Seeking assistance with LNG tank venting
Thread Content
Our plant has been operating smoothly for over a year now, using a mixed refrigerant cooling system. The designed daily gas intake is 600,000 standard cubic meters, but it is rare for the plant to operate at full capacity. Recently, the storage tanks have been continuously emptied, yet the temperature of the incoming liquid is three to four degrees lower than it was during normal operations last year. The nitrogen content in the raw gas, as per the design, is 0.8%, but currently it is around 1.5%; last year it was also around 1%. What’s most strange is that the pressure behind the flow control valve leading to the storage tanks drops from time to time. It used to be consistently 1 kilogram of pressure, but now it occasionally drops to 0.8 or 0.7 kilograms. Whenever the pressure drops, the temperature of the LNG entering the storage tanks also decreases, while the pressure inside the storage tanks rises. The pressure before the flow control valve is 4.1 Mpa. Experts, please take a look and see what the cause is
The decrease in pressure inside the storage tank indicates that its operating pressure has dropped; it is likely that an excessive amount of gas was released, which caused the pressure in the tank to fall. At the same time, as the pressure decreases, the amount of gas that flashes off also increases, exceeding the capacity of the BOG compressor to handle it, which results in unstable pressure in the storage tank.
It can’t be that reason; the liquid inlet pressure also decreases when the storage tank is not emptied, and there’s no pattern to it at all
Because the liquid that enters the tank is at a lower temperature than the liquid already inside the tank; in other words, the incoming liquid cools down the tank, and PV=NRT. Of course, the pressure in the storage tank decreases when the temperature of the gas stored there drops.
We have encountered this problem before as well. Generally speaking, it happens because the BOG vapor from the storage tank is recovered, and since this vapor contains a higher amount of nitrogen, most of what is recovered is nitrogen. At a pressure of 4.1 Mpa before throttling, it is in liquid form; once the pressure behind the product valve drops to around 80 Kpa, the liquid nitrogen vaporizes, and this process absorbs heat, causing the LNG temperature to drop to around -163°C. As a result, the substance behind the product valve becomes a mixture of gas and liquid. The pressure of the water column entering the large tank decreases, which in turn lowers the pressure behind the valve. In fact, you can conduct an experiment: stop the compressor used for BOG recovery (that is, do not recover the BOG vapor but instead discharge it directly to the flare), and after one hour, observe the temperature changes before and after the JT valve. By comparing these values, you will understand what’s happening
So how can this working condition be improved? Reduce the nitrogen content in the feed gas? Should the storage tank be changed to allow liquid inlet from above for BOG extraction? All of our BOG goes into the gas system and is not liquefied for storage in tanks. However, the design allows only about 400 cubic meters to be extracted per hour, which is far not enough. Is it feasible to reduce the liquid outlet temperature in the supercooled section?
Since your BOG enters the gas system directly without being recovered, nitrogen accumulation will not occur. This is either because the nitrogen content in the feed gas is really high, or because the insulation performance of the storage tanks is poor (this can be determined during shutdowns when all LNG flow in the pipelines is stopped). Reducing the temperature inside the cryogenic tank is another option, but it’s not worth it as it will lead to a significant increase in energy consumption. It is recommended that you install another large compressor to extract the BOG, with a capacity of at least 1000 m3 per hour.
The insulation performance of the storage tank is fine; when no liquid is flowing in, the amount of leakage is much less compared to when liquid is flowing in and then being released. The nitrogen content in the raw gas used in the design is 0.8%; a level of 1.1% is also acceptable during operation. Recently, the gas supply has not been very stable – it can reach 1.8% at times, with an average level of around 1.5%. Adding another BOG unit would not be feasible, as we only have one boiler for generating fuel gas, as well as a heat transfer oil heater and a natural gas heater for dehydration; the amount of gas used per hour is not high, around 400 cubic meters. Is there any other way?
The temperature in our storage tanks has reached -167°C, and we have to drain them several times a day. The tonnage consumption used to be 1510, but now it’s almost 1600 – it’s a real headache. Changing the inlet fluid made it a little better
What’s your tonnage consumption? Here, the power consumption per ton is 580 kWh, and the gas-liquid ratio is 1360 m3 per ton. In such a case, if there are industrial, commercial, and residential users in the area surrounding the plant, it might be advisable to supply them with BOG; it’s okay if the price is lower
Is the gas-liquid ratio that low? By tonnage consumption, I mean the gas-liquid ratio; our design specifies 1500 for that value. It seems this problem won’t be solved in the short term