THERMOPLASTIC LINED PIPE
FOR CORROSIVE CHEMICAL APPLICATIONS
ABSTRACT
Pipe lined with thermoplastic resins have been
successful in overcoming the corrosion problems of a wide range
of applications encountered in the chemical, petrochemical,
and semiconductor industries. This paper discusses the use of
PP, PVDF, ECTFE, and PTFE lined pipe in these industries. Applications
for these materials and their limitations are discussed.
INTRODUCTION
The first plastic lined pipe appeared in the U.
S. in the late 1940's. The first material used was Saran (PVDC).
As more demanding applications developed with higher temperatures
and more corrosive chemicals other materials such as chlorinated-polyether,
polypropylene (PP), polyvinylidene fluoride (PVDF), ethylene-chlorotrifluoroethylene
(ECTFE), and perfluoro alkoxy (PFA) appeared and found their
area of application.
Advantages of Plastic Lined Pipe
Plastic lined pipe has the following advantages:
1. Plastic lined pipe represents one of the most
widely used forms of a composite structure. The mechanical properties
and low cost of carbon steel are combined with the chemical
resistance of the plastic lining material.
2. The properties of the liner material such as
chemical resistance and continuous service temperature can be
readily tailored to the specific conditions of the application
while still maintaining the use of low cost steel for the mechanical
and impact properties.
3. The excellent release properties and non-fouling
properties of some plastic materials reduce fouling, the resulting
pumping energy losses, and required downtime for clean out.
4. Plastic lined pipe is safer from a mechanical
abuse standpoint than self-supported plastic pipe. Self-supported
plastic is not permitted in critical applications by some chemical
companies for safety reasons.
5. Plastic pipe, either lined or self-supported,
results in improved product purity due to the lack of metallic
contamination. This is of particular importance for the semiconductor,
pharmaceutical, and fine chemical industries.
6. Plastic lined pipe is not sensitive to changes
in chemical resistance due to process changes from oxidizing
to reducing atmosphere as some of the metals are.
7. Plastic pipe, lined or self-supported, is not
subject to crevice corrosion or chloride pitting as some of
the metals are.
8. Significant cost/performance benefits exist
for plastic lined pipe compared to corrosion resistant metal
pipe, especially considering fully installed cost.
9. Most plastic lined pipe has a built-in gasket
eliminating the need for a separate gasket to prevent leaks,
simplifying installation and maintenance. The gasketed surface
has the same chemical resistance as the plastic liner.
10. Plastic lined pipe can be field installed
faster than metal pipe with crews that are properly trained.
Disadvantages of Plastic Lined Pipe
The disadvantages of plastic lined pipe are as
follows:
1. The current maximum continuous use temperature
of plastic lined pipe is <500°F.
2. The maximum chemical resistance for plastic
lined pipe is on the protected inner surface. External corrosion
can occur in poorly maintained plants due to poor housekeeping.
3. Creep and permeation are factors for consideration
in the selection of a plastic liner material. These two factors
are not normally encountered with metal piping.
4. Abrasion resistance of some plastic materials
is inferior to metals. Temperature and solids content of the
process fluid can significantly affect performance.
5. Plastic lined pipe is more sensitive to non-uniform
heat tracing (hot spots) than is metal pipe.
6. Maintenance personnel need to be properly trained
as to the proper techniques for field fabrication of non-standard
lengths of plastic lined pipe.
The performance advantages of plastic lined pipe
far outweigh the disadvantages
DESIRED PROPERTIES OF A PLASTIC LINER MATERIAL
1. Easily processed into plastic lined pipes,
valves, and plastic lined fittings to permit the supplying of
a complete corrosion resistant system.
2. Broad use temperature range.
3. Excellent permeation resistance.
4. Low coefficient of expansion.
5. Low creep characteristics.
6. Good abrasion resistance.
7. Excellent stress crack resistance.
8. Excellent chemical and solvent resistance.
9. Low cost per cubic inch.
ENGINEERING CONSIDERATIONS
Plastic lined pipe is made in the U S to meet
ASTM F1545. This industry specification currently covers plastic
lined pipe and plastic lined fittings. This specification covers:
1. Material properties.
2. Pipe and fitting characteristics including
welds, finish, and gasket requirements.
3. Dimensions and tolerances.
4. Testing and inspection.
5. Venting requirements.
6. Workmanship.
The U. S. plastic lined pipe industry, through
this specification and the committee working on their revision
and updating, is attempting to furnish you, the end user, with
a reliable, cost effective product. Plastic lined pipe can be
fabricated with either a loose or locked in liner. A locked
in liner reduces the effect of the difference in expansion coefficient
between the metal and the plastic liner. A loose liner, when
extensively thermal cycled, can result in cracking at the flange
surface since that is the only place where the liner is restrained.
Loose liners tend to roll back and forth at the flanges when
heated to an elevated temperature due to the differences in
expansion between the metal and plastic liner. Loose liners
tend to collapse when operated under high vacuum at elevated
temperatures. In addition, loose liners provide a space for
gas build-up between the liner and the pipe resulting in corrosion
and liner collapse. Weep holes can also create problems of structural
corrosion of the metal pipe. Weep holes are required with PTFE,
FEP, and PFA due to their poor permeation characteristics.
LINER THICKNESS
Plastic liner thicknesses are typically in the
range of .1-.3 inch. The liner thickness represents a compromise
between cost, permeation, cold flow, service life, and vacuum
performance. A thicker liner will result in lower permeation,
longer service life, and improved vacuum performance. Cold flow
increases with thickness and so, obviously does cost.
LENGTH AND DIAMETER
Standard plastic lined pipe is available in the
U. S. in diameters up to 24 inches and lengths of 10 and 20
feet. Lengths longer than 20 feet are not available due to fabrication
and thermal expansion problems.
JACKETING MATERIAL
Plastic lined pipe is commonly sold with carbon
steel as the jacketing material. However, ductile iron, 304SS
and 316SS are available as a means of controlling external corrosion.
The use of carbon steel is the reason for the -20°F lower limit
for the use of plastic lined pipe. As you may be aware, carbon
steel pipe becomes brittle at temperatures below -20°F. The
use of stainless steel pipe jackets or other non-ferrous metals
permits plastic lined pipe to be used at lower temperatures.
PLASTIC LINED FITTINGS
For plastic lined pipe to be attractive and viable,
a total system is required including tees, elbows, crosses,
standard reducing and special configurations. The method of
fabrication of the plastic lined fittings is key to the total
system reliability. The ASTM specifies that no welding is permitted
on the plastic lined fitting after the liner is molded into
place. Some of the plastic lined fittings on the market do not
comply with this part of the ASTM with a resulting loss in reliability.
PRESSURE AND VACUUM LIMITATIONS
Plastic lined pipe is available in 150 and 300-pound
pressure rated pipe. Higher-pressure pipe is available on special
order. Bonded plastic lined pipe can operate at full vacuum.
Vacuum collapse capability of loose plastic lined pipe is dependent
on liner thickness.
7. When the heated liner is soft and pliable to
the touch it is ready to flare. The heated flaring die is then
positioned onto the heated liner and hydraulic force applied to
flare the plastic liner.
End covers should not be removed until plastic
lined pipe is ready to be bolted into position or sealing
faces may become damaged or distorted. If covers are removed
for inspection, they should be replaced as soon thereafter
as possible. Never use heat or a chisel to remove bolts as
the plastic face may be damaged.
Gaskets are not required with Fusibond piping
except when connected to a flange face of another material.
Gaskets of the elastomeric type are usually most suitable
and should always be approximately the same ID as the pipe.
Bolts should be tightened using specified torques
as a guide. Use the criss-cross method shown; never clock
or counter-clockwise. Installation in cold weather may require
higher bolt torques.
When assembling plastic lined piping, always
use new nuts and bolts. Threads must be cleaned and lubricated
and washers are suggested under the turned element (head or
nut). Bolts should be cross-torqued evenly.
NO WELDING OR FLAME CUTTING should be
done close to the metal pipe or fittings unless adequate precautions
are taken to prevent their being exposed to excessive heat.Vent
holes should not be plugged with paint, cement, etc. The vent
holes are necessary only with PTFE/PFA to release gases which
may be generated at elevated temperatures and become trapped
between the liner and housing. If not vented, these gases
may collapse the liner. Vent holes are placed under the flanges
for safety. This is essential when flange covers are used
with hazardous materials.
To preserve the sealing faces and protect them
from damage while not in use, end covers or blind flanges
should always be installed immediately on all plastic lined
piping items that are removed from service.
DESIGN AND FABRICATION DEFECTS
As the old saying goes, "a chain is only as
strong as its weakest link". The performance and longevity
of a plastic lined pipe system is only as good as the design
and fabrication details that went into its construction. Common
flaws encountered in commercially available plastic lined
pipe and plastic lined fittings are as follows with the indicated
effects:
1. Sharp edges, non-radius surfaces-points of
stress concentration with a resulting tendency to stress crack
on thermal cycling.
2. Welding after liner insertion-severe damage
to the plastic liner.
3. Poor weld penetration, usually 50% or less-resulting
in poor mechanical reliability of the fitting or pipe coupled
with thermal damage to the liner.
4. Thin or non-uniform lining-permeation occurs
at the thinnest spot in the lining
5. Highly stressed lining-tends to either cold
flow or stress crack.
6. Foreign particles in plastic liner-need for
ongoing QC.
7. Non-bonded liner-tends to cold flow and collapse
under high vacuum or gas "bulge" due to permeation.
8. Non-tested liner-supplier should perform
final QC on all lined components and not rely on statistical
acceptability.
9. "Mechanical" picked lining-reduces the effective
liner thickness due to the metal projections embedded into
the liner.
10. Sharp projections, blow holes in pipe or
fitting-quality of the lined pipe or fitting is only as good
as the metal part that it is being inserted into.
In the event of repeated field failures an end
user should consider sectioning of the failed fitting to determine
the cause of failure. Continued replacement is not always
the answer.
TESTING AND QC PROCEDURES
The procedures used to furnish quality assurance
by this company are as follows:
1. All pipe and fittings are visually inspected
for any imperfections prior to lining.
2. Interior of pipe and fittings are smooth,
clean and free of burrs or any other imperfections. All corners
in contact with the lining are radiused and the welds ground
smooth.
3. All welding operations are performed prior
to lining.
4. After lining, all pipe and fittings must
individually pass a minimum of 25,000-volt non-destructive
electrostatic spark test.
5. That portion of the lining forming the flange
gasket-sealing surface must be free of scratches, dents or
any defect measuring greater than 10% of the lining thickness.
6. After thorough inspection, all fittings and
pipe spools must have the plastic raised face protected with
minimum ½" thick plywood end covers bolted in place. The covers
should not be removed until the pipe or fitting is ready for
installation. If protective covers are removed for inspection,
they should be replaced as soon thereafter as possible.
APPLICATIONS FOR PLASTIC LINED PIPE
PVDC (Saran) was the first plastic lined piping
material introduced to the marketplace. It found a niche in
the steel industry in acid pickling handling hydrochloric
and sulfuric acids. Deionized water service for the food,
pharmaceutical, and semiconductor industries were also areas
of application. As temperatures and performance requirements
increased, some of these applications declined.
PP was the next plastic lined piping material
introduced. It has taken some of the applications for PVDC.
It is now used in the steel industry for acid pickling at
temperatures too high for PVDC. Food processing, handling
materials such as fructose and other syrups, is an area of
application for PP. Water and wastewater treatment is a large
area of application for PP.
PVDF is much more chemically resistant than
the two preceding materials. PVDF is widely used to handle
aggressive chemicals such as strong acids, chlorine, and bromine.
It is widely used at this time in the semiconductor industry
to handle high purity water and process chemicals where both
chemical resistance and non-contaminating characteristics
are important. PVDF is also widely used in the pulp and paper
industries.
ECTFE is being used in applications similar
to those with PVDF where higher temperatures or concentrations
make PVDF marginal. ECTFE is used to handle chlorine, caustic,
hypochlorite, and sulfuric acid. It is being used for these
applications in the chlorine, pulp and paper, and chemical
industries.
PTFE and PFA are being used to handle a wide
range of chemicals generally above 300° F. Industries using
these materials include chemical, petrochemical, food, pulp
and paper, chlorine, and semiconductor.
SUMMARY AND CONCLUSIONS
Commercial plastic lined pipe has been available
in the U.S. since 1940. A range of materials is now available
to cover temperatures up to 500° F handling a wide range of
chemicals. Plastic lined pipe is cost and performance competitive
with stainless steels and high nickel alloys. Additional advantages
for plastic lined pipe include improved product purity due
to the lack of metallic contamination, which is of particular
importance in the semiconductor, pharmaceutical, and fine
chemical industries. .
