Depressuring Unit - Technical Information
The Depressuring Unit simulates the time-pressure-temperature relationships which occur when a vessel is depressured through a relief or control valve. The initial phase of the vessel contents may be either a vapor or a vapor-liquid mixture. Several different valve flow models and vessel configurations are available. Several heat input models are also available to simulate the pressuring of the vessel by a fire or other means. An optional external makeup stream may also be specified.
Calculation Method
The depressuring calculations occur during normal flowsheet convergence. Calculation options include procedures from API standard 2000, API Recommended Practice 520, isothermal blowdown, rigorous or semirigorous blowdown, fire relief calculations, or user-defined heat input model. The calculation model employs an incremental approach using discrete time steps.
Feeds and Products
A depressuring unit may have any number of feeds. The combined feed streams are mixed adiabatically to give the initial composition, temperature, and pressure of the fluid in the depressuring vessel. The resulting combined feed must be vapor or mixed liquid and vapor. Optionally, you may specify one of the feed streams to serve as a constant-rate makeup feed. The initial composition of the liquid and vapor inside the vessel is calculated in accordance with the following guidelines:
If a liquid holdup is specified:
For a mixed phase feed, the composition of the liquid phase will be set equal to the composition of the liquid portion of the feed, and the vapor phase composition will be set equal to the feed vapor composition.
For a liquid phase feed, then the initial vapor composition in the vessel will be set equal to the vapor in equilibrium with the feed liquid at its bubble point temperature.
If no liquid holdup is specified, the initial vessel total fluid composition, temperature, and pressure will be set to the feed stream conditions.
For an all liquid feed stream to the depressuring unit, the temperature will be set to the bubble point temperature of the feed stream at the feed pressure and the vapor composition will be set to the composition in equilibrium with the bubble point liquid.
The product stream from a depressuring unit is the vent gas product at the specified calculation time steps. The depressuring unit will create as many products as are entered or are calculated, whichever is the lesser of the two. Product streams may be generated as a user option, but the calculations are not performed until output time.
Fluid in Dense Phase at Initial Conditions
To consider the liquid flow through the valve, the initial feed is flashed to check if it is liquid only phase. If so, pseudo Pc is calculated to check if the feed P is above Pc. Then the fluid is considered as dense fluid. At initial time step, the dense liquid flow through the valve is considered and the flow calculations are performed using the selected valve model for the valve. At each time step, the depressurizing vessel contents are flashed to check if the contents are in liquid only phase and above the critical conditions to consider it as dense liquid and to allow the liquid flow through the valve. If the fluid is not in dense phase, then liquid flow through the valve should not be allowed.
Related Topics
Problem Specific Calculation Sequence