Ameer Randolph
Stripping is the final stage in bulk liquid pumping from a tanker ship's tanks and pipelines. It is a critical operation that requires careful planning and the use of specialized equipment such as stripping pumps, cargo compressors, and temperature sensors. The goal of stripping is to ensure that all cargo liquid is removed from the tanks, piping, and other parts of the cargo system. This is important because any remaining liquid can continue to give off vapour, hindering subsequent purging or gas-freeing processes. Proper stripping procedures can vary depending on the design of the tanks and the type of cargo being handled, with certain chemical cargoes requiring special attention due to their potential environmental impact.
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What You'll Learn
The importance of stripping pumps
Stripping pumps are an important component of fuel tanker ships. They are used to remove the last remnants of cargo from the ship's tanks, piping, and other parts of the cargo system. This process is known as "stripping" or "cargo stripping". It is a critical operation that ensures the safety and efficiency of the ship.
One of the key purposes of stripping pumps is to prevent the loss of suction in the main cargo pump. When a pump loses suction, the rpm increases dramatically, and immediate action is required to avoid damage. By using a stripping pump, the remaining cargo can be drained, and the main pump can be kept functioning optimally.
Different types of pumps are used for stripping, depending on the design of the ship and the type of cargo. For example, a vac-strip pump creates a vacuum in the main cargo pump and the separator tank, which feeds the pump and creates an artificial positive cargo head. This type of pump is often used in conjunction with a centrifugal cargo oil pump. Another type of pump used for stripping is an eductor, which operates on Bernoulli's principle and has no moving parts, making it easy to maintain.
The proper use and maintenance of stripping pumps are crucial for the safe and efficient operation of a fuel tanker ship. It ensures that the ship complies with environmental regulations by minimising the residue of potentially harmful chemical cargoes. Additionally, effective stripping helps to reduce the risk of pollution and claims for the ship owner. Overall, stripping pumps play a vital role in the final stage of bulk liquid pumping from a fuel tanker ship, ensuring that the ship's tanks and systems are left liquid-free.
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How to strip cargo tanks
Stripping is the final stage in bulk liquid pumping from a tank or pipeline. It is a necessary procedure on board liquefied gas carriers before changing cargoes or gas-freeing.
To strip cargo tanks, careful trimming or listing of the ship can assist in draining liquid. This is especially helpful if the product is trapped in the centreline bulkhead corrugations. A large stern trim helps with drainage.
If pumps are used for discharge, the pump discharge valve should be throttled toward the completion of discharge to maintain suction at the minimum liquid level. Manufacturers' instructions should be consulted for the liquid level at which throttling should be started and the pump pressure that must be maintained during the later stages of pumping to obtain maximum stripping. Each pump should be kept under continuous control during stripping to obtain the best results without pumps running dry.
Even with the good operation of cargo pumps, some liquid will remain in the tanks at the termination of pumping. In the case of ships whose cargo tanks can accept overpressure, further stripping of liquid may be achieved by increasing tank pressure to push out the liquid through the piping system ashore. Alternatively, all stripping may be collected in one of the tanks for subsequent discharge ashore.
The use of cargo compressors, taking suction from other tanks, will ensure that all tanks and associated piping systems are left liquid-free. Proper stripping of tanks should be checked by the bottom sampling line or temperature sensors. In the case of ships with cargo tanks designed for pressures only slightly above atmospheric, stripping by pressure alone is not possible. On such ships, the remaining liquid should be boiled off by introducing hot vapour from the cargo compressors to the bottom of the tanks through puddle heat coils. During such operations, the tank pressure must be closely observed to avoid exceeding the relief valve set pressure.
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The role of pump throttling
Stripping is the final stage in bulk liquid pumping from a tanker ship's tank or pipeline. It is a procedure that involves removing all cargo liquid from the tanks, piping, reliquefaction plant, and any other part of the cargo system. This is important because any remaining liquid will continue to give off vapour, frustrating subsequent purging or gas-freeing.
Manufacturers' instructions should be consulted to determine the optimal liquid level at which throttling should be initiated and the pump pressure that needs to be maintained during the later stages of pumping to achieve maximum stripping. By carefully throttling the pump discharge valve, the pump can continue operating without running dry, ensuring that the maximum amount of liquid is removed from the tanks and piping system.
Additionally, pump throttling plays a vital role in ensuring the safe and efficient operation of the pumps during the stripping process. By controlling the pump's discharge valve, the pump's pressure and flow rate can be adjusted to prevent sudden losses of suction, which can cause the rpm to increase dramatically. Proper pump throttling, along with good planning and adherence to safety procedures, contributes to the overall effectiveness of the stripping process and helps protect the environment by minimising the discharge of chemical cargo residues.
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Safety considerations
Stripping in a fuel tanker ship refers to the final stage in bulk liquid pumping from a tank or pipeline. This process is often necessary to avoid suction loss.
- Before changing cargo or gas-freeing, it is crucial to remove all cargo liquids from tanks, piping, and any other parts of the cargo system. Residual cargo liquid can release vapour, hindering subsequent purging or gas-freeing processes.
- Proper stripping of tanks should be verified using the bottom sampling line or temperature sensors.
- For ships with cargo tanks designed for slightly higher pressures than atmospheric pressure, stripping by pressure alone is insufficient. In such cases, the remaining liquid must be boiled off by introducing hot vapour from cargo compressors to the bottom of the tanks. Tank pressure must be closely monitored during this process to prevent exceeding the relief valve set pressure.
- When pumps are used for discharge, the pump discharge valve should be adjusted towards the end of the discharge to maintain suction at the minimum liquid level. Refer to the manufacturer's instructions for specific guidelines on throttling and pump pressure maintenance.
- Even with efficient cargo pumps, some liquid may remain in the tanks. For ships with cargo tanks capable of accepting overpressure, increasing tank pressure can force out the remaining liquid through the piping system.
- The use of a vac-strip pump or a stripping pump is vital to prevent pump malfunction due to suction loss. If a pump loses suction, it should be stopped immediately, and corrective measures should be taken to supply cargo oil to the suction side.
- Good planning and efficient discharge procedures are essential for safe tanker operations. This includes information exchange between the ship and the terminal before arrival at the discharge port and the completion of a ship-shore checklist once the vessel is tied up.
- Fire safety is a critical consideration for tankers. The International Convention for the Safety of Life at Sea (SOLAS) includes stringent fire safety provisions for tankers due to the increased risk of fires and explosions on board ships carrying oil and refined products.
- To prevent accidental pollution, the International Convention for the Prevention of Pollution by Ships (MARPOL 73/78) has implemented regulations regarding subdivision and stability, ensuring the ship's survival in various loading conditions.
- The 1983 MARPOL amendments prohibit the carriage of oil in the forepeak tank, the most vulnerable point in a collision.
- To protect the marine environment, chemical cargoes must be discharged until there is minimal residue left onboard. Modern chemical tankers are designed with efficient stripping in mind, featuring smooth-walled cargo tanks and corrugated bulkheads.
- Accidents involving oil tankers have highlighted the need for additional international measures to eradicate substandard vessels and prevent environmental disasters.
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Training and courses
The Seably platform offers a course on Safe Cargo Handling with a focus on FRAMO pumps. This course covers the fundamentals of stripping, including the use of an eductor or positive displacement pump, and cargo pumps to remove liquids from tanks. It also teaches the process of throttling the pump discharge valve, regulating air from the inert gas system, and monitoring tank levels to ensure complete stripping. Additionally, the course covers the use of the optional vacuum drain to remove residual cargo after stripping.
The Seafarers International Union provides tanker course descriptions that satisfy training requirements for endorsements such as Tankerman-Assistant DL and LG, as well as basic liquefied gas tanker cargo operations. These courses include modules on LNG firefighting, confined space awareness, LNG ship operations, personal safety, LNG safety, and liquefied gas cargo pump operations.
The EMARINE Academy offers a course on Stripping Pump Operation in Oil Tanker Ships. This course takes learners from the basics of the stripping pump, including its class, types, capacity, and associated special lines, to the advanced understanding of its role and efficiency. It provides a step-by-step guide on lining up the stripping pump onboard, ensuring learners can confidently perform this task without harming any machinery or lines. The course also covers the operation of the stripping pump with illustrative equipment pictures and videos.
In addition to these specialized courses, basic vocational support programs are offered throughout the year, providing foundational knowledge for roles like AB, QMED, FOWT, Third Mate, Tanker Assistant, and Water Survival. These programs serve as a stepping stone for further training in the field of fuel tanker ship operations and cargo stripping.
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Frequently asked questions
Stripping is the final stage in bulk liquid pumping from a tank or pipeline.
Stripping is necessary to ensure that all cargo liquid is removed from the tanks, piping, reliquefaction plant and any other part of the cargo system.
Any remaining cargo liquid will continue to give off vapour and will frustrate subsequent purging or gas-freeing.
A vac-strip pump is used to avoid the pump misbehaving. If a pump loses suction, the rpm will increase dramatically.
A stripping pump is used to drain the last remnants of cargo into the slop tank.
