Reference Manual

1−13
D Proper alignment of the valve stem or shaft
within the bonnet bore.
D Applying a constant packing stress through
Belleville springs.
D Minimizing the number of seal rings to reduce
consolidation, friction, and thermal expansion.
The traditional valve selection process meant
choosing a valve design based upon its pressure
and temperature capabilities as well as its flow
characteristics and material compatibility. Valve
stem packing used in the valve was determined
primarily by the operating temperature in the
packing box area. The available material choices
included PTFE for temperatures below 93°C
(200°F) and graphite for higher temperature
applications.
Today, choosing a valve packing system has
become much more complex due to the number of
considerations one must take into account. For
example, emissions control requirements, such as
those imposed by the Clean Air Act within the
United States and by other regulatory bodies,
place tighter restrictions on sealing performance.
Constant demands for improved process output
mean that the valve packing system must not
hinder valve performance. Also, today’s trend
toward extended maintenance schedules dictates
that valve packing systems provide the required
sealing over longer periods.
In addition, end user specifications that have
become de facto standards, as well as standards
organizations specifications, are used by
customers to place stringent fugitive emissions
leakage requirements and testing guidelines on
process control equipment vendors. Emerson
Process Management and its observance of
limiting fugitive emissions is evident by its reliable
valve sealing (packing and gasket) technologies,
global emissions testing procedures, and
emissions compliance approvals.
Given the wide variety of valve applications and
service conditions within industry, these variables
(sealing ability, operating friction levels, operating
life) are difficult to quantify and compare. A proper
understanding requires a clarification of trade
names.
Figure 1-23. Single PTFE V-Ring Packing
A6161-1
Single PTFE V-Ring Packing (Fig.
1-23)
The single PTFE V-ring arrangement uses a coil
spring between the packing and packing follower.
It meets the 100 ppmv criteria, assuming that the
pressure does not exceed 20.7 bar (300 psi) and
the temperature is between −18°C and 93°C (0°F
and 200°F). It offers excellent sealing performance
with the lowest operating friction.
ENVIRO-SEAL PTFE Packing
(Fig. 1-24)
The ENVIRO-SEAL PTFE packing system is an
advanced packing method that utilizes a compact,
live-load spring design suited to environmental
applications up to 51.7 bar and 232°C (750 psi
and 450°F). While it most typically is thought of as
an emission-reducing packing system,
ENVIRO-SEAL PTFE packing is, also, well-suited
for non-environmental applications involving high
temperatures and pressures, yielding the benefit of
longer, ongoing service life.
ENVIRO-SEAL Duplex Packing
(Fig. 1-25)
This special packing system provides the
capabilities of both PTFE and graphite
components to yield a low friction, low emission,
fire-tested solution (API Standard 589) for
applications with process temperatures up to
232°C (450°F).