PIPELINE CLEANING AND BLOWING
ON CHEMICAL TANKERS - CAPT AJIT VADAKAYIL
Always ensure the
medium used for blowing a pipeline is suited for the chemical inside.
While line clearing it
is important to listen , rather than checking static pressure on the gauges.
A large 6 inch diameter
pipeline when blown uphill against the trim will cause the air to skim over the
cargo inside the pipeline , as air tends to find and remains at highest level
and liquid at the lowest level. This is why super stripping is done with small diameter
1.5 inch pipelines where a steady air blow will suffice.
With large diameter
pipelines it is necessary to pressurize the pipeline and release suddenly , to
squirt the liquid off. This is done many times till the flow of liquid past the
opening valve is no longer heard or detected.
Submarine terminals for
annex 1 CPP do not allow chemical tankers to blow air , as air inside the
submerged hose can break it off its underwater moorings and damage it. In such
cases water plug must be given . It is necessary to be prepared for this in
advance.
Lines used for cargo
operations must be cleared and pressure released to prevent solidification, polymerization,
pitting and danger to personnel connecting/ disconnecting blanks and hoses or
taking samples.
After cargo work is
over the ships crew must clear the pipelines. A free flowing cargo like
sulphuric acid if allowed to slosh between port and stbd manifolds at sea
during rolling , can damage the butterfly valve discs. Hence it is important to
clear dead ends too . Heated cargoes will freeze and cause delays. Line
clearance must be planned well.
Many off specification
"First foots" have resulted from contaminates remaining in the ships
cargo line system. It is therefore most important that all aspects of line
cleaning are included in the ship's tank cleaning program and are clearly
defined.
Crossover manifold, one
must ensure that both sides of the manifold are washed out simultaneously with
the tank.
If fitted, drop lines,
restricted gauging pipes and stripping lines must be washed simultaneously with
the tank.
If any of the valves
fitted in the cargo line have plugs, same should be removed. All such valves
should be opened and closed at regular intervals. This assists the wash water
in flushing the valve face and casing.
All drain cocks in the
line should have the dust caps off, and the drain cocks flushed at regular
intervals.
Blind flanges must be
loosened at the end of tank washing to ensure proper cleaning.
Tank vent lines must be
cleaned out during the rough washing as required.
All vapor return lines
and associated equipment must be thoroughly cleaned and dried.
After completion of
tank washing, if circumstances permit: all valves, plugs and drains should be
left in the open position. This allows the line(s) to drain and assists in
drying.
The tank inspection,
after washing, must include all lines and fittings associated with the tank as
outlined in "Tank inspection."
When a secondary
cleaning method is found necessary, the same cleaning method must also be
applied to all tank associated piping, including vapor return equipment.
A program for final
tank preparation can now be carried out (i.e.) fresh water rinsing the tank(s),
steaming the cargo lines, vent lines, drop lines, stripping lines, restricted
gauge pipes and any other fittings required for such preparation.
The tanks should be
fresh water rinsed before line steaming commences.
Line steaming: Check
the steam condensate for color and chlorides; if any contaminants found:
"clear the system."
Connect suitable size
steam hoses to each side of the manifold of the line to be steamed. Steam the
line to the pump stack drain valve, with the pump stack cargo line valve
initially closed.
During the line
steaming, the steam pressure must be sufficient to ensure a flow of steam at
the pump stack drain valve outlet and not just condensate.
All valves and drain
valves including the pump stack valve should be opened and closed at regular
intervals during the line steaming. This assists in removing any entrapped
residues and dissolves chloride salts left by the sea water washing. To prevent
damage to seals, packings, etc., never force a valve which is heated to open or
close. The coefficient of expansion of the seal is different from that of the
casing and if force is used, the seal can be damaged. When a valve does not
properly function when heated, allow the valve to cool prior to opening or
closing.
With difficult cargo ( like black heavy furnace oil)
residue or line design, using a steam injector fitted to the manifold and
injecting 5 liters of toluene has given very good results.
The steam should be
sampled at regular intervals at the pump stack drain valve outlet and
condensate checked for contaminants. This will give an indication that the
steaming has achieved the cleanliness
required or that further cleaning and/or steaming is required.
Vent lines must always
be fresh water rinsed and/or steamed upon completion of tank washing.
When steaming vent
lines, ensure that the steam is coming out both pressure and vacuum ends of the
line, open and close drain valves vapor return connections at regular
intervals, and ensure a positive steam flow at these outlets.
Drop and stripping
lines should be steamed simultaneously when steaming cargo lines. Line steaming
must be closely monitored to ensure that line and valve seals and packing are
not damaged by excessive heat. Do not use excessive force to open or close a
valve which is heated.
For draining cargo
line—the chief officer must be PERSONALLY aware of the 3 D configuration of pipelines.
Deck airline must
always be drained from the lowest position..
Line blowing must
always be discussed in the pre-loading meeting and agreement put down in
writing. To be done with air/ steam/ nitrogen. Ensure cargo does not enter the
air/ steam/ nitrogen line.
Line
Blowing
Line blowing will be
carried out using air or Nitrogen as required according to the cargo characteristics.
Most terminals will
want to clear the shore line and hose all the way to the ships tank to expedite completion. Often the
shore will clear their own lines by
pigging then blowing from the dock line.
When clearing lines
from the shore tanks a large volume of air will be used and the effect of this air on the tank levels
must be considered . Hence preferably blow the shore lines into one single
slack tank , and this tank must be ullaged last to allow the cargo to settle
down.
Air will skim through
the big pipelines much faster than liquid and
bubbles rising from the bottom drop line will expand very quickly having
a considerable effect on the levels
within the tank.
Line blowing into tanks
that are loaded to 98% (or close to 98%) should
therefore be avoided wherever possible.
Estimate the line content of shore hose and ships line to the cargo tank
Blow
Procedure
Note:
It must be understood that large dia pipelines will cause the air to skim on
top while the liquid will remain at the bottom, as it always finds the highest
point and liquid always finds the lowest point. That is why super strip lines
are small dia pipelines.
Manifold valve
open.
Drop line valve
closed.
Shore slowly builds up
pressure in the shore line and hose. (The pressure should not
exceed 80% of the ship’s lines
MWP).
Watch pressure increase
at the pressure gauge fitted outside the manifold valve.
Slowly open dropline
valve, about 10% should be enough no more than
20%.
Listen for the sound of
cargo draining into tank.
When you hear air
bubbling into the tank it is time to close the drop valve.
First foot sample fails
if line cleaning is not OK. (That is why you must take the manifold first foot sample at loadport with manifold valve shut as far as possible ). Throttle
manifold valves sufficiently so that the lines are chock-a-block with water and
dischg to annex 2 overboard from both the sides. Also flush drops, MMC pipes, superstrip lines, vent
lines , drain cocks etc. After completion of steaming keep all valves , plugs and drains open to dry. Use steam to flush PV
stack.
When flushing lines
operate the valves open/ shut a couple of times to clean out the valves.
Before commencing
discharge arrangements are to be agreed for the blowing back of cargo. Once agreed there must be no
deviation form the agreed blow back procedure without further discussion and
another written agreement. After the blow crack open the drop valve and the
discharge valve to release air pressure from the deck cargo line.
PPE and goggles, face
shield must be worn by crew involves in line blowing. Good communications are a
must. Any personnel not involved must clear the area. Risk assessment must be
done for aggressive , toxic and carcinogenic cargoes.
Before arrival
discharge port with high MP cargoes it is a good practise to tap with a spanner
( for a clear ring ) the jetty side of
the manifold and low areas of
dischg delivery pipelines to check if the lines are clear, so that
you have sufficient reaction time if
something needs to be done to clear frozen part of lines. If the lines are frozen the tap will cause a
dull thunk.
The vent systems are as
important for cargo
work as the liquid
lines. Heated high MP cargo fumes can reduce the dia of the vent cooled by icy
winds, which then must be cleared by steam.
-------CAPT AJIT
VADAKAYIL ( 29 YEARS IN COMMAND )
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