Marine Cargo Pumps and Piping Systems

Marine cargo pumps and piping move liquid cargo between tanker tanks and shore facilities. Centrifugal main pumps in a pump room handle bulk crude and product discharge at 3,000 to 10,000 m³/h per pump. Hydraulically-driven deepwell units, one per tank in the Framo arrangement, eliminate the pump room on chemical and some product tankers. Positive-displacement stripping pumps clear the final residues. The piping connects everything through a ring main or direct-line header, a crossover network, and the cargo manifold.

The Cargo Transfer Task

A crude oil tanker at a single-point mooring in the Arabian Gulf can discharge 80,000 tonnes of cargo in 18 to 24 hours. That requires sustained flows of 12,000 to 18,000 m³/h across the manifold, typically using three or four main pumps in parallel. Each pump needs enough suction head to avoid cavitation, the manifold and deck piping must handle the back-pressure from the shore system, and the inert gas plant must replace every cubic metre of discharged cargo with inert gas to keep the ullage atmosphere below 8 percent oxygen throughout.

Loading is simpler from a pump standpoint: shore pumps drive cargo through the manifold and the ship acts as a passive receiver. The ship’s crew controls trim and stability by sequencing which tanks fill first, while the watch monitors flow rate, tank levels via ullage tapes or radar gauges, and the high-level alarms. The cargo officer’s loading plan accounts for the back-pressure the shore system must overcome at the end of filling when tank heads are high.

Internal transfer, done at sea, moves cargo between tanks for stress or stability reasons. The ship’s own cargo pumps handle this, typically routing through the crossovers and the common suction header.

Centrifugal Main Cargo Pumps

How centrifugal cargo pumps work

A centrifugal cargo pump converts motor shaft rotation to fluid velocity through an impeller, then converts velocity to pressure in the volute casing. For cargo service, the impeller is typically a semi-open or enclosed type in cast iron or duplex stainless steel, depending on cargo compatibility. Head developed follows the familiar curve: high head at low flow, falling as flow increases to the best efficiency point (BEP), then further. The pump will not self-prime; the pump room must be below the tank bottom to provide a positive static suction head at all cargo levels.

Installed capacities on VLCCs typically run 3,000 to 5,000 m³/h per pump at 120 to 150 m of total head. A VLCC typically ships three or four main centrifugal pumps. Product tankers in the 50,000 DWT range typically carry three pumps rated 1,000 to 2,000 m³/h each. Operating point shifts with each stage of discharge as tank levels fall and suction head diminishes; pump operators throttle the discharge valve to track the curve without hitting cavitation.

Drive arrangements

Steam turbine drives dominated crude tanker cargo pump rooms from the 1950s through the 1990s. A turbine running at 3,500 to 5,000 rpm, geared to the pump, can absorb sudden surge loads without the motor protection issues that trip electric drives. Steam turbine cargo pumps are still specified on some modern VLCCs because the boil-off recovery systems produce steam that would otherwise be wasted. Wartsila (which absorbed Hamworthy) and Shinko Ind. are principal suppliers of steam-turbine cargo pump packages for large crude tankers.

Electric motor drives are now dominant on product tankers and on most new builds. A typical 2,500 m³/h pump uses a 500 to 900 kW induction motor running at 1,450 or 1,750 rpm through a flexible coupling. Variable-frequency drives (VFDs) allow pump speed modulation to match the discharge curve to the shore system without valve throttling losses; Sulzer (now Sulzer Pumps), Shinko, and DESMI supply electric-driven units. The motor must be rated for the hazardous pump room atmosphere: typically EEx d or EEx e to IEC 60079 standards.

Materials

Carbon steel castings with nodular iron impellers handle crude oil, heavy fuel oil, and similar hydrocarbons. For product tankers carrying refined fuels, 316L stainless steel wetted parts reduce contamination risk. Chemical tankers fitted with centrifugal pumps rather than deepwell units need the wetted path in the alloy matched to the cargo: duplex stainless steel (UNS S31803) for most inorganic acids; Hastelloy C-276 for chlorinated solvents; lined casings for concentrated sulphuric acid service.

Deepwell Submerged Pumps: The Framo System

Architecture

Framo Engineering, established in Bergen and now part of TechnipFMC, introduced the hydraulically-driven submerged pump concept in the 1970s. The principle: a hydraulic motor at the bottom of a tubular column drives a centrifugal pump directly in the cargo. The drive shaft and any rotating seal that could leak cargo vapour into the engine room are eliminated. The hydraulic circuit carrying high-pressure oil from the deck-mounted hydraulic power unit (HPU) through the column to the motor is entirely separate from the cargo; the motor housing is a pressure vessel containing the hydraulic oil, with a mechanical seal between motor and pump that keeps the hydraulic oil out of the cargo.

One Framo unit serves one cargo tank. On a 19,000 DWT chemical tanker with 18 tanks, that means 18 HPU connections, 18 discharge valves, and 18 suction strainer service points but no pump room, no pump room ventilation fans, no pump room gas detection array, and no shared suction manifold. The trade-off is that a hydraulic oil leak inside a pump column in a cargo tank is an immediate cargo contamination event.

Framo units for chemical tankers typically deliver 100 to 400 m³/h at 100 to 150 m of head. Some product tanker installations scale to 600 to 800 m³/h. Each pump’s speed is infinitely variable by adjusting the HPU oil flow and pressure; discharge rate control is hydraulic rather than valve throttling, which reduces heat generated in the cargo.

Electric deepwell pumps

Some chemical tankers carry electrically-driven deepwell pumps rather than hydraulic units. The motor is explosion-proof and submerged in the cargo tank with a hermetic seal; Svanehoj (Denmark) is a principal supplier of this type for chemical and product tankers. Electric deepwells avoid the hydraulic oil contamination risk but require more complex electrical penetrations through the tank top and EEx certification for operation in a zone 0 (inside the tank) environment.

Svanehoj’s deepwell pumps for LPG and chemical service deliver 100 to 600 m³/h. The motor runs at the tank liquid temperature, which simplifies thermal management compared to a pump room motor but requires careful motor insulation for the specific cargo and its temperature range.

Positive-Displacement Pumps for Stripping and Viscous Cargoes

The stripping problem

A centrifugal pump will cavitate and lose suction when the available net positive suction head (NPSH-A) falls below the pump’s required NPSH-R. On a large crude tanker, as the cargo level drops toward the bottom of the suction bell, NPSH-A can fall below 1.5 m while NPSH-R for the main pump is 3 to 5 m. The pump then vapour-locks, and the remaining cargo cannot be transferred.

Stripping pumps are positive-displacement machines (typically reciprocating piston or rotary twin-screw types) that can operate against high suction lift and handle two-phase flow without damage. They pick up where the centrifugal pump stops, handling the last 0.5 to 2 percent of tank volume. MARPOL Annex II Regulation 13 requires that residue after stripping not exceed specific limits for each noxious liquid substance category; Category X cargoes require residue below 0.1 m³ per tank. Meeting that limit requires either efficient deepwell stripping at the tank bottom or a dedicated stripping pump with proper suction arrangements.

Eductors

An eductor uses the venturi effect from a high-pressure motive fluid (typically cargo discharged by the main pump) to create suction and lift residue from the tank bottom. Eductors have no moving parts, no seals, and minimal maintenance. They are permanently installed on many crude tankers as the stripping device: the main pump drives motive flow at 3 to 5 bar through the eductor nozzle, generating suction at the cargo stripping connection at the lowest point of the tank. Once main pump flow drops too low to drive the eductor effectively, a small positive-displacement stripping pump finishes the job.

On Framo-equipped chemical tankers, each deepwell unit typically includes an integral stripping stage: a small impeller below the main impeller that operates at the very low flow rates required for final stripping. This avoids a separate stripping pump and its associated piping and valve train.

Screw pumps for viscous cargoes

Twin-screw positive-displacement pumps handle cargoes that centrifugal pumps cannot: bitumen at 150 to 200°C, heavy fuel oil at 50°C with viscosity above 380 cSt, molten sulphur at 130 to 145°C, and palm oil or tallow above their pour points. The screw pump generates pressure by progressive cavity action rather than velocity, making it inherently gentle on cargo and capable of high viscosity service without efficiency penalty. IMO Pump of Sweden and several European manufacturers supply screw-type cargo pumps rated to 2,000 cSt and above for asphalt and bitumen tankers.

The tanker stripping cargo tank calculator and cargo stripping rate calculator address the quantitative aspects of stripping performance.

Pump Types: Applications Summary

Pump Room Design and Safety

Layout and design intent

The pump room on a crude or product tanker sits at the aft end of the cargo section, below the main deck, with the pump floor typically 6 to 10 metres below the keel of the cargo tanks. This depth ensures positive static head to the centrifugal pump suctions at all cargo levels. Suction lines from each cargo tank run through cofferdam bulkheads into the pump room, where they connect to the pump suction manifold through individual gate or butterfly valves. Discharge lines rise from each pump through the pump room top to the cargo deck header.

The pump room is a hazardous area zone 1 under the IEC 60079 classification because cargo vapours can be present continuously during cargo operations. All electrical equipment must be certified for the zone: EEx d (flameproof) or EEx e (increased safety) at minimum. Earthing cables bond all metallic equipment to the ship’s structure to prevent static discharge.

SOLAS ventilation and gas detection requirements

SOLAS Chapter II-2 Regulation 4.5 requires continuous mechanical ventilation in tanker pump rooms providing at least 20 air changes per hour when the room is occupied and at least during cargo operations. Fans must be remote-started from outside the space and must not recirculate air from any enclosed cargo area. Fixed gas detectors calibrated to the cargo vapour must trigger audible and visual alarms at the pump room entrance and on the bridge at 10 percent of the lower explosive limit (LEL). A second alarm level at 30 percent LEL triggers an automatic shutdown of non-essential electrical equipment in the room in some class rules.

ISGOTT 6th Edition recommends that the pump room be checked with portable gas meters before entry during any cargo operations and that a second person stands by at the entrance. Entry under a confined-space permit with documented atmospheric testing for oxygen, LEL, and toxic vapours (H₂S on crude service) is mandatory.

Pump room bilge system

Cargo can enter the pump room through corroded pipe flanges, valve stem packings, or mechanical seal weeps. The pump room bilge system collects these leaks. MARPOL Annex I requires that any oily water from the pump room bilge, if discharged, pass through an oil-content monitor and not exceed 15 ppm oil in the effluent; in practice, most pump room bilge water goes to slop tanks for shore reception. Fixed bilge level alarms alert the bridge to liquid accumulation that could indicate a serious leak. The remotely-operated bilge pump allows emergency bilge emptying from outside the space.

Cargo Piping Arrangements

Ring main versus direct line systems

A ring main (also called a common header system) connects all cargo tanks to a single large-bore header running longitudinally along the ship. Any pump can draw from any tank, and crossover valves allow simultaneous operations from different tank groups through different manifold connections. On a VLCC with a ring main, three pumps can simultaneously discharge cargo from tanks in the port forward, starboard midships, and port aft groups through three separate manifold connections to maximise shore reception flow. The manifold on large crude tankers typically has six to eight connections (three to four per side), each rated for flows up to 5,000 m³/h and designed to OCIMF arm loading specification.

A direct line system gives each pump or pump group a dedicated header without shared crossovers. Cross-contamination between cargo grades is physically impossible if the valves are correctly shut. This arrangement suits chemical tankers where the IBC Code Chapter 5 requires strict segregation: each cargo is loaded, carried, and discharged through its own dedicated pump and piping, cleaned only by its own pump and cleaning system, and never sharing a wetted surface with an incompatible cargo.

Manifold design and OCIMF specifications

The cargo manifold is the interface between the ship and shore facilities. OCIMF’s Recommendations for Oil Tanker Manifolds and Associated Equipment (currently in its 5th Edition, 2019) standardises manifold flange dimensions, presentation heights, setback distances, and drip tray arrangements. Key requirements: the manifold centreline must be 2.0 to 2.5 m above deck, the manifold reducer flanges must accept ANSI 150 lb or ANSI 300 lb arms depending on the cargo pressure class, and each manifold connection must have a MARPOL-compliant drip tray with a capacity of at least 0.5 m³.

Presentation flanges for crude tankers are typically 20 to 24 inches (500 to 600 mm) nominal bore. Product tanker manifolds run 10 to 16 inches (250 to 400 mm). Chemical tankers may have manifolds as small as 4 inches (100 mm) for individual cargo grades where the cargo quantity per grade is modest.

Crossovers and valves

Crossover valves allow the piping system to route cargo around a failed pump, combine flow from multiple tanks through one pump, or segregate sections for simultaneous multi-grade loading. The crossover line itself is typically one size smaller than the main header. On a product tanker carrying five different clean petroleum products, the crossover design must ensure that the valves can be physically locked open or shut for each cargo grade to prevent accidental contamination, which is an audit point in the vetting inspection.

Gate valves dominated cargo piping through the 1980s because of their low resistance when fully open. Butterfly valves with pneumatic or hydraulic actuators replaced them on most modern tankers because they are faster to operate from the cargo control room (CCR) and integrate with the automated valve position indication system. Remotely operated valves on critical sections, with valve position indicators visible at the CCR and audible alarm on failure to travel, are standard on tankers built to modern class rules.

Manifold pressure and loading rate calculation

During loading, the back-pressure at the manifold depends on the shore pump characteristics, the hose or arm friction losses, the static head of cargo in the filling tanks, and the friction in the ship’s own filling lines. The tanker loading plan product calculator and the tanker loading rate top-off calculator cover loading rate management at the critical top-off phase.

Stripping, Eductors, and the Final Percent

Why stripping matters commercially

A VLCC carrying 2 million barrels of crude oil retains roughly 800 to 1,500 tonnes of crude in the tank bottoms after the main pumps stop. At $80 per barrel that's$4.7 million to $8.8 million of cargo the receiver doesn’t get. The on-hire survey (OBQ, on-board quantity) and the discharge survey (ROB, remaining-on-board) difference determines the final outturn figure the charterer pays for. Efficient stripping directly affects the financial settlement.

Stripping pump arrangements

On a pump room vessel, the stripping pump sits in the pump room on its own suction manifold, connected to stripping lines running along the bottom of the cargo tanks. These stripping lines terminate at the lowest point of each tank, typically at a sump at the aft end of the tank floor. On double-hull VLCCs with a pronounced deadrise (transverse slope of the inner bottom), the stripping sump sits at the centreline where cargo pools under gravity.

After the main pump loses suction, the cargo officer opens the stripping valve for each tank in sequence, starts the stripping pump, and monitors flow rate on the pump discharge gauge. Flow drops as the tank empties; below about 10 m³/h, the tank is considered stripped and the valve is closed. The entire stripping operation can take one to three hours depending on the number of tanks and the effectiveness of the sump arrangement.

Eductors: no moving parts, no seals

An eductor installed at the pump room suction manifold uses motive flow from one of the main pumps (at roughly 15 to 20 percent of main pump capacity) to generate suction through the stripping line. The Bernoulli-based ejector action can handle two-phase cargo and vapour mixtures without damage, unlike a centrifugal impeller. Eductors sized for 300 to 600 m³/h motive flow are typical on Aframax-class crude tankers. The eductor approach adds no moving parts to the pump room, which is an operational advantage in a hazardous space where maintenance access requires formal confined-space procedures.

Integration with the Inert Gas System

The marine inert gas system and the cargo pump and piping system share the cargo tank as a common workspace but must be kept hydraulically separate to prevent cargo contamour from reaching the IG plant. The standard arrangement runs the IG distribution header along the cargo deck at a higher elevation than the cargo tank vents, with non-return valves (usually a combined pressure-vacuum breaker and liquid seal on each tank) preventing back-flow.

As the main cargo pumps discharge product and tank ullage volume grows, the IG plant (typically a flue gas scrubber unit or a nitrogen membrane generator) injects inert gas at a rate matching the discharge rate. SOLAS Chapter II-2 Regulation 4.5.5 requires the IGS to be capable of supplying inert gas at a rate of at least 125 percent of the maximum discharge rate of the cargo pumps. On a VLCC with three 5,000 m³/h pumps, that demands an IG plant capable of at least 18,750 m³/h. The inert gas supply capacity check calculator evaluates this compliance requirement.

During loading, the IG system is not supplying gas but the tanks are blanketed with the inert atmosphere from the previous discharge. The IG pressure control valve maintains a small positive pressure (typically 100 to 500 mm water gauge) to prevent air ingress through the cargo manifold connections during loading rate changes. Pressure-vacuum relief valves (P/V valves) on each tank top open to the atmosphere only if the IG system fails and tank pressure or vacuum exceeds the structural design limit.

Integration with Crude Oil Washing

Crude oil washing (COW) uses the cargo itself, pumped through fixed washing machines installed inside crude tanks, to dissolve and wash down the waxy residues that coat tank surfaces after cargo discharge. COW requires the main cargo pumps to continue running at moderate pressure during the washing cycle while the tanks are simultaneously being stripped of washed residue. The piping arrangement for COW therefore uses the existing cargo pump discharge header as the supply for the washing machines, with a separate small-bore COW line branching from the discharge header and connecting to the washing machine nozzle inlets on each tank.

MARPOL Annex I Regulation 33 and the Crude Oil Washing Operations and Equipment Manual (COWE Manual) required by Regulation 29 govern the COW procedure. The COW cycle typically runs for three to four hours per tank, overlapping with the main discharge so total port time is not extended. The marine tank cleaning and crude oil washing article covers the washing technique in detail. Relevant calculators include the crude oil washing cycle calculator and the crude oil washing bottom wash calculator.

Integration with Cargo Tank Heating

Heated cargoes, including heavy fuel oil above its pour point, bitumen at 150 to 180°C, molten sulphur at 130 to 145°C, and vegetable oils above their cloud points, require the cargo tank heating system to maintain viscosity within pumpable limits throughout the voyage. The cargo pump system and the heating system are operationally coupled: if cargo cools below the minimum pumpable temperature, the main pumps cavitate or overload on viscous drag, and discharge is impossible regardless of the piping arrangement.

On a bitumen tanker, the sequence before discharge begins with verifying that the cargo temperature at the pump suction level is above the minimum pumpable temperature specified for that grade. For straight-run bitumen, that minimum is typically 160 to 180°C; for polymer-modified bitumen, 170 to 200°C depending on grade. The chief officer starts the heating system to raise cargo temperature 12 to 24 hours before arrival at the discharge port. Only after temperature is confirmed adequate does the discharge begin, usually with the screw cargo pumps at reduced speed to avoid flow-induced cooling of the suction lines.

Chemical Tanker Cargo Pump and Piping Specifics

IBC Code Chapter 5 requirements

The IBC Code, adopted under SOLAS and MARPOL for ships built after 1 July 1986 and carrying noxious liquid chemicals in bulk, imposes specific requirements on cargo pump arrangements in Chapter 5. Ships carrying products with a vapour pressure above 0.28 bar absolute at 37.8°C must have cargo pumps that don’t expose the cargo to the atmosphere at any point in the pumping cycle, specifically prohibiting open-impeller centrifugal pumps with atmospheric suction. Deepwell units with closed impeller and pressure-tight discharge satisfy this requirement.

For chemical tankers carrying products in IBC Code Category X (the most hazardous category, requiring pre-discharge prewash), stripping efficiency is directly regulated. Regulation 16.2.4 of MARPOL Annex II requires that for Category X substances the cargo tanks be washed and the tank washings discharged to shore reception, with the pump and piping system capable of demonstrating a stripping efficiency that leaves less than 0.1 m³ per tank. The chemical tanker stripping efficiency calculator and chemical tanker stripping residue calculator evaluate compliance with these limits.

Material compatibility and segregation

A chemical tanker carrying 18 different cargoes simultaneously may have 18 different wetted material requirements. AISI 316L stainless steel handles most organic acids, alcohols, esters, and chlorinated solvents at ambient temperatures. Duplex stainless steel (2205) adds strength for high-pressure or low-temperature services. Hastelloy C-276 handles concentrated hydrochloric and sulphuric acids that attack 316L. Some phosphoric acid grades and fluorinated compounds require lined tanks and PTFE-lined piping.

Each cargo grade on a chemical tanker has its own dedicated pump, dedicated suction and discharge lines, dedicated manifold valve, and dedicated drip tray. The pump and piping must be cleaned and certified cargo-free before loading the next grade in that tank if the products are incompatible. The cargo compatibility matrix specified in the IBC Code Appendix guides the planning of cargo combinations; the commercial pressure to maximise grade count per voyage makes compatibility management a practical challenge.

Vapour recovery connections

Many chemical and product tanker cargoes have significant vapour pressures at cargo temperatures. Loading these cargoes without vapour recovery would release volatile organic compounds (VOCs) to atmosphere, violating terminal permit conditions and MARPOL Annex VI Regulation 15 on VOC management. Chemical tanker piping includes a vapour return connection at the manifold: a separate small-bore line (typically 150 to 200 mm) that routes displaced vapour from the tank ullage back to the shore facility’s vapour recovery unit. The vapour return rate calculator addresses the flow sizing for this connection.

Gas Carrier Cargo Pump Systems

LPG carrier pumps

Liquefied petroleum gas (LPG) carriers typically carry fully refrigerated propane at minus 42°C or butane at minus 2°C, or pressurised LPG at ambient temperature. Fully refrigerated LPG ships use deepwell pumps very similar in principle to chemical tanker deepwell units but rated for cryogenic temperatures and propane or butane compatibility. Stainless steel construction throughout, with shaft seals able to handle the thermal cycling between ambient air and minus 42°C cargo.

The IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk) Chapter 5 specifies cargo pump requirements for gas carriers: pumps must be located so they don’t communicate cargo to the engine room, the pump casing must be rated for the maximum possible cargo pressure, and emergency shutdown valves must be capable of remote closure within 30 seconds. Pumps on fully-refrigerated ships must handle vapour-liquid mixtures that form as boil-off accumulates during long pump-down cycles.

LNG carrier cryogenic pumps

LNG carriers use submerged electric cargo pumps installed inside the cargo tanks (membrane or Moss tanks) and operating at minus 162°C in liquid methane. These are centrifugal units with hermetic motor seals; the motor stator is submerged in LNG, which acts as the cooling medium. Nikkiso and Cryostar supply most of the LNG carrier submerged pump market. Each Moss-type spherical tank typically has one or two main cargo pumps rated 1,500 to 2,500 m³/h and a separate smaller stripping/spray pump at 100 to 200 m³/h used for heel management and tank cool-down before cargo loading.

The boil-off gas (BOG) generated during the pump-down cycle as pump heat input warms the remaining LNG is managed through the ship’s BOG compressor and reliquefaction system or used as fuel in dual-fuel main engines. The tanker cool-down calculator covers the refrigeration demand during tank cool-down before LNG loading.

Net Positive Suction Head: The Fundamental Pump-Selection Constraint

NPSH is the governing constraint for centrifugal pump selection and operation. NPSH-available (NPSH-A) is determined by:

where $P_{tank}$ is the tank ullage pressure, $P_{vapour}$ is the cargo vapour pressure at the pump suction temperature, $\rho$ is the cargo density, $h_{static}$ is the vertical distance from the cargo surface to the pump centreline (positive when tank is above pump), and $h_{friction}$ is the suction piping friction head loss.

At the start of discharge, $h_{static}$ is high (full tank), NPSH-A is comfortable, and the pump runs well inside its curve. As the tank empties, $h_{static}$ falls. At the same time, friction losses in the suction piping may increase slightly as the flow distribution changes. On a VLCC with tanks 18 to 20 m deep and pump room 8 m below the tank bottom, the initial NPSH-A at full tank might be 25 to 28 m; at 1 m of cargo in the tank it might fall to 4 to 6 m. If the pump’s NPSH-R is 4 m, the margin has nearly disappeared. Throttling the discharge valve at this point is counterproductive (it increases flow instability); the correct response is to reduce pump speed or switch to stripping mode.

The NPSH available calculator and NPSH required calculator support this analysis.

Maintenance and Class Surveys

Routine inspection intervals

Framo deepwell pump hydraulic oil samples are taken at intervals specified by the manufacturer, typically every 3,000 to 5,000 running hours, and tested for water contamination, metal particles, and viscosity breakdown. Oil contamination with cargo is confirmed by chemical analysis if the cargo smells or colour appears in the HPU system. A hydraulic oil-to-cargo contamination event triggers immediate pump isolation and hull notification to the charterer and P&I club.

Centrifugal cargo pump bearings on pump room units are typically rolling-element types (spherical roller or angular contact ball) running in flooded oil baths. Bearing temperature is monitored continuously; a rising trend above the normal operating temperature (typically 60 to 80°C on the bearing housing) signals developing wear or lubricant degradation. Vibration monitoring, increasingly done with permanently mounted accelerometers, identifies imbalance or misalignment before bearing failure.

Mechanical seals on pump room shafts are the most frequent failure point on electric-driven cargo pumps. A double mechanical seal with a barrier fluid (typically a compatible light oil) prevents cargo from reaching the pump room atmosphere. Barrier fluid pressure is monitored continuously; a drop in pressure indicates a seal leak, triggering an alarm and pump shutdown before cargo vapour is released to the hazardous space.

Class survey requirements

IACS Unified Requirement P2 (Cargo Pumps Survey) requires that cargo pumps be opened and internally examined at the special survey interval, typically every five years. The survey covers impeller wear, wearing ring clearance (comparing actual to manufacturer’s new-build tolerance), shaft seal condition, bearing clearances, and casing wear. If clearances exceed 1.5 times the new-build tolerance, replacement is required before the class certificate is endorsed.

Cargo piping is pressure-tested at the annual survey using air or water at 1.5 times the design working pressure or at the original hydrostatic test pressure, whichever is lower. Pipe thickness measurements by ultrasonic testing at five-year intervals confirm that corrosion rates stay within the class-approved allowance. On older ships where original pipe thickness is marginal, class surveyors require a pipe replacement program rather than accepting further corrosion.

Common failure modes

Cavitation damage manifests as pitting erosion on the suction face of the impeller blades, typically starting at the leading edge and progressing inward. A cavitating impeller produces a distinctive crackling noise audible in the pump room or through the pump casing. Continued operation with cavitation reduces pump output, accelerates erosion, and can cause complete impeller failure within hundreds of hours. The operating procedure response is to reduce discharge valve opening (if the cavitation is suction-head-limited) or reduce pump speed.

Impeller wear from abrasive cargo particles (sand or scale entrained in crude oil, catalyst fines in some product cargoes) increases the impeller-to-wearing-ring clearance, reducing pump efficiency and maximum head. A 10 percent increase in clearance beyond the new-build value typically reduces pump output by 5 to 8 percent; a 50 percent increase reduces output by 20 to 30 percent and may prevent achieving the charter party’s discharge rate.

Limitations

The information here reflects general tanker industry practice and the principal regulatory frameworks current as of 2026. Specific pump and piping specifications vary by vessel design, cargo grade, flag state, and class society. Actual NPSH margins must be calculated using the specific pump curves, cargo properties, and pipeline geometry for the individual vessel; the NPSH formulas given here illustrate the parameter relationships, not a design tool.

The Framo and Svanehoj descriptions reflect publicly available technical documentation from those manufacturers. Detailed engineering data for specific pump models requires direct reference to manufacturer specifications and approved service documentation. Cargo compatibility guidance here is indicative; the IBC Code compatibility tables and the specific cargo tank cleaning guide (for example, the Tanker Cleaning Guide published by the Chemical Distribution Institute and the Cefic working group) must be consulted for any commercial cargo operation.

MARPOL Annex II stripping limits are stated correctly for Category X substances as of the 2006 amendments; vessels built before 1 July 1986 may operate under earlier residue limits. Flag state and port state regulations may impose stricter requirements than the IMO minimums in specific jurisdictions.

This article covers liquid cargo pump and piping systems. Gas carrier systems for cryogenic service involve additional code requirements under the IGC Code that go beyond the summary given here; those systems warrant a dedicated article.

See Also

• Oil Tanker
• Chemical Tanker
• LNG Carrier
• Marine Inert Gas Systems
• Marine Cargo Tank Heating Systems
• Marine Tank Cleaning and Crude Oil Washing
• IBC Code
• IGC Code
• MARPOL Annex I: Oil Pollution Prevention
• MARPOL Annex II: Noxious Liquid Substances
• Marine Bilge and Ballast Systems
• SOLAS Chapter II-2: Fire Protection, Detection and Extinction

Related Calculators

• System: Main Cargo Pump Centrifugal / Deepwell
• NPSH Available
• NPSH Required
• Cargo Stripping Rate
• Tanker Stripping: Cargo Tank
• Chemical Tanker Stripping Efficiency
• Chemical Tanker Stripping Residue
• Tanker Discharge Plan
• Tanker Loading Plan (Product)
• Tanker Loading Rate: Top-off
• Inert Gas Supply Capacity (SOLAS II-2 Reg 32)
• Vapour Return Rate
• Crude Oil Washing Cycle
• Cargo Heating Coil Steam Rate
• Cargo Steam Heating Time