At the 2018 Emerson Exchange conference, Emerson’s David Tuttle presented Preventing Overpressure Disasters in Supercritical Steam Generation.
David comes to Emerson from the Crosby brand of valves that joined Emerson as part of the Pentair Valves & Controls acquisition.
Boiler safety valves are the last line of defense against a catastrophic overpressure event. A safety valve opens at a preset pressure and discharges steam from the boiler to atmosphere. The set pressure overcomes a heavy spring keeping the valve closed.
ASME has a boiler and pressure vessel codes that mandates 3rd party inspectors to verify safe operations.
Steam at 3200psi is called supercritical. Boiler manufacturers have made supercritical boilers for decades. There is no regulatory guidance for calculating the supercritical steam flow through safety valves. Various suppliers have their own methods of calculation.
Emerson Pressure Relief engineering proposed a method for calculating supercritical steam flow as an isentropic process. Steam flow through the nozzle with gradually increasing velocity until sonic flow is achieved in the throat of the nozzle.to calculate the mass flow in the nozzle.
Modern safety valve designs include a nozzle in the valve inlet. At the throat the flow is sonic. To calculate the mass flow in the nozzle throat, the velocity and the density of the steam must be calculated. The maximum mass flow occurs when the ration of the nozzle throat velocity to the specific volume of the throat is maximized.
These mass flow capacities must be tested and verify per ASME and ISO 4126 to obtain rated relieving capacity. The boiler code dates to the early 1900s due to numerous boiler explosions that were occurring.
Making changes in the boiler code is not easy. A proposal driven by the Emerson pressure relief engineers developed for supercritical steam capacities was recently added. It provides end users with the assurance that safety valve sizing above 3200 psi is performed and validated consistently across all pressure relief valve manufacturers.
