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40 Guidelines for the layout of pumping and piping systems for efficient stripping
Geldigheid:01-02-1987 t/m 30-04-2000Status: Was geldig

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An explanatory note for the design of cargo systems and stripping systems of ships subject to Regulation 5A (1) of Annex II of Marpol 1973/1978 and to Resolution MEPC 18(22)-Standards for Procedures and Arrangements for the Discharge of Noxious Liquid Substances.

Date of issue: February 1987

Subject to alterations

Guidelines for the layout of pumping and piping systems for efficient stripping.


Subtitle

An explanatory note for the design of cargo systems and stripping systems of ships subject to Regulation 5A(1) of Annex II of Marpol 1973/1978 and to Resolution MEPC 18(22)-Standards for Procedures and Arrangements for the Discharge of Noxious Liquid Substances.

Contents
1. Purpose and limitations
2. Type of liquids to be handled by stripping systems
3. Location of main amounts of systems
4. Principle of stripping systems
4.1.General
4.2.Ship attitude
4.3.Cargo piping layout
4.4.Internal diameter of stripping lines
4.5.Deepwell pump with footvalve
4.6.Submerged pump
4.7.Positive displacement pump
4.8.Gaseous pisten pump
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40.01 Purpose and limitation


the purpose of these guidelines is to provide guidance for the design of cargo piping and efficient stripping systems on chemical tankers subject to Regulation 5A (1) of Annex II of Marpol 1973/1978 and to Resolution MEPC 18(22) - Standards for Procedures and Arrangements for the Discharge of Noxious Liquid Substances. These guidelines are limited to the parcel tanker type, provided with a cargo pump in each tank and shore connections at midlength of the cargo area and to systems for small depth ships equipped with on-deck positive displacement cargo pumps.

Facilitation of tank and piping system cleaning and drying is a prime consideration in the choice of a stripping system. However this is outside the scope of these guidelines. In general the measures required for proper stripping of the piping system will facilitate cleaning and drying of such systems.

It is recommended that shipowners and piping system designers carefully consider the implications of tankwashing methods on the required piping layout.
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40.02 Type of liquids to be handled by stripping systems.

For the denomination of liquid categories the terminology as defined in the Standards for Procedures and Arrangements for the Discharge of noxious Liquid Substances is used. Stripping systems should be designed to handle low viscosity, non-solidifyinf liquids, not containing particulate matter.
High viscosity, non-solididying liquids may be handled by stripping systems but this may imply increased stripped time. In no case the prewash mandatory by virtue of the above mentioned Standards for such liquids can be replaced by stripped.
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40.03 Location of main amounts of residue.


After unloading a cargo tank having contained a low viscosity, non-solidifying substance the main amounts of residue are to be found on the tank bottom, in the suction well if any anmd in the cargo piping, provided that the tank construction and vent piping layout is such that no cargo liquid be retained by stiffeners, girders or in vent piping.
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40.04 Principles of stripping systems


4.1. General
All stripping systems do rely on gravity forces to drain liquid from cargo piping and tank bottoms. Therefore during stripping operations the tankbottom and the cargo piping shall have a positive downward slope towards the suction point or stripping line as applicable. Experiments have shown up- hill blowing of cargo piping to be ineffective.

4.2. Ship attitude.
At the end ofa cargo tank unloading operation the tankbottom shall have a positive downward slope towards the suction point. In the longitudinal direction this slope should be at least 0.5 per cent (5 mm per metre). In the transverse directionthis slope should be at least 1 per cent (10 mm per metre). This requirement directly affects ship design and should therefore be considered in an early stage of design. Ballast arrangements to control ship attutide may be required.
Instrumentation showing ship attitude should be provided in the cargo control room or in anothersuitable location.

4.3. Cargo piping layout.
In all cases during the stripping of cargo piping such piping should have an effictive downward slope towards the suction point of 1 per cent (10 mm per metre). Maximum adverse ship attitudes should be taken into account.
Most tankers are designed to have a trim by the stern during unloading. This implies that the downward slope towards the in-take suction point of the cargo lines of tanks forward of the shore connection may be reduced by stern trim. A prudent approach will be to give those lines a downward slope relative to the design summer loadline of the maximum stern trim in per cent plus 1 per cent.
In the transverse direction the downward slope of the piping relative to the design summer loadline should be the maximum ship”s list in per cent plus 1 per cent.
Dead ends pockets where cargo may be trapped should be avoided as far as possible. Where dead ands or pockets are unavoidable plugged or capped drain cocks should be fitted.
Connecting cargo lines to a main line requires careful consideration to avoid pockets.

4.4. Internal diameter of stripping lines.
Succesful stripping depends largely on the correct choice of the stripping line internal diameter.
Undersized stripping lines will cause unacceptable stripping times. Oversized stripping line will retain too much residue.
For Design purpose a stripping times of 10 minutes associated with a back pressure of 1 bar at the delivery and may be used.
Where on-deck positive displacement cargo pumps are used for stripping purpose small bore stripping lines may cause the pump to run dry and cause damage. Minimum flow requirements should be obtained from the pump manufacturer.
Experience up to now indicates that except in case of on-deck main cargo pumps of the positive displacement type are being used the internal diameter of the stripping line should be between 35 and 50 mm.

4.5. Deepwell pump with footvalve.
Figures 1, 2 and 3 show schematically the deepwell pump arrangement for three operating modes:
fig.1: loading;
fig.2: discharging: and
fig.3: line stripping.
In cause solidifying, highly viscosity substances or liquids containing particulate matter are to be carried the stopvalve (1) should be closed before loadline.

4.6. Submerged pump.
Figures 4,5,6 and 7 show schematically the submerged pump arrangement for four operating modes:
fig.4: loading;
fig.5: dischargeing;
fig.6: stripping riser; and
fig.7: stripping cargo line.
The separated stripping operations for the pump riser and for the cargo line are intended to reduce the time rhe pump must be kept running without flow. This also reduce the total stripping time required because riser stripping should be done under moderate iar or inert gas pressure to avoid a pump bow- out. the cargo line may be stripped under full working pressure, subject to accetance of the shore installation.
In case hight viscosity substances, solidifying substances or substances containing particulate matter are carried stop valve (1) should be kept closed.

4.7. Positive displacement pumps
4.7.1. Ships od small depth may be equipped with on-deck positive displacement pumps, which may also be used as stripping pumps. Provision is made to drain the main cargo lines into the tank, after which the main cargo lines are shut off and the stripping lines connected to the cargo pump.
4.7.2. Separate in-tank positive displacement stripping pumps with an on-deck actuator have been proposed. In this case the stripping line may remain permanently connected to thestripping pump, except in case high viscosity substances solidifying substances or substances containing particulate matter are carried.

4.8. Gaseous piston pump.
A schematic arrangement of the gaseous piston is shown in fig.8 to 12. Essentially the gaseous piston pump is a receiver, fitted withsuction and delivery lines, both provided with non-return valves. Through a third line vacuum and pressure may ne exerted on the receiver. Under vacuum the receiver is filled with liquid and under pressure the receiver contents are moved into the delivery line. The alternate application of vacuum and pressure may be compared to a machenical piston, hence the name ”gaseous piston pump”.
A timing mechanism to regulate the alternate application of vacuum and pressure should be provided. For reasons of efficiency an intermediate phase in which the pressurized air or inert gas is vented into the tank before vacuum is applied, may be added. Vacuum may be obtained from an ejector driven by water delivered by a ballast pump.
The non-return valves should be fitted with a valve lifting device to facilitate draining and drying after tank cleaning. The non-return valve nr. (1) should also be fitted with means to keep it closed in case high viscosity substances, solidifying substances or substances containing particulate matter are carried. Figures 8, 9, 10, 11 and 12 show schematically the arrangements for five operating modes:
fig.8: loading;
fig.9: dischargeing;
fig.10: stripping cargo line;
fig.11: line draining;
fig.12: tank stripping.
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