Natural gas transported through common carrier pipelines must meet a moisture specification of 7 pounds of water per MMscf. Gas is usually dried to meet this requirement by scrubbing with a concentrated glycol solution. Figure 6-1 shows a standard glycol contactor tower, regenerator, and pump.
Gas flows into the bottom of this tower where entrained water and naphtha drop out and are withdrawn under level control. The upflowing gas is contacted with the circulating glycol and dried. The glycol is pressured from the contractor to the regenerator, where it is heated to its boiling point to drive off water. Typically, 100 pounds of circulating glycol absorbs 3—4 pounds of water. After cooling, the reboiled glycol is pumped back to the contractor tower.
On the surface it would not seem possible that much could go awry with such a simple system. But, of course, the experienced process operator knows that it is only a matter of time for anything that can go wrong to go wrong. As a case in point, consider the operation of the glycol circulating pump.
This ingenious positive displacement pump is driven by expanding gas withdrawn along with the wet glycol, from the contactor tower (see Figure 6-1). The speed of this pump is set by a small valve that controls the amount of expanding gas emitted into the pump. An operator judges the amount of glycol circulation based on the audible strokes made by the pumps internals. The quicker the strokes, the greater the glycol circulation.
But suppose the pump has developed mechanical problems that reduce the volume of glycol normally pumped per stroke? Or perhaps the pump internals have deteriorated to the point that glycol circulation has stopped. Since glycol drying units are not normally equipped with flow meters on the circulating glycol, how can the process operator of the troubleshooting engineer recognize the problem.