UNDERSTANDING PLASTICS
Here’s how the test applies to the
design of a plastic valve or valve body:
You start with the requirement for operating use. If the manufacturer wants a
warranty of at least 10,000 hours of
use, you go back to the RTI rating for
the appropriate mechanical property
and find the point that gives 10,000
hours of use while still maintaining 50%
of the original property value. You then
design in a safety factor of two to three
times.
Another important thermal property
is the Deflection Temperature Under
Load (DTUL). This property is used to
determine short-term heat resistance,
and it helps to distinguish between
materials that can support a load at elevated temperatures. For a valve component or the valve itself, it’s essential to
assess the load level along with the
operating temperature before making a
material selection.
WATER AND CHEMICAL
RESISTANCE
The hydrolytic and chemical environments also need to be defined for valve
applications. Because valves regulate
the flow of so many different types of
fluids, understanding their chemistry is
vital for selecting the right materials,
whether those materials are metal or
plastic. Hydrolytic stability is particularly important because water has very
aggressive effects on materials. Many
engineering plastics look good on paper
but quickly break down when exposed to
water.
If a valve operating environment
includes water, information needs to be
obtained from the engineering plastics
supplier on the effects of hydrolysis. In
addition, data such as flowing water
versus stagnant water needs to be examined. Also, without proper design, the
combination of flow and increasing temperature can lead to premature parts
failure. Internal components can very
well be subjected to oxidative attacks by
chlorinated water, a fact that is often
discovered too late.
MANAGING PART STRESS
Design engineers with considerable metals experience can overlook the effects
Figure 2. Semi-crystalline and amorphous
thermoplastics
of creep on plastic parts, particularly at
elevated temperatures. Creep is the tendency of a material to deform when
stress is applied over a long period of
time. It can significantly affect long-term performance of plastic components, especially those components
operating under high stress or heat.
Creep testing measures strain as a
function of time at a constant temperature and constant load. The slope of the
curve is the creep deformation rate of
the material. Figure 3 shows that even
though the short-term properties of different plastics may be similar, they can
react very differently to stress applied
over time.
Creep rupture testing is similar to
creep testing except that higher stresses
are used to determine the time needed
to cause failure. Data is plotted to generate creep rupture curves at different
temperatures. This information can then
be extrapolated to predict long-term
hydrostatic strength (LTHS).
Creep rupture testing also can determine limitations due to hydrolytic and
Figure 3. Creep resistance over time
oxidative stability. Significant changes
in the slope of the creep rupture line can
occur through structural changes in the
material, and these changes indicate
upper limits have been reached. This can
result in different design considerations.
Based on LTHS values, engineers can
determine appropriate design stresses
and safety margins for pressurized
applications.
PLASTICS’ FUTURE ROLE
When considering engineering plastics
for a valve application, it is important to
partner with material suppliers. Be sure
to understand test methods and review
what the results mean with that supplier.
Review the chemical environment and
the potential effects of moisture. Confirm operating environment temperatures. Since engineering plastics’
mechanical properties are time- and
temperature-dependent, no one should
rely on single point data or data sheets
alone. Perform mechanical analysis over
a temperature range and range of times,
and be sure to consider stresses in parts
operation. Addressing such issues goes a
long way toward creating a successful
valve design with engineering plastics.
Plastics have played an increasingly
larger role in the design and manufacture of valves. Their overall performance and reduced cost have made them
a highly attractive alternative in the last
quarter century over traditional materials like brass and bronze. In particular,
engineered thermoplastics, which boast
higher performance, have made strong
inroads and will continue to penetrate
the market because of their exceptionally high strength, light weight and corrosion resistance. Also, their broad property profile and time-tested use have
enabled users to effectively meet the
stringent requirements of the valve
industry. VM
PATRICK NEEL is a senior business development
representative for Solvay Advanced Polymers, LLC
( www.solvayadvancedpolymers.com), a supplier of
high-performance thermoplastics, based in
Alpharetta, GA. Neel has more than 20 years of
experience in R&D and application development in
the engineering resins field. He is a member of the
Society of Plastics Engineers and ASTM. Reach
him at 704.752.9538 or pat.neel@solvay.com.
