The first oddity I noticed was the noise from the vents associated with the individual reboilers. As Figure 6-1 shows, the expanding gas, used to drive the glycol pumps is also used as fuel to reboil the glycol. The excess gas not burned in the reboiler is vented under pressure control to the atmosphere. When the efficiency of the glycol pump is reduced due to mechanical problems, two factors act to increase excess gas venting:
• The reboiler firing rate drops because less glycol must be reheated.
• The amount of gas flowing from the tower to the glycol pump increases because there is less glycol liquid to restrict the flow of gas.
Hence, the net result of a reduction in glycol circulation rate due to reduced pumping efficiency is increased venting of excess natural gas. Of the six vents (one for each reboiler), only one was blowing hard. I also observed that the main burner on this particular reboiler was rarely on. Note: Temperature control on glycol reboilers works hke your home heater—either full on or full off. Lack of firing on a glycol reboiler—that is, low reboiler heat duty—is another indication of a low glycol circulation rate.
The usual cause of glycol pump failure is deterioration of the O ring seals. Next morning, I requested that the suspect pump be overhauled. While this work proceeded, I continued my investigation.