Online Answers for Boiler Breakdowns
If a boiler experiences a shut down, for example, the online software is capable of storing all data points it has received every second during the four seconds prior to the event. It saves this data for each of the last five shutdowns. At company headquarters, a master computer receives all of the data.
The information includes operational settings, operational history, and diagnostic information. It then configures this data to:
When a boiler goes down, any of 27 possible alarms or cautions is automatically transmitted to headquarters. Miura relays a summary of this information during normal working hours to the customer’s authorized service company. The service company is then armed with critical data to more quickly and efficiently isolate the problem and effect repairs, before a technician even goes to the site.
All of its boilers with the XJ1 expanded enunciator can take advantage of the Miura Online Maintenance (MOM) system. Fee options are a yearly contract or per-use charge. The customer only needs to provide a dedicated telephone line. MOM can handle up to 37 boilers on a single line.
What MOM ChecksThough the system is capable of conveying real-time operating data on every connected boiler, at any time, this explanation will just address an event that triggers an alarm.
An alarm always causes a shutdown. And one cause of boiler shutdown is low-water cutouts. Low-water cutouts can result from three different causes:
1. The amount of total dissolved solids (TDS) in the water is too high.
2. Steam is being pulled off the boiler faster than it’s being made.
3. The feedwater pump is bad.
To identify which of the three is causing the low-water cutout, MOM looks at the status of the low-water cutout probe in each of the four seconds before shutdown. It then selects reports on all operating data related to the cutouts.
These reports are called “signatures.” They present the four seconds of each item as a bar graph. The graphs show the status of the steam pressure, flame signal, water levels, surface blowdown valve, feedwater pump, damper position, and conductivity sensor. The signature also shows the last four seconds’ measurements of the scale monitor, temperatures, etc.
Next, the accumulated alarm histories are compared to the current alarm. Finally, MOM’s 31-day chart shows rates of change over the past month, such as if and how quickly the feedwater ratio was dropping.
With the full picture of the boiler’s operating history and shutdown status, the manufacturer can isolate which of the three low-water cutout causes needs to be corrected.
Analyzing the SignatureIn one application, a textile manufacturer was seeing its Miura boiler go down on low water every 20 minutes. The boiler was only two years old, so age wasn’t a factor. Miura recommended using MOM.
The textile manufacturer installed a dedicated phone line, ran a phone cable from the control box (XJ1 expanded enunciator) to the jack, flipped the dipswitch, and there, revealed to MOM, was all the data accumulated in the XJ1 since the boiler was installed.
In the snapshots that the XJ1 had of the boiler’s status in the four seconds before each of the previous five shutdowns, the system revealed that low-water cutouts were occurring because of high TDS.
The signature was always the same for each low-water cutout, showing high TDS. Total dissolved solids were above 5,000 microsemens. It showed that the feedwater pump was running up until some of the shutdowns.
It didn’t matter whether the boiler was in low fire or high fire, the signature showed that the boiler just shut down, sending an alarm. This set of events is indicative of a low-water cutout due to water treatment problems. In other words, total dissolved solids are too high.
With the above information, the other two possible causes of low-water cutouts were eliminated.
If the problem had been too much steam drawn off the boiler, the signature would have been different. TDS would be consistently below 4,000 microsemens. The “pump-on” and “pump-off” probes would have been dry. The boiler would be in high fire with the pressure dropping. The feedwater pump would be on, trying to keep up with steam demand, but it wouldn’t be able to, so the pressure would drop and the water, being pulled out of the water tubes, would cause low-water cutout.
Then there is the final possible cause, which is a difficult one to identify. That is whether the feedwater pump capacity is dropping or not.
This was checked out by looking at MOM’s 31-day chart. The feedwater ratio should be at 2:1 or higher at all times. This ratio indicates there is twice as much water going in as steam coming out. If this ratio keeps dropping, there is a problem with cavitation or a plugged strainer, and this will result in low-water cutout.
So why was TDS so high and causing the low-water cutouts?
Steam output goes down as the amount of TDS goes up, but the system still needs the same amount of steam, no matter what. So, it keeps trying to draw the steam so much that the water itself will start to lift off the boiler. TDS has to be limited in order to maintain a specified pressure.
For example, in the range of 100 to 130 psi, you don’t want to have more than 4,000 microsemens of solids in the water. The only way to maintain that level is to take out a certain amount of solids through a blowdown.
Blowdown rates are typically from 4% to 7% on a boiler. So at 10,000 lb of steam per hour, the user is blowing down (flushing out) 400 to 700 lb of water through the boiler’s water tubes, once an hour. The textile manufacturer had set the blowdown rate at 1%, and using only 1% of a boiler’s water wouldn’t flush enough solids out of the tubes.
Simple SolutionMark Utzinger, manager of Miura’s U.S. operation, noted, “To show our customer that the blowdown rate was the cause of the low-water cutouts, we remotely set the blowdown rate at 10% (the maximum rate). We asked them to flush the whole boiler down. In other words, we had them stop the boiler, clean it completely out, and fill it back up. It stopped their low-water cutouts.
“Then we backed off to 5% to see how good their system was, to see if their water was normal or not. Even at 5%, they were fine. You want the blowdown rate as low as possible because then you are blowing down less hot water. As a minimum, you want 4%. What the U.S. Department of Energy recommends is 4% to 7%.”
The problem was solved.
Publication date: 02/05/2001