9 Points to Safe Steam Piping
To ensure that an accident is not waiting to happen the next time someone opens a valve or puts a steam line into service, there are nine methods for screening high-pressure (below 300 psig) piping systems typically encountered in industrial steam plants. These methods identify safety issues related to design and/or installation of pipe, valves, flanges, and components used in these piping sys...
To ensure that an accident is not waiting to happen the next time someone opens a valve or puts a steam line into service, there are nine methods for screening high-pressure (below 300 psig) piping systems typically encountered in industrial steam plants.
These methods identify safety issues related to design and/or installation of pipe, valves, flanges, and components used in these piping systems. Methods below provide insights for understanding and conducting a simple screening of a steam piping system.
1. Pipe basics
It is important to understand piping system design and the consequences of being wrong. Let's start with the pipe itself. Most pipe is manufactured at a steel mill from sheet steel. In many cases, the mill rolls the sheet into tubes and welds it.
Piping designations differ. An example is ASTM (American Society for Testing and Materials) A 53B, ASME (American Society of Mechanical Engineers) SA 53 B. SA 53 B is the designation for plain black-steel pipe mostly used in industry. Piping also comes in different schedules (or wall thicknesses). The schedule would be 40 or 80. Wall thickness for typical 6-inch schedule 40 pipe increases from 0.280 inches to 0.432 inches for schedule 80. Schedule 40 is what's commonly required by B31.1 for pressure piping in most service conditions discussed below. B31.1 and ASME code, Section I, provide calculations that the designers use to determine the safe working pressure of the piping based on its type, thickness, and minimum diameter. Schedule 80 follows good practice for most condensate systems since they are a severe duty compared to steam. Condensate is likely to contain carbonic and/or other mild acids that tend to erode condensate piping over time. Using thicker piping from the beginning builds in a factor of safety.
2. Boiler system ratings
In most states and jurisdictions, steam conduit is classified as high-pressure piping when it exceeds 15 psig. ASME Section I (power boiler) and ASME code B31.1 (pressure piping) codes are the main construction rules defining high-pressure piping issues and requirements for construction and installation. Any high-pressure piping system's pipe, valves, and components have to be rated for boiler MAWP (maximum allowable working pressure). ASME requires a nameplate containing this data to be affixed to the boiler. Pipe attached to a steam discharge flange must also be built to withstand the boiler's MAWP.
3. Pressure check
In multiple boiler installations, design rules typically apply to all pipe through the second stop valve from the discharge flange of the boiler and are governed by ASME Code Section I and B31.1. Beyond discharge of the second valve, pipe must be rated as required by the applicable jurisdiction. This may be the boiler MAWP or at least the setpoint of the highest system safety relief-valve. There are also specific rules for stamping or identifying pipe falling within the code's boundary jurisdiction for the boiler. Typically a serial number, "certified by" and pressure information will be stamped on pipe, or a nameplate banded onto the pipe.
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 flange MAWP, but a designation that allows a certain pressure use based upon the installation and temperature to be encountered. Pressure and temperature ratings can be found in ASTM A 105 B 16.5 specification tables.
Flange materials can also be tricky; always be sure to use 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 application.
5. Fastener issues
Fasteners should be rated at least Grade 8. Bolt grades indicate the tensile strength resisting the force that must be applied for the proper assembly of, and working pressures encountered by, the components. Bolts are graded by a standard marking system shown on their heads; you should be able to see markings identifying its grade. Be aware that cheap fasteners can mean forgeries that can cause injury or death.
Also, take care with threaded rods and studs. Studs are not simply off-the-shelf threaded rod cut to size; studs should be marked with an end stamping indicating the grade/type-of-material, in effect its tensile strength. It should match the needs of the application. You can find out more about fasteners and their ratings through the National Fastener Distributor Association.
6. Joining methods
Pipe is assembled with other pipe, fittings, and flanges either by welding or threading. There are specific codes describing the permissibility of threading or welding. With welding you also need to be aware of several options. Flanges and fittings are either slip-on or weld-neck. Slip-on fittings slip onto the end of the pipe. The flange is then welded 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. With butt-welded or weld-neck flanges, the two pieces—flange and pipe—are prepped and then welded together, with full penetration (a welder carefully lays a bead and builds up layers around the entire surface of the gap between the two pieces).
Socket welding describes when a slip-on fitting, usually used for small diameters, is inserted into the fitting until it bottoms out. The pipe is then pulled back from the bottom and welded to the fitting. Failure to pull the pipe back can cause weld failure due to stress.
7. Valve, fitting 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 (example 6 inches SA 234 Gr WPB or 1 inch 3000M A105 B 16.5 with the logo). The pressure rating must at least equal the design MAWP. Again ASME/ASTM SA/A 234 or SA/A 105 B 16.5 gives the specific requirements for these fittings.
Valves will also be marked with their pressure rating along with the type of service/usage. ASME codes specify which types of valves and fittings are permitted and their proper service applications in pressure piping applications.
8. Welding regulations
Welding on pipe, fittings, flanges, and pressure vessels must only be done by a certified person. Welding on pressure piping must be performed following qualified welding procedure specifications. It is the responsibility of the installer to have welding procedures 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).
National Board Inspection code, which is required for repairs of pressure equipment, also includes American Welding Society (AWS) standard welding procedures. AWS standard welding procedures must be followed or the weld can be deemed to be unqualified and may have to be removed.
9. Qualified welders
Welders must also be process qualified under ASME code Section IX. Once qualified and certified, the welder can only weld within the variables listed on that welder's performance qualification record.
The welder must also weld within the process at least once every six months or the qualification expires. Welders will mark their welds with stamping to identify the person welding specific joints. This stamping should be on each system, but could be obscured by insulation.
Along with the ASME required stamping, national boards' registration numbers might also be found. National Board of Boiler and Pressure Vessel Inspectors maintains a database of all registered pressure items. By providing the board with the national board number, original manufacturer, and year built—as indicated on a nameplate—it can provide a great deal of information about the piping and original design. This is a very important source when inspecting, repairing, or replacing items.
Proper repair or replacement should ensure the same integrity—if not better—as when the boiler and pressure piping or equipment was originally constructed.
This article originally appeared in Pharmaceutical Processing ; www.pharmpro.com
John R. Puskar, P.E, is principal at CEC Combustion Services Group,