Storage Tank Protection_Jump Start

Jump Jum p Sta Start: rt: Stor Storage age Tan Tank k Pro Protec tectio tion n in Asp Aspen en HYS HYSYS YS and Asp Aspen en Plu Pluss • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

A Brief Tutorial (and (and supplement to training and online documentation) documentation) Anum Qassam, Product Management, Aspen Technology, Technology, Inc. Jennifer Dyment, Product Marketing, Aspen Technology, Inc.

© 2013 Aspen Technology, Inc. AspenTech , aspenONE , the aspenONE aspenONE logo, and the Aspen leaf logo are trademarks of Aspen Technology, Inc. All rights reserved. ®

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Jump Start: Pressure Relief Valve Sizing in Aspen HYSYS ••••••••••••••• ••••••••••••••••• •••••••••••••••••• ••••••••••••••

Contents Introduction .................................................................................................................................................. 1 Initial Setup ................................................................................................................................................... 2 Adding a Storage Tank .................................................................................................................................. 2 Tank Design Tab............................................................................................................................................4 Initial Set Up..............................................................................................................................................4 Tank Design Inputs....................................................................................................................................6 Insulation Details ......................................................................................................................................7 Normal Venting Tab ...................................................................................................................................... 8 Initial Set Up..............................................................................................................................................8 Normal Venting Inputs............................................................................................................................10 Normal Venting Results .......................................................................................................................... 10 Emergency Venting Tab .............................................................................................................................. 11 Initial Set Up............................................................................................................................................11 Emergency Venting Inputs......................................................................................................................12 Emergency Venting Results.....................................................................................................................14 Conclusions ................................................................................................................................................. 14 References .................................................................................................................................................. 14 Additional Resources .................................................................................................................................. 14

© 2013 Aspen Technology, Inc. AspenTech , aspenONE , the aspenONE logo, and the Aspen leaf logo are trademarks of Aspen Technology, Inc. All rights reserved. ®

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

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 2000. 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

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AspenTech support website (support.aspentech.com)

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AspenTech courseware available in on-line and in-person versions

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AspenTech business consultants

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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. Please refer to the previously published Jump Start Guide: Relief Sizing in Aspen HYSYS and Aspen Plus v8.6 for additional details on any of the aforementioned topics.

Either Aspen Plus or Aspen HYSYS can be used to follow this jump start guide. Following the jump start guide with the example file “PRESSURE RELIEF STARTER.hsc” in Aspen HYSYS or the example file “Safety Analysis without PRD.bkp” in Aspen Plus is strongly recommended. Both files are on aspenONE Exchange.


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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, users can use the “Safety Analysis without PRD.bkp” file instead. Make sure that the correct units are selected when specifying parameters when prompted. To begin, the user 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 the user can 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. The user can also get to the Tank Manager by selecting the Storage Tank Protection heading on the Navigation Pane, shown in Figure 2.

Figure 1: Open Tank Manager from Ribbon

Figure 2: Open Tank Manager from Navigation Pane

The user should now see the Tank Manager. Click the Add button to add a new storage tank, as shown on Figure 3.


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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. The user must 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

Quick Tip The user can quickly add a storage tank and open 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 The user should now see the Storage Tank form, which is shown in Figure 6.


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Figure 6: Tank Design tab

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. These parameters are highlighted in Figure 7.

Figure 7: Tank Parameters for Reporting Purposes Only


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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 The user can 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 Re lieving Gas Molecular Weight. To select t a stream from the simulation environment, check the Use Stream box, shown in Figure 9.

Figure 9: Use Simulation Stream to Represent Tank Fluid Then, a dialog box will appear. In the dialog box, select the stream from the simulation environment that best represents the fluid in the tank. For the user that is following along with the example file, select stream FeedStream. Then click the OK button. The steps are highlighted in Figure 10.


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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.


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Figure 12: Tank Design Sample Inputs (1) Barometric Pressure: This value is used to adjust gauge pre ssures 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) Max. 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 products in the tank. F or products stored above 40°C with vapor pressure of less than 5 kPa, the evaporation rate is added to the outbreathing t erm. Use zero when the maximum operating temperature is less than 40°C (104°F).

Insulation Details This section describes how to take cre dit for insulation on the tank (if needed). I f no insulation is present, then Insulation Area Fraction is 0 and all other parameters are deactivated. Figure 13 shows sample values for each parameter. Each parameter is then described in detail beneath the figure.


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Figure 13: Insulation Sample Inputs (1) In the Uncontained Area Fraction field, type a real value between 0 and 1. The default value is 1. This value tankes into account the reduced heat transfer to the environment for double wall tanks. The value is calculated as the ratio of the tank surface area not inside of the containment tank to the total tank surface are a. Note that this parameter is available after installation of v8.6 Cumulative Patch 1. (2) In the Insulation Area Fraction field, type a real value between 0 and 1. The default value is 0. This value 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 < A insulated / Atotal < 1 for partially insulated tanks. (3) In the Insulation Thickness field, type the wall thickness of the insulation layer. (4) In the Thermal Conductivity (Normal) 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. (5) In the Thermal Conductivity (Fire) field, type the value of the Ther mal 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.

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.


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Figure 14: Normal Venting Tab There are instances where normal venting calculations are not applicable (for floating roof tanks, for example). The user 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


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Normal Venting Inputs In order to continue the inbreathing and outbreathing calculations, the user must specify the PRDspecific calculaton parameters. Figure 17 shows sample values for each parameter. Each parameter is then described in detail beneath the figure.

Figure 17: Normal Venting Sample Inputs (1) API 2000 Edition : Select the API 2000 edition (upon which the inbreathing and outbreathing th

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calculations will be based). User can select the 6 edition, 7 edition, or the 7 edition Annex A th

(which is the same as the 5 edition). (2) 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. (3) 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 protecting for vacuum conditions. (4) Maximum Liquid Inflow : Maximum volumetric tank-filling rate; used to determine the out breathing flow. (5) Maximum Liquid Outflow : Maximum volumetric tank discharging rate; used to determine the inbreathing flow. (6) Tank Vapor Molecular Weight: Molecular weight of vapor above the liquid content o f the tank. The default value is 28.8.

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.


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Figure 18: Normal Venting Sample Results (1) Liquid Movement: The total Inbreathing/Out-breathing volumetric flow, expressed as flow of air (2)

(3) (4) (5)

(Nm3/h Normal conditions at 0°C/32°F or SCFH standard conditions at 15.6°C/60°F. Thermal: Normal inbreathing originating from a maximum decrease in the vapor-space temperature leading to contraction or condensation of vapors. Normal out -breathing originating from a maximum increase in the vapor-space temperature leading to contraction or condensation of vapors. Total: Sum of thermal and liquid transfer effects. Preliminary Device Area: The preliminary area required for the calculated volumetric Inbreathing and Outbreathing relief load. 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 em ergency 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.


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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 Emergency Venting Inputs In order to continue the emergency venting calculations, the user must specify the PRD -specific calculation parameters. Figure 17 shows sample values for each parameter. Each parameter is then described in detail beneath the figure.


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Figure 21: Emergency Venting Inputs Notes: •

Wetted area calculations are in accordance with API 2000 rules and may differ from API 521.

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Standard flame height is different for the two API standards.

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The design pressure (that is, 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 2000. 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 insulation or fireproofing equipment. Type a re al value between 0 and 1. T he default value is 1. Use 0 for total protection and 1 for no prote ction. (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).


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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 contact. Calculated as a percentage of the total surface area and/or area be low 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 re lief 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 now provides rigorous determination of the required fire relief load. Users can quickly and easily size relief devices for the fire overpressure scenario.

References 1. American Petroleum Inst., “Venting Atmospheric and Low-pressure Storage Tanks”, ANSI/API Std th

2000, 6 Ed., November 2009

Additional Resources


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Public Website:

http://www.aspentech.com/products/aspen-HYSYS.aspx

Online Training:

http://www.aspentech.com/products/aspen-online-training AspenTech YouTube Channel:

http://www.youtube.com/user/aspentechnologyinc


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Storage Tank Protection_Jump Start

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