TL;DR: Boilers are pressurized, blowing down a boiler can expose process water to the atmosphere. Drastic pressure changes can cause water to turn into steam and reading the conductivity of steam is difficult.  Follow your manufacturer's piping recommendations for skimmer line automatic blowdown.

Disclaimer: This guide is meant to serve as a reference for water treatment professionals.  It assumes the individual is qualified and trained to work with the equipment in this guide.  None of the tips outlined should supersede any local laws or on-site safety protocols.  When working with equipment always take the appropriate safety precautions this industry can expose you to hazardous chemicals, pressurized vessels, extremely hot surfaces, electrical hazards and more.  Always use appropriate personal protective equipment (PPE) and seek assistance on any task which may be beyond your qualifications.

During the warmer months our non-process boilers are often laid up until the cold comes back.  As winter approaches, the memory of those troubled accounts with constantly fluctuating conductivity, chasing calibration, replacing probes, adjusting globe valves, switching out orifice plates and more comes back to haunt our routine service visits.  

Chasing conductivity in a boiler is one of our most common winter time technical calls.  Conductivity is an extremely important measurement when it comes to monitoring the water quality in our boilers. We want to ensure we are maximizing water reuse to conserve both water and fuel while mitigating scale and particulate settling.  This is one of the more challenging things to diagnose and fix when on the phone and often we find ourself physically going on-site to fix the issue and train on some of the fundamentals around how the probes work, appropriate blowdown piping manifolds, and just the basics on the ideal gas law.  The following should serve as a good place to troubleshoot conductivity in your boilers.

1. Check Calibration: If you feel like you are chasing calibration the first step would be to verify controller conductivity values to your calibrated handheld. Note: Conductivity is a temperature compensated measurement.  When checking measurements it is important to allow for instrumentation to "Get to temp" for an accurate measurement.
2. Verify Wiring: Depending on your controller, you may know immediately if there is an issue with the conductivity probe through a probe error.  However, some controllers have solely analog probes and you may need to double check the wiring.  Ensure you are avoiding long cable runs of weak analog signals.  Longer runs allow for electrical noise to interfere and compromise your signal integrity.
3. Inspect Probe: If you have verified an issue with your measurement and confirmed your wiring is correct next would be pulling the probe for inspection.  Recall what the probe looked like new, if you notice haziness to the probes finish, rust build-up, and/or probe separation/de-lamination.  Build-up on a probe can often be removed from a simple cleaning, consult the manufacturers manuals for their suggested cleaning methods.  If your probe is showing signs of de-lamination or separation it might be time for a new probe.
4. Piping Configuration: Boiler controller manufacturers publish recommended piping configurations in their manuals.  They will indicate pipe lengths, orientations, and necessary piping elements to set your precise piece of instrumentation up for a repeatable way of consistently measuring conductivity.  They typically include a ball valve/solenoid, a pressure regulating mechanism like an orifice plate or needle/globe valve, and the probe.  Verify order, orientation, and spacing in your installation matches the manufacturer recommendations.  The correct piping configuration may depend on your controllers programming and the sample mode being used.
5. Back Pressure: Conductivity probes are designed to measure the conductivity of liquid. Opening a blowdown valve can create a drastic swing in pressure which in turn can cause a phase state change of process water turning into steam.  One of the most common problems when it comes to chasing conductivity calibration is ensuring your blowdown manifold has enough back pressure to keep our sample in liquid state.  Emphasis here is on the flow regulating device. Either slowly close adjustable globe/needle valves or replace orifice plates until the conductivity measurement stabilizes to a consistent reading.
6. Controller Programming: Controllers have a few different ways to measure conductivity and the correct configuration will depend on often the size of the boiler and a few other factors.
• Continuous Sample: This mode is reserved for large boilers.  To have a continuous sample going to drain the boiler needs to be large enough to "cycle-up" fast enough where the slow continuous bleed does not inhibit steam production and the boiler will still overshoot conductivity setpoint.
• Timed Sample: This is a commonly performed method where you will intermittently open the blow down valve to allow enough time for an "At-temp" measurement.  Spacing out the timings enough as not to preemptively dispose of good hot water while sampling frequently enough to catch a system before the scaling tendency get too high is and important balance.
• Timed Sample and Hold: This is probably the best mode to use when you are chasing steam flash.  This timer works much like timed sample, however, the physical measurement is taken with the blowdown valve closed which should pressurize the blowdown manifold. Our process solution is more likely to be in liquid state to promote a healthy measurement.
7. Routine Monitoring: No instrumentation is calibrated once and good forever.  Ensure you build in routine calibration checks, probe inspections, and preventative maintenance.  Your conductivity probe will require some routine cleaning and eventual replacement.  Establish some guidelines around this and identify which systems are outliers to your others and apply this troubleshooting guide.

This is only a brief troubleshooting guide and by no means comprehensive. Skimmer lines can often be impacted by the boiler itself, from the level of the water in the boiler to the presence of nucleate boiling.  There are some tips around flooding the skimmer line to ensure fluid measurement also.  As you can tell from the length of the article getting this dialed in can be time consuming and throw a multi-hour delay into your routine service routine.  If you are through this article and still struggling you can at least in confidence either contact us, your internal support team or the manufacturer for additional support.