In refineries and
petrochemical plants, valves and related components often come into contact
with process media. This contact can result in many different types of issues—typically related to chemical
attack—along with
premature corrosion and erosion. While all valve vendors can provide advice
regarding correct construction materials, in some cases more is required in the
form of lab testing to ensure compatibility between wetted parts and process
media.
This article will discuss
how plant personnel can work with vendors to address this issue prior to
purchase, and a supporting example is included.
Complex interactions. Unwanted interactions
between wetted parts and process media typically occur due to the improper
selection of construction materials. This is due to the unknown presence of substances
in the process media, as well as a lack of knowledge regarding the interaction of
wetted parts and process media. These interactions can be quite complex,
requiring intimate knowledge of not only the process media and contaminants,
but also possible interactions with different materials of construction.
Process plant personnel may sometimes select wetted
parts without an intimate knowledge of their process media, especially
contaminants. While the main media may be benign with respect to interactions
with wetted parts, contaminants are often present and can cause issues. While a
safety data sheet (SDS) is typically available and will describe the process
media fully, it often will not fully detail contaminants.
If the wrong materials of construction are selected
inadvertently, the part/component may fail prematurely, causing unplanned
downtime. Because the parts are wetted and come into contact with the process
media, replacement often requires shutting down all or part of the process.
Improperly selected parts can also emit hazardous
fumes due to chemical attack from the process media, presenting a potential
health risk to personnel, as will be shown in the example.
Addressing the issue. Valve vendors typically have
experts on hand to evaluate the interaction of the parts they supply with
process media. The usual process involves examination of each SDS, followed by
recommendations regarding the proper materials of construction for all wetted
parts. This procedure works well in most cases, but not all.
For processes where the
exact state of the media is not known at all times, further precautions should
be taken. The first precaution is a thorough examination of the process media
and all contaminants, at all stages of the process (i.e., not just steady state).
These other states can include startup, shutdown, feedstock variations and grade
changes, among others. Feedstock variations and grade changes are becoming more
prevalent for a variety of reasons, making examination of these areas more
important. After the valve vendor is informed regarding the process media and
contaminants, it can make recommendations regarding the proper materials of
construction.
In some cases, lab testing
will be required—some
valve vendors offer this as a service. This testing exposes the wetted parts to
the process media and contaminants over a period of time to detect unwanted
interactions.
While no evaluation
methodology is perfect, and premature failures of wetted parts may still occur
for various reasons, consultation with the valve vendor beforehand, along with
testing if required, will address most issues.
An issue that can commonly
go undetected without prior examination beforehand is failure due to liquid metal
embrittlement (LME), which is often not considered during the material
selection stage as it is rarely encountered in the process industries.
LME explained. LME mainly occurs when a particular liquid metal
(e.g., mercury, gallium, indium, rubidium, cesium, francium, lithium, zinc) comes
in contact with another susceptible metal. It can take place at either low or
high process temperatures, and typically occurs after a period of exposure.
LME effects have been known for more than 100 yr,
although they have often been overlooked when selecting wetted parts for
process plant applications. The effect predominantly manifests as ductility
reduction of the metal, which can lead to cracking when suitable stress is
applied. Mercury, gallium and indium, in particular, have been shown to cause embrittlement
of high-strength steels.1
Not all metal alloys are embrittled when they come
into contact with liquid metal, so LME should be considered during the selection
process to ensure correct materials of construction. Mercury is the most
discussed due to its widespread manifestation, both within processes and from
the atmosphere.
In some geographical regions, mercury can be
present in natural gas at levels between 200 micrograms per cubic meter (mg/m3)
and 300 mg/m3,1 and mercury concentrations can reach up
to 600 ppm in crude oil.2 In addition to its presence in process
media, mercury can deposit on metal surfaces as elemental mercury when it is
released into atmosphere.
Mercury melting/freezing and boiling points are –37.9°F (–38.8°C) and 674.1°F (356.7°C), respectively.
Reactivity is limited to certain compounds and metals as mercury is inert at
times, with limits. It does not give up its two outer valence electrons, therefore
it tends to form weak covalent bonds with other elements, when feasible. If
this were to occur, it typically takes place in liquid form at or near-normal
room temperature.
When mercury is combined with metals, it forms
amalgams (e.g., as previously used for dental fillings in combination with copper,
silver and zinc). While mercury also forms amalgams easily with aluminum, gold
and silver, it does not form amalgams with transition metals, except for copper
and zinc. It can react with almost all halogens—including fluorine, chlorine, bromine and iodine—forming halogen salts,
which can affect process and personnel health.
While LME is not common, it
can pose significant risks to the process and personnel. For example, a wetted
part may fail due to LME, a situation typically requiring analysis by plant
personnel to determine the cause. The failure can result in process shutdown
and possibly harmful emissions, but other issues can arise during the analysis if
lab personnel are not aware of the possible presence of mercury or other
harmful elements.
Filter assembly case issues. A filter assembly case installed
in a petroleum refinery had been in service for less than 10 mos before
cracking occurred and was observed. The filter assembly specification showed materials
of construction as copper alloy free-cutting brass ASTM B16-C36000.
FIG. 1 shows the filter assembly after removal from the
process. Visual observation revealed four longitudinal unbranching cracks on
the larger section, with one crack opening wider than the others. The external
surface had a few greenish areas due to atmospheric corrosion and the
subsequent formation of copper oxide. Cracks originated from the inside of the
filter outlet side of the filter assembly threaded portion.
Positive material
identification was carried out using the OXFORD XRF X-MET7500 method on the
filter assembly, and results indicated the filter material conformed to ASTM
B16-C36000 leaded brass, as shown in TABLE 1.
Low magnification
observation of the filter assembly was performed using a digital microscope. The
filter assembly was manually opened, revealing a main fracture surface. The
opened filter assembly outlet side had a main crack at the second thread root
from the head, and all cracks had a brittle profile. Further observation
revealed silvery particles scattered around the main crack and on the remaining
threads (FIG. 2).
At this stage of the investigation, elemental analysis of the silvery particles
was conducted.
Scanning electron
microscope (SEM) observations and semi-quantitative energy dispersive X-ray (EDX)
microanalysis confirmed that the silvery particles were in fact mercury. Mercury
was found within the crack and on all areas on the surface under evaluation.
Mercury was found on all
open surfaces of the main crack fracture surface and on the threads. Exposure
of mercury was predominately on the side exposed to the process media, with migration
to the outer surface due to the cracks.
Since mercury was detected, further testing was stopped
because the lab was not equipped to handle hazardous substances. From the
investigation, it is evident that mercury LME-induced cracking was the mode of
failure. Progressive crack growth occurred during service, initiating from the
second engaged thread root.
The exact mercury concentration in the process media
was unavailable, but the filter internal surface exposure was probably progressive
after installation because brass contains copper and zinc, and both amalgamate
with mercury.
Hazardous substances, such as mercury, should be
handled in closed areas with no air flow to avoid exposure to vapors.
Metallographic sample mounting using a high-temperature press machine will
cause mercury to spread into air after removal of the sample from press during
cooling. This can endanger personnel and contaminate the air as it is carried
away.
Takeaway. Selecting the proper materials of construction for
wetted parts can be complex—in
some cases, possible interactions with process media can be overlooked. The consequences
of improper selection can include unplanned shutdowns and harmful emissions.
Valve vendors can provide
assistance (including testing) to improve material section. While no evaluation
is perfect, trained experts can help plant personnel reduce the risk of
premature failure or wetted parts, improving safety, uptime and regulatory
compliance. HP
LITERATURE CITED
ALI BABAKR is a Senior Principal Materials Engineer for research and development at Emerson. Dr. Babakr has previously held various positions with Emerson and other companies as a subject matter expert dealing with metallurgical and failure evaluation, material selection and corrosion studies. He has field experience in the petroleum and petrochemical industries, and he participates in various ASTM and AMPP subcommittees. Dr. Babakr holds an MS degree and PhD in metallurgy from the University of Idaho, and a BS degree in chemistry from Huston-Tillotson University in Austin, Texas.
JIM GRIFFIN is a Director of research and development at Emerson. Dr. Griffin previously worked in reactor safety analysis with Commonwealth Edison before joining Emerson in 1995. He has more than 30 yr of experience in materials, acoustics, cavitation, flame detonation, additive manufacturing and pressure regulator design. Dr. Griffin earned his PhD in nuclear engineering from the University of Missouri – Columbia, and an MS degree in the same specialty from the Massachusetts Institute of Technology. He also holds a BS degree in nuclear engineering from the University of Buffalo, and an MBA degree from the University of Iowa.