SOUR SERVICE

What is Sour Service?

Sour service is well defined as when considerable amount of H2S, disulfide, or other sulfur present in fluid concentration of 2 mg/L or more in the water phase as equivalent to meeting the NACE criteria. It is also known as Hydrocarbon service.

In general, the environment containing water and H2S meeting the definition of sour environments in NACE MR0175 are considered Sour.

Caution: H2S is extremely toxic and must be handled with extreme care

This terms are used in Petroleum Industry to cover a range of damage mechanisms that can occur due to H2S in process environments. As a result the damage that causes are defined as below,

Hydrogen Induced Cracking (HIC):

The mechanism, related to hydrogen blistering (wet H2S cracking), that produces subsurface cracks parallel to the surface and step wise cracks of a metal due to high concentration of hydrogen.

HIC is evaluated in terms of crack length ratio (CLR), crack thickness ratio (CTR) and Crack Sensitivity Ratio (CSR) as per the NACE Standard.

Sulfide Stress Cracking (SSC):

Brittle failure by cracking under the combined action of susceptible micro structure, tensile stress and corrosion in the presence of water and hydrogen sulfide. Hardness is one of the primary factor in SCC susceptibility

Some environmental conditions known to cause SSC are those containing free water (in liquid phase) and:

(a) >50 ppmw dissolved H2S in the free water, or

(b) Free water pH <4 and some dissolved H2S present, or

and some dissolved H2S present, or

(d) >0.0003 MPa absolute (0.05 psia) partial pressure H2S in the gas in processes

with a gas phase.

Stress Corrosion Cracking (SCC):

In a corrosive environment, the slow growth of cracking / brittle failure of a metal produced by the combined action of mechanical stress and deterioration in the presence of chloride, sulfide and H2S especially at elevated temperature.

Stress-Oriented Hydrogen Induced Cracking (SOHIC):

It occurs in severe wet, sour service and can occur in carbon steel pipe and plate that is resistant to HIC and SSC.

Hydrogen Blistering (HB):

It is the growth of subsurface cavities in metal, bulged like blisters, as a result of significant increase in internal pressure due to hydrogen molecular gas formation close to the surface. The equilibrium pressure of molecular hydrogen in contact with atomic hydrogen is sufficient enough to rupture the material.

What is the role of NACE in Petroleum Industry?

National Association of Corrosion Engineers (NACE)

NACE provides guidance on which corrosion resistant alloys and materials should be used to prevent the corrosion. In Oil and Gas, generally NACE MR0175 and MR0103 is preferred to select the materials.

In order to select the correct NACE standard requirement it is necessary to differentiate the SSC occurrence in sour environments conditions experienced in upstream (oil and gas production) and downstream (refining and gas processing) of the petroleum industry.

Basically we can categorize by pH-value of the environments to differentiate sour service from upstream and downstream of the petroleum industry which is covered by NACE MR0175.

High pH-value will occur mostly in oil and gas production sour environments which is caused by high concentrations of the bisulfide ion.

Low pH-value is reveal in wet sour H2S environment in refinery which is caused by low concentrations of the chloride ion.

It is extremely important to note that Based on the environmental conditions it is users responsibility to defining the environment conditions for the materials selection as per the required NACE standards.

NACE MR0175 / ISO 15156:

It was developed for the prevention of Sulfide Stress Cracking (SSC) due to H2S in oil and gas production systems. It was generated for the most aggressive H2S containing environment which also known as sour gas environment. It address the requirements and recommendations for selection and qualification of materials for H2S service in oil and natural gas production. It is globally recognize as ISO 15156.

It protect metallic components from Sulfide stress cracking, Stress corrosion cracking, and other mechanisms of cracking caused by H2S. Avoiding the equipment failures and Minimize the health and safety risk to the public, personnel and the environment.

NACE MR0103:

The purpose of NACE MR0103 is to resists the sulfide stress cracking (SSC) in sour refinery process environments that contain wet hydrogen sulfide (H2S). The term “wet H2S cracking” as used in the refining industry covers a range of damage mechanisms like sulfide stress cracking (SSC). It is used generally in refinery construction.

NACE MR0103 was developed specifically for the downstream (refining and gas processing) industry which is outside the scope of NACE MR0175.

Prevention / Control of Corrosion:

In order to prevent from corrosion,

· Minimizing Hydrogen content.

· Low internal Stress.

· Increase in Material Strength, and Hardness.

· Increase in Mechanical strength as per the service requirements.

· Using the materials as per the recommendation of the standards.

· Controlling the environment.

· Isolating the components from the sour environment.

· Using appropriate anodic or cathodic polarization.

· Hydrogen Bake-out.

· Cathodic Protection.

· Good solidify micro structure of the Material.

· Post-weld heat treatment (PWHT)

It serves two purposes.

a) As a tempering process, it reduces the hardness of the weld deposit and the heat affected zone (HAZ).

b) As a stress relieving process, it reduces residual stresses in the weldment through stress relaxation. Both of these effects tend to reduce the probability of failure due to SSC.

Example:

For carbon steel piping, the following services require PWHT,

i. All wet sour services (NACE MR 0103, MR 0175 / ISO15156) when weld hardness exceeds 200 BHN.

ii. All wet sour services (NACE MR 0103, MR 0175 / ISO15156) and causing HIC when weld hardness exceeds 200 BHN.

· Seamless pipe, forging and casting are considered to be resistant to HIC.

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