A refrigerant compressor blocked discharge scenario is used to size the flare systems for baseload LNG plants. This scenario is set up by closing a valve at the compressor discharge, causing the pressure in the discharge piping to rise to the pressure relief setting. At this point the pressure safety valves (PSVs) open and the refrigerant flows to the flare system. A peak relief flowrate and pressure are calculated as this event proceeds and the flowrate is used to determine the flare piping diameter, the height of the flare and its distance from the plant to protect workers from thermal radiation.

This scenario can be easily simulated in steady-state mode because the pressure relief setting is known by design constraints. However, dynamic simulation of the LNG refrigeration circuits has shown that it is possible to reduce the amount of refrigerant flared or eliminate it altogether. The key to this scenario is what is happening before the relief valves open and how the plant reacts to the pressure buildup. An interesting option is to bogdown the gas turbine and helper motor that drives the compressors and trip them on low speed before the PSV to flare reaches its set pressure.

The gas turbine's maximum available power can be reached faster by increasing the mass flowrate through the compressor stages, so that the compressors demand more power. The additional mass flowrate can be achieved by opening either the existing anti-surge control (ASC) valves or additional valves (e.g. a Recycle PSV) if they exist in the plant. When the compressor power demand reaches the gas turbine's maximum available power, the turbine reduces speed in order to maintain a power balance between itself and the compressors. In turn, this lower speed reduces the head achieved by the compressor and causes a maximum in the discharge pressure. The speed reduction also reduces the maximum available power and the gas turbine continues to reduce speed until it trips on low speed.

The gas turbine's trip could remove the refrigerant compressor blocked discharge case as the flare sizing case, allowing the flare and its header to be sized to a less severe scenario leading to important economic savings. This paper will examine this effect on a propane refrigeration circuit of a typical LNG C3/MR plant.