1. Steam Piping Ratings of Boiler SystemsIn most states and jurisdictions, steam piping is classified as high-pressure piping when it exceeds 15 psig. The main construction code used to define high-pressure piping issues and requirements for construction and installation is published by the American Society of Mechanical Engineers (ASME) Section I (power boiler) and ASME code B31.1 (pressure piping) codes.
2. MAWP: What Is It?The piping, valves and components of any high-pressure piping system have to be rated for the maximum allowable working pressure (MAWP) of the boiler. ASME requires a nameplate be affixed to the boiler with this information on it (figure 1). The piping attached to the steam discharge flange must also be built to withstand the MAWP that the boiler can generate. In multiple boiler installations, the design rules typically apply to all piping through the second stop valve from the discharge flange of the boiler and are governed by ASME Code Section I and B31.1. After the discharge of the second valve, the piping must be rated as required by the applicable jurisdiction. This may be the MAWP of the boiler or at least the setpoint of the highest safety-relief valve protecting the system.
There also are specific rules for stamping or identifying the piping that falls within the code boundary jurisdiction of the boiler. Typically the serial number, “certified by” information and pressure rating will be stamped on or a nameplate banded to the pipe.
3. Some Piping BasicsIt is very important that you understand what you are looking at -- and the consequences of being wrong -- when it comes to evaluating piping systems. I'll start with the pipe itself. Most pipe you will encounter starts its life at a steel mill in sheet form. In many cases, the mill rolls the sheet into tubes and welds it. There are different piping designations; one example is ASTM SA 53B. ASTM means American Society of Testing Materials, and SA 53B is the designation for the plain black steel pipe mostly used in industrial applications.
Piping also comes in different schedules, or wall thicknesses. The schedule or thickness would be schedule 40 or 80. The wall thickness for typical 6" schedule 40 increases from 0.280 to 0.432" for schedule 80. Schedule 40 is what is commonly called for in B31.1 for pressure piping in service conditions that apply to this article.
B31.1 and ASME code, Section I, have calculations that steam system designers use to determine the safe working pressure of the piping based on its type, thickness and minimum diameter. Schedule 80 is called out as good practice for most condensate systems since they are a severe duty as compared to steam. Condensate is likely to
contain carbonic and/or other mild acids, which tend to erode condensate piping over time. Making this kind of piping thicker from the start builds in a factor of safety.
4. Joints and Joining MethodsPipe gets assembled to other pipe, fittings and flanges by either welding or threading. There are specific code requirements that describe when it is permissible to thread or when welding must be used.
With welded joints, you also need to be aware of several other variables. Flanges and fittings are supplied as either slip-on or weld-neck devices (figure 2).
A slip-on fitting is just that: slipped onto the end of the pipe. The flange then is welded up around the contact points on the inside and outside of the pipe and the flange. Socket welding is a term used to describe when a slip-on fitting, usually used for small diameters, is inserted into the fitting until it bottoms out. Then, the pipe is pulled back from the bottom and welded to the fitting. Failure to pull the pipe back can cause welds to fail from stresses. Slip-on flanges are not considered as strong a joint as weld-neck or butt-welded connections.
With weld-neck or butt-welded joints, the two pieces -- flange and pipe -- are prepped and then welded together with full penetration through the weld construction. (In other words, a welder carefully lays a bead and builds up layers around the entire surface of the gap between the two pieces.)
5. Flanges and Their RatingsYou also need to be aware of flange pressure ratings. The ratings and certification information are usually stamped on the circumference of the flange. A 150 lb flange indicates a pressure temperature rating. This is not the MAWP of the flange but a designation that allows a certain pressure to be used based upon the installation and temperature to be encountered. Pressure/temperature ratings can be found in ASTM A 105 B 16.5 specification tables.
6. Flange MaterialsFlange materials also can be tricky. You always want to make sure you are using carbon steel flanges with the proper rating; A 105 B 16.5 is a typical carbon steel flange used in pressure-piping applications. Cast iron flanges are too brittle and could break in this kind of application.
7. Fastener Issues: When a Bolt is Not Just a BoltThere is a standard marking system for fasteners. When you look at a bolt head, you should see markings that identify its grade or rating. Be aware that cheap fasteners can mean forgeries that can cost someone their life. You can find out more about fasteners and ratings by visiting the National Fastener Distributor Association’s web site.
Fasteners should be rated at least Grade 8. This means they have a tensile strength that could withstand the force that must be applied for the proper assembly of the components.
Also, be aware of threaded rods and studs. Studs are not simply some supply house off-the-shelf threaded rod cut to size. Studs should be marked with a stamping in the end that indicates they are a special grade and type of material that is of sufficient tensile strength. They might have a marking like “AB or HV” stamped into the end.
8. Valve and Fittings RatingsValves and fittings should have their pressure rating cast into them or marked as required by the applicable material specification (figure 3). Many pipe fittings are marked with a manufacturer's logo or insignia, size, and schedule rating: an example would be 6" SA 234 Gr WPB or 1" 3000M A105 B 16.5 with the company's logo. The pressure rating must be at least equal to the design MAWP. Again ASME/ASTM SA/A 234 and SA/A 105 B 16.5 give the specific requirements for these fittings.
Valves also will be marked with their pressure rating along with the type of service permitted. The ASME codes specify which types of valves and fittings are permitted to be used and their proper service applications in pressure-piping applications.
9. Welding ConsiderationsWelding on pipe, fittings, flanges and pressure vessels must only be performed by someone with the proper credentials following qualified welding procedure specifications. It is the responsibility of the installer to have welding procedure specifications that are certified to meet the applicable ASME code construction. (Refer to ASME code Section I and B 31.1, and ASME Code Section IX for welding procedure specifications.) The National Board Inspection Code, which is required for repairs of pressure equipment, also includes the American Welding Society's (AWS) standard welding procedures. Every detail of the AWS's standard welding procedures must be followed when welding or the weld can be deemed an unqualified weld and in jeopardy of having to be removed.
10. Qualified PeopleWelders also must be qualified to the requirements of ASME code Section IX. Once qualified and certified, the welder can only weld within the variables listed on their welder's performance qualification record. The welder must weld within the process at least once every 6 months or the qualification expires. Records must be kept to prove that the welder had welded at least once every 6 months. Welders are required by ASME to mark their welds with stamping to identify which person welded which joint.
In addition to the welders' stamping, you may also find registration numbers from the National Board of Boiler and Pressure Vessel Inspectors on your piping. The National Board maintains a database of all registered pressure items. You can call them with the National Board number, original manufacturer and year built (as indicated on a nameplate), and they can tell you about your high pressure system and its original design. When inspecting, repairing or replacing items, a check with the National Board can help ensure the proper repair or replacement, securing the same safety integrity as when the boiler, pressure piping or pressure equipment was originally constructed. PH