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Safety Relief Valve FAQs

Safety Relief Valve FAQs

Companies that repair, test, maintain or...

An Update on U.S. Valve-Related Standards

An Update on U.S. Valve-Related Standards

It takes much time and effort by many pe...

Diagnostics for Commissioning and Startup

Diagnostics for Commissioning and Startup

Many facilities have found value in perf...

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Safety Relief Valve FAQs

Safety Relief Valve FAQs

Monday, 14 August 2017  |  Lyndon Garrick

Companies that repair, test, maintain or supply valves routinely receive inquiries from end users about safety-relief valves. Here are a few questions...

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Industry Headlines

Forum Energy Technologies Acquires Multilift

2 DAYS AGO

Forum Energy Technologies, Inc. has acquired the stock of Multilift from Pelican Energy Partners, a Houston-based oilfield services focused private equity fund, and management. Based in Houston, Multilift manufactures the patented SandGuard and the Cyclone completion tools; products that extend the ...

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ValvTechnologies Names Teele Director of Global Quality Management

4 DAYS AGO

ValvTechnologies, Inc. has appointed Michael Teele director of global quality management. Based in Houston, Teele will be responsible for setting and maintaining global quality standards, systems and processes, as well as serving as a leadership champion of continuous improvement. With a deep technic...

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Executive Order to Speed Up Infrastructure Project Approval

2 DAYS AGO

President Trump signed an executive order intended to roll back “rules regarding environmental reviews and restrictions on government-funded building projects in flood-prone areas as part of his proposal to spend $1 trillion to fix aging U.S. infrastructure,” Reuters reports .

“Trump's...

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DuPont and Dow Chemical Set Closing Date for Merger

3 DAYS AGO

DuPont and The Dow Chemical Company announced that all required regulatory approvals and clearances have been received, that all conditions to closing of their merger of equals have been satisfied, and that their merger of equals will close after the market closes on August 31, 2017.

“The compa...

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U.S. Factory Output Declined in July

1 DAY AGO

Manufacturing output edged down 0.1% in July. The index for durables decreased 0.5%. Among durable manufacturing industries, with the indexes for primary metals and for furniture and related products each dropped more than 1%. The index for other manufacturing (publishing and logging) moved down 0.4%....

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Empire State Manufacturing Index Highest in Nearly Three Years

3 DAYS AGO

Business activity grew strongly in New York State, according to firms responding to the August 2017 Empire State Manufacturing Survey. The headline general business conditions index climbed fifteen points to 25.2, its highest level since September 2014. The new orders index rose seven points to 20.6 a...

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Materials Selection for Deepwater Gate Valves

spr11_deepsea_fig1Figure 1. A typical subsea installation showing valves, manifolds and jumpers

With the discovery of oil and gas in water depths thousands of feet below the surface, selection of valves is more important, difficult and complicated. Gate valves, which are often used in subsea applications, are available today in a wide selection of materials, but choosing them requires knowledge of new challenges and established standards.

In years past, the materials used to handle corrosive service in the sea faced mainly the challenges of hydrogen sulfide (H2S), carbon dioxide (CO2) and chlorides. With deepwater well drilling, the newer subsea systems being drilled also need to handle chemicals that will minimize paraffin, asphaltene, hydrates and scale formation as well as provide corrosion inhibitions. These chemicals, however, have adverse effects on metallic and non-metallic materials, and the problem is compounded when materials have to handle produced fluids, annular fluids and the injected chemicals. Also, with subsea systems, the effects of hydrogen embrittlement from the cathodic protection system have to be taken into account. For this reason, choosing the materials to be used in gate valves for subsea is especially challenging.

WHAT GOES INTO THE CHOICE

In selection of materials for subsea gate valves, the following must be considered:

  • Composition of produced fluids in contact with valves and internal parts—all wetted parts
  • Service temperatures
  • Operating pressure ranges
  • Galvanic effects from contact of dissimilar materials
  • Crevice corrosion at seal and flange faces
  • Temperature and chemical resistance for non-metallic materials
  • Cathodic protection (CP) on materials
  • Effectiveness of coatings on materials
  • Weldability for weld overlay
  • Material availability and cost
  • Compatibility of materials with injected fluids

 


VALVE BODY MATERIALS

Several organizations provide recommendations for the selection of materials for valves. These include the National Association for Corrosion Engineers (NACE) and American Petroleum Institute (API).

NACE only covers metallic material requirements for resistance to sulfide stress cracking (SCC) for oilfield equipment, which is not intended to include design specification. (Other forms of corrosion and other modes of failure are outside the scope of NACE’s standard and should be considered in design and operation of equipment.) NACE also has requirements for low-alloy materials exposed to sour service. For example, the organization requires that hardness for alloy materials be limited to HRC 22 maximum. Nickel content is limited to 1% maximum, and NACE also has proposed heat treatment such as normalized, normalized and temper, and quench and temper.

API has several standards, such a specification 17D “Specification for Subsea Wellhead and Christmas Tree Equipment,” which uses the material requirements of API 6A.

Specification API 6A covers a number of specific areas for subsea valves, including strength, impact and quality testing. Strength level depends on the pressure rating of the equipment. For example, for flanged end connections, equipment used to pressure levels of 10,000 psi must be manufactured from material having a minimum yield strength of 60,000 psi. Equipment exceeding 10,000 psi pressure must be designed using equipment with specified yield strength of 75,000 psi (refer to API 6A Table 5.2).

Once the fluids that will be produced have been determined, valve selection can occur. Besides the challenges the fluids will produce, as well as the temperatures and pressures involved, the service conditions must also be considered. This includes how long the equipment might be exposed to seawater. Alloy steel will handle most benign conditions, including low CO2 for short periods of time, but even short seawater exposure can cause corrosion of critical components. This is especially true if seawater is trapped in those components and cannot be flushed out in a timely manner. Even with benign conditions, there is need for long-term life—in many cases over 25 years.

Valves as specified using API and NACE standards to handle strength and corrosive requirements can be grouped as follows with typical materials and applicable service conditions:


Stainless-steel Valves
spr11_deepsea_fig2Figure 2. Typical subsea tree assembly

When environments call for stainless steels such as 410 and F6NM, they may have similar corrosion resistance in oilfield environments; however, they have significant differences in weldability. Stainless 410 in the wrought and welded condition has lower impact toughness than F6NM. Welds of 410 have lower toughness, and depending on the operation, F6NM is often used if there is a risk of Joule Thomson effect (the temperature change of a gas or liquid forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment) at the wellhead. Even though stainless steels such as 410 and F6NM have good corrosion resistance and can handle mild corrosive conditions, weld overlay of critical sealing surfaces with corrosion-resistant alloy (CRA) is used to minimize pitting.


Duplex Stainless-steel Components

Although duplex stainless steels have good corrosion resistance in most environments, the use of these materials is limited for wellhead equipment because of the danger associated with sigma formation during heat treatment in large section thicknesses. Improper heat treatment not only results in poor corrosion resistance, but also poor toughness property. Duplex stainless steels require a satisfactory balance between ferrite and austenite both in the wrought and welded structures. Super duplex is specified where the Pitting Resistance Index (PRE) exceeds 40, whereas duplex is specified for thin components.

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