PETRONAS TECHNICAL STANDARDS
DESIGN AND ENGINEERING PRACTICE
MANUAL
GUIDELINE FOR CALCULATING
SAFETY DISTANCES IN LPG
STORAGE AND HANDLING
INSTALLATIONS
PTS 20.162
JANUARY 1988
PREFACE
PETRONAS Technical Standards (PTS) publications reflect the views, at the time of publication, of PETRONAS OPUs/Divisions.
They are based on the experience acquired during the involvement with the design, construction, operation and maintenance of processing units and facilities. Where appropriate they are based on, or reference is made to, national and international standards and codes of practice.
The objective is to set the recommended standard for good technical practice to be applied by PETRONAS' OPUs in oil and gas production facilities, refineries, gas processing plants, chemical plants, marketing facilities or any other such facility, and thereby to achieve maximum technical and economic benefit from standardisation.
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GUIDELINES FOR CALCULATING SAFETY DISTANCES IN LPG STORAGE AND HANDLING INSTALLATIONS
CONTENTS
1. Introduction
2. Assessment of Fire Situations
2.1 Audit of the Facilities
2.2 Selection of Leakage Scenarios and Assessment of their Consequences 2.3 Radiation Criteria for Personnel Protection
2.4 Vapour Cloud Explosion
2.5 Boiling Liquid Expanding Vapour Explosion (BLEVE)
2.6 Selection of Leak Reduction Measures and Methods to Mitigate the Effects
3. Consequence Assessments
3.1 Introduction
3.2 Calculation of Flow Rates
3.3 Vapour Jets - Dispersion and Fires 3.4 Two-phase Jets - Dispersion and Fires
4. Worked Example
4.1 Description of Facility
4.2 Audit of the Facility and Choice of Scenarios
4.3 Consequence Assessment, Analysis and Proposed Action
5. References
TABLES
1. Discharge Areas for 'REGO'. 'FISHER', and other pressure relief valves 2. Relief Valve Fire and Radiation Flux Data for Propane
FIGURES
1. Schematic of Model Facilities
2. Example PLUMEPATH Dispersion Profile for Butane 3. Distances to LFL for Propane and Butane Releases
4. Distances to 1.5 kW/m² Radiation Flux for Vapour and Liquid Horizontal Butane Jet Fires 5. Distances to 5 kW/m² Radiation Flux for Vapour and Liquid Horizontal Butane Jet Fires 6. Distances to 8 kW/m² Radiation Flux for Vapour and Liquid Horizontal Butane Jet Fires 7. Distances to 13 kW/m² Radiation Flux for Vapour and Liquid Horizontal Butane Jet Fires 8. Distances to 32 kW/m² Radiation Flux for Vapour and Liquid Horizontal Butane Jet Fires 9. Distances to 44 kW/m² Radiation Flux for Vapour and Liquid Horizontal Butane Jet Fires 10. Flame Lengths for Vapour and Liquid Horizontal Butane Jet Fires
11. Distances to 1.5 kW/m² Radiation Flux for Vapour and Liquid Vertical Butane Jet Fires 12. Distances to 5 kW/m² Radiation Flux for Vapour and Liquid Vertical Butane Jet Fires 13. Distances to 8 kW/m² Radiation Flux for Vapour and Liquid Vertical Butane Jet Fires 14. Distances to 13 kW/m² Radiation Flux for Vapour and Liquid Vertical Butane Jet Fires 15. Distances to 32 kW/m² Radiation Flux for Vapour and Liquid Vertical Butane Jet Fires 16. Distances to 44 kW/m² Radiation Flux for Vapour and Liquid Vertical Butane Jet Fires 17 . LPG Depot Layout - Worked Example
18. Flow scheme - Worked Example
19. Nozzle details of Propane Sphere Worked Example 20. Nozzle details of Butane Sphere Worked Example
1. INTRODUCTION
One of the main differences between the recently-issued Supply and Marketing (SM) LPG Manual P a r t 2 S e c t i o n 0 3 P T S 3 0 . 0 6 . 1 0 . 1 2 . L P G B u l k S t o r a g e Installations (hereafter referred to as the Manual/PTS) and previous issues is the requirement to relate siting of equipment to the radiation flux levels that would be experienced from fires in the installation. This approach applies to LPG bulk storage installations with individual tanks of 135 m3 and above and is consistent with the Institute of Petroleum Model Code of Safe Practice, Part 9, Liquefied Petroleum Gas, Volume 1, dated February 1987.
The covering letter which was sent to all companies with the Manual/PTS suggested that all sites should compare the design of their facilities with the new standards and consider whether any aspects should be modified. Site layout is one aspect that in many cases will be difficult, if not impossible, to alter. Therefore, it will be important to examine whether a fire could endanger human life, equipment or property, inside or outside the site. If this is recognized as a possibility, it will be necessary to consider changes to the design to reduce the probability of the incident and/or provide additional protection to people and equipment. In a completely unacceptable situation it may be necessary to shut down the installation and transfer the activities elsewhere.
Since the Manual/PTS does not indicate how the evaluation should be carried out, or provide information on the calculation of thermal radiation levels, an inter-functional team has produced this set of guide-lines with the assistance of Thornton Research Centre. It has been written in three parts:
Section 2. Assessment of Fire Situations Section 3. Consequence Assessments Section 4. Worked Example
It should be noted that although these guide-lines have been produced as an aid for evaluating the layout requirements for bulk storage installations with individual tanks of 135 m3 and above, they are also in general applicable to layout aspects of all other sections of plants handling pressurized LPG at Marketing and Manufacturing installations (e.g. loading/ unloading ships, bulk road vehicles or rail tank wagons and filling cylinders). As stated in the Manual/PTS, operating companies may choose to apply these guidelines to installations with individual tanks of less than 135 m3.
2. ASSESSMENT OF FIRE SITUATIONS 2.1 AUDIT OF THE FACILITIES
Section 03.02.01.02 of the Manual/PTS covers safety distances. It states that possible leak sources should be identified and their rate of leakage and duration assessed. The first step is the identification of such leak sources. This requires a systematic evaluation of the design against the Manual/PTS and a review of the operating/maintenance procedures for the installation.
The assessment should be carried out by a team of three or four people, who between them have a good knowledge of the design, operation and maintenance of the facility. They should have available up-to-date flow schemes and engineering information about the equipment before they start.
The team should study the facility, line by line and piece of equipment by piece of equipment, and consider whether there could be any circumstances which might lead to a leak of LPG. This should be done in an imaginative way with the team considering the usual operating conditions at the site and also any unusual conditions which they can conceive.
They would consider, for instance:
−
possible high or low temperatures−
possible high or low pressures−
overfilling of storage vessels, bulk lorries or rail tank wagons−
the effect of impurities in the LPG (e.g. water)−
the wrong grade of LPG (e.g. C3 instead of C4)−
the effect of incorrect operation or maintenance−
incorrect material selection or equipment fabrication−
malfunctioning equipment−
impact (e.g. vehicles)The procedure outlined above is a simplified version of the more structured HAZOP study, where the team leader takes the team through a series of guide words for each part of the plant which they are examining.
2.2 SELECTION OF LEAKAGE SCENARIOS AND ASSESSMENT OF THEIR CONSEQUENCES
In section 03.02.01.02 (c) of the Manual/PTS it states that the evaluation of leak sources should take into account failure modes, likelihood and consequences. It is not possible to give detailed guidance on the likelihood of particular leak scenarios, because the probability that an event will occur will depend on the design of the specific facility and the quality of its operation and maintenance. However, a review of serious incidents that have occurred in the LPG industry shows that they have usually been the result of the following situations:
−
Product discharge through a relief valve (including overfill).The Manual/PTS requires in section 03.02.01.02 (a) that product discharge to atmosphere through relief valves on LPG storage vessels should be considered as leakage scenarios in all cases. This requirement is consistent with the Institute of Petroleum Model Code of Safe Practice, Liquid Petroleum Gas, Volume 1. The rationale behind this requirement is that these relief valves are the only devices in an LPG facility which are designed to be able to vent large quantities of LPG to atmosphere.
−
Flange leak or other joint leak (see Section 4 for typical leakage hole sizes)−
Pump seal leak (see Section 4 for typical leakage hole sizes)−
Open drain valve−
Rupture of small bore connections (e.g. breakage of an instrument line)−
Hose leak or rupture (e.g. vehicle pullaway)It is recommended that these failure modes should always be considered for consequence calculations, unless the audit team has very good reasons to discount them. Other situations will be possible on many installations. These could include the leak of a vessel or pipeline due to internal or external corrosion, or the effect of an external incident such as vehicle impact. Inclusion of these scenarios for consequence calculations will depend on the team's judgement of whether their likelihood is greater or less than that of the failure modes listed above.
Using the information given in Section 3 the team should next assess the flow rate of the leaking liquid or vapour. Factors in the design which may control the flow should be taken into account.
An assessment should also be made of the time that the release will last. This will depend on the presence of operators, the accessibility of manually-operated isolating valves and the existence of remotely-operated valves. The likelihood of ignition will depend in part on the time that a flammable mixture persists.
When a leak or spill occurs the hydrocarbon vapour will disperse, forming a cloud. The distances to the lower flammable limit can be obtained from Section 3. Should the cloud extend beyond the site boundary or to another area where ignition sources are not controlled, measures will be necessary to limit the size of the release.
Flash calculations for liquid butane and propane suggest that product leaks may form significant liquid pools. However, experimental work and field trial studies by Thornton Research Centre have established that the jet formed when these materials are released entrains considerable amounts of air. Small droplets of liquid LPG are formed. These evaporate rapidly and the result is a cold vapour cloud with no significant pool formation. The only situation for which pool formation can be envisaged is with butane in cold climates when its vapour pressure is low. At the same time the discharge velocity must be low, as in the case of a leak in the discharge line from a storage vessel, when the driving force is provided largely by the head in the vessel. Therefore, in nearly all leakage situations ignition of the leak will result in a jet fire, rather than a pool fire.
Finally the distances to various levels of radiation intensity from ignited leaks can be read from the tables associated with Section 3. The results should then be compared with the criteria given in Figure 03.02.01.02 of the Manual/PTS, which are discussed in Section 2.3.
2.3 RADIATION CRITERIA FOR PERSONNEL PROTECTION
In Figure 03.02.01.02 of the Manual/PTS maximum radiation flux levels are given for personnel inside the site boundary and for the situation at the site fence. The notes attached to this figure only give brief guidance on the choice of type of area. In order to give additional guidance the following amplified notes have been prepared.
The acceptability of maximum heat flux levels is based, in part, on the person's ability to escape, since injury is a function of heat flux and exposure time. Employees engaged in the location's activities should be trained in what to do in an emergency and will be healthy and active. On the other hand, members of the public at or near a location are unlikely to know what to do and may also include the full community age and health range.
2.3.1 Plant boundary situations
−
Remote Area. An area where there is a low likelihood of people. Those likely to be present will be fit but may be lightly clothed. There is no shelter available but escape is both easy and obvious.−
Urban Area. An area where there is a strong possibility of people of full community age and health range being present. They will be fully clothed. There is no shelter available but escape is easy or only slightly hindered (e.g. need to cross a road).In a situation where there is no site fence, such as an automotive LPG station, it may be necessary to relate the size of the fire to the time needed to get away from the heat. For instance, not more than 30 seconds should be required to move from a radiation intensity of 5 kilowatts/ metre² (kW/sq.m) (second degree burns in ca. 30 secs) to an intensity of 3 kW/sq.m (second degree burns in 60 secs) and a further 90 seconds to get to an intensity of 1.5 kW/sq.m.
−
Critical Area. This is the same as an urban area, but with hindered means of escape.2.3.2 Plant areas
− Process Area. Those likely to be present will be healthy and trained in emergency procedures. They will be fully clothed and will be able to be clear of the area within one minute.
− Protected Work Area. This refers to permanent buildings where personnel are obliged to remain in order to operate plant, but may be exposed through glass. It may also provide a refuge for those escaping from the fire. The radiation level refers to the building exposure.
− Work Area. There is minimal shelter from the fire and slightly hindered escape. Those present will be healthy and be fully clothed.
− Critical Area. This is one where an operator may have to be present for short times occasionally, e.g. to check the state of equipment. He will be trained in what to do if a fire starts, but escape routes may be hindered because of plant complexity.
2.3.3 Flash fires
When a cloud of hydrocarbon vapour ignites the initial flash fire will be of high intensity, but of such a short duration that only people actually enveloped in it will be seriously burnt.
2.4 VAPOUR CLOUD EXPLOSION
If an LPG installation is designed to the current Manual/PTS and other Group standards for handling LPG, or has been adequately updated, the probability of a leak or spill being large enough to result in an explosion will be very low. In addition, trials carried out by research organizations have shown that vapour cloud explosions in an open situation are very unlikely. If LPG vapour enters an area where equipment or pipework are closely spaced, or enters a building, drainage system or another confined space, an explosion may well be possible.
Vapour cloud explosion is a complicated subject, and is still an active research topic. If a company considers that it is necessary to review the possibility of such an event, it should consult CHSE for advice.
2.5 BOILING LIQUID EXPANDING VAPOUR EXPLOSION (BLEVE)
This type of explosion occurs when a ductile vessel, containing a liquid whose vapour pressure is well above atmospheric, ruptures. Because the vessel is made of a ductile material its shell tears, generating a relatively small number of large fragments. If the liquid in the vessel is flammable and the rupture has been caused by heat from an external fire weakening the wall of the vessel above liquid level, the BLEVE produces a buoyant fireball. The size of the fireball, its duration and the intensity of its radiation are determined by the total contents of the product in the vessel. The pieces of the vessel can travel several hundreds of metres. This is the situation usually associated with the occurrence of a BLEVE.
In the past a BLEVE has been considered as an unrealistic event for an installation designed to Group standards. However, it must be recognized that BLEVEs occur somewhere in the world, either with transport tanks or in fixed installations, at the rate of about one every two years. Therefore, the possibility has to be recognized, particularly by PETRONAS companies with older installations .
If the installation meets the requirements of the Manual/PTS the probability of a BLEVE will be low enough to be considered unrealistic, because these standards have been developed specifically to eliminate the possibility of the vessel walls being overheated. However, if some of the requirements are not met and/or if operating procedures are not strictly enforced, the probability could be much higher. A critical review of the design, operation and maintenance of the installation should be carried out if any concern is felt by a company. CHSE would be prepared to assist in such a review.
Another possible cause of vessel failure is severe over pressurization, probably associated with vessel imperfections due to faults in the material of construction, faults in its fabrication, or possibly due to internal corrosion. Failure for these reasons is considered extremely unlikely for a vessel installed to the requirements of the Manual/PTS and operated correctly. If a source of ignition is also present a fireball similar to that produced by a BLEVE will occur.
2.6 SELECTION OF LEAK REDUCTION MEASURES AND METHODS TO MITIGATE THE EFFECTS
If any of the leakage scenarios that have been examined are found to give unacceptable radiation levels at the site boundary or within the facility it will be necessary to consider measures that will either reduce the probability of the release, reduce its length of time, or reduce the radiation level. In the first place the requirements of the Manual/PTS should be applied. However, if these are not practicable other methods may be considered. The suggestions given below are not exhaustive; some will be more suitable for Marketing locations, while others may be preferred by Manufacturing locations. It must be recognized that there may be occasions when it is not practicable to improve the installation to an adequate level of safety. In that case it may be, necessary to shut down or relocate the facilities.
− Installing hydrocarbon gas detectors/alarms, possibly interconnected to emergency shut down valves.
− Incorporating fusible links in the actuating systems for emergency shut down valves.
− Installing secondary emergency shut down valves with a mode of failure non-common to the primary valves.
− Welding more of the pipework and valves.
− Connecting all relief valves to a flare or vent system.
− Providing fire protection/tank cooling for small tanks as well as for large tanks.
− Providing fire protection for adjacent equipment.
− Pressurization or sealing of nearby buildings containing sources of ignition (e.g. an electrical substation).
− Using breakaway couplings or drive-away protection in road and rail tank filling/discharge systems.
− Installing vehicle impact barriers.
− Using load cells/weigh bridge to reduce the chance of overfilling bulk lorries or rail tank wagons.
− Using computer controlled loading/unloading.
− Replacing hoses with loading arms.
3. CONSEQUENCE ASSESSMENTS 3.1 INTRODUCTION
This part of the Guide-lines provides a range of dispersion and fire hazard assessments to complement the leakage scenarios described in Section 2. The structure which has been adopted is intended to mirror the main classes of hazard which can arise, which are vapour and two-phase jet releases. However, a prerequisite for carrying out dispersion and fire hazard assessments is the calculation of the relevant mass flow rates. The three resulting sections are headed:
− Calculation of flow rates - vapour leakage flows - liquid leakage flows
− Vapour jets, dispersion and fires
− Two-phase jets, dispersion and fires
3.2 CALCULATION OF FLOW RATES
The following sections provide calculation methods for most leakage situations. For a general treatment of the calculation of leakage flows, the reader is referred to the review written by Ramskill and entitled "Discharge rate calculation methods for use in plant safety assessments" (Ref. 1).
The leakage from equipment at LPG installations will be of two types. It may come from the vapour space of vessels or from lines handling LPG vapour. when only vapour flow has to be considered. Alternatively, the leak may be from lines or equipment handling liquid LPG. In these cases liquid flow or two-phase flow may occur. Vapour leakage flow calculations and liquid/two-phase flow calculations are described in the following subsections.
3.2.1 Vapour leakage flows
Vapour leakage flows may be divided into two categories:
− Flow through relief valves and other cases where choked flow occurs.
− Flow through holes where choked flow does not occur.
Choked flow occurs where the ratio of upstream and down-stream pressure is greater than a critical value. For propane and butane this occurs where:
Pr essure in pipe or vessel
Atmospheric pressure
≥
1.8
In practice this will be the likely situation at many installations. For ease of calculation it is recommended that all vapour leaks are treated as choked flow and handled as described below, although this may be a conservative assumption. If required, calculation methods for unchoked flow can be found in Reference 1.
These flows should be sufficiently accurate for most leakage situations. However, in complex cases, e.g. where there is a large hole in a pipe which is more than, say, 20 m from the vessel which is the source of pressure, pipe friction losses will reduce the flow rate. In these cases reference should be made to the review by Ramskill (Ref. 1).
The calculation of choked flow is demonstrated by the operation of a relief valve. The relief valve on a partly filled vessel of LPG will open as soon as the pressure in the vapour space of the vessel has risen to the relief valve set pressure. This could for example be due to the effect of a fire close to the vessel. The formula to be used for the flow calculation is that derived from API RP 520 - "Recommended Practice for the Design and Installation of Pressure Receiving Systems in Refineries" (Ref. 2). The equation is derived from a general equation for conditions of critical flow and may be written:
W
KAP
C
M
T
where:
W is the flow rate (Kg/s), C is the gas/vapour constant, K is the discharge coefficient, A is the discharge area (m²),
P is the vessel design pressure x 1.2 (N/m²), (or upstream pressure for holes in equipment) M is the molecular weight,
T is the relief valve inlet temperature (K). The value of C is 145 for propane and butane.
The discharge coefficient (K) of a relief valve varies with the inlet and disc shape and also lift characteristics. It should be taken as 0.9.
Tables 1(A), (B) and (C) give the discharge area (A) for typical relief valves.
Equation (1) may also be used for vapour flow through holes in equipment. In this case the value for the discharge coefficient (K) should be 0.8.
For the case of tanks affected by fire, equation 1 reduces to the following working equations (2) + (3):
For propane tanks: W =
419 KAP
(Kg/s) (2) For butane tanks:
W = 367 KAP
(Kg/s) (3)
In this case the relief valve inlet temperature (T) has been taken as 100°C, not theequilibrium temperature for the pressure at which the relief valve is blowing. The temperature of 100°C is based on experimental fire engulfment trials.
3.2.2 Liquid leakage flows
Simple liquid flows may generally be calculated by the use of Bernoulli's equation. Examples include leaks at temperatures well below the boiling point, or orifice type leaks driven by the product head. In general, however, leakage flows will be two phase in nature with varying vapour to liquid ratios.
Propane and propane/butane mixtures are generally handled as liquids well above their atmospheric boiling points, so that a large fraction will flash during emission. With butane, however, it is more likely that on occasion the temperature may fall below its boiling point at atmospheric pressure. Whilst this condition increases the likelihood that the leakage flow has a higher liquid percentage, significant vapour emission will still arise. This may be understood by examining Figure 01.02.08.01 of the Manual/PTS, which shows the absolute vapour pressures of those light hydrocarbons which are the principal components of commercial LPG.
In general terms, the calculation of flashing LPG flows from pressurized systems is complex since rapid evaporation of the liquid can take place before, during and after emission. The precise form of emission, pipe rupture, flange leak, valve leak, hose burst, etc., can play a major part in conditioning the resultant flow.
For simplicity the equations presented in these Guidelines are adequate for nearly all leakage situations that an assessment team could meet. More generally, however, the calculation of two phase flows is complex so that where greater precision in calculation becomes important the matter should be referred to PETRONAS.
Appendix 3 of the Institute of Petroleum Model Code of Safe Practice, Part 9, Liquefied Petroleum Gas, Volume 1, describes the calculation of release rates based on the application of a typical simple equation which assumes a homogeneous equilibrium two-phase flashing liquid release from the orifice. The discharge coefficient has been taken as 1.0. These assumptions will obviously produce leakage rates at variance with those calculated using the equations set out herein.
3.2.2.1 Overflows through relief valve vents
This case can arise when the relief valve opens as a result of product being pumped into the vessel after it is full. Maximum liquid flow rate into the vessel should be used for the consequence calculations.
It is recommended that the capacity of the relief valve is checked for this flow, since changes to the LPG handling system may have been made since the system was designed. The recommended method is to be found in section 3.17 of API RP 521. The flow formulae to be used are presented in Appendix C of API RP 520 (Ref. 3).
3.2.2.2 Flows from broken equipment
When pressurized LPG is released from containment the discharge is usually a two-phase mixture of vapour and liquid. The behaviour of such a discharge is difficult to analyse and not fully understood. There is a maximum discharge rate which exists for a two-phase mixture. This occurs at some critical pressure ratio between the upstream pressure and the exit pressure. Several methods have been proposed to evaluate the critical discharge rate of a two-phase flow from a pipe. Here the simple equations described in Ramskill's review are used . The equation to be used is dependent on the length/diameter ratio (L/D) of the leakage path. In all the equations used here the discharge coefficient is takenas 0.6.
Three leakage path situations may be considered: For L/D = 0
We have orifice flow and the flow rate is described by:
M = 0.6 A
2
ρ
(
P
o−
P
a)
( 4 ) whereM is mass flow rate (kg/s) Po is upstream pressure (N/m 2 ) Pa is atmospheric pressure (N/m2) A is area of hole (m2)
ρ
is density (kg/m3)Examples of this situation are small holes in pipes or equipment, e.g. due to corrosion For 0 < L/D <12
For this situation the flow rate will lie between that for orifice flow and that for two phase equilibrium flow. For most flange leakage cases as well as pump seal failures, the L/D value lies in this ranges. The conservative figure given for orifice flow should be used unless a more accurate figure is required. In which case refer to Ramskill ( Ref.1)
In the scenarios that are listed in the in the worked example , orifice flow (i.e. L/D = 0 ) has been used.
For L/D ≥ 12
The critical mass flow rate can be calculated using the two phase equilibrium flow model given below. For more detail see Ramskill (Ref.1)
(1). Calculated the critical pressure Pc at The pipe exit as :
(2) The critical temperature Tc ( °K ) corresponding to the satured choke pressure
Pc is then found using vapour pressure data.
(3) Assuming thermodynamic equilibrium, the vapour mass fraction which would flash off from the liquid is calculated :
m = 1 – exp (
λ
−
c
(T1 –Tc )) (8)
where
m = vapour mass fraction
c =
liquid specific heat (j/Kg) at Tc
λ
= latent heat of vaporization of liquid (J/Kg) at TcT1 = reservoir storage temperature (°K)
Note : If m is negative then liquid flow only will occur and equation (4) should be used to calculated the flow rate.
(4)
Assuming homogeneous mixing and so slip between the phases, the mixture density is calculated as :
ρ
c
ρ ρ
m
g
m
= +
−
−1
1
1 (9) whereρg is the vapour density at Tc and Pc (kg/m 3
) ρ1 is the liquid density at Tc and Pc (kg/m
3
)
(5) The standard discharge formula is then used to calculate the critical flow rate :
M = 0.6 A
2
ρ
c o c( )
P P
−
( 10 ) where M is the mass flow rate ( kg/s)This calculation should be used for large leaks and ruptures of lines and hoses, where two-phases flow will form upstream of the break. Note that if the leak is fed by a pump the flow rate could be determine by the pump capacity rather than the flow regime. In the case of a pipe rupture the escaping LPG will probably be fed from both sides of the break, e.g. from the pump as well as from the receiving vessel.
3.3 VAPOUR JETS – DISPERSION AND FIRES 3.3.1 Relief valve vapour releases
Single –phase (vapour only) emission is assumed from vessel relief valves, as generally observed under fire engulfment conditions. The case of vessel overfill and subsequent two phase emission through the pipe is taken as a special case of the two-phase jets and fires dealt with in the next section.
For the discussions of relief valve dispersions and fires which follow ,dispersion and jet fire radiation calculations have carried out for wide range of emitted vapour flow rates, which cover all sizes of tanks in general use within Group operations. The selected flow rates are:
Propane : 2.5, 5, 7.5,10,15, 20, 25, 30, 40 ( kg/s) Butane : 2.5, 5, 7.5, 10,15, 20, 25, 30 ( kg/s)
In deriving the various consequences it is necessary to make various assumption in regard to the particular facility and equipment. In order to carry out calculation to complete Tables 2 and 3,typical tanks sizes, valves sizes, vent stack height and length have been used (see Figure 1) . Where the facility under examination differs from this model facility, interpolation between the calculated figures will be necessary.
3.3.1.1 Dispersion
As the source of emission under consideration in this section is vapour only, the PLUMEPATH dispersion package has been used for emissions well above ground level. An example is shown in Figure 2 for butane emitted from a vertical relief valve vent pipe. Dispersion plumes have been calculated for the mass flow rates set out above. In all these cases mixing with air occurs rapidly and it may be assumed that flammable plumes do not reach down to the ground. Therefore no dispersion data is presented here for vapour emission from relief valves.
3.3.1.2 Fires
The radiation fields generated by relief valve fires have been modeled with computer packages which have been developed by Thornton Research Centre and validated by experimental studies. Tables 2 and 3 present distances to the critical ground level fluxes of 1.5, 5, 8 and 13 kW/m² and to the critical tank top level fluxes of 8, 32 and 44 kW/m², as identified in the Manual/PTS.
A typical wind speed of 5 m/s has been assumed in all calculations. It should be noted that if wind speeds are considerably higher radiation flux levels close to the stack can be significantly higher. If this could be critical, further radiation calculations will be required for higher wind speeds in which case PETRONAS should be consulted.
As may be seen from the Tables the higher flux levels are frequently not achieved. In addition to the critical distance data, the Tables also present flame lengths, flame lift-off and stack outlet height above grade.
3.3.2 Other vapour releases
For the dispersion of vapor releases from sources which do not behave as relief valve releases, in that they may be horizontal and near the ground, dispersion calculations will be very unreliable. This is because the presence of other equipment in the area will cause unpredictable air movement. The distance to lower flammable limit (LFL) will be smaller than the conservative estimates which can be obtained by using the data in Figure 3 for liquid releases.
For the assessment of vapour fires reference should be made to the vapour fire curves shown in Figures 4 to 16 (a description is given in section 3.4.2). As propane is less radiative than butane these results may be used for propane as well.
3.4 TWO-PHASE JETS - DISPERSION AND FIRES
Two-phase jets may arise from a wide range of events in a facility and may lead to a variety of consequences. As set out in Part One of these Guide-lines, the following situations have been identified as having the potential for the creation of a serious incident.
− flange (joint) leak
− pump seal leak
− open drain valve
− small bore pipe rupture
− hose leak or rupture
− tank overfill/relief valve discharge
As may be seen from this list, a more disparate collection of release situations is possible in comparison to the relief valve cases considered in the previous section. Again consequence information is presented calculated for a wide range of mass flow rates at the source. The only geometrical factor which has been introduced to help differentiate two phase discharge cases is the division into horizontal and vertical emissions.
A wide range of flow rates to cover the range and scale of the principal incidents listed above have been considered.
3.4.1 Dispersion
Before describing the dispersion calculations adopted and results obtained, some words of caution are necessary. The calculation of highly-turbulent flashing two-phase flows is an active and difficult research field. The guidance provided here is therefore subject to revision as better models are developed and validated.
The principal purpose of the dispersion calculations is to assess the extent of the flammable cloud which will be formed. The distance to the lower flammable limit (LFL) is thus the main parameter calculated. This then enables a judgement to be made in regard to facilities engulfed by the flammable plume and the proximity to the site boundary. As mentioned in Part One, the initial consequence of ignition of this cloud will be a transient flash fire which although of high intensity will only seriously affect people within it. The major effect of a flash fire will be to initiate a jet fire or, much less likely, a pool fire.
Dispersion distances for dense gases, such as LPG vapours, are strongly dependent on the nature of the surface over which the gas disperses. The relevant parameter used in the models, called the surface roughness length, can be estimated. A reasonable conservative value relevant to typical LPG facilities is 0.1 m.
The vertical temperature gradient in the atmosphere has a considerable effect on gas dispersion. Strong surface cooling, under clear skies at night, and a low wind produce stable conditions. Weather conditions are denoted by letters A to F (after Pasquill
-
discussed in Ref. 4). The most stable conditions are denoted by the letter F. Neutral conditions, under cloudy skies or in higher winds, are most common, andare given the letter D. Strong sunshine in the daytime, with low winds, produces unstable temperature gradients, the most extreme being denoted by A. In these circumstances, gas will dilute in a shorter distance than for D stability. The largest dispersion distances are found in F stability weather. Calculations are normally performed for 5 m/s wind speed and D stability (5D) to give typical average results. 2 m/s wind and F stability (2F) represents a typical worst case.3.4.1.1 Horizontal dispersion
The source for the dispersion calculations was assumed to be a flashing jet of pressurized liquid propane or butane. No dilution due to the jet has been assumed. For horizontal jets directed downwind, this dilution is counteracted by the fact that the jet pushes the gas further downwind. For other jet orientations, the results are conservative (predicted dispersion distances greater than actual).
Figure 3 presents distances to LFL for the 5D and 2F atmospheric conditions for a surface roughness factor of 0.1 metre.
3.4.1.2 Vertical dispersion
Calculation methods for two-phase vertical releases are still being developed. In the meantime the horizontal dispersion calculations described above may be taken as a worst case analysis of the vertical dispersion case. Where this failure mode is critical, PETRONAS should be asked to advise.
3.4.2 Fires
The radiation fields generated by two-phase Jets have been calculated with computer models which have now been partially validated by large-scale jet fire trials. A typical worst case wind speed of 5 m/s has been assumed in all calculations.
Results for varying mass flow rates are presented for butane in Figures 4 to 16. As propane is less radiative than butane, these results may be used for propane also.
3.4.2.1 Horizontal jet fires
Curves showing the distance to critical flux levels for a range of mass flow rates are presented in Figures 4 to 9. These give radiation fluxes at ground level. The flame shapes and lengths are different for liquid and vapour fires. Two limiting curves are shown, the upper for liquid and the lower for vapour fires. Depending on the precise ratio of liquid to vapour, actual cases will lie between the two extremes. In the high momentum jet fires which these Figures represent, jet momentum decreases with increasing distance along the jet until at a given point buoyancy forces become dominant and the flame lifts off the ground. This phenomenon enables an effective jet impingement distance to be defined.
Figure 10 shows the horizontally projected flame lengths calculated for the range of flow rates.
3.4.2.2 Vertical jet fires
In general, vertical jet fires may be expected to have a smaller radiative impact than the corresponding horizontal jet fire. A set of curves showing the downwind distance to critical flux levels for vertical jet fires with a wind speed of 5 m/s is presented in Figures 11 to 16.
These do not give exactly the same figures as Tables 2 and 3 because there is no stack pipe and the emission conditions are different.
4. WORKED EXAMPLE
In order to demonstrate the application of Sections 2 and 3 of the Guidelines, a worked example of a study at an existing facility has been completed. This part of the Guidelines follows the assumptions, considerations and calculations involved in that example.
4.1 DESCRIPTION OF FACILITY
The facility is an LPG depot which is supplied by barge via the local river. It comprises a barge unloading berth, bulk storage for both commercial propane and commercial butane, and bulk road vehicle loading. The unloading berth is separated from the remainder of the facility by a motorway. Figures 17 and 18 show the depot layout and a simplified flow scheme, respectively.
4.1.1 Barge unloading berth
The unloading berth includes two 100 mm dia loading arms, one for liquid service, the other for vapour. Barge unloading is performed using a shore-based compressor. The loading arms are hydraulically operated from a control station at the berth. Each arm contains several swivel joints and the connection with the barge is via a flanged joint. The arms are not equipped with a breakaway coupling or other quick release device.
On the shore side of the loading arms, the line from each arm bifurcates for either propane or butane service. The vapour line is equipped with an hydraulically-operated ball valve before the line bifurcates. This valve is actuated electrically from the compressor station. The liquid line has no such valve.
As soon as each line bifurcates the four lines (i.e. propane liquid, butane liquid, propane vapour, butane vapour) are each fitted with two flanged manually-operated ball valves. Each liquid line is then fitted with a flanged non-return valve. There are several small bore fittings for instruments, draining and hydrostatic pressure relief in this area.
Once the pipelines are clear of the valve locations, the two vapour lines are lagged to prevent vapour condensation.
Firefighting facilities at the barge berth consist of dry chemical fire extinguishers. A hydrant is located on top of the river bank some 30 metres from the loading arms. This hydrant is partially obstructed from the berth by a building housing fire pumps which take suction from the river. A fire alarm is located on the berth.
There is no emergency shutdown or leak detection system in place on the berth apart from the remote operated valve on the vapour line.
4.1.2 Bulk storage
The vapour pipelines to the barge berth are routed via a compressor which takes suction either from the vapour space on one of four butane spheres (each of 1 025 m3 capacity) or from the vapour space of one propane sphere (1 750 m3 capacity). The liquid filling line into the bottom of each butane sphere also doubles as a liquid withdrawal pipe. The liquid filling line into the propane sphere is directed into the top of the vessel.
The four butane spheres each have two flanged connections directly under the vessel. These connections are liquid inlet/outlet and drain. The liquid inlet/outlet is fitted with a flanged hydraulically-operated ball valve followed by a Shand and Jurs hydraulic valve. The valve nearer the sphere includes a fusible link which is designed to cause the valve to fail closed under fire engulfment conditions. The drain connection is equipped with a flanged manually-operated ball valve followed by a spring-loaded dead-man ball valve. The drain line then extends beyond the periphery of the sphere. This line is lagged as is the liquid inlet/ outlet line.
The top connections into each butane sphere comprise vapour line, pressure relief valves, Whessoe contents gauge and maximum fill level float gauge. All connections are flanged. The vapour line is equipped with an hydraulically-operated ball valve, again actuated electrically from the compressor station. The Whessoe contents gauge is local readout only. The maximum fill level float gauge is linked to the compressor station such that in the event of overfill the compressor will cut out.
The propane sphere has a welded line from the base of the sphere to the primary valve which is located at the edge of the bunded area some ten metres from the periphery of the tank. The primary valve is flanged and is remotely operated. It is followed again by a Shand and Jurs valve. A catchpot is located downstream from these valves and a lagged drain line connected to the pot. The drain line is fitted with a manual valve followed by a spring loaded valve.
The top nozzles into the propane sphere are as for the butane spheres plus the liquid inlet line. The liquid inlet line is fitted with an hydraulically-operated ball valve with electrical actuation from the compressor station as above.
All pressure relief valves discharge direct to atmosphere. There is no flare or vent system. All spheres are located in separate low-bunded areas. All are equipped with a water spray sprinkler system.
The nozzle details for the propane and the butane spheres are indicated in the attached
Figures 19 and 20.
4.1.3 Bulk road vehicle loading
Product stored at the depot is pumped into bulk road vehicles. The bulk loading facility includes a swivel-jointed loading arm for liquid transfer. There is no vapour return line. Connection with the road vehicle is made using an Acme coupling. The loading arms are equipped with a manually-operated valve adjacent to the Acme coupling. There is no breakaway coupling fitted or other driveaway protection/prevention. A remotely-operated valve is installed in the line upstream from the loading am. This valve is operated from an emergency stop located directly adjacent to the loading point. Loading is performed using a preset turbine meter. The loading area is protected from vehicular damage by highway guard-railing.
Firefighting facilities at the loading area consist of hand held fire extinguishers plus adjacent fire hydrants. There is no sprinkler system nor are there fire water monitors.
4.1.4 Depot operation
Barge unloading is completed under the control of a shore-based supervisor in conjunction with barge crew. The supervisor does not stay at the barge berth during the operation; he is mostly at the compressor station which is located over 100 metres away from the berth and not in direct line of sight. The supervisor checks the pipelines and the berth about once per hour during unloading operations. The unloading rates are approximately 180 m3/hour for propane and 140 m3/hour for butane. At the completion of unloading, liquid lines are blown clear of liquid as far as is possible, using the plant compressor.
The compressor station is equipped with a control panel which enables the supervisor to set the remotely operated valves on the storage tanks and on the vapour line at the barge berth. The supervisor thus controls transfer of product into the spheres. As stated above, there is no remote readout of product level in each tank, only a high level overfill cutout. Filling level is controlled by observing the local readout from the Whessoe gauges. The bulk road vehicle loading operation is controlled by the vehicle driver. The depot supervisor is usually not present during loading operations. The driver does not have access to storage tank valves, only to valves in the delivery pipework. Loading rate is approximately 20 m3/hour.
4.2 AUDIT OF THE FACILITY AND CHOICE OF SCENARIOS
The audit of the facility was completed by a team of four people including the depot operations superintendent, a company safety adviser and two LPG engineering specialists. The team spent a day on site gathering information and inspecting the layout, operation and maintenance of the facility. Depot staff were able to advise on operations and maintenance procedures and engineering staff provided layout drawings, flow schemes and technical data (i.e. pump curves, pressure relief valve data, etc.).
Having assembled all relevant information the team then studied the facility to determine those situations which might result in leakage of LPG. The approach adopted was to commence with the barge berth and then follow the pipe track to the compressor station, into the storage vessels, and then out to the bulk vehicle loading point. Using the procedure set out in Section 2.1 the team identified a number of leakage scenarios which they considered to be credible and/or the consequences potentially severe
.
4.2.1 Barge berth 4.2.1.1 Scenario 1
There are no protective devices on the loading arms (e.g. breakaway couplings etc.) to prevent damage or rupture in the event of excessive barge movement. It was therefore decided that loading arm rupture should be considered. There are no emergency shutdown valves on the barge so full bore continuous flow driven by the pressure in the barge tank was adopted. There are similarly no emergency shutdown valves on the liquid lines to the storage tanks on shore. Although the non-return valve in these lines decreases the probability of liquid flow from the pipelines, it cannot be relied upon for emergency shutdown purposes. The pipeline must therefore also be included as an additional leakage source.
Scenario 1: Loading am rupture. Leak fed by barge storage and lines on shore. 4.2.1.2 Scenario 2
The barge berth area contains many flanged joints, the loading arms contain several swivel joints, and there are numerous small bore connections in the area. The number of joints and connections warrants the consideration of a flange or joint leak.
As the liquid line is blown clear after each delivery, there is no need to consider the effects of barge impact during berthing.
Scenario 2: Flange, swivel joint, or small bore connection leak during barge
4.2.1.3 Scenario 3
The pipelines from the barge berth to the storage vessels run parallel and adjacent to the motorway for some distance before crossing under the motorway. The pipe track is lower than the motorway and it is conceivable that either a vehicle or goods from a vehicle could leave the motorway and impact upon the pipelines.
Scenario 3: Damage to pipelines due to vehicular or vehicular goods impact. 4.2.2 Pipe track
4.2.2.1 Scenario 4
The pipelines from the barge berth to the storage tanks are flanged along the entire length. Where the pipelines enter the depot after crossing under the motorway there are a number of redundant valves and connections which, as above, warrant the consideration of a flange leak in this area. The pipelines enter the depot at the south west corner of the property. A concrete block wall in excess of two metres high separates the depot from the neighbouring property.
Scenario 4: Flange leak on pipe track at entry point into depot. 4.2.3 Storage vessels
4.2.3.1 Scenario 5
The pipelines at the barge berth and throughout the depot are not clearly marked to indicate product carried. There is no interlock in the valving system to prevent propane being delivered into the butane pipework. Given the above, it is quite conceivable that propane could be delivered into the butane spheres. The butane spheres are not designed for propane vapour pressure.
Scenario 5: Propane delivered into butane rated spheres. 4.2.3.2 Scenario 6
The butane spheres are fitted with flanges on the sphere side of the primary valve on the liquid inlet/outlet and on the drain line. Given the inability to control a leak from these flanges and the sphere inventory, leaks from these flanges must be considered.
Scenario 6: Leaks from flanged joints on butane spheres on sphere side of primary valve.
4.2.3.3 Scenario 7
Based on Scenario 6, a fire fed from a flange leak underneath the butane spheres must be considered. This fire may in turn lead to overpressurisation of the vessel and consequent vapour discharge through the vessel relief valve(s).
Scenario 7: Vapour release from butane sphere pressure relief valve due to fire engulfment.
4.2.3.4 Scenario 8
The Whessoe level gauges on the butane and propane spheres are only of the local indicator type. They are not equipped with alarm or emergency shutdown features. The lack of a remote readout at the control point in the compressor station is likely to lead to an estimation of ullage by the depot supervisor and the possibility of error.
The maximum fill level gauge can only be tested by exceeding the safe filling level. The gauge is not tested and its reliability is therefore questionable.
The level instrumentation as described above on both the butane and propane spheres is inadequate. Overfill of the spheres must therefore be considered.
In the event of overfill, liquid will flow through the vapour suction line to the compressor. The knockout drum adjacent to the compressor is not equipped with any liquid level alarms or trips. Liquid could therefore enter the compressor giving a liquid stroke. This would cause a major leak.
Scenario 8: Overfill of propane/butane spheres leading to major leak at compressor.
4.2.3.5 Scenario 9
The propane sphere is fitted with a flanged connection on the liquid outlet line on the sphere side of the primary valve at the edge of the bunded area. As for the butane case a leak from this flange must be considered.
Scenario 9: Leak from flanged joint on propane sphere on sphere side of primary
valve.
4.2.3.6 Scenario 10
A fire fed from the flanged joint leak on the propane sphere in Scenario 9 may lead to overpressurisation of the vessel and consequent vapour discharge through the vessel relief valve(s).
Scenario 10: Vapour release from propane sphere pressure relief valve due to
fire engulfment.
4.2.4 Product transfer 4.2.4.1 Scenario 11
The pumps for transfer of product from the storage spheres to the bulk vehicle loading point are located at the edge of the bunded area approximately 10 metres from the tank shell. Each pump is fitted with a mechanical seal but not with a throttle bush. Total failure of a pump seal is not a common event but neither is it rare enough to discount.
Scenario 11: Leak from pump seal due to total failure of seal. 4.2.4.2 Scenario 12
The compressor station is an area of complicated pipework again with many flanged joints and small-bore connections.
Scenario 12: Flange or small-bore connection leak at compressor station. 4.2.5 Bulk road vehicle loading
4.2.5.1 Scenario 13
As for the barge berth there are no protective devices on the loading am at the bulk road vehicle loading point to prevent damage or rupture in the event of a driveaway. Loading am rupture is therefore included. The only emergency stop button in this area is located adjacent to the loading point. It would probably beinaccessible in the event of loading am rupture
.
The bulk vehicles are equipped with emergency stop buttons on each corner of the tank frame. However, given that a positive action is required to activate these buttons and that such action is required adjacent to a large leak. the vehicleshould still be considered as a leakage source.
Scenario 13: Loading arm rupture. Leak fed by delivery pump and bulk road vehicle storage tank.
4.2.5.2 Scenario 14
The loading area also contains many flanged and swivel joints and small-bore fittings.
Scenario 14: Flange, swivel joint or small bore connection leak at bulk road vehicle loading point.
4.2.5.3 Scenario 15
The coupling between delivery pipework and vehicle is a vulnerable area in that the driver may not make the coupling correctly or a sealing ring may be damaged or missing. The coupling must be made at each delivery. The frequency of connection warrants the inclusion of possible leakage.
4.3 CONSEQUENCE ASSESSMENT, ANALYSIS AND PROPOSED ACTION
In this section each of the leakage scenarios is considered in turn and a consequence assessment is completed as described in Section 3. The acceptability of the consequences are then considered and possible actions suggested to reduce the probability of the scenarios, reduce their impact by minimising the leakage rate, and/or provide additional protection to people, property and equipment.
The possible actions suggested for this example should not be seen as 'correct answers' for these scenarios. Neither are they exhaustive. They are merely typical of the types of action which might be relevant to a particular situation. Actions should be selected according to specific site requirements.
With the exception of vapour release from pressure relief valves, the consequence assessments have only been completed for leaks from liquid pipelines since these leaks give more severe consequences than for the vapour case.
If the proposed actions for offending scenarios in liquid lines are not to be also applied for the vapour cases then these cases must be considered in their own right.
4.3.1 Scenario 1: Barge berth loading arm rupture
Calculations have only been completed for the propane case since the vapour pressure, release rate and consequent hazards are higher than for butane.
Leak = Two phase leak from 100 mm dia hole From Section 3.2.2.2
Adopt L/D >12 Pc = 0.55 Po
For Po choose equilibrium vapour pressure at 15°C =
9 x 105 N/m2 (Ref Fig 01.02.08.01 of Manual /PTS) Pc = 0.55 x 9 x10 5 = 4.9 x 105 N/m2 Tc = 268°K m = 1 – exp [
λ
−
c
( T1 – Tc)]with c = 2 407 J/kg°K (Ref Fig 01.02.11.01 of Manual /PTS) and
λ
= 383 000 J/Kg (Ref Fig 01.02.10.01 of Manual /PTS)m = 1 - exp
−
−
2 407
383 000
( )
288 268
= 0.12 Mixture density ρc =m m
gρ ρ
+
−
−1
1 1 ρc =0 12
9 3
0 88
535
1.
.
.
+
−(Ref Tables 01.02.04.01 and 01.02.0.03. of Manual /PTS.) = 69kg/m3
Critical flow rate M = 0.6 A
2
ρ
c(
P
o−
P
c)
= 0.6 xπ
x
0 1
x
x
x
4
2
69
9 10
4 9 10
2 5 5.
(
−
.
)
= 35.4 kg/secFrom Figure 3 estimate distance to LFL as follows: 5D conditions - 105 m
2F conditions - 190 m
Probability of early ignition is high. Vehicles on motorway most likely ignition source.
From Figures 4 to 16 distances to radiation flux levels from the ignited mixture are as follows:
Horizontal Jet Fires Vertical Jet Fires
1.5 kW/m² 140 m 120 m
5 kW/m² 110 m 65 m
13 kW/m² 90 m 40 m
This scenario is clearly unacceptable, even though the leak considered above is fed by the barge storage alone. The leakage rate would be much greater if the shore based pipelines were also to contribute to the leak.
The consequences of this incident are so severe that the leakage rate and duration should be reduced. The probability of the incident should also be reduced.
Possible means of achieving the above are:
• Emergency release couplings on loading arms.
• An inter-related system of remotely-operated fail-safe emergency shutdown valves at
the termination of the barge pipework and on the berth on the shore side of the loading arms.
• Inclusion of fusible links in emergency shutdown systems.
• Supervision during entire unloading operation at the berth.
• Improved fire protection/fire fighting and gas detection/ gas dispersion facilities.
• Construction of a water curtain adjacent to the motorway.
4.3.2 Scenario 2: Flange., swivel joint, or small-bore connection leak at barge loading berth
In this scenario, typically consider one segment of a flange gasket blown out. Based on a number of pipe sizes from 50 mm to 300 mm diameter. the average leakage path area equates to an effective hole diameter of 12 mm. A small-bore connection leak would give a similar case. As above, the calculation is only completed for the propane case.
From Section 3.2.2.2 Adopt L/D = 0 M = 0.6A
2
ρ
(
P
o−
P
a)
= 0.6 xπ
x
0 012
x
x
x
4
2
510
9 10
1 10
2 5 5.
(
−
)
= 2.0 kg/secFrom Figure 3 estimate distance to LFL as follows: 5D conditions - 25 m
2F conditions - 35 m.
The elevation of the barge berth is lower than the motorway and the berth is elevated above the river. The above distances will therefore be conservative because LPG vapour is heavier than air and the vapour will tend to fall to the river.
From Figs 4 to 16 distances to radiation flux levels from liquid fires are as follows: Horizontal Jet Fires Vertical Jet Fires
1.5 kW/m² 55 m 45 m
5 kW/m² 40 m 30 m
8 kW/m² 40 m 25 m
13 kW/m² 35 m 20 m
The scenario is unacceptable. The radiation flux levels on the motorway particularly are excessive.
The probability of leakage should be reduced.
The pipework in the barge berth area contains many joints and small bore connections. Possible action to reduce the probability of leakage includes:
• Rationalisation of pipework to reduce number of joints/ connections.
• Replace flanged joints with welded joints.
In addition, possible action to prevent the vapour cloud reaching the motorway and/or to protect the motorway from the effects of an ignited leak includes:
• Construction of vapour barrier wall.
• Installation of water curtain with or without automatic actuation triggered by gasdetectors on the berth.
4.3.3 Scenario 3: Damage to pipelines due to vehicular or vehicular goods impact
In this scenario the range of damage caused by impact could range from shearing the pipelines to springing a flange. Typical leakage rates for these two extremes have already been considered in the above two scenarios.
As both the above scenarios are unacceptable then this scenario is also unacceptable. Possible actions are:
• Install reinforced highway guard-railing adjacent to pipe track.
• Bury pipelines in this area.
• Install remote operated emergency shutdown valves at either end of vulnerablepipework.
4.3.4 Scenario 4: Flange leak on pipe track
The leakage rate in this scenario will be as for the similar case at the barge berth (i.e. M = 2.0 kg/sec). Where the pipelines enter the depot there is significant local confinement, especially in a disused compressor shed. Confinement raises the possibility of vapour cloud explosion in the event of ignition. The probability of ignition is high due to the adjacent motorway and offices. This scenario is unacceptable.
Possible actions:
• Reduce probability of leakage as for Scenario 2.
• Examine need for disused building and demolish if possible and/or investigate other means to reduce confinement.
4.3.5 Scenario 5: Propane delivered into butane rated spheres
In order to calculate the vapour pressure of a mixture created by delivering propane into the butane spheres, a worst case of assuming 100 per cent propane at a receipt temperature of 25°C into an empty sphere has been adopted.
The vapour pressure of propane at 25°C is 11.5 x 105 N/m² which is higher than the design pressure of the butane sphere of 7.3 x 105 N/m² . This is also the start-to-discharge pressure of the relief valves. The fully open pressure (accumulated pressure) is 8.0 x 105 N/m².
The scenario is unacceptable. If product contamination is a credible scenario, then the relief valves should be sized to relieve sufficient propane vapour to avoid over-pressurization of the vessel.
Possible actions to prevent product contamination are:
• Clear and positive identification of pipelines throughout the depot and particularly at the barge berth.
• Introduction of a valve interlock system to ensure only one set of valves is open at the barge berth.
• Installation of in-line densitometers linked to an alarm/ shutdown system to detect incorrect product in pipelines.
4.3.6 Scenario 6: Leaks from flanged Joints on butane spheres on sphere side of primary valve
The leakage in this scenario will again be based on leakage from an equivalent hole diameter of 12 mm. From Scenario 2 M = 0.6A
2
ρ
( )
P P
o a−
= 0.6 xπ
x
0 012
x x x
4
2
577 3 1 10 1 10
2 5 5.
( . )
−
= 1.1 kg/secFrom Figure 3 estimate distance to LFL as follows: 5D conditions - 15 m
2F conditions - 25 m
From Figs 4, 5, 6, 7, 9 distances to radiation flux levels from liquid fires are: Horizontal Jet Fires
1.5 kW/m² 40 m
5 kW/m² 30 m
8 kW/m² 25 m
13 kW/m² 25 m
44 kW/m² 20 m
Vertical jet fires will impinge on the vessel shell in all cases. No radiation distances are therefore given.
In the event of ignition the radiation effects on the sphere and on adjacent spheres exceed 44 kW/m² and are therefore unacceptable.
Possible actions to reduce the probability of this scenario are:
• Modification to sphere pipework and provision of correct valve to remove flanged joint upstream of primary valve.
4.3.7 Scenario 7: Vapour release from butane sphere pressure relief valve
From Section 3.2.1, for fire-engulfed butane tanks
W =
367
KAP
kg/sec
Spheres are equipped with two pressure relief valves although only one valve is lined up at any time. The valves are labelled as 6R10.
From Table 1c for R orifice, Discharge area = 103.23 x 10-4 m² W =
0 9 103 23 10 7 3 12 10
367
4 5. . . .
x x x x x
− = 22.2 kg/sec.Section 3.3.1.1 describes that flammable vapour plumes from vertical relief valves may be assumed not to reach ground level. The distance to LFL is therefore not applicable in this instance.
From Table 3, distance downwind to radiation flux levels are: Tank Top Radiation Flux
8 kW/m² - 45m
44 kW/m² -Ground Level Radiation Flux
1.5 kW/m² - 95 m
5 kW/m² - 40 m
13 kW/m² -
-The above flux levels indicate that the 5 kW/m² radiation contour crosses the site boundary. Whilst this boundary should probably be described as an urban area, the region adjacent to the boundary is not developed and a higher radiation intensity could be tolerated. Given that the flux level at the plant boundary is not much greater than 5 kW/m², this scenario is considered to be acceptable and the existing location of the butane sphere closest to the plant boundary does not warrant any action. Any future development outside the plant should be monitored as construction of a hospital, school or other facility difficult to evacuate at short notice would require a reappraisal of the situation.
4.3.8 Scenario 8: Overfill of propane/butane spheres
Overfill of the propane or butane spheres will not result in product leakage from the spheres but will result in liquid flow through the suction line to the compressor.
A liquid stroke in the compressor is unacceptable. Possible actions are:
• Installation of remote readout level gauge in transfer control area.
• Installation of level gauge capable of registering low level, high level and high-high level, with alarms and inlet valve shutdown devices attached. Gauges should permit regular testing to ensure satisfactory operation.
• Regular testing of maximum fill level gauge.
• Installation of liquid level emergency alarm on knockout drum with compressor trip.
4.3.9 Scenario 9: Leak from flanged joint on propane sphere on sphere side of primary valve
This scenario as per Scenario 2:
M = 2.0 kg/sec. Distances to LFL:
5D conditions - 25 m 2F conditions - 35 m
Distances to radiation flux levels from liquid fires are:
Horizontal Jet Fires Vertical Jet Fires 1.5 kW/m² 5kW/m² 8 kW/m² 13 kW/m² 44 kW/m² 55 m 40 m 40 m 35 m 30 m 45 m 30 m 25 m 20 m 10 m
The radiation flux level on the propane sphere in the event of an ignited leak is excessive, although the location of the primary valve remote from the sphere mitigates the impact on the sphere. Possible actions are as for Scenario 6.
4.3.10 Scenario 10: Vapour release from propane sphere pressure relief valve due to fire engulfment
From Section 3.2.1 for fire engulfed propane tanks:
W =
419
KAP
kg/sec
The propane sphere is equipped as per the butane spheres with two pressure relief valves although only one is lined up at any time. The valve is labelled 6R10.
From Table 1C for R orifice: Discharge area = 103.23 x 10-4 m² W =
0 9 103 23 10 15 5 12 10
419
4 5. . . .
x x x x x
− = 41.2 kg/secAs for the butane case, the distance to LFL is not applicable as the flammable plume may be assumed not to reach ground level.
From Table 2, distance downwind to radiation flux levels are: Tank top Radiation flux
8 kW/m² - 50 m
44 kW/m² -
-Ground level radiation flux:
1.5 kW/m² - 110
5 kW/m² - 40
13 kW/m² -
-As for the butane case, the 5 KW/m2 radiation contour crosses the site boundary, The figures are similar for both the propone and butane case and the points set out for the butane case are therefore also applicable to this case.
4.3.11 Scenario 11: Leak from pump seal due to total failure of seal
Adopt L/D = 0
For pumps without throttle bushing, adopt effective diameter hole =17mm M = 0.6A
2
ρ
( )
P P )
o a−
(a) For Propane
M = 0.6 x
