Orbital Welding
ISPE's
Pharmaceutical Engineering
Article of the Year
Orbital Welding in Compliance with the
New ASME Bioprocessing Equipment (BPE) 1997 Standard
by Barbara K. Henon, Ph.D.
This article discusses ASME's BPE-1997 Standard that relates directly
to orbital welding of tubing which directly or indirectly come into
contact with the product. This includes weld criteria, weld
documentation, inspection and qualification of welding procedures to
the Standard. The article also addresses other parts of the standard
such as material selection, surface finish of tubing, and dimensional
tolerances of fittings to be installed by orbital welding.
Introduction
Standards are important determinants of quality in fabrication. A
decade ago, welding for pharmaceutical applications would have
simply been qualified to ASME Section IX of the Boiler and Pressure
Vessel Code with reference to ASME B31.3. This assured the structural
integrity of the weldments, but was not specific as to the quality of the
weld surface on the inside of tubing which is essential for bioprocess
applications. The 3A Sanitary Standards, first implemented by the
Dairy industry in the US in the 1950s, offered guidelines for materials
and fabrication techniques that mandated fully penetrated welds in
sanitary piping systems, and attempted to set guidelines for
workmanship and quality control in recognition that weld quality was a
determining factor in the ability to maintain a piping system in a
sanitary or hygienic condition. However, with the emerging
biotechnology industry, it was felt that new standards were needed to
meet the higher quality demands for the more complex, often delicate,
and very costly bioengineered products. In particular, it was felt that
there was a need for design criteria for equipment to enable it to be
effectively cleaned and sterilized, and an emphasis on assuring weld
surface quality once the requirement for strength was met. A need for
standardized definitions also was recognized as well as the need to
integrate existing standards for vessels, piping, appurtenances, and
other equipment for the bioprocess industry without infringing on
those other standards. In response to the special needs of the
biopharmaceutical industry, the American Society of Mechanical
Engineers (ASME) has developed a new standard to provide guidelines
for the design and fabrication of facilities in which pharmaceutical
products are manufactured by means of bioprocess technology.
Figure 1. Welding operator on site at Fiocruz Instituto de Technologia in Brazil using
an orbital tube welder in the fabrication of a facility for the production of vaccines. A
water cooling unit, placed underneath the power supply, is used to cool the weld
heads during welding.
The purpose of this discussion is to familiarize the reader with the
concepts and intent of the BPE Standard, and the rationale for the use
of orbital welding in the fabrication of bioprocess equipment and piping
systems. It also is intended to guide those wishing to qualify orbital
welding procedures to this Standard. It is not intended to be complete,
nor to cover all parts of the Standard which pertain to welding, nor
topics covered by the Standard but unrelated to welding, nor to be a
substitute for the Standard which can be obtained from the ASME.1-4
Figure 2. An orbital weld of two electropolished 31 6L stainless steel tubes. The
tubing was 2.000 inches (SQ8 mm) OD with 0.065 inch (1.65 mm) wall.
The weld bead is fully penetrated to the ID of the weld joint. The weld bead is
uniform in width without arc wander and the weld profile is fairly flat on the OD and
ID. There are no visible cracks or porosity. Light oxidation can be seen in the Heat
Affected Zone (HAZ) of the weld. The oxygen level of the argon ID purge was 8 ppm.
The new ASME Bioprocessing Equipment Standard, ASME BPE-1997,
was released in October of 1997, but the process of writing this
standard began much earlier when individuals at the 1989ASME Winter
Annual Meeting determined that there was interest in the industry for
such a standard, and approached the ASME Council on Codes and
Standards to develop a standard for the design of equipment and
components for use in the growing biopharmaceutical industry. The
project was approved by the Council and a directive issued to the
Board on Pressure Technology to initiate the project. The scope of the
proposed standard included the design, materials of construction,
inspection, testing of vessels, piping, and related accessories such as
pumps, valves, and fittings for use in the biopharmaceutical industry.
The ASME standard was not intended to replace existing standards,
but wherever possible, the new ASME standard was expected to
reference existing standards that are applicable to biopharmaceutical
equipment design and fabrication. Throughout the writing of the
standard, close relationships were established with other standards
groups including the European CEN, ASTM, and 3A Dairy Standards.
The writing of the standard was accomplished by a Main Committee
and several subcommittees which met several times each year until
the standard was completed. These subcommittees were: General
Requirements; Design Relating to Sterility and Cleanability of
Equipment; Dimensions and Tolerances; Material Joining; Surface
Finishes; and Seals.
Applicability
The BPE Standard applies to all parts of equipment and piping that
contact either the product, raw materials, or product intermediates
during process development, scale-up, or manufacturing and all
equipment systems that are a critical part of product manufacture.
This includes systems such as water-for-injection (WFI), clean steam,
purified water, ultrafiltration, and intermediate product storage. Piping
systems or parts of the system that do not contact the finished
product are not covered by the BPE Standard. Pressure vessels, and
steam-sterilized systems or any other systems which require pressure
operation must conform to all applicable requirements of ASME Section
VIII and ASME B31.3, Process Piping. Qualification to the BPE
Standard does not exempt users in any case where municipal,
provincial or federal codes regulating pressure vessels and piping are
in effect. The requirements of these codes must be met. Welding is
used as a joining method throughout biopharmaceutical plants. It is
used in the construction of pressure vessels and bioreactors as well as
in the joining of piping systems. Orbital welding, which uses the Gas
Tungsten Arc Welding (GTAW) process, is not used for pressure
vessels or tanks as other welding processes are more suitable for this
purpose. However, the GTAW process is frequently used for the
product contact side of a vessel because of its high quality while faster
methods are typically used for subsequent passes.
In this standard, tube and pipe are differentiated with tube referred to
by its nominal OD, and pipe referred to by diameter and schedule.
Orbital welding may be used to weld either tube or pipe. The
dimensional tolerances of tubing makes it more suitable for
autogenous orbital welding, but autogenous orbital welds of schedule
pipe are practical for wall thicknesses of up to 0.154 inches (2 inch
schedule 40). Orbital welding of pipe on applications requiring the
addition of filler to the weld can be done using equipment with wire
feed capabilities in sizes from 1/2 inch NPS and up. Orbital welding is
commonly used to join tubing in piping systems for WFI, clean steam,
and product lines. It also is used to connect tanks and vessels to the
piping systems in the construction of equipment such as multi-effect
stills as well as to connect bioprocess equipment mounted on skids.
Typical tubing diameters for biopharmaceutical applications are 1 to 4
inches, but 1/2 inch OD instrumentation tubing (or smaller) may be
orbitally welded and autogenous orbital welds with enclosed weld
heads can be done on tube diameters up to 7 inches OD. Autogenous
orbital welds of tube sizes up to 8 inches have been done for
pharmaceutical installations using the more complex orbital welding
equipment with filler wire capabilities which is generally used for larger
diameter pipe. This type of equipment can be used for autogenous
welds on pipe sizes up to 14 inch schedule 10. The decision on
whether to use tube or pipe for a particular application would be based
upon the pressure requirements of the system, surface finish
requirements, availability of fittings with the proper end-preparation
for orbital welding, etc.
Figure 4. Fermentor used for the production of antigens installed in a Class 100,000
fermentation room in a facility for the production of vaccines. Orbital welding was
specified for the WFI piping, chilled water air and clean steam. Photo courtesy of
Fiocruz Instituto de Tecnologia em Imunobiologicos in Brazil
Biotechnology / Pharmaceutical
Design Concepts for Cleanability and Sterilizability
A key feature of the BPE Standard is the concept of hygienic design
where hygienic is defined as "of or pertaining to equipment and piping
systems that by design, materials of construction, and operation
provide for the maintenance of cleanliness so that products produced
by these systems will not adversely affect human or animal health." In
keeping with this concept, the Design Part of the Standard had as its
objective to describe and outline accepted practices which have been
shown to result in the fabrication of bioprocessing equipment that is
both cleanable and sterilizable. It makes the distinction between
preferred, recommended, and not recommended designations for
particular designs and fabrication practices.
Bioprocess equipment is generally designed to be cleaned-in-place
(CIP) rather than being disassembled for cleaning (cleaned-out-ofplace or COP). The BPE Standard addresses only automated or manual
CIP processes and automated steam-in-place (SIP) processes, but not
hot water, 176°F (80°C) sanitizing, or other methods of sterilization.
In order for CIP and SIP to be effective, the inner surfaces of piping
and equipment must be smooth and free of crevices so that it is
cleanable and resists to the extent possible the colonization by
microorganisms. Equipment also must be designed and fabricated so
that deadlegs are held to an absolute minimum where a deadleg in a
piping system is defined as a pocket, tee, or extension from a primary
piping run that exceeds a defined number of pipe diameters (L) from
the ID of the primary pipe (D). For bioprocessing piping systems an
L/D ratio of 2:1 is considered to be achievable for most valve and
piping configurations. A deadleg is undesirable because it provides an
area of entrapment which may not be reached by cleaning or
sterilizing procedures and thus leads to contamination of the product.
It has been shown that it takes more heat over a longer period of time
to achieve sterilization in an air pocket than in spaces that are reached
by steam since moist heat is much more effective in achieving
sterilization than dry heat.12 Cleaning solutions used for CIP must be
able to reach all parts of the system at sufficient velocity to ensure the
removal of soil or deposits by chemical and/or mechanical cleaning,
and all joints must be capable of being CIP cleaned and SIP sterilized.
Figure 3. Concepts of the BPE Standard place an emphasis on design for drainability
and sterilizability of bioprocess systems to prevent growth of unwanted
microorganisms. An illustration of this concept is shown in the vessel which features
a sloped rather than flat bottom for ease of draining. Piping is also sloped to allow
draining by gravity alone.
Drainability is important for maintaining bioprocess systems in a clean
and sterile condition. Fluid remaining after draining becomes a
colonization site for bacteria or other microorganisms creating an
unacceptable bioburden to the system. Sites where fluid accumulates
also may become a corrosion-initiation site adding additional
contaminants to the system. The design Part of the BPE Standard calls
for holdup volume, or that amount of liquid which remains in the
system after draining is complete, to be minimized by design. For a
piping system to be drainable, lines must be sloped to no less than 3
inches per 50 ft (75 mm per 15 m) or 0.5% as a minimum with a
recommendation for 1/4 inch per foot (20.8 mm per m) or
approximately 2% for short runs and 1/8 to 1/16 inch per foot for field
runs, skids, and transfer panels.
Connections to equipment shall use acceptable hygienic design
connections as agreed to by the owner/user and the manufacturer
(contractor) which means that threaded connections must not be used
where they would come in contact with the process since the machined
threads are likely to entrap soiled material and provide areas that
cannot be reached by cleaning. Similarly, the number of bolts must be
held to a minimum.
The optimum design for cleanability and sterilizability of bioprocess
systems will have a minimal number of connections. For those
connections that are required, buttwelded connections are to be used
wherever possible since welded connections present the lowest risk of
contamination. For welding of biopharmaceutical tubing in the US, the
ends of the tubing are mechanically squared, butted together, and
typically joined in an autogenous weld using automatic orbital tube
welding equipment. An autogenous or fusion weld is one which is fused
without the addition of filler metal. Welding is not limited to tube-totube welds, but is used to join tubes to fittings and is recommended
for joining all hygienic valves into the process line.
When ferrules and ferrule connections are used, it is recommended
that they be as short as possible to ensure that deadlegs are
minimized. Any fittings used must be designed so that the interior is of
a flush design or with a bulge concavity not exceeding 5% of the wall
thickness so that a readily cleanable and drainable surface is exposed
to the bioprocessing fluid.
Figure 5. A small orbital weld head being used to weld 1/2 inch diameter tubing on a
bioreactor skid. The tubing supplies gases such as oxygen to the reactor. Photo
courtesy of B. Braun Biotech.
Although welds are the most secure type of joining method, the
surfaces of the weld bead on the inside of the tubing must be smooth
and crevice-free in keeping with the concept of hygienic design. Any
lack of fusion, porosity, or excessive buildup of material on the ID may
provide a site which is inaccessible to cleaning or sterilization
procedures. While hygienic systems can be achieved with manual
welds, orbital welds are generally smoother and much more likely to
provide a crevice-free surface on a repeatable basis.
Materials
For bioprocessing systems, the materials of construction for product
contact surfaces must be fairly inert so as not to contaminate or
otherwise affect the integrity of the product. The materials must be
compatible with the bioprocessing environment including the
temperatures reached during steam sterilization which are saturated
steam at a minimum temperature of 266°F (130°C) for a duration of
at least 100 hours under continuous steady state conditions. The
materials also must be able to withstand high operating pressures and
endure harsh chemical and mechanical cleaning procedures. The
material recommended for use in bioprocessing systems is type 316
stainless steel (UNS 31603), or where it is to be welded, type 316L.
Other higher alloy materials such as AL6XN or 2205 duplex may be
used where higher corrosion resistance is needed, or other materials
may be used as agreed upon by the purchaser and manufacturer.
However, materials shall be identified by an industry recognized
standard. Type 316 and 316L belong to the 300 series of stainless
steels which are included in P number 8 according to the classification
in ASME Section IX BPVC in which base metals with similar properties
are grouped. The 300 series was selected by 3A for use in the dairy
industry because it was cleanable and reasonably corrosion resistant,
and that standard has been applied by the pharmaceutical industry.
Although type 304 was the most commonly used alloy in the past, it
has gradually been replaced by types 316 and 316L for hygienic
applications because these alloys, although very similar to 304 in
chemical composition, contain 2-3% molybdenum which significantly
increases the corrosion resistance. Fluids used in bioprocess systems
are frequently high in chloride concentration presenting a challenge to
the corrosion resistance of stainless steel systems.
Figure 6. An orbital weld of a short sanitary ferrule to a pulled tee on 316L stainless
steel tubing. The use of short welded ferrules is recommended to keep deadlegs to a
minimum and this is easily accomplished with orbital welding technology.
The choice of 300 series was fortuitous for welding since it is easily
weldable and machinable. Compared to higher alloys, the loss of
corrosion resistance by autogenous welding for 316L is minimal.
Autogenous welding is less costly than welding with filler not only
because the expense of filler is avoided, but also because orbital
welding equipment for autogenous welding is less expensive than
orbital welding equipment which has the capability of adding filler wire
to the weld. Thus a material that can be autogenously welded is an
economical choice. Furthermore, welding procedures are fairly simple
since no preweld nor postweld heat treatments are required. These
properties make 316 stainless steel a practical and cost effective
choice for bioprocess systems.
The chemical composition of type 316 stainless steel is specified by
ASTM 269, Seamless and Welded Austenitic Stainless Steel Tubing for
General Service; ASTM A270, Specification for Seamless and Welded
Austenitic Stainless Steel Sanitary Tubing and ASTM A 312/A 312M,
Seamless and Welded Austenitic Stainless Steel Pipes. The L version of
316 has a lower carbon content which is 0.035 rather than 0.08% in
standard 316. The low carbon version is preferred for welding because
it reduces sensitization or carbide precipitation which may occur in the
heat-affected-zone (HAZ) of welds during welding. In the temperature
range of 800 to 1500°F, which occurs at some distance at either side
of the weld, carbon comes out of solution in the austenite grains and
migrates to the grain boundaries. Chromium migrates from the grain
boundaries to combine with the carbon as chromium carbide leaving
the grain margins depleted of chromium. Since the corrosion
resistance of austenitic stainless steel depends upon a uniform
distribution of chromium throughout the metal, depletion of chromium
at the grain boundaries makes it susceptible to corrosion. The reduced
carbon version of 316 does not completely suppress the formation of
chromium carbide, but results in less loss of chromium during welding,
and therefore, less loss of corrosion resistance
WELD PUDDLE PROFILES
Figure 7. Left: The element sulfur has a profound effect on the fluid dynamics of the
weld pool. When sulfur is in the medium to high range of AISI 316 SS, heat from the
arc is transferred downwards in the weld pool resulting in good penetration. When
the sulfur content is below 0.008 heat is transferred towards the edges of the weld
pool resulting in shallow penetration with a tendency towards OD concavity.
Right: When one heat is low in sulfur and the other is in the medium to high range,
the arc may deflect towards the low sulfur heat In extreme cases, the weld bead
may miss the joint on the ID.6,9
The element sulfur, which is present as a contaminant in stainless
steel, has a significant effect on weldability as well as on the quality of
surface finish that it is possible to achieve on any particular heat of
base material. Although the sulfur content of AISI 316 and 316L
stainless steel has a maximum of 0.030% sulfur, the BPE Standard has
limited the sulfur content for weld fittings and tubing to between 0.005
and 0.017%. The reasons for this restriction are complex. Sulfur at
higher concentrations promotes a tendency towards cracking and gives
a rough appearance to the weld bead. Sulfur combines with
manganese in the base metal to form manganese sulfide inclusions.
During processing, these inclusions are rolled out and appear in
scanning electron micrographs as "stringers." Corrosion tends to occur
preferentially at the inclusion sites and when the inclusions are
removed during electropolishing or passivation they leave a pitted
surface.7 For this reason, tubing manufacturers which have to meet
stringent surface finish requirements for the semiconductor and
bioprocess industries prefer materials which are very low in sulfur.
However, when the sulfur concentration gets into the very low levels,
i.e., less than 0.005%, the material often becomes difficult to weld.
The fluid dynamics of the weld pool changes when the temperature
coefficient of surface tension goes from positive to negative at low
sulfur concentrations.6 This has the effect of transferring heat in the
weld pool outwards from the center of the weld so that when heat is
applied the weld pool becomes wider before becoming deeper and the
weld is shallow with a tendency to form a concave weld bead. With
higher sulfur concentrations, heat is transferred in an inward direction
resulting in deeper penetration with a narrower weld bead which is
desirable. The higher sulfur concentration makes it easier to get a
satisfactory weld.
In compliance with the BPE Standard ASTM has added A270-95a
Supplementary Requirements, S2 Pharmaceutical Quality Tubing 316,
a supplement to ASTM A270 which limits the sulfur content to between
0.005 and 0.017%. This is a compromise which avoids the very lowest
sulfur material which is difficult to weld, but restricts higher sulfur
contents which are known to result in higher non-metallic inclusions
and lower corrosion resistance. Users should be aware that welding of
very low sulfur heats of material to heats in the intermediate to high
levels of sulfur may result in deflection of the welding arc towards the
low sulfur heat.9 This may result in a weld bead that fully penetrates
the low sulfur side, but misses the weld joint. This problem is
minimized by the sulfur requirements of the BPE Standard, but users
should be aware of sulfur contents when welding unmatched heats
together such as would be the case for the welding of tubes to fittings
or to valves.
Surface Finish
The Surface Finish Part of the BPE Standard defines the criteria for the
specification of interior surface finishes for vessels, distribution
systems, and other components having contact with the product.
Surface anomalies which would be detrimental to the process are
defined and classified as a basis for acceptance or rejection of the
material. Type 1 anomalies are those caused by an inherent property
of the material such as non-metallic inclusions, while anomalies of
Type 2 are caused by inadequate control of processing which would
include scratches, grind marks, localized corrosion from inadequate
removal of residuals, etc. Tables with acceptance criteria for the
various anomalies are presented. Welds are classified according to
whether the interior is to be ground smooth and flush on the ID with
removal of visual pits and defects, ground smooth but not flush with
the pits and defects in place, or as-welded with no grinding.
The smoothness of the surface finish is quantified by Ra
measurements using a profilometer. For tubing Ra Average is used
which is derived from readings taken at four cross sections
approximately 90 degrees apart. The Ra, or roughness average is the
log of the arithmetic mean of the surface profile. It is usually
expressed in µ-inches (or µ-meters) where the Ra of As-Drawn and/or
Mechanically polished tubing may be from 20 µ-inches (0.5 µ-meters)
to 30 µ-inches depending upon the classification group with an Ra max
of 30 - 45, µ-inches again depending on the classification. Tubing that
has been mechanically polished followed by electropolishing will have
an Ra of from 10 µ-inches up to 20 µ-inches depending on the
classification with an Ra max of 15-30 µ-inches. Similar tables of
readings for fittings and valves are presented.
It should be noted that welds on mechanically polished and
electropolished tubing invariably have a rougher surface than that of
the unwelded base metal. Although the smoothness of the ID weld
surface is somewhat material dependent, orbital welds will in most
cases be considerably smoother than manual welds on the same
material.
Dimensional Tolerances (DT) for Automatic Weld Fittings
The DT Part of the Standard includes tables showing acceptable
dimensions for stainless steel hygienic clamp type fittings and 316L
tube fittings to be joined by automatic welding. Fittings for automatic
welding of pipe sizes are not included. The specifications for the
dimensional tolerances for weld fittings to be used for orbital welding
are important because the production of consistent high quality orbital
welds depends upon consistent fit-up between tube and fitting in the
orbital weld head. Good fit-up in turn demands consistently good end
preparations of tubing and fitting as well as consistency of wall
thickness and diameter.
Fitting tangent lengths for orbital welding were based upon dimensions
of the weld heads of the various orbital welding equipment
manufacturers. Ideally, the straight length of the fitting should be long
enough to reach from the outside edge of the tube clamp inserts which
hold the tube or fitting in the weld head to the center of the weld head
where the tungsten electrode is typically positioned. With some orbital
weld heads, it is possible to relocate the tungsten closer to the edge of
the head to accommodate fittings with shorter stick-out.
Figure 8. An example of an orbital weld of a tube-to-an-elbow for use in a bioprocess
application.
Photo from B. Braun Biotech.
The fitting ends must be square cut without burrs or breaks.
Squareness is measured as a distance from a line drawn perpendicular
to the tangent plane to the edge of the fitting which is 0.005 in (0.127
mm) for a 1/4 inch tube and .008 inches for a 2 inch tube, etc. If tube
or fitting ends are not square, there will be a gap between the ends
when they are positioned in the weld head, and the resulting weld may
appear concave or, in severe cases, the arc may completely fail to fill
the gap.
It is expected that the nominal wall thickness of the fitting be the
same as that of the tube it is to be welded to. Welding current for
orbital welding is based upon the wall thickness of the weld joint with
approximately 75 amps of primary welding current and 25 amps of
background current in the first level for a wall thickness of 0.065
inches. Inconsistent wall thicknesses will result in inconsistent welds.
The wall thickness of fittings outside the control portion, such as the
outside radius of an elbow, shall have at least 65% of the nominal wall
thickness. The OD dimension is important since if the OD of the fitting
is different from that of the tube, there will be a ridge on the ID of the
weld that may interfere with drainability of the system.
When welding is used in the manufacture of fittings for this standard,
the welds must meet the weld criteria of the Standard. Fittings,
whether welded or not, must meet specified pressure ratings. Buttweld
fittings made to this Standard would be 316L stainless steel unless
otherwise agreed to by the owner/user and contractor. Weld fittings
must be labeled with the heat number of materials used to make each
part of the fitting. This is important to know in case of incompatibility
between heats when welding tube to fitting. If the fitting is small, code
numbers may be used for heat identification. The surface finish
specifications for weld fittings must be in compliance with the surface
finish part of the Standard, and fittings are to be protected by either
sealed plastic bags or end caps and shrink wrap for shipping and
handling.
Material Joining (MJ)
The MJ Part of the Standard applies to the joining of bioprocessing equipment which
includes pressure vessels, tanks, and any vessels designed and built to the ASME
Boiler and Pressure Vessel Code, Section VIII, Division 1; piping built to ASME B31.3;
tubing and fittings, and is intended to be used in conjunction with these standards as
they apply. The MJ Part, as with the other parts of the Standard, covers only those
process systems which contact bioprocessing products or product-process streams.
For the purpose of this discussion, only those systems which can be orbitally welded
will be considered. Although joining methods other than welding are mentioned, at
this point the MJ Part is concerned almost exclusively with the welding of 316L
stainless steel. Where weld surfaces that contact the process are to be finished after
welding, welding processes are limited to the arc or high energy beam (electron beam
and laser beam) processes defined in ANSI/AWS A3.0. Where weld surfaces that
contact the process are to be used as is, that is in the as-welded condition, welding
processes are limited to the inert-gas arc processes (such as gas tungsten arc welding
and plasma arc welding) or the high energy beam (electron beam and laser beam)
processes. The Standard recommends that every effort be made to use an automatic
or machine welding process such as an orbital tube welder. However, where size or
space does not permit access with the weld head, manual welding may be used, but
only by agreement between the owner/user and contractor. All welding processes are
to be qualified to section MJ-8 of the BPE Standard.
Weld and finish samples are recommended before a job is begun to demonstrate that
the desired quality can be obtained. The use of test coupons was recommended by
the 3A standards in the 1950s and such test results demonstrated that the manual
welder had the skill to make an acceptable weld. The use of test coupons is important
for orbital welding because with orbital welding, it is possible to establish weld
parameters to obtain an optimal quality weld on a particular heat of material that once
established can be repeated with a high degree of accuracy from weld to weld
indefinitely. Because of heat-to-heat variations in stainless steel, it is recommended
that weld samples be done on the actual heat of material to be used on the job. For
example, the degree of smoothness of the weld bead, the amount of weld bead
penetration at a particular current setting, and even a tendency towards discoloration
during welding is influenced by the quantities of trace elements in the base material.
Although the degree of repeatability is assured by the accuracy of the power supply,
other factors which affect the weld also must be controlled in order to obtain weld
consistency. These include consistency of tube and fitting end preparation,
dimensional tolerances of tubing and fittings including valve stubs, cleanliness of
materials, geometry and condition of tungsten electrodes, and workmanship
standards. The standard provides for sample welds to be made on a regular basis as
decided upon by the owner/user and contractor to demonstrate that the orbital
welding equipment has been set up and is functioning properly, and that the purging
procedures are effective. It is recommended that sample welds be made at the
beginning of each work shift, whenever the purge source bottle is changed, and when
the automatic or machine welding equipment is changed in some way such as by a
change of weld head.
Discoloration of the weld and HAZ may occur during welding as a
result of oxidation. Oxidation, or heat tint as it is sometimes called, is
known to cause a reduction in the corrosion resistance of stainless
steel in proportion to the amount of oxygen contamination of the ID
purge gas. The BPE Standard calls for the amount of discoloration of
the ID surface to be minimized such that the weld bead is free of color,
but with some light discoloration of the HAZ accepted provided that it
is tight to the surface. The amount of discoloration to be allowed is
subject to agreement between the owner/user and the contractor. It
should be noted that color-free orbital welds are the norm for the
semiconductor industry that routinely purifies purge gas to the low
parts per billion levels of contaminants. The standard argon purge gas
supplied for welding generally contains trace amounts of oxygen in the
low parts per million range which should be no greater than 7 ppm,
and also contains minimal amounts of moisture. In Europe, the
standard for hygienic tubing discoloration is "light straw color" which
occurs at about 20 ppm oxygen in the ID purge gas. It is possible to
obtain color free welds on many lots of material with good purging
techniques with standard argon purge gas with 1-2 ppm of oxygen and
moisture. However, on some heats of material, particularly in the
larger sizes, there is often a light blue "halo" or yellowish tint in the
HAZ. It is more difficult to purge larger diameter tubing, and also more
difficult to purge mechanically polished tubing than tubing that has
been electropolished.10
The inert gas shield provided by an enclosed weld head offers more
protection of the weld OD from oxidation during welding than an open
frame type of head. It also protects the ID of the weld since the tubing
ends may separate somewhat during the welding process allowing the
exchange of gases between the ID and OD of the tube. If the tubing
OD is open to atmosphere rather than to an inert gas environment
provided by an enclosed head, there is a greater potential for
contamination of the weld.
In the semiconductor industry and increasingly in biopharmaceutical
installations, the use of gas purifiers and an oxygen analyzer to verify
that the ID purge gas has not picked up contaminants before reaching
the weld joint has become an accepted practice. Many mechanical
contractors have experience in both the semiconductor and bioprocess
industries, and are very familiar with high-purity purging techniques.
This is an area that will need further attention in the future. The
semiconductor industry is concerned enough with loss of corrosion
resistance and contamination to experiment with heats of stainless
steel with low manganese in order to prevent the precipitation of
manganese on the HAZ which is associated with some of the
discoloration. There is no clear standard in any industry for detecting
color on welds since it may be visible in some lights but not other. One
semiconductor manufacturer inspects their orbital welds with specified
intensities of visible light and also with blacklight to detect
discoloration. The biopharmaceutical industry has not yet reached this
level of concern, and perhaps for many applications, it is unnecessary.
However, it is important to understand that the technology is already
in place for achieving a very high level of weld quality which will result
in the optimal corrosion resistance for the material.
(b) Misalignment (Mismatch)
(c) OD Concavity
(d) ID Concavity (Suckback)
(e) Lack of Penetration: None Allowed
(f) Convexity
Figure 9. Acceptable and unacceptable weld profiles.
Weld Inspectors and owner/users are to be allowed free access to
work areas while work assigned to them is in progress as well as
relevant certifications and documentation. An inspection plan,
including the types of examinations to be made, shall be agreed upon
in advance of the job by the owner/user and contractor. Inspectors
who perform borescopic examinations are required at the owner/users'
discretion to meet the requirements of ASME B31.3 paragraphs 340.4,
342.1 and 342.2, and may be certified as an AWS-QC1 Certified
Welding Inspector. All welds are to be visually inspected on the OD
and at least 20% of the welds selected on a random basis for each
section are to be inspected on the ID either directly or with a
borescope. Random inspections are important since welds of tube-tofitting in which different heats are welded together are more likely to
present a problem than tube-to-tube welds. Welding defects which
may be repaired by rewelding include lack-of-penetration for which
one additional pass is permitted and lack-of-fusion. However, it should
be noted that each successive pass on a weld adds to the heat input
and may result in a significant loss of corrosion resistance.
Other defects must be cut out or otherwise repaired at the owners'
discretion. Experienced contractors have been able to reduce orbital
weld reject rates to very low levels. One mechanical contractor was
able to reduce the reject rate for orbital welds in bioprocess
applications over a period of several years from 1.8 to 0.2%.
Weld Qualification to the ASME BPE-1997 Standard requires that the
welds be certified to ASME Sect. IX of the BPVC, and meet the
requirements of B31.3 Process Piping. For this, there must be a QA
manual and a QA program in effect as well as a set of general weld
standards which reference the BPE Standard. Qualified weld
procedures must be developed, preferably on the actual heats of
materials to be used. The orbital weld schedule which lists weld
parameters such as welding currents, pulse times, rotational speed,
and times is frequently used as part of the Procedure Qualification
Record (PQR).
Figure 10. Weld qualification and quality control for orbital welding involves the use
of sample welds known as test coupons made before the start of a job and at
specified intervals during the job. Once acceptable procedures have been developed
for orbital welding, consistent high-quality welds can be produced with a high degree
of repeatability. Photo courtesy of Binsky and Snyder Mechanical Contractors.
Test coupons must be generated and subjected to destructive testing.
Although existing standards have been used to qualify systems which
have been installed by orbital welding, those standards were written
with manual welding in mind, and this has lead to some confusion
when qualifying autogenous orbital welds. Autogenous orbital welds
are expected to be flush on the OD and ID, and do not require grinding
in preparation for destructive testing. The tests include bend tests over
a certain-sized radius in which two each of root and face bends are
required to determine the ductility of the weldment. The test coupons
must be carefully inspected after bending for cracks in the weld or
HAZ. Tensile testing must be done to demonstrate that the base
material still meets the minimum tensile strength (PSI) specified after
welding. If the test specimen breaks in the weld, but still meets the
PSI rating for the material, the weld is considered good. The limits of
the weld range are determined by taking the minimum wall thickness
qualified and then allowing up to two times the thickness with an
unlimited diameter range. Radiography is not required by the BPE
Standard unless it is required for another applicable standard.
When satisfactory test results have been obtained, the results must be
recorded on the Weld Procedure Specification (WPS) Form QW-482,
and the PQR Form QW-483, or equivalent forms. For welder
certification, each welder must submit either 6 inches of linear weld (a
1.000 inch OD weld is 3.14 linear inches) for testing or multiple
coupons, but no more than 4. Welder test results must be recorded on
the Welder Performance Qualification form QW-484. Signatures of test
personnel and a supervisor are required in confirmation of test results.
Forms are available from the ASME. Each time a new welder is
certified, he or she must repeat the above procedures for each and
every weld which may be evaluated by either a bend test or X-ray.
Passivation
Very little attention is directed towards passivation by this standard. It
merely states that passivation of all welded systems shall be done
after cleaning and prior to use if agreed to by the owner/user and the
manufacturer. Passivation is typically done to improve corrosion
resistance which is accomplished by the removal of any free iron or
other contaminants that remain on the product-contact surface after
fabrication and welding. Passivation is done according to ASTM 380
which specifies the procedures used to treat the completed system
with a nitric acid solution or a proprietary mixed chelant formulation.
Passivation of welded surfaces is important because welding disturbs
the passive film on stainless steel which is responsible for maintaining
the corrosion resistance. During welding, the chromium in the weld
area is greatly reduced, while the iron content is increased which
disturbs the favorable chrome/iron ratio. Passivation restores the
natural chrome/iron ratio, and much of the corrosion resistance that is
lost by welding.10 However, passivation only affects the outer 50 Å? of
the weld surface. Deep heat tint, which has been correlated with a
considerable loss of corrosion resistance which occurs when the ID
purge gas contains oxygen above the low parts per million range,
cannot be removed by passivation. For optimal corrosion resistance of
welded tubing systems, the best possible purge should be used on the
weld ID followed by a proven passivation procedure.
Documentation of weld qualification results including the WPS, PQR,
and welder WPQ are to be presented to the owner/user at the
completion of the job and retained for a period of at least 3 years. In
addition, weld maps of bioprocessing components and weld inspection
logs which must include the type of inspection and the date must be
maintained, and the welder identification must be recorded either on
the weld inspection log or on the weld map. This type of information as
well as the serial numbers of the weld head and power supply may be
entered into some orbital welding power supplies and printed out on
the weld schedule sheet and used as part of the documentation.
Similar information is frequently recorded on the OD of the tubing or
pipe next to the weld and used in conjunction with the weld log in case
of a question at a later date. Other records, such as Material Test
Reports (MTRs) listing the chemical composition and test data of the
heats of materials used, surface finish test reports, and results of
pressure testing, passivation, and other documentation required by the
Section on Design, also shall be presented to and retained by the
owner/user for the required period.
Although it has been released, work on the BPE Standard is far from
over. A Main Committee meeting as well as subcommittee meetings
were held in March in Philadelphia to coincide with INTERPHEXUSA'98.
Addendums for the Standard are planned for October of 1998 and
1999 with a new addition expected by the year 2000. The meetings
were well attended attracting new recruits to expand and modify the
Standard in response to industry needs. Whereas earlier codes and
standards were written with manual welding in mind, other standard
writing groups are now in the process of updating their standards to
include references to orbital welding. The Baseline® Pharmaceutical
Engineering Guide, Volume 4: Water and Steam Systems Draft of
October 30, 1997 includes recommendations for orbital welding, and
the AWS standards writing group also will reference orbital welding.
Orbital welding has become the accepted joining technology for the
high purity industries, but its suitability for other industries, such as
the food, dairy and beverage industries which have in common the
need to eliminate bacterial growth and minimize corrosion, should be
apparent.
Figure 11. Orbital welds for biopharmaceutical applications are inspected visually on
the OD, while a borescope is used to inspect a specified percentage of welds on the
ID. On the above installation a videoendoscope was used to store video images of
the welds for FDA documentafion. Photo courtesy of Pfizer Animal Health
References
1. ASME Section IX of the Boiler and Pressure Vessel Code.
2. ASME Bioprocessing Equipment Standard (BPE-1997) available from
the American Society of Mechanical Engineers, 22 Law Drive, Box
2900, Fairfield, New Jersey 07007-2900. Tel: 800-843-2763; Fax:
201-882-1717; E-Mail: Infocentral@asme.org.
3. ASME Section VIII.
4. ASME B31.3, Process Piping.
5. ASTM A270, Specification for Seamless and Welded Austenitic
Stainless Steel Sanitary Tubing.
6. Burgardt, P. and C.R. Heiple. Interaction Between Impurities and
Welding Variables in Determining GTA Weld Shape. The Welding
Journal, June, 1986.
7. Burton, C. Effect of Stainless Steel Melt Methods on the Corrosion
Resistance of Welds in UHP Gas Lines. Presented at Semicon West,
San Francisco, California, 1997.
8. Cohen, R. M. Identifying Trace Elements Affecting the Weldability of
316L Stainless Steel Tubing. FUTURE FAB International Volume 2.,
1997.
9. Fihey, J.L., and R. Simoneau. Weld Penetration Variation in GTA
Welding of some 304L Stainless Steels. Presented at the American
Welding Society Conference, Kansas City, MO., 1982.
10. Grant, A., Henon, B.K., and F. Mansfeld. Effects of Purge Gas
Purity and Chelant Passivation on the Corrosion Resistance of Orbitally
Welded 316L Stainless Steel Tubing. Pharmaceutical Engineering,
Jan/Feb, Mar/Apr, 1997.
11. Pollard, B. The Effects of Minor Elements on the Welding
Characteristics of Stainless Steel. The Welding Journal, September,
1988.
12. Young, J. The Engineering of Sterilization and Decontamination.
Presented at the ASME Bioprocess Technology Program, Bioprocess
Equipment Technology, Bioprocess Equipment Design. University of
Virginia, 1992.
This article was adapted from a presentation conducted at the Primer
Simposium Internacional de Soldadura '98. Monterrey, Nuevo Leon,
Mexico. March 14, 1998.
About the Author
Barbara Henon is Manager of Technical Publications at Arc Machines,
Inc. Dr. Henon received her Ph.D. in biological sciences from the
University of Southern California, and did postdoctoral research in
neurophysiology at the Beckman Research Institute of the City of Hope
in Duarte, California. An AWS member since 1988, and a member of
ISPE, she also is active in the ASME and is the former Chairman of the
ASME Bioprocess Engineering Subdivision. Dr. Henon has been a
member of the ASME Bioprocess Equipment Main Committee and
Material Joining Subcommittee since 1989 which, in collaboration with
other subcommittees, recently published the new ASME BPE-1997
Standard for the design and fabrication of bioprocessing equipment
and piping systems.
Her primary responsibility at Arc Machines, Inc. is writing articles on customers'
applications of orbital welding in a variety of industries which have in common a need
for high quality and high purity piping systems. Dr. Henon is experienced in welding
education with 12 years of field experience training orbital welding operators.