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Reprinted from Chemical Engineering Progress
By Rich Greene.
Pharmaceutical Plants: Choose Your Material
If you've ever opened a faucet at home to find that the water coming out
had a brown color, then you know part of the problem — rust from the
iron in your pipes. While this is bothersome at home, it is unacceptable
at a pharmaceutical plant, particularly in a purified-water line.
Pharmaceutical plants have been traditionally using Type 316L stainless
steel (SS) as the preferred material-of-construction. Since Type 316L
SS contains about 70% iron, it can rust under aggressive conditions.
(The "L" is for low carbon (< 0.03%), needed to help prevent
intergranular attack.) In high-purity water systems, deposits on stainless
surfaces are thin and are referred to as "rouge" due to their
reddish-brown hue. However, rouge is only part of the problem. One
way to eliminate rouge is by using plastics instead of stainless steel.
Selecting the most suitable material of construction for pharmaceutical
pipes, equipment and other systems is a complex issue that has
aroused debate among those in the pharmaceutical community. To
help clarify this issue, the New Jersey section of the International
Society of Pharmaceutical Engineers (ISPE; Tampa, FL; www.ispe.org)
held a forum on plastics vs. stainless steel at its meeting in Somerset,
NJ (June 11, 2002). "Choosing the optimum material is far from easy,"
says George Black, a communications consultant who organized the
forum. "A similar problem was faced by the semiconductor industry
more than 10 years ago. The decision that pharmaceutical companies
must make is more complex," he says.
For instance, the water-quality standards required for the process fluids
used in the manufacture of sophisticated electronic equipment could
readily be judged by performance-testing the units. "With performance
as the judgment criterion, the increased benefits offered by the
thermoplastic materials were easy to evaluate and justify in terms of
quality and cost," says Black. "Plastics won this battle because they
proved cost-effective and met or exceeded equipment test standards."
The effect of impurities in pharmaceutical water is another story. Every
change in the manufacturing procedure or equipment that might affect
the purity of water for ingestion or injection in a human being has to be
validated before use. This validation requires extensive testing beyond
Designers of pharmaceutical and biotechnology manufacturing and
processing systems must overcome more than just a natural resistance
to change. They need to address problems such as getting
internal-management approval, justifying capital costs involved in
making the change, securing validation, and dealing with the potential
liability, if these changes affect the health of their customers. As
Shakespeare so aptly put it, "There's the rub."
Will the pharmaceutical industry do what the semiconductor
manufacturers did — and switch to plastics? The choice will depend on
the material's ability to resist leaching, withstand heat, and show good
structural integrity. But by far, the key issue is avoiding contamination
(see the sidebar).
The culprits behind rouge are dissolved iron and oxygen. Elemental iron
(Fe+…) is insoluble in water, but oxygen converts it into Fe+†, which
precipitates as either Fe•O– or Fe(OH)–. One gram of iron will cover an area
of 65 x 65 ft… (6.04 m… x 6.04 m…), three atoms deep. This is a huge surface.
Oxygen can enter a system through seals, such as on a pump impeller or by
diffusion through plastics, which are permeable to gases.
Rouging in water-for-injection (WFI) systems can occur due to active
corrosion, such as on non-stainless steel components. But far more
frequently, rouging results from simple oxidation of trace quantities of
dissolved iron. Engineers who are familiar with water treatment will recognize
this as aeration. "It may be far easier to prevent aeration and the resulting
precipitation than to eliminate trace iron from process streams and systems,"
says ISPE forum speaker David O'Donnell, manager, technical services, for
Rath Manufacturing Co. (Janesville, WI; www.rathmfg.com). "While plastics
certainly are iron-free, if there are other components somewhere in a system
that are made of steel, there still can be a problem," he says. "Even some
'high-end' plastics are actually permeable to gases (e.g. O•, N•, He, H•, CO•)
and many solvents. It is this permeability that leads over time to blistering
behind plastic-lined items," says O'Donnell.
In general, there are three types of rouge. Type 1 is due to the
dissolution of steel, such as is found on a pump impeller. Type 2 results
from active corrosion and Type 3 is due to high temperatures.
Passivation is a technique commonly used by pharmaceutical
companies to rid a system of rouge. "Passivation cleans out rouge, but
it doesn't prevent Types 1 and 3 from happening again. If oxygen
permeates the system, rouging can recur," says O'Donnell.
Type 2 is often the result of improper welding techniques that leave a
heat-affected zone (HAZ) near the weld. Type 2 rouge strikes the HAZ,
not the weld itself. "During fabrication, manual field welding should not
be allowed," says O'Donnell. "Use automatic orbital welding whenever
possible to prevent heat tints from forming. Follow this with
post-passivation acid treatment and then neutralize with an alkaline
rinse," he recommended.
The 300 series stainless steels are sensitive to chlorides at a pH of 6.5-8
and at temperatures less than 140°F (60°C). Although Type 316L
tolerates about 1,000 mg/L of chlorides, care must be taken in wet-dry
zones, where concentrations can reach 26,000 ppm in the worst case
(for magnesium chloride).
Still, stainless steel hasn't become the historic material-of-choice
because it presents problems. It does have numerous positive
attributes. It is impermeable to oxygen, other gases and solvents. It
has 10 times the thermal conductivity of plastics, which makes it well
suited for heat-transfer surfaces. Stainless steel is strong, too. Its yield
and tensile stresses are at least 10 times those of thermoplastics, and it
requires about 800°F (427°C) to initiate creep. Many plastics creep at
room temperature. Therefore, plastic piping must be supported
properly to ensure its integrity. Steel pressure ratings are generally 20
times higher than those for plastic. "In practice, a 1 in. O.D. 300-series
stainless-steel pipe can withstand 1,600 psi (11,032 kPa), while a similar
plastic pipe can tolerate a maximum pressure of 75 psig (5.17 bar), but
only at room temperature. At 200°F (93°C), plastic loses half of its
strength," says O'Donnell.
PROPERTIES OF PLASTICS VARY
While Type 316L has basically one set of properties, those of plastics
vary from type to type. Improved properties generally come at a price,
so the less-costly plastics, such as polyvinyl chloride (PVC) and
polypropylene (PP), generally do not perform as well as the
fluoropolymers, namely polyvinylidene fluoride (PVDF), Type 316L's
main competitor in pure-water systems. PVDF melts at 352°F (178°C),
allowing it to be steam-sterilized, but rigid PVC starts to decompose
near the boiling point of water, making sterilization possible only by
chemical means. "PVC and PP must be sterilized with hydrogen
peroxide or chlorine, which both require a rinse afterward. PVDF not
only tolerates steam, but also withstands sterilization by ozone. Ozone
has a half-life that is measured in minutes, so no post-sterilization
cleaning is needed," says Gary Dennis, worldwide market manager,
technical polymers for Atofina Chemicals (Philadelphia, PA; www.
atofina.com). Still, PVC and PP are inexpensive and are common in
chemical process industries (CPI) plants.
COST VS. PERFORMANCE
Fluoropolymers are available that can outperform PVDF in temperature
resistance, such as perfluoroalkoxy (PFA) resin, which can be used at
operating temperatures to 500°F (260°C). But these formulations are
expensive and usually exceed the needs of a pharmaceutical plant.
"PVDF is among the hardest fluoropolymers and has among the highest
tensile strength in this family of plastics. It even has better abrasion
resistance, often referred to as particulation, than stainless steel," says
Dennis. Particulation is measured by the amount of polymer abraded
from a surface by a rotating wheel. PVDF's particulation is 5-10
mg/1,000 cycles of rotation, while that for SS is about 50 mg/1,000
Fluoropolymers have excellent chemical resistance including to
deionized water, high thermal stability and they resist degradation by
sunlight. Also, since they have low coefficients of slip, microorganisms
(notably, fungi and bacteria) generally do not grow on them. On the
contrary, metal surfaces cannot easily be smoothed out to a degree that
can compete with plastics. Microbial-induced corrosion (MIC) is not
uncommon in chemical plants, (i.e., in heat exchangers), and can take
place on micropolished stainless-steel surfaces in pharmaceutical
facilities. In chemical plants, biocides can be added to process water to
prevent MIC in cooling towers and heat exchangers. Not so in an
ultrapure water system.
Although quite smooth, polymer surfaces can contain molecules that
can leach out into water streams. In its virgin form, PVDF is highly pure
and contains no additives. Thus, nothing will leach out. But polymers
such as PVC and PP can have additives. These include plasticizers, heat
stabilizers and flame retardants.
PVDF, however, is not an inexpensive plastic, but piping and vessels
fabricated out of it, or lined with it, are said to be about 10% cheaper
than comparable all-stainless systems. Although neither material (PVDF
or stainless steel) is perfect, each has substantial advantages and
proven track records in pharmaceutical plants, as well as in other CPI
installations. "Since pharmaceutical plants traditionally use stainless
steel, switching to plastics isn't going to be easy," says Black. But the
switch may be on.
"Eventually, the semiconductor industry embraced the high purity and
corrosion-resistance advantages of plastics. It is only a matter of time
before the pharmaceutical industry follows suit," says Rick Bolger,
marketing manager for Plast-O-Matic Valves (Cedar Grove, NJ; www.
plastomatic.com). Obviously, some applications will forever be
stainless steel, due to high-pressure and temperature considerations.
But for many applications, change is inevitable. "We find that interest is
growing for thermoplastics in the pharmaceutical community,
particularly for homopolymer PP and a number of fluoropolymers,
including PVDF," says Bolger.
The debate will result in an education process on both sides: The plastic
piping industry has historically been geared toward the needs of the
core CPI segments and toward semiconductor manufacturers. "These
needs don't necessarily apply to pharmaceutical industries," says
Bolger. "It's likely that some new products will arise as a result." By the
same token, the pharmaceutical engineer will need to learn how to
design a plastic system, including understanding the materials and the
numerous joining techniques available. The plastic piping industry is
positioned to solve contamination problems, and the pharmaceutical
engineer is geared toward improving quality and efficiency, so plastics
may make further inroads into this area. Some companies have had
plastic pure-water systems in-place, and have already found success
PLASTICS VS. STAINLESS: THE DEBATE GOES ON
"The question has been raised as to the suitability of sealless,
magnetically-coupled pumps for use in pharmaceutical and other
high-purity water systems. The reason for concern is the potential for
bacteria formation in the gap between the inner magnet and the
containment can. This gap is frequently less than 0.03 in. Current
thermoplastic mag-drive designs, however, offer clearances of 0.09-0.10
in. Further, they provide wide, open fluid passages for the continuous
flow of fresh liquid, as well as drain plugs to assure complete clearing of
the passages within the pump and in the suction piping back to the
shutoff valve." — Dan Besic, chief engineer, Vanton Pump & Equipment
Corp. (Hillside, NJ), 2001 technical seminar.
PVDF and PP plastic systems are inherently superior to stainless steel in
that they are manufactured from 100 % pure resin. Unlike stainless
steel alloys, there is virtually no difference in the chemical composition
of manufactured lots of material. The welding process or chemical
cleaning procedures do not degrade the corrosion resistance of
thermoplastic systems." — Roger Govaert, Asahi/America, and Albert
Leughamer AGRU Kunstsofftechnik, in Ultrapure Water, Dec. 2001.
"Both plastics and metal have logical industrial applications. Metals are
favored for larger, industrial systems operating at higher temperatures.
Under these conditions stainless steels and the higher alloys make a lot
"Plastics have very low strength and temperature tolerance relative to
metals. Many WFI systems operate at 180°F (82°C) and use steam
cleaning protocols. Metals tolerate steam cleaning with no difficulty.
Plastics typically soften considerably at such temperatures.
"WFI systems should be designed to minimize oxygen to avoid this.
Avoiding the use of plastics would be one key step, as plastics are
permeable to oxygen. The use of nitrogen blanketing in accumulation
tanks would be another step, as would the use of magnetically-coupled
pumps (no rotating shaft seals exposed to process steam).
"Metals have no significant flow limitations. Plastics suppliers often use
10-12 ft/s (3.048 - 4.587 m/s) as a maximum flow speed, particularly on
plastic-lined items to avoid damage." — Dave O'Donnell, manager,
technical services, Rath Manufacturing Co. (Janesville, WI)
AN EXISTING PVDF INSTALLATION
As evidence of the possible wave of the future, Christ, Ltd. built a
purified water-treatment plant and distribution system for Sulzer
Orthopedics in Winterthur, Switzerland. The plant has been in
operation since 1966. Christ was awarded the contract because it had
successfully built similar pharmaceutical facilities requiring purified
water that met European Pharmacopoeia and other guidelines on sterile
products. High requirements were set in regard to the distribution
system. Design factors included:
• No deadspaces within the system
• Low surface roughness of piping material and welding seams
• Dynamic operation of the system with a high velocity
• Periodic disinfection with ozone.
The complete distribution loop was installed using bead- and
crevice-free, high-purity PVDF piping. The pure-water plant easily and
continuously meets the requirements set by Sulzer Orthopedics in
regard to conductivity, capacity and yield, and the water-purity quality
meets and surpasses the requirements set by United States
Pharmacopeia (USP). For example, the target conductivity for the water
was set at <10 μS/cm and the plant's diluate is below detection limits.
The chloride target was 5.3 ppm, and, it too is below detection limits.
In the 1950, Vanton developed a revolutionary all-plastic pump for use in conjunction with the first heart-lung device. The design limited fluid contact to only two non-metallic parts: a plastic body block and a flexible liner. This was the birth of our Flex-I-Liner rotary pump. Its self-priming sealless design made it an industry standard for the handling of corrosive, abrasive and viscous fluids as well as those that must be transferred without contaminating the product. Vanton now offers the most comprehensive line of thermoplastic pumps in the industry.
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Vanton Pumps (Europe) Ltd.
Unit 4, Royle Park
Congleton CW12 1JJ