3 Mar 05 19:32
Deriming an ASU is performed in three steps:
1. Warm up the cold box
2. Dry the cold box
3. cool down the cold box for re-start
1. WARMING THE COLD BOX:
Reduce your air flow and shut down the expander
Dump your liquids from the columns
Open all the liquid and gas purges on the cold box. Open the non-condensible purge fully on the main condenser.
You will see the purges ice up. Wait for the purges to thaw out, this will give you a good indication that the cold box temperature is above freezing.
Nonetheless, check your temperature, especially in the sub-cooler area, there might be some trapped liquid that does not allow air circulation.
2. DERIMING THE COLD BOX
Take a set of P&ID's, a highlighter and study the cicuits. Some circuits may require that air is passed in one direction and then in the other to make sure that you have circulation.
Your main condenser should be the first item to be derimed and the last to be buttoned-up. It is the most critical part of the plant.
The expander should be derimed in counter flow to the normal flow path (outlet to inlet). Make sure the turbine does not spin and make sure the seal gas and oil skid are turned on.
Adjust the derime gas for maximum flow but also for minimum pressure. I always limit my derime flow to a whisper, like a small breeze on my hand. The lower the pressure of the derime gas, the more moisture you will pick up. Screaming derime flows out of the purges and vents are an exercise of futility, a waste of power and time.
I personally never use a heater, the risk to damage the aluminum exchangers is too great. There are exceptions to this rule when dealing with newer plants (falling film vaporizers especially). Check the plant manufacturer's instructions.
Start checking the purges and vents for a dew point below -90F (-70C). As you reach such a dewpoint at a given purge or vent, close it (again, except the non-condensible vent). If a dew point takes for ever to come down, be patient, this means that liquid water is slowly evaporating from somewhere. If a purge does not have any flow when it should, it means that it has an ice block (which is a whole other ball of wax), do not ignore it.
As you close a purge or a vent, mark the date and dewpoint on the P&ID and check the new flow path. Make sure you do not send a wet stream back into a dry area inadvertently.
3. COOL DOWN THE COLD BOX FOR RE-START
When all dewpoints are good, fire up the turbine, watch the temperatures go down in the exchangers.
As you see the purges and vents frosting up, close them so that the cold gas goes to the warmer areas. However do not close the non-condensible vent or the lower column sump purge. The lower (or high pressure) cloumn sump purge is where the last hot gas devils are chased out.
Do not inject liquid until all the temperatures are at design and the plant is ready to make liquid.
I hope this helps.