Hydrocarbon Processing - August 2022 - 83

Maintenance
and Reliability
A. KITTUR, Saudi Aramco, Ras Tanura, Saudi Arabia
Pressure swing adsorption vessels fatigue analysis
The hydrogen (H2
) plant in a refinery
produces high-purity (99.9+ vol%) H2
products using a licensed pressure swing
adsorption (PSA) purification process
to fulfill the H2
hydroprocessing facilities, as shown in
FIG. 1. The H2
cess consists of two trains. The PSA process
purifies H2
makeup needs of refinery
recovery/purification proby
filtering heavier gases/
impurities from the process stream as it
passes through an adsorber vessel. Each
train has 12 identical adsorbers in four
groups of three vessels. Each vessel goes
through four stages in a cycle: adsorption,
depressurization, desorption (regeneration)
and repressurization. Adsorption at
high pressure pulls all the heavier gases/
impurities from the feed gas, producing
99.9% pure H2
. The vessels can handle
40,000 cycles/yr of pressure/de-pressure
between 50 psig and 335 psig. This fluctuating
pressure range allows for a fatigue
design life of 20 yr.
Problem statement. The original equipment
manufacturer (OEM) design completed
during the manufacturing of the
vessels was based on expectations of the
pressure cycling range being between 5 psi
and 340 psi. The operating cycle time was
anticipated to be 12.08 min, which would
result in 870,000 cycles in 20 yr. The fatigue
design life was acceptable with negligible
cumulative usage factors on almost
all sections of the vessel except at the joint
located between the skirt, and the bottom
head was at 87.6%. As a result, the adsorber
vessels were designated at 20 yr of
design fatigue life with a safety margin of
12.4% at the previously mentioned location,
all based on the nominal thicknesses
of the vessel components. Since the PSA
vessels were nearing their end of fatiguedesigned
life based on the OEM design,
a complete replacement of 24 vessels was
required as they were approaching 20 yr in
service. However, since the H2
plant has
been in operation, the PSA vessels have
experienced changed operating conditions,
wherein the actual pressure cycling
range was reduced to between 16 psig to
285 psig, as shown in FIG. 2.
For an accurate and representative
remaining life assessment of the PSA
vessel, the thickness of the vessel components
was available, and the presence
of objectionable flaws had been detected
and geometrically characterized by nondestructive
examination (NDE)-based
inspections. The inspections confirmed
that all 24 inspected vessels showed good
health, apart from two vessels with linear
indications on the shell parent plate and
the circular and longitudinal weld seams.
These factors established the need for a
fitness-for-service (FFS) assessment. The
estimated OEM fatigue design life calculations
were deemed completed during the
manufacturing of the vessels as an unacceptable
technical basis for the forthcoming
replacement strategy of PSA vessels.
A stress-fatigue evaluation in accordance
with the American Society of Mechanical
Engineers (ASME) Sec-VIII3
Div-2/American Petroleum Institute
(API)-5791
Annex B1.52 was required for
the components (head, nozzle, flanges,
shell and skirt) of the PSA vessels in presFIG.
1. Pressure swing adsorption in a H2
plant.
sure cyclical service to evaluate protection
against failure from cyclical loading.
In the assessment, embedded flaws were
identified in the vessel wall during the inspection
findings and classified as cracklike
flaws. In addition to the stress-fatigue
analysis, the API-5791
FFS assessment
was required to establish the behavior of
the embedded flaws detected during the
inspections of the vessel components.
Based on the estimated fracture toughness
properties of the vessel shell, this
analysis would predict the future crack
growth and further validate the life extension
of the vessel.
Finite element modeling. The finite
element method (FEM) with continuum
elements was utilized in this analysis,
which provided the total stress distribution
in the vessel components. The PSA
vessel was modeled as a 30° 3D-solid
FIG. 2. Operating pressure trends of PSA.
Hydrocarbon Processing | AUGUST 2022 83
Hydrocarbon Processing - August 2022

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