vapor loss from crude oil tankers

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VAPOR LOSS FROM CRUDE OIL TANKERSInfluence by design of venting system

Reference document: INTERTANKO’s “Guidelines for The Control of a Multiphase

Crude Oil Cargo For Cargo Operations and Handling ” (March 2001) and

INTERTANKO’s VOCON on the same subject (undated).



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

Guide on minimizing vapor loss by designand operational procedures

Pres-Vac Engineering A/ SSvanevang 3-5 •DK 3450 Allerød •DenmarkPhone +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 NB Y B A S I C D E S I G N

PagePagePagePage 1 111 of of of of 3333

Comments on how tominimize vapor loss bybasic design and re-thought procedures

IntroductionIn short, the Publication lists two issues, which combined cause the vast majority of vaporloss during voyage:

A. Apparently what must be termed as distrust in the efficiency and reliability of p/ vvalves, leading to:

B. Procedures involving manual de-pressurization, - as a consequence of A. above, -with closing pressures below the vapor pressure of the crude oil cargo.

NOTE: The referenced INTERTANKO publications are assumed known to the readerin 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 cargoand environmental concerns, can be reduced to a fraction of the current level if ventingsystems are designed (and used) with these issues in mind. Retrofits are possible withoutstructural changes at a negligible cost. Further, safety aspects will be improved byfollowing the views and opinions in this paper.

DISCLAIMER: The views and opinions expressed in thispaper are entirely those of Pres-Vac Engineering A/S andare provided without assuming any responsibility in anyform, shape, or nature whatsoever.





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

• a retroactive and mandatory concept of secondary venting, 1 and

• a new ISO standard for marine p/ v valves, which is mandatory for installationsafter July 1, 2002 (15364 is sort of semi-retroactive).

This leads to the following:

• if the venting system could maintain a constant tank pressure above the pressureequilibrium of the cargo; and

• if the venting system was reliable and offered close to nil maintenance,

the two issues listed above as being the main cause of the loss of vapor would beeradicated. The thesis of this memo is that a properly designed venting system using p/ vvalves of a reliable and maintenance friendly design under the auspice of the ISO standardcan make redundant the practice of manual de-pressurization. It is further the thesis thatnot only would this conserve enormous volumes of cargo otherwise lost at great expense,but environmental and safety aspects will be improved considerably, too. The much-

discussed avenue of installing Vapor Control Systems for onboard use could most likelybecome 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 followingconclusions:

1:

Whether or not the IMO initiative in regard of developing a valve standard is sufficientin regard to establish a new level of trust and comfort with regard to the reliability of p/ v valves, and in the affirmative;

2:

Whether or not manual de-pressurization can be completely avoided and what designparameters would be necessary for the appropriate venting system and equipment forthis to succeed.

1 Equipment which is not taken into consideration in the Publication. However, with good reasonbecause the p/ v breakers referred to in the Publication have generally been accepted as secondaryventing means according to the new regulations, despite their failing to meet the most basic requirementsof SOLAS II-2, rule 59. The irony is that the specific tanker cited by the Australian DOT as featuring aninadequate venting system – on basis of which SOLAS was revised - has been approved as being in fullcompliance with the revised regulations as is. A full report on the SOLAS amendments is available at

www.pres-vac.com





Available means to reduce vapor loss Three simple but extremely effective means are available that have potential to significantlyreduce the loss of vapor from crude oil tankers during voyage:

1. Revised operational procedures with regard to the necessity of manual de-pressurization, if needed at all.

2. Increased opening setting and closing pressure of pressure relief valves - andcorresponding changes to related system components, e.g. alarm settings.

3. Appropriate dynamic flow characteristics of said pressure relief valves.

In the following, the operating procedures onboard crude tankers are assumed to be asdescribed in the aforementioned INTERTANKO publications

According to the INTERTANKO publication, tank pressure is generally maintainedbetween 1,000 mm WG and 400 mm WG during voyage. This restriction is adhered tobecause of lack of trust with regard to the pressure/vacuum valves, thus calling for manualpressure controlling via a central riser.

The report reads:

“However, results and observations for the practice aboard tankersreveal that tanker commands do not generally rely upon P/ V valvesfor over pressure control and will release pressure manually throughone of the previously discussed onboard systems when the overpressure reaches about 1,000 mm WG. This practice, being absolutelynecessary to protect the vessel’s structure, reflects good seamanshipby preventing abnormal wear to a safety device. The results andobservations from the practice aboard tankers with regard to theselected pressure for closing of the manually opened releasemechanism seem to show a pressure of approximately 400 mm WGas the commonly selected. It is the determination of this pressure thatneeds to be defined such that unnecessary release of bothhydrocarbon and Inert gases is avoided .”

As outlined in the above, we need to consider 1) what the situationis in regard to availability of reliable p/ v valves and 2) how toavoid manual de-pressurization. The first issue must be answered

T H E

A P P R O A C H

I N T E R T A N K O

P U B L I C A T I O N

Where does thisleave us? K E Y

Q U E S T I O N S





satisfactorily in order to deal with the second issue in a proper manner.

According to the VOCON cited in the foregoing, the recommended solution is to establishthe vapor pressure for each cargo and limit the manual de-pressurization to where the boil-off begins. This, however, would appear a not optimum solution compared to a ventingsystem that completely eliminates the need to do manual de-pressurization. It isrecommended to stop the de-pressurization at 800 mm WG. With reference to Fig. 5 itwould, 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 necessarywith emphasis on IMO MSC/ Circ. 677/ 1009, cf. ISO 15364.

Is it fair not to trust thep/v valves?

The answer is definitely affirmative, - p/ v valves cannot be trusted, generally speaking. Thisstatement, 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 testedwith no regard whatsoever to the actual working environment. Further, in the completeabsence of internationally accepted quality assessment instruments, the single criterion forselection and acceptance has been type approvals. In reality, this has left design of p/ vvalves completely at the discretion of each manufacturer with little if any interest paid byowners, administrations, classes, and other bodies once a valve is type approved. Thecriterion for type approval (since 1984) is fire testing in a laboratory, which has absolutelynothing to do with the reliability issue. Only a tiny fraction of all p/v valves will besubjected to a real fire incident, whereas they all are functioning as balancing relief valves inpractice. Design priorities have been upside down.

When commands do not trust the p/v valves, it is not because they fear a flash-backincident happening; they fear that the valves will not manage to balance the tank pressureand therefore they involve themselves and conduct manual pressure adjustments.Considering the high number of over-pressure incidents reported to the IMO inconnection with recent revisions of SOLAS, the commands’ lack of trust would appearfully justified, from a general point of view.

One issue that has been brought up on earlier occasions is the potential misunderstandingthat may occur when breather valves are misunderstood for full flow valves. However, bynow, ISO 15364 leaves the capacity issue at the responsibility of the buying entity whensubmitting the mandatory sizing data required by the section of the ISO standard titled“Ordering Information”. There should consequently be no misunderstanding possiblewhen interpreting the “Master’s loading chart” required by IMO MSC/ Circ. 731.





A lot of p/ v valves are prone to mal-function because of insufficient tolerance to cloggingby cargo vapor deposits, IG residue, corrosion, and freezing water. Some valve designscome with net clearances of less than 1 mm between moving parts and inner walls. Itshould be quite obvious that the maintenance required for keeping such devices in goodworking order is prohibitive. Some valves have check-lifts that push the discs 1/100 of thefull stroke, leaving check-lifting close to being an illusion. Some valves are totally dependingon internal drain holes, which cannot be controlled from the outside and therefore requiregas-freeing and removal. Other valves are so complicated to inspect and dismantle thattime is prohibitive for maintenance. And other designs are so vulnerable to over-icing thatthey 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 overly trust by the various parties involved with regard to the traditional typeapproval procedures according to IMO regulations without realizing that they simply donot cover the many aspects related to practical use of the equipment.

In response to a number of over-pressure incidents, IMO decided to amend SOLAS andrequire 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 theapplication of secondary venting systems, the SOLAS amendment has probably had little if any effect. Especially considering that the not-trusted p/ v valves do not gaintrustworthiness 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 flowtesting. Since regular flow testing according to IMO MSC/ Circ. 677/1009 satisfies 15364,the new test requirements are of interest only with regard to non-SOLAS vessels. 15364 ismandatory by reference in IMO MSC/ Circ. 1009 for all valve installations on or after July1, 2002.

The purpose of 15364 is to highlight technical issues of importance to in-serviceperformance, i.e., issues, which are not covered by type approval testing. The explicitpurpose of IMO having this standard prepared was the number of incidents caused bymalfunctioning p/v valves, i.e., existing designs are not performing well enough, againgenerally speaking, and new designs should be expected.

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 testrequirements expressing themselves in one-page certificates. The approving body shouldtherefore issue a Product Review Document (PRD) outlining the design specifics of agiven product, leaving it at the discretion of the user to decide whether or not it will besuitable for the specific installation using ISO 15364 as an assessment tool. The approvingbody and the manufacturer have not determined compliance for a specific application; thatis the responsibility of the person or entity reviewing the PRD, i.e., eventually the owner byvirtue of the ISM code.

By accepting a given design of equipment, thus assuming an obligation to adhere to therequired maintenance level, malfunctioning due to jammed discs and blocked gas passage

ways should in principle be impossible because the equipment has been evaluated and

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V E N T I N G

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O W N E R I S

R E S P O N S I B L E





found satisfactory by the owner. It may therefore be argued that if the practice still is toconduct manual de-pressurization, the due diligence of the valve chosen was notsatisfactory.

It should be noted, however, that the wordings applied in type approval certificates are notnecessarily consistent and precise. Some certificates will show ISO 15364 compliancewithout further comments, which is obviously a misleading mistake to some degreebecause there is no such thing as general ISO 15364 compliance. – That would be purenonsense. ISO 15364 is a design and descriptive code to be used as a tool for theconcerned owner wanting to scrutinize a valve for suitability. There is no limitation as towhat can be approved and termed “ISO 15364 complying.” As long as the product isadequately described as per ISO 15364, the owner has the information needed to assess theproduct for suitability, thus, assume responsibility. If an owner is satisfied by, as anexample, net clearances allowing for inside fouling of 0.1 mm, so be it. Everything fits therequirements of ISO 15364 as far as the maintenance and operational conditions are laiddown and made available to those evaluating and accepting.

All current PRES-VAC valve designs have been described and certified according to ISO15364. But they are far from equally suitable for all applications. They fall in different pricecategories and are designed for different types of vessels. The final responsibility forselecting the proper piece of equipment for a vessel is not with the manufacturer, but withthe buyer, owner, and user, with whom the maintenance and operational issues are vested.

Responsibility distribution?As an example of the distribution of responsibility under the auspice of the ISO standard,please refer to the scanned images of a CE type approval and a USCG type approval in thefollowing, noting the reservations in regard of suitability, a subject left for theuser/ buyer/ owner to decide.

This is the core of the new approach. Suitability was not a concern in the past when theexistence of a non-descriptive type approval was sufficient. Now, the issues that arerelevant for how the valve will perform in-service, how maintenance is to be carried out,and how frequently, is all left for the buyer/ user/ ship owner to consider and accept.

L I M I T E D

C E R T I F I C A T E





Figure 1

S U M M A R Y O F

C E A N D U S C G

C E R T .





Figure 2

The type approval certificate itself will probably provide close tonothing. As an example, however, with some element of guidanceto 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

Where to find thenecessaryinformation?





establish suitability. The mere existence of a type approval certificate is just the first step. Technical acceptance is depending on a review of the specific product for the specific job.

An owner’s approach

From an operational point of view, an owner should beconcerned 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 SP A S S A G E - W A Y SA N D A L L O W E DT H I C K N E S S O FD E P O S I T S

This issue is the most important with regard tolowering maintenance and avoiding stuck discs. Thedistance between discs and inner walls, spindles andbushings, and configuration of drains are extremelyimportant. The Buyer can select net clearances from1.5 to 30 mm from the PRES-VAC range andwith/ without drains. The allowed inside thickness of deposits vary from 0.1 mm to 5.0 mm in the PRES-

VAC range.2 .A C C E S S T OI N S P E C T I O N

The possibility to inspect the valve’s complete insidefor fouling and deposits should be easy and convenientto carry out. Ideally, removing one hood shouldsuffice.

3 .A C C E S S T OR E P L A C E W E A RP A R T S

Seats and discs should be replaceable by removing theworn out part without further disassembling. Whenseats have been replaced, perfect alignment of the newseat to the disc should be a feature of the system. Seatsthat are held in place by Loctite®or thread or screws

will not allow for adequate alignment unless they aremachined after installing.3 . 1M A I N T E N A N C E All necessary maintenance should be possible to carry

out with the valve in place.

When considering a valve design, the possibility of inside accumulation of condensate, drains blocking up,etc. should be considered and accepted.

4 .C H E C K - L I F T I N GA N D I C E - L A Y E R

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

What are the mainissues to look for?




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ISO 15364 item Owner’s requirement be full and the disc positions visually indicated.

The possible ice-layer thickness within the PRES-VACrange varies from 5 to 20 mm.

5 .D I S C P O S I T I O NI N D I C A T I O N

Should be visually indicated on the valve from anydistance and angle of observation.

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

The operator shall be able to check the thickness of fouling from the outside and determine whether tocontinue with inside inspection and possibly cleaning.

7 .P R E S S U R E D R O P 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. Itshould be considered that the normal venting rate ishalf the required venting rate for VECS, which wouldoften cause the operation to take place in the zonewhere the pressure peak normally is.

8 .C L O S I N GP R E S S U R E

Should be above the inert gas replenishing pressureand above the vapor pressure of the cargo to eliminateboil-off.

Part conclusion 1If traditional methods are still applied in the selection process of marine p/ v valves,nothing will change and vapor loss continue because the operator is justified in his decisionnot to rely on the venting equipment, generally speaking.

If the issues listed in ISO 15364 are considered properly, noting that most existing designswill fall short of meeting reasonable requirements, the valves selected are trustworthy. This

opens a new window for solving the credibility issue, especially considering that the generalpurpose of the ISO standard is specifically to improve valve performance, which implicitlymeans foregoing existing designs being the very cause of the situation that lead toredundancy venting being enforced.

It should be recalled that – at any time of the selection process - a valve design review is notconducted by the manufacturer, the yard, the class, or the administration. No, a certificatelisting the relevant documentation describing the valve is supposedly presented to theowner and the examination hereof is entirely his responsibility. As an example, a givenvalve design may be appropriate for a chemical carrier, but not necessarily for use on aBitumen or crude oil tanker and vice versa.

O W N E R ’ S

T A S K





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

What valve characteristics are right? This issue is of particular interest because of the tremendous impact it has on the amountof released cargo vapor during voyage.

Basic understanding of how different designs work is necessary for reference. Pressurerelief valves are generally made with the following characteristics:

i. Modulating , i.e., with a rise in pressure above the nominal setting proportional tothe vented volume. These valves are typically vacuum relief valves andconventional in- or end-of-line valves. However, some high velocity vents aremodulating over part of the flow range, e.g. half the rated capacity. This will takecare of hammering, but not necessarily chattering or fluttering.

ii. Full lifting , i.e., with an instant reduction of pressure drop over the valve to a valuebelow the nominal setting due the 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 toarrest flame is seriously endangered when the loading rate is relatively smalland/ or when the valve is under the influence of pressure surges in the pipingcausing unstable movement of the disc (known as “hammering”). Full liftingvalves are typically old designs of high velocity vents, but some are also used as in-line valves when pressure drop is an issue relevant to the design of the pipingarrangement. A full lifting, weight-loaded high velocity valve will always sufferfrom hammering/ chattering/ fluttering, especially bad under the influence of smallbore piping or long piping. 2

iii. Controlled blow-down , i.e., an instant reduction of the pressure drop over the valve bya value corresponding to the differential between opening and closing pressures,i.e., the blow-down value. This behaviour will cause the valve to relief over-pressure and then close at the tank pressure corresponding to the net-closing forceof the valve. These valves are typically magnet-controlled valves, often highvelocity valves or in-line valves used when pressure drop is an issue relevant to thepiping lay-out.

iv. N on-hammering weight-loaded high velocity valves , i.e., a valve that is modulating until thegas flow justifies full exposure of the orifice to atmosphere in terms of effluxvelocity, and then transforms into full lifting. These valves typically feature the

2 Hammering was determined a danger for flash-back in high velocity vents when IMO initiated a testseries of type approved equipment. This lead to the requirement for non-hammering valves, which is

refined and elaborated in EN test requirements.

T E R M I N O L O G Y





same sort of action seen in full lifting valves, but differently configured so that the“pop-off” does not occur until the gas volume is sufficient. These valves yield apressure 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. Untilfully open, probably until half the rated capacity, the design is not stable againstfluttering and chattering 4. At some pipe configurations, these valves are not ableto maintain adequate efflux, which should be shown in the certification if doneproperly.

v. N on-oscillating high velocity valves , i.e., a valve that shows no pressure surges for aspecified piping configuration. These valves are new comers to marine use andtypically feature a combination of controlled blow-down mechanisms and weight-loaded non-hammering designs, i.e., a magnet-controlled opening of the valvedealing with unstable disc movement at small flow rates to eliminate chatteringand fluttering and a delayed pop-off action for high flow rates. These valves willyield a certain “negative” pressure drop, i.e., the system pressure will be reduced10-20% below the valve’s nominal setting, until the valve is fully open when thesystem pressure will stabilize at the valve’s nominal setting. This performanceallows for the use of less diameter piping and/ or higher venting rates on acomparable basis. These valves are typically high velocity p/v valves.

The loss of vapor during voyage, for instance caused by sloshing or thermal variation, isdetermined by the valve’s opening characteristics and its closing pressure. A valve that fallswithin the category of non-oscillating high velocity valves as described above features a variationof design adjustments that can be called 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 issue in regard of VCS operations because the calculated venting rate is often twice the

loading rate, but the vented media is to some extent inert gas more than heavy density gas.

V A P O R L O S S





Pressure

Flow

1

3

2

4

5

6

Figure 3

1 Opening setting: 1,800 mm WG Adjustable: 1,600 – 1,800 2 Closing pressure: 1,500 mm WG Adjustable: 1,400 – 1,600 3 SOLAS venting capacity @ pressure: 1,000 m 3 /h (=Design point 1)4 VECS venting capacity @ pressure: 2,000 m 3 /h (=Design point 2)5 Maximum venting capacity without pressure increase: 2,000 m 3 /h

6 Pressure differential available for increased loading rate or reducedpipe diameter: 300 mm WG

NUMBERS ABOVE ARE FOR THE EXAMPLE ONLY

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 followingparameters can be considered:

Opening setting : Should be higher than tradition calls for. The operational marginnormally left between the valve’s opening setting and the alarmsetting, which generally has been reserved for the pressureincrease over the valve, can be waived. – And without impairingsafety because this type has no pressure increase over thesetting. As an example, the setting of the valve can be raisedfrom 1,400 mm WG to 1,800 mm WG with a 100 mm WGmargin to the alarm point. With the subject valve type, the tank





pressure will not exceed 1,800 mm WG, which is no differentthan the normal picture.

Traditional weight-loaded non-hammering valve (opening andclosing lines respectively)

in comparison with new non-oscillation combination valve

10

12

14

16

18

20

22

0 500 1000 1500

Nm³/h

Figure 4

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

Closing pressure : Should be selected based on the pressure drop conditions of thevent piping with a view to minimize the valve’s opening andclosing cycles (hammering or non-oscillation), and at the sametime with consideration to limiting the loss of vapor, i.e.,

contradicting interest. If a setting of 1,800 mm WG is chosen, asuitable closing pressure for a crude oil tanker would be, say,1,500 mm WG.

Operating pressure : If the above recommendations and equipment type would beconsidered, the tank pressure during voyage would stabilize at alltime between 1,500 and 1,800 mm WG.

Measured pressure : The following chart is from the INTERTANKO publicationand shows pressure peaks over a 50 day period closing in ataround 1,400 mm WG, which is the traditional setting pressurefor valves used onboard crude oil tankers. This is absolutely not

ideal from a cargo conservation point of view.





Figure 5

As can be seen, temperature is crucial to the development of tank pressure, but this can hardly be controlled.

Proposed changes: By increasing the setting and by using valves with controlledover-pressure and a high closing pressure, refer to Fig. 3 forreference, the following would be the scenario:

Tank pressure @ fullflow rate

Safety margin to p/vbreaker set @

2,000

ITEM

mm WG m 3/h mm WG

Traditional set-pressure

1,400 1,800 – 1,900 100 - 200

Proposed set-pressure 1,800 1,600 – 1,800 200 – 400

Closing pressure Tank differential

Non-hammeringweight-loaded design

600 - 800

Non-oscillating design 1,500 - 300

Figure 6





The above numbers can be displayed differently for a better overview:

Tank pressure (savepiping pressure drop)

mm WG

Proposed non-oscillatingtype

Traditional non-hammering type

2000 Alarm/ Liquid breaker setting

1900 Max working pressure

1800 Opening setting

Max working pressure

1700

1600

1500 Closing pressure

1400 Opening setting

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 beconsidered.

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





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

• If the de-pressurization continues down to the pressure where the cargo’s boil-off rate is being vented, the situation is in reality becoming non-inerted and no end-of-line flame screen is capable of functioning as a device to prevent the passage of flame in such conditions.

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

Part conclusion 2Equipment is available now, designed and constructed specifically to the new ISO standardthat should allow the vessel commands to trust the performance, unlike in the past whenmany designs have been installed that certainly did not deserve the slightest degree of trust.

However, certification is not a green card. The new ISO standard is vague and merely of adescriptive nature. However, the owner can and should use it as a tool and agenda for hisproduct review in order to select equipment that works with a minimum of maintenanceand a high level of insensitivity to deposits and corrosion. It must be recalled as the mostimportant issue that any valve can be ISO 15364 certified. Certification does not imply thatthe use is recommended, only that the valve has been considered and described inaccordance with the ISO standard. Judgment into suitability is entirely at the owner’sdiscretion, not with the yard, class, or manufacturer.

Final conclusionEquipment is now available in the market, which provides the owner compliance with allthe issues addressed above (1-8). In practice this leaves the crew with much enhanced levelof comfort because maintenance is drastically reduced, inside fouling layer can be checkedfrom 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 beconsiderably higher than in the past. The nominal setting can even be fine-tuned by thecrew without removing the valve from the point of installation.

If this kind of ISO compliant equipment is used, the traditional hesitation in trusting anddepending on the valves can be overcome. Considering the physical properties of the crudeto the increase valve opening setting and closing pressure, there should be no need formanual de-pressurization.

In practice, the following should be applied:

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.

N O G R E E N

C A R D





Pressure setting of the valves should be increased and dynamic valve characteristicschosen that allow for this without sacrificing safety by eliminating the usual pressure peakduring operation. In other words: a venting system that maintains the tank pressure at nohigher than the valve’s nominal setting,.

Alarm setting (pressure) can remain as today, i.e., slightly above the increased openingsetting proposed above.

Flow characteristics of the chosen equipment should allow for a limited blow-downvalue to conserve the loss of vapor normally associated with sloshing and thermal variation.

--o0o--

New buildings:

If the above 3 issues are adopted, loss of vapor during voyage will be reduced to a fractionof the levels seen today. There will in principle be no extra cost.

Existing vessels:

New valves will be required at an approximate cost of US$ 2,000 - 4,000 per tank, and thealarm and liquid p/ v breaker settings may need adjustment. The payback time so small thatit will not be worth mentioning.

--o0o--

It should be noted, however, that without owner’s examination for ISO 15364 compliancefor the actual application, the sought after reduction of vapor loss is by all likeliness notachieved. The most important issue, however, is the level of owner effort required whenspecifying the equipment configuration and lay-out because the philosophy behind is notrequired by regulations, leaving it’s exploitation at owners’ initiative for their own benefit.

Copenhagen, November, 2001

_____________________ Eric Aarestrup Sørensen

PRES-VAC ENGINEERING A/ S

vapor loss from crude oil tankers

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