Gas 2024 Issue

Residue gas compressor

Air cooler

Residue gas compressor

Air cooler

To sales

Location 1

Location 2

Inlet gas

Cold plant

Recirculation piping

Dust lters

Dehydrators

Liquid product

Dryout f low path

O-line

Dryout f low path

O-line

Figure 3 Recirculation piping

Figure 4 Possible locations of pressure-reducing devices for dryout

Recirculation piping A recirculation line connecting the residue gas line down- stream from the residue gas compressor(s) to the inlet gas piping upstream of the dehydrators should be included to complete the closed loop. For designs that include a cold plant bypass, the plant bypass can also be designed to function as the recirculation line during dryout. Figure 3 shows the typical recirculation piping connection points. Pressure-reducing device and location The location of the pressure-reducing device to take the plant pressure drop upstream of the cold plant is dependent on the dehydrator regeneration system design and source of regeneration gas. When selecting the pressure-reducing device location, the cold plant designer must be mindful of the location cho- sen for dehydrator regeneration gas supply and return to ensure the regeneration compressor has enough compres- sion head to return the wet regeneration gas to the plant. For example, in plant designs that use inlet gas for regen- eration, the pressure-reducing device must not be located between the dehydrators and the piping take-off for the regeneration gas supply. The regeneration gas compres- sor will not be able to overcome the pressure difference. Figure 4 shows possible locations to consider. Typically, a temporary flow orifice, manual valve, or auto - mated valve is used as the pressure-reducing device. For example, if a temporary orifice is to be installed at Location 2 in Figure 4, the temporary orifice can be installed down - stream of one of the molecular sieve dust filters inside its isolation valves (see Figure 5 ). Typically, two full-flow dust filters (one filter in service and one spare filter) are included as part of the dehydrator design package, allowing easy orifice installation and removal. The orifice plate should be of suffi - cient thickness to handle the pressure drop, which in many cases will exceed 500 pounds per square inch (psi) or 35 bar. The pressure-reducing device should be sized to take the full plant pressure drop at the maximum dryout flow rate. The maximum dryout flow rate is set by the maximum flow the cold plant J-T valve can pass when fully open with a pressure drop of ~30 psi (2 bar). The dryout gas flow rate will end up being approxi - mately 30% of the cold plant nameplate capacity without

introducing too much pressure drop across the fully open J-T valve. Typically, even a single full plant rate residue compressor can be operated on its surge control line at reduced speed to provide a pressure rise of less than the normal operating pressure rise. The pressure-reducing device can then be sized for the expected pressures when running the compressor at reduced speed on its surge line. The surge control valve will then remain in control in parallel with the dryout loop flow for the duration of the dryout procedure. If multiple machines are used, it may be possible to use one machine with its surge control valve closed during dryout. The closed-loop operating pressure on the residue com- pressor suction side must be high enough to clear the low-pressure shutdowns for the compressor. The discharge pressure will be determined by the permissible operating point for the compressor. Drain valves During the model review, identify all low points around major process equipment and add drain valves as required. Hard pipe all low-point drains to a closed-drain system. Design the low-point drains to be easily accessible. The low-point drains will be used periodically to drain any col- lected free water from the system. A one-inch drain is the minimum size recommended. There must not be any inter- nal projection at the drain connection. The turboexpander and booster compressor low points are always inside the machines’ isolation valves and, there- fore, are not available to drain free water during dryout. As a result, the lowest spot around both machines may move

Temporary orifice in place of b lind f lange sp acer

Dust lter

To c old plant

Figure 5 Detail for temporary orifice

Gas 2024