API 620 / 650 PDI IN THE LNG TANKS

[gasnaturel]

Hi,
Please I want to know the utility of PDI in The LNG Tanks, a limite value in normal operation, and how to control it when the BOG compressors TRIP?
seeing the API 620 / 650 they talk only about construction issues, ei: when we lost one compressor the PDI goes from -2mbarg to + 9mbarg please who can tell me what should we do or am I in safe area?
regards

 

[IFRs]

Is your tank built to API 625 ?

 

[gasnaturel]

This is all DATA Standard for Our Tank :

LNG Storage Tank Standard for inner tank API 620 Appx Q. Note: 2
Type of Full Containment tank 9% nickel inner,
PC outer wall,
concrete roof
Note: 3
Net capacity inner tank (between normal min. and
normal max. operating levels at operating
temperature)
150,000 m3
Internal diameter inner tank (ambient temperature) By Tank
Contractor
m Note:4
Height inner tank (ambient temperature) By Tank
Contractor
m Note:4
Minimum width of annular space
(When using cellular glass insulation)
1.0 m
Corrosion allowance: (steel enclosed by outer tank) 0 mm