API 650 seismic "overturning" calculation assumes the development of two plastic hinges, one at the junction to the shell and the other at some distance inward from the shell. As a result of this model, a part of bottom and shell can be lifted off the ground and the weight of the content will resist overturning.
Details are given in a famous paper "Basis of seismic design provisions for welded steel oil storage tanks", authors Wozniak and Mitchell and In "Aboveground Storage Tanks" by Myers, chapter 6.1.5.
Based on this model, one can calculate the effective lift-off; see E.7.3.1 Method for Estimating Tank Uplift.
As you can see, API 650 does not consider a correlation of such value with Earthquake magnitude, vertical acceleration, etc, so I would consider E.7.3.1 gives the maximum tank response vertical displacement.
Returning to your question, the lift off is vertical (of course!) and theoretically this vertical deflection has nothing to do with nozzle's centerline elevation. I would add that the horizontal displacements should be increased to account for drift of the tank, and the drift depends on that elevation - see the case when the nozzle is top shell located.
As EC.7.3 PIPING FLEXIBILITY explains, the design displacements of Table E-8 "are a compromise of practical design considerations, economics and the probability that the piping connection will be at the point of maximum uplift. If one "estimated" the tank uplift using the simplified model in the appendix, the uplift will often exceed the values in Table E-8 unless the tank is in lower ground motion regions".
However in this particular case the values given by Table E-8— Design Displacements for Piping Attachments are referred as "ASD Design Displacement mm (in.)".
Also API 650 E.7.3 PIPING FLEXIBILITY says
" Unless otherwise calculated, piping systems shall provide for the minimum displacements in Table E-8 at working stress levels (with the 33% increase for seismic loads) in the piping, supports and tank connection. "
"The piping system and tank connection shall also be designed to tolerate 1.4Cd times the working stress displacements given in Table E-8 without rupture, although permanent deformations and inelastic behavior in the piping supports and tank shell is permitted."
Perhaps it may be helpful to cross-reference API 650 Appendix-E to the provisions of ASCE-7 section 15.7, and notably subsection 15.7.4 "Flexibility of Piping Attachments", also Table 15.7-1 "Minimum Design Displacements for Piping Attachments".
API-650 is a general specification for designing welded steel storage tank for oil storage
Large size storage tank
Temperature: -40F to 500F
Pressure: up to 2.5 psig
Design and Construction of large welded low pressure storage tank
Large size storage tank
Temperature: -325F to 250F
Pressure: up to 15 psig
API 12D is field fabricated tank of production liquids
Smaller in size compared to API 650/620
Temperature: Ambient
API 12F is shop fabricated tank of production liquids
Smaller in size compared to API 650/620
Temperature: Ambient
API 12F is intended to use for oilfield service mostly
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