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Control systems

MAN B&W ME-C series for LNG Carriers. Control systems

Generally, the temperature in the nitrogen loop decides the quality of N2 in the coolant circuit.
Increasing or decreasing the amount of nitrogen in the loop changes the cooling capacity. The amount is changed by injecting or withdrawing nitrogen from the receiver. If the cooling capacity is too high, the inlet expander temperature will decrease. The control valve to the receiver at  the compressors discharge will open to withdraw the nitrogen from the main loop. Correspondingly, if the cooling capacity is too low, the inlet expander temperature will increase. The control valve from the receiver to the compressor suction side will open to inject nitrogen into the main loop.

The relationship between cooling capacity and pressure changes is based on the fact that a turbo compressor is a constant volume flow machine. When the suction pressure is changing, the mass flow is changing and, correspondingly, the cooling capacity. The pressure ratio for the compressor is constant and independent of the suction pressure. Even if the cooling capacity is reduced, the outlet expander temperature will be nearly the same.
The BOG cycle is an independent loop. The cargo tank pressure is kept approximately constant by varying the mass flow through the compressor. The boil-off compressor will be a two-stage centrifugal compressor with diffuser guide vanes (DGV) for controlling the capacity. There is DVG on both stages, and they work in parallel, controlled by the same signal. 

Redundancy is required by the International Classification Society Association (IACS), as discussed later. The requirement is fulfilled if one of the following options is installed:Thermal oxidizer or flare system capable of burning the maximum boil-off rate.Two 100% reliquefaction plant with one cold box, comprising the following:

Two BOG-compressor units
(two-stage centrifugal compressor)
Two N2-compressor/expander units 
(three-stage integrated gear  centrifugal compressor with one expander stage)
One cold box
One LNG phase separator
One LNG forced return pump
Auxiliary systems
Which one to operate of the two BOG-compressor units and N2-compressor/expander units can be freely chosen by operating the applicable valves. Change-over of equipment is done manually, and must be done only when the machinery is shut down. Simultaneous parallel operation of the equipment will not be possible.
As the reliability of today’s steam turbine driven LNG carriers is considered high, an alternative system must not deteriorate the availability of the LNG carriers. The reliquefaction system therefore only uses proven components – first class, high quality with extensive references. The low-duty compressors in the RS system are the same as used on all LNG carriers today. The refrigeration cycle is in operation on the LNG carrier S/S LNG Jamal, and the 3-stage compressor with expander is operating on FPSOs and in onshore process plants. The proposed cold box (plate fin heat exchanger) is widely used in onshore cryogenic installations. An availability, which is at the same level or better than ship machinery in general.

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