DBB Valves in severe service applications
Having worked on many projects in the past, it seemed that many piping engineers tend to stereotype the term “severe service” on their valve requirements.
In a given process or system, only few can be considered as such, but they are the ones that pose the greatest design challenge. Each type of valve (and there are many of them) has several parameters to consider.
I can only mention these salient points that defines severe service relevant to Double Block & Bleed (DBB) Isolation Ball Valves:
Temperature
High temperature is common to power generation and steam service. It requires temperature ratings from 400 C° to even up to 800 C°. As such, it needs special attention when it comes to design and material selection that conforms the allowable thermal stress.
A careful consideration of the ASME B16.34 standard is a must. ASMEB16.34 lists the max. allowable Working Pressure relative to the temperature and material. Standard connections like flange as per B16.5 also sets ratings and limitations.
Due to the inherent limitation of non-metallic materials like PTFE or PEEK, it is very common to find a metal-to-metal seated valve on high temperature application.
Extreme Corrosion
Corrosion is inevitable in most process industry but more so in the upstream & midstream Oil & Gas applications. H2S and other corrosive substances can be quantified but the industry can only lessen corrosion in a piping system.
High nickel alloys such as Alloy 400, C276 or 625, known to be corrosion-resistant alloys, are the typical go-to materials for such corrosive applications and environments. Weld overlays and cladding on the valve are now becoming more familiar in the industry and are used for larger valves.
A special design called “Atmospheric Sealing” is a proprietary design introduced to the offshore market. Here corrosion is also mitigated from the outside environment. For example, Chloride-Induced Stress Corrosion Cracking (CSCC) is a common problem on offshore platforms in the Middle East. Besides that, high salinity water from the sea can influence severe corrosion outside the valve body and sealing elements.
Erosion
This may have to do with flow characteristics such as the fluid velocity, turbulence and pressure drop which can speed up erosion. But a Double Block & Bleed Valve (DBB) is used as an isolation or ON-OFF valve, abrasive media is the common suspect that cause this problem.
Abrasive media can cause scratches on the spherical ball and on the typical soft inserts. Once these scratches are magnified from opening/closing operations, a hazardous and costly leakage cannot be avoided. Metal-to-metal seated isolation valves are more suitable for this application. Besides, applying hard-facings, such as Stellite, Tungsten Carbide, and Chromium Carbide are the common wear-resistant methods used in the industry.
Also bear in mind, there are two basic differences when selecting the correct Double Block & Bleed (DBB) Valve design for your application. Have a look at this infographic to understand the differences.
Historical data
Plant managers and plant operators usually know the valves performance history. For existing installation, it is important that data sheets and operating conditions/parameters be reviewed and compared against actual or experienced-based data. From this exercise, a severe service application can be identified and then the appropriate valve design applied.
End users may then take into considerations the commercial aspect, uptime, reliability, safety, fugitive emission requirements and most important the Total Cost of Ownership (TCO).
(Image source: © pichitstocker / Fotolia)