As high-performance and engineered ball valves become more prevalent due to better sealing and reduced emissions, now is the perfect time for an overview of valve products and what should be referenced when specifying and ordering.
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 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-seated ball valves up to nominal pipe size 24 and class 600 rating) guide the industry.
"By at least a 10:1 ratio, 'soft-seated' ball valves dominate the refining and chemical markets," explained Barry Hoeffner, director of quarter-turn products at Ladish Valves. "This is evolving as the costs of metal-seated valves come down, but in relatively clean services under 450 degrees Fahrenheit, this has been and will likely continue to be the most cost-effective solution."
There are many options for seating material in these valves, although the vast majority used in the refining and chemical industries are based on a form of Teflon™/ polytetrafluoroethylene (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 reinforced 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, there will be very undesirable results.
When should a severe or critical service metal-seated ball valve be considered? This is a rapidly expanding segment of the market that is widely misunderstood.
"I believe this is the correct answer: When process requirements exist calling for ball valves that operate and seal in conditions outside the range of typical products, test protocols and industry standards," Hoeffner said. "This can mean many things to many people, and you must take care to not simply throw money at a problem hoping it goes away."
In the petrochemical, chemical and refining industries, metal seats would be the preferred choice for coking services, high-pressure hydrogen and gas isolation, catalyst handling, reactor isolation (manual and automated), "high-cycle" ball valves, heater isolation, etc. These are places where reliable, tight shut-off is important, and "soft-seated" valves will not function due to high temperature, pressure, wear, solids, etc.
It is important to understand that metal-to-metal sealing in ball valves is a misnomer. If uncoated metal balls and seats are in direct contact, they would be damaged due to galling when operated and would never seal well. In reality, it is the ball and seat coatings that provide the sealing and allow the valve to cycle without galling. These coatings are generally lapped for tight shut-off and must resist chemical and thermal attack from the process in which the valve is installed.
The key to reliability of "metal-seated" valves in any process application is the selection of the right coating and the lapping/grinding process to "mate" the sealing surfaces.
"While there is no such thing as 'zero leakage' in valves, by clearly defining the process objectives and properly specifying these products, the greatest reliability can be achieved," Hoeffner said.
Meet Barry Hoeffner and the Ladish Valve team on Feb. 8 at The Bell Tower on 34th in Houston during the PVF Roundtable's quarterly networking meeting.
For more information or to register, visit www.pvfroundtable.org.
