Vessel Blowdown Systems: Assurance Framework for Operate Phase

Arun Bhattacharya, Nikhil Barbare and Rao Vasantharao Australian Gas Technology Conference, Perth 26th July 2012

Australian Gas Technology Conference, 26th July 2012Disclaimer: Views expressed in this paper are personal views of the authors only and do not necessarily represent the views of their Employers.

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Role of Blowdown systems in Hydrocarbon Industry

Blowdown Systems- Improved guidance in Design PhaseOperate Phase Assurance framework Opportunity for improvement Some Aspects on Suggested Framework

Role of Vessel Blowdown Systems in Hydrocarbon Industry

Blowdown systems in Hydrocarbon IndustryBlowdown system releases hydrocarbon inventory in Emergency shutdown event to flare, to prevent escalation that may cause damage to personnel, plant and equipment.

Typical Blowdown System Components: Blowdown Valves, Restriction Orifices (BDV/RO) Flare SystemWV2 WV1 SSV SCSSV Single Largest Well

Safety Criticality of the Blowdown Systems:

Trigger: Emergency scenario such as fire. Blowdown systems are usually automatically activated Escalation Potential: Vessel impacted by fire Consequences: Vessel rupture if the vessel metal strength cannot withstand the internal pressure due to external severe heat input (potential jet fire) Role of Blowdown systems: Prevention of vessel failure by rapid depressurisation

Blowdown systems: A critical safety barrier for which Design Practices are evolving over the years ???

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Role of Blowdown systems in Hydrocarbon Industry

Blowdown Systems - Improved guidance in Design PhaseOperate Phase Assurance framework Opportunity for improvement Some Aspects on Suggested Framework

Guidance in Engineering Design Phase

Improved Guidance in Design phase in recent past for Vessel Blowdown SystemsEarlier API 521Fire Case Generic with more stress on Pool Fire Achieving 690 kPag or 50% design pressure in 15 min

API 521 (2007)Credible Fire with sections for Pool and Jet Fire Criteria determined by on vessel survivability analysis

Depressurisation Criteria for Blowdown System Design

Inputs from Vessel survivability analysis play a key role in determining safe depressurization rate and time under the latest API Standard.Let us look in a bit more detail at the inputs available from Vessel survivability studies

Vessel Survivability StudiesEstablish hydrocarbon release rates, release durations, credible fire cases, associated fire impacts on the vessels Use suitable data on fire heat flux, vessel thickness and estimate the rate of rise in vessel wall temperature and reduction in material yield stress properties at elevated temperatures Plot vessel tensile strength decay with time/ temperature- Max Allowable Vessel Pressure (MAVP) Generate vessel internal pressure profile during blowdown cycle under fire scenarioWhat next with these two curves??

Vessel Survivability StudiesPlot the vessel internal pressure decay curve on vessel tensile strength decay with temperature curve.Pressure (N/mm2) 40 35 30 25 20 15 10 5 0 0 250 500 750 1000 1250

MAVP and Vessel Pressure vs time

Rm - Tensile Strength Ry - Yield Strength

MAVP Vessel Pressure

time (s)

Two important conclusions come out of this analysis

Conclusions from Vessel Survivability40 35 Pressure (N/mm2) 30 25 20 15 10 5 0 0 250 500 750 time (s) 1000 1250 Pressure (N/mm2)

4035

MAVP Vessel Pressure

MAVP

30 25 20 15 10 5 0 0 250

Vessel Pressure

MAVP < VP

500 750 time (s)

1000

1250

Vessel Survives

Vessel Fails !!

Decisions on the Blowdown system sizing are made depending on the vessel survivability (of course keeping in mind the Flare capacity issues).Apart from these two, there is a third input available Critical monitor time

Critical Monitor TimeCritical Monitor Time is the time in the vessel depressuring cycle when the vessel metal yield strength comes closest to vessel internal pressure. This point indicates the most vulnerable point of time for vessel survivability and thus is an important point of interest for Operational Assurance Framework development.MAVP and Vessel Pressure vs time40 35 Pressure (N/mm2) 30 25 20 15 10 5 0 0 250 500 time (s) 750 1000 1250

MAVP Vessel Pressure

These inputs from design phase form an important basis for defining the assurance framework in Operate Phase.

1 2 3 4

Role of Blowdown systems in Hydrocarbon Industry

Blowdown Systems - Improved guidance in Design PhaseOperate Phase Assurance framework Opportunity for improvement Some Aspects on Suggested Framework

What are the attributes for Operate Phase Assurance??

Operate Phase: Attributes of AssuranceOperate Phase Assurance:

Confirming whether the Blowdown System actual performance meets the design criteria Capturing any performance degradation issues. Measuring the decayed pressure at the right point of time Working with easytomonitor procedures for use by Operations

No International Standard guidelines are yet available for setting up assurance criteria

Operate Phase: Required quality of Assurance

Assurance based on earlier API 521 criteria ( 690 kPag in 15 min) does not provide the right level of assurance based on vessel survivability study (Vessel may depressurise in 15min but could still fail before). Decayed pressure at 15th minute also may not provide the right level of assurance for systems where critical monitor time is less than 15 min.

Linking Design and Operate PhasesDesign phase Heat input Depressurisation Starting pressure Credible Fire Model Trip set point Operate Phase Cold Blowdown Normal operating pressure

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How to correlate the Design and Operate Phases ??

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Role of Blowdown systems in Hydrocarbon Industry

Blowdown Systems - Improved guidance in Design PhaseOperate Phase Assurance framework Opportunity for improvement Some Aspects on Suggested Framework

Suggestions to adapt Design Correlations to Operate Phase are discussed further

Suggested Correlations: Design and Operate PhasesAssurance Regime ParameterCold conditionAppropriate time point Initial Pressure (cold blowdown conditions)

Suggested Correlation Methodology

Using depressurisation models of design phase, derive the cold blowdown decayed pressure profileUse the Critical monitor time wherever relevant Adjust for starting pressure level ( lower than the trip set pressure) using the following generic equation:

PAct, inst = PMOP, inst* (PAct / PMOP)

where

PAct, inst = Instantaneous vessel pressure for a cold blowdown or fire scenario PMOP, inst= Instantaneous vessel pressure for Maximum Operating Pressure (e.g. PZHH) PAct , PMOP = Starting pressures: Actual, Maximum Operating Pressure What is the acceptable criteria?

Acceptable CriteriaObserved Vessel internal pressure < Predicted cold blowdown pressure Blowdown Performance Assurance Criteria Met!

This methodology offers a quick and easy field verification method based on acceptable vessel pressure during the depressurization cycle at a specified time (CMT). A simple pressure data check can check the health of blow-down system performance of a vessel.

Conclusions This paper presents a methodology to correlate the Design Phase objectives and Operate Phase assurance requirements. Suitability and adequacy of Blowdown Systems can be easily confirmed during regular performance monitoring Blowdown System degradation can be identified for existing systems as well as for Brownfields modifications based on analysis of vessel failure characteristics and system blowdown performance It is also intended to initiate a discussion among interested Professionals on various options/methodologies for a robust Operational Assurance framework in this regard.

Thank You !!!Any Questions???