Emergency Depressurization PRG - pr.GEN.0007 [PDF]

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DESIGN CRITERIA

EMERGENCY DEPRESSURIZATION

PRG.PR.GEN.0007 Rev. 0 May 2010

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010 sheet 2 (14)

INDEX

1

SCOPE AND PURPOSE

3

2

REFERENCE DOCUMENTS

3

3

DEFINITIONS

3

4.

ACTIVITIES DESCRIPTION

4

4.1

WHY EMERGENCY DEPRESSURIZATION SYSTEM IS REQUIRED.

4

WHERE EMERGENCY DEPRESSURIZATION IS REQUIRED.

4

4.3

SECTIONALIZATION

6

4.4

DESIGN CRITERIA

8

4.2

4.4.1

General Philosophy

4.4.2.

Design capacity of the depressurization system

8 10

Fire case 10

4.5

Hot Depressurization

12

Other Scenarios

12

4.4.3

13

MDMT (Cold Depressurization).

GENERAL ARRANGEMENT

14

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010 sheet 3 (14)

1

SCOPE AND PURPOSE

This document provides the design guidelines to be followed for emergency depressurization, unless otherwise specified in Client’s Criteria or by Licensors and Suppliers. The application of this Design Criteria shall be subjected to accurate review of the results by a qualified and competent designer to avoid unnecessary over sizing or design not adequate to the scope. 2

REFERENCE DOCUMENTS

PRG.GG.GEN.0001

Choice and Definition of Design Temperature and Design Pressure.

API-521 / ISO 23251

Pressure – relieving and Depressurization System.

3

DEFINITIONS

BLEVE

Boiling Liquid Expanding Vapour Explosion

Depressurization

Depressurization of a plant or part of a plant, and equipment usually via vapour depressurization valves.

Sectionalization

Isolating sections of a unit to limit the volume that is exposed to leaks, thereby making depressurization more effective.

MDMT

The Minimum Design Metal Temperature is the lowest temperature that the equipment may possibly reach during start-up or shutdown and exceptional cool down events caused by high rate depressurization, and it shall be defined for all piping and equipment.

NLL

Normal Liquid Level

Settling out pressure

Equalized pressure throughout compressor loop after compressor shutdown

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010 sheet 4 (14)

4.

ACTIVITIES DESCRIPTION

4.1

WHY EMERGENCY DEPRESSURIZATION SYSTEM IS REQUIRED.

Emergency depressurization facilities are specified to accomplish at least one of the following objectives:

a) To reduce the risk of catastrophic equipment failure and/or BLEVE during fire exposure.

b) To reduce the risk of equipment failure due to internal exothermic reaction (i.e. runaway process reaction).

c) To reduce the amount of released material if there is a loss of containment caused by a leak and leading to an unacceptable safety hazard.

d) To rapidly bring the facility into a ‘safe’ state, in the event of other emergency scenarios such as loss of instrument air or power failure;

Emergency depressurization system installation, when required, is to be provided in addition to relief valve, as the latter cannot depressurise a vessel or a system, but it can only limit the pressure, during upset condition, at its set point value (plus accumulation, according to the applicable code). 4.2

WHERE EMERGENCY DEPRESSURIZATION IS REQUIRED.

Emergency depressurization system should be foreseen for the application listed as follow:

a) Process equipment system containing volatile or flammable liquids susceptible to flashing under normal operating conditions that can cause a bleve based on credible failure scenario. The minimum quantity of liquid above which depressurization shall be provided, should be given by plant owner / user. If this information is not admissible, during Feed phase optimisation/sectionalisation studies can be done on the basis of economics/operation considerations. The following two exceptions applies:

1. For LPG storage, depressurization facilities are usually not applied due to the very large depressurization loads that would result, and because liquid pool fires are deemed highly unlikely (no

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010 sheet 5 (14)

flanges below the liquid level, sloping floor, remote impounding, etc.) and BLEVE is prevented via active and/or passive fire protection.

2. When the installation of depressurization valves and disposal systems is impractical or impossible (i.e.: Natural Gas Liquid transfer lines). In this case, precautions should be taken to reduce the risk of leakage and fire by minimizing the number of flanges.

b) High pressure process units (operating above 17.5 barg [250 psig], as per API 521 / ISO 23251), processing toxic and/or flammable liquids and gases.

c) Processes in which an exothermic reaction can lead to exceed the equipment design conditions and eventually loss of containment in a relatively short time (i.e. hydrocracking process)

d) Pressure vessels containing flammable or toxic gas that can be exposed to fire shall be evaluated on a case-by-case basis. During fire scenario it is expected that these vessels may collapse even before reaching the relief valve set pressure. In these cases depressurization is one mean to reduce the risk of vessel failure.

e) Process sections that have the potential for developing a significant flammable or toxic cloud if there is a leak.

f) Other applications should be evaluated through risk assessment, taking into consideration if depressurization the equipment, section, etc. is an effective measure to lower the risks (i.e. reduce the consequences or reduce the likelihood of escalation).

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010 sheet 6 (14)

4.3

SECTIONALIZATION

Sectionalization is a mean to make depressurization more effective by limiting the inventory of equipment that is within the system. Due to this reason Sectionalization: •

Effectively reduces the risk of escalation by stopping or limit the release in case of fire or loss of containment to the involved zone in order to minimize the loss of process product and the depressurization system load. An appropriate plant layout, including fire walls and protection, may be required in order to avoid that two different fire zones are involved in the same fire emergency.

•

Optimizes investment costs: (i.e.: by reducing size of flare system or number of shutdown valves). Plant depressurization should be carried out in the way that peak loads from the different zones are not simultaneous.

Sectionalization involves the use of emergency isolation valves, to stop flows from one system into another. Sectionalization activity shall be carefully evaluated since generally introduces additional valves, hence, additional costs, leak points and additional weight, as well as maintenance for the plant owner. Sectionalization may create ‘trapped inventories’ that are isolated from drain, relief or depressurization connections. Moreover this shall be evaluated against the benefits of sectionalization to find a lowest cost and risk solution. The sectionalization shall be established as follows: a) The unit/section/equipment that requires depressurization facilities shall be identified using the criteria in (4.2).

b) The size/volume of the system to be depressured shall be defined including all of the equipment that are normally involved in the same unit operation area. EXAMPLES: 1. A distillation column with its related reboiler, condenser, accumulator and pump systems. 2. A large pressure vessel in a refrigeration loop that includes the open path with the compressor and potentially other equipment.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010 sheet 7 (14)

c) Where the size of the system and its plot area is large it may make sense to sectionalize this into smaller systems (subject to cost evaluation).

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010 sheet 8 (14)

4.4

DESIGN CRITERIA

4.4.1

General Philosophy

The Following scheme summarize the depressurization workflow:

Depressurization

Sizing

Minimum Temperature

Cold Case Fire Hot depress. Air Failure Others

In generally depressurization calculation shall pursuit the following aims: 1)

Design capacity of the depressurization system (Flare header / sub header, flare system) A number of different scenarios, shall be evaluated: i.e. Fire case, Hot depressurization (Run way reactor), instrument Air Failure etc. See API 521 (ISO 23251) for a comprehensive list of depressurization causes.

2)

Minimum Design Metal Temperature of the Systems. Because the effect of pressure reduction lower the temperature of the system.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010 sheet 9 (14)

Emergency depressurization typically meets the following criteria: Depressurise the equipment to 7 barg [100 psig] or 50 % of design pressure, whichever is lower, within 15 min. API 521 (ISO 23251) prescribes that the criteria of depressurization to 7 barg, or 50 % design pressure, within 15 min as suitable for pool fire exposure of vessels with a wall thickness greater than 25 mm but may not be suitable for thinner vessels, in which a faster depressurization rate might be required. The use of vessel/piping passive fire protection (e.g. fire resistant insulation or fireproofing) may reduce the temperature rise rate of the fire-exposed equipment. API 521 (ISO 23251) takes into account this fact by reducing the “environmental factor”, F (equal to 1 for un-insulated equipment only), which might reduce the heat input and hence the relief load during depressurization in case of calculation exceeding the design flare load. The thermodynamics path to be selected during depressurization is isenthalpic; except for the fire case, in which Duty is calculate according API 521 (ISO 23251). In case Process simulator requires the PV term work contribution it shall be equal to 0.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010

sheet 10 (14)

4.4.2.

Design capacity of the depressurization system Fire case

The Designer shall evaluate the depressurization rate in case of fire, if applicable, since it normally determines highest flowrate. Input data for the depressurization calculation in fire condition are: a) Liquid inventory (NLL for vessel as per API 521 (ISO 23251)); b) Vapour volume of the system; c) Wetted surface d) Initial condition ( Temperature and Pressure)

Calculation should be performed based on the following input; the initial pressure in the equipment under consideration is the relief valve set pressure, or the pressure, which will be reached after 15 minutes of fire exposure in case of manual emergency depressurization, or the maximum operating pressure in case of automated emergency depressurization. In case of a Compressor system, the settling out condition should be selected. The initial temperature is the one reached at corresponding Pressure for each case explained above, to calculate it, dynamics simulation should be arranged through adequate software.

Vapour depressurization calculation take into account the following:

a) Vaporization due to heat input from the fire; b) Vapour density variation during pressure decrease and temperature change c) Liquid flashing due to pressure decrease. (This factor applies only when a system contains liquid at or near its boiling point).

Heat flux input shall be calculated according to API-521(ISO 23251) (for pool-fire impingement), and should be taken into account for the duty calculation that the wetted surface decreases in time while simulation is going on. An other method is according Stefan-Boltzmann formula (for jet-fire impingement) that allow to take into account radiation, forced convection, flame temperature from any kind of external fire to the item. The final condition, as a general case, is 7 barg [100 psig] or 50 % of design pressure, whichever is lower within 15 min.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010

sheet 11 (14)

The above consideration are summarized in the table here

Automated Depressurization

Initial Pressure

Initial Temperature Duty

Maximum

Temperature

operating

Corresponding @

Pressure

Initial Pressure

Pressure reached Manual

after 15 min of fire.

Depressurization

(limited to the relief valve set pressure)

Temperature after 15 min of fire

Final Condition

API – 521

7 barg [100 psig] or

(ISO-23251)

50 % of design

or

pressure whichever

Time

15 min

Stefan-Boltzman is lower API – 521

7 barg [100 psig] or

(ISO-23251)

50 % of design

or

pressure whichever

15 min

Stefan-Boltzman is lower

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010

sheet 12 (14)

Hot Depressurization

The designer should consider also depressurization without fire, in order to evaluate emergency scenario such as runaway reaction. Hot depressurization is performed to mitigate/stop the effect of a runaway reaction. Normally two systems are installed; one for fast depressurization and one for slow depressurization. The two systems has different function; the fast depressurization is used in particular in case of fire or loss of recycle compressor for prolonged time in order to immediately stop the reaction, while the slow one is normally used in case of short loss of recycle compressor, or loss of feed, in order to decrease the pressure in the reactor and so the kinetic of reaction. If the opening of the slow depressurization valve is not enough to reduce the pressure/temperature in the system, the quick depressurization valve can be opened simultaneously. The final condition, as general case, is 25 % of design pressure within 15-30 min with a max depressurization rate of 10.5 bar/min each valve (limit give in order to reduce the flare load).

In order to calculate the depressurization rate consider that the pressure drop vs time as a logarithmic plot. So the slope of the line can be calculated considering the following expression: Slope = (ln Pf – ln Po)/tdep Where: Po is the operating pressure, Pf is the target pressure at the end of the depressurization, tdep is the request max time for depressurization. With the slope is possible to calculate the pressure drop in the first minute of depressurization and so the max flowrate to size the depressurization facilities such as, valve and orifice plate size. dP/min = Po *(1-eslope)

Other Scenarios

The designer should consider also depressurization without fire, in order to evaluate emergency scenario such as: power/instrument air failure, operator mistake, etc. In these scenarios, the initial conditions are, generally, those established in the system immediately after isolation of the system.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010

sheet 13 (14)

4.4.3

MDMT (Cold Depressurization).

Cold depressurization calculations are performed to evaluate the Minimum Design Metal Temperature (MDMT). Cold Depressurization is performed based on following initial conditions; the Initial Pressure is the normal operative pressure or settling out pressure for compressor. Temperature is the operating one. The final condition to evaluate the MDMT is the minimum backpressure of the relevant flare system, or atmospheric pressure if the system is connected to the atmosphere. A particular scenario of cold depressurization is the depressurization after prolonged shut-down. If required by contractual requirements, the Designer should evaluate also the Depressurization after prolonged shutdown. In this case the Initial Temperature is the minimum ambient temperature and the initial Pressure is the one according the ambient temperature (the pressure should decrease due to Gas condensing). The final condition is atmospheric pressure.

Initial Pressure

Initial Temperature Duty

Final Condition

Time

to evaluatre MDMT Cold

Operating

Operating

Depressurization

Pressure

Pressure

-

Minimum flare back -

pressure, or Atmospheric Pressure

Depressurization

Pressure@

Minimum ambient

after prolonged

T ambient

temperature

Shut-down

-

Minimum

flare

-

pressure, or Atmospheric Pressure

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent

PRG.PR.GEN.0007

Rev. 0

Data

May 2010

sheet 14 (14)

4.5

GENERAL ARRANGEMENT

The minimum requirement of a Blowdown valve is described below: •

Air Close/Air Failure Open (AC,AFO)

•

Tight Shut-Off (TSO)

•

Reduced bore (if there is not other contractual requirement)

Depressurization valves should fail in open position and maintain their integrity during the duration of the emergency. For this reason either they shall be located outside fire area or, they shall be protected against fire (fireproofing of body and actuator). The installation of a dedicated air bottle, allowing 3 complete valve strokes, shall be evaluated in order to operate the depressurization valve even in case of instrument air failure or to avoid simultaneous or spurious opening.

Note CSO: Car seal Open (is a minimum requirement).

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 File code: CRIDESBI.DOT Data file: PRG_PR_GEN_0007_R00_E_F.DOC CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Third parties or used for purposes other than those for which it has been sent