Technical Tips Volume 1 Number 8
Tips on Pressure Vessel Leaks
Recently a Client called to ask what might be the cause of loud noises
coming from the vicinity of the Subcooler in a Closed Feedwater Heater. Here
is a summary of the possible causes and actions the plant may take to deal
with the problem:
CAUSES
- Tubes passing through the endplate of the subcooler are not expanded tightly
in place to allow them to expand and contract axially as the heater works.
The annular space between the tubes and the holes in the endplate will allow
steam to intrude into the subcooler if the tube surface contained within
the endplate is insufficient. In some very old Closed Feedwater Heaters,
the subcooler end plate is too thin (less than 3-inches thick) to condense
steam that passes around the annuli. Consequently, when there is sufficient
pressure differential between the condensing zone and the subcooling zone,
uncondensed, wet steam will enter the Subcooler. The wet steam erodes the
cross-flow baffles, especially in the vicinity of the tube holes. As the
tube holes enlarge due to the erosion, the effect is to significantly increase
unsupported the tube span, making the tubes more vulnerable to vibration
and subsequent failure. As the tube holes enlarge due to the erosion, of
the tubes making the tubes more vulnerable are subject to vibration and subsequent
failure. Current practice is to specify a minimum of 3-inch thick end plates
as shown in the figure and modest pressure drops across the end plate. It
might seem to be reasonable to plug tubes as they begin to leak. But this
is self-defeating because doing so does not allow for cooling/condensing
any steam passing through the annuli. A better way to deal with this problem
is to fix it as shown below in Fixes. As shown in the sketch, the clearance
between the tube and hole is ~0.0045-inch. We recommend holding all end plate
drilling to the TEMA Special Close Fit Tolerances with no exceptions for
out of tolerance holes that the TEMA Standards allow for drilling tubesheets
(4% oversize).
- The welds of the Subcooler shell to the flat roof plate have failed as
shown in the photograph.
- The welds of the Subcooler to the back face of the tubesheet have failed.
Purchasers can avoid failed weld problems by specifying in their procurement
specification that all welds of the Subcooler enclosure to the roof plate and
to the back face of the tubesheet must be full penetration welds, are fluid
penetrant examined and that the Manufacturer perform a standing water test
on the enclosure to verify that the welds are intact.
FIXES
- Fix the problem with thin end plates by having the tubes expanded into
the end plate so they just contact the hole interiors, thereby reducing the
volume of steam that can pass between the tubes and hole.
- Fix the problem with failed longitudinal welds by cutting a window in the
shell of sufficient size to expose the whole roof plate-to-closure weld.
Grind out the failed weld. Attach clamps to pull the roof plate to contact
with the enclosure, reweld and examine the weld with fluid penetrant. Replace
the removed section in the shell and hydrostatically test. Note that this
kind of repair meets the National Board Inspection Code (NBIC) requirements
for repairs. Most jurisdictions require such work to be performed only by
Repair Organizations that possess the NBIC-issued R Symbol stamp and Certificate
of Authorization.
- Attempts to fix the problem of failed Subcooler enclosures to tubesheets
have not bee successful. Various schemes have been tried, including injecting
sealants at the position of the failed welds. But to our knowledge, none
have been successful. We recommend replacing the heater, or in the case of
a heater installed in the condenser neck, replacing the bundle and channel.
COMMENT
It goes without saying that careful and proper level control
is the best way to operate Closed Feedwater Heaters. Improper or inadequate
level control can also lead to Subcooler problems. But this Technical Tip does
not address such problems.
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