DESC: Cargo tank washing process

FIELD OF THE INVENTION

The present invention relates to the washing process of a stainless steel cargo tank. More particularly the present invention relates to a method for minimizing the corrosion effect of sulphuric acid on the stainless steel cargo tank.

DESCRIPTION OF THE RELATED ART

Generally, for the transportation of aggressive acids like sulphuric acid, phosphoric acid, nitric acid etc., tanks made up of stainless steel are used on ships. The corrosion effect of sulphuric acid on the tank carrying sulphuric acid is negligible when the walls of the tank are made of stainless steel. The tank needs to be cleaned internally every time after unloading the cargo as traces of the sulphuric acid is left on the walls and corners of the tank. The cleaning of the tank makes it ready for loading of another cargo other than sulphuric acid in the tank that has to be dispatched to different location.

It is well documented that the corrosion effect of sulphuric acid on stainless steel is minimal either at high concentrations (more than 70%) or at low concentrations (less than 20%) as compared to the concentration within the range of 20 percent to 70 percent. Hence, sulphuric acid of more than 80 percent concentration is stored and transported in the chemical tanker. It is also known that the corrosion rate of the metal is more at a higher temperature than at the room temperature. The above mentioned conditions, high temperature and dilution, aggravate the corrosion rate of stainless steel tanks.

There is no standard procedure of washing the cargo tank. Various ship owners and operators have generated their own company specific washing procedure. The conventional procedure of washing the cargo tank is to wash the tank first with fresh water for a short period followed by washing with clean sea water for about two hours to remove all traces of acid and then finally rinsing the tank wall with fresh water. The drawback of this process is that fresh water reacts with traces of sulphuric acid on the walls on the tank thus diluting it and bringing the concentration below 70 percent. This enhances the corrosion of the stainless steel. Also the reaction of water and sulphuric acid is an exothermic reaction which further supplements the corrosion of the tank walls.

Fresh water is needed on board primarily for human consumption and is also used as feed water for boilers. Hence there is a need to save as much fresh water as possible on board. Usage of fresh water for washing the tanks not only lead to shortage of fresh water when needed for primary purposes but also very costly as compared to readily and freely available sea water.

In light of the foregoing discussion, there exists a need for an innovative process for washing the cargo tanks, which not only minimizes the corrosion of stainless steel but also helps in saving of fresh water on board.

SUMMARY

An object of the present invention is to minimize the corrosion effect of sulphuric acid on a stainless steel cargo tank.

An embodiment of the present invention provides a method of washing the stainless steel cargo tank after unloading a liquid from the cargo tank. Firstly the stainless steel cargo tank cargo tank is stripped and clean sea water is sprayed using a plurality of jet ejectors for a predetermined time interval. As soon as the spraying starts, a pump is set in operation to discharge the diluted sulphuric acid out of the cargo tank continuously. This is followed by manually spraying of clean sea water. Fresh water is sprayed at the end of the process to removes all traces of clean sea water. In another embodiment, caustic soda solution is sprayed on a plurality of weld seams and secluded joints, in case if minute traces of sulphuric acid are retained followed by washing with fresh water. And finally, the stainless steel cargo tank mop dried. In another embodiment, a pH sensor can also be provided to check the pH value of the liquid discharging out of the cargo tank.

BRIEF DESCRIPTION OF DRAWINGS

The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. Embodiments of the present invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the scope of the claims, wherein like designations denote like elements, and in which:

Fig. 1 shows a flowchart of the washing process of a stainless steel cargo tank according to an embodiment of the invention; and

Fig. 2 shows a block diagram of a system used in the washing process of the stainless steel cargo tank;

Fig. 3 is a graphical representation showing comparison of rate of corrosion of Steel SS316L with fresh water and sea water at elevated temperature; and

Fig. 4 is a graphical representation showing comparison of rate of corrosion of Steel SS304 with fresh water and sea water at elevated temperature.

DETAILED DESCRIPTION OF EMBODIMENTS

As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “an article” may include a plurality of articles unless the context clearly dictates otherwise.

Those with ordinary skill in the art will appreciate that the elements in the Figures are illustrated for simplicity and clarity and are not necessarily drawn to scale. For example, the dimensions of some of the elements in the Figures may be exaggerated, relative to other elements, in order to improve the understanding of the present invention.

There may be additional components described in the foregoing application that are not depicted on one of the described drawings. In the event such a component is described, but not depicted in a drawing, the absence of such a drawing should not be considered as an omission of such design from the specification.

Before describing the present invention in detail, it should be observed that the present invention utilizes a combination of system components which constitutes a process for washing a stainless steel cargo tank. Accordingly, the components and the method steps have been represented, showing only specific details that are pertinent for an understanding of the present invention so as not to obscure the disclosure with details that will be readily apparent to those with ordinary skill in the art having the benefit of the description herein.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting but rather to provide an understandable description of the invention.

A process of washing a stainless steel cargo tank is shown in a flow chart 100 of Fig. 1 according to an illustrative embodiment of the invention. The method for washing a cargo tank 202 as shown in the flowchart 100 can be performed using a system 200 shown in the block diagram of Fig. 2. The process of washing the cargo tank 202 is carried out to minimize the corrosion effect of sulphuric acid on cargo tank made of stainless steel. It should be appreciated that the process can also be used for minimizing the corrosion effect of any other liquid which is acidic in nature. It should also be appreciated that the term stainless steel cargo tank 202 and cargo tank 202 can be interchangeably used in the current disclosure.

Initially in the first step 102, the cargo tank 202 is stripped as per the guidelines of procedures and arrangements manual set by the user. The stripping is performed after unloading sulphuric acid from the cargo tank 202. Stripping is done to minimize the quantity of sulphuric acid deposits in the cargo tank 202. Stripping can be done using any of the conventional method present in the art.

In the next step 104, the cargo tank 202 is equipped with a plurality of jet ejectors 204. The plurality of jet ejectors 204 is configured to spray clean sea water inside the cargo tank 202 for a predetermined time interval. The plurality of jet ejectors 204 are attached to a supply line 206 as shown in Fig. 2. The other end of the supply line 206 is configured to suck in the clean sea water. In the current invention, the clean sea water refers to the sea water free of dirt, dust and industrial chemicals. The use of clean sea water in the first step is economical as the clean sea water is abundantly available off shore.

In the next step 106, as soon as the plurality of jet ejectors 204 start spraying clean sea water in the cargo tank 202, a pump 208 is set in operation. The pump 208 is present at a discharge line 210 of the cargo tank 202 as shown in Fig. 2. The pump 208 should remain in operation at all times throughout the washing process so as to continuously discharge sulphuric acid diluted in sprayed clean sea water from the cargo tank 202 as per MARPOL Annex – 2. Marine Pollution (MARPOL) Annex – 2 is an international marine environmental convention which deals with control of pollution by noxious liquid substances.

When the pump 208 starts operating, the diluted sulphuric acid starts flowing in the discharge line 210. Normally, the discharge line 210 includes a plurality of valves (not shown). The plurality of valves should be in complete open position at all times throughout the washing process. The plurality of valves if left partially opened can be damaged by the sulphuric acid diluted in the washed clean sea water.

In an embodiment of the invention, a pH sensor 212 is present at the end of the discharge line 210. The pH sensor 212 is configured to continuously check the pH of the discharged water. The pump 208 is kept running till the pH of the discharged water is detected to be in the neutral range. For precautionary purposes spraying of clean sea water is continued for another half an hour after the pH of the washed water is detected to be about seven.

According to another embodiment of the invention, after the first washing with clean sea water, the cargo tank 202 can be gas freed for user’s entry inside the cargo tank 202. At step 108, manual washing of the cargo tank 202 with clean sea water is done. For manual cleaning the user should wear appropriate personal protective equipment before entering in the cargo tank 202. In an example, the manual cleaning can be performed by the person using handheld hose spraying clean sea water. The manual washing of the cargo tank 202 is performed especially under the heating coil, (not shown), pump stack, on the U clamps and other blind sectors to ensure the removal of all deposits of sulphuric acid.

Finally at step 110, all the traces of clean sea water from the cargo tank 202 are removed using fresh water. The plurality of jet ejectors 204 can also be configured to spray fresh water in the cargo tank 202.

The traces of sulphuric acid can be retained despite of washing the cargo tank 202 thoroughly with more than adequate quantity of sea water for a good length of time and followed with washing by fresh water. There is a possibility that a plurality of weld seams (not shown) and secluded joints of the cargo tank 202 might retain small traces of sulphuric acid. At step 112, it is checked that whether the traces of sulphuric acid are present in joints or not. If the traces of sulphuric acid are present, then step 114 is followed or else step 116 is followed according to another embodiment of the disclosure. At step 114, the caustic soda solution is used to remove the remaining traces of the sulphuric acid solution. Caustic soda solution is manually sprayed around the plurality of weld seams and secluded joints followed by washing with fresh water. And finally at step 116, the cargo tank 202 can be dried using mop.

It has been observed in laboratory experiments that the rate of corrosion of stainless steel is very negligible when sulphuric acid is diluted with clean sea water as compared to fresh water as shown in graphs in Fig. 3 for stainless steel type SS316L and Fig. 4 for stainless steel type SS304. The observed result holds true for concentrations of sulphuric acid in the range of 20 percent to 70 percent as well. Also the corrosion of stainless steel is found to be negligible when experiment was conducted as elevated temperature, in the range of 65 degree Celsius to 75 degree Celsius. Hence an inference was made based on the experiments that stainless steel (SS316L and SS304) resist corrosion better with sulphuric acid diluted with clean sea water than with fresh water.

The present invention has been described herein with reference to a particular embodiment for a particular application. Although selected embodiments have been illustrated and described in detail, it may be understood that various substitutions and alterations are possible. Those having ordinary skill in the art and access to the present teachings may recognize additional various substitutions and alterations are also possible without departing from the spirit and scope of the present invention, and as defined by the following claim. ,CLAIMS:1. A process of washing a cargo tank after unloading a liquid from the cargo tank, the process comprising:
stripping the cargo tank to minimize the quantity of the liquid in the cargo tank;
equipping the cargo tank with a plurality of jet ejectors, the plurality of jet ejectors configured to spray clean sea water for cleaning the liquid deposited in the cargo tank for a predetermined time interval;
starting a pump present at a discharge line of the cargo tank as soon as the plurality of jet ejectors start spraying the clean sea water, the pump is configured to continuously discharge the liquid diluted in clean sea water;
manually cleaning the cargo tank with clean sea water; and
cleaning with the cargo tank with fresh water to remove traces of clean sea water, wherein the pump is further configured to continuously discharge the liquid diluted with the fresh water.

2. The process of claim 1, further comprising operating the pump until the pH of the liquid diluted with sea water and fresh water is reached to a predefined neutral range of pH

3. The process of claim 2, wherein the neutral range of pH is about seven.
4. The process of claim 1, wherein the liquid to be unloaded is an acid.
5. The process of claim 4, wherein the acid is sulphuric acid.
6. The process of claim 1, wherein the cargo tank is made up of a stainless steel material.
7. The process of claim 5, wherein the stainless steel is at least one of SS316L and SS304.
8. A process of washing a cargo tank after unloading a liquid from the cargo tank, the process comprising:
stripping the cargo tank to minimize the quantity of the liquid in the cargo tank;
equipping the cargo tank with a plurality of jet ejectors, the plurality of jet ejectors configured to spray clean sea water for cleaning the liquid deposited in the cargo tank for a predetermined time interval;
starting a pump present at a discharge line of the cargo tank as soon as the plurality of jet ejectors start spraying the clean sea water, the pump is configured to continuously discharge the liquid diluted in clean sea water;
manually cleaning the cargo tank with clean sea water;
cleaning with the cargo tank with fresh water to remove traces of clean sea water, wherein the pump is further configured to continuously discharge the liquid diluted with the fresh water;
manually spraying mild caustic soda solution on a plurality of weld seams and joints followed by washing with fresh water for removing traces of the liquid; and
mop drying the cargo tank.
9. A system for washing a cargo tank after unloading a liquid from the cargo tank having a discharge line, the system comprising:
a plurality of jet ejectors configured to spray clean sea water for cleaning the liquid residue deposited in the cargo tank for a predetermined time, the plurality of jet ejectors also configured to spray fresh water to remove traces of clean sea water; and
a pump present at the discharge line of the cargo tank as soon as the plurality of jet ejectors start spraying the clean sea water, the pump is configured to continuously discharge the liquid diluted in clean sea water and in fresh water.