The hydrocarbon fluid produced from a well is made up of many components ranging from methane, the lightest and most gaseous hydrocarbon, to some very heavy and complex hydrocarbon compounds. Because of this, whenever there is a drop in fluid pressure, gas is liberated. Therefore, pressure control is important.
The most common method of controlling pressure is with a pressure controller and a backpressure control valve. The pressure controller senses the pressure in the vapor space of the pressure vessel or tank. By regulating the amount of gas leaving the vapor space, the backpressure control valve maintains the desired pressure in the vessel. If too much gas is released, the number of molecules of gas in the vapor space decreases, and thus the pressure in the vessel decreases. If insufficient gas is released, the number of molecules of gas in the vapor space increases, and thus the pressure in the vessel increases. In most instances, there will be enough gas separated or “flashed” from the liquid to allow the pressure controller to compensate for changes in liquid level, temperature, etc., which would cause a change in the number of molecules of gas required to fill the vapor space at a given pressure. However, under some conditions where there has been only a small pressure drop from the upstream vessel, or where the crude GOR (gas/oil ratio) is low, it may be necessary to add gas to the vessel to maintain pressure control at all times. This is called “make-up” or “blanket” gas. Gas from a pressure source higher than the desired control pressure is routed to the vessel by a pressure controller that senses the vessel pressure automatically, allowing either more or less gas to enter the vessel as required.