WP_Fire Case BLOWDOWN_FINAL.pdf

BLOWDOWN Tech Technology nology in Aspen HYSYS Getting Gett ing Started Guide for Fire Case Depressurization Depressur ization Analysis Anum Anu m Qassam, Senior Produ Product ct Manag Manager, er, Safety Analysis

Introduction..................... ........................................... ............................................. ..........................2 Audience ......................................................... ....................................... ....................................2 Associated Files ..................................................... ........................................... ...........................2 Learning Objectives ................................ ............................................ .........................................2 Specifying Heat Transfer Model ................................. ................................................ ..........................3 API 521 Heat Flux Model ................................................... ............................................. .................3 Overview ..................................................................................... ........................................... ....3 Data Requirements...................................................................................... ................................3 Constant Heat Flux Model ............................ ........................................ ...........................................6 Overview ..................................................................................... ........................................... ....6 Data Requirements...................................................................................... ................................6 Design Orifice for Fire Case ................................................... ............................................. .................9 Overview ..................................................................................... ........................................... ....9 Procedure....................................................... ......................................... ...................................9 Conclusion .................................................... ............................................ ....................................... 16

© 2016 Aspen Technology, Inc. CONFIDENTIAL -- For Interna l Use Only

A pool fire on a syste m often represents the scenario which results in the greatest peak mass flow rate that the blowdown valve (BDV) must handle (since the liquid will likely vaporize over the course of the fire, causing a rise in system pressure, which both introduces more possible vapor to the flare system and establishes a greater driving force to the disposal system). This tutorial will show you the key steps of setting up a de sign study for a BDV using BLOWDOWN Technology in Aspen HYSYS.

This is an intermediate-level tutorial intended for process engineers who engage in the desi gn or revalidation of blowdown valves; this tutorial assumes that you have already revi ewed the beginnerlevel Computer-Based Training available on the support website, and that you have f amiliarity with simulating streams and unit operations in Aspen HYSYS.

Deployed with Aspen HYSYS is an example file that shows how to use the BLOWDOWN Analysis tool to design an orifi ce for the fire emergency depressurization case. 

To foll ow along with this tutorial, open up the associated file BLOWDOWN Fire Case Depressurization.hsc.

This document will describe the following key steps in using the BLOWDOWN Analysis tool for designing a blowdown valve for the fire case: 

Specifying the Heat Transfer Model



Designing the BDV Orifice


The first and most important step is to identify (either using local regulatory guidelines, or internal company guideli nes) which heat flux model to use when si mulating the fi re. The BLOWDOWN Analysis tool allows you two industry standard heat flux model s: (1) API 521 Heat F lux Model outli ned in Section 4.4.13.2.4.2 of API 521 (6e) and (2) a constant heat flux in accordance with global standards (e.g., NORSOK). This section detail s the steps for specifying either heat flux model. Note: This text, for brevity, will refer to the “API 521 Heat Flux Model.” To be exact, this is the empirically-determined heat flux model outlined in Section 4.4.13.2.4.2 of API 521 6e. Note that alternative fire modelling methods are outlined in other sections of API 521 6e, but this is n ot the intende d interpretation of the terminology “API 521 Heat Fl ux Model”.

This section describe s how to simulate a pool fi re in BLOWDOWN in accordance with guidance in the 6th edi tion of API 521. This section also provides some engineering guidance -- but always confirm that your analysis is aligned wi th local regulatory and internal company gui delines.

In order to analyze a system exposed to a pool fire in accordance with Section 4.4.13.2.4.2 in API 521 (6e), first select Apply to liquid as the Heat Flux Method on the Heat Transfer tab of the unit operation exposed to the pool fire as shown in Figure 1. Some guidance for the data requirements is provided below Figure 1 – more detail s and descriptions are available in the F1 Help in Aspen HYSYS.


Figure 1. Apply to liquid data requirements

#

Data

1

In Fire Zone?

2 3

Fire Heat Flux Method Fire Heat Flux

Description This check box must be sele cted in order to analyze depressurization for a system under fire. Apply to liquid is the appropriate selection to model fire i n accordance with API 521 6e Secti on 4.4.13.2.4.2. The (calculated) constant heat flux that is appli ed to the liqui d inventory at each time step during the run. This selection affects the C constant in the API heat flux model . The

4

Adequate

determination of what is ”adequate ” drainage is up to the engine er, but in

Drainage &

general the re should be drainage that carries hazardous liquids away from

Firefighting

the vessel and provides sufficient protection via a firewater system or similar.


Per API 521 (6e), for an open fire, the wetted area should be raised to the 0.82 power. For a confined fi re, re-radiation of the heat due to surrounding 5

Open Fire

walls or equipment may cause higher heat flux es than an open fire . Therefore, conservatively, when this check box is cleared, the wetted surface area is raised to the 1.0 power (versus 0.82). This selection allows you to model the heat transfer between the fluid in the vapor phase and the environment. When using the API 521 heat flux model, the recommendation is to select None, ignoring any heat transfer that may

Vapor Zone 6

occur between the vapor inventory and the environment. This is because the

Ambient Heat

API 521 heat flux model was empiri cally derived, whereas BLOWDOWN’s

Transfer

heat transfer model between the fluid and the environment is based on fundamental heat transfer models. Combining the two di fferent models may result in unpredi ctable results – results which may not accurately represe nt reality. The environmental factor is a number between 0 and 1 which effectively reduces the fire heat flux to the liquid. When taking credit for insulation for

7

Environmental (F) Factor

the API heat flux model, the recommendation is to estimate an F factor when taking credit for insulation ( versus specifying a layer of insulation or cladding). If you need assistance in estimating this paramete r, sections 4.4.13.2.7.2 through 4.4.13.2.7.4 of API 521 6e provide guidance. When includi ng insulation, BLOWDOWN will rigorously model heat transfer between the fluid, any cladding, the wall, the insulation, and the environment to more accurately predict wall temperatures using the

8

Include Insulation

insulation’s conductivity, diffusivity, and thickness. For the fi re case using the

API 521 heat flux model, i t is not recommended to rigorously model the insulation, as there is no guarantee that combining the API 521 empirical model wi th fundamental heat transfer models will result in any realisti c temperature predictions of the insulation or wall. When includi ng cladding, BLOWDOWN will rigorously model heat transfer between the fluid, the cladding, the wall, any insulation, and the environment to more accurately predict wall temperatures using the

9

Include Cladding

cladding’s conductivity, di ffusivity, and thickness. For the fire case usi ng the

API 521 heat flux model, it is not recommended to rigorously model the cladding, as there is no guarantee that combining the API 521 empi rical model wi th fundamental heat transfer models will result in any realisti c temperature predictions of the cladding or wall.

After specifying the heat flux parameters, confirm that you have accurately captured the initial volume holdup in the system on the Initial Condition tab, as shown in Figure 2. This effects the wetted surface area calculation, which then impacts the calcul ated heat flux using the API 521 heat flux model.


Figure 2. Specify initial liquid holdup

Note: The discussion in this section was around vessels which contain a liquid holdup, since there is expl icit guidance for such systems in API 521 6e. Annex A of API 521 6e also indicates that the API heat flux model “can also be used to calculate the pressure profil e for vessels, piping, and

other equipment exposed to fuel -controlled pool fire.” In the BLOWDOWN Analysi s tool, you may specify the same heat transfer parameters for a pipe that you can for a vessel. It is up to each engineer to determine whether the use of the API heat flux model for wetted piping is appropriate.

This section describes how to simulate a pool fire in BLOWDOWN using a constant heat flux to the outer “layer” of the vessel . This section also provides some engineering guidance -- but always confirm that your analysi s is aligned to your company’s guidelines.

The Apply to wetted wall and Apply to all wall options for the Heat Flux Method both allow you to define a constant heat flux to the outer layer of the unit operation (see Fi gure 3).

Figure 3. Constant Heat Flux Model Options

Data

Description

Apply to

This selection applies the constant heat flux onl y to the outside layer of the liquid

wetted wall

(or wetted) zone of the vessel. If the ve ssel becomes fully vapor-filled over the


course of the depressuri zation, then no heat will be applied to the system (see Figure 4). Apply to all

This selection applies the constant heat flux to the enti re vessel outside layer. This

wall

is the only heat flux method which is valid for simulating fire heat flux to a vaporfilled system.

The above Heat Flux Method selections differ with respect to the fluid zone to which the heat flux i s applied, as shown in Figure 4.

Figure 4. Heat Flux Method Zone Applicability

Some guidance for each of the data requi rements is provided below Figure 5 – more details and descriptions are avail able in the F1 Help in Aspen HYSYS.


Figure 5. Apply to all wall data requirements

#

Data

1

In Fire Zone?

2 3

Fire Heat Flux Method Fire Heat Flux Vapor Zone

4

Ambient Heat Transfer

Description This check box must be sele cted in order to analyze depressurization for a system under fire. Apply to all wall is the most common way to model fi re heat flux to the vessel wall surface. The (specified) constant heat flux that is applied to the entire outer laye r of the vessel at each time step during the run. This selection allows you to model the heat transfer between the fluid in the vapor phase and the envi ronment. This is rigorously calculated when Apply to all wall is selected as the Heat Flux Method, but may be excluded if Apply to wetted wall is selected. When includi ng insulation, BLOWDOWN will rigorously model heat transfer between the fluid, any cladding, the wall, the insulation, and the

5

Include Insulation

environment to more accurately predict wall temperatures using the insulation’s conductivity, diffusivity, and thickn ess. When taking credit for

insulation, take care to ensure that the insulation i s fireproof. Including the effects of insulation is generally less conservative.


6

Material

This selection allows you to model the insulation as Cellular Glass or to User Define a custom material. When User Defined is selected as the Material, you must provide the

7

Thermal Conductivity

thermal conductivity of the i nsulation. This parameter specifies the degree to whi ch the material conducts heat. Note that Table 6 in Section 4.4.13.2.7.3 of API 521 6e provides thermal conductivity values for typical thermal insulations. When User Defined is selected as the Material, you must provide the

8

Thermal Diffusivity

thermal dif fusivity of the insul ation. This parameter measures the ability of a material to conduct thermal energy relati ve to its abili ty to store thermal energy. Several publications on heat transfer provide diffusivity values for a variety of materials. When includi ng cladding, BLOWDOWN will rigorously model heat transfer

9

Include Cladding

between the fluid, the cladding, the wall, any insulation, and the environment to more accurately predict wall temperatures using the cladding’s conductivity, diffusivity, and thickness.

When designing a blowdown valve, the objective is to identi fy the minimum required orifice size which will satisfy the design criteria. The design criteria may be based on regulatory standards or a more detailed stress rupture analysis. Generally speaking, the design depressurization rate is determined based on the rate needed to ensure stress rupture is not an immediate concern during a pool fire scenario, since the pool fire scenario generally results in a higher depressurization rate than non-fire situations. This section will hi ghlight, using an example, how to design an orifice for the fi re case using BLOWDOWN Technology in Aspen HYSYS. In this example, the API 521 design guidance is used, whi ch recommends that for carbon steel vessels of 1” or greater thickness, reducing the equi pment pressure to 50% of the

design pressure within 15 minutes is appropriate.

To design a BDV orifice, an iterative procedure is needed where the pressure at 15 minutes is tracked. The orifice si ze is iteratively decreased or increased, and then the analysis re-run, in order to determine the minimum orifice size. The HYSYS Adjust block makes the iterative solving procedure for solving the minimum orifice diameter much easier. This section wi ll describe the procedure to use the BLOWDOWN Analysis tool with an Adjust block in HYSYS to minimize the orifice size. The steps of the procedure are listed below and described in detail in this section: © 2016 Aspen Technology, Inc. CONFIDENTIAL -- For Interna l Use Only



Run the BLOWDOWN Analysis with an Initial Orifice Guess  Set up the Adjust Block 

Select the Run BLOWDOWN on Input Change check box  Run the Adjust solver

This section detail s how to specify the orifice diameter and set up the Run Controls tab; this section assumes that the fire heat flux to the vessel i s modelled according to the API Heat Flux Model. 

For this example , use an estimate of 1” diameter for the orifi ce as shown in Figure 6.

This is an i nitial guess. As a general rule, use an orifice diameter where the BDV is likely choked. The diameter must be smaller than the inlet l ine to the orifi ce, as well as the outlet line from the orifice.

Figure 6. Run BLOWDOWN with initial orifice diameter guess



On the Run Control tab, specify the halt condition time as 15 minute s (900 seconds) and the halt pressure as atmospheric, as shown in Figure 7. Do not use the halt pressure to represent the pressure design criteria (50% of the design pressure). We will use the HYSYS Adjust block to meet this requirement.


Figure 7. Halt Conditions Specification



Run the analysis. Run the analysis one ti me through to ensure that there are no errors with the run.

This section wil l illustrate how to set up the HYSYS Adjust block. The Adjust block will be use d to iterative ly change the orifice diameter used in the BLOWDOWN Analysis tool. It will monitor the final BLOWDOWN pressure at the end of 15 minutes and adjust the diameter appropriately to try to meet the pressure design criteria (50% of the syste m design pressure, in this case). 

From the Model Palette (shortcut key is F4), add the Adjust block to the HYSYS main flowsheet, as shown in Fi gure 8.

Figure 8. Add HYSYS Adjust Block



Double-click the ADJ-1 icon to open the Adjust block.




Select the BLOWDOWN Orifice Diameter as the Adjusted Variable, as shown in Figure 9. The HYSYS Adjust block functions much like Goal See k in Excel. The objective is to iterate on the orifice diameter, until the final pressure at 15 minutes is 50% of the design pressure.

Figure 9. Specify Adjusted Variable



Select the BLOWDOWN Final Pressure as the Target Variable, as shown in Figure 10. The Final Pressure is the pressure calculated by BLOWDOWN at the Halt Time specified on the Run Controls tab.


Figure 10. Specify Target Variable



Specify the pressure design criteria as the Specified Target Value, as shown in Figure 11. In thi s example, a target pressure value of 375 psia will be used. This pressure should represent the design pressure criteria for the system, which in thi s example is 50% of the system design pressure.


Figure 11. Specify Target Value as Pressure Design Criteria



On the Parameter tab, change the Tolerance, Step Size, and Iterations, as shown in Figure 12. Suggested values for a reasonable initial tolerance and step size are shown in Figure 12 below. The step size should be smaller than the initial ori fice guess, and the pressure tolerance should be gene rous in the first pass.

Figure 12. HYSYS Adjust Block Recommended Parameters




Double-click on the BLOWDOWN icon, and check the box “Run BLOWDOWN on Input Change” This selection allows the HYSYS Adjust block to “control” the BLOWDOWN Analysis tool . If this check box i s cleared, the HYSYS Adjust block will not work properly with the BLOWDOWN tool.

Figure 13. Allow Adjust Block to control BLOWDOWN block



Click the Start button on the HYSYS Adjust block. This step can take some time. You should monitor the convergence on the Monitor tab. I recommend keeping the Adjust block number of iterations small (5), so that you can monitor if the system is convergi ng, and make corrective actions as neede d. In thi s example, as shown in Figure14, the system ran for 11 iterations until adequate convergence was reached.


Figure 14. Final Orifice Size Result

The optimal size for the orifi ce is 1.536 inches, whi ch is when the final pressure at 15 minutes is 374.644 psia. The BDV vendor can select the next standard orifice size up from this in order to ensure that the system is adequately sized for the fire scenario.

The BLOWDOWN Analysis tool is the i ndustry-validated technology to assess cold-temperatures in equipment & process piping. It has also been used in industry to desi gn blowdown valves for the fire case scenario. The objective of this inte rmediate-level tutorial was to demonstrate how to use the HYSYS Adjust block with the BLOWDOWN Analysis tool in order to design the BDV for the fi re case scenario. Your continued f eedback helps us here at Aspe nTech create knowledge solutions that provide value. © 2016 Aspen Technology, Inc. CONFIDENTIAL -- For Interna l Use Only

You can provide f eedback on this and all other content: 

Contact AspenTech support  On the support website (www.support.aspentech.com), at the end of every solution is a link to provide feedback (see Figure 15).

Figure 15. Provide fe edback on Support Websi te



Inside the product, you can click on the Resourcestab, click the All Content button, and select Request new content to provide feedback on existing content or recommend new content.

Figure 16. In-Product Request for New Content


WP_Fire Case BLOWDOWN_FINAL.pdf

Recommend Documents