HIGH PRESSURE HYDRAULIC SYSTEMS

INTRODUCTION TO BASIC HYDRAULICS

The study of hydraulics deals with the characteristics & use of liquids. Hydraulic system first came in to use in 17 th century. Renowned French scientist Pascal discovered that pressure applied on a confined fluid is transmitted equally in all directions. Liquid is practically non‑compressible & pressure in any confined hydraulic system is same, at any position. Based on this principle hydraulic systems came in use for transmitting power, multiplying force & modifying motions.

There are different mediums available to transmit power from one point to another. E.g. – Electricity, Fluids such as steam, air or liquids.

All mediums have their advantages & disadvantages. Some of the advantages of hydraulic systems are :

a)  Stepless speed control of the driven machinery such as cargo pumps.

b)  All moving parts are lubricated by hydraulic oil, hence less wear & tear.

c)  Driven machinery can be placed anywhere required. It eliminates requirement of  pump-room in a tanker. It also helps

     to reduce the length of the cargo pipelines and number of cargo valves.

d)  Liquid used in the system also cools down the system. Generated heat is taken by the liquid, which is removed in the

     cooler. 

e)  Small dimensions & low weight of the components.

f)   It reduces the risk of fire as compared to electricity.

BASIC HYDRAULIC SYSTEM

There are four basic components in any hydraulic system, which are hydraulic pump, hydraulic motor, pressure relief valve & a reservoir tank. See fig.1.1

Hydraulic pump creates the flow in the system, motor creates the pressure, relief valve holds the pressure in the system up to certain maximum allowed & reservoir tank for replenishment of oil in the system & to allow expansion of oil due to temperature changes.

Hydraulic system may vary in size & complexity but the four basic components are always there.

OPEN LOOP SYSTEM

Refer fig. 1.2, which is an open hydraulic system. For practical reasons, a filter, oil cooler & pressure gauges are required to put in the circuit.

There are various types of open hydraulic ‑systems, such as ‑

DIRECT HYDRAULIC CIRCUIT

A simple direct hydraulic circuit is shown below in fig. 1.2

In this system a pump can run only one motor. However it is often necessary to have more than one motor run, but not at a time. In this case we can use a directional valve to lead the oil flow from pump to one motor or the other.

COMBINED DIRECT HYDRAULIC CIRCUIT

Refer fig. 1.3 below, here a number of pumps are shown driven by one prime mover through a gear arrangement. This system is cheaper because you spare pumps & drives for the pumps.

CENTRAL RING LINE SYSTEM ‑ Refer fig. 1.4 below

The combined direct hydraulic circuit leads us to a central ring line system where a number of pumps deliver oil to a common main pressure line. From this line we can run any number of motors till there is sufficient quanfity of oil in the line. However to avoid overspeeding of motors we must have an oil flow restrictor before the motor named as flow control valve. This central system gives us the highest degree of flexibility & also it is cheap to install, as it requires less piping system than the direct circuit system. On the other hand it is necessary to maintain minimum pressure in the ring main line required to satisfy the highest consumer.

CLOSED LOOP HYDRAULIC SYSTEM

The only difference between open & closed system is that, only a small part of the system oil passes through the reservoir & is fed back by the feed pump. This makes the closed system a little complicated, but it has advantages as higher efficiency & a smaller reservoir.

Similar to open loop hydraulic circuits there are different types of closed loop circuits such as direct closed circuit, combined direct closed circuit & central ring line closed circuit.

Refer fig. 2.1, 2.2 & 2.3 .

HYDRAULIC OIL

The medium used in a hydraulic system is hydraulic oil. It has four main functions to perform

• Transmit power from the hydraulic aggregate to the consumers.

• Lubricate all moving parts.

• Transfer all generated heat to the cooler.

• Transfer all dirt & other particles to the filter.

To maintain hydraulic system in good condition for the lifetime, it is of utmost importance that oil is maintained in good clean condition. Type of oil used depends upon the design & intended use of the system. A typical high‑pressure hydraulic system used for cargo pumping uses ISO VG 46 grade of hydraulic oil.

CARE OF OIL

▪ Never mix oil of different grades.

▪ Replenish clean fresh oil through the filling filter.

▪ Check & drain reservoir tank for any water.

▪ Test oil for water content with onboard test kit.

▪ Take regular samples, generally every three months & land for complete analysis including particle count

▪ Monitor regularly the pressure drop across the filter. Renew filter elements before the bypass valve opens at the set

  differential pressure. Use only original filter elements.

SAMPLING

▪ Always take sample from the same point, which should be clearly marked as sampling point.

▪ Take a clean representative sample. Drain oil from sampling point about a liter or two before taking sample. Flush the

   bottle before taking sample.

▪ Take sample when system is in use at normal operating parameters.

▪ Always use sampling bottles provided by the maker. Ensure these bottles are stored separately & used only for

  sampling purpose. These bottles should be kept closed during storage to avoid ingress of any foreign particles, which

  may give wrong results during analysis.

▪ Proper procedure for sampling & dispatching the sample must be followed as per the manufacturer's instructions.

OIL ANALYSIS

Analysis results must be examined carefully for following :

▪ Particle count is very important. If system is operated with high particle content it will damage the system components,

  which becomes an expensive affair.

  Number of particles per 100ml > 5 micron ‑ 32,000, Max. 64,000.

  ­Number of particles per 100 ml > 15 micron ‑ 2,000,Max. 4,000.

▪ High water content ‑ It increases wear down in the system, increases rust in the system, damages the additives of the

  oil & causes clogged filter for some types of hydraulic oils.

▪ Chlorides ‑ High chlorides will have the same effect as water. In addition it will accelerate corrosion in the system.

▪ Viscosity ‑ Any change in viscosity indicates mixing with other liquid. Reduced viscosity indicates insufficient shear

   stability. Increased viscosity indicates excessive oxidation.

▪ Zinc & Phosphorus ‑ This is a normal antiwear additive in the oil. A decreased level will cause increased wear in the

  system.

FILTERS & STRAINERS

Hydraulic pump & motor have very fine clearances between moving parts. Even very small particles of dirt or metal will damage the machinery. Hence to achieve‑ long life of the hydraulic system it is very important to maintain oil in clean condition. To achieve this, filters & strainers are used in the system.

Strainer is a coarse filter mounted in the suction line of the pump. It is usually of 100 to 150 micron fineness. It is due of the fact that a fine filter will create too high a pressure drop to be accepted in the suction line of the pump.

A high ‑pressure fine filter can be mounted after the pump or before the consumer. This type of filter must withstand the high pressure in the system & therefore it is expensive.

In all hydraulic systems it is must to have a return filter. Normally a full flow filter of 10 micron fineness is used. This filter has a built‑in adjustable bypass valve, which normally adjusted to open at 0.6 to 0.8 bar pressure differential across the filter  elements must be replaced before the differential pressure reaches this value.

These filters are also fitted with magnetic rods for arresting iron particles & should be inspected & cleaned regularly.

In bigger systems a separate return filter box outside the tank‑is generally provided instead of incorporating the filters in the tank.

A TYPICAL HIGH PRESSURE HYDRAULIC SYSTEM

Ref. Hydraulic Diagram for engine room & deck as provided on some tankers.

This system is used in two ways either as a high pressure system or a low‑pressure system. High‑pressure system maximum pressure is 240 bar & low pr system maximum pressure is 50 bar.

High pressure system has total three power packs, which are installed in parallel to supply high pr hydraulic oil to the system. There are two main power packs which are diesel engine driven with a clutch & gearbox arrangement. Third power pack is electric motor driven direct coupled single pump of variable delivery type.

The term power pack is defined as a complete set of hydraulic pumps, gearbox, clutch & other accessories & attachments. Prime mover for1hydraulic pumps can be either diesel engine or electric motor. Diesel driven main power packs ran have three to four hydraulic pumps, either all variable delivery type or may be combination of two fixed‑& ‑two variable delivery type pumps.

All power packs deliver oil to main pressure line from where a number of hydraulic motors can be driven to run cargo pumps or deck machinery as long as sufficient oil delivery from power pack is available. Oil is returned back to the system via a filter & cooler.

Low pressure system is also provided with three power packs installed in parallel. Each is an electric driven direct coupled single hydraulic pump of fixed displacement type. Low pr system is used for cargo heating. It can also be used for ballasting, deballasting, tank cleaning or operating winches if required. But as pressure is limited to 50 bar only, full capacity of pumps or winches can not be utilised.

A change over switch from system 'A' (High pr system) to 'B' (Low pr system) is provided on cargo control room panel. Main power packs in high‑pressure system can not be used when low‑pressure system is in use. Only electrical power pack no.3 can be used along with low‑pressure system depending upon the type of installation on your ship.

Ensure to stop all power packs before changing over the system.

A feed pump continuously supplies oil at inlet of hydraulic pumps at 5‑bar pressure. One pilot pump is always kept running even when main power packs are not in use to avoid ingress of ‑any air or moisture in the system.

There is a flow control valve before every cargo pump for controlling oil flow to hydraulic motor for cargo pump.

Storage cum expansion tank for system oil is provided in the system. Feed & pilot pumps take suction from this tank. Oil from system is returned back to this tank. Complete unit of feed pumps, pilot pumps & tank is termed as auxiliary hydraulic unit.

OPERATION OF THE SYSTEM

STARTING THE SYSTEM –It must be decided in advance that how many pumps to be run & accordingly whether one or two power packs will be required.

In Engine Room ‑­

-     Start diesel engine for power pack.

-     Increase engine rpm up to 750 before clutching in. As per company operational check list rpm is to be increased to

      775, but it has been experienced that during operation there is always fluctuations in demand & supply which causes

      engine rpm to fluctuate & at 810 rpm over‑speed trip operates.

-     Check cooling water flow through gear oil cooler is normal.

-     Check gear oil pressure is normal (about 5 bar ) & gear oil splash from inspection window.

-     After checking all is normal, running noise, vibrations, no leaks etc., inform duty officer in cargo control room to clutch

      in  power pack.

-   After power pack is clutched in check clutch oil pressure is normal (about 25 ‑ 30 bar).

-   Observe variable hydraulic pump's tilt angle & its operation is normal.

In Cargo Control Room

‑­   Start feed pump & check feed pressure after it is stabilized that it is normal (about 5‑5.5 bar).

-   When engine available lamp is illuminated on the panel, press clutch in button. Clutch in lamp is illuminated &

    system pressure rises from zero to 30‑40 bar.

-   Turn system pressure controller potentiometer knob from zero towards increase very slowly to increase system

    pressure in steps of 10 bar only, till required system pressure is achieved, which is generally 200 ‑210 bar. Maximum

    system pressure up to 240 bar can be increased when required.

Emergency / manual operation ‑‑ In case system pressure does not increase from ccr remote controller, then it can be done manually by pilot pressure controller in engine room.

STARTING OF CARGO PUMPS

-  After system is ready for use, cargo pumps can be started slowly one by one.

-  Initially observe closely that cargo is going, from all the tanks which are being pumped. It may happen that cargo from

   one tank is going to another (There are no non‑return valves provided in cargo pump discharge line as same lines are

   used for cargo loading as well).

-  Once required number of pumps have been started up to required pressure &  discharging is settled, check hydraulic

   pressure on pump gauges & system pressure. As a rule system pressure should be kept 10‑15 bar more than the

   required pressure at the pump for discharging & not always 210‑240 bar.

-  If initially only one power pack was started & now more pumps have to be started which exceeds one power pack

   capacity then second power pack has to be started & clutched in.

For clutching in second power pack all pumps must be stopped & system pressure is  reduced to minimum.

Similarly when two power packs are running & one has to be stopped, all pumps must be stopped, system pressure is reduced to minimum & then one power pack is declutched. After declutching one power pack, system pressure can be increased again & pump restarted.

STOPPING THE SYSTEM

Duty officer in cargo control room

• Stop all pumps.

• Reduce system pressure slowly to minimum.

• Declutch power packs.

• Stop feed pump & keep pilot pump running.

• Inform engine room that finished with power pack & can be stopped.

Duty engineer in engine room ‑

• Reduce engine rpm from 750 to 720.

• Waft for a few minutes before stopping the engine till exhaust temperatures are reduced.

• Stop cooling water for hydraulic oil cooler.

HYDRAULIC PUMP

Piston pumps are used, as they are able to discharge a bigger flow & operate at a higher pressure than the vane pumps.

In a bent axis piston pump the cylinder block rotates with the drive shaft, but at an offset angle.

Piston rods are attached to the drive shaft flange by ball joints & are forced in & out of their cylinder bores as the distance between the drive shaft flange & the cylinder decreases or increases. A link shaft keys the cylinder block to the drive shaft, to maintain alignment & assure that they turn together. The link shaft does not transmit force.

These piston pumps can be of fixed displacement type or variable displacement type.

In fixed displacement pumps angle of pump axis is fixed & delivery can not be varied.

In variable displacement pumps by changing the angle of pump axis (swivel angle), delivery change is achieved. Variable delivery pumps are preferred over fixed delivery pumps in high pressure hydraulic system installed on board.

Swivel angle of pump is hydraulically controlled with spring centering via the control cylinder. A bigger angle will give more oil flow, but it will also require a bigger torque on the drive shaft to maintain the same pressure.

Piston pumps are highly efficient units available in a wide range of capacities. Because of their reversability they have an advantage for some applications. However due to very fine clearances between their closely fitted parts & finely machined surfaces, cleanliness &‑good quality oils are vital for their long service life.­

These pumps are not so ‑simple to service & repair as vane pumps are because of their complex design. However as long as oil is kept free from impurities, no service is required normally.

HYDRAULIC MOTOR

Construction of a hydraulic motor is exactly same as a hydraulic pump. Hydraulic motors are designed for a certain capacity to perform a particular function, hence they are of fixed maximum capacity & rpm is controlled by flow control valve installed before them to control oil flow to hydraulic motor.

A hydraulic motor used in the system is exactly same as a fixed displacement hydraulic pump.

Refer diagram as shown on next page.

GEAR BOX & CLUTCH

 The gear ‑ is designed to transmit power from engine to hydraulic pump. In an engine driven power pack, engine is usually coupled to the gear input shaft by means of a flexible coupling. For the input shaft, the drive is through a hydraulically operated engine isolating clutch in constant mesh with an output wheel fitted to the gear output shafts.

 The hydraulic pumps are coupled to the gear output shafts by means of flexible couplings.

 Oil pressure for gear lubrication & clutch operation is achieved through attached gear oil pumps, which are gear driven from the gearbox input shaft, so that oil pressure is available as soon as engine is started. Oil pumps take suction from gearbox oil sump through a suction strainer & delivers through a discharge filter to pressure regulating valve, to control valve & to various lubricating points on ‑the gear. Oil from the control valve is directed to the distributor on the clutch shaft & along the port in the shaft to engage the clutch.

  All bearings used in gearing arrangement are ball & roller bearings. Bearings are splash lubricated from rotation of gear as well as overquirt of oil from the attached oil pump used for gear lubrication.

The clutch ‑ main function of clutch is to isolate the pumps from the engine.

This is a twin cone type oil operated clutch, consisting of two inner & two outer members. Outer members are bolted together & carry the gear wheel through flange couplings. Outer unit is free to rotate on the ball bearings of the input shaft. Two inner members are carried on longitudinal splines on the input shaft, & each has a friction lining fitted on its cone. Four special springs keep the two inner members together when clutch is in disengaged position. Clutch is engaged by directing oil pressure in to the chamber between the inner members, which then slide on the splines until they engage with the outer members. The working contact is maintained by oil pressure until it is released.

GEREBOX & CLUTCH ARRANGEMENT

PILOT PRESSURE CONTROLER

To keep a balance between oil delivery & consumption, a pressure control valve is needed. This valve is stepless remote control from cargo control room. When system pressure reaches the set value of the pressure control valve, the valve will open for oil to the pump angle control cylinder. The swivel angle will now balance out. If oil consumption from cargo pumps is increased, system pressure will drop slightly & oil flew through the pressure control valve will drop. The swivel angle will now increase until a new balance is achieved between oil delivery & consumption. By means of this system, oil delivery from pumps will always be maintained same as the oil consumption.

Refer drwg. This valve is an assembly of four valves attached together

a) Solenoid valve for remote control from cargo control room

b) Valve for emergency manual operation (Normally open)

c) Relief valve (Adjusted to set system pr 240 bar)

d) Operating valve

Emergency manual operation ‑ When system pressure is not developed with remote control

solenoid valve, follow below mentioned procedure ‑

1)   Open relief valve fully.

2)   Close manual emergency operation valve.

3)   Start closing relief valve very slowly & check that system pressure is coming up. Set system pressure as required.

FLOW CONTROL VALVE

A flow control valve is provided before every cargo pump for controlling oil flow to pump hydraulic motor, hence rpm of cargo pump. As these valves are controlling a good amount of oil flow, some extra power is required to drive these valves. Oil pressure from pilot system is supplied to flow control valve for its operation. The main purpose of flow control valve is ‑

•     To start & stop the cargo pump

•     To control speed (rpm) of cargo pump

•     To prevent overspeeding of cargo pump

Cargo pump ran be operated & rpm controlled either from cargo control room or locally with a local control valve mounted on flow control valve.

To operate the pump remotely from cargo control room ‑ Ensure local control valve is in closed position.

To operate the pump locally ‑ Ensure that pump start lever in cargo control room is in zero or stop position. Open the local control valve fully & then put the pump start lever in cargo control room to maximum position. Now pump can be operated by closing local control valve slowly.

Different types of flow control valves are fitted for different types of pumps.

Adjustments of flow control valves are factory set & not to be disturbed at all.

CENTRIFUGAL PUMP

Centrifugal pump works on the principle of centrifugal force.. Rotation of pump impeller causes the liquid it contains to move outwards from the centre to the circumference of the impeller. The revolving liquid is impelled by centrifugal effect. It can only be projected in to the casing around the periphery of the impeller if other liquid in the casing can be displaced. Displaced liquid in moving from casing to the delivery line causes flow in the discharge side of the system. The liquid in the impeller & casing is essential for its operation. In moving out from impeller to casing by centrifugal effect, it drops the pressure at the centre to which suction line is connected for supply of liquid to be pumped. It is termed as the volute pump due to the shape of its casing. The object of the volute is to reduce the velocity of liquid after it leaves the impeller, and so convert part of its kinetic energy to pressure energy.

Pump Discharge Characteristics ‑

For a centrifugal pump it can be shown that the theoretical relationship between discharge head “H” and throughput 'Q' is a straight line. Throughput is minimum when head is maximum. Because of the eddy losses caused by impeller blade thickness & other mechanical considerations there will be some head loss, increasing slightly with throughput. These losses together with friction losses due to fluid contact with the pump casing and inlet & impact losses result in the actual H/Q curve as shown below.

The discharge characteristics of centrifugal pump are obtained by measuring the throughput 'Q' with increase in head “ H” during a test at constant speed. The actual discharge characteristics provide important information for the designer of a pumping system.

From power curve 'HP/Q' we see that minimum power is consumed by the pump when there is no flow & discharge head is maximum. This equates to the discharge valve being closed. Maximum pressure of the pump, with discharge valve closed, is only moderately higher than the working pressure. That is the reason a relief valve is not necessary for a centrifugal pump.

CARGO PUMP

Centrifugal pump is used for cargo pumping driven by hydraulic motor with all required pipe connections etc, Complete pump system has been subdivided in. three main parts ‑ Top cover plate, pipe stack & pump unit.

Top cover plate : Installed on top cover plate are ‑ Flow control valve for pump speed control with pilot pressure connection for remote operation & local control valve for local operation, Hydraulic oil inlet & outlet connections, Cargo outlet connection, stripping connection & purging connection for pump cofferdam i.e. air/inert gas inlet & exhaust connections.

Pipe stack : A hydraulic pipe unit and a cargo discharge pipe forms the pipe stack. Hydratflic pipe unit is made of three concentric pipes leading from top cover plate to pump unit. Inner, pipe is high pressure hydraulic pipe, outside of this pipe is low pressure oil return pipe & outermost is the cofferdam which segregates cargo from hydraulic oil.

Pump unit : it consists of a hydraulic motor driven single stage, vertical, centrifugal pump. In the pumphead there is a cofferdam connected to cofferdam in pipe stack. Cofferdam purge pipe is connected to the pumphead cofferdam. Any leakage of oil whether cargo or hydraulic oil will be blown through by air or inert gas to the cofferdam purge pipe. Shaft seal arrangement is made to‑ segregate cargo & hydraulic oil. For repairs & overhaul of pump refer relevant instruction manual onboard.

PURGING OF PUMP COFFERDAM

Purging of cofferdam is one of the most import ant operation because it is the only way to check condition of pump seals. If it is done regularly & necessary action is taken in time in case of large leakage, a troublefree operation of cargo pump is obtained.

Purging is important to detect leaks & monitor condition of shaft seals & to avoid that leakage is blocking the cofferdam. The following procedure is followed for purging

‑ Prepare a container below the exhaust trap to collect leakage.

- Check that the cock & bottom of exhaust trap is not clogged.

- Check that the drain hole from the relief valve on the purging valve block is open.

- Connect air or inert gas supply to the purging valve. Maximum pressure 7 bar.

- Drain the supply line of any condensate.

- Take necessary precautions to avoid danger from dangerous cargoes. Use safety gear as required e.g. safety

   clothing &   goggles etc.

- Open air/inert gas supply line.

- Check that air/inert gas is coming out of the exhaust trap vent line indicating cofferdam is open.

- Relief valve on the purging block is set to open at pressure 3.0 to 3.5 bar, hence a small leakage is normal.

- Drain exhaust trap in between to collect any oil.

- Purge cofferdam in several sequences as required & drains exhaust trap in between each sequence.

- Disconnect air/inert gas supply.

- Close exhaust trap drain valve.

- Measure the amount of leakage oil, evaluate & log purging results in the log sheet.

Purging Intervals :

- Immediately before cargo loading.

- One to two days after loading. If no leakage then purge after every 10‑15 days. If there is cargo leak in any pump

  cofferdam, purge same every day.

- If pumps are used for cargo recirculation during the voyage, cofferdam must be purged before & after start & stop of

  the pump.

- Immediately before & after every discharging.

Evaluation of purging results :

Cargo leakage ‑ Cargo in cofferdam can come from flange face seals in pipe stack or pump head, shaft seal (Double lip cargo seal), and cracks in pipe stack or purge pipe. A small leakage about 0‑.5 Itr/day during pump operation is normal. For short period of time, high leakage peaks can occur. If leakage is more than 2 Itrs/day, then pump must be purged a couple of times daily to confirm & reason found.

Hydraulic oil leakage ‑ Hydraulic oil in cofferdam can come from flange face seals in pipe stack or pump head, crack in pipe stack or pump head, pump shaft mechanical seal & lip seal. A small leakage rate of about 0.25 Itr/day from seal is normal during pump operation. For short periods of time a higher leakage peak can occur. If leakage rate is increasing above acceptable level, pump must be purged a couple of times daily & inspected to find the reason of leakage.

PRESSURE TESTING OF COFFERDAM

If purging of cofferdam has indicated that leakage is more than the accepted level & action must be taken for rectification, the first thing you have to do is identify the leakage. The best way is to pressure test the cofferdam.

Never start dismantling the pump unless until you know if, what & where is the problem.

 A standard pressure testing kit with blank flanges is provided on board.

Refer instruction manual onboard.

CARGO LEAKAGE ‑ If there is a cargo leak it does not mean that leak is from cargo seal only & you open up the pump & change cargo seal while leak may be somewhere else. This is just a waste of time & money. So identify the leak first. Cofferdam can be pressure tested by blinding off the purging medium relief valve with rubber gasket. Disconnect cofferdam's riser pipe flange on top cover plate & install a test flange with pressure gauge. Connect the purging medium (air) to the test flange & increase the pressure to 3 ‑ 3.5 bar max. See figure 1.

If cargo leak was heavy & pressure is falling down rapidly, then most probably there is a crack in cofferdam riser (purge) pipe or pipe stack.

If leakage was not much & pressure is not falling then after about 5 minutes check all flanges around cargo seal, riser pipe & all other connections for possible leakage. Use soapy water for better detection of possible leakage.

If it is not possible to detect leakage & pressure of 3 bar is stabilised for a long period of time, it is possible that cargo seal is worn out. The reason of pressure not falling down may be, that the 3 bar pressure was pressing the upper lip (towards cofferdam) of the cargo seal around pump shaft & there was no leak during testing. Dismantle cargo seal for inspection & renewal. If this is not the case & it is difficult to detect leakage, then you have to split the cargo pump & pressure test the components separately.

HYDRAULIC OIL LEAKAGE ‑ Under normal circumstances hydraulic oil leak is very seldom. Hydraulic oil leak can be due to mechanical seal failure, crack in pipe stack which can occur if there are abnormal vibrations in the pipe stack or corrosion in seal ring grooves after long service.

In case of hydraulic oil leak you normally have to split the pump for testing & identifying the leak. See figure 2.

CARGO OPERATIONS

LOADING THROUGH THE PUMP

0

- Maximum acceptable loading pressure is 8 bar at top cover plate. The pump is equipped with an anti‑rotation device

  enabling the pump to be used as a drop line for cargo loading. It is very important to check the pressure gauge on

  flow control valve for any indication of hydraulic oil pressure during cargo loading. Any rise in hydraulic pressure

  on gauge during cargo loading indicates that pump is turning. Stop loading through the pump. Anti‑rotation device must

  be checked as soon as possible.

- If a separate drop line is installed, start loading through the pump only until 1 ‑foot sample is taken, and the cargo tank

  level is above the drop line outlet. Then open valve to drop line & continue loading through both the pump & the drop

  line.

CARGO DISCHARGING

‑ After starting high pressure hydraulic system, increase system pressure to 150 bar.

- Start cargo pump slowly & let it run with hydraulic pressure 40‑50 bar for about 1‑2 minutes.

- Raise the pump's discharge pressure above manifold pressure to avoid back flow in to the tank. Then open the cargo

  pump discharge valve.

- Increase the pump hydraulic pressure until required discharge pressure or capacity is achieved. If required increase the

  system pressure. The system hydraulic pressure should be maximum 10‑15 bar above the highest consumer

  pressure in order to minimise energy consumption, noise level, wear & tear.

- Pump capacity should be controlled by the pump hydraulic pressure & not by throttling the cargo discharge valve or any

  other valve in the cargo piping system. This can be achieved easily by keeping all cargo pump controllers in maximum

  position & regulate the main hydraulic system pressure until required discharge rate is achieved. If necessary each

  cargo pump must be balanced individually by decreasing/increasing cargo pump hydraulic pressure.

   Generally it is recommended to run as many pumps in parallel as practical at a reduced hydraulic pressure

   rather than just a few pumps with maximum hydraulic pressure.

-  At the end of the discharge, cargo pump may run dry & suck air which will be indicated by vibration in pump & hunting

   in hydraulic pressure. This can easily be avoided by reducing the pump hydraulic pressure & throttling the discharge

   valve as required.

CARGO PUMP OPERATIONAL ADVICE

Operation of cargo pumps in its optimum range is imperative to get maximum lifetime of the pumps & to operate at minimum noise.

STRIPPPING OF CARGO TANKS & CARGO PIPE

‑ After cargo discharging is complete, stripping is done by cargo pumps only. Best possible stripping results are obtained with minimum manifold back pressure. Air or inert gas is used to pressurize cargo pump discharge pipe & pump is run at slow speed locally.

- Close cargo pump main discharge valve.

- Open stripping line valve to discharge line.

- Connect air or inert gas hose to the connection provided on top cover plate of pipe stack for pressurizing the cargo

  discharge pipe.

- Start pump slowly & keep about 60‑80 bar pressure. This depends on practical experience. Open air/inert gas to pipe &

  maintain 4‑5 bar pressure. It has been observed that if pressure is less or if 2‑3 tanks are being stripped together, then

  stripping is not good.

- A booming noise is heard when cargo tank is empty.

- Close stripping valve.

- Shut air/inert gas.

- Stop pump.

TANK CLEANING

‑ A separate tank cleaning pump is provided for tank cleaning operation. To keep a dry tank top & still avoid dry running of

  cargo pump during tank washing, the capacity of tank cleaning machines & cargo pump discharge capacity must be

  kept equal. To achieve this increase or reduce the cargo pump hydraulic pressure accordingly until capacities are

  balanced.

- Before tank cleaning is finished, close cargo pump discharge valve & open stripping valve to allow an increase in water

  level of the tank. Run pump at about 100 bar pressure against closed discharge,valve for 1‑2 minutes to clean the pump.

- Remember to flush the tank & pump with fresh water if sea water has been used for tank cleaning.

CARGO HEATING

Cargo heating is done by means of deck mounted steam heaters & forced cargo circulation through submersible cargo pumps. Advantages of this system as compared to heating coils in tank are

‑ Good mixing of hot & cold cargo in tanks.

- Easy regulation of temperature in each tank.

- Clean tank surface & hence easy tank washing.

- Small heating surface

- High efficiency.

- Forced circulation & high velocity of cargo through heater means lower surface temperature & risk of carbonising

  avoided

- As cargo is passing the heating surface at high velocity, the temperature of heating medium can be kept higher than

  coils in tank

- Easy maintenance (on deck only)

- In combination carriers (OBO) heating coils have to be removed from tanks every‑time for loading dry cargo & installed

  again before loading heating cargo.

- Heater is exposed to cargo only when heating is required

Cargo heating operation –

Refer hydraulic diagram & its description on page 10, 11 & 12, three electric motor driven hydraulic pumps are provided in the system for the purpose of cargo heating.

For circulation of cargo through heaters, cargo pumps require hydraulic pressure of 40‑60 bar. Low pressure pumps give a fixed system pressure of 50 bar.

- Check available data of maximum recommended temperature of both heating medium & cargo

- Change over the switch on ccr panel from high pressure system to low pressure system operation

- Start any one or two or all three pumps as required Check system pressure is maintaining 50 bar Start cargo pump & ensure circulation through heater before heating medium is opened

- To avoid overheating, never start heating without confirming proper cargo flow through heater

- Shut off heating medium before pump is stopped.

- Drain heaters properly after heating operation is completed.

- Do not use sea water for cleaning heaters. Steam connections are provided for cleaning heaters.

Very few cargoes require heating only for the purpose of becoming pumpable. In a tanker where cargo pumps are located in the pumproom, high frictional losses occur through the suction pipe while pumping viscous cargoes. Heating is required to reduce viscosity to overcome problem of frictional losses. With submersible cargo pumps this problem is avoided & viscous cargoes can be pumped, although at reduced capacity. Heating can therefore be only limited to the extent required or for those cargoes which are not pumpable in cold condition.

Heat loss is basically proportional to the temperature difference between cargo & ambient temperature of air / sea. When no cargo heating is done, the temperature drop is approximately 1‑2 deg c per day. Most heating systems are designed to raise temperature by 5‑6 deg c per day. Heating operation is expensive. Saving achieved by faster unloading of cargo due to reduction in viscosity by heating, is much less than the money spent on heating.

CARGO    HEATING

OPERATIONAL ADVICE

  Heating by means of deck mounted heat exchangers and forced circulation by submerged cargo pumps

PORTABLE CARGO PUMP

A hydraulically operated portable cargo pump is generally provided on board for use in case any main cargo pump becomes inoperative & can not be repaired. This is a submersible type centrifugal pump & is of high capacity with low discharge head. Discharge capacity is reduced very much if back pressure is high. That is why it is better to use this pump for transferring cargo from one tank to another & not for discharge directly to manifold. Pump is supplied with hydraulic hoses, connections & a flow control valve for regulation of pump capacity.

PRECATIONSTO TAKE –

- Never connect the portable pump to the main system without using a flow control valve for speed control of the pump.

- Always connect the oil return side first & then the pressure side.

- Be sure that the snap‑on couplings are secured by means of the locking ring.

- Ensure that the handwheel on the starting head of the flow control valve is secured by the locking screw & in closed

  position.

- Test run the pump on deck to ensure that oil flow is free & pump runs normal.

- After use, always clean the pump & flush with fresh water.

- While disconnecting the oil hoses, pressure side to be disconnected first & then return side.

- Pump, hoses & couplings must be stored properly in clean condition & in safe & secured manner so that it is easily

  accessible & ready for use next time.

- As this unit is only required in emergencies, hence may not be used for long time. Unit must be tested for proper

  operation at least once or twice a year.

Refer relevant instruction manual on board before using the pump.

SYSTEM MAINTENANCE

Hydraulic systems generally do not require any major routine overhauls, if system is operated carefully, & hydraulic oil condition is maintained good. A proper watchkeeping & good observation of the system during operation ensures earlier detection of any trouble to avoid major breakdown.

Major overhaul of hydraulic pumps, motors, flow control valves, gearbox & clutch system etc, are generally done by the makers, as they are precision jobs & require proper tools, equipment & expertise.

ROUTINE INSPECTIONS / PREVENTIVE MAINTENANCE

- Once a month check magnetic chip detectors on hydraulic pumps for any metallic particles, which will indicate wear  

  down or damage in pumps.

- During operation of the system, note down the differential pressure across the filter. At full load on the system

  differential pressure up to 0.3 bar is normal. At 0.8 bar differential pressure filter elements must be renewed.

- Every three months system oil sample must be landed for analysis including particle count.

- Maintain hydraulic oil in good & clean condition.

- Whenever system is stopped for any maintenance, purge the system for any air after restarting. Also deairate the

  system on regular basis.

- Gear oil cooler & filters to be cleaned as required.

-  Gear oil to be tested for any water content every month with onboard test kit.

- All alarms & trips must be tested regularly.

- Cargo pump & pipe stack cofferdam to be purged before & after cargo loading as well as discharging. Any cargo or

  hydraulic oil in cofferdam will indicate seat leakage. Findings must be recorded by chief officer in the log sheet provided,

  which is to be sent to the company after completion of voyage.

- Check coupling between hydraulic pumps & drive shaft from gearbox. These coupling are either grease filled or oil filled.

  Oil filled couplings are splined couplings & it is very important that they are always filled with oil as per recommendation. - Check‑condition of cargo pump against performance curve at least once a year.

- Never start dismantling any component unless until you know if, what & where is the problem.

- Cleanliness is of utmost importance.

SOME PROBLEMS OBSERVED IN A TYPICAL HYDRAULIC SYSTEM

A.   Unable to run any one cargo pump at full rpm or not at all ‑ i.e. oil pressure at pump inlet does not increase to maximum or not at all whether started locally or from ccr.

      Reasons : 1)  Local control valve may be leaky. Renew same.

                       2)  Orifice of corresponding pump at orifice block in pilot oil line in engine room is clogged fully or partially.

                            Remove & clean.

                       3)  Orifice at pump start handle in ccr is clogged. Remove & clean.

Generally 1) or 2) is the reason, check them first & then check 3) as to open orifice at start handle in ccr is difficult & time consuming.

B.   Hunting of cargo pump during starting ‑ i.e. Unable to run pump at low pressure steadily. Pressure & hence pump rpm keeps hunting continuously. But at higher pressure pump operates normally. Either the reason is 2) or 3) as mentioned above or orifices of pump flow control valve is partially clogged. Remove & clean. Sometimes it has been observed that none of the orifices are clogged, then take advice from the makers. Check the size of the orifice which is normally 100 i.e. 1.0 mm dia. This may require replacement to size 80 i.e. 0.8 mm.

Do not replace any orifice without advice from the makers.

                                                                                                                                            

C.   Fluctuation / continuous hunting in system pressure when increased to required     setting – OR             system

      pressure does not increase to maximum even with pressure control potentiometer on maximum ‑ Generally

      problem lies with pilot pressure controller.

      Normally one spare is always there. Replace & test the system. There are a lot of orifices in pilot pressure controller

      which gets clogged partially & have to be cleared. Be very careful in overhauling this part as this is one of the most

      important component of the system.