David W. Spitzer
David W. Spitzer

Which carbon steel flanges below are not acceptable for a flowmeter when the maximum pressure is 15 bar (approximately 218 PSIG)?
A. 150 lb.
B. 300 lb.
C. 600 lb.
D. 900 lb.

At first glance, it would seem that the 150 lb. flange would not be acceptable (Answer A) and that all of the remaining flanges are acceptable. However, this problem is not quite that straightforward.
The actual flange pressure limit is dependent upon the fluid temperature. As the fluid temperature increases, the flange metal weakens, and the maximum pressure limit of the flange decreases. Standards are available that list the maximum pressure limits at different temperatures.  

In particular, at ambient conditions (under 38 C), a 150 lb. carbon steel flange is rated to 285 PSIG (approximately 19.6 bar). Therefore, a 150 lb. flange is acceptable at relatively low temperatures. However above approximately 200 C, the 150 lb. flange pressure rating drops below 15 bar and would not be acceptable (Answer A). Similarly, a 300 lb. flange is rated at approximately 15 bar at 475 C.  
There is insufficient information to definitively answer the question in this case.

Additional Complicating Factors
Other factors may override the temperature issue described above. For example, in some services, higher flange ratings than necessary may be used to reduce the probability and magnitude of leaks that might occur over the life of the system.

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.

Mr. Spitzer can be reached at 845 623-1830 or 
www.spitzerandboyes.com. Click on the “Products” tab to find his “Consumer Guides” to various flow and level measurement technologies.