mechanical seal failure in pumps with API plan 52.

[I2P]

Friends,
We havew a centrigugal pump handling condensate, and the pump has plan 11 and plan 52 for sealing fliud. Recently we have had a seal failure. I have a few basiic questions.
1.0 On the sight glass provided on the seal return line to the seal pot,we could never observe the circulation of fluid.When the pump vendor was contacted during commissioning he said that circulation is happening but is not too distinctly visible.Also there was no significant difference between the seal fluid supply and return line temperature. Again when this was pointed out to the vendor he said that since the fluid is at ambient temperature there wont be any heat buildup. Also we have not provided any cooling water supply to the seal pot as per his recommendation. Upon inspection after failure we found the seal had cracked, and root cause investigation is under progress.I am fairly convinced thatif the seal has been rightly installed then the flow would be clearly visible in the sight glass, and if flow is ensured, then irrespective of process fluid temperature being ambient( around 30 deg C), there would be a heat build up and the return seal fluid would be at a higher temperature.Am I right in thinking that it is a case of seal fluid not circulating at all which has led to seal failure.
Presently we have replaced the seal with a new one from another vendor and trial was taken and we observed circulation from sight glass and also seal return line getting appreciably hotter than the supply.
2.0 Now our problem is that having not connected cooling water to the seal pot we have no means of cooling the seal fluid. Since we do not have cooling water utility we propose to provide external fins on the seal return line.We want to make our own inhouse calculation and arrive at the fin area required. Could anybody suggest how to make these calculations to determine the no, dia,of fin?
Since it has become a warranty issue I would appreciate if someone could clearly tell me that our thinking is right that seal failure has occurred due to improper installation?
Secondly I want to know what can be the cause of fluid not circulating if externally the connections seem ok. Within the seal what is the motive force that forces the seal fluid back to the seal pot?
Thanks to all in advance.

 

[pmover]

from memory (which is subject to correction),
1) yes, the seal fluid should have been seen in flow sight glass. no or little flow will likely result in seal or pumping ring failure. a seal reference or std is API-682?

2) Cooling the sealing fluid. the best solution is to obtain a seal pot with exchanger installed. by the time you determine cooling required and mfg a "southern engr'd" exchanger, obtaining a seal pot with exchanger may be the best economical solution. that being written, ...

the duty or amount of heat to remove from seal fluid is:
q = mass flowrate (m) x heat capacity of fluid (Cp) x temperature difference (dT)
next determine the log mean temperature difference (lmtd).
solve for UA from eqn: q=UA x lmtd
from source books (i.e. GPSA), determine a reasonable and conservative U value and then determine the A (area) needed.

other than that, good luck.
-pmover