Questions regarding the three-valve assembly
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When a three-valve manifold is used in conjunction with a differential pressure transmitter, what is the sequence in which the valves of the manifold should be opened?
When the three-valve assembly is in use, close the balance valve and open the positive and negative pressure valves to prevent the diaphragm from being subjected to pressure in only one direction.
1. Close all instrument valves, including those in the three-valve assembly. 2. Open the balance valve of the three-valve assembly. 3. Open all valves on the low-pressure side, including the instrument gate valves and those on the low-pressure side of the three-valve assembly. 4. Open all valves on the high-pressure side. 5. Close the balance valve
Some materials I’ve seen state that when starting up, one should first open the balance valve, then the high-pressure valve; after that, close the balance valve and open the low-pressure side shut-off valve. I’m not sure which of these is correct.
The correct sequence for opening and closing the three-valve assembly is as follows: The steps to open the three-valve assembly are: ① First, open the high-pressure valve； ② Close the balance valve ； ③ Finally, open the low-pressure valve. The steps to shut down the three-valve assembly are: ① First, shut off the low-pressure valve ； ② Open the balance valve ； ③ Finally, close the high-pressure valve. The general principle is: when the transmitter is in operation, when switching the three-valve assembly, it is not allowed for the condensate or isolation fluid inside the pressure guide tube to be lost ； Try not to subject the transmitter to one-way pressure. When the transmitter is shut down, the root valve and the high and low pressure valves of the three-valve assembly should be closed, while the balance valve should be opened to ensure that the pressures on both sides of the transmitter’s measurement chamber are equal, maintaining a state of balance.
The balance valve was opened first to prevent excessive pressure from being applied to one side of the instrument, which could cause damage to its interior
It's best to understand it from a conceptual perspective. The opening of the balance valve indicates that the signal values at both ends of the differential transmitter are “0”, meaning the pressure difference is “0”. This method can be used to check if the instrument is zeroed. If this equalizing valve is closed, it means a pressure difference has been created across the differential transmitter. Usually, the pressure in the positive-pressure tube is higher than that in the negative-pressure tube; in this case, the indicated value is above the “0” level of the measurement point. In abnormal conditions, if the negative pressure valve is opened first, the pressure in the negative pressure line will become higher than that in the positive pressure line, and the indication value will drop below the “0” mark. For a negatively varying structure, this pressure is in the opposite direction; it is uncertain whether it has the same capacity to withstand pressure as positive pressure. The positive pressure difference is the maximum pressure difference that can be tolerated within the allowable range. To avoid the possibility of a reverse pressure difference in the instrument, do not open the negative pressure valve first. For indicating instruments, the original displays were mostly mechanical instruments, not digital ones. These instruments generally cannot be below the “0” position; if they are, it will cause damage to the indication system. Additionally, some digital display instruments may also have issues with reverse pressure signal limitation protection. Therefore, the correct procedure must be followed in accordance with the instructions provided with the instrument. For reference.
Following the standard sequence of operation, one should first open the positive pressure valve and close the balance valve, then open the negative pressure valve; the reverse order is applicable when they are not in use. When not in use, both the positive and negative pressure valves are closed, while the balance valve remains open to maintain a pressure difference of 0 between the two sides.
The negative pressure side of my differential pressure transmitter is connected to the atmosphere. When it is not in use, the shut-off valve on the high-pressure side, the balance valve, and the shut-off valve on the low-pressure side are all closed. Is this approach correct?
The balance valve cannot be closed; when it is open, the positive and negative pressure sides are connected, resulting in no pressure difference. To measure pressure and create a pressure difference, the balance valve must be closed during operation in order to separate the positive and negative pressure chambers. . .