Hydrocarbon Processing - July 2022 - 29

Special Focus Valves, Pumps and Turbomachinery
G. WELSFORD, ValveMan, Exton, Pennsylvania
The effects of highly corrosive fluids
on valves and disasters that can be avoided
Some of the highly corrosive fluids
in the oil and gas industry contain the
following compounds: carbon dioxide
(CO2
), hydrogen sulfide (H2
S), chlorides
and moisture. These corrosive fluids
have different chemical interaction
mechanisms with the valve's body and
demonstrate distinct behavioral tendencies
when in contact with specific metallic
materials. In crude oil, the concentration
of H2
S is between 0.05% and 15%.
Sulfide concentrations above 0.15% are
sufficient to cause sulfidation corrosion
in plain steels and low-alloy steels.
CO2
CO3) once
becomes corrosive after its reaction
with moisture, where it converts
into carbonic acid. The kinetics of this
reaction becomes favorable for the formation
of carbonic acid (H2
partial pressure of CO2
reaches 0.5 bar,
which is often the case in the industry.
H2
CO3 is a weak acid, and it becomes aggressive
in acidic regions when it attacks
steel, creating iron carbonate. Lastly, the
chloride ions and their concentration in
the pipeline should be accurately determined
since they have one of the fastest
adsorption rates to the steel surface.
Once they penetrate through the coating
layer, their reaction with metal oxides is
very rapid. As a result, chlorides often
cause a localized breakdown, a corrosion
mechanism commonly known as pitting.
Corrosion-resistant check
valve
materials. One of the optimal anticorrosive
materials for a valve's body (such as
the industrial check valve shown in FIG. 1)
is stainless steel 316, due to the 2%-3%
molybdenum content that improves
316's corrosion resistance. Unreactive,
non-metallic materials are suitable for
other wetted components, such as valve
seals, seats or linings. Piping systems with
high corrosion rates due to the availability
of chloride traces in the fluid use valves
made from alloys with higher pitting resistance
equivalent numbers (PREN).
Alloys with higher PREN values are more
resistant to corrosion than steel.
Any severe service valve must provide
sufficient sealing throughout its
service life. While graphite-based packing
materials provide excellent sealing
at higher temperatures, they are likely to
undergo galvanic corrosion. An alternative
to such corrosive flow operations is
using polytetrafluoroethylene (PTFE)
valve packings.
Monel, an alloy of nickel, has better
corrosion and chemical resistance
compared to steel-based valve materials.
Other alloys that perform well in corrosive
and aggressive fluid service are Alloy
C-276, which is commonly referenced
as Hastelloy C-276 and is composed of
nickel-molybdenum-chromium alloys
and tungsten. Inconel is an alloy of nickel
and chromium and resists oxidation at
high temperatures and pressure.
Corrosion effects upon check valves.
The safety of severe service pipelines
relies on the integrity of check valves.
These valves guarantee directional and
volumetric control of hazardous fluids
while preventing leakage. When corrosion
occurs, the valves weaken, increasing
the probability of fluid media leakages
and fugitive emissions of toxic gases.
Valve components consist of dissimilar
materials (metals and non-metals) that
react differently to the corrosive service
media. Valve corrosion occurs in any or a
combination of the following ways.
General corrosion. This occurs due
to chemical or electrochemical reactions
within the valve. Concentrations of sulfur,
chlorides or carbonates mix with moisture
and oxygen in the piping system to
form weak acids that attack metal surfaces
FIG. 1. The use of optimal anticorrosive materials, such as stainless steel 316, is suggested
for equipment like industrial check valves.
Hydrocarbon Processing | JULY 2022 29

Hydrocarbon Processing - July 2022

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