At this time, no national standards exist that clearly define severe service valves (SSVs) or set them apart from general purpose valves.
Most experts agree that SSVs are identified by applications, and that these applications challenge the valve’s ability to provide a minimum acceptable level of performance over a minimum acceptable duration. All valve design functions require basic information, but for those valves destined for severe service, it is imperative to have a comprehensive understanding of the factors that affect their in-service performance. A recognized definition would benefit users through improved process performance and increased profitability, safety and environmental protection.
A Demonstrated Need
The following case study highlights the need for such a definition.
Major Copper, Uranium and Gold Mine. This mine has a unique slurry and tailings system, which operates at a pH of 0.9 to 1.2. The slurry is abrasive, corrosive, scale forming and slightly radioactive. Valve failures reduce the plant’s mechanical reliability and also contribute to safety and environmental challenges, resulting in additional costs for maintenance and repairs and negatively impact the total cost of ownership of the valve assets.
Problem. Push-through style knife gate valves (Figure 1) had recurring issues, including:
- Frequent seat failures as often as every two months or 140-160 cycles
- OEM-recommended lubrication system maintenance
- Additional associated maintenance costs totaling approximately $25,000 every 2-3 months
- Valve body blow outs
- Slurry leakage
- Recurring blockage of the bottom drain port
Solution. The challenge was to find a valve that would last longer and require little to no maintenance so the team could concentrate its efforts on more critical equipment and at the same time reduce the risk of catastrophic failure and the need to stop production.
We visited the site in 2012 and after reviewing the application, recommended the following specification:
- SAF2507 body and gate, Xylan coated
- Chrome carbide overlay on the gate tip and bore of the valve
- Valve bore matched to the pipeline
- SAF2507 upstream wear ring with tungsten carbide overlay
- Aflas primary seal, metal to metal secondary seal
A trial valve was installed in May 2013 (Figure 2). Bi-monthly inspections were completed with the final inspection in December 2013, after 6.5 months of operation and 303 cycles.
During the trial period, there was zero maintenance and zero failures and the valve proved a reliable bi-directional seal when needed. The final inspection showed no visible wear on the gate tip, seat or wear ring.
Successful Results. As of January 2015, the same trial valve has been in service for nearly 20 months with zero maintenance and zero failures. The valve’s ability to achieve the full class 300 pressure rating with thinner face-to-face dimensions was impressive. These dimensions made removal and inspections easier, quicker and more cost effective and the valve has given the site much more reliable tailings disposal isolation, dramatically reducing maintenance costs.
Moving Toward a Standard Practice
This case study is a typical example of how important it is to have the right valve for the job. Many of today’s industrial processes put tremendous stresses on valve applications. By having a clearly defined severe service valve category, users would have confidence that the products they employ can withstand the rigors required to get the job done. This in turn leads to a more reliable and more cost-effective operation.
Developing the specific technical standards to define a severe service valve remains the challenge. That is where the Manufacturers Standardization Society (MSS) plays an important role. The MSS is the approving authority and has several options available.
The MSS can utilize a special task force that would allow interested members a way of concentrating on the development of specific standards. Such a task force could include MSS members and external participants from industry and government. It would develop and approve a draft Severe Service Valve Standard Practice that would then be presented to the MSS Coordinating Committee, which would review the Standard Practice and any comments it receives before giving final approval. After this committee approves the draft it is considered approved by the MSS and its sent to publication and made available to industry.
Another option is for the MSS to create a new committee consisting of representatives involved with severe service valves. This usually starts with a proposal to the MSS Board via the MSS Executive Director. The Board could then take action to create a new committee and assign its first Chair.
A third option is to establish a task force within an existing MSS committee, which would also begin with a proposal by the committee member to begin work on a new Standard Practice.
The MSS notes that that a proposal for a new standard or a new committee could come from a source external to MSS and its members. The open proposal would be sent to the MSS Executive Director and the Board would consider the request. This is not a regular occurrence but does demonstrate that the MSS is open to proposals from the industry, even outside of its membership.
Once a new committee or task force has been established it would utilize various technical information to determine the Standard Practice, including class/size designation, construction, materials, testing and marking.
The time it takes to get a new Standard Practice approved varies depending on subject matter and whether issues arise that need to be resolved. If everything falls in to place the new standard can be completed and published within four months but typically the process takes between 6-12 months.
Ross Water is president of CGIS and serves on the Manufacturers Standardization Society (MSS) as a task force member on C-114 Steel Valves and C-409 Knife Gates. He is currently increasing awareness of severe service valves at conferences and to industry organizations.