Hydrocarbon Processing - November 2021 - 85

Valves, Pumps and
Turbomachinery
D. LEAVITT, Emerson Automation Solutions, Colorado
Springs, Colorado; and J. GREMILLION, Emerson
Automation Solutions, Baton Rouge, Louisiana
How to select the proper valve for reliable
performance in critical/severe service applications
Critical and severe service applications
demand automated valves that perform
reliably under punishing conditions, such
as high temperatures, high cycle rates,
high shutoff pressures, high velocities,
very long required service lives and zeroleakage
requirements. These applications
may handle poisons, corrosives, slurries,
heavy particulates or other very dangerous
or difficult-to-handle process media.
Selecting an automated on-off valve
that operates reliably under these conditions
with these media can be very challenging,
especially if the valve is critical to
plant operations.
While there are several valve technologies
to choose from, a triple offset valve
(TOV) is a good choice for many critical
and severe service applications (FIG. 1),
particularly when zero-leakage performance
is required.
This article will familiarize the reader
with the internal seal design and capabilities
of this valve type, and will show
why it is a good fit for critical and severe
service applications.
Before discussing the valve design
in detail, it is important to clarify some
misconceptions about leakage rates with
metal-seated valves and to define the test
requirements used to verify performance.
Zero-leakage shutoff as defined per
API 598 and ISO 5208 standards.
Many critical and severe service applications
require a bidirectional, zero-leakage
shutoff utilizing metal-to-metal seats. This
is a very difficult requirement to meet, and
the American Petroleum Institute (API)
598 standard does not have a metal-seated
specification for zero leakage, only providing
a zero-leakage specification for resilient
seated valves. To add to this confusion,
terms are used throughout the valve
industry (e.g., bubble tight, drop tight and
leak tight) that technically have no true
shutoff rating or definition. In addition,
many engineers are under the misconception
that Class 4 shutoff is zero leakage. It
is not, as Class 4 is a leakage rate standard
applied to control valves but not to the
shutoff capability of isolation valves.
The International Organization for
Standardization (ISO) 5208 standard
differs in the way it categorizes valve seat
performance standards. ISO 5208 simply
qualifies shutoff within a category, rather
than the type of seat. For bidirectional,
zero-leakage applications, valves should
be qualified to ISO 5208 Rate A.
Some vendors state that their valve is
" tested to API 598, " while failing to mention
which test standard within API 598
was used. The API 598 Resilient Seat test
standard specifies zero bubbles and zero
drops of water during the bidirectional
seat leakage test. Valves successfully passing
this test standard are considered to
have zero leakage. It is this test protocol-API
598 Resilient Seated-that is
usually applied to TOVs, even though
these valves are metal seated.
There are other test protocols within
API 598 for metal-seated valves that have
an allowable leakage rate, and the allowable
leakage increases significantly with
valve size. While these leakage rates may
be acceptable for some processes, they
are unacceptable for many critical and severe
service applications. Processes such
as pump isolation, bypass valves, headers,
emergency block valves and reactor isolation
valves will all place pressure on valves
from either direction, so a bidirectional,
zero-leakage seal is a clear requirement.
Understanding triple offset technology.
TOVs employ three offsets in their
design (FIG. 2). The goal of the design is
to have the valve disc seal meet the mating
body seat with no rubbing until the
components meet and touch simultaneously
over the entire 360° of the seat and
seal. This is accomplished by offsetting
the shaft upstream of the seating surface
and moving the shaft slightly to one side
of the centerline of the pipe. These first
two offsets eliminate any rubbing during
most of the valve travel, and the third offset
ensures performance over the remaining
valve travel.
FIG. 1. A TOV is often the best choice
for critical and severe service applications
requiring zero leakage.
Hydrocarbon Processing | NOVEMBER 2021 85
Hydrocarbon Processing - November 2021

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