ML20058A356
| ML20058A356 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 10/16/1990 |
| From: | Israel S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML17348A608 | List: |
| References | |
| TASK-AE, TASK-T90-14 AEOD-T90-14, NUDOCS 9010260017 | |
| Download: ML20058A356 (7) | |
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AEOD TECHNICAL REVIEW REPORT UNIT:
Turkey Point TR REPORT NO. AEOD/T 9014 DOCKET NO.:
50-250 DATE: October 16, 1990 LICENSEE:
Florida Power & Light EVALUATOR / CONTACT: S. Israel SUBIECT:
SEAL PROBLEMS IN BORIC ACID TRANSFER PUMPS -
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SUMMARY
Problems with seals and the seal water system in the boric acid transfer pumps at Turkey Point were reviewed for potential generic implications. SCSS and NPRDS databases were queried to identify other plants with boric acid pump problems. Contact with seven other plants disclosed that they did not use seal water systems for their pumps because they use single mechanical pump seals rather than the double seal used at Turkey Point. Seal failure does not appear to degrade the pump performance, but it does require corrective action.
. Discussions with plant personnel indicated chronic housekeeping and maintenance problem with the 12 w/o boric acid in the system. One utility is changing over to sealless pumps as
- part of their modification to 4 w/o boric acid in the system. This would eliminate the housekeeping and maintenance needed for these pumps.
DESCRIPTION OF EVENT On September 10,1989, a boric acid transfer pump (BATP) was declared out of senice at -
Turkey Point'because the nitrogen pressure in the seal pot could not be monitored. Accordin to the LER (Ref.1), this particular event was caused by a maintenance person, When removing the nitrogen pressure indicator from the seal pot, the wrench slipped off the
" rounded flats" inducing vibration in the indicator. The vibration resulted in the pointer falling off the indicator. This circumstance revealed that the BATPs were previously techni-cally " inoperable" when the nitrogen pressure indicators are removed to refill the seal pot L
(about once a week).
i According to the LER, i
"A nitrogen presst.re between 35 psig and 40 psig is maintained in the seal pot, in part, to ensure seating of the inner seal faces and minimize inner seal leakage, FPL has determined that as long as the BATP seal water system is " closed", loss of the seal pot nitrogen pressure would not render the BATP " inoperable". Any leakage through
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the BATP inner seal faces'into the seal water system would tend to pressurize the seal '
pot and limit the amount of leakage to a value which would not significantly affect the BATP minimum required flow rate. However, whencver the seal pot nitrogen indicator is removed, the seal water system is an "open" system. BATP inner seal leakage during pump operation with the seal water system "open" would not be enough to affect the minimum required flow rate of the BATP, however, the inner seal leak would result in lost inventory from the BAT. For this reason, FPL considers a a
BATP to be " inoperable" while refilling the seal pot."
This was the fifth Turkey Point LER associated with the BATP seals since 1987.
BACKGROUND The.BATP is a small pump (about 50 gpm with a 50 psi head) for moving 12 w/o boric acid into the charging system from batching tanks. One utility indicated they had about 140 of this type of pump on site, mostly in the radwaste system. Mechanical seals used in these pump are manufactured by several different vendors. A given pump model may have different stuffing boxes to accommodate different types of mechanical seals.
A functional diagram of the seal sssembly in the Turkey Point BATPs is shown in Figure 1 It is a double mechanical seal with a seal water system that pressurizes the plenum tms.o H the inner and outer seals. The seal pot contains clean water which is pressurized to about 40 psig by nitroger. cover gas. This plenum pressure is higher than that of the boric acid on the l
other side of the inner seal.
i The seal pot insures that there will be inleakage across the inner seal and thus, precludes formation of boric acid on the highly polished seal face. There also will be leakage across L
the outer seal face because the pressure in the plenum is higher than that of the environment.
The rotating part of the seal is' spring loaded against the stationary part to allow for shaft movement relative to.the stuffing box. The higher pressure in the inner plenum also imposes a hydraulic load on the primary seal faces.
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4 The secondary seals, some form of o-ring, are also loaded by the higher pressure in the plenum. These secondary seals provide a leak free, static interface between the rotating s part and the shaft and between the stationary seal p&rt and the stuffing box.
l All mechanical seals leak normally to lubricate the rubbing surfaces at the seal interface.
Early in seal life, this leakage is not noticeable because of the small flow rate. As time goes by, the leakage increases until a decision is made to replace the seal assembly. Accordin Ref. 2, seals can last up to two years if properly designed,-fabricated, installed, and main-tained.
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ANALYSIS The BATP seal problems at Turkey Point were first reported in May 1987 when a failed seal-resulted in massive nitrogen intrusion into the boric acid system (Ref. 3). Gas binding of the BATPs prevented emergency boration of both Turkey Point units until the problem was
-understood and corrected. The cause of that event was an unisolated nitrogen supply connected to the seal pot so that nitrogen flowed into the boric acid system after the BATP
. seal failed and the seal pot was empty. The corrective actions included isolating the nit supply line to the seal pot and a review of the existing seal design to identify worthwhile improvements, in April ~1988, another seal failure occurred at Turkey Point on a different BATP and was reported in Ref 4. In that instance, the nitrogen supply to the seal pot was isolated. No gas intrusion into the boric acid system was mentioned in the LER. The seal failed again on the same pump in August 1988. The LER (Ref. 5) stated that the seal pot would be removed in December 1988. A seal pot on a different BATP failed in August 1989. The corrective action noted that the licensee was unable to find a replacement seal assembly tha, ooesn't require cooling water. An improved seal water system was supposed to be implemented '
May 19%) (Ref. 6). In September 1989, the only BATP aligned to service one of the reactors was declared inoperable because of a failed pressure indicator on the seal pot as noted above.
. (Ref.1). The issue was further complicated by the discovery that removal of the pressure indicator to add water to the seal pot made the BATP technically inoperable. This inoper-ability had not been recorded during prior maintenance activities when water was added to the seal pot.
1 A revision (Ref. 7) to the September 1989 LER withdrew the commitment to develop an.
improved seal water system. Instead, the licensee committed to maintaining BATPs, with adequate water in the seal pot, aligned to the two units. In addition, the licensee also stated,
" A cartridge type single seal compatible with twelve (12) weight percent boric acid cannot be located. FPL is currently pursuing a boric acid concentration reduction program which will allow plant operation at or below four (4) weight percent boric acid. Cartridge type single seals are available which are compatible with this boric
' acid concentration. Plant operation at or below four (4) weight percent boric acid would also eliminate the need for a BATP seal water system."
It appears that the seal water system made it difficult to maintain the BATPs. This support system was developed to preclude the 12 w/o boric acid from leaking past the mechanical seal on the BATP, Heat tracing on the boric acid system maintains a temperature around 160*F to keep the boric acid in solution. The pump seals can not be heat traced. Consequently, without a seal water system, there was a chance that boric acid crystals would form at the seal face and could damage the seals when the pump is started.
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The seal water system at Turkey Point requires a double seal in order to pressurize the plenum betweer the inner and outer seals. The major function of seal water in contact with
- the inner seal is to lubricate the seal interface with clean water during pump operation. Thus, i
there is continuous leakage of the seal water into the boric acid system during pump opera-tion. Similarly, the seal water lubricates the outer seal, in this case, the leakage is outside the pump. When a seal is new, the leakage is not noticeable. As the seal deteriorates from use, the leakage increases and may be observed when the seal is degraded excessively. Norm less than a gallon of water is added to the seal pot each week so the observed leakage rates are small.
Discussions with personnel at seven other plants provided additional insights into boric acid systems. First, not all plants consider the boric acid pumps safety related. Consequently, component failures in this system may not be reponed by all plants to NPRDS. Second, none of the other plants use a double mechanical seal on the BATP sa they do not have seal water systems. It appears that Turkey Point introduced what they considered to be a design im-provement (compared to other plants) and have had intermittent problems because of it.
' Third, most of the licensees contacted plan to reduce their 12 w/o boric acid systems to 4 w/o because of chronic problems with this material. Only one plant-site contacted already uses w/o boric acid.
Operating experience at the eight plant-sites indicates that the seals on the BATPs need to be -
1 replaced about every 12 months, even at Turkey Point. Mechanical pump seals are very demanding devices. The faces are highly polished to minimize leakage during operation. In
- order to work properly, the faces of the two parts of the seal must be perpendicular to the pump shaft and the shaft must be concentric with the stuffing box. ' Axial and radial shaft movement at the seal location must be minimized to slow down seal deterioration. Piping loads at the pump inlet can cock seal elements causing rapid seal failure. Particles intro-duced to the seal interface are an operating concern because they can scratch the highly polished surfaces which results in excessive leakage.
One plant reported seal replacements on two month intervals. This is excessive compared the ' general experience at'the other plants contacted. The utility is changing seal design They noted that neoprene was used for the secondary seals in the seal assembly at this pla Neoprene's service temperature is very close to the operating temperature of the boric acid system (160*F). Viton will be substituted for the o-rings. Prairie Island used to have seal lifetimes of three to four months. About four years ago they changed their seal design and initiated a controlled installation procedure so that they now have seal lifetimes on the order i
of 12 months.
Because of chronic housekeeping problems, Zion is replacing its BATPs with a sealles design which will eliminate leakage. This change is part of a modification associated with reducing the boric acid to 4 w/o Sealless pumps are widely used in the chemical industry to contain toxic materials. They are also used to a lesser degree in the radwaste systems in som nuclear plants. As noted earlier, one licensee indicated they had 140 utility pumps with me-4
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chanical seals on site, mostly in the radwaste system. Since all mechanical seals leak at an ever increasing rate with continued operation, they pose housekeeping and maintenance i
chores.
The BATP seal system (including seal water) at Turkey Point does not appear to pose a!
significant safety concern except for potential blanketing of the system with nitrogen. Th j
i nitrogen issue was highlighted in an information notice (Ref. 8) and the licensee instituted administrative procedures to isolate the nitrogen source after the seal pot is pressurized i
9). Absent the nitrogen issue, the safety impact of BATP seal failures at Turkey Point shl be no different than that at other plants which utilize single mechanical seals instead of the -!
double used at Turkey Point. De leakage associated with the seal does not appear to seriously degrade the performance of the pump; it appears to be a housekeeping and main' tenance problem.
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CONCLUSION l
i The seal water system used on the BATPs at Turkey Point may be unique. The safety concern associated with the nitrogen cover gas used in the seal water system 'was previ addressed in an information notice and implementation of administrative procedures at the
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plants. Seal leakage itself does not appear to significantly degrade pump performance.
Mechanical seals on pumps similar to the BATPs are prone to leakage that introduces a l
i continuing housekeeping and maintenance chore. Several utilities have taken steps to redu seal problems by changing seal designs and utilizing sealless pumps.
REFERENCES 1.
Florida Power and Light, Licensee Event Report 250/89-13, Turkey Point, Unit 3, October 9,1989.
2.
J. Rams 4ca, "How to choose and iristall mechanical seals," The Chemical Engineerinj Guide to Pumps, McGraw Hill Publications Co., New York, N.Y.,1984 3.
U.S. Nuclear Regulatory Commission, Inspection Report No. 50 250/87-28, Turkey Point, Unit 1, July 17,1987.
4 Florida Power and Light, Licensee Event Report 250/88-05, Turkey Point, Unit 3, May 13,1988.
5.
Florida Power and Light, Licensee Event Report 250/88-19, Turkey Point, Unit 3, Septemoer 21,1988.
6.
Florida Power and Light, Licensee Event Report 251/89-09, Turkey Point, Unit 4, September 22,1989.
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7 Florida Power and Light, Licensee Event Report 250/89-13, rev 1, Turkey Point, Unit 3, May 29,1990.
8.
U.S. Nuclear Regulatory Commission IAss of Emergency Boration Capability Due to Nitrogen Gas Intrusion, Information Notice N0. 87-57, November 6,1987.
9.
Florida Light and Power, Licensee Event Report 250/87-17, Turkey Point, Unit 3, June 26,1987.
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