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

Hi - Lo

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.

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.

Hi-Hi Exchanger

Channel Closer

THREADED LOCK RING

TUBESHEET

SHRINK RING

SHELL SIDE GASKET

CHANNEL SIDE GASKET

INTERNAL CYLINDER

INTERNAL FLANGE

SPLIT RING

INTERNAL SEEVE

SET SCREW

SET SCREW

OUTER COMPRESSION RING

INNER COMPRESSION RING

CHANNEL COVER

SET SCREW

PARTITION PLATE

GASKETRETAINER

GASKETCHANNEL BARREL

TUBES

SHELL

NOZZLE GLANDNOZZLE GLAND

PUSH ROD

INTERNAL CYLINDER

GASKET

PARTITION COVER

INTERNAL FLANGE

SET SCREW

SPLIT RING

INTERNAL SEEVE

CHANNEL GASKET

OUTER COMPRESSION RING

INNER COMPRESSION RING

CHANNEL COVER

GASKET RETAINER

THREADED LOCK RING

PUSH ROD

SET SCREWPUSH ROD

SET SCREW

OUTLET NOZZLE

INLET NOZZLE

TUBESHEET

SHRINK RING

CHANNEL BARREL

TUBE BUNDLE

SHELL

Hi-Hi Exchanger

Exploded View

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

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.

RIGIDITY

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

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Thank you