Handling of insulating gas

Last modified by Xwiki VePa on 2024/02/07 07:36

Follow these guidelines when transferring the insulating gas (N2+CO2 mixture) from the EGP tank to the reservoir tanks S1 and/or S2.

Preparatory steps

Start with the safety preparations. Also be aware that the gas mixture may be suffocating when leaked in the room air in large amounts. Therefore have the portable oxygen-deficiency monitor with you all the time. If you hear unexpected noise that may indicate gas leak, go to the nearest exit and tell others to do so also.

  1. Check that all the valves (manual and motor-driven) are closed
  2. Check that the pressure gauges at various places along the gas transfer line and on the tanks are functioning properly and show reasonable pressure values. Note that most of the pressure gauges indicate overpressure, not absolute pressure (0 means 1 bar absolute, -1 vacuum). Note that some of the gauges may be disconnected from the system by a closing valve and don't indicate a correct pressure.
  3. Check the status of tanks S1 and S2, their gauges typically show a value of about 0.5 to 0.8 bar (gauge). The content may vary from a (known) mixture of N2+CO2 to air, vacuum, or completely unknown.
  4. If there is any doubt on the composition of gas in the S1 or S2 tank or in any components of the gas transfer line, you must first pump vacuum in the tanks and/or in the relevant parts. See separate instructions.
  5. See the simplified flow diagram and check the location of the various valves and their motor controls.
  6. Avoid sudden opening/closing of valves in order to prevent pressure shocks and/or turbulent flow.

Insulating Gas Valves1.pdf

 Simplified process flow diagram.

Transfer of gas from accelerator tank (EGP) to reservoir tank(s)

  1. Please note that the volume of the EGP tank is 80 m3 and S1 and S2 are each 45 m3 only. Don't try to squeeze all the gas from EGP to only one reservoir tank. Yes, I have tried, it triggers the safety valve, and you'll loose the gas!
  2. Open valve V71 to the buffer tank (volume 1 m3) in the compressor room.
  3. Open (slightly, about 30 degrees) valves V27 (S1) and V29 (S2) at the inlet of the reservoir tanks. The line is marked with the text "TAKIMMAINEN" and is nearest to the wall behind the tanks.
  4. Go to generator hall (K2) and check the status of the valves there. All must be closed (V66, V71, V07, V09)
  5. Open motor valve V03 by pressing the button in the panel. This valve opens completely, as indicated by the green light (if not broken). You'll hear some hissing sound when the gas tube fills with gas. The gauge on the tube should show the full tank pressure.
  6. Open valve V66 slightly (perhaps 30 degrees or less) to let the gas into the long tubing towards the S1/S2 and the buffer tank. Adjust the opening/closing of the valve that the hissing sound is tolerably loud. Try to avoid turbulent flow and pressure shocks.
  7. Observe the pressure gauges at K2 and in compressor room. The latter should be about half of the tank pressure.
  8. Let the gas flow and open the valves at S1/S2 and valve V66 more to compensate the diminishing of the flow rate due to diminishing pressure difference. At some point you may open all valves completely.

S2_01.pngS1_02.pngPressure gauges of reservoir tanks before gas transfer.

V71.pngValve V71 in open position, pipe into buffer tank (compressor room).

K2_KP01V03.pngMotor valve V03 control (in open position)


EGP_gauge-pressure.jpgPressure_gauge_K2_V03.jpgPressures at K2: Accelerator tank (left), Gas transfer line (right).

V66_open.jpgOpening valve V66 slightly (at level K2).

Pressure_gauge_1m3_tank.jpgPressure at compressor room, gauge attached at the 1 m3 buffer tank.

S1+S2_valves.pngValves V27 (S1) and V29 (S2) in open position, gas flowing freely from accelerator tank through "TAKIMMAINEN" line.


Use of the compressor for gas transfer (from EGP tank to S1/S2)

Here we assume that the valves have the same status as in the previous section. If unsure, please check.

The rest of the gas must be transferred by using the SF6 gas compressor located at the compressor room. You can monitor the gas flow rate in the decreasing pressure of the accelerator tank.  After 1 to 2 hours, there is no free flow anymore.

Forced gas flow using the compressor must go through the absorption dryer to ensure the extreme dryness of the gas.

  1. Check the status of the valves before and after the compressor. Open slowly the two valves.
  2. Open the valves at the absorption dryer (two labeled valves and two unlabeled near the bottom)
  3. Open motorized valve V24 using the control panel in the compressor room to balance the pressures on the gas tubes. Also open the gauge valves near V24.
  4. Close valves V27 and V29 at S1 and S2.
  5. Open valves V28 (S1) and V30 (S2) completely.
  6. Open the three valves at the cooling water distribution box (one for compressor, one for intercooler, and one for aftercooler)
  7. Open the valves to the cooling water booster pump and start the pump.
  8. Start the compressor for a few seconds, observing the oil pressure gauges of the compressor. A recommendation of the manufacturer is to rotate the wheel manually for a couple of rounds, but who cares!
  9. If compressor OK, start again, observing the oil pressures (difference must be greater than 3.1 bar), and the gauge pressures at valve V24.
  10. The compressor is running and you ensured that the valves from the EGP tank through compressor and gas dryer to the reservoir tanks S1/S2 are all open.
  11. Close the bypass valve V24 and observe the pressures at V24 starting to change; one decreasing, the other increasing as the gas is compressed from EGP tank to S1+S2 through the dryer. (If the high-pressure line is blocked by accidentally forgetting a valve closed, the pressure increases until the safety valve at the compressor exit fires, or the compressor stops!)
  12. Check the pressures at various points (EGP tank pressure, buffer tank, V24, gas dryer, S1 and S2) during the gas transfer.
  13. Monitor the compressor temperatures and oil pressures.
  14. Monitor the EGP tank pressure through the automation system until saturation reached (or the compressor stops automatically). There is a low-pressure limit switch at the inlet of the compressor anyway.
  15. After compressor shut down, close the valves V28 (S1) and V30 (S2) and V03 (EGP tank).
  16. Open the bypass valve V24 and start the compressor again for a cooling period of 15 to 30 min. in this step the remaining gas in the tubing (outside the tanks!) is just circulated through the gas dryer and the gas coolers just in order to cool the compressor down to about 40 ºC.
  17. Shut down the compressor, the cooling water booster pump, and close the cooling water valves you opened above
  18. Close the valves before and after the compressor.

Use of the vacuum pumps (Roots blowers)

The pressostat installed at the gas transfer line shuts down the compressor when the inlet pressure goes below atmospheric pressure. The last 80 m3 or so of the insulating gas must be pumped by vacuum pumps into the atmosphere. Follow these guidelines when evacuating EGP tank, tanks S1 and S2, and any other parts of the gas transfer system. The following instructions apply for evacuation of the EGP tank:

  1. Is the pumping to vacuum really necessary? If it isn't don't do it! The possible reasons for pumping to vacuum could be
    1. You need to enter the tank for inspection or service work and the tank has to be vented with room air to atmospheric pressure. Evacuate first to vacuum
    2. The tank or gas line has been vented with room air
    3. You are not sure whether or not the system contains room air or insulating gas
    4. You need to fill the accelerator tank with CO2+N2 mixture after the a tank opening.
  2. Check that the pressure in the parts of the system you plan to pump is below XX bar. The vacuum pump valve wont open otherwise. And should not!
  3. Check that the exhaust valve of the vacuum pump (V09) is open and that the other valve to pressure line (V07) is closed.
  4. Check that valve V66 is closed, if you are not pumping the transfer tubing or S1/S2 etc. at the same time
  5. Close the venting valve at the pump exhaust
  6. Open the water cooling and start the booster pump.
  7. Wear hear protection, the pumps are noisy!
  8. Start the pumping station and check all is OK (oil levels, cooling water etc.)
  9. If pumping the accelerator tank (EGP) open valve V03 and check that the pressure is low enough for the Roots blowers. If not, pump with compressor first.
  10. Open motor valve V05 and observe pressures at various points.

Note that the mechanical gauges on the DN50 tubing are not suitable for vacuum measurements. Please attach a Pirani gauge or similar to monitor the vacuum at a suitable point,e.g., at the gas filter near the V10 motor valve at the tank bottom (go to K3 level and climb the stairs up in  the analyzing magnet hall). It is usually enough to pump down to about 15 mbar. There is always some water in the system that takes forever to get rid of by vacuum pumping. If you are venting the tank anyway, there is no use of pumping longer than for about 3 hours. In case your next step is to fill the tank with insulating gas after service, you may continue a bit longer. The most effective way to get rid of the moisture is to transfer gas from S1/S2 into EGP tank and circulate the gas through the absorption dryer using the compressor. A suitable pressure is about 5 bar. At that pressure the belt can be run continuously to speed-up the drying process.

Venting of the accelerator tank

Before the service entrances can be opened, the tank must be vented to atmospheric pressure and ventilated carefully.

  1. Check that the valves V1 and V3 are closed (all others, too!)
  2. Check that there is nobody in the analyzing magnet hall and especially on the grid platform below the accelerator tank. The huge hissing noise may cause heart-attack on sensitive persons or falling on the steep stairs. Block the entrance with a rope and a "DO NOT ENTER" plate.
  3. Open the motor valve V26 via the control panel at level K2.
  4. Open the valve completely as indicated with the green light (if not broken).
  5. Let the valve open for several hours, perhaps even overnight or over weekend.
  6. When finished, don't forget to close valve V26 again. If not completely closed, it may become a problem later!

Regeneration of the absorption dryer

The absorption dryer contains molecular sieve material that absorbs the moisture from the insulating gas. After one full tank of gas (80 m3  at 16 bar) is dried, the material is saturated and must be regenerated. A separate document gives the guidelines in detail (with figures) to complement the check list. See also the PID of the LN2 tank and related equipment.

Be prepared to stand by for about 6 hours. The process may not be left running unattended for more than one hour or so. Make notes during the process.

  1. It's important that the direction of the gas flow is always from the LN2 tank to the dryer. Open the valves in correct order to ensure this!
    1. The pressure at the expansion tank (compressor room) should be kept at 4 to 5 bar.
    2. The pressure at the gas line ("TAKIMMAINEN") near the LN2 evaporator should be smaller than the pressure in the LN2 tank but larger than the pressure at the expansion tank
  2. Close V24 (motor valve)
  3. Close V15 and V17
  4. Open V18 (using the wrench attached)
  5. Open V13.1 and V13
  6. Close the valves near the bottom of the gas dryer (unnumbered)
  7. Locate the dryer exhaust valve (blue) that opens automatically when the regeneration starts.
  8. The temperature gauge at the top of the dryer must be monitored during the process and should not go above 130 - 150 ºC. However, sometimes the temperature still rises after heating is switched off (see below). It may go up to 190 ºC, but don't worry!
  9. Go out to the LN2 tank but don't close the two LN2 valves in front of the tank (#2 and V23 are essential for the generation of the N2 gas)
  10. Open slightly the inlet valve (# 11 in the schematics) to the evaporator H1 (if not already open from last time use). This valve controls the rate of LN2 evaporated, but rarely needs adjusting.
  11. Open the valve (V79) after the evaporator about 20 to 30 degrees. Prepare to adjust this valve later...
  12. In the injector hall, open the valve between other evaporator (H2) and the main gas line, about 30 to 45 degrees max. You may need to adjust this one too!
  13. Monitor the pressure at 1.a. and 1.b. above to increase to about 3 bar (the exhaust valve of the dryer doesn't operate with less)
  14. Start the regeneration: switch ON and press the green button, the lamp "ELVYTYS" lit.
  15. The blue dryer exhaust valve opens and you hear the hissing noise of gas flow. If not, something went wrong and you need to check the status of the valves.
  16. Don't leave yet, stand by and wait for a stationary state to develop (in 15 to 30 min).
  17. Observe the pressures at relevant points and adjust the valves, after LN2 evaporator (step 11) and at the main gas line (step 12).
  18. Note that a too high gas flow prevents the proper heating of the absorber, but a too low results in overheating.
  19. The regeneration process switches to cooling mode automatically when the temperature reaches about 130 ºC. It may take about 3 hours for heating and another 3 hours for cooling.
  20. The process is completed when the exhaust gas is cooled to about 70 ºC. The inlet and exhaust valves of the dryer are automatically closed.
  21. Don't forget to close the valves at the LN2 tank evaporator (12) and in the main gas line. Close valve V79 first. The valve #11 may stay open.

Notes

  • The molecular sieve material stores quite an amount of heat. It's important to let it cool enough before you start drying the insulating gas.
  • The thermal conductivity of the absorbent is rather poor. This explains the frequently observed phenomenon that in the cooling phase the temperature of the exhaust air still increases, approaching 190 ºC. The thermostat has already switched off the heater, but the heat conduction still continues resulting to larger thermal transfer area between the gas flow and the hot absorbent material.
  • The absorbent material nearest to the heater element first absorbs most of the heat, and the water is released and transported away with the gas flow. Temperature of the absorbent remains low until most of water is released. Contact area between hot surface and gas flow is rather small. As the absorbent dries the temperature rises and heat is transported both via conduction and convection to absorbent and gas flow.
  • Sometimes the regeneration process stops prematurely to overheating ("YLILÄMPÖ"). Just let it cool for a while before trying again but close the valve V79 at the LN2 evaporator first.
  • The whole process of regeneration is rather delicate, depending on the amount of water in the absorbent, the heating power and the flow rate of the cooling N2 gas. Currently, the only parameter that can be controlled is the gas flow rate determined by the inlet pressure. Too high heating power results in disintegration of the absorbent grains, a low heating power means longer regeneration times. Think how the gas flow rate affects?
  • The automatic control of the process would need a highly non-linear adaptive system, I guess.

LN2_PID.pdf

 Simplified PID of the LN2 system.

Regeration_of_Gas_dryer.pdf

 Guidelines with figures.

Adding CO2 gas in the system

Read the instructions for safe handling of CO2 from the supplier (in English or in Finnish). See also section Gas Safety

  1. Order 360 kg of CO2 from Linde Gas (or whoever the contract supplier happens to be!),  a set 12⨉50 L bottles of BIOGON C.
  2. Connect the flexible tube from the CO2 heater/pressure regulator to the bottle set. Don't forget the seal, and tighten.
  3. To prevent too much air entering the system, flush the tubing a few times. Don't know how? Open bottle valve to pressurize, close valve, release gas, repeat a few times. Don't inhale the gas, it's not toxic, but suffocating.
  4. Tighten the connector and open the bottle valve. Check for leaks.
  5. Connect the gas heater plug to 230 VAC outlet. Heat of evaporation is 348 kJ/kg at 1 bar.
  6. Adjust the pressure regulator to value higher than in the gas line
  7. Open the manual closing valve to the gas line and observe the pressure gauge in the gas line.
  8. You can add the gas to either S1 or S2 (open the corresponding valves) or directly on-the-fly to the gas drying process (see below).
  9. Adjust the pressure regulator and the closing valve to keep the gas flow steady and small enough to prevent the freezing of the closing valve or the no-return valve. They are not cryogenic, sorry.
  10. When finished, close the valves in reverse order, disconnect the heater, close the bottle valve and disconnect the connecting flexible tube.
  11. Don't forget to return the bottles to the supplier as soon as possible. Every day counts.

Notes

  • The CO2 contains some water (< 50 ppm) and the gas has to be dried as a mixture (< 30%) with the N2 insulating gas.
  • Don't try drying pure CO2 alone, it may result in compressor failure, as solid crystals are formed. If you need deeper understanding study the relevant phase diagrams of thermodynamics.
  • If we have a P = 1 kW heater, the evaporation of the whole 360 kg takes t = 360 kg x 348 kJ/kg / 1 kW = 34.8 hours. If there is no heater, the required energy is taken from the environment, and results in freezing at various places!
  • Pure CO2 is a liquid at pressures above 5.2 bar. At lower pressures, only solid and gaseous phases are possible. The bottom of the gas bottles freeze and it's quite impossible to squeeze the rest out of the bottles. Just close the bottle valve and stop.

Adding N2 gas in the system

  • Follow the guidelines given in the section Regeneration of gas dryer for operation of the LN2 tank and related valves.
  • Ensure that the pressure in the gas line is always at least 1 bar smaller than in the LN2 tank.
  • You can add N2 gas to either tanks S1 or S2 or directly on-the-fly to the gas when filling the EGP tank or during the gas drying process (see below).

Original guide for the insulation gas system (in Finnish)

Contains some information in the documentation style of early 1980's. Not really necessary, but if you are curious!

Eristekaasu_ohje.pdf


Related articles

Related articles appear here based on the labels you select. Click to edit the macro and add or change labels.

Unknown macro: contentbylabel. Click on this message for details.


Unknown macro: details. Click on this message for details.