- Published on Tuesday, 24 June 2014 14:58
- Written by Javier Vergara
While the American Petroleum Institute (API) and other organizations published a variety of valve standards covering gate, ball, check, butterfly and plug valves for decades, the first for globe valves came out in 2013. It is API 623, Steel Globe Valves—Flanged and Butt-welding Ends, Bolted Bonnets. The size range covered by the valves in this standard is 2-24 inches, and the standard covers ANSI classes 150 through 2500.
Gate valves are governed by API 600 while check valve design is covered in API 594. Before the new standard API 623, it was common practice in the refining and petrochemical industries to see API 600 wall thicknesses referenced for a globe valve on a specification sheet even though API 600 only covers gate valves. Wall thickness was really the only portion of API 600 requirements that could be applied to globe valves. (The other requirements of API 600, which govern stem diameters, packing sizes and gasket styles and design, did not apply.)
However, one of the biggest concerns facing the API 623 task force was with the issue of stem diameters. Since the force required to close a globe valve is much greater than a gate valve, the globe valve stem diameter requirements are often much larger. While each manufacturer had its own design requirements, an industry standard minimum for stem diameters of globe valves did not exist. API 623 has set that standard.
Additionally, globe valves are often provided with stems made of austenitic stainless steels (i.e., 304ss, 316ss, 347ss). These materials are not as strong as the standard 410 stainless-steel stem material, so when the austenitic stems are specified, they must be larger in diameter or they can bend or buckle. The new globe valve standard addresses this concern by including minimum stem diameters that will work with austenitic stainless steels.
MINIMUM WALL THICKNESS
Before API 623 was created, the minimum wall thickness for globe valves was based on recommendations in the American Society of Mechanical Engineers (ASME) B16.34 standard. The minimum wall thickness specified in the new globe valve standard mirrors API 600 specifications and is greater than ASME B16.34. This additional wall thickness produces a heavier globe valve pattern and provides additional corrosion and erosion resistance. The added wall thickness also helps to avoid deflection in the bonnets of larger-diameter globe valves, a phenomenon that often causes globe valve leakage because of the disc rising off the seat as the bonnet expands upward.
The new heavy-pattern globe valve will be useful in a variety of industries, including refineries, power generation and the chemical industry. In this era of less-than-optimal casting quality, the extra wall thickness will be appreciated by many end users.
The standard is applicable to a variety of body materials as selected from ASME B16.34 Groups 1 and 2. These material groups include plain carbon steels, stainless steels and chrome-molly alloys. The standard does not apply to ductile or cast iron valves.
SEATS AND SEATING REQUIREMENTS
As a valve design standard, API 623 affects all aspects of the globe valve. Since these valves are used primarily for throttling and regulating flows, the flow rate of various manufacturers should be close to API 623 globe valves. Although globe valves have a convoluted, restrictive flow path that affects fluid friction and flow rate, minimum seat diameters have been established that will standardize one of the key variables in the flow rate equation.
Like in other API valve standards, there is a table of trim materials for API 623. This trim chart is similar to the one found in API 600. Materials include, but are not limited to, 410, 316, 304, Alloy 20 and others.
There are several options for gaskets and gasket materials including:
- Metal ring joint
- Spiral wound gasket with filler
- Corrugated metal with graphite facings
One of the more popular valves found on the output side of steam boilers is the stop-check valve, which combines the operations of both a globe valve and check valve in one valve body. Stop-check and angle valves are within the scope of API 623 and are included in this new standard. However, pressure-seal valves are not.
Creation of any standard is a uniquely exacting process, one that takes the input of many people. It took about three years and countless hours from the standards team, which was led by Steve McJones of BP Global, to publish API 623.
However, the result provides a first-of-its-kind guidance that should be appreciated by anyone who wants the best-designed globe valves for the job.