Open Rack LNG Vaporizer
Objective
In order to reduce construction costs and improve the efficiency of operation and maintenance, we developed and introduced a low-cost, easy-to-use open rack LNG vaporizer drawing on our expertise in design, construction, and operation. This technology won the prize of the Japan Gas Association in 2006.
Present Situation
Applying new technologies including a new optimum design technique, Tokyo Gas recently upgraded its dependable, time-proven ORV to produce a new high-performance model, HiPerV, at its Ohgishima LNG Terminal. The first unit, completed in November 2000, has a capacity of 200t of LNG per hour, with reduced construction costs and improved efficiency of operation and maintenance.
Features
1.Reduction of construction costs using optimum design technique
Taking as our starting point the star-fin heat exchanger tubes developed in 1984, we completely overhauled the conventional empirical design approach and develop a new numerical analysis-based optimum design technique to efficiently determine the best tube shape for heat transfer efficiency. Developed following this approach, the new HiPerV’s tubes have larger diameters and more efficiently shaped internal and external surfaces. A special feature of these tubes is that their fin grooves are not easily blocked even when heat-resistant ice forms on their exteriors, enabling them to convey uniformly large volumes of seawater. Only half the normal number of tubes is required as a result, greatly reducing construction costs and installation space.2.Improvement in operation
The vaporizer itself is made of aluminum alloy and the connecting pipes are made of stainless steel. As the body and pipes used to be connected by flanges, the vaporizer had to be kept in standby mode in order to keep the flanges constantly cool even during shutdown by maintaining a slight flow of LNG so as to prevent LNG leaking from them at startup. In order to enable rapid startup from ordinary temperature without the need for maintaining in standby mode, a transition joint was developed in 1985, and this joint was introduced for practice in 1985. This joint consists of layers of aluminum, titanium, nickel, and stainless steel, and is designed to absorb thermal stress by combining them in descending order of coefficient of thermal expansion. It is now the standard joint used for the connections between vaporizers and piping, significantly improving both operational efficiency and safety.
3.Improvement of maintainability
ORVs use seawater as a heating medium, which places their tubes in a corrosive environment. Conventionally, therefore aluminum-zinc alloy coating had been applied to prevent corrosion. However, this tends to wear off and requires regular repair. Since 1998, therefore, we are using tubes that are given an anti-corrosion cladding at the extrusion stage. This cladding consists of an aluminum-zinc alloy that functions as a sacrificial anode. As tubes clad in this material do not need periodical repair, this has helped significantly reduce maintenance costs.
Outlook
LNG demand is expected to continue to grow, both in Japan and overseas, creating a need for more reliable and economical vaporizers. This type of ORVs with imporved efficiency of operation and maintenance, developed by Tokyo Gas, has become the mainstream in LNG vaparizors and more than 300 units are aleady adpoted. The optimum design technique, transition joint, and cladded tube described here are expected to make major contributions to reducing construction and maintenance costs for the many users planning to build new or replacement vaporizers.
