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.
During tank washing, a positive line pressure should be maintained to ensure that all surfaces of the line are washed. The manifold valves should be adjusted to maintain the positive pressure.
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 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
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 )