“Ideal” Manifolds…
Not So Ideal?
By Jeffrey Bozanic, NSS-CDS 181, NSS 22353
Fellow
October 2005
The Benjamin Conversion manifold, or as it is more popularly
called, the dual valve manifold, was utilized for cave diving and other
environments in which regulator redundancy was deemed beneficial throughout the
late 1970s to the mid-1990s. This manifold was a vast improvement over the
pre-existing manifolds of the day, as they allowed two independent regulator
systems to be used on the same set of doubles. Thus, if a regulator failure
occurred (either first or second stage), the diver still had a viable option for
self-rescue from the cave. During this time, cave divers worried about a few
possible failures that could still result in catastrophic gas loss from the
primary gas supply. These included:
- Burst
disk failure
- Sudden, massive failure of one of the cylinder neck o-rings
which seals the manifold
- Loss
of integrity of the manifold itself
These concerns lead to the development of the “Ideal” or
isolation manifold, which allowed the two cylinders to be isolated from each
other, maintaining at least part of the gas in the event of one of the failures
listed above. It was considered a vast improvement, and very quickly replaced
the use of the “unsafe” dual valve manifold. It is the primary manifold used
today for all forms of technical open circuit diving. Yet, my opinion is this
valve does not add safety, rather it significantly reduces it.
During the twenty or so years in which the dual valve
manifold design was in use, there was only one recorded failure of the type
listed above that occurred while diving. This event occurred during a cave dive
while using a Sherwood manifold incorporating a metal-to-metal seal. Immediately
prior to the dive, the double cylinders were accidentally knocked off the
preparation platform. They fell about three feet to the ground, landing on the
manifold. The manifold was closely examined prior to diving, but was not
leaking, and the divers elected to dive. After the cylinder pressure had been
reduced to about 1500 psi, the manifold catastrophically failed, and both divers
exited successfully sharing gas from the remaining rig. It was suggested at the
time that the fall caused a displacement cylinders relative to each other, which
did not manifest itself until the pressure reduction allowed the metal-to-metal
seal to shift and lose integrity.
In addition, in 30 years of accident data collection, there
are two instances of in-water burst disk failure recorded. Both events occurred
with cylinders that had been pressurized beyond the working pressure of the
cylinders (in one case almost to the hydrostatic test pressure!), and occurred
within minutes of the cylinders being placed in the water (prior to cave
penetration, while in a safe environment). Also, in both instances, the burst
disks had not been replaced in many years. It can be hypothesized that the old
disks had metal fatigue from small flexing associated with repetitive filling
and emptying over the years, and failed due to thermal shock when placed into
relatively cold water after being sun warmed on the surface. Cave divers used to
alleviate this risk by double disking or soldering the disks shut, but these are
not recommended procedures. A far better practice is to replace all burst disks
annually. As both of these incidents occurred at the surface, prior to beginning
the dive, an isolation manifold would not have benefited the divers, since they
would have called the dive anyway.
In contrast, since the isolation manifold was introduced in
the early 1990’s, there have been many, many incidents related to misuse of the
manifold. Most of these have been rectified without harm to the divers involved,
but all of them had the potential for very serious consequences. The types of
problems associated with this design of manifold along with representative case
histories include:
- The isolation valve being closed prior to
the dive.
Case #1: This involved a cave
diver who began the dive with 3,000 psi (200 bar) in his doubles. He and his
buddy did a S-drill prior to descending, indicating that both regulators were
working fine. About 15 minutes into the dive, he noted that the pressure on his
SPG was not dropping as expected. He reached up, opened the isolation manifold,
and watched as his pressure dropped from 2,700 psi (180 bar) to 1,700 psi (110
bar). He called the dive, and exited the cave with no further incidents.
What happened was that the diver
was in the practice of always leaving his isolation manifold open. However, when
he had it filled, apparently the fill station operator closed it. Thus, only one
cylinder was being utilized during the dive. The diver using the cylinders did
not check the isolation valve, since it was “always” open. The pressure drop
seen was due to the S-drill usage, BC and drysuit inflation, and cylinder
cooling after being placed into the water.
- The isolation valve being closed during
filling of the cylinders.
Case #2: A cave diver planned a
nitrox dive to a depth of 110 ffw. Prior to the dive he analyzed his cylinders
and found that he had EAN32, as expected.
He proceeded to a depth of 50 ffw, whereupon he began to experience
symptoms of CNS oxygen toxicity. He immediately began sharing gas from his
buddy, and aborted the dive.
After examining the cylinders on
the surface, the team found EAN32 in one cylinder, and 100% oxygen in the
second. Apparently, at some time during the blending process, the isolation
valve was shut, resulting in only one cylinder being properly prepared. This was
the cylinder that was analyzed, and so everything appeared normal prior to the
dive. At no time prior to the dive did the diver check the isolation valve.
- Roll off of the left manifold valve.
Case #3: A cave diver swimming
through a tight cave passage experienced a sudden failure of his gas supply. He
switched regulators, and aborted the dive. After surfacing, he found that left
manifold valve was closed. It had been open prior to the dive, as evidenced by
his utilization of that regulator for the entire period up to the sudden supply
failure. His forward movement through the overhead environment resulted in the
“auto-shutdown” of the valve, as the hand wheel turned shut off as it scraped
across the ceiling.
These failures are only representative of those in the files,
and related to me anecdotally from other sources. Cases like this are very
numerous, and any of them could have resulted in a fatality. In my opinion, it
is only a matter of time until one does.
One might argue that these incidents did not need to occur,
and that it was the divers’ fault for not checking the isolation valve prior to
their dives. I do not disagree with this. However, when a piece of equipment
opens itself up to a multitude of cases of “pilot error,” while not providing
any concrete improvement in other areas of safety, then the net result is one of
additional risk with a commensurate reduction in safety. For this reason, and
the history of misuse of the manifolds in the field, my belief is that we should
go back to using the standard dual valve manifold of the 1980’s or adopt another
type of technology.
NOTE: This is one of a series of articles planned for Underwater Speleology, NACD News, and other journals of
interest to the technical diving community which will discuss findings from the
combined accident analysis files collected by the cave diving community.
About the Author:
Jeffrey Bozanic
P.O. Box 3448
Huntington Beach, CA
92605-3448
(714) 775-4462
E-mail: JBozanic@HQonline.net
Jeff was certified as a NAUI Instructor in 1978, and for the NSS-CDS in 1983.
He is certified to teach diving for the NSS-CDS, IANTD, TDI, and NAUI. Jeff is
active in teaching cave, rebreather, nitrox, technical nitrox, and trimix diving
courses. Together with his wife, Rebekah, he has maintained the combined
accident files for the cave diving community (a joint project of the NSS-CDS,
NACD, and IUCRR). He has published extensively on diving education topics, with
heavy emphasis on cave diving safety techniques. He has edited/reviewed many
diving textbooks, and is the author of Mastering Rebreathers. He has served on
several Boards of Directors in the diving community, including as Chairman of
the NSS-CDS and as Vice Chairman of NAUI, and as Treasurer on the AAUS Board.
Jeff has received the NAUI Outstanding and Continuing Service Awards; the Silver
Wakulla, Abe Davis, Henry Nicholson, and International Safe Cave Diving Awards;
the SSI Platinum Pro 5000 Award, and is a NAUI Hall of Honor inductee.
Article Copyright
2005 Jeffrey Bozanic, All Rights Reserved
