Hydrocarbon Processing - October 2022 - 31

Special Focus Plant Safety and Environment
G. E. BOHME, Sulphur Experts Inc., Calgary, Alberta, Canada
Why sulfur plants fail: An in-depth study of
sulfur recovery unit failures-Part 1
Equipment malfunction or an unplanned
shutdown of a sulfur recovery
unit (SRU) can have a significant effect
on a production company's profitability,
along with an equally serious impact on
personnel safety and the environment.
The goal of this article is to establish the
highest-probability threats to-and to focus
industry attention on-the reliability
of sulfur recovery facilities. These threats
were identified by analyzing hundreds of
cases of SRU failures investigated by the
author's company over the last 30 yr to
understand the root causes of these failures.
Failure pathways include corrosion,
plugging, temperature excursions, explosions
and process gas/fluid releases. This
article also offers the most effective strategies
to prevent these failures.
The SRU. The role of the SRU is to convert
the hydrogen sulfide (H2
S) present
in various industrial process gases-such
as acid gas, sour water stripper (SWS)
gas and other gases with sulfur species-
to elemental sulfur. Government regulatory
agencies typically specify the sulfur
recovery efficiency (the percentage of
inlet H2
S to be recovered as elemental
sulfur) required for each facility, or they
specify the maximum allowable sulfur
species emissions (H2
S, sulfur dioxide,
or total
sulfur species) allowed from
each facility. The SRU technologies that
are available to meet these regulatory
requirements, and the operating conditions
required to achieve optimal efficiencies,
are well known, and many other
articles have dealt with the various reasons
why SRUs may fail to achieve their
efficiency/emissions requirements.
This article will not focus on the topic
of regulatory failures but will address
physical/mechanical failures of SRUs.
Physical failures can seriously damage
SRU equipment, minimize SRU throughput
or shutdown an SRU completely,
causing oil/gas/chemical production
losses and associated profitability and
repair costs. These physical failures can
also have serious environmental and
safety implications, with the worst cases
resulting in injury or death.
The author's company has provided
consulting services to sulfur recovery and
related industries (such as amine treating,
sour water stripping and dehydration) for
more than 50 yr, with documented cases
going back more than 30 yr. During this
period, the author's company has conducted
more than 3,000 projects in more
than 60 countries, and currently manages
between 200 and 300 projects per year,
including a constantly increasing number
of mechanical failure investigations.
While this historical work does not cover
the entirety of SRU failure cases, it is
likely the largest collection of SRU failure
case studies in the world and can therefore
provide a valuable window into the
frequency of, and the repercussions associated
with, various failure mechanisms.
This article will focus on the highest
probability threats to SRU facilities and
on general strategies to identity and mitigate
each of the different failure mechanisms.
Note: The case numbers cited in
the article are based on relatively serious
incidents only, usually for cases that resulted
in shutdowns, lost production, serious
equipment damage and recordable
incidents. While more minor cases in all
failure categories are still extremely common,
these were excluded from the case
counts to better focus on the results.
FAILURE TYPES
For the purposes of this article, physical
and mechanical failures in SRUs have
been divided into five main categories (in
FIG. 1. Examples of SRU corrosion
Hydrocarbon Processing | OCTOBER 2022 31
order of prevalence): corrosion, plugging,
temperature excursions, process fluid
(gas/liquid) release and explosions. A
description of each category, along with
case number estimates, examples, consequences
and recommendations for investigation
and prevention, are presented in
each of the following five sections.
Corrosion. SRU vessels and piping are
mostly constructed from carbon steel.
SRU condenser tube welds, condenser
tubes and vessel corrosion allowances
are usually on the order of 0.125 in.
(125 mil), meaning that corrosion rates
of a few mil/yr can be accommodated
and dealt with during scheduled turnarounds.
However, higher corrosion rates
can result in a loss of containment in days,
weeks or months-resulting in unscheduled
shutdowns and possible process fluid
releases. Corrosion scenarios in SRUs
(FIG. 1) typically fall into four categories.
These include the following:
* High-temperature H2
S sulfidation
° Corrosion rates become
concerning around 300°C
(572°F) (9 mil/yr)1
° Corrosion rates increase rapidly
as temperatures increase [e.g.,
37 mil/yr at 400°C (752°F)]1
° Normal process temperatures
at or above this range exist
in the reaction furnace and
wasteheat boiler (WHB), as
well as in some catalyst beds
Hydrocarbon Processing - October 2022

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