It is important that the valve specifier understands ANSI Class ratings and pressure ratings; and they must clearly designate the appropriate standard class, special class, or intermediate class rating when specifying valves.
When Valtek designates a valve pressure class according to ANSI B16.34, the following standard designations are generally used: Class 150, 300, 600, 900, 1500 or 2500. These designations apply to valves with flanged, buttweld, “Grayloc”, socketweld, or NPT threaded end connections.
In addition to the above mentioned classes and end configurations, ANSI B16.34 designates a standard Class 4500 for buttwelding end valves. Buttwelding end valves may also be upgraded to Special Class 150, 300, 600, 900, 1500, 2500, and 4500. Special Class ratings are available when certain non-destructive examination requirements are met for buttwelding end valves. Properly applied Grayloc-style end connections are also suitable for many high pressure applications. Therefore Valtek offers Special Class and standard Class 4500 valves with Grayloc and buttweld end connections.
ANSI B16.34 also permits standard and Special Class 400 ratings with appropriate end connections. Although available by special request, Valtek does not offer the Class 400 rating as a standard option.
ANSI B16.34 also permits the use of intermediate ratings for buttweld end valves. For example: A customer requires a WCB body material for service at 300OF and 6500 psi. ANSI B16.34 provides a method for determining an intermediate rating for this valve (a buttweld end valve) so that it would not be necessary to require a Class 4500 valve rating (with greater size, weight and cost). This method is described in detail in Annex F of ANSI B16.34-1981. Using this example, the method in Annex F would allow the valve specifier to use an intermediate rating of Class 3300. Although designing to intermediate ratings increases engineering time and adds pattern time when castings are required, it minimizes weight, size and material costs for some applications.
The intermediate rating method can also be used to establish pressure classes greater that Class 4500. However, the valves specifier must be careful not to interpret a 6600 psi pressure rating as a Class 6600 which has a maximum working pressure of 13,200 psi.
The control valve user normally specifies the body material, which is often the same material as the pipe. The most common choices of body material are carbon steel, chrome-molybdenum steel and stainless steel.
Carbon steel is the most commonly used material for bodies. It handles most non-corrosive liquids and gases up to 800 degrees Fahrenheit for continuous service, or to 1000 degrees Fahrenheit for occasional service. (See ANSI B16.34 for specific temperatures and pressures.) Carbon steel can be used for most condensate and steam services.
Chrome-moly steel is used for higher temperatures and pressures than carbon steel, including such services as high pressure steam or flashing condensate which requires corrosion and errosion resistance. Chrome-moly is stronger than carbon steel and, in some cases, is as strong as stainless steel. It costs less than stainless steel, but is not as corrosion resistant.
Stainless steel is specified for high temperature services (1000 degrees Fahrenheit and up) or in corrosive applications. It is more corrosion resistant than either carbon steel or chrome-moly.
Special alloys—such as Hastelloy B and C, Monel, nickel and titanium—are also available. Consideration should be given to the types of material that have been used successfully in the past for similar applications.
Valtek is cautious about choosing materials for the user. Except in the most basic applications, the control valve user should specify the body materials.
Because of the lack of actual correct fluid data, Valtek does not recommend body material or guarantee corrosive resistance or fluid compatibility. Valtek has no control over fluid composition.
Once the body material has been chosen, check the pressure/temperature tables in ANSI B16.34 (found at the end of this section) to determine the application limits for the selected material.
The valve body can be cast, forged, wrought or fabricated. Castings are usually the first choice in standard sizes and ratings. Forgings are used for smaller sized Mark Two valve bodies. (Generally these valves have high pressure ratings—1500, 2500 and 4500 for those special materials not available in castings.) Barstock bodies are recommended when delivery is critical and a casting or forging is not available. Fabricated bodies are a convenient way to manufacture large angle valves.
Bonnets are generally manufactured from barstock of the same material as the body. The exception is smaller (6-inch or less) low pressure chrome-moly valves, where a stainless steel bonnet is normally standard. The basic rule is to specify the general type of bonnet material only, not the ASTM (American Society of Testing Materials) specification number, because Valtek’s standard material varies according to valve size and pressure rating.
Material specification is based upon the ASTM specifications listed in ANSI B16.34 for standard service valves, and ASME (American Society of Mechanical Engineers) specifications. ASME codes are preceded by the letter “S,” such as SA216-WCB.
The tables in this section are from ANSI B16.34 and B16.24 (bronze). They determine the valve body’s pressure rating, such as Class 150, 300, 600, etc. The design or maximum pressure and temperatures (as given by the control valve user) should be used. Pressure and temperature values may be extrapolated between the lines.
The “special class” tables allow higher pressure and temperature values for weld-end valves with some nondestructive examination of critical areas of the body and bonnet.
Table 5-II provides the applicable ASTM codes for Valtek’s common body materials.
Table 5-I: Valve Body Material Temperature Limits (OF)
|*Carbon Steel (Grade WCB)||-20||1000|
|Carbon Steel (Grade LCB)||-50||650|
|Carbon Moly (Grade WC1)||-20||850|
|1-1/4 Cr - 1/2 Mo (Grade WC6)||-20||1000|
|2-1/4 Cr - 1 Mo (Grade WC9)||-20||1050|
|5 Cr - 1/2 Mo (Grade C5)||-20||1100|
|9 Cr - 1 Mo (Grade C12)||-20||1100|
|Type 304 (Grade CF 8)||-425||1500|
|Type 347 (Grade CF8C)||-425||1500|
|Type 316 (Grade CF8M)||-425||1500|
|3-1/2 Ni (Grade LC3)||-150||650|
* The carbon phase of carbon steel may be converted to graphite upon long exposure to temperatures above 775° F. (Check applicable codes for maximum temperature rating of various materials.)
TABLE 5-II: Material Standards
Table 5-III: Valve Body (Pressure Containment) Materials
* Valtek standard for cast bodies ** Valtek standard for Mark Two bodies