Stacking BPE Clamp Fittings Can Cause Unforeseen Problems
An Exmple of How the Accumulation of Tolerances Can Affect the Squareness of ASME BPE Fittings
The industry wide adoption of the ASME BPE standards have brought a host of benefits to the biopharmaceutical process industry. But, merely specifying the use of ASME BPE fittings in a piping assembly does not necessarily guarantee proper fit up when building a high purity piping assembly. Recently, Holland worked with a customer in need of pharmaceutical grade custom piping assemblies and the associated documentation for a piece of downstream bioprocessing equipment they were building. Their design incorporated custom process piping assemblies as well as many standard ASME BPE compliant grade sanitary clamp fittings. Holland supplied the standard fittings and fabricated the custom piping in our shop. All items were inspected at our facility and found to be within ASME BPE standards.
When the piping assemblies were installed there were some significant fit up issues. The custom assemblies fit up to the machine with no issues. However, where multiple standard ASME BPE tees clamped together there were fit up issues. There was such a stack up of tolerances that the piping manifolds were more than ½ off. The stack up of tolerances also led to incorrect pitch angles as well. Angles that were to be 3 degrees downward slope were now found to be 1.8 degrees upward slope. The piping would never drain.
The tees were within specifications per ASME BPE standards. The specifications for the angular difference on the face of a 1.5 tee can be as much as 1.3° per end. This could potentially create a 2.6° difference between the opposing end connections. Stacking several tees exasperates the problem even more. The attached picture demonstrates this phenomenon.
We solved the problem by manually straightening all of the tees so that the ferrule faces were now within 0.1° of being straight.
Holland was eventually able to fix the problem but the question remains, Are ASME BPE dimensional tolerances good enough for every situation found in bio-pharmaceutical processes?. We would recommend:
Avoid designs where multiple clamped fittings are stacked together.
If you cannot avoid stacking fittings, check the tolerances of each fitting you are using. You may need to manually straighten them for you system to fit together.
The highest risk of this phenomena happening is when you stack multiple tees. We would recommend using fabricated manifolds whenever possible.