spreadsheets and mainly derived from applying thermodynamic principles to estimate the transient profiles of gas phase depressuring systems. This model could help

in understanding the thermodynamics of depressurisation, and the results from this model may be used as references for checking the results from a new simulator or a new ver- sion of a known simulator. While depressurised segments contain gas phase, liquid phase, or multiphase systems, the model discussed herein is applicable to depressurised seg- ments with single or multicomponent gas phase only and will need to be expanded for a multiphase system. In addition to emergency situations such as fire requiring the segment in the fire zone to be depressurised, de-in - ventory of the same segment can also be expected during normal shutdown and maintenance or in the event of inad- vertent opening of the depressuring valve. In these cases of depressurisation without external heat input, for gases such as LNG feed gas in the region of positive Joule Thompson (JT) coefficients, the adiabatic expansion from a well-insu - lated segment will result in the maximum temperature drop and generally becomes the governing scenario for specify- ing the minimum metal design temperature requirement of the system. This design requirement is essential for plants such as LNG facilities with gas streams at low normal oper- ating temperatures. Calculation model-adiabatic The total volume (V, ft³) of an isolated segment to be depressurised typically involves the volumes of equipment items and piping between the isolation valves, which are typically automatic or remotely operated on-off valves. The total volume remains unchanged during the depressuri- sation, but the fluid mass (M, lbm) inside the volume will decrease. Equations 1 , 2 and 3 describe the time-depend- ent M during depressurisation:

1200

1000

y = 202.1796x R = 0.9999

-1.3250

800

600

(Eq 1)

400

where r is the gas density (lb/ft³) and m is the mass flow rate (lb/hr). The transient mass flow rate m out of the iso- lated segment varies with the segment pressure (P) and the outlet or backpressure of the depressuring orifice discharg - ing to a flare system. For most of the high-pressure or LNG plant segments to be depressurised, the initial pressures (P o ) are much higher than the typically targeted pressure of

200

0.2

0.4

0.6

0.8

Specic volume, ft/lb

Figure 1 Pressures vs specific volumes

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PTQ Q4 2023