When screening high pressure (below 300 psig) piping systems normally encountered within industrial steam plants, safety issues related to the design and/or installation of pipe, valves, flanges, and components used in these systems must remain a top concern. Although not an all-inclusive guide to safe practices, the following provides insights for understanding and conducting a simple screening of a steam piping system. Taking the time to review what is installed is important for every steam plant operator to help make sure that an unexpected accident is not waiting the next time someone opens a valve or puts a steam line into service.
Steam Piping Ratings of Boiler Systems
In 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.
MAWP – What is it?
The piping, valves, and components of any high pressure piping system have to be rated for the MAWP, Maximum Allowable Working Pressure, of the boiler. ASME requires a nameplate be affixed to the boiler with this information on it. 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 set point of the highest safety relief valve protecting the system. There are also specific rules for stamping or identifying the piping which falls within the code boundary jurisdiction of the boiler. Typically the serial number, “certified by”, and pressure will be stamped or a nameplate banded to the pipe.
Some Piping Basics
Understanding to look at and the consequences of being wrong when it comes to evaluating piping systems is extremely important. Most pipe 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: ASTM A 53B, ASME SA 53 B. ASTM means American Society of Testing Materials, SA 53 B, is the designation for plain black steel pipe often used within the industry. Piping also comes in different schedules (wall thicknesses). The 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’s commonly called for in B31.1 for pressure piping. B31.1 and ASME code, Section I, have calculations that designers use to determine the safe working pressure of the piping based on its type, thickness and minimum diameter. Schedule 80 is a good practice for most condensate systems since they are much harsher than steam. Condensate contains 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.
Joints and Joining Methods
Pipe 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. Within the welding world, be aware of several other possibilities. Flanges and fittings come as either slip on or weld neck. Slip on fittings are just that, slipped onto the end of the pipe. The flange is then welded up around the contact points on the inside and outside of the pipe and the flange. Slip on flanges are not considered as strong a joint as weld neck or butt welded connections. In the case of a butt welded or weld neck flange, the two pieces, flange and pipe, are prepped and then welded together with full penetration through the weld construction (a welder carefully lays a bead and builds up layers around the entire surface of the gap between the two pieces). 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.
Flanges and Their Ratings
Flange pressure 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 which allows a certain pressure use based upon the installation and temperature to be encountered. Pressure/Temperature ratings can be found in ASTM A 105 B 16.5 specification tables (www.astm.org).
Flange materials can also be tricky. Always ensure 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.
Fastener Issues, When a Bolt is Not Just a Bolt
Fasteners could be another problem area. Fasteners should be rated to at least a grade 8, which means they have a tensile strength that could withstand the force that must be applied for the proper assembly of the components. There is also a standard marking system for fasteners that identify its grade or rating. Be aware that cheap fasteners can mean forgeries that can cost someone their life. Find out more about fasteners and ratings by going to the National Fastener Distributor Association’s website at www.nfda-fastener.org. Also, be aware of threaded rods and studs. Studs are not simply a 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 may have a marking like “AB or HV” stamped into the end.
Valve and fittings ratings
Valves and fittings should have their pressure rating cast into them or marked as required by the applicable material specification. Many pipe fittings are marked with a manufacturer’s logo or insignia, size, and schedule rating (ie: 6” SA 234 Gr WPB or 1” 3000M A105 B 16.5 with the logo). The pressure rating must be at least equal to the design MAWP. Again ASME/ASTM SA/A 234 or SA/A 105 B 16.5 give the specific requirements for these fittings. Valves will also be marked with their pressure rating along with the type of service permitted to be used. The ASME codes specify which types of valves and fittings are permitted to be used and their proper service applications in pressure piping applications.
Welding on pipe, fittings, flanges, and pressure vessels must only be done by someone with the proper credentials. Welding on pressure piping must be performed 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 AWS (American Welding Society, www.aws.org) standard welding procedures. Every detail of AWS standard welding procedures must be followed when welding or the weld can be deemed not a qualified weld and be in jeopardy of having to be removed.
Welders must also 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 also 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 will mark their welds with stamping to identify which person welded which joint. This stamping could be there on your system but possibly obscured with insulation.
Along with the ASME required stamping, the National Boards registration numbers might also be found. The National Board of Boiler and Pressure Vessel Inspectors is located in Columbus, OH. They maintain 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 what you have and the original design. This is very important when inspecting, repairing or replacing items. The proper repair or replacement should ensure the same safety integrity as when the boiler, pressure piping or pressure equipment was originally constructed (if not better).
Take a little time to use the information above and learn more about what you already have. High pressure steam piping systems are nothing to play with. Make sure the job is done right, use professionals and understand the tasks that you’re performing as well as the risks associated with them.
Honeywell Combustion Safety, formerly CEC Combustion Safety, LLC, has been in business since 1984. With engineers and staff members that sit on Code committees such as ASME CSD-1, NFPA 56, NFPA 85, NFPA 86, and NFPA 87, our inside expertise is integrated within all of our practices and our global reach ensures that customers around the world are kept safe. Contact us at +1 216.749.2992 or visit www.combustionsafety.com for additional information.