150844970 Vapor Loss From Crude Oil Tankers

VAPOR LOSS FROM CRUDE OIL TANKERS Influence by design of venting system

Reference document: INTERTANKOs Guidelines for The Control of a Multiphase

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

INTERTANKOs VOCON on the same subject (undated).

P R E S - V A C E N G I N E E R I N G A / S

Guide on minimizing vapor loss by design and operational procedures

Pres-Vac Engineering A/S Svanevang 3-5 DK 3450 Allerd Denmark Phone +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

Page Page Page Page 1111 of of of of 3333

Comments on how to minimize vapor loss by basic design and re-thought procedures

Introduction In short, the Publication lists two issues, which combined cause the vast majority of vapor loss during voyage:

A. Apparently what must be termed as distrust in the efficiency and reliability of p/v valves, 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 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 systems are 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.

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.



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

a retroactive and mandatory concept of secondary venting, 1 and

a new ISO standard for marine p/v valves, which is mandatory for installations after 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 pressure equilibrium 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 be eradicated. The thesis of this memo is that a properly designed venting system using p/v valves of a reliable 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 much-discussed 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 the IMO initiative in regard of developing a valve standard is sufficient in 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 design parameters would be necessary for the appropriate venting system and equipment for this to succeed.

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



Available means to reduce vapor loss Three simple but extremely effective means are available that have potential to significantly reduce 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 - and corresponding 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 as described in the aforementioned INTERTANKO publications

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 because of lack of trust with regard to the pressure/vacuum valves, thus calling for manual pressure controlling via a central riser.

The report reads:

However, results and observations for the practice aboard tankers reveal that tanker commands do not generally rely upon P/V valves for over pressure control and will release pressure manually through one of the previously discussed onboard systems when the over pressure reaches about 1,000 mm WG. This practice, being absolutely necessary to protect the vessels 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 pressure for closing of the manually opened release mechanism seem to show a pressure of approximately 400 mm WG as the commonly selected. It is the determination of this pressure that needs to be defined such that unnecessary release of both hydrocarbon and Inert gases is avoided.

As outlined in the above, we need 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 issue must be answered

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Where does this leave us? K E Y

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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 establish the 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 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? The answer is definitely affirmative, - p/v valves cannot be trusted, generally speaking. 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 type approval (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 a real fire incident, whereas they all are functioning as balancing relief valves in practice. Design priorities have been upside down.

When commands do not trust the p/v valves, it is not because they fear a flash-back incident happening; they fear that the valves will not manage to 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 a general point of view.

One issue that 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 Masters loading chart required by IMO MSC/Circ. 731.



A lot of p/v valves are prone to mal-function because of insufficient tolerance to 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 the discs 1/100 of the full stroke, leaving check-lifting close to being an illusion. Some valves 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 overly trust by the various parties involved with regard to the traditional type approval procedures according to IMO regulations without realizing that they simply do not 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 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 has probably had little if 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. 677/1009 satisfies 15364, the new test requirements are of interest only with regard to non-SOLAS vessels. 15364 is mandatory by reference in IMO MSC/Circ. 1009 for 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.

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 the PRD, i.e., eventually the owner by virtue of the ISM 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

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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 be noted, however, that the wordings applied in type approval certificates are 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 a valve for suitability. There is no limitation as to what can be approved and termed ISO 15364 complying. As long as the product is adequately described as per ISO 15364, the owner has the 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 they are far from equally suitable for all applications. They fall in different price categories and are designed for different types of vessels. The final responsibility for selecting the proper piece of equipment for a vessel is not with the manufacturer, but with the 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 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 the buyer/user/ship owner to consider and accept.

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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 to nothing. As an example, however, with some element 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

Where to find the necessary information?



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 owners approach

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 Owners 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 O F D E P O S I T S

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 PRES-VAC range.

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

The possibility to inspect the valves complete inside for fouling and deposits should be easy and convenient to carry out. Ideally, removing one hood should suffice.

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

Seats and discs should be replaceable by removing the worn out part without further disassembling. When seats have been replaced, perfect alignment of the new seat to the disc should be a feature of the system. Seats that are held in place by Loctite or thread or screws will not allow for adequate alignment unless they are machined after installing.

3 . 1 M 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 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

What are the main issues to look for?



ISO 15364 item Owners requirement

be full and the disc positions visually indicated.

The possible ice-layer thickness within the PRES-VAC range varies 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

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

6 . F O U L I N G I 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 to continue 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. It should be considered that the normal venting rate is half the required venting rate for VECS, which would often cause the operation to take place in the zone where the pressure peak normally is.

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

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 because the operator is justified in his decision not to rely on the venting equipment, generally speaking.

If the issues listed in ISO 15364 are considered properly, noting that most existing designs will 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 general purpose of the ISO standard is specifically to improve valve performance, which implicitly means foregoing existing designs being the very cause of the situation that lead to redundancy venting being enforced.

It should be recalled that at any time of the selection process - a valve design 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 vice versa.

O W N E R S

T A S K



In short, if the owners 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 be argued that if the procedure of manual de-pressurization continues, the chosen valves are not the right ones.

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

Basic understanding of how different designs work is necessary for reference. Pressure relief valves are generally made with 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 necessarily chattering or fluttering.

ii. Full lifting, i.e., with an instant reduction of pressure drop over the valve to a value below 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 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 in-line 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 the influence of small bore piping or long piping. 2

iii. Controlled blow-down, i.e., an instant reduction of the pressure drop over the valve by 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 over-pressure and then close at the tank pressure corresponding to the net-closing force of the valve. These valves are typically magnet-controlled valves, often high velocity valves or in-line valves used when pressure drop is an issue relevant to the piping lay-out.

iv. Non-hammering weight-loaded high velocity valves, i.e., a valve that 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 a danger for flash-back in high velocity vents when IMO initiated a test series 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 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 some pipe configurations, these valves are not able to maintain adequate efflux, which should be shown in the certification if done properly.

v. Non-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 feature a combination of controlled blow-down mechanisms and weight-loaded 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. These valves will yield a certain negative pressure drop, i.e., the system pressure will be reduced 10-20% below the valves nominal setting, until the valve is fully open when the system pressure will stabilize at the valves nominal setting. This performance allows for the use of less diameter piping and/or higher venting rates on a comparable basis. These valves are typically high velocity p/v valves.

The loss of vapor during voyage, for instance caused by sloshing or thermal variation, is determined by the valves opening characteristics and its closing pressure. A valve that falls within the category of non-oscillating high velocity valves as described above features a variation of 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 m3/h (= Design point 1) 4 VECS venting capacity @ pressure: 2,000 m3/h (= Design point 2) 5 Maximum venting capacity without pressure increase: 2,000 m3/h

6 Pressure differential available for increased loading rate or reduced pipe 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 following parameters can be considered:

Opening setting: Should be higher than tradition calls for. The operational margin normally left between the valves opening setting and the alarm setting, which generally has been reserved for the pressure increase over the valve, can be waived. 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



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

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

in comparison with new non-oscillation combination valve

10

12

14

16

18

20

22

0 500 1000 1500

Nm/h

Figure 4

The crude oils 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).

Closing pressure: Should be selected based on the pressure drop conditions of the vent piping with a view to minimize the valves 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 a setting of 1,800 mm WG 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 pressure during voyage would stabilize at all time between 1,500 and 1,800 mm WG.

Measured pressure: 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 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 controlled over-pressure and a high closing pressure, refer to Fig. 3 for reference, the following would be the scenario:

Tank pressure @ full flow rate

Safety margin to p/v breaker set @

2,000

ITEM

mm WG m3/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-hammering weight-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 (save piping pressure drop)

mm WG

Proposed non-oscillating type

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 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 risers end-of-line flame



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 pressure where the cargos 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 to unhealthy vapour.

Part conclusion 2 Equipment is available now, designed and constructed specifically to the new ISO standard that should allow the vessel commands to trust the performance, unlike in the past when many 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 a descriptive nature. However, the owner can and should use it as a tool 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 issue that any valve can be ISO 15364 certified. Certification does not imply that 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 owners discretion, not with the yard, class, or manufacturer.

Final conclusion Equipment is now available in the market, which provides the owner compliance with all the issues addressed above (1-8). In practice this leaves the crew with much enhanced level of comfort because maintenance 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 crew without removing the valve from the point 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, 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.

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C A R D



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 valves nominal setting,.

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

Flow characteristics of the chosen equipment should allow for a limited blow-down value 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 fraction of 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 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 owners 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 by regulations, leaving its exploitation at owners initiative for their own benefit.

Copenhagen, November, 2001

_____________________ Eric Aarestrup Srensen

PRES-VAC ENGINEERING A/S

Documents

150844970 Vapor Loss From Crude Oil Tankers