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PRESS RELEASE - January 2005

Crucial matter of valve selection
Crucial matter of valve selection Bestobell argues that the selection of valves in LNG carrier sampling and purging applications is in need of a new approach, especially in view of the rapidly changing nature of LNG shipping By Andrew C Brown, Director, Bestobell LNG Kevin Fretwell, Technical Director, Bestobell LNG Ian Morris, Product Development Manager, Bestobell LNG Frances Crosland, Project Manager, Bestobell LNG The LNG carrier fleet is set to more than double in a few years and each new ship will be expected to operate at a level of efficiency unheard of until recently.

The choice of the wrong valve technology will severely compromise this goal of high operating efficiency. The installation of unsuitable valves carries with it a high risk of leaks, expensive valve maintenance and safety concerns during the lifetime of the ship. The specific issue considered here is the design of LNG valves used in sampling and purging applications. Because the advent of larger size LNG carriers has introduced much bigger ship design variations, it is more importance than ever that the correct cryogenic valve technology is selected.

Why are ball valves currently specified for sampling and purging on LNG carriers? Over 30 years ago, when the LNG industry was young, naval artchitects and marine engineers looked to the more established very large crude carrier (VLCC) designs to provide a lead on the choice of valves. VLCCs had relied successfully on ball valves to manage the flow of oil around shipboard cargo systems, distribution manifolds and shoreside terminals. As a result, the same valve technology was applied to the new breed of LNG carriers emerging in the 1960s and 1970s.

No apparent consideration was given to the difference in performance of such valves when handling very cold liquefied natural gas. The only criterion was that the valves supplied were fire-safe. LNG carrier design is a relatively conservative discipline. It has changed little over the last 40 years, including in the area of valve technology. As a result, the use of ball valves is still the norm. In the meantime, the general cryogenics industry has adopted globe valves for the storage of cryogenic liquids, including LNG. Bestobell considers LNG carriers to be, effectively, temporary cryogenic storage facilities.

Cryogenic liquids, including LNG, have a completely different performance profile to that of oil when handled in bulk. Bestobell would never specify ball valves in leakage-critical cryogenic storage applications, for a number of reasons. Ball valves are mainly specified in process applications where pressure and flow are critical. They are designed to remain permanently open, hence the size of the orifice of the ball is the same diameter as the pipe. A ball valve will only be operated in a shutdown or bypass situation. In the 1960s an operator handling cryogenic liquids was involved in a major accident when a sampling valve was fully opened.

The valve in question was a ball valve, and the subsequent enquiry established that the accident was due to the specification of that valve. As a result of the lessons learned, some ship operators have since employed cryogenic globe valves in their sampling and purging lines and for other applications requiring tight shut-off. In industrial cryogenic applications globe valves are favoured for sampling and purging applications because of the level of control they offer, i.e. they enable the operator to control and throttle the flow and pressure during these operations.

Advances in design technology over the years have resulted in modern globe valves with greater safety margins than their predecessors. For example, the anti-blowout stem retention system, which nullifies what was once a weakness of the globe valve, is now a common feature on such devices. In addition, the round bonnet with the slip-on flange not only facilitates installation in areas of limited accessibility but also, more importantly, provides an antidote to the thermal contraction and expansion of the seals that occur at very low temperatures. The arrangement thus ensures a long service life for the unit. The use of conventional bonnet designs in such applications would result in leakage over time.

Both ball valves and valves fitted with a bonnet welded to the stem become distorted around the seal face over time, resulting in leakage. Of the benefits delivered by globe valve technology, none is more important than longevity. Despite the extremes of operation, there are still cryogenic globe valves in regular use that have provided safe service in over 40 years of continuous operation. The same cannot be said of a ball valve in cryogenic service. While the majority of LNG carriers still employ ball valves in their sampling and purging lines, if shipowners were to utilise the latest risk assessment techniques as a basis for their decisions, the dangers of choosing ball valves would be highlighted.

These risk assessment techniques would make use of the expertise accumulated by cryogenic valve manufacturers over many years in their work to develop products able to perform maintenance-free over the longest periods practicable. Such development work entails researching the performance of materials under huge temperature variations; studying flow characteristics; measuring operational wear and tear; and creating the appropriate technological solutions. Our own research and development work has proven the advantages of globe valves over ball valves for sampling and purging.

These advantages include lighter mass, lower cost, reduced risk of leaks, low maintenance and improved safety. Not all globe valves are the same. Rising costs and other commercial considerations have prompted certain manufacturers to introduce globe valves whose design does not take adequate account of the cryogenic environment. Based on non-cryogenic designs, such globe valves do not allow for thermal expansion and contraction in the body design and are also susceptible to liquid lock, a major safety concern.

In summary, when choosing a valve for sampling and purging on LNG carriers, such a valve needs to possess the following characteristics: - cryogenic suitability - ability to throttle the liquid flow for increased control and safety - a full-bore design - zero leakage design - a loose bonnet construction to allow thermal expansion and contraction to occur without disrupting the seal between the body and the headwork - increased sealing area with protection from particle ingress - fire-safe approved to BS 6755 Part 2 and BS EN ISO 10497:2004 - revolving disc to ensure non-rotating seat contact. - metal to metal seat to ensure tight shut-off at all times - a one-piece, anti-blowout spindle manufactured to a high-strength design especially for LNG service It is incumbant upon the valve manufacturer to educate the market on the selection of the correct valves for particular applications.

In addition, it is the responsibility of shipowners and shipbuilders to ensure that the correct product is specified and that the future of liquefied gas transport is not compromised by unnecessary risks.

"Bestobell LNG - specificed by those who plan for safety"