The case of the tripped auxiliary low water cutoff, this weeks case

RayWohlfarth

In this case, the auxiliary low water cutoff keeps tripping and the reset button has to be pushed to restart the boiler. The boiler is a commercial steam boiler with two low water cutoffs. The primary low water control, a McM 150, has two functions. It cycles the boiler feed pump to maintain water in the boiler. If the water continues to drop, the same control will shut off the burner. The auxiliary low water cutoff is mounted slightly lower and is a manual reset control. Why was the auxiliary low water cutoff failing? I blew down the primary low water cutoff and it operated the pump and cut power to the boiler.
As usual, I will let you know Friday at 6am EST

EdTheHeaterMan

How Slightly Lower is Slightly Lower?
Lets assume that it has worked fine in the past and this is a new condition... What is the chance that some service was completed between the time that there was no problem and the now? What is the chance that the feed pump is not feeding as quickly as it used to feed? What is the chance that the water in the return tank is too low so when the feed pump engages there is no water to feed? Did someone close a valve for a service or maintenance and forget to open it all the way? Was there as part replacement on the feed valve/pump that has a time delay where the original was not delayed? Were adjustments made to the 150 that brings the pump on at a later stage in the boiler cycle? I would start to look at why the water is not returning to the boiler based on the action of the primary LWCO connection/logic to the feed pump.

ethicalpaul

My shot: Oil was introduced into the boiler water from new pipe work and this caused surging with a sudden drop in water level that triggered the manual reset--when examined after the fact, everything seemed to be in order. They would have had to be watching the gauge glass at the time of the surging to see it occur.

retiredguy

I think that @EdTheHeaterMan and @ethicalpaul both covered everything that probably caused the problem but let me add one thing; something has changed with the quality of the water and now the water line surges rapidly causing the aux low water to cut out. The burner shuts off and the water line returns to it's normal level. As a side note, I hope that the aux low water is on it's own boiler tapping and preferably on the opposite side of the boiler as it should be and not installed using the same boiler tapping as the primary low water control.

mattmia2

I'm gonna go with partial vacuum sucking the water out of the boiler after the cycle ends. There are a lot of ways this can happen, with a feed tank usually something about a check valve or vacuum breaker,

EBEBRATT-Ed

The above comments by others have pretty much covered everything. Surging water level would be my best guess.

JUGHNE

I have had the MCM 150 call for water from the feeder pump, the inrush of water surged the level enough that the 150 float dropped down farther and shut the burner down for a bit.

Cutting down/throttling the water feeder input prevented this.

Is this what is happening, boiler water level surging up and down as feeder pumps to trip the lower LWCO?

SlamDunk

We have probe style Aux. LWCO with manual reset. If the LW probe was cut too short, you will see burner lock out before or at same time float will cut burner off. The cut off was instant, as soon as probe was out of water.

However, The manufacturer of the aux lwco made a change to that controller by letting probe be dry for a minute before locking out. The reason was surging, or slow feedwater, causing nuisance shut downs in the middle of the night and angry maintenance people complaining about going from sheets to streets.

Sometimes, you just have a problem with the feedwater system.

JUGHNE

Also, if the float in the secondary LWCO has a small leak and partially filled with water, the pumping surge may have tipped it over the edge to lock out.

RayWohlfarth

Well, this weeks case was a head scratcher to me. I discovered a small buildup of scale inside the float chamber. It wasn't enough scale to stop the float from dropping when the blowdown valve was opened but it was enough to stop it from dropping under normal operating conditions. Apparently, I wasn't the only one who saw this problem because several boiler insurance companies have mandated testing the low water cutoff by simulating actual operating conditions. Its called the "Evaporation Test" https://youtube.com/watch?v=HgpD6xVPuQo

SlamDunk

There is a test we must perform for our state inspectr at least once a year where we close the feed water valve, bottom drain the boiler and wait for both cutoffs to perform. There must be water visible in sight glass when aux lwco is activated. You test everything withthis test but mostly, you ensure aux lwco probe isnt too long. Per manufacturer, minimum of 9" of water must be over top row of fire tubes. Per inspector, water column miniscus must be visible when aux trips.

RayWohlfarth

@SlamDunk This isn't required by our inspectors yet but several insurance companies are requiring it. I think its a great idea