High velocity vent in a patented non-oscillating version to ISO 15364:2007 / MSC Circ. 677 and ISO 16852
Mark IV SuperGreen high velocity valve is the result of dedicated
research into creating the ideal compromise between loss of tank vapour and non-oscillating behavior, all in a simple package that
appeals to low maintenance. It features a combination of magnet
power and air breaks, which are unique and patent applied for.
It is truly non-oscillating as it complies with the two second
no-metal-to-metal contact definition from ISO 15364:2007. At the
same time, and this is the real accomplishment, it does not emit
tank vapour to atmosphere unless the set-pressure is reached, at
which point it will only emit excess pressure, unless a true
emergency is happening when the primary full capacity unit will open.
Normally, the blow-down (difference between
opening and re-seating pressure) must be larger than the pressure drop
over the vent line, which can render the design useless from the point
of a VOC Management Plan as huge volumes of cargo will boil-off and be
emitted for no purpose to atmosphere. INTERTANKO and others have
estimated the VOC loss on the crude carrier to equal 0.25% of the cargo.
The Mark IV SuperGreen can reduce VOC loss for all practical purposes to nil during voyage.
The vent features two discs: a small spring loaded one for thermal
variation. It creates a rising pressure in level with rising capacity,
i.e., it is a modulating valve. It will only emit what's necessary in
order to maintain tank pressure at or just above it's set-pressure.
The main valve, however, is a full-lifting type due to its magnet
characteristics. To render it non-oscillating and without a drastic
blow-down, it features a novel air break. As an example, it will open
full at set-pressure 0.21 bar and re-seat at 0.15 bar. A comparative
competitor valve for the same pipe length would not re-seat until the
tank pressure is down at 0.10 bar and that's what would happen every
time the competitor valve opens.
From a practical point of view, the valve's wear and tear is for
all practical purposes limited to the small thermal valve, which has a
stroke of 1-2 mm and a moving weight of a few kg. In other words: seat/disc
wear and pitting is reduced to a mere fraction of the usual level, where
grinding/replacement can be necessary every 1-2 years. When overhaul is
necessary, the entire inside trim can be lifted out in one lot, replaced,
and the crew can easily remove seats and discs for repair or replacement.
There is no need to involve the factory or to use expensive service engineers.
Main features of type Mark IV SuperGreen:
● Complies with the Marine Equipment Directive Amendment 9 (EN 12874/ISO 15364:2007 and ISO 16852)
● Approved for long vent pipes as non-oscillating (2 second requirement)
● Free from any blow-down during voyage, i.e., ideal VOC Management Plan contribution
● Only one moving part
● Simple, rigid design and construction
● Low maintenance cost - maintenance done by crew onboard
● Full bore clearance through valve body
How to benefit from SCANVENT’s dual nozzle design
Most importantly it must be stressed that leakage and VOC loss are two very different issues:
Definition of leakage rate:
All metal-to-metal seat/disc systems suffer a certain leakage rate.
That’s the unpleasant reality and why ISO 15364:2007 for marine p/v
valves Section 6.1 reads:
“The maximum gas leakage rate shall be
provided and expressed as the volume in standard air that may leak
from the valve at 80% of the nominal setting.”
A metal-to-metal seat/disc is so sensitive that a human hair
will cause a leakage rate that can be felt at the tip
of your finger. Obviously, as hardened sot and other particles pass
and as the seat and disc slam against each other, perfect alignment
is vanishing and the leakage rate will gradually increase.
When SCANVENT staff first worked with leakage rates some 20 years
ago, this resulted in portable test rigs and test procedures.
Resilient seals were brought into the picture to reduce leakage
rates, but when spherical seats/discs came some 10 years ago, the
leakage rates were dramatically reduced and things went back to
metal-to-metal arrangements. The following procedure is applicable
for the most used valve type with a spherical arrangement and shows
an ISO 15364 Section 6.1 leakage rate of 2 liters/min from a VLCC
Definition of VOC loss rate:
If the venting system does not operate during voyage, the total
volume released will of course equal the leakage rate, but that’s
VOC loss in question. Industry studies headed by Intertanko suggest a
total VOC loss rate during voyage of 0.25% of the cargo. This is
not the 2 liters stray emission mentioned above! All full-lifting valves have a
blow-down, i.e., the difference between opening and closing pressure.
Due to the requirements for non-oscillating performance, the blow-down
must be quite large and is often 50-70% of the opening setting.
Every time such a valve opens, because of thermal expansion or
sloshing, the ullage space pressure is reduced accordingly bringing
the pressure below the vapor pressure of volatile gases such as Methane
(a super ozone-depleting gas many times worse than CO2). Then the cycle
repeats itself. The same happens when a mast riser valve is opened too
early and/or shut too late.
What the SCANVENT design brings:
The secondary valve will release over-pressure only; it has no blow-down.
Its capacity is relatively small for in-voyage
venting needs only. Put differently, the tank pressure never reach the
set-point of the primary valve and therefore de-pressurization of
the ullage space does not take place.
The VOC Management Plan can state: “No VOC release below the VOC valve’s
set-point takes place during voyage and therefore the in-voyage VOC handling
cannot be improved as it equals only what must be released for the
integrity and safety of the ship.”
Now, having once and for all solved the VOC issue on a no-cost-no-brainer
basis, the bottleneck moves on to the unavoidable leakage rate from big bore
valves. The SCANVENT design utilizes a standard spherical seat/disc arrangement and
therefore the leakage rate is what the industry standard must be assumed
to be. However, bearing in mind that the primary valve functions only under
special conditions and is not fluttering during voyage, we
have gone back to the 20 year old resilient seal system. The result is amazing,
and a “must have” considering the low cost.
The two video sequences to the right show the unavoidable leakage rate from
the industry standard spherical seat/disc arrangement and for the same but with
our resilient seal. The lifetime
of the seal is impressive because the o-ring is a static installation: the
weight of the disc is still carried by the metal seat and does not compress the o-ring.
The o-ring is a readily available industry standard in NBR or Viton and is
easily replaced when needed, for instance when the valve is cleaned, and a
safety pin can be inserted to allow one crew member to perform the replacement while
doing the regular cleaning.