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Technical Tips Volume 1 Number 4
Tips on Sleeving Feedwater Heaters
The process of sleeving closed feedwater heaters is the insertion
of thin tubes (sleeve material) into the existing inlet tubes of a feedwater
heater, then expanding the inserts into the tubes. Expanding the sleeve into
the tube produces a residual interfacial fit pressure between the sleeve
OD and tube ID. Typical sleeve thicknesses are in the range 0.01-inch to
0.03-inch thick, depending upon material of construction and thickness of
the original tubes. Sleeving the inlets of closed feedwater heaters and condensers
which has long been used to protect against inlet-end erosion is increasingly
widespread in the power generation industry. More recent developments are:
(1) restoring tubes to service that have been plugged because of perforations
in discreet, identifiable locations. (2) restoring tubes to service that
have been plugged because their walls have become so thin that continued
operation presents a hazard of failure, and (3) bridging failures in discreet
location of tubes that are otherwise intact.
Technical Tip 1: Choose the Most Suitable Sleeving Method
Roller expanding Historically, sleeves have been roller expanded into tubes. The adequacy of such expansion must be established by experimentally correlating rolling torque with sleeve-to-tube tightness because making measurements to deduce percent wall reduction of the thin sleeves is impractical and torque sensing to control the degree of rolling is not very precise.
Hydraulic expanding Hydraulic expanding is preferable for expanding sleeves into tubes. Because of its precise control and there is less risk of causing the tube to bulge because of over-expanding than with rolling. Unlike rolling equipment, in which torque may drift from the original setting, hydraulic expanding equipment maintains the set expanding pressure within tolerances of ±1.25% to ±2.5% depending upon the yield strength of the sleeve. Hydraulic sleeve expanding pressure is best determined experimentally using mock-ups.
Hydraulic Sleeving Equipment The
HydroPro ®SleevePro strain
control System, pictured below, is one of the most advanced on the market.
The system tracks the stress/strain curve of the sleeve electronically sensing
the points at which the sleeving material yields and makes contact with the
parent tube. After the sleeve contacts the parent tube, the system controls
additional water volume to limit the radial expansion of the sleeve and parent
tube, thus creating a tight interfacial fit by utilizing the elastic capacity
of the parent tube.
Some contractors who are qualified to perform sleeving are American Power Services of Erlanger, Kentucky, Framatome ANP, Lynchburg, Virginia, and CTI Systems, Stratford, Connecticut.
Technical Tip 2: Calculation Procedures for Estimating Pressure Drop and Heat Transfer
- From the Manufacturer’s data sheet determine the pressure drop and heat transfer for the design basis conditions.
- Calculate the increased pressure drop for the number of tubes to be sleeved.
- Calculate the lost heat transfer capacity that can be recovered after the plugged tubes are sleeved.
It is customary to follow the methods of the Heat Exchange Institute, Cleveland, Ohio HEI Standard for Closed Feedwater Heaters 6th Ed., March 1998 when performing these calculations.
Estimating Pressure Drop for Sleeving Pressure drop in a tube is the sum of entry and exit losses of feedwater into and out of the tube end and friction losses in the straight lengths and U-bend turns. Usually, when the purpose of sleeving is to prevent tube end erosion, the sleeves are short and there is sufficient pressure drop available so that any increase resulting from sleeving does not materially affect the operation.
Click below to download MGT’s 3 copyrighted Excel spreadsheets, programmed to estimate thermal effects and pressure drop:
Thermal Effects of Sleeving SS Case without sleeved DC
DeltaP due to sleeving 100 tubes with full length sleeves
DeltaP due to sleeving all tubes with 18-inch sleevesDownload All 3 Excel files (15 KB Zip file)
Note that when full length sleeves are installed to restore to service previously plugged tubes the cross-sectional area for feedwater flow increases because the plugs are removed. Therefore, the pressure drop is less after sleeving than when the tubes are plugged. The means to perform such calculations is provided on the Excel spreadsheet referred to above.
Heat Transfer Effects of Sleeving Feedwater Heater Tubes
Oxide films on the sleeve exteriors and tube interiors and areas of
possibly incomplete contact between sleeves and tubes increase the
overall resistance to heat transfer. The resistance of the barrier
between tube and sleeve is called contact resistance. Reduction in
the amount of recovered duty is the result of additional resistance
to heat transfer of the sleeve metal wall and the contact resistance.
Based on unpublished data, contact resistances appear to be about:
0.0011 hr-sq ft deg F/Btu for Stainless Steel type 304 tubes sleeved
with SS 304 sleeves, 0.0013 hr-sq ft deg F/Btu for Admiralty Brass
tubes sleeved with 90-10 Copper-Nickel, 0.0046 hr-sq ft deg F/Btu for
Admiralty Brass Tubes sleeved with SS 304 sleeves, and 0.0026 hr-sq
ft deg F/Btu for ™Monel tubes sleeved with Monel sleeves. Readers
should be aware that the listed contact resistances have not been established
by extensive testing. These estimated resistances indicate that for
austenitic stainless steel tubes sleeved with stainless sleeves and
for admiralty brass tubes sleeved with 90-10 copper nickel sleeves,
recovery of surface is about 89% effective; for Monel tubes sleeved
with Monel sleeves, recovery is about 80% effective; and for admiralty
brass tubes sleeved with austenitic stainless sleeves, recovery is
about 70% effective.
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