In addition to operational advantages
the TOV offers a significant savings in
weight and space, which will have a
positive impact in the overall spending
on valves. The simple rule is that the
larger the size and the higher the rating, the larger the savings a TOV can
offer. With reduced weight, especially
when using exotic materials, the total
cost of ownership can be reduced.
Figure 4. Computational fluid dynamic image of a 3-inch ASME Class 300 valve
For many valves, bubble-tight is adequate, but this fails to recognize that the
valve is leaking. Bubble-tight, in accordance with FCI 70-2 Class V or Class
VI or 598 and API 6D metal-seated
means “allowable leakage.” For example, FCI 70-2 cl.VI for an 8-inch valve
allows 40 bubbles per minute when tested at 3. 5 bar air; 598 leakage is measured in drops per minute (liquid) and
bubbles per minute (air), while 6D and
FCI 70-2 leakage is measured in milliliters per minute.
ADVANTAGES OF A TOV
TOVs offer several advantages compared to other valves typically used in
most upstream applications. Compared
to a soft-seated ball valve, the TOV is
inherently fire safe with a non-rubbing
rotation. No safety factors are required
in actuator sizing and the valve opens
against full delta P (no equalizing pressure is required). It requires less maintenance, offers better control capabilities, and ultimately reduces the space,
weight and cost.
A soft-seated ball valve is also zero
leakage. However, the soft components
are limiting in terms of applicability and,
more importantly, the life expectancy of
the valve. A TOV achieves the same performance with metal-to-metal sealing,
virtually no temperature limitation and a
much more extended product life.
If necessary, a TOV can open against
full-rated differential pressure in all
classes up to ASME Class 1500. This is
not possible with ball valves where it may
be necessary to have a small bypass line
to equalize upstream and downstream
pressure to open the valve. This additional pipe and valves will not only increase
the overall cost of the unit but will add
potential points of leakage and failure.
Because of the weight and space
savings, always an important consideration offshore, installation of TOVs is
easy and simple when compared to ball
valves. The valves are easy to operate
and are an excellent choice for ESD
(emergency shutdown) and HIPPS
(high-integrity pressure protection system) applications. Whatever the
required closing time is, it can be
achieved and maintained for a very
long time with a TOV. TOVs are used as
standard for trip applications by major
This article is focused on the zero
leakage capabilities of TOVs in isolation applications. However, TOVs also
can be used for control applications.
Furthermore, the triple eccentric
design offers advantages over traditional high-performance butterfly
valves. One TOV can be used at the
same time for both control and isolation, replacing the two valves normally
used (butterfly for control and ball for
isolation). Conventional ball valves are
not used in control applications.
While replacement of ball valves in
downstream applications is common,
there is still reluctance to using TOVs
in upstream and offshore applications.
A common perception is that when a
TOV is fully open there is risk of damaging the seal ring because of the high
velocity of the media. However, a computational fluid dynamic image of a 3-
inch ASME Class 300 valve shows that
the valve has the lowest media velocity
near to the seal ring both upstream and
downstream (Figure 4).
Another perception is that when the
TOV is fully open, there may be a considerable pressure drop because of the
presence of the disc and shaft in the
middle of the line. A calculation with
oil at 90 bar and a velocity of 3 meters
per second in a 14-inch ASME Class
600 TOV shows that once the valve is
in the fully open position, the pressure
drop in this particular case would be
only 0.07429 bar.
Triple offset valves are a serious,
robust and reliable alternative to traditional ball valves for many offshore
applications in the upstream oil and
gas segment. TOVs can provide reductions in weight and space and substantial cost savings with medium and
large valves on the topsides of floaters
and platforms, as well as in
FPSO/floating storage and off-take
PIETER KOK is upstream market manager at
Tyco Flow Control ( www.tycoflowcontrol.com).
Reach him at firstname.lastname@example.org.