Boiler does not switch to the second stage. Troubleshooting

Greetings! Here is another article about boiler malfunctions and this time we will analyze an interesting case when the Miura boiler does not switch to the second stage in automatic mode, while there are no visible problems or alarms.

In fact, the problem was that the boiler could not increase pressure and stop, it was working all the time. After that, it was noticed that it did not use the second stage (High Combustion) to supply more fuel to the injector.

How does the boiler work?

In this article, we will consider the Miura boiler, but such troubleshooting will of course work for other boilers, since the operating principle is approximately the same for all of them.

Simple logic diagram of the Miura boiler operation

In short, the following should happen: the boiler is in automatic mode - the steam pressure switch (upper setting) gives a signal to start - the fan starts, the damper opens (35 seconds of blowdown) - the ignition transformer gives a spark through the electrodes - the pilot fuel supply valve opens (Pilot Valve) - ignition occurs (Flame Eye - the "eye" sees the flame) - the transformer stops giving a spark - the first stage fuel supply valve opens (Low Combustion Valve) - the pilot valve closes - the second stage supply valve opens (High Combustion Valve) according to steam pressure (pressure switch - lower setting) and the damper opens - the boiler builds up the required pressure according to the pressure switch and the fuel supply stops, the furnace is purged for 35 seconds and the fan stops.

Miura Boiler Control Panel

Auxiliary boiler is one of the most dangerous mechanisms on a ship, so the protection and interlock circuits, as well as alarms, are the largest and most complex.

Boiler operation options depending on steam pressure switch settings

Above, I described in simple terms what happens in the boiler without technical details. This information is enough to understand the principle of operation of the boiler, but it is not enough to understand the entire logic of its operation, what processes follow each other in time, how electrical circuits check each other for errors and failures in operation, how protection against incorrect operator actions works, etc.

We will not go into this in detail, since there is a lot of information about the boiler. For this, the vessel usually has all the necessary instructions.

And if your vessel does not have instructions, I recommend contacting our closed telegram channel "Marine Engineering Manuals", which has most of the manyals for the most popular marine boilers.

Why can't we go into logic?

The answer to this question is very simple - because we always have an electrical circuit diagram at hand that will help us understand all the processes occurring in the boiler. Even if you do not know the entire logic of the boiler, but you know how to read electrical diagrams, you can easily detect and fix any problem.

In fact, you need to understand that the fuel supply to the nozzle (sprayer) occurs with the help of a solenoid valve, which opens if the required voltage is applied to its coil. This is what we will start from in our search.

Solenoid valves (L - Low Combustion, H - High Combustion)

There is no power coming to the solenoid valve "H - High Combustion". We found this out using a multimeter. If the necessary power was coming, but it did not open, then the problem is either in the coil or in the valve itself. This did not solve the problem, so we will open the electrical circuit diagram.

Boiler operation diagram (attention to the High Combustion solenoid valve)

So, let's go and figure out the diagram. In this case, we are interested in the High Combustion solenoid valve, on the diagram it is 20VH. We measured with a multimeter that 220 V power is not supplied to it (the control circuit uses 220 V).

220V control circuit

Next, we go along the entire circuit to this solenoid valve and find out what is not giving the command to receive power to the 20VH coil. The first thing that catches the eye is the open contacts LMX, 20VHX and 20VLX. We need to figure out what kind of relay this is and whether they receive power during the boiler startup process.

Relay 20VHX

We also pay attention to the 20VL coil of the first stage solenoid valve. We know that the boiler is working at the first stage, which means that the coil is receiving power and this means that its circuit is working, and in this circuit there is our 20VLX contact.

Relay LMX

Thus the whole circuit up to this contact and the contact itself works, and the problem with the second stage is only in the contacts of the 20VHX and LMX relays.

Relay 20VHX

Relay 20VHX in our circuit is responsible for the second stage, in automatic mode it should receive power from the PLC and close its contact in the solenoid coil circuit. Interestingly, in automatic mode it does not receive power.

In manual mode it receives power from the circuit with open contacts 43MQX2 and 20DX. Let's figure out what kind of relay this is.

Relay 43MQX2

Relay 43MQX2 is energized when the boiler is started in manual mode. Relay 20DX is a DM damper motor relay, which is energized in automatic mode by the PLC and in manual mode by the normally closed contact 20VPX, the open contact 43MQX2 and the second stage High Combustion switch.

Relay 20DX under power (LED light is on)

At this stage, it is time to switch the boiler to manual mode and see how the circuit works. Manual boiler mode (also known as emergency mode) starts the boiler according to a simplified scheme, it practically does not involve PLC, as well as some protections. In fact, only the relay contactor part works. This often helps to identify the problem or at least reduce its search to a narrow area.

In manual mode, relay 20VHX receives power (in automatic mode it does not) and closes its contact in the circuit of the 20VH valve coil, but the coil still does not receive power and there is no transition to the second stage.

The 20VHX relay is powered in manual mode (LED is on)

Only the LMX relay contact remains. Notice how I led you away from the problem that was obvious. In fact, it often happens that you follow a false trail and waste a lot of time on it.

The LMX relay is powered when the LM limit switch is activated. Is the limit switch not working?

In my search, I acted the same way as I write in this article, I noticed this limit switch right away, but did not check it, and first went along another circuit and eventually returned to it. What activates the limit switch? This limit switch is closed by the damper when it opens to supply air to the firebox for the second stage. More fuel = more air.

Damper motor (with damper adjustment)

Damper end switch (inside the box)

Our boiler uses two dampers and, accordingly, two motors. One damper DM2 operates on the pilot and first stage, the second DM, which can be adjusted with bolts, operates on the second stage.

This DM motor is powered by the 20DX relay. Without power, the normally closed contact of 20DX always supplies power to this motor and it rotates in one direction. When power is supplied to the 20DX relay, it closes its normally open contact and the motor begins to rotate in the other direction - to open the damper (the power supply circuit changes). As a result, the damper opens, activates the end switch and thus the LMX relay is activated.

Damper motor

The damper motor is a single-phase asynchronous motor Sertec Corporation 8RH2-DW30S993 with a capacitor start. The connection diagram shows that the circuit has a starting winding and a working winding. To change the direction of rotation, it is necessary to swap the wires of the starting winding (two terminals connected via a capacitor).

It remains to check the operation of this circuit. From the manual mode, we found out that the 20DX relay receives power and closes its contact in the 20VHX relay circuit, which means it is necessary to make sure that power is also supplied to the DM damper motor. It turned out that power is supplied to the motor, but the shaft with the damper does not rotate.

Checking the power supply at the DM damper motor terminal block

DM damper shaft does not rotate

After removing the motor, it turned out that the rotor was stuck in one position and does not rotate.

By the way, in my practice there was a case when the coupling fell apart and the half coupling, which you see in the photo above, was torn off, the motor shaft rotated, but the damper was in one place. So this boiler part needs to be checked periodically. The case was on the Monarch injector (Weishaupt Monarch Burner).

Disassembling the motor showed that the Japanese bearings were stuck in one position. In our case, there was a new motor in stock, which was installed and successfully tested. As a result, the boiler started working at the second stage.

Video of the boiler and damper motor in operation

Disassembling the motor

By the way, before removing the motor, I made sure that the capacitor C2 has sufficient capacity to start the motor (0.8 μFarad).

Damper capacitors

Boiler operation logic

As you understood from this troubleshooting, the PLC did not supply power to the 20VHX relay because the LM end switch of the DM damper did not operate. If I had immediately turned to the boiler operation logic, I would have found the problem faster, but I relied on reading the diagram and switching the boiler to manual mode.

The LMX relay contact gives a signal to the PLC

Here is a hint (the circuit diagram above), the normally open LMX relay contact gives a signal to the PLC when the LM end switch of the DM damper is activated. Thus, it “says” that fuel can be supplied, air has entered the furnace. All these processes (air supply and fuel supply) occur almost instantly, but the logic of the sequence is still observed. If the end switch is not activated, the fuel will not flow.

The LMX relay is powered (LM limitswitch is closed)

Note that the LED lights help to understand whether the relay is activated or not, but it is also important to rely on your own measurements with a multimeter. It happens that the relay is activated, but its contacts do not work correctly (they close or open the wrong ones). Therefore, always use a multimeter to check the correct operation of the circuit.

In the Miura boiler manual, you can also find the boiler operation logic, section "Program Simulation Diagram".

Program Simulation Diagram (second stage operation)

This is where you can see how the second stage (High Combustion), the damper motor (DAMPER) and the limit switch (LMX) are interconnected. The second stage will not start until all the conditions for starting are met, otherwise High Combustion STOP (DAMPER Abnormal). Unfortunately, as is often the case, there is no such alarm on the panel and you have to do your own troubleshooting.

Nevertheless, the problem was fixed. Once again, I will remind you that first of all it is important to be able to read the circuit diagrams, because the systems on different ships may be different, but the electrical circuit diagrams work the same. For example, Miura and Aalborg boilers are completely different boilers, but if you know how to read the circuit diagrams, then for you the difference between them will be only visual.

There are, however, nuances in the diagrams of different manufacturers. For example, Japanese, Chinese or Korean diagrams are read slightly differently, and there may be different designations of relays and contacts.

Reading Japanese Schematics

Japanese schematics are very simple in this regard. For example, look at the normally open contact of the LMX relay, underneath it is written "6 - D", this means that the contact is on page "6", and the letter "D" corresponds to the column in which it is located (similar to a chessboard). Also under the relay there is always a table with all the relay contacts that are on the circuit diagram. For example, the LMX relay also has a contact "11 - D" on page "11" in the "D" column and it is also normally open (this is the contact that gives a signal to the PLC). From this table you can understand what kind of contacts the relay has: normally open or normally closed. This principle is used in most electrical schematics, not only Japanese ones.

By the way, if you don't know how to read electrical diagrams on a ship, I recommend reading the article "How to read electrical diagrams on a ship? Main engine shaft turning gear operation diagram".

So what happened to the motor?

As you already understood, the motor bearings stuck, but it did not burn out! Unlike the case with the diesel generator regulator motor. And this means that you can try to replace the bearings. Unfortunately, we did not have new 608Z bearings in our spare parts stock, but from experience working with small bearings, we decided to try to disassemble the old non-working portable grinders.

608Z grinder bearing

608Z bearings

It turned out that such bearings exist, although they are Chinese, but still "live". Therefore, we installed the bearings, connected the motor according to the circuit diagram with a capacitor and returned it to working condition. The motor was put in reserve for emergency use.

Checking the motor

Perhaps such bearings will not work for long, because they are not original, and besides, the motor heats up a lot during its operation, and it is unknown what kind of lubricant is used in them. By the way, despite this, we still ordered a new motor!

Video of the motor running with Chinese bearings

That's all! I hope the article was useful for you. If you have any questions, write in the comments! Thank you for your attention.

More articles about boilers on the site:

• Boiler safety devices. Quick check for the surveyor
• Abnormal Fire. The boiler does not start. Troubleshooting
• Pressure switch. The fuel pressure switch on the boiler has failed. What is an abnormal fire?