VFK
Mechanical
- Aug 13, 2023
- 16
An existing BEU heater is being re-purposed.
* The tubeside fluid will change from hydrocarbon gas to liquid hydrocarbon.
* Either the existing U-tube-bundle will be plugged with mechanical plugs (20% of tubes) or a new U-tube bundle will be procured. Either way, there will be the same number of tubes in-service.
* The existing tubesheet is CS 217 mm thick with 8mm DSS W/O and is extended for bolting. The 206 DSS U-tube rows are 38.1mm OD x 2.77mm thk and strength welded and lightly expanded.
* The heater was designed to ASME-VIII-1 2007 A09 addendum.
* For the new U-tube bundle, the tube layout will be identical to the existing except the inner 3 U-tube rows will be missing. In other words, the untubed lane width UL1 (ASME ref) will increase from 114 mm to 256 mm (refer to attached image).
* The design temperature and pressure will be unchanged (refer to attached image), but the tubeside operating pressures and temperatures will change marginally.
* An HTRI simulation has been completed to verify the thermal performance (duty) of the heater and vibration risks for both new bundle and plugged bundle as well.
* The original tubesheet design calcs appear to have been completed in accordance with ASME UHX rules.
My worry is doubling the tubesheet untubed lane area may induce higher differential thermal stresses in the axial and radial directions within the tubesheet at the centre and in the vicinity between the perforated tubesheet and untubed lane. This is based on my assumption that increasing the untubed lane area will reduce the heat transfer through the central region of the tubesheet compared to the original design.
From my reading of the Heat Exchanger Design Handbook by Thulukkanam, it appears the tubesheet design rules in ASME and TEMA are based on thin plate theory. And for U-tubes only pressure loads are considered when sizing the tubesheet. ASME UHX-11.5 concerns itself with the untubed lane dimensions, but it appears this is a correction factor for the ligament efficiency. So it would seem the "usual" design tools are not set-up to perform checks for differential thermal stresses within the tubesheet.
Q1 - should i be worried? / is it necessary/prudent to analyse the differential thermal stresses within the tubesheet when the thickness is +200mm thick?
Q2 - If so, is there an existing software module available, or is FEA the only method?
* The tubeside fluid will change from hydrocarbon gas to liquid hydrocarbon.
* Either the existing U-tube-bundle will be plugged with mechanical plugs (20% of tubes) or a new U-tube bundle will be procured. Either way, there will be the same number of tubes in-service.
* The existing tubesheet is CS 217 mm thick with 8mm DSS W/O and is extended for bolting. The 206 DSS U-tube rows are 38.1mm OD x 2.77mm thk and strength welded and lightly expanded.
* The heater was designed to ASME-VIII-1 2007 A09 addendum.
* For the new U-tube bundle, the tube layout will be identical to the existing except the inner 3 U-tube rows will be missing. In other words, the untubed lane width UL1 (ASME ref) will increase from 114 mm to 256 mm (refer to attached image).
* The design temperature and pressure will be unchanged (refer to attached image), but the tubeside operating pressures and temperatures will change marginally.
* An HTRI simulation has been completed to verify the thermal performance (duty) of the heater and vibration risks for both new bundle and plugged bundle as well.
* The original tubesheet design calcs appear to have been completed in accordance with ASME UHX rules.
My worry is doubling the tubesheet untubed lane area may induce higher differential thermal stresses in the axial and radial directions within the tubesheet at the centre and in the vicinity between the perforated tubesheet and untubed lane. This is based on my assumption that increasing the untubed lane area will reduce the heat transfer through the central region of the tubesheet compared to the original design.
From my reading of the Heat Exchanger Design Handbook by Thulukkanam, it appears the tubesheet design rules in ASME and TEMA are based on thin plate theory. And for U-tubes only pressure loads are considered when sizing the tubesheet. ASME UHX-11.5 concerns itself with the untubed lane dimensions, but it appears this is a correction factor for the ligament efficiency. So it would seem the "usual" design tools are not set-up to perform checks for differential thermal stresses within the tubesheet.
Q1 - should i be worried? / is it necessary/prudent to analyse the differential thermal stresses within the tubesheet when the thickness is +200mm thick?
Q2 - If so, is there an existing software module available, or is FEA the only method?
