A. BABAKR and J. L. GRIFFIN, Emerson, McKinney, Texas
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.