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© Genesis Oil & Gas Consultants Ltd

Relief and Blowdown Systems

Are you sure you can rely on them?

Colin Deddis, June 2013

This document is created for general illustration only and is not intended as a substitute for specialist advice in relation to

any particular situation. All liability is expressly disclaimed.


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• Relief and blowdown systems as layers ofprotection.

• Relief and blowdown systems’ functionality.

• What can go wrong?

– Overview

– Relief valve provision and set pressure

– Inadequate design capacity

– Liquid accumulation

– Vibration

• Design verification.


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Process

Design

Critical

Alarms &

Human

Intervention

Emergency

Shutdown &Safety

Instrumented

Functions

Physical

Protection

e.g. Pressure

Relief

Post Release

PhysicalProtection

e.g. Blowdown

EmergencyResponse

HA

Z

A

R

D

C

O

N

S

E

Q U

E

N

C

E

Basic

Process

Control

TOP EVENT


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Relief and Blowdown Systems- Functionality

HP Flare Drum

Process systems

Relief

valves

Blowdown

valves

Manual

valves

Rupture

disks

Control

valves

High pressure “wet” flare system

High pressure “cold” flare system

LP Flare Drum

Low pressure flare system

Liquids to process system

Liquids to process systemor LP Flare Drum


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Relief and Blowdown Systems - Functionality

Emergency SourcesRelief devices

Depressurisation

Operational SourcesStart-up/shutdown

Process upsets

Maintenance vents


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Relief System Incidents (70 examples)The Institute of Petroleum Guidelines for the Safe and Optimum Design of Hydrocarbon Pressure Relief and Blowdown Systems. 1999

Pressure build-up/

chokes

Vibration

Slugging/hydraulic

hammer

Reverse flow

Incompatible fluids

HP/LP interfaces

Materials of

construction

Capacity of flareVent ignitionPurge ratesJet reaction forces

Liquid hold-up/

flare drum sizing

Design specification

Installation/

maintenance

Operation

Control and

instrumentation

Layout and drainage

Relief valve provision,

sizing and set pressure

Management of change


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Pressure Relief Devices PerformanceInspecting the inspection data, TCE , December 2011/January 2012, Flower & Jones

% Set Pressure Number of Valves % Valves

130% 253 3.6

Test data for ~7,000 operational relief valves period 2000 - 2008


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Relief System Sizing Basis

• Flare sized for full flow relief from single train

• Export route can be common for both trains

• Simultaneous blocked outlet of both trains discounted on basis of:

• segregated air supplies to SDV-001 and SDV-002 &

• air accumulators on each valve with single check valves in the supply line

SDV-001

SDV-002

To export

To export

Train 1

Train 2

To flare

To flare

Full flow reliefBlocked outlet

Full flow relief

Blocked outlet

FC

FC


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Actual air supply to SDVs


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Liquid Accumulation/Slugging


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Vibration


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Complex Failures

PAH

gas

SWS

SWR

Heat Exch.LAH ESD

ESDV

Closeddrain

HP Flare

Drum

LP flare

drum

Over-board

ESDV

disc failure

no alarm

tell-tail blocked?water

enters

drum

does not

trip

seawater

pumps

not tightshut-off fills fills

overfills

no level

>LAH

press =

4 barg(no alarm)

closed

What happened?

liquid @+40m

Source: HSE Safety Alert 01/2008


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STHE Overpressure Protection – the “problem”

Two major hazards associated with bursting disk failures:

• Impairment of relief system – liquid inflow & overfill.

• Incident escalation - reverse rupture leads to uncontrolledhydrocarbon release from systems connected to the relief system.


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STHE Overpressure Protection – the “problem”

• Increased use of bursting disks to protect STHEs over past 10 to 15years

• Estimated frequency of guillotine tube rupture

– 0.0009 per unit per year (~1 per 1,100 years)[1]

• Frequency of bursting disk failures protecting STHEs

– 7 incidents in 13 years (~50 exchangers)

– 0.011 per unit per year (~1 per 90 years)[2]

• Future growth in numbers of high pressure STHEs requiring

overpressure protection

• Has the balance of risk shifted?

1. IP Guidelines for the Design and Safe Operation of Shell & Tube Heat Exchangers to Withstand the Impact of Tube Failure, Aug 2000

2. Estimate based on incident details collated by Energy Institute JIP


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Relief Systems Design Verification Process

Agree Scope of Work

Data Gathering

Design Verification

Competency Assessment

Presentation of Findings and Recommendations

Agreement of Actions and Closeout Priorities

Disposal SystemDesign

Flare Capacity

Liquids Handling

Design

Specification

HP/LP Interfaces

Management of

Change

Relief Device

Selection

Relief Load

Relief Device Inlet

Piping

Design &

Construction

SIL & LOPA AnalysisFlare Stack / Vents /

Headers & Tail Pipes

Emergency Shutdown

Purge Rates Materials Selection

Control &

Instrumentation

Overpressure

Protection

Flare Radiation

Emergency

depressurisation

Operation


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Relief System Design Verification

Categorisation of Findings

Pressure build-

up/ chokes8%

Vibration3%

Slugging/hydraulichammer

2%Reverse flow

3%

Incompatible fluids4%

HP/LP interfaces2%

Materials ofconstruction

6%

Capacity of flare3%

Vent ignition1%

Purge rates3%

Jet reaction forces0%

Liquid hold-up/flare drumsizing 5%

Design specification19%

Installation/maintenance2%

Operation1%

Control andinstrumentation

6%

Layout and drainage1%

Relief valve provision,sizing and set pressure

22%

Management ofchanges, modifications

and upgrades10%

15 Reviews, 551 Findings


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Relief Device – Design Verification

70.3%

29.4%

0.3%

313 Relief Devices - Findings

Relief Valves Adequately Sized

Governing Relief Case Incorrect

No Relief Provision


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64%

36%

Inlet Line Loss Checks

Inlet line Losses Acceptable

Inlet losses unacceptable(>3% of set pressure)

Relief Device – Design Verification


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Summary

• Relief and blowdown systems are complex pressure systems

• Design constraints can be violated; impairing the relief and blowdownsystems.

• Design verification can be successfully deployed to determine

system robustness prior to operation.

• Design verification findings indicate some systemic design flaws.


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Thank-you

Any questions?


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References

1. The Institute of Petroleum Guidelines for the Safe and Optimum Design of

Hydrocarbon Pressure Relief and Blowdown Systems, 1999

2. IP Guidelines for the Design and Safe Operation of Shell & Tube Heat

Exchangers to Withstand the Impact of Tube Failure, Aug 2000

3. Inspecting the inspection data, TCE , December 2011/January 2012, Flower

& Jones

4. HSE Safety Alert 01/2008 Flare system impaired by cooling water loss

through bursting disc failure on an intercooler heat exchanger.

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