A pressure vessel is any closed container built to hold a fluid at a pressure meaningfully different from ambient - and in oil and gas, that covers a huge share of the equipment: separators, scrubbers, knockout drums, and more. Because stored pressure is stored energy, these vessels are heavily engineered and code-governed. This guide explains what a pressure vessel is, how it is designed and rated, and where you find them.
Pressure Vessel in one line: A pressure vessel is a closed container designed and built to safely hold gases or liquids at a pressure substantially above (or below) atmospheric. In oil and gas it includes separators, scrubbers, knockout drums, and treaters, and it is designed to a code such as ASME Boiler and Pressure Vessel Code Section VIII.
The distinction that matters is atmospheric versus pressurized. A stock tank vents to near-atmospheric pressure and is not a pressure vessel. A separator that holds gas and liquid at tens or hundreds of psi is a pressure vessel: it must contain that pressure with a strong, thick-walled shell and heads, engineered so it will not rupture. Regulatory thresholds vary, but a common line is that vessels above roughly 15 psig of design pressure fall under pressure-vessel codes.
Because compressed fluid stores energy, a pressure-vessel failure can be violent. That is why these vessels are designed with defined material, wall thickness, weld, and inspection requirements, and are protected by relief devices - unlike an atmospheric tank, which is a low-energy container.
In North America, pressure vessels are typically designed, fabricated, and inspected to the ASME Boiler and Pressure Vessel Code, Section VIII. A compliant vessel carries a code stamp (the U stamp) and a nameplate stating its key ratings. The most important of these is the maximum allowable working pressure (MAWP) - the highest pressure the vessel is rated to hold at a stated temperature. The vessel must never be operated above its MAWP.
To guarantee that, every pressure vessel is fitted with a relief device - a pressure safety valve (PSV) or rupture disc - set at or below the MAWP, which opens to release fluid before pressure can exceed the vessel's rating. Design temperature matters too, because material strength falls with heat and steels can become brittle in the cold.
Pressure vessels are everywhere in production and processing: two- and three-phase separators, gas scrubbers, inlet and compressor suction/discharge knockout drums, filter and coalescer vessels, amine and glycol contactors, and heater-treater shells. Each holds process fluid at pressure and each has its own MAWP and relief protection.
Operationally, the live values that matter are pressure, level, and temperature inside the vessel - measured by field transmitters wired to an RTU, flow computer, or PLC. A cloud SCADA such as Merobix reads those digitized tags over Modbus, DNP3, or OPC UA, so operators can watch separator or scrubber pressure and level and be alarmed on high-pressure or high-level conditions remotely. The SCADA supervises and alarms; the vessel's mechanical relief device provides the ultimate overpressure protection.
A pressure vessel is a closed container engineered to safely hold gases or liquids at a pressure substantially different from atmospheric. In oil and gas that includes separators, scrubbers, knockout drums, and treaters, all designed to a code such as ASME Section VIII and protected by relief devices.
A storage tank operates at or near atmospheric pressure and is a low-energy container, while a pressure vessel is built to hold significant internal pressure, often tens or hundreds of psi. Pressure vessels have thicker code-stamped shells, an MAWP rating, and mandatory relief protection that atmospheric tanks do not.
MAWP is the maximum allowable working pressure - the highest pressure the vessel is rated to hold at a stated temperature, shown on its code nameplate. The vessel must never be operated above its MAWP, and its relief valve or rupture disc is set at or below MAWP to prevent that from happening.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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