The caller on the phone said something like this: “I just received the pump you sold me, and it’s got 150-pound flanges. This pump is going to be putting out 550 feet of head. That’s 238 pounds per square inch (psi).” What the consultant could have said (but resisted for the sake of diplomacy) is, “No, it doesn’t have 150-pound flanges. There’s no such thing.”
This confusion began in 1927, when the American Engineering Standards Committee (the original name of the group now known as the American National Standards Institute) published Standard B16e. It was this first standard that codified flanges as 150-pound, 300-pound, etc. Various reviews and technical advances resulted in several updates to the standard over the years, but it was not until 1973 that the obsolete nomenclature was replaced with the term “class” to denote operating pressure ranges of the various flanges. But this allowed plenty of time for the former description to become well-entrenched and misunderstood.
Currently, American Society of Mechanic Engineers (ASME) B16.5 and its ductile iron counterpart, B16.42, are detailed and complex, and they contain comprehensive information regarding flange dimensions, bolting, gaskets, materials, temperatures, styles and more. Although they provide significant detail, some basic misunderstandings of flange ratings persist.
For example, what is the pressure rating of that “Class 150” flange? It is certainly not 150 pounds per square inch gauge (psig). It actually varies quite a bit with temperature and materials of construction, but let’s concentrate on the usual “cold water, non-shock” rating. Per the standards, this “base” rating is for temperatures between minus 29 C and 38 C (minus 20 F and 100 F). For a pump cast in American Society for Testing and Materials (ASTM) A395 60-40-18 ductile iron, the flanges, if designed to Class 125 dimensions, are considered to be Class 150 flanges and rated to 250 psi—that is a head of 578 feet. So a 250-psi application does not require Class 250 flanges. And there is a healthy margin associated with this upper limit. The ASTM B16.42 standard requires that flanges of any pressure-containing vessel, cast in ASTM A395 ductile iron and equipped with Class 150 flanges, be able to withstand a hydrostatic test of at least 1.5 times the rating. This would correspond to a pressure of at least 375 psig.
Does this mean that a Class 150 ductile iron flange subjected to a pressure of 375 psi is in danger of rupture? No; the pressure rating is not based on a mechanical failure of the flange. Rather, it is based on the ability of the flange to accommodate clamping forces adequate to retain a gasket seal without leaking.
If a flange is too thin or has too few bolt holes, it will distort at some pressure. It will not maintain adequate face flatness to allow proper gasket retention with conventional gaskets. But this does not mean the vessel (pump case) to which the flanges are attached is capable of withstanding this pressure. The pump case could rupture at 250 psi, but should that happen, you can be fairly certain the flanges will not be the weak link. In other words, unless the pressure is significantly higher than the ASME standard rating times its safety factor, the flange should not be the point of concern. The pump manufacturer should make the working pressure limit of the pump very clear.
How were the flange ratings determined? Regarding ductile iron flanges, as governed by ASME B16.42, the ratings were derived from equations found in “Non-mandatory Appendix B, Methods for Establishing Pressure-Temperature Ratings.”
The ambient rating equation is shown in Equation 1. It is worth noting (and illustrative of the conservative nature of the published flange ratings) that using this equation for a Class 150 flange, constructed of ASTM A395, 60-40-18 ductile iron, produces a flange rating of 315 psig. This is 65 psi higher than the rating as published in the tables found in the standard. Using the standard’s requirement of a hydrostatic test pressure of at least 1.5 times the rating, this would mean the flange can safely handle 473 psi. This is not intended to advocate using Class 150 ductile iron flanges for duty at 473 psi, but applying the requirements and methods of the ASME B16.42 seems to arrive at this value.
This standard pertains specifically to castings that conform to ASTM A395, “Standard Specification for Ferritic Ductile Iron Pressure-Retaining Castings for Use at Elevated Temperatures.” Within this standard, only one grade is noted, the 60-40-18. However, the same grade exists under standard ASTM A536 and possesses the same mechanical properties. It would require a careful reading of both standards to determine if ASTM A536 60-40-18 “conforms to” the requirements of ASTM A395. However, because the flange ratings are based upon the mechanical properties of the materials, it would be difficult to imagine how the latter material would not meet the requirements of ASTM B16.42.