A NACE Compliant Valve

In the world of industrial valves, selecting the right choice for your specific application is crucial to ensure optimum performance. One key factor in this decision-making process is the compliance with industry standards like NACE (National Association of Corrosion Engineers). This article reviews requirements for a NACE compliant valve, then summarizes some key elements of the MR0175 and MR0103.

What is a NACE Compliant Valve?

A NACE compliant valve is one with a design that meets requirements as per NACE MR0175/ISO 15156 and NACE MR0103/ISO 17945. Meeting these requirements makes them capable of withstanding corrosive environments, especially sour service. The term “sour service” refers to corrosive environments where there is exposure to a relatively high percentage of hydrogen sulfide (H₂S). In such environments, hydrogen atoms can preferentially seep into hard materials like high-strength metals and heat affected zones (HAZs) that have not been stress-relieved. As hydrogen builds up in these materials, there is a corresponding pressure build up, resulting in cracks and eventual failure. This phenomena is referred to as hydrogen induced cracking (HIC), and in the presence of stress, the failure could be by sulfide stress cracking (SSC).

Using NACE compliant valves in such situations helps to extend equipment life, reduce maintenance costs, and enhance safety. This is particularly useful where valves are in hard-to-reach areas like offshore oil and gas facilities in which intervention activities come at a significant cost.

NACE MR0175

NACE MR0175 was initially released in 1975, covering requirements relating to materials for resistance to sulfide stress cracking. There have been several revisions over the years until its merger into ISO 15156 in 2003 along with significant restructuring. The restructuring includes coverage for chloride stress corrosion cracking, alongside new restrictions and requirements for using Corrosion Resistant Alloy (CRA). Since the 2003 document, the task of determining acceptability of a material was split into two separate functions:

• • Function 1: The manufacturer must ensure that the valve meets the metallurgical and manufacturing requirements so that it is permissible for sour service use.

• • Function 2: The end-user must ensure valve is compatible on the basis of specific service environment parameters such as pH, chlorides, temperature, H₂S partial pressure, etc.

The implication of this change is that a compliant material prior to the 2003 merger may no longer qualify as compliant. For example, using the 2002 edition, 316 SS can serve in sour service as long as it is annealed, free of cold work, and with hardness less than HRC 22. But from the 2003 edition, this material satisfies only Function 1. It requires further assessment on the basis of Function 2 and qualifies if the service has a chloride concentration below 50 mg/L. Also, the operating temperature should be less than 60 and the H₂S partial pressure below 100 kPa. Thus, it is necessary to mention the revision of MR0175 referenced in any material certification.

Summary of MR0175 Requirements

The requirements in this standard cover a vast number of materials with details including:

• • General principles for the selection of crack-resistant materials.

• • Cracking resistant carbon and low-alloy steels, as well as the use of cast irons.

• • Cracking resistant CRAs (corrosion-resistant alloys) and other alloys.

Requirements in this standard are robust, but the following sections review just a few.

Material Hardness Requirements

Steel with high levels of hardness are susceptible to HIC because of their microstructure. So, the NACE MR0175 limits the allowable hardness for each type of steel as the table below shows.

Type of Steel	Grades	Permissible Hardness (HRC)
Ferritic	405,430, 409, 434, 436, 442, 444, 445, 446, 447, 448	≤ 22
Martensitic	F6NM	≤ 23
Austenitic	S20910	≤ 35
Precipitation Hardening	S66286	≤ 35

Material Chemical Properties

The chemical properties of a material influences its ability to operate in sour service. For example, steel with higher carbon content results in increase in hardness and susceptibility to failure in sour service. So, the MR0175 limits certain chemical elements for a material to be certified.

Chemicals	Content
Carbon	C ≤ 0.10%
Sulfur	S ≤ 0.002%
Phosphorus	P ≤ 0.020%

Heat Treatment

The heat treatment condition of a material influences its performance as it relieves stress and minimizes susceptibility to SSC. However, only certain heat treating processes are recommended for each material group. For example, allowable heat treatment conditions for carbon steels include:

• • Hot-rolling.

• • Annealing.

• • Normalizing.

• • Normalizing and tempering.

• • Normalizing, austenitizing, quenching, and tempering.

• • Austenitizing, quenching, and tempering.

In addition, cold-worked carbon steel materials that produce outer fiber deformation greater than 5% must undergo stress relief to attain hardness below 22 HRC.