Vapor Loss From Crude Oil Tankers

VAPOR LOS LOSS S FR FROM OM CRUDE CRUDE OIL OI L TANK TAN K ERS Infl nfluencebydesign of venting ntingsystem

Reference document: INTERTANKO’s “Guidelines for The Control of a Multiphase Crude Oil Cargo For Cargo Operations and Handling INTERTANKO’s VOCON on the same subject (undated).

(March 2001) and

”

P RE RE SS - VA VA C E N G I N E E R I N G A/ A/ S

Guide on minimizing vapor loss by design and operational procedures

Pres-Va -Vac Eng Enginee ineering A/ A/ S Svanevang 3-5 •D •DK 3450Alle llerød •D •Denmark Phone hone +45 48 17 40 55 •Fax +45 48 17 17 88 

V A P O R L O S S M I N I M I Z A T I O N B Y B A S I C D E S I G N

Comments on how to minimize vapor loss by basic design and rethought procedures DISCLAIMER: The views and opinions expressed in this paper are entirely those of Pres-Vac Engineering A/S and are provided without assuming any responsibility in any form, shape, or nature whatsoever.

NOTE: Thereferenced INTERTANKO publications are assumed known to the reader in advance. In the following, said publications are referred to as the “Publication.”

ABSTRACT: This paper shows that release of VOC during voyage, causing loss of cargo and environmental concerns, can be reduced to a fraction of the current level if venting systemsare designed (and used) with these issues in mind. Retrofits are possible without structural changes at a negligible cost. Further, safety aspects will be improved by following the views and opinions in this paper.

Introduction In short, the Publication lists two issues, which combined causethe vast majority of vapor lossduringvoyage: A. Apparently what must betermed as distrust in theefficiencyand reliability of p/ v valves, leading to: B. Procedures involving manual de-pressurization, - as aconsequence of A. above, with closing pressuresbelow the vapor pressureof the crudeoil cargo.

Page 1 of of 3


Thesis & object The issue of distrust in p/ v valves is historically well founded, for which reason the IMO recentlyintroduced: •

aretroactiveand mandatoryconcept of secondary venting, 1 and

•

a new ISO standard for marine p/ v valves, which is mandatory for installations after July1, 2002 (15364is sort of semi-retroactive).

This leads to thefollowing: •

if the venting system could maintain a constant tank pressure above the pressure equilibriumof the cargo; and

•

if the venting systemwas reliable and offered close to nil maintenance,

the two issues listed above as being the main cause of the loss of vapor would be eradicated. The thesis of this memo is that a properly designed venting system using p/ v valves of areliable and maintenance friendly design under the auspice of the ISO standard can make redundant the practice of manual de-pressurization. It is further the thesis that not only would this conserve enormous volumes of cargo otherwise lost at great expense, but environmental and safety aspects will be improved considerably, too. The muchdiscussed avenue of installing Vapor Control Systems for onboard use could most likely become obsolete in an instance and great savings achieved economically and safety-wise. The object of this memo is therefore aimed at providing support for the following conclusions: 1:

Whether or not theIMO initiativein regard of developingavalvestandard is sufficient in regard to establish a new level of trust and comfort with regard to the reliability of p/ v valves, and in theaffirmative; 2:

Whether or not manual de-pressurization can be completely avoided and what design parameters would be necessary for the appropriate venting systemand equipment for this to succeed.

1 Equipment which is not taken into consideration in the Publication. However, with good reason

because thep/ v breakers referred to in the Publication have generally been accepted as secondary venting means according to the new regulations, despite their failing to meet the most basic requirements of SOLAS II-2, rule 59. The irony is that the specific tanker cited by the Australian DOT as featuring an inadequate venting system– on basis of which SOLAS was revised - has been approved as being in full compliance with the revised regulations as is. A full report on the SOLAS amendments is available at www.pres-vac.com

Page 2 of of 4


Available means to reduce vapor loss T H E

Threesimplebut extremely effectivemeans areavailablethat havepotential to significantly reducethe loss of vapor fromcrude oil tankers during voyage:

A P P R O A C H

1. Revised operational procedures with regard to the necessity of manual depressurization, if needed at all. 2. Increased openingsettingand closingpressure of pressurerelief valves- and corresponding changes to related systemcomponents, e.g. alarm settings. 3. Appropriatedynamic flow characteristicsof said pressurerelief valves. In the following, the operating procedures onboard crude tankers are assumed to be as described in theaforementioned INTERTANKO publications

I N T E R T A N K O P U B L I C A T I O N

According to the INTERTANKO publication, tank pressure is generally maintained between 1,000 mm WG and 400 mm WG during voyage. This restriction is adhered to becauseof lack of trust with regard to the pressure/vacuumvalves, thus calling for manual pressurecontrolling via acentral riser. Thereport reads:

“However, results andobservations for thepractice aboard tankers reveal thattankercommandsdonotgenerallyrelyuponP/ V valves for over pressure control and will releasepressuremanually through oneofthepreviouslydiscussedonboardsystemswhentheover pressurereachesabout 1,000mmWG. This practice, being absolutely necessary to protect the vessel’s structure, reflects good seamanship by preventing abnormal wear to a safety device. The results and observations from the practice aboard tankers with regard to the selected pressurefor closing of the manually opened release mechanism seem to show apressure of approximately 400 mm WG asthecommonlyselected.Itisthedeterminationof thispressurethat needstobedefinedsuchthatunnecessaryreleaseofboth hydrocarbonandInertgasesisavoided.”

K E Y Q U E S T I O N S

Where does this leave us?

As outlinedin the above, weneed to consider 1) what the situation is in regard to availability of reliable p/ v valves and 2) how to avoid manual de-pressurization. The first issuemust be answered

Page 3 of of 5


satisfactorily in order to deal with thesecond issuein aproper manner. Accordingto the VOCON cited in the foregoing, therecommended solution is to establish the vapor pressure for each cargo and limit the manual de-pressurization to wherethe boiloff begins. This, however, would appear a not optimum solution compared to a venting system that completely eliminates the need to do manual de-pressurization. It is recommended to stop the de-pressurization at 800 mm WG. With reference to Fig. 5 it would, however, appear that the loss of vapor would only be reduced by 1/ 3 in volume, although it must be noted that exact numbers are impossible to give. In reflecting on the above, a review of current requirements for p/ v valves is necessary with emphasis on IMO MSC/ Circ. 677/ 1009, cf. ISO 15364.

Is it fair not to trust the p/v valves? Theanswer is definitely affirmative, - p/ v valves cannot betrusted, generallyspeaking. This statement, coming from a p/ v valve maker, may appear somewhat strange. However, reality of the matter is that this kind of safety equipment is generally designed and tested with no regard whatsoever to the actual working environment. Further, in the complete absence of internationally accepted quality assessment instruments, the single criterion for selection and acceptance has been type approvals. In reality, this has left design of p/ v valves completely at the discretion of each manufacturer with little if any interest paid by owners, administrations, classes, and other bodies once a valve is type approved. The criterion for typeapproval (since 1984) is fire testing in a laboratory, which has absolutely nothing to do with the reliability issue. Only a tiny fraction of all p/ v valves will be subjected to areal fire incident, whereas theyall are functioning as balancing relief valves in practice. Designpriorities havebeen upside down. When commands do not trust the p/ v valves, it is not because they fear a flash-back incident happening; theyfear that the valves will not manageto balance the tank pressure and therefore they involve themselves and conduct manual pressure adjustments. Considering the high number of over-pressure incidents reported to the IMO in connection with recent revisions of SOLAS, the commands’ lack of trust would appear fully justified, from ageneral point of view. One issuethat has been brought up on earlier occasions is the potential misunderstanding that may occur when breather valves are misunderstood for full flow valves. However, by now, ISO 15364 leaves the capacity issue at the responsibility of the buying entity when submitting the mandatory sizing data required by the section of the ISO standard titled “Ordering Information”. There should consequently be no misunderstanding possible when interpreting the“Master’s loading chart” required by IMO MSC/ Circ. 731.

Page 4 of of 6


W H Y

V A L V E S M A L -

F U N C T I O N

A lot of p/ v valves are proneto mal-function becauseof insufficient toleranceto clogging by cargo vapor deposits, IG residue, corrosion, and freezing water. Some valve designs come with net clearances of less than 1 mm between moving parts and inner walls. It should be quite obvious that the maintenance required for keeping such devices in good working order is prohibitive. Some valves have check-lifts that push thediscs 1/100 of the full stroke, leaving check-lifting closeto being an illusion. Somevalves are totally depending on internal drain holes, which cannot be controlled from the outside and therefore require gas-freeing and removal. Other valves are so complicated to inspect and dismantle that time is prohibitive for maintenance. And other designs are so vulnerable to over-icing that they must be inspected and ice-freed every watch. The list goes on and on. What is an un-welcomed fact, however, is that these designs are all type approved and – formally - rightfully so. The problem causing this situation is lack of interest, lack of standards, and overlytrust bythe various parties involved with regard to thetraditional type approval procedures according to IMO regulations without realizing that they simply do not cover the many aspects related to practical useof the equipment.

S E C O N D A R Y V E N T I N G

In response to a number of over-pressure incidents, IMO decided to amend SOLAS and require secondary venting systems and at the same time initiated a new ISO standard (15364) for marine p/ v valves. Due to the numerous interpretations accepted into the application of secondary venting systems, the SOLAS amendment hasprobably had littleif any effect. Especially considering that the not-trusted p/ v valves do not gain trustworthiness by doubling, cf. page 2, footnote 1. Introduction to ISO standard 15364

The standard contains some test requirements, but they are few indeed and only for flow testing. Since regular flow testing according to IMO MSC/ Circ. 677/1009satisfies 15364, the new test requirements are of interest only with regard to non-SOLAS vessels. 15364 is mandatory by referencein IMO MSC/ Circ. 1009for all valve installations on or after July 1, 2002. The purpose of 15364 is to highlight technical issues of importance to in-service performance, i.e., issues, which are not covered by type approval testing. The explicit purpose of IMO having this standard prepared was the number of incidents caused by malfunctioning p/ v valves, i.e., existing designs are not performing well enough, again generally speaking, and new designs should be expected. P R O D U C T R E V I E W

D O C

O W N E R

I S

R E S P O N S I B L E

Due to the endless design approaches available, not to mention different applications, issues such as reliability and in-service performance cannot be dealt with by means of test requirements expressing themselves in one-page certificates. The approving body should therefore issue a Product Review Document (PRD) outlining the design specifics of a given product, leaving it at the discretion of the user to decide whether or not it will be suitable for the specific installation using ISO 15364 as an assessment tool. The approving body and the manufacturer have not determined compliance for a specific application; that is the responsibility of the person or entity reviewing thePRD, i.e., eventually theowner by virtueof theISM code. By accepting a given design of equipment, thus assuming an obligation to adhere to the required maintenance level, malfunctioning due to jammed discs and blocked gas passage ways should in principle be impossible because the equipment has been evaluated and

Page 5 of of 7


found satisfactory by the owner. It may therefore be argued that if the practice still is to conduct manual de-pressurization, the due diligence of the valve chosen was not satisfactory. It should benoted, however, that the wordingsapplied in typeapproval certificatesare not necessarily consistent and precise. Some certificates will show ISO 15364 compliance without further comments, which is obviously a misleading mistake to some degree because there is no such thing as general ISO 15364 compliance. – That would be pure nonsense. ISO 15364 is a design and descriptive code to be used as a tool for the concerned owner wanting to scrutinize avalve for suitability. There is no limitation as to what can be approved and termed “I SO 15364 complying.” As long as the product is adequately described as per ISO 15364, the owner hasthe information needed to assess the product for suitability, thus, assume responsibility. If an owner is satisfied by, as an example, net clearances allowing for inside fouling of 0.1 mm, so be it. Everything fits the requirements of ISO 15364 as far as the maintenance and operational conditions are laid down and made available to those evaluating and accepting. All current PRES-VAC valve designs have been described and certified according to ISO 15364. But theyare far from equally suitablefor all applications. They fall in different price categories and are designed for different types of vessels. The final responsibility for selecting the proper pieceof equipment for a vessel is not with themanufacturer, but with thebuyer, owner, and user, with whomthemaintenanceand operational issues are vested.

Responsibility distribution? L I M I T E D C E R T I F I C A T E

As an example of the distribution of responsibility under the auspice of the ISO standard, pleaserefer to the scanned images of aCE typeapproval and aUSCG typeapproval in the following, noting the reservations in regard of suitability, a subject left for the user/ buyer/ owner to decide. This is the core of the new approach. Suitability was not a concern in the past when the existence of a non-descriptive type approval was sufficient. Now, the issues that are relevant for how the valve will perform in-service, how maintenance is to be carried out, and how frequently, is all left for thebuyer/ user/ ship owner to consider and accept.

Page 6 of of 8


S U M M A R Y C E

A N D

O F

U S C G

C E R T .

Figure1

Page 7 of of 9


Figure2

Where to find the necessary information?

The type approval certificate itself will probably provide close to nothing. As an example, however, with someelement of guidance to the ship owner, the above scanned images show certificates with reference to ISO 15364. What is important, however, is the reference to the manual and the so-called Product Review Document, which the User is obliged to examine to

Page 8 of of 10


establish suitability. The mere existenceof a type approval certificate is just the first step. Technical acceptanceis dependingon areview of thespecific product for thespecific job.

An owner’s approach What are the main issues to look for?

From an operational point of view, an owner should be concerned about the following main issues (not listed in priority and not exhaustive):

ISO 15364 item

Owner’s requirement

1 . N E T - F R E E G A S P A S S A G E - W A Y S A N D A L L O W E D T H I C K N E S S D E P O S I T S

O F

2 . A C C E S S T O I N S P E C T I O N

3 . A C C E S S T O R E P L A C E W E A R P A R T S

3 . 1 M A I N T E N A N C E

This issue is the most important with regard to lowering maintenance and avoiding stuck discs. The distance between discs and inner walls, spindles and bushings, and configuration of drains are extremely important. The Buyer can select net clearances from 1.5 to 30 mm from the PRES-VAC range and with/ without drains. The allowed inside thickness of deposits vary from 0.1 mm to 5.0 mm in the PRESVAC range. The possibility to inspect the valve’s complete inside for fouling and deposits should beeasyand convenient to carry out. Ideally, removing one hood should suffice. Seats and discs should be replaceable by removing the worn out part without further disassembling. When seats havebeen replaced, perfect alignment of thenew seat to the disc should be afeatureof thesystem. Seats that are held in place by Loctite®or thread or screws will not allow for adequate alignment unless they are machined after installing. All necessary maintenance should be possible to carry out with thevalvein place. When considering a valve design, the possibility of inside accumulation of condensate, drains blocking up, etc. should beconsidered and accepted.

4 . C H E C K - L I F T I N G A N D

I C E - L A Y E R

Ideally, this should be done by turning built-in handles that will work regardless of over-icing. Stroke should

Page 9 of of 11


ISO 15364 item

Owner’s requirement

befull and thedisc positions visually indicated. Thepossibleice-layer thickness within thePRES-VAC rangevaries from 5 to 20 mm. 5 . D I S C

P O S I T I O N

I N D I C A T I O N 6 . F O U L I N G I N D I C A T I O N

7 . P R E S S U R E

8 . C L O S I N G P R E S S U R E

D R O P

Should be visually indicated on the valve from any distance and angle of observation. The operator shall be able to check the thickness of fouling from the outside and determine whether to continuewith inside inspection and possibly cleaning. Should be non-existent over the entire flow range to reduce pipe costs, increase safety margin in case of mal-operation, and allow for increased loading rates. It should be considered that the normal venting rate is half the required venting ratefor VECS, which would often cause the operation to take place in the zone where thepressure peak normally is. Should be above the inert gas replenishing pressure and above the vapor pressure of the cargo to eliminate boil-off.

Part conclusion 1 If traditional methods are still applied in the selection process of marine p/ v valves, nothing will change and vapor loss continue becausethe operator is justified in his decision not to rely on the venting equipment, generally speaking. O W N E R ’ S T A S K

If the issues listed in ISO 15364 are considered properly, noting that most existing designs will fall short of meeting reasonablerequirements, thevalvesselected are trustworthy. This opens anewwindow for solving thecredibility issue, especiallyconsidering that the general purposeof the ISO standard is specifically to improvevalveperformance, which implicitly means foregoing existing designs being the very cause of the situation that lead to redundancy venting being enforced. It should berecalled that – at anytimeof the selection process - a valvedesign review is not conducted by the manufacturer, the yard, the class, or the administration. No, a certificate listing the relevant documentation describing the valve is supposedly presented to the owner and the examination hereof is entirely his responsibility. As an example, a given valve design may be appropriate for a chemical carrier, but not necessarily for use on a Bitumen or crude oil tanker and viceversa.

Page 10 of of 12


In short, if the owner’s due diligence examination of the valve shows adequate reliability for extended service, there should be no excuse for not trusting the valves. If trust is not created, the examination has not been carried out appropriately. It may therefore beargued that if the procedure of manual de-pressurization continues, the chosen valvesare not the right ones.

What valve characteristics are right? This issue isof particular interest because of the tremendous impact it has on the amount of released cargo vapor during voyage. T E R M I N O L O G Y

Basic understanding of how different designs work is necessary for reference. Pressure relief valves are generally madewith the following characteristics: i.

Modulating , i.e., with a rise in pressure above the nominal setting proportional to

the vented volume. These valves are typically vacuum relief valves and conventional in- or end-of-line valves. However, some high velocity vents are modulating over part of the flow range, e.g. half the rated capacity. This will take care of hammering, but not necessarilychatteringor fluttering. ii.

Full lifting , i.e., with an instant reduction of pressure drop over the valve to a value

below the nominal setting duethe effect of an extra lifting area around the disc. These valves feature pop-off characteristics and provide huge, instant capacity. However, for high velocity valves, the efflux velocity will vary and the ability to arrest flame is seriously endangered when the loading rate is relatively small and/ or when the valve is under the influence of pressure surges in the piping causing unstable movement of the disc (known as “hammering”). Full lifting valves are typically old designs of high velocity vents, but some are also used as inline valves when pressure drop is an issue relevant to the design of the piping arrangement. A full lifting, weight-loaded high velocity valve will always suffer from hammering/ chattering/ fluttering, especially bad under theinfluenceof small bore piping or long piping. 2 iii.

Controlled blow-down , i.e., an instant reduction of the pressuredrop over the valveby

a value corresponding to the differential between opening and closing pressures, i.e., the blow-down value. This behaviour will cause the valve to relief overpressure and then closeat the tank pressurecorresponding to the net-closing force of the valve. These valves are typically magnet-controlled valves, often high velocity valves or in-linevalves used when pressuredrop is an issuerelevant to the pipinglay-out. iv.

N on-hammering weight-loaded high velocity valves , i.e., a valvethat is modulating until the

gas flow justifies full exposure of the orifice to atmosphere in terms of efflux velocity, and then transforms into full lifting. These valves typically feature the 2 Hammering was determined adanger for flash-back in high velocity vents when IMO

initiated atest seriesof type approved equipment. This lead to the requirement for non-hammering valves, which is refined and elaborated in EN test requirements.

Page 11 of of 13


samesort of action seen in full lifting valves, but differently configured so that the “pop-off” does not occur until the gas volumeis sufficient. These valves yield a pressure increase of 10-30% above their nominal setting at most flow volumes 3. As the designation indicates, this system is used in high velocity vents only. Until fully open, probably until half the rated capacity, the design is not stable against fluttering and chattering 4. At somepipe configurations, these valves are not able to maintain adequate efflux, which should be shown in the certification if done properly. v.

N on-oscillating high velocity valves , i.e., a valve that shows no pressure surges for a

specified piping configuration. These valves are new comers to marine use and typically featureacombination of controlledblow-down mechanisms and weightloaded non-hammering designs, i.e., a magnet-controlled opening of the valve dealing with unstable disc movement at small flow rates to eliminate chattering and fluttering and a delayed pop-off action for high flow rates. Thesevalves will yield a certain “negative” pressure drop, i.e., the system pressure will be reduced 10-20% below the valve’s nominal setting, until the valve is fully open when the system pressure will stabilize at the valve’s nominal setting. This performance allows for the use of less diameter piping and/ or higher venting rates on a comparable basis. Thesevalves are typically high velocity p/ v valves.

V A P O R

L O S S

The loss of vapor during voyage, for instance caused by sloshing or thermal variation, is determined by thevalve’s opening characteristics and its closing pressure. A valvethat falls within the category of non-oscillating high velocity valves as described above features avariation of design adjustments that can becalled upon to limit the loss of vapour. A representative chart would appear as follows next page:

3 Requiring larger pipe bore or reduced loading rate. 4 This can be an issuein regard of

VCS operations becausethe calculated venting rateis often twice the loading rate, but the vented media is to some extent inert gas more than heavy density gas.

Page 12 of of 14


Pressure

5

1 3

4

6

2 Flow

Figure3

1 2 3 4 5 6

Opening setting: 1,800 mm WG Closing pressure: 1,500 mm WG SOLAS venting capacity @ pressure: 1,000 m3/h VECS venting capacity @ pressure: 2,000 m3/h Maximum venting capacity without pressure increase: 2,000 m3/h Pressure differential available for increased loading rate or reduced pipe diameter: 300 mm WG

Adjustable: 1,600 – 1,800 Adjustable: 1,400 – 1,600 (=Design point 1) (=Design point 2)

NUMBERS ABOVE ARE FOR THE EXAMPLE ON LY

By carefully sizing the pressure relief valve, and by using in particular a non-oscillating high velocity valve that has been specifically designed with ISO 15364 in mind, the following parameters canbeconsidered: Opening setting:

Should behigher than tradition calls for. Theoperational margin normally left between the valve’s opening setting and the alarm setting, which generally has been reserved for the pressure increaseover the valve, canbewaived. – And without impairing safety because this type has no pressure increase over the setting. As an example, the setting of the valve can be raised from 1,400 mm WG to 1,800 mm WG with a 100 mm WG margin to the alarm point. With the subject valve type, the tank

Page 13 of of 15


pressure will not exceed 1,800 mm WG, which is no different than thenormal picture.

Traditional weight-loaded non-hammering valve (opening and closing lines respectively) in comparison with new non-oscillation combination valve

22 20 18 16 14 12 10 0

500

1000

1500

Nm³/h

Figure4

The crude oil’s vapour pressure will most often create a tank pressure approaching 1,400 mm WG, i.e., 16.7 psia according to the experience gathered at PRES-VAC. According to the Publication, figures of 800 and 1,000 mm WG are mentioned and over duration up to 1,400. (The tank pressure in question is the concept known as equilibrium pressure ). Closingpressure:

Should beselected based on the pressure drop conditions of the vent piping with a view to minimize the valve’s opening and closing cycles (hammering or non-oscillation), and at the same time with consideration to limiting the loss of vapor, i.e., contradicting interest. If asetting of 1,800 mmWG is chosen, a suitable closing pressure for a crude oil tanker would be, say, 1,500 mm WG.

Operating pressure:

If the above recommendations and equipment type would be considered, the tank pressureduring voyagewould stabilize at all timebetween 1,500and 1,800 mm WG.

Measuredpressure:

The following chart is from the INTERTANKO publication and shows pressure peaks over a 50 day period closing in at around 1,400 mm WG, which is the traditional setting pressure for valves used onboard crudeoil tankers. This is absolutely not ideal from acargo conservation point of view.

Page 14 of of 16


Figure5

As can be seen, temperature is crucial to the development of tank pressure, but this can hardlybecontrolled. Proposedchanges:

By increasing the setting and by using valves with controlled over-pressure and a high closing pressure, refer to Fig. 3 for reference, the followingwould bethe scenario:

Tank pressure @ full flow rate

ITEM

Safety margin to p/v breaker set @ 2,000

mm WG

m /h

3

mm WG

Traditional setpressure

1,400

1,800 – 1,900

100 - 200

Proposed set-pressure

1,800

1,600 – 1,800

200 – 400

Closing pressure

Tank differential

Non-hammering weight-loaded design

600

- 800

Non-oscillating design

1,500

- 300

Figure6

Page 15 of of 17


Theabovenumbers can bedisplayed differentlyfor abetter overview: Tankpressure(save piping pressure drop)

Proposed non-oscillating type

Traditional nonhammeringtype

mmWG 2000

Alarm/ Liquid breaker setting

1900 1800

Max working pressure Openingsetting Max workingpressure

1700 1600 1500

Closing pressure

1400

Openingsetting

1300 1200 1100 1000 900 800 700 600

Closing pressure

Note on the safety aspects The practice of doing manual de-pressurization could raise certain safety aspects to be considered. •

During the de-pressurization, the entire safety of the vessel is to a certain degree depending on the flame arresting capability of the mast riser’s end-of-line flame

Page 16 of of 18


screen. This kind of equipment is very often in poor working conditions because of the vulnerability to corrosive attack by the sulphuric acid forming when inert gas and ambient moist mix. •

If the de-pressurization continues down to the pressurewhere thecargo’s boil-off rate is being vented, the situation is in reality becoming non-inerted and no endof-line flame screen is capable of functioning as adeviceto prevent the passageof flamein such conditions.

•

The practice of de-pressurization is also a concern in regard of crew exposure to unhealthy vapour.

Part conclusion 2 Equipment is availablenow, designed and constructedspecificallyto thenewISO standard that should allow the vessel commands to trust the performance, unlike in the past when many designs havebeen installed that certainly did not deserve the slightest degree of trust.

N O

G R E E N C A R D

However, certification is not a green card. The newISO standard is vagueand merely of a descriptive nature. However, the owner can and should useit as atool and agenda for his product review in order to select equipment that works with a minimum of maintenance and a high level of insensitivity to deposits and corrosion. It must be recalled as the most important issuethat any valvecan beISO 15364 certified. Certification doesnot implythat the use is recommended, only that the valve has been considered and described in accordance with the ISO standard. Judgment into suitability is entirely at the owner’s discretion, not with the yard, class, or manufacturer.

Final conclusion Equipment is now available in the market, which provides the owner compliance with all theissues addressed above(1-8). In practicethis leavesthe crew with much enhancedlevel of comfort becausemaintenance is drastically reduced, inside fouling layer can be checked from the outside, and all necessary maintenance can be done with the valve mounted, including replacement of discs and seats. Further, the new equipment will handle the full venting rate without ever exceeding the nominal setting and the blow-down value can be considerably higher than in the past. The nominal setting can even be fine-tuned by the crewwithout removing thevalvefrom thepoint of installation. If this kind of ISO compliant equipment is used, the traditional hesitation in trusting and depending on the valves can be overcome. Considering the physical properties of the crude to the increase valve opening setting and closing pressure, there should be no need for manual de-pressurization. In practice, thefollowing should beapplied: Operation procedures should call for tank pressures to be as high as the valve setting, before manual de-pressurization is considered in the first place.

Page 17 of of 19


Pressure setting of the valves should be increased and dynamic valve characteristics chosen that allow for this without sacrificing safety by eliminating the usual pressure peak during operation. In other words: a venting system that maintains the tank pressure at no higher than the valve’s nominal setting,. Alarm setting (pressure) can remain as today, i.e., slightly above the increased opening settingproposedabove. Flow characteristics of the chosen equipment should allow for a limited blow-down valueto conserve the loss of vapor normally associated with sloshing and thermal variation.

--o0o-Newbuildings:

If the above 3issues are adopted, loss of vapor during voyagewill be reduced to afraction of the levels seen today. There will in principle beno extra cost. Existing vessels:

New valves will be required at an approximate cost of US$2,000 - 4,000 per tank, and the alarm and liquid p/ v breaker settings may need adjustment. The payback time so small that it will not be worth mentioning. --o0o-It should be noted, however, that without owner’s examination for ISO 15364 compliance for the actual application, the sought after reduction of vapor loss is by all likeliness not achieved. The most important issue, however, is the level of owner effort required when specifying the equipment configuration and lay-out because the philosophy behind is not required byregulations, leavingit’s exploitation at owners’ initiativefor their own benefit.

Copenhagen, November, 2001

_____________________ Eric Aarestrup Sørensen PRES-VAC ENGINEERING A/ S

Page 18 of of 20

Vapor Loss From Crude Oil Tankers

Recommend Documents