Pressure relief flashing liquids

A vapor poeket on the exchanger s low-pressure side can create a cushion that may greatly diminish the pressure transient s intensity. A transient analysis may not be required if sufficient low-pressure side vapor exists (although tube rupture should still be considered as a viable relief scenario). However, if the low-pressure fluid is liquid from a separator that has a small amount of vapor from flashing across a level control valve, the vapor pocket may collapse after the pressure has exceeded the fluid s bubble point. The bubble point will be at the separator pressure. Transient analysis will prediet a gradually inereasing pressure until the pressure reaches the bubble point. Then, the pressure will increase rapidly. For this ease, a transient analysis should be considered. 

The calculation method can be selected by application of the decision tree in Figure 9.2. The liquid temperature is believed to be about 339 K, which is the temperature equivalent to the relief valve set pressure. The superheat limit temperatures of propane and butane, the constituents of LPG, can be found in Table 6.1. For propane, T, = 326 K, and for butane, T i = 377 K. The figure specifies that, if the liquid is above its critical superheat limit temperature, the explosively flashing liquid method must be chosen. However, because the temperature of the LPG is below the superheat limit temperature (T i) for butane and above it for propane, it is uncertain whether the liquid will flash. Therefore, the calculation will first be performed with the inclusion of vapor energy only, then with the combined energy of vapor and liquid. 

Vapor pressure data, P, of pure components are important in calculations relating to vapor-liquid phase equilibria, e.g., in the simplest case to predict the pressure in a closed vessel containing a specific liquid or mixture of liquids. P data are required for bubble point and most dew point computations. These values are used in flash calculations involving mass transfer operations. Clearly the design of pressure requirements for storage equipment requires knowledge of the vapor pressure of the components as does the design of appropriate pressure relief... 

If the vessel with the leaking relief valve is liquid full with light hydrocarbons (ethane, propane, or butane), then a leaking relief valve will be quite obvious. The line downstream of the faulty pressure relief valve will be covered with ice. The ice is atmospheric moisture freezing on the line to the flare. This is not due to a Joule-Thompson expansion, but due to the auto-refrigeration of the volatile hydrocarbon liquid as it flashes to a vapor in the flare header piping. 

A.12. The bleed condenser, which is a vessel connected to the pressurizer and maintained at a lower pressure in normal operation, should be fitted with passive relief devices (e.g. rupture discs, rehef valves or safety valves operated by pilot valves) capable of transmitting steam, hquids and flashing liquids, since the condenser may be flooded in the event of large discharges of fluid from the RCS or the pressurizer. In the design of the bleed condenser, account should be taken of the range of pressures and temperatures of the RCS. 

The reaction force for a liquid SRV is much less than for gas and is normally of very litde concern. Liquids are not compressible and do not expand when lowered in pressure as do gases and vapours. Nevertheless, care needs to be taken when the liquid is flashing during relief. 

Let s first define exactly what we are talking about Two-phase flow describes a condition whereby a flow stream contains fluid in the liquid phase and at the same time in the gas or vapour phase. Flashing flow occurs as a result of a decrease in pressure, and all or a portion of a liquid flow changes into vapour. It is possible for both flowing conditions, two-phase and flashing, to occur simultaneously within the same application. The complexity of the issue results from the fact that this condition is never stable and constantly changes during a relief cycle. 

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