Coupling Seizure

[bhaskar5150]

In our DCU plant, we have 2 nos of FD fans (identical) for supplying combustion air to the burners with the following data :
Flow : 10.896 m3/sec, Static pressure :4700 pa (479 MMWC), Rated power : 61.25 KW
Temp : 35C, RPM : 980, Motor rating : 75KW. Impeller type : Backward curved.

Each fan has a IGV at suction for flow control & discharge damper is full close or full open & has a full close command on individual motor trip. Heater has a trip on low combustion air pressure in individual ducts (4 passes) to the burners.(25MMWC trip pressure against a normal of 34MMWC in individual passes).

Fan-A has a inherent problem of coupling failure (more than 3 times, last one in Jun'16).It is a practice here to run both fans in parallel since its commissioning. Present flow is around 6.6 M3/sec & static pressure is 410 MMWC. This is done by throttling the IGVs of both the fans.(Control is single command for both IGVs) In all previous cases, the coupling bolts & shims break getting uncoupled with the motor but motor still running (doesn't trip), therefore doesn't closes the discharge damper & flow from Fan-B goes to Fan-A & reverse rotates the fan. (This is obvious & only a after effect of coupling seizure). Then, discharge damper of Fan-A is closed manually.

Another important fact is that normally after every start-up, one fan (Fan-B in last 3 cases) is started first as the flow requirement is less initially & after a certain time interval the 2nd fan (Fan-A in last 3 cases) is started and IGVs of both the fans are adjusted accordingly to get the desired flow & static pressure.

From above, it can be concluded the Fan-A starts against a back pressure (created by the static pressure of Fan-B). Is it because of this that the Fan-A never recovers under parallel operation, operates on the left of the peak pressure of the fan curve and goes under stall. Due to this flow reversal in fan-A, coupling shims & bolts breaks as they are weakest components in that line. Also, the stalling zone is not defined by the manufacturer in the individual fan curve.

If above is the cause, how to know & confirm it.

We are finding it very difficult to access the parallel performance of the fans and have no idea how the combined curve would behave under the present operating conditions.

Any advice/suggestion to get a permanent solution to this problem.

Thanks in advance.

 

[don1980]

I think it's extremely unlikely that this coupling is failing because the fan is started up against the backpressure of the other fan. For the listed flow rate and static head, the resulting force load on the coupling is very low.

I think it's much more likely that the coupling failures are due to misalignment. I suggest you get a skilled millwright to check the installation.

 

[georgeverghese]

In my experience, the FD fans are kept running for all cases on fired heater trip except when there is confirmed process coil tube rupture or when exhaust stack temp is at TSHH.

Since there is no such thing as low flow surge on a low dp FD fan (check fan curve), it may be okay to close the discharge damper on each of the 2 fans at the same time as the command to emergency stop the fans??

 

[bhaskar5150]

Thanks Don & George for your valuable comments.

Rightly pointed out that the fan is not supposed to go under surge considering such low pressure levels.

Also, alignment was done during last shutdown, reported OK. No abnormality is reported from vibration checks also (before the failure). Bearing temps are also normal.

One thing to note here is that at present both the fans are running (6.6 M3/sec - combined flow & static pressure is 410 MMWC at common duct) & current reading at local ammeter in both the fans is 75A. From the current reading,the approx motor output (fan BKW) is 43 KW. From the manfg's individual power curve, flow of each fan is coming as 7 m3/sec. So, the total flow should ideally be 14 m3/sec (current IGC opening = 40% equal on both fans). However, total flow as indiacted in DCS is 6.6 m3/sec only. From this, can I presume that one fan is over powering the other(In this case, Fan-B overpowering the Fan-A) & therefore, flow reversal taking place in Fan-A.

 

[don1980]

I suggest you go over to the mechanical engineering forum for further help with diagnosing the coupling failure problem. A low-pressure centrifugal fan is not going to overload the coupling, regardless of how you operate the fan and the flow dampers. This failure is explained by mechanical factors rather than operating conditions. There's probably something wrong with the alignment of the shafts or coupling hubs, or with the mounting (vibration) of the motor or fan.

If both fans are identical, and they're both running at the same speed with the IGVs in the same position, then you can't get reverse flow through one of the fans. Each one is putting up the same head. Different adjustments on the outlet dampers will cause dissimilar forward flow from each fan, but neither fan is capable of sending a reverse flow through the other one. Regarding your flow rate comparisons, note that fan flow rates are referenced to the inlet side. It sounds like you're comparing outlet side volumetric flow against the vendor curve value for inlet flow.

 

[georgeverghese]

Also suggest we check the calibration / verify the total flow FT calibration is as it should be and that it is operating correctly.