Jump Start: Storage Tank Protection in Aspen HYSYS and Aspen Plus

Transcription

1 Jump Start: Storage Tank Protection in Aspen HYSYS and Aspen Plus A Brief Tutorial (and supplement to training and online documentation) Anum Qassam, Product Management, Aspen Technology, Inc. Jennifer Dyment, Product Marketing, Aspen Technology, Inc.

2 Table of Contents Introduction Initial Setup Adding a Storage Tank Tank Design Tab Initial Set Up Tank Design Inputs Insulation Details Normal Venting Tab Initial Set Up Normal Venting Inputs Normal Venting Results Emergency Venting Tab Initial Set Up Emergency Venting Inputs Emergency Venting Results Conclusions References Additional Resources

3 Introduction Without appropriate safety precautions, low pressure storage tanks can fail due to excessive pressure or vacuum. The Safety Environment in Aspen HYSYS and Aspen Plus provides calculation methodologies to quantify the required normal and emergency venting capacity of a low-pressure storage tank in accordance with the API This is not meant to be used as a stand-alone reference document. AspenTech recommends that a range of other resources be referenced to give the new user a comprehensive view of how to use Aspen HYSYS and Aspen Plus. These may include: Jump Start Guide: Relief Sizing in Aspen HYSYS and Aspen Plus V8.6 AspenTech support website (support.aspentech.com) AspenTech courseware available in on-line and in-person versions AspenTech business consultants Additional jump start guides available on a variety of related topics This document provides a detailed overview of the steps required to determine the normal and emergency venting requirements for low pressure storage tanks within the Safety Environment of Aspen HYSYS and Aspen Plus. Aspen Plus or Aspen HYSYS can be used to follow this jump start guide. Use the example file PRESSURE RELIEF STARTER.hsc in Aspen HYSYS or the example file Safety Analysis without PRD.bkp in Aspen Plus. Both files are available on aspenone Exchange. Initial Setup All examples and screenshots in this guide are based on the example file PRESSURE RELIEF STARTER.hsc in Aspen HYSYS. If more familiar with Aspen Plus, use the Safety Analysis without PRD.bkp file instead. Make sure that the correct units are selected when specifying parameters when prompted. To begin, you should: 1. Open the file PRESSURE RELIEF STARTER.hsc or Safety Analysis Without PRD.bkp 2. Run the case (Aspen Plus only) 3. Enter the Safety Analysis Environment Adding a Storage Tank This section describes how to add storage tanks to the Safety Environment. Open the Tank Manager by clicking on the Storage Tank Protection button on the Home tab of the ribbon, shown in Figure 1. You can get to the Tank Manager by selecting the Storage Tank Protection heading on the Navigation Pane, shown in Figure 2. 1

4 Figure 1: Open Tank Manager from Ribbon Figure 2: Open Tank Manager from Navigation Pane The Tank Manager should be visible. Click the Add button to add a new storage tank, as shown in Figure 3. Figure 3: Add a Storage Tank on the Tank Manager Tab Clicking the Add button will result in a new row added to the Tank Manager. Open up the Storage Tank tab by either (1) selecting the newly-created entry and clicking the Edit button (see Figure 4) or (2) double-clicking on the Tank Manager row to open up the Storage Tank tab. Figure 4: Open a Storage Tank Form 2

5 Tip: You can quickly add a storage tank by opening up the Storage Tank tab by right-clicking the Storage Tank Protection heading on the Navigation Pane and selecting Add Storage Tank from the menu that appears, as shown in Figure 5. Figure 5: Adding a Storage Tank from Navigation Pane You should now see the Storage Tank form, which is shown in Figure 6. Figure 6: Tank Design tab 3

6 Tank Design Tab This section details the various inputs in the Tank Design tab, and the effect of the input on the normal and emergency venting calculations. Initial Set Up In the Tank Design tab, the Tank Name, Service, and Tank Design Code do not affect the storage tank calculation. These inputs are only for reporting purposes their parameters are highlighted in Figure 7. Figure 7: Tank Parameters for Reporting Purposes Only The Tank Type determines which equations are used to calculate the tank volume and tank exposed area to determine the heat input for the emergency venting calculation. Available values for the Tank Type are shown in Figure 8. Technical details on the calculation are available in the F1 help. Figure 8. Tank Type Available Values Select a Stream from the simulation environment to represent the fluid inventory in the tank. The Safety Environment will use the selected Stream's properties for the following variables: Tank Vapor Molecular Weight, Relieving Gas Temperature, and Relieving Gas Molecular Weight. To select a stream from the simulation environment, check the Use Stream box, shown in Figure 9. Figure 9: Use Simulation Stream to Represent Tank Fluid In the dialog box that appears, select the stream from the Simulation Environment that best represents the fluid in the tank. If following along with the example file, select the stream FeedStream, then click the OK button. These steps are highlighted in Figure 10. Figure 11 displays the completed setup. 4

7 Figure 10: Select Reference Stream Dialog Box Figure 11: Example of Completed Storage Tank Set Up Tank Design Inputs This section details the tank-specific parameters necessary for the normal and emergency calculations. Figure 12 shows sample values for each parameter. Each parameter is then described in detail beneath the figure. Figure 12: Tank Design Sample Inputs 5

8 1. Barometric Pressure: This value is used to adjust gauge pressures in this tank system only. 2. Design Pressure: Maximum pressure that the tank wall and structure can handle. 3. Operating Pressure: Pressure during the normal operation of the tank. This value must be less than the Design Pressure of the tank and the Set Pressure of protecting devices. 4. Diameter: Diameter of the tank. 5. Height: Height of the tank. 6. Maximum Operating Temperature: Maximum possible temperature during the normal operation of the tank. This value is used to determine the Inbreathing/Outbreathing flow relationship used and Thermal C-factor for Normal Venting calculations. This value must be less than the design temperature of the tank. 7. Latitude: Select the latitude of the system, which is used to determine the Y-factor in the thermal Inbreathing/Outbreathing in the Normal Venting calculations. 8. Vapor Pressure: Select the type of Vapor Pressure, which is used to calculate the C-factor for the thermal Inbreathing/Outbreathing in the Normal Venting calculations. 9. Evaporation Rate: Evaporation rate of the products in the tank. For products stored above 40 C with Vapor Pressure of less than 5 kpa, the Evaporation Rate is added to the Outbreathing term. Use zero when the Maximum Operating Temperature is less than 40 C (104 F). Insulation Details This section describes how to account for insulation on the tank (if needed). If no insulation is present, then Insulation Area Fraction is 0 and all other parameters are deactivated. Figure 13 shows sample values for each parameter, along with descriptions for each below. Figure 13: Insulation Sample Inputs 1. Insulation Area Fraction: In this field, type a real value between 0 and 1. The default value is 0 and is calculated as the ratio of the insulated surface area of the tank to the total tank surface area (including the shell and the roof of the tank). Use 1 for fully insulated tanks, 0 for non-insulated tanks, and 0 < Ainsulated / Atotal < 1 for partially insulated tanks. 2. Insulation Thickness: In this field, type the wall thickness of the insulation layer. 3. Thermal Conductivity (Normal): In this field, type the value of the Thermal Conductivity, λ, of the insulation material at operating temperature conditions. This value is used in the Normal Venting calculations to get the thermal breathing insulation reduction factor. 4. Thermal Conductivity (Fire): In this field, type the value of the Thermal Conductivity, λ, of the insulation material at fire temperature conditions (538 C or 1000 F). This value is used in the Emergency Venting calculations to obtain the environmental (F) factor. 6

9 Normal Venting Tab This section describes how to calculate the normal venting requirements (Inbreathing and Outbreathing). Initial Set Up Click on the Normal Venting tab shown in Figure 14 in the example file. Figure 14: Normal Venting Tab There are instances where Normal Venting calculations are not applicable, such as with floating roof tanks. You can disable the Normal Venting calculations for a tank by unchecking the Calculate Normal Venting Size check box. For the purposes of this example, the box will remain checked (see Figure 15). Figure 15: Disable Normal Venting Calculations The Normal Breathing Device Name and Breathing Device Type highlighted in Figure 16 are for documentation purposes only. Figure 16: Normal Venting Parameters for Reporting Purposes Only 7

10 Normal Venting Inputs In order to continue the Inbreathing and Outbreathing calculations, specify the PRD-specific calculaton parameters. Figure 17 shows sample values for each parameter, along with descriptions for each below. Figure 17: Normal Venting Sample Inputs 1. Set Pressure: Type a real value between 0 and 15 psig. This is the pressure at the inlet of the device, at which the relief device starts opening under service conditions. 2. Vacuum Set Pressure: Type a real value between 0 and 15 psig that is less than the tank's Operating Pressure. This value is the pressure at the inlet of the relieving device that protects vacuum conditions. 3. Maximum Liquid Inflow: Maximum volumetric tank-filling rate; used to determine the Outbreathing flow. 4. Maximum Liquid Outflow: Maximum volumetric tank discharging rate; used to determine the Inbreathing flow. 5. Tank Vapor Molecular Weight: Molecular weight of vapor above the liquid content of the tank. The default value is Normal Venting Results After you type values for the basic information and the Normal Venting Inputs, calculated values for Inbreathing and Outbreathing appear for the parameter shown in Figure 18. Each parameter is then described in detail beneath the figure. Figure 18: Normal Venting Sample Results 1. Liquid Movement: The total Inbreathing/Outbreathing volumetric flow, expressed as flow of air (Nm3/h normal conditions at 0 C/32 F or SCFH standard conditions at 15.6 C/60 F). 2. Thermal: Normal Inbreathing originating from a maximum decrease in the vapor-space temperature leading to contraction or condensation of vapors. Normal Outbreathing originating from a maximum increase in the vaporspace temperature leading to contraction or condensation of vapors. 3. Total: Sum of thermal and liquid transfer effects. 8

11 4. Preliminary Device Area: The preliminary area required for the calculated volumetric Inbreathing and Outbreathing relief load. 5. Preliminary Calculated Size: The preliminary orifice diameter required for the calculated volumetric Inbreathing and Outbreathing relief load. Emergency Venting Tab This section describes how to calculate the emergency fire venting requirement. Initial Set Up Click on the Normal Venting tab shown in Figure 19 in the example file. Emergency Device Name and Emergency Device Type, highlighted in Figure 19, are for documentation purposes only. Figure 19: Emergency Venting Tab There are instances where Emergency Venting calculations are not applicable (no flammable inventory within tank farm, for example). The user can disable the Emergency Venting calculations for a tank by unchecking the Calculate Emergency Venting Size check box. For the purposes of this example, the box will remain checked (see Figure 20). Figure 20: Disabling Emergency Venting Calculations 9

12 Emergency Venting Inputs In order to continue the Emergency Venting calculations, the user must specify the PRD-specific calculation parameters. Figure 21 shows sample values for each parameter, along with descriptions for each below. Figure 21: Emergency Venting Inputs Notes: Wetted area calculations are in accordance with API 2000 rules and may differ from API 521. Standard flame height is different for the two API standards. The Design Pressure (higher or lower than 1 psig) heavily affects the wetted area calculation for larger tanks. 1. Set Pressure: Pressure at which the relieving device starts to open upon service conditions. Type a real value between 0 and 15 psig. 2. Bottom Tan Above Grade: The vertical distance to the vessel from the nearest surface which could contain a pool of flammable liquid. 3. Flame Height From Grade: Maximum height the flame would reach around the exterior of the tank. The default value is 30 ft. 4. Calculate Environmental (F) Factor: Select Yes or No from the drop-down list. The default value is No. If you select Yes, the Environmental (F) Factor is calculated using the fire insulation properties from the Tank Design tab and API If you select No, you must manually type the Environmental (F) Factor. 5. Environmental (F) Factor: The factor used to account for tank protection reducing the fire relief rate, such as by insulation or fireproofing equipment. Type a real value between 0 and 1. The default value is 1. Use 0 for total protection and 1 for no protection. 6. Relieving Gas Temperature: Temperature of the vapor leaving the tank at the inlet of the relieving device. This temperature is usually assumed to be the boiling temperature of the liquid stored in the tank. 7. Relieving Gas Molecular Weight: Molecular weight of relieving vapor that vaporizes from the stored liquid. 8. Fluid Latent Heat: The latent heat of vaporization of the stored liquid at relieving conditions. The default value is 27.7 kcal/kg (or 50 Btu/lb). 10

13 Emergency Venting Results After you type values for the basic information and the Emergency Venting Inputs, calculated results appear, as shown in Figure 22. Figure 22: Emergency Venting Results 1. Calculated Exposed Area: Wetted Exposed Area of the tank. This is the area of the portion tank which is in contact with both the flame and the liquid. Calculated as a percentage of the total surface area and/or area below the maximum flame height. 2. Calculated Heat Input: Heat Input from exposure to fire. Calculated from a range of formulae that depend on the Design Pressure and Exposed Area. 3. Calculated Relieving Flow: Venting requirement. This is the total flow that the relieving device is required to vent off the tank to ensure complete protection. 4. Preliminary Device Area: The preliminary area required for the calculated volumetric relief load. 5. Preliminary Calculated Size: The preliminary orifice diameter required for the calculated volumetric Outbreathing relief load. Conclusions API 521 recommends that all vessels under 25 ft. in elevation be protected for the fire overpressure scenario. Depending on the protected system, varying levels of rigor may be required in determining the required relieving load. The Safety Environment in Aspen HYSYS and Aspen Plus provides rigorous determination of the required fire relief load. Users can now size relief devices for the fire overpressure scenario quickly and easily. References 1. American Petroleum Inst., Venting Atmospheric and Low-pressure Storage Tanks, ANSI/API Std 2000, 6th Ed., November

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