In the photo you’ll see a steam to heating water shell and tube heat exchanger (wrapped in silver faced insulation). In the background you see an orange control valve actuator for the incoming steam line. Below the Heat Exchanger you see two black devices, which are the steam traps for the heat exchanger.
The deficiency here is that the condensate after the traps needs to be lifted (see that vertical insulated piped with the 15psi condensate sticker on it). That is the steam condensate leaving the traps and being returned to the condensate collection tank elsewhere in the mechanical room.
The reason that ‘lifting’ of condensate after the steam traps is an issue, in short, is that there may not be sufficient pressure to push (or lift) the liquid condensate through the steam traps and up that vertical pipe the 13 feet or so it needs. There is a steam control valve that modulates the amount of steam going into the heat exchanger depending on the desired temperature for the heating water that the steam is heating. That steam control valve modulating also fluctuates the steam pressure going into the heat exchanger. At minimal demand, there would be significant pressure drop occurring over that valve which means not enough steam pressure to push any collected condensate through the steam traps or let alone ‘lift’ the condensate up that vertical section of pipe the 12+ feet into the overhead to get back to the condensate collection point.
This manifested itself in very erratic temperature control for the heating water system. A resolution was needed as soon as possible. For failure to provide complete condensate drainage via gravity to a condensate collection tank will lead to poor temperature control (as mentioned), and potentially damaging water hammer:
- Poor Temperature Control: Any lift in the condensate return piping after the trap discharge requires a positive pressure to develop in the heat exchanger shell to provide condensate drainage. For this to occur, condensate needs to back up in the heat exchanger shell until enough tube surface is covered by condensate to build a positive steam pressure. However, when the positive steam pressure develops to move the condensate through the steam trap and up the vertical condensate return line, over temperature can occur from the steam remaining in the shell. The resulting condition will show a wide range of outlet fluid temperature from the heat exchanger tube side. [Note: During testing of the HHW system poor HHW temperature control was experienced, the reason appears clear based on the above].
- Water/Condensate Hammer: The lift of condensate, or back pressure caused by the static leg of condensate in the steam trap return piping can flood the heat exchanger shell and cause severe water hammer as steam enters the flooded shell. The resulting water hammer can damage the steam trap, the steam regulating valve, and the heat exchanger tubes. It can also cause the heat exchanger and trap gaskets to blow out.
For gravity drainage from the HEX steam traps to the Condensate Collection Tank to occur; the vertical elevation of the Heat Exchangers would have needed to be quite high (i.e. above 7+ feet). Vertical space within the low ceiling mechanical room is very limited. At this point, elevating the heat exchangers to achieve this gravity slope of the condensate from the traps does not seem feasible as it would require a significant amount of re-piping of both heating water, and steam supply piping, etc. HEA recommended that design team review and consider a solution such as the addition of steam motive pump traps to pump the condensate from the heat exchangers to the condensate return unit receiver. This would allow the condensate to gravity drain out of the traps at atmospheric pressure, then be pumped to the condensate collection point which is at higher elevation.
This solution of installation of steam motive condensate pumps was carried out and proper temperature control was restored.