Heated sample Line (HSL) heaters come in two styles – Parallel and Series.  Parallel heater HSLs can be purchased in bulk or purchased longer than needed and cut in the field to length.  Typically they come with ends unfinished and a kit supplied by the manufacturer is used to finish and seal the lines after they are cut to length.  Series heater HSLs typically are specified for the application and come finished on both ends.  Cutting – up to 10% of the length – can be done, however it is not recommended by the manufacturer.  CiSCO prefers to install the series heater because:

1.  Parallel heater lines only come in single phase power.  Series heater lines are available in single and three phase.  Since the HSL is one of the largest power users in a CEMS, it is much easier to balance the power between the three power phases with a 3 – phase line.  A single phase line puts all the power draw on one of the three phases.

2.  The parallel heater lines have segmented heaters, while a series heated line has a continuous heater.  A segment (or a number of segments) can fail, causing a “cold spot’ in the sample line allowing sample condensation and unless the segment that fails is where the alarm thermocouple is located, there will be no indication of failure.  This does not happen with the series heater, the entire line is heated or is cold providing positive failure indication.

 

Shutdown and Maintenance:

Refer to you O&M Manual for what to do with the heated sample line during long unit shutdowns and for periodic maintenance.

 

Temperature Control:

CiSCO has used various methods of temperature control for Heated Sample Line over the years.  The current system is to have the line controlled via the PLC.  This allows for user inputs (via the Operator Interface Terminal) for the operating temperature and alarm points.  Additionally this system allows for a HSL control setpoint for when the turbine is operational, and for a different setpoint when the turbine is down.  This is done by having an On Line/Off Line signal determine which setpoint to control to.

During turbine downtime, the CEMS pulls ambient air through the HSL.  Since this is much colder than the flue gas, the HSL will try to heat the air and thus run most of the time.

 

Heated Sample Line Life:

 

The HSL life is quite variable but experience indicates 8-12 years is typical.