Ball Valve Seat Selection for the Refining and Chemical Industry
As a manufacturer of high performance and engineered ball valves, we are often asked about what industry standards affect these products and what should be referenced when specifying and ordering. The answer to that question is not nearly as straightforward as with the gate, globe, and check valves that have historically been the primary go-to valves in refining.
With the more common ‘rising stem’ valve types, standards such as API 600, API 602, API 603, etc. have taken a lot of the guesswork out because much of the design and sealing methods are well defined. However, as ball valves become more prevalent due to better sealing and reduced emissions, reliance on standards alone is not sufficient. Assurance that the installed product meets process requirements relies on much more.
By Barry Hoeffner – Ladish Valves
There are several standards that influence the design and performance of ball valves. At a basic level, ASME B16.34, ASME 16.10 and others govern the dimensions and wall thickness (among other things) of most all valves used in the refining and chemical industries. Standards like API 641 (emissions for quarter-turn valves), API 607 (fire testing for both ‘soft’ and metal seated quarter-turn valves), and API 608 (requirements for metal ball valves generally up to NPS 24 and class 600 rating) guide the industry.
None of these standards address the suitability of sealing materials for individual processes. Since these valves are typically used in applications where sealing is critical to the process, there are several open questions that must be addressed when specifying ball valves.
• Valve Body Material: this will be determined by the pipe specification and process requirements, and I almost always clearly stated.
• Valve ‘Trim’ Material: internal components must be compatible with the process to resist attack, and it must have the mechanical integrity to operate safely and reliably.
• Seat and Seal Material: this should be compatible with the process conditions and confirmed with the end user.
• Appropriate Seat Rating: pressure / temperature acceptability.
While body and trim material is critically important, in this article the focus will be on the seat and seal material.
Representative pressure & temperature rating of common materials.
‘Soft Seated’ Valves
By at least a 10:1 ratio, ‘soft seated’ ball valves dominate the refining and chemical markets. Although this is evolving as the costs of metal seated valves come down, soft seated’ ball valves will likely continue to be the most cost-effective solution in relatively clean services under 450ºF.
There are a wide array of options for seating material in these valves. The vast majority used in the refining and chemical industry are based on a form of Teflon ™ / PTFE. Apart from the base material, there are molecularly enhanced versions and compounds filled with glass, carbon fiber and/or graphite. One problem that exists in many specifications for these types of valves is the vague term ‘RPTFE’ which (not so simply) indicates reenforced PTFE. Without information on the type of reinforcement, this can be misleading. For example, if a request is made for RPTFE in a plant with HF Acid (which dissolves glass) and a valve with glass-filled PTFE is installed, the results will be very undesirable.
In addition, manufacturers offer seat and seal materials like PEEK, Nylon, PCTFE, etc. for specific process requirements. All have certain properties that, when combined with specific seat designs, create allowable maximum (and minimum) temperature ranges and differential pressure.
It is well past the scope of this article to provide an application guide to all these seat types, but if involved with selection or specification of these seat and seal materials, here are some guidelines to consider:
• Always verify that the seat and seal materials are rated for the maximum or minimum design temperature, and the maximum differential pressure in the system where the valve is to be installed. All reputable manufacturers publish this information.
• If specifying reenforced PTFE seat material, note any material limitations or requirements around the ‘filler’.
• If automating, confirm the torque requirements of the specific seat material provided. This can vary greatly.
• Whenever possible, include the design temperatures and pressures in the specifications. This is especially important in cryogenic or other extreme conditions.