Excerpts from the ASME PVP 2016 technical paper An Improved Design of Threaded Closures for Screw-Plug Heat Exchangers by Haresh Sippy, MD, TEMA India Ltd Excerpts from the ASME PVP 2017 technical paper Design of Threaded Closures for High Pressure Screw-Plug Heat Exchangers designed to ASME Section VIII Div. 2 by Haresh Sippy, MD, TEMA India Ltd
Hi - Hi The Hi-Lo design being obsolete, we have converted all Hi-Lo to Hi-Hi in the BORL project, with the licensor Chevron s consent. The reasons for obsolescence are: 1. It is 10-30% heavier/ costlier than the Hi-Hi design. The position of the first baffle shifts and the length of the shell and tube bundle increases due to the presence of a girth flange and the shifting of the nozzle. 2. It is much more expensive to maintain. Besides, shell removal takes longer than bundle removal and requires space at the rear. Hi - Lo 3. The studs are big in size and difficult to open. Often, round-headed nuts are provided to reduce the BCD. These become impossible to open. Servicing is difficult; efficiency and life is reduced. 4. Replacing the tube bundle is as good as replacing the entire exchanger, as the tube-sheet is integrated with the channel barrel. 5. It is very hazardous, as the gasketed joint is likely to leak to the atmosphere. 6. The piping has to be disconnected in order to remove the shell whereas as in Hi-Hi the tube bundle can be removed after the internals are taken out.
SET SCREW SHRINK RING SPLIT RING CHANNEL SIDE GASKET TUBESHEET SHELL SIDE GASKET THREADED SET LOCK SCREW RING SET SCREW INTERNAL CYLINDER INTERNAL FLANGE INTERNAL SEEVE Hi-Hi Exchanger Channel Closer CHANNEL COVER INNER COMPRESSION RING GASKET RETAINER PARTITION PLATE OUTER COMPRESSION RING GASKET CHANNEL BARREL SHELL TUBES
OUTLET NOZZLE SHELL CHANNEL BARREL SHRINK RING INTERNAL FLANGE SPLIT RING INTERNAL CYLINDER TUBESHEET GASKET RETAINER OUTER COMPRESSION RING INNER COMPRESSION RING INTERNAL SEEVE CHANNEL GASKET NOZZLE GLAND NOZZLE GLAND TUBE BUNDLE INLET NOZZLE Hi-Hi Exchanger Exploded View CHANNEL COVER THREADED LOCK RING PUSH ROD GASKET PARTITION COVER SET SCREW PUSH ROD PUSH ROD SET SCREW SET SCREW
Tube-Sheet to Shell joint leakage in a Hi-Hi construction. With corrosive fluid on tube side. This Joint / Gasket is design for and is also tested at ambient temperatures only. It fails in service because the Spiral wound gasket, crushes under the load it's not designed for, the extra load on the gasket is due to differential thermal expansion between the Low Alloy steel channel barrel wall and the stainless steel internals. We then adopt a metal gasket with groves filled with graphite epoxy known as Kamprofile.
Figure below shows the deformation of the threaded end in the radial direction, of the channel barrel when the end cover was subject to the channel pressure. Bell-Mouthing It can be seen by analysis that the magnitude of bending stress can amount to three times the average longitudinal stress in the undercut area. This phenomenon of bell-mouthing can be logically understood and supported by calculations to show that the combined bending and longitudinal stresses on the undercut surface of the threads produces a wedging action that results in radial displacement causing dilation of the channel. Therefore, this dilation that is a cause of reduction in shear area has to be restricted by using higher wall thickness in the threaded region. This higher wall thickness of the channel barrel immensely shoots up the cost of the forging. TEMA India has thus adopted its own technology of added steel to overcome this phenomenon of bell mouthing.
Our Kamprofile Gaskets - solid metal gaskets with grooves and layer of graphite Our Spiral Wound Gaskets metallic wound strips with graphite as fillers
Even though the threaded closures comply with the requirements of ASME Code, there are still problems of jamming of screw threads due to lack of RIGIDITY. We have a situation where a pressure vessel which apparently meets all Code rules and satisfactorily passed the Code required hydrostatic test, seems to malfunction after a short period of operation. As ASME gasketed joint designs are based on stress considerations, due to critical requirements of preventing intermixing of shell side and tube side fluids, an additional study was made to determine if the existing design would meet the requirement for leak tightness as per EN 1591-1. Leak tightness becomes a mandatory requirement in such exchangers which are used in oil refineries to produce clean fuels. For assessing the leak tightness of the joint, calculations were made as per EN 1591-1 which is also incorporated in EN 13445 Part 3. It was found that the applied gasket force was in excess of that required for obtaining leak tightness under the specified design conditions including effects of progressive distortion due to frequent re-assembly. To maintain uniform pressure on the gasket it is imperative that applied bolt load to be transferred through a RIGID internal cylinder.
In the year 1999/2000 the Prototype of the modified design developed by Tema India Ltd. was built to conduct several tests at high pressures to verify and fruitfully adopt this thicker section on threaded Area of the closure.
Screw Plug Heat Exchanger Prototype
Frontal view Screw-Plug Prototype
Shrink Fitting in Process Channel Barrel after PWHT
Thread Cutting in Progress
A view of channel internals inserted
HHPS Vapour / Treat Gas Exchanger
RIGIDITY Summary To prevent leakages and consequent intermixing of fluids, and ensure ease of maintenance and opening of the plug, all we need can be explained using one word - RIGIDITY. Based on the type of construction, this RIGIDITY has to be maintained on various components. This is achieved as follows: 1 All Screw Plug Heat Exchangers RIGIDITY on the channel barrel is obtained by the Added Steel Ring on the threaded portion of the channel barrel, thus increasing the wall thickness 2 On Hi-Hi Tube Hi Screw Plug Exchangers RIGIDITY of the threaded portion of the channel barrel, as mentioned above RIGIDITY of the gasket that may get deformed or crushed and cause of leakage. The much-needed RIGIDITY can only be provided by a solid metal gasket with grooves filled with graphite (Kamprofile) to achieve resilience. 3 Only on Hi-Hi Shell Hi Screw Plug Exchangers RIGIDITY of the threaded portion of the channel barrel, as mentioned above RIGIDITY of the gasket, as mentioned above As the net differential pressure acts on the tube sheet in a direction against the pressure exerted by the bolts, the internal cylinder across the nozzle openings is likely to suffer deformation. Therefore, RIGIDITY is mandatory on the internal cylinder. Hence, it is imperative to make the cylinder RIGID by way of design that there is almost no deformation observed in the internal cylinder at the nozzle openings area. This ensures that the bolt load and the additional load due differential thermal expansion between the Low Alloy steel channel barrel and the stainless steel internals is transferred onto the gaskets uniformly to give a leak-proof shell to the tube-sheet joint.
To download the paper An Improved Design of Threaded Closures for Screw Plug Heat Exchangers, visit http://proceedings.asmedigitalcollection.asme.org/ proceeding.aspx?articleid=2590185 The copyrights have been assigned to ASME. The paper will be available online December 2017 at http://proceedings.asmedigitalcollection.asme.org Connect with me on www.linkedin.com/in/hareshsippy/ https://twitter.com/hareshsippy/ https://www.facebook.com/hareshksippy/ www.hareshsippy.com
Thank you