Lakewood Instruments boiler controllers give you great flexibility in operation schemes. You can choose from three boiler water treatment automation control schemes:
- Continuous Sample – a steady, small blowdown that substantially increases via setpoint control.
- Sample/Cycle – a periodic cycle of checking conductivity, blowdown if needed, then wait for another cycle.
- Sample Hold – a creative way to handle systems that are prone to steam flashing and therefore unreliable conductivity readings.
A graphic comparing some relevant points appears at the end of this blog post.
A “Setpoint” and a “Deadband” are programmed into the controller. The system always has a small amount of bleed through the main flow restriction device, plumbed in parallel with the Motorized Ball Valve (MBV). When the controller reads a conductivity above the setpoint, the MBV will open for additional blowdown. When the setpoint is satisfied, the valve will close, and the controller monitors system conductivity. The conductivity reading is always active with this method. Plumbing is shown below:
A “Setpoint”, “Sample Time” and “Cycle Time” are programmed into the controller. Using a 2-minute sample and a 60 minute cycle as examples, the controller will open the MBV for a period of 2 minutes (Sample). At the end of that time, the controller will compare the live conductivity against the setpoint. If the conductivity is below the setpoint, the controller will close the valve. If the conductivity is above the setpoint, the valve will remain open until the setpoint is satisfied. At that point, the “Cycle” time will begin. The displayed conductivity reading is only active during a sample. During the Cycle, the reading on the controller is the last reading before the valve closed. The sensor is not taking a live reading during the Cycle. Plumbing (for both Sample/Cycle and Sample Hold) is shown below:
The Blowdown relay will energize for a user selected time called the Sample Time. At the end of the Sample Time the Blowdown relay will turn off and the controller will go into a 30 second Hold Time. The Hold Time allows time for the sample to stop flashing before actually reading the conductivity value. At the end of the Hold Time, the controller will go into a Reading Time of 15 seconds. At the end of the Reading Time, the controller will compare the conductivity reading to the setpoint. If the conductivity is higher than the setpoint, the controller will go into a user selected Blow to Resample Time and re-energize the Blowdown relay then repeat the above sequence. Once the setpoint is satisfied, the controller keeps the Blowdown relay turned off for a user selected time called the Cycle Time. At the end of the Cycle Time the Blowdown relay will once again go into the Sample Time and repeat the above.
When to Use Each Cycle:
- Small boilers (under 1000 lb/hr blowdown rate) typically use the Sample/Cycle blowdown control scheme using the PL5 (1/2”)or PL575 (3/4”) plumbing kit.
- Larger boilers (over 1000 lb/hr blowdown rate) typically use the Continuous blowdown scheme using the PL6 (1/2”) or PL675 (3/4”) plumbing kit.
- Boilers that are prone to steam flash can use the Sample/Hold (available on the Model 150 & NEXSYS controllers) which helps to eliminate erratic conductivity readings. This scheme also uses the Sample/Cycle plumbing kits.
By choosing the proper control scheme, you can optimize your boiler water treatment automation to achieve the best balance of energy efficiency, chemical usage and accurate measurement.