Case Study: Rotary Gate Valve0 pages

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Revolutionary New Valve

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Rotary Gate Valve

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FMC/StatoilHydro - Ormen Lange project

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Case Study

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Excellent

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Power & Industrial

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Solutions T

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POWER & INDUSTRIAL

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There are very few genuine new

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developments in the valve world, new

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ideas are mostly developed along a

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theme, however, one completely new

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concept in valve design is the Rotary

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Gate Valve developed by Weir Valves &

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Controls UK Ltd part of the Weir Power

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& Industrial Division in the UK.

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The valve has been successfully supplied

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to FMC/StatoilHydro for the Ormen

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Lange project in Norway where there

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was a particular problem in controlling

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regenerated Mono Ethylene Glycol (MEG)

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injection to prevent hydrate formation.

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Subsea equipment has been supplied

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for over 25 years, with more and more

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discoveries are being declared viable in

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deep and ultra deep waters. The Ormen

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Lange project situated off the north western

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coast of Norway was developed in water

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depths of 1,100 metres on the edge of

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the Arctic Circle and has an ambient sea

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temperature which can reach -2°C. Gas

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produced from the field contains entrained

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water which if left untreated would form

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hydrate and potentially block up the

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process lines. Gas from the Ormen Lange

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project is piped to an onshore terminal

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which is 120 kilometres from the subsea

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template.

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Hydrate Formation

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Hydrates are an ice-like lattice structure

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of hydrocarbons, which form when the

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entrained liquids are taken below the

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hydrate formation curve. When these

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occur, equipment is usually required to

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be brought to the surface for removal or

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'melting' of the hydrates. Downtime is very

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costly to the producer, so it is important to

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prevent the formation of hydrates. MEG is

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used as a type of anti-freeze to inhibit the

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expected water content in the hydrocarbon

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at the given well head pressure, in order

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to eradicate hydrate formation in the gas

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production line. Even 5-10% loss of MEG

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injection can lead to hydrate formation and

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it is critical to maintain injection, therefore

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the valve used has to be totally reliable.

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Hydrates in the process line

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Valve Development

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The valve was originally developed to

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isolate flow, with the ultimate advantage

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of not having a cavity (zero displaced

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volume during stroke). This is significant,

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because the valve will not drift on failure of

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the operator, and will not be detrimentally

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affected by a cavity, which could

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otherwise become pressure locked and/

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or blocked by hydrates of line debris. The

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turndown ratios far exceed those of more

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traditional controlling devices, and whereas

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conventional control and choke valves have

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an inherent leakage, the rotary gate valve

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gives zero leakage since the gate is based

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on conventional through-conduit gate valve

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technology.

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So Why Was a New Style of Valve

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Required?

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MEG dosage is probably the most critical

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process on applications where hydrates

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are present, so if the dosage valve was

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to fail then the operation would have to

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be shut down and the hydrates cleared.

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The Ormen Lange project was the first

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to use regenerated MEG and therefore

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no other valve design had been tried in

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the application. Other valve types were

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considered (described below), but the

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Rotary Gate Valve was selected due to its

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superior design.

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Screw and Piston Design

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On screw and piston design applications

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pressure loss is achieved by the flow as it

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passes along a course screw thread. This

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design has a low tolerance to impurities and

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can very quickly become damaged or stuck

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along the threaded area.

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Needle and Piston Design

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The needle and piston design valve relies

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on a two stage needle and seat to achieve

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a pressure drop. This design is not normally

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used on applications with high levels of

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impurities, as the impurities bridge across

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the flow orifice and erode the needle.

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Spline Choke Valves

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The spline choke valve has to distribute the

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pressure drop across a segmented plug. The

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plug clearances of this design means that

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the valve is often subject to blockages, and

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impurities in the flow mean that erosion can

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take place across the valve plug.

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The Rotary Gate Valve

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Weir Power and Industrial Case Study 1

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