Calculation of Aspen safety valve performance under fire conditions
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Last edited by xyq1983 on 2021-2-18 13:26. Condition: 200m3 horizontal pentane (isopentane) storage tank, stored at full pressure; safety venting calculated for fire conditions； Pentane operating conditions: 30°C/corresponding saturated pressure ； Design conditions for the storage tank: 50°C/0.4 MpaG. First, start Aspen Plus and add isopentane in the property module; then define the methods for determining those properties. After that, go to the simulation module to create new streams – the feed stream F, the vapor phase V, and the liquid phase L. Define the feed stream with a flow rate of 2000 kg/h, at 30°C and 8 bar pressure (this will not affect the design of the safety valve). Define the parameters for the flash tank: a pressure drop of 0, with adiabatic flashing. Run the calculation simulation; once it’s complete, proceed to the safety analysis module. There are two ways to do this: either select “Add relief device” from the menu bar, or choose “Safety analysis” from the modules on the left side. On the left side here, select to enter the safety analysis module; once inside, simulated scenarios and options related to safety relief will be displayed. Right-click on module “B” to create a safety relief device along with the corresponding flow paths ； Choose L here. This step is primarily intended to introduce the relevant physical properties of pentane; the specific parameters for safe venting can be set later. After clicking \"Add\", PSV-1 will appear. The values here are taken from the data in the simulation module, but they can also be customized, with the software performing calculations based on the manually set parameters. Further refine the equipment information here. This design relies on manual input; for complex systems and multiple operating conditions, a comprehensive analysis is required to make decisions. Next, create the discharge condition. Click on the condition label to enter and create a condition. Aspen offers comprehensive condition analysis; if the equipment is part of a process plant, it is necessary to take into account the upstream and downstream processes, as well as conduct comparisons and analyses of risks under different conditions. Here, users are allowed to create multiple operating conditions for analysis. Condition analysis is a very complex process; for the analysis of different overpressure risks, one can refer to relevant standards and specifications such as Appendix B of GB150, HG-T 20570.2-95 \"Installation and Selection of Safety Valves\", API520 Part I (2020), API521 (2020), SY/T10043-2002 (which is largely consistent with API521), and SY/T10044-2002 (which is largely consistent with API520). It is recommended to consult the API standards; the Aspen safety analysis module follows these API standards strictly, especially when dealing with fire scenarios and multiphase flow discharge calculations. There are still differences between domestic standards and the parameters specified in API. There will be time in the future to discuss this with everyone. The fire condition is selected for this design. Hydraulic calculations can be performed here for the valve size and pipeline system dimensions. Select here, then click to open the status. It is mainly divided into 4 sections here (the screen is too small to display everything). The first section deals with the definition of the discharge conditions; here, the reference fluid is selected, and the software will calculate the physical properties based on the set discharge conditions. Due to the fire condition, the discharge pressure is assumed to be 21% here (see API520 or HG20570). The table above is taken from HG20570; the cumulative backpressure is 10% of the set pressure, and this parameter depends on the type of valve (see API520 or HG20570) ； The selection of the calculation method is usually automatically matched by the software based on the referenced logistics and discharge conditions. However, for complex systems, human judgment is still required; the default method is HEM (the translation of this term in Chinese is a bit problematic; it actually refers to average calculation based on mixed homogeneous conditions. One can look up articles online regarding the HEM calculation method, or we can discuss it together later). Under these operating conditions, the discharge state may be accompanied by liquid. The discharge temperature is calculated using the discharge pressure software (saturated phase). The second part deals with the setting of surface parameters and environmental factor parameters under fire conditions; these settings affect the heat data generated by the external fire on the system, thereby influencing the amount of release. Discharge volume selected based on calculated value ； The calculation method chosen is based on the wetted area; this term is really tricky to translate. The wetted area index is the default value set by the system. Referring to API521, the original source of this data can be found in NPFA30 (the National Fire Protection Association), in the sections related to above-ground storage tanks. Let’s discuss this together. The calculation method of domestic standards implies this formula. Below is the calculation for the latent heat value; the default is “yes”” ； The latent heat value defaults to 116.3 kj/kg; this is the system’s default value. I’ve been struggling for days trying to find the source of this value, and I would appreciate it if anyone who knows the answer could provide some guidance. The default final vaporization rate is 10%; this is related to the duration of the fire, and a case-by-case analysis is required. When the wetted area remains constant, the vaporization rate stays fixed, and this has no impact on the diameter of the safety valve. The other options are mainly for setting the internal hydraulic conditions; the default values are used here. The third section relates to equipment parameters and installation-related information; multiple devices can be added here, and the details should be filled in based on the actual information of those devices. In our case, it is a horizontal 200 m3 storage tank, with a full liquid level of 3.8 m and a foundation elevation of 0.8 m. As for environmental factors, there is no insulation layer nor sprinkling system here, so the value is 1. For more details, refer to API521. The remaining fields can be left at their default values. Once all fields are filled in, the software performs the calculation automatically. Part four mainly shows the calculated discharge diameter. Click to select, and the software will automatically match it. If valve manufacturers provide emission coefficient data, it can be filled in based on the supplier’s data. Thus, the calculation of the safe discharge volume and throat diameter under fire conditions is complete. It is possible to query the properties of fluids; here the discharge temperature is shown to be 89.87°C. The design temperature of the equipment should be adjusted accordingly. Pipeline calculations are carried out using the relevant tags, and the design is based on the actual installation conditions of the pipelines. It is necessary to verify the critical flow velocity and back pressure values. Avoid exceeding the speed of sound and excessive back pressure (dynamic back pressure), as this can cause vibrations and reduce the discharge capacity.
This post was last edited by xyq1983 on 2021-2-17 08:01, using the data tables generated by Aspen Basic Engineering
The vaporization latent heat should be the latent heat assumed when 10% of the stream vaporizes. For logistics with a very low compressibility coefficient, the HEM method is recommended.
Does latent heat change with the vaporization rate?
Does latent heat change with the vaporization rate?
This post was last edited by xyq1983 on 2021-2-18 at 16:18. It’s not clear what needs to be explained; I don’t quite understand it from this picture. This graph allows one to look up the latent heat value of pure substances, and this latent heat value is a function of temperature. This is likely the system’s default value of 116.3 kj/kg = 50 BTU/lb; the software’s calculation results are quite accurate, and it can be used as an initial value in certain calculation processes. But when calculating the breathing valve, this value doesn’t change; I don’t understand why, and I couldn’t find any relevant explanations in the help files. Let’s discuss this together. Hybrid computing is more complex; I’ll find time to do a comparison under different operating conditions.
Specifying a Fire Relief Load Calculation: Wetted (API)
In wetted cases, boiling liquid enters the vessel as a result of an external pool fire. You can calculate the relief load for a vessel that contains liquid under relief conditions by either estimating the latent heat of the multicomponent liquid based on the energy required to vaporize a certain mol fraction, or by manually specifying the latent heat value. Aspen Plus uses the methodology outlined in one of the four viable options listed in the “Guidelines for Pressure Relief and Effluent Handling Systems” by the Center for Chemical Process Safety (CCPS). The Wetted calculation method estimates the latent heat of vaporization based on the energy required to vaporize a specified portion of the liquid present in the vessel on a molar basis. The required relief load is calculated in accordance with API 521, 6th Edition, Section 4.4.13.2.4.2. Optionally, you can enable prediction of two-phase relief flow by using the “Calculate Vapor/Liquid Disengagement” dropdown list. To view the relevant equations, refer to Wetted Fire Calculations.
Specifying Calculation Parameters for Wetted Fire Cases
To specify the calculation parameters for wetted fire cases:
On the Scenario Setup tab, in the Relief Load section, select “Wetted (API)” from the Calculation Method dropdown list. In the Wetted Area Exponent field, you can adjust the value used for fire calculations involving partial containment; the default value is 0.82. This default value is appropriate for open fires, where neither the fire nor the vessel is located within a confined space. Under certain conditions of partial containment (such as the presence of embankments or walls up to the vessel’s height), a value of 1 is recommended, as stated in API 521, 6th edition (2014), Section 4.4.13.3. From the Drainage and Firefighting dropdown list, choose whether drainage and firefighting measures are Present or Absent; the default value is Absent. Your selection affects the constant in the fire heat flux equation outlined in API 521, Section 4.4.13.2.4.2. From the “Calculate Latent Heat?” dropdown list, decide whether to specify the latent heat value directly or to calculate it based on the energy required to vaporize a specified portion of the selected fluid. It is recommended to estimate the latent heat for mixtures with a wide boiling point range; the default value is Yes. In the Latent Heat field, enter the desired value; the default value is 116.3 kJ/kg (Btu/lb). The latent heat value must be greater than 0. In the Initial % Vaporized field, specify the initial mol percent of the fluid that will be vaporized during the calculation; the default value is 0 mol%. In the Final % Vaporized field, specify the final mol percent of the fluid that will be vaporized; the default value is 10 mol%, and this value must be greater than that specified in the Initial % Vaporized field. From the Sensible Heat dropdown list, choose whether to include sensible heat in the latent heat calculation; the default selection is Exclude. From the “Calculate Vapor/Liquid Disengagement” dropdown list, select one of the following options: No (the default choice) or Yes. When you select “Yes,” vapor/liquid disengagement is calculated using DIERS methods to determine whether a two-phase relief phase will occur. See Specifying Vapor/Liquid Disengagement Calculation Parameters below. The total exposed area and the total required relief load are displayed in the Fire Load Calculation Results table.
In wetted cases, boiling liquid enters the vessel as a result of an external pool fire. You can calculate the relief load for a vessel that contains liquid under relief conditions by either estimating the latent heat of the multicomponent liquid based on the energy required to vaporize a certain mol fraction, or by manually specifying the latent heat value. Aspen Plus uses the methodology outlined in one of the four viable options listed in the “Guidelines for Pressure Relief and Effluent Handling Systems” by the Center for Chemical Process Safety (CCPS). The Wetted calculation method estimates the latent heat of vaporization based on the energy required to vaporize a specified portion of the liquid present in the vessel on a molar basis. The required relief load is calculated in accordance with API 521, 6th Edition, Section 4.4.13.2.4.2. Optionally, you can enable prediction of two-phase relief flow by using the “Calculate Vapor/Liquid Disengagement” dropdown list. To view the relevant equations, refer to Wetted Fire Calculations.
Specifying Calculation Parameters for Wetted Fire Cases
To specify the calculation parameters for wetted fire cases:
On the Scenario Setup tab, in the Relief Load section, select “Wetted (API)” from the Calculation Method dropdown list. In the Wetted Area Exponent field, you can adjust the value used for fire calculations involving partial containment; the default value is 0.82. This default value is appropriate for open fires, where neither the fire nor the vessel is located within a confined space. Under certain conditions of partial containment (such as the presence of embankments or walls up to the vessel’s height), a value of 1 is recommended, as stated in API 521, 6th edition (2014), Section 4.4.13.3. From the Drainage and Firefighting dropdown list, choose whether drainage and firefighting measures are Present or Absent; the default value is Absent. Your selection affects the constant in the fire heat flux equation outlined in API 521, Section 4.4.13.2.4.2. From the “Calculate Latent Heat?” dropdown list, decide whether to specify the latent heat value directly or to calculate it based on the energy required to vaporize a specified portion of the selected fluid. It is recommended to estimate the latent heat for mixtures with a wide boiling point range; the default value is Yes. In the Latent Heat field, enter the desired value; the default value is 116.3 kJ/kg (Btu/lb). The latent heat value must be greater than 0. In the Initial % Vaporized field, specify the initial mol percent of the fluid that will be vaporized during the calculation; the default value is 0 mol%. In the Final % Vaporized field, specify the final mol percent of the fluid that will be vaporized; the default value is 10 mol%, and this value must be greater than that specified in the Initial % Vaporized field. From the Sensible Heat dropdown list, choose whether to include sensible heat in the latent heat calculation; the default selection is Exclude. From the “Calculate Vapor/Liquid Disengagement” dropdown list, select one of the following options: No (the default choice) or Yes. When you select “Yes,” vapor/liquid disengagement is calculated using DIERS methods to determine whether a two-phase relief phase will occur. See Specifying Vapor/Liquid Disengagement Calculation Parameters below. The total exposed area and the total required relief load are displayed in the Fire Load Calculation Results table.
In the case of a mixture, as the amount of evaporation increases, the components in the liquid phase become heavier, and the discharge temperature rises.
I tested it with water, and the values for the latent heat of vaporization were different, which indicates that the software calculated them using different components.