Research Project Assessed in AFT Impulse
Lamar University – Beaumont, TX
With growing climate concerns, the world looks to natural gas to as a clean way to meet energy demands. Liquefied natural gas (LNG) is liquefied by cooling the gas to -160°C or below (<-256°F), loaded into special tankers for transport, then turned back into gas at its next terminal. The LNG loading process occurs through loading arms, constructed of expensive lowtemperature alloys for cryogenic operation and designed to consider the moving vessel the arm is attached to. An LNG terminal might have two to five loading arms for different purposes. In an emergency shutdown, flow through these arms must be suddenly halted by closing a sequence of valves, opening vent valves, and tripping transfer pumps. The sudden deceleration of flow results in pressure surge, potentially exceeding pipe design pressures and causing extreme pipe stress, further worsening an emergency.
A research team at Lamar University led by Xinyu Liu evaluated the surge results from an LNG loading emergency shutdown. The system, as constructed in AFT Impulse, is found in Figure 1. In addition to a scenario with the emergency system working as intended, the team considered five cases where the system malfunctioned: 1. Emergency system working properly 2. Pumps fail to trip 3. Vent valves fail to open 4. Pumps fail to trip and vent valves fail to open 5. 1 of 4 pumps fail to trip 6. 1 of 3 vent valves fail to open The team also evaluated valve characteristics for the closing and opening valves. These characteristics can drastically impact a surge response, so the team performed sensitivity analysis for comparison.
The maximum surge pressure in each case was compared to the pipe’s design pressure of 220 psia (15 bar). Only in the worstcase scenario where both the pumps fail to trip and the vent valves fail to open was this design pressure exceeded (Table 1). While surge pressure did not exceed the design pressure in most cases, the resulting transient piping forces created major concerns. The imbalanced force would cause stress, deflection, and vibration as the pressure wave transmitted through the system. The worstcase with pumps running and vent valves closed saw forces exceed 12,000 lbf (53 kN) near the transfer valves. The team exported their force results from AFT Impulse into ANSYS for a time history analysis and to assess displacement, vibration, and pipe stress (Figure 2).
ELEMENTS OF SUCCESS
The team emphasized the value of performing system sensitivity analysis with a simulation model, especially with the dangerous consequences of surge during an emergency. By considering forces in addition to pressures, the team could reveal further pipe stress concerns.
Unaddressed pressure surge and pipe stresses caused by an emergency shutdown could cause a secondary emergency condition
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