For vessels containing compressed air, the requirements for overpressure protection are found in 29 CFR 1910.169(a)(2)(ii)¹, which specifically references ASME Boiler and Pressure Vessel Code (BPVC), Section VIII.²

All equipment used in providing and utilizing compressed air are required to have pressure safety valves in accordance with ASME BPVC Section VIII.  This includes sizing the pressure safety valves to limit the accumulation within the vessel to 10% above the MAWP while relieving.  Because the regulation does not prescribe the overpressure scenarios, our common practice of performing the typical broad-ranged overpressure protection analysis for these systems is important to identify the potential overpressure scenarios.  Of course, the most common design basis is the blocked outlet case for those installations where the air compressor is a positive-displacement type machine.

The applicability of fire as a credible overpressure scenario is one question that commonly arises for air receivers that are deemed to be in a fire zone.  Air receivers are commonly vapor-filled, and thus have an unwetted surface area.  API Standard 521 §4.4.13.2.4.3 provides calculations for this situation³, although it is very common to simply indicate a nominal relief requirement like for hydraulic expansion, for which any sized relief device is adequate.

Even for those cases in which no overpressure scenarios are identified, and overpressure protection is provided by system design, we will typically not invoke ASME Section VIII UG-140.  Not only does the OSHA regulation require a pressure safety valve be installed, ASME Section VIII UG-140 (b)(1) indicates that, at least in most instances, UG-140 cannot be applied to vessels exclusively in air service.⁴

In addition to the lifting device requirements of ASME Section VIII §UG-136(a)(3), there are a couple of noteworthy installation requirements found in 29 CFR 1910.169(b)(3), specifically the following:

• No block valves allowed between the receiver and its pressure safety valve
• All safety valves have to be “tested frequently and at regular intervals”

The prohibition of a block valve on the inlet makes the testing of the pressure safety valves difficult, to say the least, requiring a shutdown and isolation of equipment in order to inspect and maintain the pressure safety valve.  This is a common justification provided to local jurisdictions for requesting an exemption and allowing the use of an inlet block valve.

See our general introduction to this series to find other systems with prescriptive requirements.