Water separation issue in storage tanks
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In the daily operation of storage tanks, draining water is one of the most fundamental tasks for operators. So, what is the sequence for opening and closing the valves when draining water from a storage tank? First method: Sequence for starting water discharge: First, open the primary water discharge valve (located on the side of the tank), then slowly open the secondary water discharge valve (the one that discharges water into the drainage pit). Sequence for ending water cut-off: First, close the secondary water cut-off valve, then close the primary water cut-off valve. Note: This operation method is a conventional one, and its advantage lies in the ability to control the amount of water used during cutting off the flow, as well as in preventing oil from leaking out at the end of this process. Second method: Sequence for starting water cut-off: First, open the secondary water cut-off valve of the tank, and then slowly open the primary water cut-off valve. Sequence for ending water cut-off: First close the primary water cut-off valve, then close the secondary water cut-off valve. Note: For this method of operation, an iron barrel must be placed at the drain outlet before cutting off the water, in order to collect the oil that is between the primary and secondary valves, which can then be recovered. Analysis: 1. The first method allows the water cut-off valve to be closed promptly, preventing oil loss. The downside is that if the associated secondary valve is closed as well, the space between the primary and secondary valves will be filled with liquid. If there are no leaks in the valves, this space becomes a sealed one; when temperatures rise in summer or heating is used in winter, this can lead to localized pressure buildup, resulting in leaks. 2. The second method does not cause pressure buildup, but there may be a slight loss of oil during the final stage of water separation; however, this does not result in any pressure buildup. So everyone can discuss which method, or a third method, should be adopted.
I’ve been using the second one. According to the description, there seems to be no dedicated water cut-off tank; only two valves are available. Therefore, it depends on the operators on site to determine the appropriate timing for cutting off the water flow. Generally, not too much oil will leak out. Here, when we separate the water, we stop the process as soon as we see oil bubbles in the water; there’s no need to make it completely clean. The pressure buildup is still quite severe.
Aren’t automatic water separation systems popular for oil tanks these days? The water cutting tank is generally used for liquefied hydrocarbon spheres, right? !
Currently, newly installed storage tanks are generally equipped with automatic water cut-off devices, while older tanks only have water cut-off valve assemblies. So, during normal operation, is the inlet valve of the automatic water cut-off device closed? As far as I can tell, the automatic water cut-off devices for atmospheric pressure storage tanks are not turned off (those on spherical tanks are turned off). So if it isn’t turned off, that means the water cut-off device at that point is a leakage site of the tank, and there is no emergency shutdown mechanism at that location. The State Administration for Work Safety has repeatedly emphasized the need to install emergency shut-off devices at the inlet and outlet of tanks in order to prevent oil leaks in case of accidents. Therefore, if the automatic water cut-off device doesn’t have a shut-off valve, can it be considered that this point constitutes a leakage site and represents a safety hazard in the event of an accident? Well, it’s back to normal. If the manual valve at the inlet of the automatic water cut-off device isn’t closed, this could pose a risk of oil leakage in case of an accident; so what about closing the manual valve of the automatic water cut-off device? The problem with turning off the automatic water cut-off device has arisen again. 1. If the automatic water cut-off function is used while it is turned off, the cutting effect will definitely be poor. 2. If the inlet valve of the automatic water cut-off device is closed, the tank will be filled with liquid; how then can we relieve the pressure? Everyone is welcome to discuss! Currently, my approach is to keep the inlet valve of the automatic water cut-off device for the atmospheric pressure tank open, while keeping the inlet valve of the automatic water cut-off device for the pressurized tank closed.
Is an automatic water cut-off device also installed in the pressure tank? I really haven’t heard of this. Generally, automatic water cutters can be installed in atmospheric pressure storage tanks. Since it is automatic, the inlet valve of the water cutter naturally opens and closes automatically. When the sensing element inside the automatic water cut-off device detects that the moisture level within it has reached a certain threshold, the valve opens automatically; when the moisture content falls below a certain level, the water cut-off device closes automatically, stopping the water cutting process. It’s not very realistic to have absolutely no oil at all. Just minimize oil leaks as much as possible. The small amount of oil carried away by the cutting water can enter the oily wastewater system through the sewer network for further recovery, thereby reducing oil losses. As for the requirements for emergency shut-off valves, they mainly refer to the inlet and outlet of the storage tank; if you insist that the water cut-off valve also counts, then that’s fine as well. Automatic water cut-off devices usually come with remote control, allowing them to be closed from a distance as well.