Recommendation for the installation of cargo strainers on LNG carriers 2nd

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

Society of International Gas Tanker

& Terminal Operators Ltd

Recommendations for the Installation

of Cargo Strainers on LNG Carriers

Second Edition October 1992

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

RECOMMENDATIONS FOR THE

INSTALLATION OF CARGO STRAINERS

ON LNG CARRIERS

Notice of Terms of Use

While the advice given in this document has been developed using the best information currently available, it is intended purely asguidance to be used at the owner's own risk. No responsibility is accepted by the Society of International Gas Tanker and TerminalOperators Ltd, or by any person, firm, corporation or organisation who or which has been in any way concerned with thefurnishing of information or data, the compilation, publication or authorised translation, for the accuracy of any information oradvice given herein or for any omission herefrom or for any consequences whatsoever resulting directly or indirectly from thecompliance with or adoption of guidance contained herein even if caused by a failure to exercise reasonable care.

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

INDEX

1. Introduction

2. General

3. Philosophies

4. Recommendations

(a) Strainer Type and Disposition

(b) Flow Characteristics

(c) Fittings

(d) Wire Mesh Size

(e) Inspection

Figure 1 Conical In-Line Dual Flow Strainer Arrangement

Figure 2 Conical In-Line Discharge Flow StrainerPage111222233

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

RECOMMENDATIONS FOR THE INSTALLATION OF

CARGO STRAINERS ON LNG CARRIERS

1. Introduction

Until relatively recently cargo strainers have not been fitted or considered necessary on LNG carriers. LNG beingessentially a "clean" cargo, filtration has been generally limited to the installation of strainers in the loading lines atthe loading terminal principally during the initial commissioning of the plant and build-up period of the project.Avoidance of contamination on the ships has relied on good housekeeping and visual inspection during constructionand subsequent maintenance periods. Incidents have occurred, however, where gross solid matter has been dischargedinto receiving terminal facilities to the extent that terminal operators now require ships to provide some form of cargostraining as a protection against serious contamination or damage to their terminal installations. At the same time,export and import terminals have recognised the part they can play in maintaining the cleanliness of cargo transfer inprotecting downstream units and in safeguarding their own interests.

2. General

The purpose of this document is to provide recommendations, based on current experience, for the type, disposition,performance and usage of LNG shipboard cargo strainers. On the assumption that existing ships in service havealready adopted philosophies and arrangements mutually acceptable within their projects, these recommendations areintended as a guide for newbuildings or existing ships presently not employed in project trading.

3. Philosophies

In current LNG trading, two broad philosophies on cargo transfer straining have emerged. One is that the ship acceptsthat a cargo delivered to it by an export terminal will be clean and, by a primary emphasis on strainers in its owndischarge, it in turn ensures a clean cargo to the receiving terminal. In a strict application of this philosophy, the shipwould not require to provide strainers during loading and any strainers on the shore side at discharge, additional to theship's strainers, would be entirely at the discretion of the receiving terminal. However, a shore export strainer cannotprotect against debris picked up in any piping and the loading arms downstream of the strainer and removing cleanshipboard strainers before loading and replacing them for discharge is an unwanted work load. Thus, ships operatingwithin this philosophy commonly have strainers which may be, and are, left in place during loading.

The alternative philosophy takes the rationale that it is in the interests of the ship, by fitting strainers during loading,to ensure that it receives a clean cargo and, likewise, in the interests of the import terminal, by means of shore sidestrainers, to ensure that it receives a clean cargo. Thus, for the ship the prime requirement is that it strains the cargo asit is loaded.

Here again, however, as an additional precaution and to minimise work load, dual-flow strainers maybe provided andbe left in place during discharge. Alternatively, some ships operating within this philosophy provide easilyinterchanged single-flow strainers separately for discharge.

Recognising the energy adsorption and consequent additional boil-off resulting from any restriction deliberatelyplaced in the way of cargo flow, two regimes of strainer mesh are generally accepted - a finer mesh for periods whengeneral contamination is more likely, as at commissioning into service or after maintenance, and a less fine mesh forperiods when the normal routine cleanliness of ships and terminals can be expected.

Strainer requirements in both philosophies are thus virtually identical in that ships generally provide strainer facilitiessuitable for both loading and discharge flow and at two levels of strainer fineness.

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

4.Recommendations

It is recommended that shipboard strainers should meet the following design and installation criteria and be used as indicated.(a)Strainer Type & Disposition

Greatest convenience is achieved if strainers are capable of being cleaned or changed with ease. For this reason, theconical, in-line strainer, inserted in the existing or marginally extended manifold piping is preferred to the basket typestrainer. Experience with the latter type indicates that it is difficult to locate and arrange for dual-flow, iscomparatively awkward to clean and may exhibit top flange sealing problems.

The recommended strainer is based on a simple in-line, basket woven, AISI 316 stainless steel wire mesh of conicaldesign. The strainer should be capable of being inserted into and removed from the manifold piping when theship/shore arms are not connected but without disturbing the ship's associated pipework support systems. An outlinearrangement of a dual-flow (loading and discharge) strainer meeting these requirements is given in Figure 1 (b).Similarly, an arrangement for a single-flow discharge strainer combining strength with lightness and utilised on someships is shown in Figure 2.

If spool-piece extensions to the existing manifold piping are required to adequately house the strainers between thepresentation flanges and the manifold valves, the spool-pieces should be such that the manifold arrangements aremaintained within the limits recommended by the OCIMF LNG Manifold Standards

(Reference 1) in respect of the position of the presentation flanges inboard from the ship's side and of the strength ofthe manifold to withstand imposed loads.

(b)Flow Characteristics

The maximum pressure drop across dual-flow strainers in the clean condition should not exceed 0.5 bar at themaximum design flow rate in either direction. In practical design, when clean, where adequate strainer housing hasbeen available, dual-flow strainers have been found to exhibit pressure drops considerably less than the maximumrecommended above.

Where a ship's strainers are to be used during loading and where there are either no shore strainers installed or theshore strainers are not of equally fine mesh, the ship's strainers should be so constructed that, if totally blocked, theywill withstand the maximum pressure which could be imposed by the cargo transfer system.

Similarly, ship's strainers to be used during discharge should be capable, when blocked, of withstanding the maximumpressure imposed by the ship's pumps. Where ship strainers are used during loading in association with shore strainersof at least as fine a mesh, the ship's strainers should be capable of withstanding a pressure differential of at least 3 barwithout collapse.

In the case of single-flow strainers, the mesh reinforcement requirements for total blockage need only apply, ofcourse, the appropriate flow direction. The pressure drop across such strainers in clean condition and with maximumdesign flow rate, again should not exceed 0.5 bar.

(c)Fittings (see Figure 1 a)

The strainer housing piping shall be provided with the following fittings:

1)

2)Local pressure gauge connections upstream and downstream of the strainer for indicating pressure differences across thestrainer. It may be necessary that one of these connections be on the loading or discharge arm connecting flange.A valved drain connection and suitable arrangements for purging the housing.

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

(d)Wire Mesh Size

Protection from contamination may be required particularly following the entry into service of an export terminal orof a ship or following any maintenance or refitting in an export terminal or on a ship which could affect the internalcleanliness of the terminal's storage/loading systems or of the ship's cargo system respectively. Whenever suchconditions apply, strainers of ASTM 60 mesh (nominal aperture of 0.25 mm) should be installed on shore, on ships oron both in accordance with the accepted philosophy, during loading and discharge of the first two subsequent cargoesfrom a terminal to which the conditions apply or the first two subsequent cargoes carried by a ship to which theconditions apply. thereafter, provided that all strainers at both loading and discharge are exhibiting a clean conditionon inspection following cargo transfer and provided that there is to be no significant increase in cargo transfer flowrate, the ship's master, in consultation and agreement with the receiving terminal, may replace the shipboard strainerswith strainers of ASTM 20 mesh (nominal aperture of 0.84 mm).

Inspection

Where shipboard strainers are of the type inserted and clamped at the presentation flange as indicated in the Figures,inspection of strainer cleanliness after loading is simple and may be carried out routinely without removal of thestrainer. Inspection of strainer cleanliness during cargo transfer will require stoppage of cargo flow and disconnectionof the cargo transfer arms and inspection after discharge will require the removal of the strainers. Such inspectionsshould be made at the discretion of the ship's master and would normally be undertaken only if the pressure lossacross a strainer significantly exceeds that experienced in the clean condition.

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Reference

(1)"Standardisation of Manifolds for Refrigerated Liquefied Gas Carriers (LNG)" Oil Companies International Marine Forum

(OCIMF). Distributed by Witherby & Co Ltd, London.

SIGTTO London Liaison Office - January 1984/Reprinted October 1992

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

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