want to know more about silicon carbide mating ring crack 2

[reallyme00]

To All,

Recently I experienced a problem(Out board stationary ring crack) with my mechanical seal. The Outboard seal start to leak after two days of startup. leakage rate was 3drops/min. we found during dismantlig that the crack(near anti rotation pin groove) at the out board stationary ring caused the leak. The crack was hard to see ie., hair line crack.

The operator started the pump one day later, the instalation of the seal on the pump. At the inboard side there were symptoms that seal could have experienced high pressure(over pressurisation). The I/B seal contact pattern was like I.D contact and scratches at the outer circumference because of abrasive entry.

Now my question is, apart from instalation error what would cause this mating ring crack? I red one answer from Mr.JJPALIN, that cold start up could lead to crack of the hard face. Could I know more that How a cold start up could crack the mating ring? What are all the other possible causes?

your answers would help me to avoid the similar failures in future. I am sure that installation error is not an issue because we assembled the seal by taking the direction of rotation in mind.

Advance thanks to all.

Seal details:
It is a double mechanical ApI plan 02/53M seal.
RPM-2900, Pumping fluid Temp- 275 Deg C,
Stuffing box pr- 8 bar.

Barrier fluid pressure - 15 bar.
Barrier fluid temp - around 70 Deg C.
Stationary ring- silicon carbide
rotating ring - metal bellow with carbon face.

 

[JJPellin]

In most instances where I have seen cracked hard faces at or near the anti-rotation pin groove, one of the following causes was found:
1. The anti-rotation pin could be too long. If the pin comes in axially into a slot in the back of the face, a long pin can bottom out in the slot and press hard against the face causing it to crack. If the pin come in radially from the OD, this is not as likely. But the pin could still be incorrectly located and hitting hard against the S-C.
2. Cold start up in a viscous product. If cold product sets up between the seal faces, the faces stick together and the torque necessary to separate the faces at start-up spins the face, with only the anti-rotation pin to stop it. The stress of the hard contact between the pin and the groove can crack the face. This is a greater concern with a motor drive since the start-up is always sudden.

I have the following recommendations. Put the seal together with no gaskets. Verify that the hard face is solidly supported in the housing and not hitting the pin. Warm the seal and pump up carefully before start-up. If this is a pump that starts automatically, a warm up line, drilled check valve, steam tracking, steam jacketing, etc. may be necessary.

 

[reallyme00]

Dear JJ,

Really, I am happy about your quick reply. Now I understand that how a cold start up could crack the hard face. For that my thanks for you. As you mentioned I checked the viscosity of the barrier fluid is only 0.37mm2/Sec. So I hope it could not be a cause for that. And I checked the pin dimension and it is well short of the length that require to touch the back of the hard face.(The pin comes in axial direction). So now I am trying to find that was there any mistake happened during installtion? ie, hard face touch the shaft during fitting, or even (by mistake)when the technician assembles the seal would have assembled in the wrong Direction of rotation. because radially, there is enough gap between the groove and the pin (the pin to hit the hard face during startup.) Thanks again.

 

[RGRAJANI]

One major contributor for the SiC ring crack is shocks to seal happened during transportation or installation. While starting a unit of the refinery I noticed approx. 12 SiC rings cracked out of 120 seals. After careful replacement problem was almost solved.