Dry gas seal
Thread Content
The last edit to this post was made by Catalyzing a Grain on 2018-10-27 at 10:57. In other words, after the compressor stops operating but the dry gas seal continues to function, the pressure in the process system connected to the cylinder body must be reduced to below 3 kilograms. May I ask, what are the effects if it exceeds three kilograms? Help needed
It would be best to include a flowchart to illustrate it, as the description alone is not very clear.
A weight of more than 3 kilograms will cause excessive stress on the contact surface between the rotating ring and the stationary ring of the dry gas seal, thereby damaging that sealing surface. This is my personal understanding; please correct me if I’m wrong.
I need to know the specific parameters of your unit; the pressure in the process system must be reduced to below 3 kilograms. My understanding is that the pressure of the sealing gas should be higher than that of the process gas, so as to create a positive flow and ensure that clean sealing gas continuously leaks from the seal into the cylinder. If the flow is in the opposite direction, dirty process gas will emerge and contaminate the seal
It involves four stages of compression: stages 1 and 2 involve one cylinder (a low-pressure cylinder, where the pressure is increased from 50 kPa to 230 kPa to 600 kPa), while stages 3 and 4 involve another cylinder (a high-pressure cylinder, where the pressure ranges from 600 kPa to 1500 kPa to 3000 kPa). Between the stage compressions, there are liquid separation tanks, washing towers, and alkali washing towers; the compressed gas is then sent to the distillation unit for separation. After the shutdown, the dry gas seal of the high and low pressure cylinders was changed to 7 kilograms of nitrogen. When using nitrogen to displace the alkali in the compression section of the tower system, the process requirements specify that the system pressure should not exceed 3 kilograms; this is also stated in the emergency shutdown procedure, which requires that the system pressure be reduced to below 3 kilograms as soon as the compressor stops operating. What is the reasoning behind this value of 3 kilograms?
This post was last edited by Suixin1025 on 2020-1-21 at 21:38: Dry gas sealing
Based on your description of the process, after the machine is shut down, the supply pressure for the dry gas seal in both the high-pressure and low-pressure cylinders becomes 7 kilograms of nitrogen, while the system pressure drops to 3 kilograms. This is done to ensure that clean nitrogen continues to flow into the cylinders, preventing the gas inside them from leaking out and contaminating the dry gas seal. As for why a pressure difference of 4 kilograms is maintained – rather than 3 or 5 kilograms – it probably depends on the specific conditions of the unit
The pressure in the cylinder body is 3 kilograms. If there is a leak in the cylinder body that is sealed by the comb-shaped seal, the leakage then occurs in the balance tube; as a result, the pressure in the balance tube will be lower than 3 kilograms (during normal operation, a control valve is used to maintain a pressure difference of 3 kilograms between the primary sealing gas and the pressure in the balance tube. After shutdown, the primary sealing gas is replaced with 7 kilograms of nitrogen, while the pressure in the balance tube remains well below 3 kilograms). In this state, the pressure of the primary seal gas supply can easily maintain a pressure difference of three kilograms with the balance tube, with a margin of safety. If the cylinder pressure is too high, which in turn leads to high pressure in the balance tube, it becomes impossible to control the 3-kilogram pressure difference between the primary sealing gas and the balance tube, and as a result, leakage from the cylinder cannot be prevented. (Each time the compressor is started, before the speed control valve is opened, the seal air source for the first stage of the dry gas seal in the high-pressure cylinder must be converted from low-pressure nitrogen to high-pressure nitrogen, in order to prevent leaks from the high-pressure cylinder. However, the primary seal air source for the low-pressure cylinder is equipped only with low-pressure nitrogen; even when the cylinder pressure reaches normal levels, using low-pressure nitrogen as the primary seal air source is sufficient to achieve sealing
When the machine is shut down, the pressure in the balance tube should be the same as that in the cylinder, right? Both at 3 kilograms?
John Crane requires that the pressure in the vessel be higher than the pressure in the flare to prevent the flare gas from flowing back into the sealing surfaces. It is better to ensure 0.3 Mpa.
After the compressor stops, the nitrogen supply to the dry gas seal must not be turned off, in order to prevent lubricating oil from entering the dry gas seal. Before the pressure in the vessel exceeds 0.7 MPa, the main sealing gas for the dry gas seal is changed to 0.7 MPa nitrogen as appropriate; the booster pump must continue to operate until the vessel is isolated and the pressure is reduced below 0.7 MPa.