Sequence of commissioning of dry gas seals in large units
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Dear sea friends, when putting the dry gas seal into operation, follow the operating instructions provided by Sichuan Niki: first activate stage 1, then stage 2, and finally isolate the gas; the sequence for deactivating it is the reverse. In practice, only isolation gas is used to block the lubricating oil. Is it necessary to enforce strict requirements regarding this order of commissioning? Back-to-back dry gas seals are shown in the figure below: The pressure of the secondary seal gas (the main seal gas) is higher. Following this sequence, should the seal gas be supplied first, followed by the pre-seal gas, and finally the isolation gas?
The isolation gas should be introduced first and then stopped to prevent contamination of the lubricating oil in the bearing compartment； ； ； Inject main seal gas and pre-stage gas again
The sequence between the isolation and oil systems is strictly specified, but the sequence among the three seal gases is not very clear.
As long as oil transportation is taking place, the isolation gas must be activated in advance; this has nothing to do with anything else. The sequence is as follows: first, the main sealing gas is supplied, followed by the pre-sealing gas. Once the main sealing gas is in place, it usually doesn’t matter what happens, including during the operation of the turning gear
Successively, it is the isolator, primary seal gas, and secondary seal gas
Establishment of the dry gas seal system: (1) Open all instrument valves and put the on-site pressure gauge into use. One set of dry gas seal filters is in operation while the other is on standby. ⑵ Isolation gas supply: Isolation gas is supplied 10 minutes before the start of oil transfer. The pressure gauges PI3184A/B on the low-pressure nitrogen pipeline system indicate a pressure of 0.45 MPa. Valves V18, V19 (or V20, V21) and V22, V23 are opened in the low-pressure cylinder, while valves V30, V31 (V32, V33) and V34, V35 are opened in the high-pressure cylinder. After pressure reduction via the pressure regulators, the pressure gauges PI3185A/B indicate a pressure of 0.4 MPa. The upstream and downstream shut-off valves of the orifice plate R02 at the low-pressure end of the low-pressure cylinder were opened, and the readings on the pressure gauge PI3187A were both 20 KPaG. The upstream and downstream shut-off valves of the orifice plate R01 at the high-pressure end of the low-pressure cylinder were opened, and the readings on the pressure gauge PI3186A were both 20 KPaG. The orifice plate R07 and the upstream and downstream shut-off valves at the low-pressure end of the high-pressure cylinder were opened; the readings on the pressure gauge PI3187B were both 20 KPaG. The upstream and downstream shut-off valves of the orifice plate R06 at the high-pressure end of the high-pressure cylinder were opened, and the readings on the pressure gauge PI3186B were both 20 KPaG. Note: Introduce isolation gas before oil transfer to prevent lubricating oil from entering the sealing surface. Similarly, the isolation gas can be turned off only after oil transport stops and there is no oil in the oil return sight glass for 30 minutes. The isolation gas must not be interrupted during normal operation. ⑶ When the primary seal gas is activated, the pressure gauges PI3181A/B on the medium-pressure nitrogen pipeline show a pressure of 6.0 MPa. In the low-pressure cylinder, valves V1, V2 (or V3, V4), and V5, V6 are opened; in the high-pressure cylinder, valves V1, V2, V3, V4, (V5, V6, V7, V8), V15, V16, V17, and V18 are opened. After pressure reduction via pneumatic diaphragm control valves, the pressure gauges PI3183A/B still show a pressure that is 0.3 MPa higher than the pressure in the unit’s balance pipe. The pressures PDICA3182 for the low-pressure cylinder and PDICA3187 for the high-pressure cylinder are both ≥0.3 MPa. Before air enters the unit, valve V13 is opened at the low-pressure end of the low-pressure cylinder, and valve V16 is adjusted; the flow meter FI3182 indicates a flow rate of 185 Nm3/h. Valve V12 is opened at the high-pressure end of the low-pressure cylinder, and valve V15 is adjusted; the flow meter FI3181 also indicates a flow rate of 185 Nm3/h. Valves V22, V23, V27, and V28 are opened at the low-pressure end of the high-pressure cylinder, and the flow meter FI3188 indicates a flow rate of 300 Nm3/h. Valves V20, V21, V24, and V25 are opened at the high-pressure end of the high-pressure cylinder, and the flow meter FI3187 indicates a flow rate of 300 Nm3/h.
(4) When the flare line is activated, if the pressure inside the compressor is greater than the pressure in the flare pipeline (PI3188A/B > 0.1 MPa), valves V34 and V38 are opened at the low-pressure end of the low-pressure cylinder; the flow meter FI3186 indicates a flow rate of approximately 0 Nm3/h. Valves V33 and V37 are opened at the high-pressure end of the low-pressure cylinder; the flow meter FI3185 also indicates a flow rate of approximately 0 Nm3/h. Valves V47 and V50 are opened at the low-pressure end of the high-pressure cylinder; the flow meter FI3192 indicates a flow rate of approximately 0 Nm3/h. Valves V45 and V49 are opened at the high-pressure end of the high-pressure cylinder; the flow meter FI3191 indicates a flow rate of approximately 0 Nm3/h.
(5) When the secondary seal gas is activated, valve V26 is opened at the low-pressure end of the low-pressure cylinder, and valve V30 is adjusted; the flow meters FI3184 and FIT3186 both indicate a flow rate of 8 Nm3/h. Valve V25 is opened at the high-pressure end of the low-pressure cylinder, and valve V27 is adjusted; the flow meters FI3183 and FIT3185 both indicate a flow rate of 8 Nm3/h. Valves V38 are opened at the low-pressure end of the high-pressure cylinder, and valve V42 is adjusted; the flow meters FI3190 and FIT3186 both indicate a flow rate of 6 Nm3/h. Valves V37 are opened at the high-pressure end of the high-pressure cylinder, and valve V39 is adjusted; the flow meters FI3189 and FIT3185 both indicate a flow rate of 6 Nm3/h. Once the unit is operating normally, the flow meters FIT3185 and FIT3186 indicate a flow rate of approximately 12 Nm3/h, while the flow meters FIT3191 and FIT3192 indicate a flow rate of approximately 13 Nm3/h. Note: The compressor must be started under pressure, with the internal pressure being higher than that of the flare pipeline; otherwise, a negative pressure difference may occur upstream and downstream of the primary seal, which can damage the seal after startup. ⑹ Connect it to the air circuit of the booster pump; open V8, V9, V41, and V42 in the low-pressure cylinder. When the booster pump P3181 is operating, adjust the pressure relief valve PCV2 so that the pressure indicated by gauge PI3189A is 0.4 MPa. At the same time, observe the booster pump – its piston should move at a frequency of 50–60 times per minute; if not, adjust V42. For the high-pressure cylinder, open V9, V10, V12, V11, V53, and V54. When the booster pump P3182 is operating, adjust the pressure relief valve PCV2 so that the pressure gauge PI3189B shows a value of 0.4 MPa. At the same time, monitor the booster pump’s piston; its operating frequency should be 50–60 times per minute. If not, adjust V54. ⑺ After confirming that the nitrogen pressure is normal, slowly open the nitrogen pressure relief valve to apply nitrogen sealing to both the front and rear bearing housings of the turbine, keeping the pressure at 0.02 MPa (it is sufficient to observe gas escaping from the vent pipe on site).
(4) When the flare line is activated, if the pressure inside the compressor is greater than the pressure in the flare pipeline (PI3188A/B > 0.1 MPa), valves V34 and V38 are opened at the low-pressure end of the low-pressure cylinder; the flow meter FI3186 indicates a flow rate of approximately 0 Nm3/h. Valves V33 and V37 are opened at the high-pressure end of the low-pressure cylinder; the flow meter FI3185 also indicates a flow rate of approximately 0 Nm3/h. Valves V47 and V50 are opened at the low-pressure end of the high-pressure cylinder; the flow meter FI3192 indicates a flow rate of approximately 0 Nm3/h. Valves V45 and V49 are opened at the high-pressure end of the high-pressure cylinder; the flow meter FI3191 indicates a flow rate of approximately 0 Nm3/h.
(5) When the secondary seal gas is activated, valve V26 is opened at the low-pressure end of the low-pressure cylinder, and valve V30 is adjusted; the flow meters FI3184 and FIT3186 both indicate a flow rate of 8 Nm3/h. Valve V25 is opened at the high-pressure end of the low-pressure cylinder, and valve V27 is adjusted; the flow meters FI3183 and FIT3185 both indicate a flow rate of 8 Nm3/h. Valves V38 are opened at the low-pressure end of the high-pressure cylinder, and valve V42 is adjusted; the flow meters FI3190 and FIT3186 both indicate a flow rate of 6 Nm3/h. Valves V37 are opened at the high-pressure end of the high-pressure cylinder, and valve V39 is adjusted; the flow meters FI3189 and FIT3185 both indicate a flow rate of 6 Nm3/h. Once the unit is operating normally, the flow meters FIT3185 and FIT3186 indicate a flow rate of approximately 12 Nm3/h, while the flow meters FIT3191 and FIT3192 indicate a flow rate of approximately 13 Nm3/h. Note: The compressor must be started under pressure, with the internal pressure being higher than that of the flare pipeline; otherwise, a negative pressure difference may occur upstream and downstream of the primary seal, which can damage the seal after startup. ⑹ Connect it to the air circuit of the booster pump; open V8, V9, V41, and V42 in the low-pressure cylinder. When the booster pump P3181 is operating, adjust the pressure relief valve PCV2 so that the pressure indicated by gauge PI3189A is 0.4 MPa. At the same time, observe the booster pump – its piston should move at a frequency of 50–60 times per minute; if not, adjust V42. For the high-pressure cylinder, open V9, V10, V12, V11, V53, and V54. When the booster pump P3182 is operating, adjust the pressure relief valve PCV2 so that the pressure gauge PI3189B shows a value of 0.4 MPa. At the same time, monitor the booster pump’s piston; its operating frequency should be 50–60 times per minute. If not, adjust V54. ⑺ After confirming that the nitrogen pressure is normal, slowly open the nitrogen pressure relief valve to apply nitrogen sealing to both the front and rear bearing housings of the turbine, keeping the pressure at 0.02 MPa (it is sufficient to observe gas escaping from the vent pipe on site).
Whether to install the primary or secondary dry gas seal first depends mainly on whether the compensation mechanism (spring) of the dry gas seal is located on the moving ring or the stationary ring. In modern series double gas seal designs, the moving ring and the shaft are usually fixed together, with the spring located in the stationary ring; it makes little difference whether the first-stage seal or the second-stage seal is activated first. If the spring is on the moving ring, secondary sealing gas is introduced first; this causes the moving ring to move backward, and the stationary ring comes loose from its seat, resulting in the failure of the primary seal. As for the isolation gas, compared to the sealing gas, there is no difference whether it is introduced first or later. Before using the lubricant, be sure to add it. To prevent lubricant from entering the dry gas seal. Note: Before introducing the process medium or using nitrogen for displacement, seal gas and pre-feed gas should be introduced first to prevent impurities present in the medium from entering the dry gas seal.
Different operations have different sequences, but there is one principle that must not be violated: protecting the dry gas seal. For example, when there is no fluid or pressure inside the unit, I need to start the oil circulation first; for that, isolation gas must be supplied before starting the oil circulation. Another example is when process fluid needs to be introduced into the compressor, in which case pre-feed gas must be supplied first to ensure that the sealing chamber remains clean and prevent the gases within the unit from contaminating the sealing surfaces. Also, if the compressor is shut down without releasing pressure, then the pre-feed gas supply must continue uninterrupted. The specific steps for putting it into use depend on the operating procedures of the compressor; there are no strictly fixed steps, as everything is done to protect the dry gas seal.
Sequence for putting the double-end face dry gas seal into operation: 1. Isolation gas 2. Main sealing gas 3. Pre-buffer gas
This is just the order of writing; in reality, it’s fine to inject the main sealing gas separately, or you can inject the main sealing gas first, followed by the isolation gas and the pre-sealing gas, or alternatively the main sealing gas, then the pre-sealing gas, and finally the isolation gas.