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.