Dynamic Simulation of Relief and Controlled Blowdown Cases

Pressure Relief load estimation is commonly estimated by taking system is approximately at "steady state condition". The estimated relief load could be excessive due to conservative assumption, unrealistic external energy inputs, etc. Current trend is to utilize Dynamic simulation in order to derive realistic but still preserve conservatism, integrity and safety of plant pressure relief and overpressure protection systems. This approach inline with API std 521 recommendation and probably the way to go in near future.

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This study presented the dynamic simulation of a gas compression system, proving the viability of operational philosophy and emergency shutdown logic with quantitative process responses in various situations. To avoid unnecessarily high peak in an initial stage of blowdown, this study employed the controlled blowdown and investigated its safety level.

This study concluded that dynamic simulation of start-up and emergency operation improved the operability of the whole process. The revealed transient behavior demonstrated that PSVs sized to API standard led to chattering because the standard gives excessive size. Choice of properly sized PSVs eliminated the chattering with a decrease in relief loads by 40%. The blowdown valves and PSVs are likely to be oversized if the API RP 521 is observed. The dynamic simulation gave precise estimates, consequently decreased the flare loads, and better safety. The controlled blowdown system mitigated the flare load to about 60% of the conventional blowdown system. Its safety was more reliable than that of the conventional, satisfying SIL 2 of IEC 61508.

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Related Topics

• Suspicious Discrepancy In Supercritical Fluid Relieving Calculation
  
• Tedious & Simple Method In Determining Specific volume For Isentropic Nozzle Flow Mass Flux
• Constant Density To Obtain Relieving Condition
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve
• API Std 520 Part 1 Dec 2008 is Released
• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• Requirement of overpressure protection devices on system design to PIPING code 

Set Pressure of Spare PSV

Anyone can tell me what will be the set pressure for the spare PSV in the multiple PSV (3 X 50%) arrangement. ? Set pressure for 1st PSV is preset at 100% whilst set pressure for 2nd PSV is 105% of 1st PSV set pressure. Should we set 3rd PSV (spare) at 100% or 105% of 1st PSV set pressure ???

Considering a PSV systems with

• PSV-1 set at 100% set pressure
• PSV-2 set at 105% set pressure
• PSV-3 is spare for PSV-1 or PSV-2.

IF PSV-3 set at 100% set pressure

i) whenever PSV-1 is removed for inspection / maintenance, PSV-3 will be put in service. Then the situation will be

• PSV-1 out of service
• PSV-2 set at 105% set pressure
• PSV-3 set at 100% set pressure

Situation remain same as before. Healthy and proper setting.

ii) whenever PSV-2 is removed for inspection / maintenance, PSV-3 will be put in service. Then the situation will be

• PSV-1 set at 100% set pressure
• PSV-2 out of service
• PSV-3 set at 100% set pressure

Situation slightly different. There is potential of chattering in case relief. However, this situation is intermittent, short period, operator aware and attended and manageable. This is acceptable.

IF PSV-3 set at 105% set pressure

i) whenever PSV-1 is removed for inspection / maintenance, PSV-3 will be put in service. Then the situation will be

• PSV-1 out of service
• PSV-2 set at 105% set pressure
• PSV-3 set at 105% set pressure

PSV-2 or PSV-3 will only start to open at 105% set pressure. This is not acceptable as relieving pressure is exceeded 100% of set pressure..

ii) whenever PSV-2 is removed for inspection / maintenance, PSV-3 will be put in service. Then the situation will be

• PSV-1 set at 100% set pressure
• PSV-2 out of service
• PSV-3 set at 105% set pressure

Situation remain same as before. Healthy and proper setting.

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RECOMMENDATION
Therefore setting spare PSV other than 100% of set pressure will possibly lead to situation where the initial relieving pressure higher than 100% of set pressure. This is against code requirement and therefore the spare PSV is recommended set at 100% of set pressure.


Related Topics

• Suspicious Discrepancy In Supercritical Fluid Relieving Calculation
• Tedious & Simple Method In Determining Specific volume For Isentropic Nozzle Flow Mass Flux
• Constant Density To Obtain Relieving Condition
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve
• API Std 520 Part 1 Dec 2008 is Released
• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• Requirement of overpressure protection devices on system design to PIPING code 

Suspicious Discrepancy In Supercritical Fluid Relieving Calculation

Pressurized liquid or vapor-liquid in equilibrium or vapor only system during normal operation, when it is expose to external fire attack, heat inputs into vessel (or system) may possibly increase internal pressure and temperature. For system with high design pressure (or Maximum allowable working pressure, MAWP), the pressure relief valve (PRV) protecting the vessel (or system) may have same set pressure as the design pressure (or MAWP). Subject to design code of the vessel, overpressure allowed by code is different from code to code. Typically for ASME unfired vessel section VIII, the maximum allowable overpressure is 10% of set pressure. This will results maximum allowable accumulation pressure (or relieving pressure) reach at 110% of set pressure.

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In some event, the relieving pressure is higher than the fluid critical pressure. For example, a CO2 injection compressor, the injection pressure at is 65 barg. The design pressure may be 72 barg and relieving pressure is approximately 79.2 barg, is higher than CO2 critical pressure of 72.9 barg. The PRV is relieving at supercritical condition.

Conventional method in determining PRV orifice size is presented in API RP-521. Part 1. It considered all “un-wetted” vessels are same regardless the fluid is supercritical, a vapor or a gas. Nevertheless, one shall take note that  this method based on the physical properties of air and the perfect gas laws with no change in fluid temperature. 

• Supercritical fluid may not follow perfect gas law
• Low compressibility of supercritical fluid (e.g. 0.5 to 0.7)
• Change in fluid temperature during relieving

API method may be conservative, there are chattering and oversized PRV and discharge problem. A  rigorous method has been discussed by R.C. DOANE in "Designing for pressure safety valves in supercritical service" published in Hydrocarbon Processing Jan 2010. This method assuming all thermodynamic paths are well defined by a Process Simulator. Thermodynamic path of fluid from operating condition to relieving and drop to back pressure to PRV may be defined by four typical steps.

1. Constant Specific Volume path (Initial to Relieving)
2. Constant Pressure path (Extended relieving)
3. Constant Entropy path (PRV relieving path)

For Step 1, earlier post "Constant Density To Obtain Relieving Condition" has discussed similar subject previously. 

For the step 3, another definition of PRV relieving path can be Isentalpic from relieving to throat follow by isentropic from throat to PRV backpressure as discussed in "Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve".

In the "Designing for pressure safety valves in supercritical service" article, table 1 "Supercritical relief valve sizing example problem—normal butane" has tabulated step-by-step calculation. This table has incorporated equation 1 to equation 10 in this article. The calculation consist of segment 1-to-2, 2-to-3, 3-to-4 and 4-to-5. In recent work in establishing similar task carried out by this example, one suspicious discrepancy is identified. Details as follow.

From segment 1-to-2 to segment 4-to-5, volumetric flow is increased from 1304 ft3/hr to 1315 ft3/hr (segment 2-to-3) and decreased in subsequent segments to 1250 ft3/hr. The volumetric flow is in the range of 1250 to 1315 ft3/hr. HOWEVER, mass flow is increased almost double from 6,374 lb/hr to 13,405 lb/hr (segment 2-to-3) and decreased in similar range (11620 to 12,371 lb/hr). Why there is significant different in segment 1-to-2 compare to other segment ?

Detail checking found that Mass Flux (G in lb/ft2s) is calculated by dividing Orifice Velocity (v) by Specific Volume (V) at orifice condition for segment 1-to-2. On the other hand, Mass Flux (G in lb/ft2s) is calculated by dividing Orifice Velocity (V) by Specific Volume (V) at outlet condition for other segments. Different method in calculating Mass Flux has created the discrepancy.

Opinion

As the flow is choked (or highest velocity for subsonic) at the PRV nozzle,  Mass Flux (G in lb/ft2s) should be calculated by dividing Orifice Velocity (v) by Specific Volume (V) at orifice condition.

Well... Above query will be raised and response will be posted once we have received it.

Related Topics

• Tedious & Simple Method In Determining Specific volume For Isentropic Nozzle Flow Mass Flux
• Constant Density To Obtain Relieving Condition
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve
• API Std 520 Part 1 Dec 2008 is Released
• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• Requirement of overpressure protection devices on system design to PIPING code

Tedious & Simple Method In Determining Specific volume For Isentropic Nozzle Flow Mass Flux

Earlier post "API Std 520 Part 1 Dec 2008 is Released" highlighted one of the main  changes was to use of isentropic nozzle flow method and/or ideal gas specific heat ratio for vapor/gas relief flow calculation.

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Isentropic nozzle flow method is considering one dimensional homogeneous flow is limiting in Pressure Relief Valve (PRV) nozzle with isentropic (adiabatic and reversible) expansion across the PRV nozzle. Maximum mass flux across the PRV nozzle will be determined follow by mass flow passing the PRV. In determining the maximum mass flux, an integration of specific volume (v) time differential pressure (dP) is required. This integration involve substantial derivation of specific volume (v) which change with pressure (P). If the system can be modelled in HYSYS, you may consider the following tedious and simplified method in determining the specific volume (v) with varies pressure (P).

Tedious method

The detailed method involve the fluid expanded isentropically across multiple expander with fix differential pressure (dP). Entire model is simulated using HYSYS with Isentropic expansion using Expanders. Stream 1 is the feed to PRV nozzle. Stream 1 is expanded isentropically (polytropic efficiency =100%) to stream 2 with fix differential pressure. Stream 2 is further expanded isentropically to stream 3, Stream 4 is further expanded isentropically to stream 4,.............

Mass density is extracted from each stream in order to use for integration of specific volume (v) time differential pressure (dP). Following is the example of specific volume (Inverse of Mass density).

Simple Method

A simple method is proposed hereafter. It basically utilize the capability of Property Table.

Step 1 : Enter stream 1 and open the Property Table Utility in Attachment tab.

Step 2 :From the Connections tab, select Variable 1 as Pressure with Incremental pressure from Outlet (e.g. 1 barg) to Inlet pressure (e.g. 12 barg).

Step 3 :From the Connections tab, select Variable 2 as Entropy with State mode. Enter the Entropy of Stream 1 here.

Step 4 : In Dep. Prop. tab, Add Mass Density. Now the system is ready for calculation. Click the Calculate button.

Now Mass density change according to pressure (with isentropic expansion) is ready.

Above method can be easily adopted for subcooled flashing, two phase flashing flow across the PRV nozzle.

Do you have better idea ?

Related Topics

• API Std 520 Part 1 Dec 2008 is Released
• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• Workbook for Chemical Reactor Relief System Sizing
• A must have book...Emergency Relief System Design Using DIERS Technology
• Requirement of overpressure protection devices on system design to PIPING code
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve

Tank Normal Venting Rate (API Std 2000) - Spreadsheet is ready

Atmospheric storage tank normal venting based on API Std 2000 6th edition, Nov 2009 has been discussed in "Tank Normal Venting Rate Estimation Using Latest Method As in API Std 2000". Ankur, an experience engineer in Chemical industry, actively involved in CheResource Forum, has take his effort and time to program it in Microsoft Excel spreadsheet. Thanks to Ankur.

As reminder, the information and methods included within this spreadsheet are presented for common sharing and intended to be used by technically skilled persons at their own discretion. We do not warrant the suitability or accuracy of these methods.

Insulation type and recommended thermal conductivity have also been included in the spreadsheet for easy reference. The insulation type and thermal conductivity as follow :

Common Insulation Materials	Average Thermal Conductivity, W/m-K
Cellular glass	0.05
Mineral Fiber	0.04
Calcium Silicate	0.06
Perlite	0.07

Products contain volatile components or dissolved gases or flashing products may result adddtional volatile component / gases / vapor. This amount will increase the outbreathing rate.

Products stored at temperature above 40 degC or its vapor pressure higher than 5.0 kPa, products evaporation rate shall be added into outbreathing rate.


Read and download spreadsheet in "Tank Normal Venting Rate Estimation Using Latest Method As in API Std 2000".

* If you find any error or improvement,  please inform us.
**If you have any useful program and would like to share within our community, please send to me.

Related Post

• Tank Normal Venting Rate Estimation Using Latest Method As in API Std 2000
• Tank Venting - API Std 2000 (Nov 2009) - Revised method and Old Method in ANNEX A
• Tank Thermal Breathing - Proposed Equation Correlate API Std 2000 (Annex A)
• Tank Normal Venting Rate Estimation Using Siddhartha Equation
• Find Latitude & Longitude Using GOOGLE MAP

Tank Normal Venting Rate Estimation Using Latest Method As in API Std 2000

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Liquid product like Chemical, condensate, etc is commonly stored in fixed roof vertical cylindrical tank. Inert gas blanketing system is provided to avoid air and moisture contact and contaminate liquid product. Liquid movement by content filling (pump-in) or emptying (pump out) and weather changes (ambient heating or cooling) will results internal pressure increase (overpressure) or decrease (vacuum) in the tank. Thus, a protecting system providing inbreathing or outbreathing gas is provided to maintain a constant pressure in the tank.

Inbreathing

Emptying (pump-out) and ambient cooling will lead to normal inbreathing. General equation to determine inbreathing flow :

V_in,air = V_pe + CV_tk^0.7 R_i

where

V_in,air = Total inbreathing in Nm3/h (Air)

V_pe = Pump-out or emptying in m3/h

V_tk = Tank capacity in m3

R_i = Insulation reduction factor
C = Factor subject to vapor pressure, average temperature and latitude (see Table 1)

Table 1 : C-factor for Inbreathing
Latitude	Vapor Pressure
	Hexane or similar		Higher than hexane or unknown
	Average Storage Temperature  (oC)
	< 25	>=25	< 25	>=25
Below 42o	4	6.5	6.5	6.5
42o to 58o	3	5	5	5
Above 58o	2.5	4	4	4

Find Latitude & Longitute Using GOOGLE MAP

Outbreathing

Filling (pump-out) and ambient heating will lead to normal outbreathing. General equation to determine outbreathing flow :

V_out,air = V_pf + YV_tk^0.9 R_i

where

V_out,air = Total Outbreathing in Nm3/h (Air)

V_pf = Pump-out or filling in m3/h

V_tk = Tank capacity in m3
R_i = Insulation reduction factor

Y = Factor subject to latitude (see Table 2)

Table 2 : Y-factor for Outbreathing
Latitude	Y-factor
Below 42o	0.32
42o to 58o	0.25
Above 58o	0.2

Special notes :
i) If products contain volatile components or dissolved gases or flashing products, perform flashing calculation to estimate quantity of volatile component / gases / vapor. This amount shall be added into outbreathing rate.

ii) If products stored temperature above 40 degC or its vapor pressure higher than 5.0 kPa, products evaporation rate shall be added into outbreathing rate.

iii) Quick check on plant latitude, check out in "Find Latitude & Longitute Using GOOGLE MAP"

Determination of  R_i = Insulation reduction factor

If no insulation, 

R_i = 1

If fully insulated including Shell and roof,

R_i = R_in = (1 + h x L_in / Lamda_in)^-1

If partial insulated,

R_i = R_inp

R_i = (A_inp / A_TTS).R_in + [1-(A_inp / A_TTS)]

 
L_in = Insulation thickness in m
Lamda_in = Insulation thermal conductivity in w/mK

h =Inside heat transfer coefficient in w/m2K  [4 w/m2K is commonly assumed]

A_inp = Insulated surface area of tank in m2

A_TTS = Total tank surface area (shell + roof) in m2

Concluding remarks

The following method is the latest method used in API Std 2000, ISO 28300 & EN 14015. Nevertheless, API Std 2000 (6th edition) still maintaining the old method in ANNEX A as an alternative approach in determining normal venting rate. Comparison and recommendation can be found in "Tank Venting - API Std 2000 (Nov 2009) - Revised method and Old Method in ANNEX A".

In determining normal tank venting rate per ANNEX A, designer can estimate tank venting rate from a table. Siddhartha has presented an equation to predict tank venting based on simple correlation, as discussed in " Tank Normal Venting Rate Estimation Using Siddhartha Equation". JoeWong has further proposed a new correlation as discussed in "Tank Thermal Breathing - Proposed Equation Correlate API Std 2000 Data" with better accuracy.

Ref :
i) API Std 2000 "Venting Atmospheric and Low-Pressure Storage Tanks", 6th Edition, Nov. 2009
ii) ISO 28300 "Petroleum. petrochemical and natural gas industries -  Venting Atmospheric and Low-Pressure Storage Tanks"
iii) EN 14015 "Specification for the design and manufacture of site built, vertical, cylindrical, flat-bottomed, above ground, welded, steel tanks for the storage of liquids at ambient temperature and above "

**********************************

Above equations have been programmed by Ankur, an experience Chemical Engineer, share with readers of Chemical and Process Technology. You may download here.

Thanks to Ankur
Download

*If you have any useful program and would like to share within our community, please send to me.
Related Post

• Tank Venting - API Std 2000 (Nov 2009) - Revised method and Old Method in ANNEX A
• Tank Thermal Breathing - Proposed Equation Correlate API Std 2000 (Annex A)
• Tank Normal Venting Rate Estimation Using Siddhartha Equation
• Find Latitude & Longitude Using GOOGLE MAP
• PSV for Shell-and-Tube HEX Tube Side Overpressure Protection against External Fire Attack ?
• Should we consider JET FIRE for Pressure Relief Valve (PSV) load determination ?
• Protective Measures against FIRE other than Pressure Relief Device (PRD)
• Extra Caution When Eliminating Overpressure by Fire Attacks 

Tank Venting - API Std 2000 (Nov 2009) - Revised method and Old Method in ANNEX A

Liquid product like Chemical, condensate, etc is commonly stored in fixed roof vertical cylindrical tank. Inert gas blanketing system is provided to avoid air and moisture contact and contaminate liquid product. Liquid movement by content filling (pump-in) or emptying (pump out) and weather changes (ambient heating or cooling) will results internal pressure increase (overpressure) or decrease (vacuum) in the tank. Thus, an overpressure and vacuum protecting system providing inbreathing or outbreathing gas is provided to maintain a constant pressure in the tank.

There are several recommended practice (RP) and standard (STD) are available to guide engineers in designing and specifying venting / relief load from a storage tank.

• API Std 2000 "Venting Atmospheric and Low-Pressure Storage Tanks"
• ISO 28300 "Petroleum. petrochemical and natural gas industries -  Venting Atmospheric and Low-Pressure Storage Tanks"
• EN 14015 "Specification for the design and manufacture of site built, vertical, cylindrical, flat-bottomed, above ground, welded, steel tanks for the storage of liquids at ambient temperature and above "

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In determining normal tank venting rate, API STD 2000, (edition 1999) allows designer to estimate tank venting rate from a table. Siddhartha has presented an equation to predict tank venting based on simple correlation, as discussed in " Tank Normal Venting Rate Estimation Using Siddhartha Equation". JoeWong has further proposed a new correlation as discussed in "Tank Thermal Breathing - Proposed Equation Correlate API Std 2000 Data" with better accuracy.

The methodology in determining normal inbreathing and outbreathing rate are different between API Std 2000 and ISO 28300 / EN14015. Groth Corporation, a well known Pressure-Vaccum relief valve manufacture has conducted a study to compare the API Std 2000 and ISO 28300 / EN14015. A few differences are identified :

 API STD 2000

• Full vacuum through 1.034 barg
• Aboveground tanks for liquid petroleum or petroleum products and aboveground and underground refrigerated storage tanks
• Fixed roof tanks
• Tank volumes up to 28,618m3
• No insulation factor considered for regular venting (emergency only)

EN 14015

• -20 mbar through 500 mbar
• Non-refrigerated tanks
• Fixed roof tanks (with or without internal floating roofs)
• No limit on tank volume
• Insulation considered for regular and emergency venting

ISO 28300

• Full vacuum through 1.034 barg
• Aboveground tanks for liquid petroleum or petroleum products and aboveground and underground refrigerated storage tanks
• Fixed roof tanks
• No limit on tank volume
• Insulation considered for regular and emergency venting

One of the interesting findings in the study was the normal inbreathing and outbreathing comparison betweeen API STD 2000 and ISO 28300/EN 14015.

(Click image for large image)
Source : Groth Corporation

Details can be found in " Introduction and ISO 28300 Presentation". Above study was comparing API STD 2000, revision 1999 and latest ISO 28300 / EN 14015. API has recently released latest API STD 200, last NOV 2009. One of the main changes was synchronizing methodology between API STD 2000 rev. Nov 2009 and latest ISO 28300 / EN 14015. 



Opinions
Previous method used in API STD 2000 still maintain in Annex A as "Alternative Calculation of normal venting requirements". This annex provides a calculation approach that may be used to design protection systems for the normal venting requirement of petroleum storage tank.

Above finding indicates minor difference in outbreathing rate between API STD 2000 and ISO 28300 / EN 14015. Both methods are acceptable for estimating outbreathing (in main text and annex A). However, present inbreathing method (as in annex A) consistently lower compare to revised method. It is always take extra precaution when you used annex A for estimating inbreathing rate.

You may be working on ongoing project priors to revision of API STD 2000 (NOV 2009). Following standard implementation spirit, you may continue to use old revision. However, it is always advisable to refer to latest API STD 2000 from safety and integrity aspect.

You may work on revamping or debottlenecking of existing plant, extra precaution shall be  taken. Whenever you have modification or new requirement to existing tank, you may have to comply to latest API STD 2000 (NOV 2009) even thought your existing tank was designed and fabricated to old revision of API STD 2000.

Related Post

• Tank Normal Venting Rate Estimation Using Latest Method As in API Std 2000
• Tank Thermal Breathing - Proposed Equation Correlate API Std 2000 Data
• Tank Normal Venting Rate Estimation Using Siddhartha Equation
• PSV for Shell-and-Tube HEX Tube Side Overpressure Protection against External Fire Attack ?
• Should we consider JET FIRE for Pressure Relief Valve (PSV) load determination ?
• Protective Measures against FIRE other than Pressure Relief Device (PRD)
• Extra Caution When Eliminating Overpressure by Fire Attacks 

Visualise PSV Flow Distribution Using CFD

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Pressure safety valve (PSV) or pressure relief valve (PRV) is commonly used to protect a pressure containment part i.e. vessel, column, etc from overpressure. It is one of the code approved type of overpressure protection devices. This type of device is reclosing type where the mechanism of devices is designed such that it will stop relief when the pressure is reduced to it reseat pressure. Besides PSV, rupture disc (RD) and rupture pin are also code approved type of overpressure devices. This type of device is non-reclosing type where it will continue to relief until all inventory is completely evacuated from the system or with operator intervention.

Earlier post "Visualise Pressure Safety Valve (PSV) Assemblies & Operation" presented some features about the PSV type and also a video clip for typical construction and operation of a conventional PSV. This post will present PSV valve disc movement and flow distribution inside a PSV.

PSV valve disc movement and flow distribution inside a PSV is simulated using CFD modeller, CFX 11.0 . Following video clip shows how a spring loaded pressure relief valve works under over-pressure condition.

You may notice that high mass flux is on the disc facing PSV outlet nozzle.

Following is another video clip shows oscillating movement of PSV disc during relieving condition.

Above oscillating movement generate a oscillating forward flow which is believe one of the cause for valve chattering. Read more about PSV chattering in "PSV Chaterring is Destructive...The ways to Prevent...".

Thanks to Songxguan

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• Several Impact of Backpressure on Conventional PRV
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• Useful Documents Related to Pressure Relief Valve (PRV) - Part 2
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 1
• Should we consider JET FIRE for Pressure Relief Valve (PSV) load determination ?
• Should we install Butterfly valve for Pressure Relief Valve (PSV) isolation ?

Rupture Disc Installation Guide Videos Reduce Premature Openning Risk

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Rupture disc (RD) is one of the non-reclosable type pressure relief device for vessel overpressure protection. It has been used in many application due to it uniqueness and special characteristic as covered in "Why Rupture (RD) Upstream of Pressure Relief Valve (PRV) ?". In some application, two RDs are placed in series to minimize inventory lost cause by premature opening of RD. Premature opening of RD may be cause by RD quality during manufacturing. Besides, premature opening of RD may be caused by wrong handling and installation of RDs.

Elfab, a RD manufacturer offer a few RD installation videos which could very useful for most operator. you may download from the following links or visit Elfab.

• Conventional Domed (CD)
• Domed Composite (DC)
• Flat Composite (FC)
• Opti-Gard (OP)
• Opti-Gard SoLo (OPS)
• Safe-Gard (SG)
• Universal Arma-Gard (AGS)

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• Why Two Rupture Discs in Series ?
• Why Rupture (RD) Upstream of Pressure Relief Valve (PRV) ?
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• Criteria for Requirement of Pressure Relief Device for Tube Rupture
• Some Comments on Providing External Insulation as Protective Measure against FIRE
• Protective Measures against FIRE other than Pressure Relief Device (PRD)
• PSV for Shell-and-Tube HEX Tube Side Overpressure Protection against External Fire Attack ?

Constant Density To Obtain Relieving Condition

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A pressure vessel or system expose to external fire, entire system may be isolated automatically by a plant wide emergency shutdown system (ESD) and emergency depressuring system will be initiated to depressure system to safe level and evacuate the inventory from high risk area (expose to external fire) to disposal system. Depressuring system is known as one of the most effective measures against external fire risk. Besides, there are others measures as discussed in "Protective Measures against FIRE...".

Pressure relief device (PRD) may not protect pressure vessel or system from external fire. Having said that PRD may serve in some circumtances to "buy time" for operator action. Besides, design code and/or local regulation demand a PRD in pressure vessel or system as ultimate protection.

One may have process simulation for normal production system. The stream for vessel or system expose to fire is at normal operating pressure and temperature. How shall one can adjust the process simulation to bring it up the relieving pressure ? One may consider a constant density method.

Constant density method
Pressure vessel expose to fire will be isolated and settled-out. Read more in "Adjusted Method For Compressor Settle Out (with Vapor & Liquid) Using HYSYS". Assuming no credit for automatic depressuring system and operator intervention, the inventory trapped in the pressure vessel will remain as trapped vapor mass (M_V0) with vapor volume (V_V0) and liquid mass (M_L0) with liquid volume (V_L0) at normal pressure (P₀) and temperature (T₀) when ESD is just initiated. Total trapped mass (M_T0) will be M_V0 + M_L0 and total trapped volume (V_T0) will be V_V0 + V_L0. Mix denstiy will be M_T0/V_T0.

Trapped inventory will be heated up with external fire. Heat added into the trapped system will cause temperature rise, more liquid vaporise and pressure rise upto relieving pressure, before pressure relief device is popped open. At relieving condition (relieving pressure and temperature), trapped vapor mass (M_Vr) with vapor volume (V_Vr) and liquid mass (M_Lr) with liquid volume (V_Lr). Total trapped mass (M_Tr) will be M_Vr + M_Lr and total trapped volume (V_Tr) will be V_Vr + V_Lr. Mix denstiy will be M_Tr/V_Tr. As there is no inventory evacuated from the system, total trapped mass (M_T0) and volume (V_T0) at normal condition will be same as trapped mass (M_Tr) and volume (V_Tr) at relieving condition.

M_T0 = M_Tr
V_T0 = V_Tr

Similar mix density (M_T0/V_T0) will remain same.

M_T0/V_T0 = M_Tr/V_Tr

This is commonly known as constant density method.

Related Topic

• Don't misunderstood depressuring
• Depressuring Flow - Quick Manual Method
• Protective Measures against FIRE other than Pressure Relief Device (PRD)
• Adjusted Method For Compressor Settle Out (with Vapor & Liquid) Using HYSYS
• Simple Manual Method for Settle Out Condition Estimation
• Simple Method For Compressor Settle Out Using HYSYS
• Depressuring - Save Some Time in HYSYS - FLARENET Iteration

API Std 521 - Errata and Addendum Released by API

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This post is intended to provide supplementary information for earlier post "API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section".

Addendum May 2008
API has released the Addendum details to API Std 521. It describes in detail the modified sections compare to earlier release in 2007. You may download Addendum May 2008 from here.

Errata June 2007
For those who don't not aware the Errata in June 2007 for API Std 521 released in 2007, you may read "ERRATA - API Std 521, Pressure Relieving and Depressuring Systems" and download Errata June 2007 from here.

Related Topic

• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• ERRATA - API Std 521, Pressure Relieving and Depressuring Systems
• Workbook for Chemical Reactor Relief System Sizing
• A must have book...Emergency Relief System Design Using DIERS Technology
• Requirement of overpressure protection devices on system design to PIPING code
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve

PRD Backpressure

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Recently there have been many discussion and argument among engineers from operating company, design and engineering, manufacturer and consulting on one simple but important term relates to Pressure Relief Device (PRD), "backpressure". A lot of terms have been used and mixed and lead to confusion. Beside, there are also some new terms created unintentionally which further confuse the discussion. This post is intended to clarify common definition used in Chemical Process Technology

Several definition of "backpressure" have been pointed out in the discussion :

i) Backpressure
ii) Superimposed backpressure
iii) Built-up backpressure
iv) Total backpressure
v) Constant backpressure
vi) Variable backpressure
vii) Accumulated backpressure
viii) Total accumulated backpressure
.
.
.
All these definition have been used in the discussion and lead to confusion and conflict.

Correct Definition and Combination

Every companies and engineers may have different definition and understanding about "backpressure". The combination of terms used within a code or company may be correct. However, one specific term used in one code or company shall not mix with another term used in another code or company.

For example, instrument datasheet is used to transfer and communicate information between an instrument engineer in design company and engineer in PRD manufacturer. The terms used possibly are Constant backpressure, Variable backpressure and Total backpressure. However, a process engineer in design company may use Superimposed backpressure, Built-up backpressure and Backpressure to transfer and communicate similar informations to instrument engineer in design company. Whenever the discussion among three parties (process engineer, instrument engineer & manufacturer engineer), correct combination of terms shall be clarified and used. Confusion will occur when different combination are used e.g. built-up backpressure mix with constant and variable backpressure.

Common Definition of "Backpressure" in Chemical Process Technology
A pressure relief valve (PRV) in "Ready-to-operate" mode and "relieving" mode during plant operation will expose to different type of pressure. "Ready-to-operate" mode is the PRV's disc keeping PRV in closed position when inlet pressure (Pi) is lower than or equal to PRV set pressure (Ps). "Relieving" mode is PRV disc away from seat allowing fluid passing the PRV valve nozzle when the inlet pressure (Pi) is higher than PRV set pressure (Ps).

As PRV may be in Ready-to-operate and Relieving mode, the "backpressure" exist at different modes will vary. In many event, understanding of this "backpressure" creates a lot of confusion among engineers in operation, design, vendor, manufacturer, etc. It is important to make the defintion clear prior to any discussion.

Superimposed backpressure (Pbs) is the static pressure that exists at the outlet of a pressure relief device (PRD) at the time the device is required to operate ("Ready-to-operate" mode). Superimposed backpressure is the result of pressure in the discharge system coming from other sources i.e. Pressure control valve (PCV), pressure relief valve (PRV), etc. Superimposed backpressure may be constant or variable.

Built-up backpressure (Pbb) is the increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device (PRD) opens ("relieving" mode).

Backpressure (Pb) is the pressure that exists at the outlet of a pressure relief device (PRD) as a result of the pressure in the PRD discharge system. Backpressure is pressure result of both superimposed backpressure caused by other sources and built-up backpressure due to the relief flow during "relieving" mode. Backpressure is the sum of the superimposed and built-up backpressures.

Pb = Pbs + Pbb

Above defintion are inline with API STD 520 Part 1, Dec 2008 - Sizing, Selection & Installation of Pressure-Relieving Devices in Refinery.

"Backpressure" will affect the performance of Pressure Relief Valve (PRV). "How "backpressure" affect PRV performance ? Read more in "Several Impact of Backpressure on Conventional PRV".

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API Std 520 Part 1 Dec 2008 is Released

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American Petroleum Petroleum (API) released the API Std 520 Part 1, Sizing, Selection and Installation of Pressure-relieving Devices in Refineries Part 1 - Sizing and Selection Eighth Edition, Dec. 2008 in December 2008. First far most important change is it became a STANDARD, instead RECOMMENDED PRACTICES. User shall remember although it became a standard, it is solely users responsibility to make sound, scientific, engineering, safe, environment friendly judgment. Neither API nor its employee, etc make warranty for the use of this standard. Detail refer to "Special Note" in relevant Standard.

Applicability
User of this standard shall understand the applicability of this standard.
This standard

• applies to the sizing and selection of pressure relief devices (PRD) used in refineries and related industries for equipment that has maximum allowable working pressure (MAWP) of 15 psig (103 kPag) or greater
• applicable to PRD protecting unfired pressure vessels and related equipment
• applicable to steady state flow of Newtonian fluids
• supplement to ASME Section VIII Pressure Vessel Code
• is not applicable Atmospheric or Low pressure storage tank (cover under API Std 2000)
• is not applicable fired vessels (cover under ASME section 1 and ASME B31.1)

Revised/Added Section
Quick glance through this document, several section are revised/added. The updated / added sections are (not limted to) :

• 3. Terms and Definitions (Revised - all definition are in sorted alphabetically)
  
• 4.2.1.3 Balanced PRVs (Revised)
• 4.2.2.3 Pilot Types (Revised)
• 4.2.2.5 Pilto-operated PRV Accessories (Added/revised)
• 4.2.3 Cold Differential Test Pressure (CDTP) (Shifted)
• 4.4.2.1.1 Buckling pin devices... (revised)
• 4.4.2.1.3 ASME Code Case 2091-3 defines... (Added)
• 4.4.2.2.1 The set pressure... (Revised)
• 4.4.3.2 & 4.4.3.3 Breaking pin... (Revised)
• 5.3.2.2 ...a) bench set pressure... (Revised)
• 5.3.4.2 Total backpressure... (Revised)
• 5.4.1.4 ...(Partly revised)
• 5.5 Development of Sizing Equations (New)
• 5.6.1 Applicability (New)
• 5.6.4.3 Balanced PRVs (New)
• 5.6.5 Alternate Sizing Procedure (New)
• Annex B (Revised)

Above are not consolidated changes. If you found any others, please share with us (click here).

Comments
There are several remarks that you may take note (when this standard is apply) :

• Use of isentropic nozzle flow method and/or ideal gas specific heat ratio for vapor relief estimation

• Use of Homogeneous Equilibrium Model (HEM) based on thermal and mechanical equilibrium for two phase relief. Nevertheless, the Leung Omega Two point method is still remained as a choice of method.
  

• Generally thermal and mechanical equilibrium can be achieved for PRV relief nozzle length more than 100mm. If shorter nozzle is used, Homogeneous Non-equilibrium Model (HNE) may be considered.

• Additional attention shall be taken when appreciable of non-condensable gas present in the flashing liquid. Leung Omega Two Point method for flashing liquid and non-condensable gas may be referred.
  

• Used of HEM method is easy but rigorous. Understanding of the thermal equilibrium for fluid properties is important and required.

• Two phase Vapor Liquid Relief method presented in Annex B have not been validated by test (accoring to API) and there is no any recognized procedure for certifying the capacity of PRVs in two phase flow service.

Related Topics

• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• Workbook for Chemical Reactor Relief System Sizing
• A must have book...Emergency Relief System Design Using DIERS Technology
• Requirement of overpressure protection devices on system design to PIPING code
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve

Confusion on API and ASME Effective Discharge Area

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Conducting overpressure scenario analysis, derivation of relief load, follow by pressure safety valve (PSV) sizing and finally selection of PSV are common activities in oil and gas project. Nowadays, API Std 521 is commonly used for sizing and hence the required API effective discharge area is the result of the calculation. PSV with effective discharge area larger than required API effective discharge area shall be selected. However, many PSVs' certified area are presented in ASME area.

How to relate API effective discharge area and ASME area ?

This is a very common question and problem face by many young engineers. Sometime engineers may wonder why a PSV with ASME "D orifice" is sufficient for a relief scenario required API "E orifice". Reason for the difference between API and ASME is dated back to 1962 when ASME Section VIII Code was changed to derate all certified relieving capacities by 10%. PSV manufacturers have decided to increase PSV flow area by 10%, instead of derating their capacity by 10%. Nevertheless the API "orifice" still remain unchanged.

A PSV with ASME flow area (A_ASME), ASME discharge coefficient (K_ASME) and API discharge coefficient (K_API),

Corrected API effective discharge area, A_API = K_ASME x A_ASME / K_API

Example,

A calculated effective area based on K_API = 0.973, A_API = 0.12 inch2. A API "D orifice" with 0.11 inch2 is insufficient. A "E orifice" with 0.196 inch2 is required.

Let check the National Board published data, a ASME "D orifice" will have K_ASME = 0.859 and flow area of A_ASME = 0.15 inch2, the equivalent API effective discharge area would be

A_API = K_ASME x A_ASME / K_API

A_API = 0.859 x 0.15 / 0.973
A_API = 0.132 inch2

The ASME "D orifice" is having API equivalent effective discharge area of 0.132 inch2 is higher than required effective discharge area of 0.12 inch2. Thus an ASME "D orifice"is still sufficient.

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