PTQ Q2 2024 Issue

 Periodically drain system low points As circulation flow is established, free water will move to the process low points where it can be drained. Mark all low-point drains in the field and on a set of piping and instrument diagrams (P&IDs) to ensure all low points are drained during dryout. We recommend maintaining an operator log listing all low-point drains and the times each was blown down for the duration of dryout. The best method for purging the plant is by opening all low-point drains and allowing any water present to purge to the closed drain system throughout dryout. However, this approach increases the amount of flaring, and having a constant purge to flare throughout dryout may not be allowed. If a constant purge is not allowed, establish a rou- tine frequency for draining all low points (say, once every two hours). Open each drain for at least 30 seconds to remove any free water at that location. The operator might be able to hear the water draining and feel the water flow as the drain valve opens. Add fresh gas as necessary to maintain the minimum system pressure during dryout.  Establish flow through stagnant piping loops Once the main dryout flow path is dry, establish flow through stagnant piping and equipment loops which would otherwise not dry out. The stagnant piping loops discussed in this paper should be designed to free-drain to the lowest point. Drain any free water collected at the piping system low point first. Use the method described in Step 3 for draining water at each process low point. Once the free water is drained, the remaining water in the loop should be small enough to be picked up and carried away by the warm dryout gas introduced. Monitoring during dryout and time to completion Once the dryout process has been established, how does the operator know if dryout is progressing and when it is complete? Establish the baseline water content in the cold plant when dryout starts, and record water content readings through- out dryout to track progress. An online moisture analyser to monitor dryout is helpful because water content readings can be collected and trended in the plant’s data historian. As discussed earlier, the recommended location to track the water content is at the outlet of the cold plant upstream of the residue gas compressor suction. In addition to monitor- ing at this location, it is helpful to have a portable moisture analyser to collect periodic readings in stagnant piping sys- tems where additional dryout gas flow had to be introduced. Figure 5 is an example of the general water content trend during dryout. As process flow paths are changed and flow is introduced into stagnant areas, do not be surprised if the water content increases at times during dryout. This is an indication that additional water has been absorbed by the vapour stream and removed from the process equipment and piping. The suggested maximum water content for a cold plant to avoid forming hydrates is 10 ppmv. Dryout is close to completion once the water content decreases below 10 ppmv, but we recommend continuing dryout for a short

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Figure 5 Example water content trend during dryout

Summary of steps  Initiate dryout gas flow through equipment  Circulate dryout gas through the main flow path  Periodically drain system low points  Establish flow through stagnant piping loops.  Initiate dryout gas flow through equipment With the J-T valve pressure controller in ‘manual’, slowly open the valve to 100% valve position. Increase the dryout gas flow, if necessary, by increasing the residue gas compressor speed until the calculated gas circulation flow target is reached. As the circulation rate is established, continue to monitor the sys- tem pressure. To maximise the velocity through the cold plant, dryout system pressure should be set as low as the residue compressor operation will allow. Add fresh gas as needed to maintain the minimum system pressure during dryout.

Establish the baseline water content in the cold plant when dryout starts, and record water content readings throughout dryout to track progress

 Circulate dryout gas through the main flow path The pressure drop across the fully open J-T valve should be low enough so that no significant cooling occurs. In most cases, a pressure drop of less than 30 psi (2 bar) should be low enough to minimise cooling in the plant. Monitor the cold plant temperatures. All the equipment will cool slightly, but the temperatures should stabilise. If equipment starts cooling significantly, slow down the res- idue compressor until the pressure drop across the fully open J-T valves decreases enough so that the cooling stops. No hydrocarbon liquid should form. If liquid accumulates in any vessel, it should be just the free water moved to the vessel by the dryout gas flow.

PTQ Q2 2024