P9 Guidelines for the Flushing of Hydraulic Systems I2

BFPA

/

P9

GUIDELINES

FOR THE FLUSHING

OF HYDRAULIC

SYSTEMS

Representing Hydraulic & Pneumati<j_wtfJ tj,ies

BF

P

A

BFPA/P9

GUIDELINES

FOR THE FLUSHING OF

HYDRAULIC SYSTEMS

Addendum

The publication mentioned throughout this document titled BFPA/P54 -Guidelines for the Comparison of Particle Counters and Counting Systems for the Assessment of Solid Particles in Liquid should be BFPA/P55.

July 2004 BFPA/P9

FOREWORD

The first issue of this document was originally published in May 1978 as AHEM/9 and was prepared by Technical Committee El - Contamination Control. At that time, members from the following companies were represented on Committee El under the Chairmanship of Mr N Way of Pall Europe Ltd: Automotive Products Ltd BHRA Dowty Hydraulic Units Ltd Fairey Filtration Ltd Fawcett Engineering Institution of Plant Engineers Lucas Industrial Equipment Ltd National Coal Board NEL

Pacific Scientific International Inc. Pall Europe Ltd

Plessey Hydraulics Sperry Vickers Group Tell-Tale Filters Ltd Y-ARD Ltd

It has now been completely revised by Technical Committee El: Chairma n: Membe rs: Mr N Way Mr R Baker* Mr M Fairhurst Mr B Goodwin Mr C D Hart Dr T M Hunt* Mr K Martin Mr S J Purser Mr R T Roderick Mr J A Smith Dr D G Tilley Mr M J Wates Mr P J Wilson Pall Europe Ltd Howden Wade Ltd BHR Group Ltd Parker Hannifin plc

Fawcett Christie Hydraulics Ltd Lindley Flowtech Ltd Parker Schrader Bellows Division Vickers Systems Ltd Flupac Ltd Pacific Scientific Ltd University of Bath Norgren Martonair Ltd UCC International Ltd

* Members of special working group who, along with M Griffin of Norson Power Ltd, I Burniston of Pall Europe Ltd, and A Whitehouse of Marioff Services Ltd, prepared the draft for the Committee's approval.

These Guidelines are intended to reflect the opinion of the Association only and a User should also consider manufacturers' instructions before using any particular product. Whilst the Association has taken all reasonable care to

ensure the accuracy of these Guidelines, no liability or responsibility in negligence or otherwise whatsoever shall be accepted by the Association, its members, servants or agents as to the content or interpretation of these Guidelines.

No part of this publication may be photocopied or otherwise reproduced without the prior permission in writing of the Association. BFPA Guideline documents are regularly reviewed and readers are advised to check the validity by contacting the Association at the address given below.

British Fluid Power Association, Cheriton House, Cromwell Park, Chipping Norton, OX7 5SR. Tel: 01608 647900. Fax: 01608 647919. E-Mail:

enquiries@bfpa.co.uk Website: http://www.BFPA.co.uk

BFPA/P9 TABLE OF CONTENTS Subject Page 1.0 Introduction 1 2.0

Decision Balance for Flushing 1 3.0

Condition of Components 2 4.0

Condition of Steel Pipes 2 5.0

Site Conditions and Storage 3 6.0

Flushing Fluids 3

7.0 Minimum Flushing Procedures Using the System Pump 3 8.0 Use of Specialist Flushing Equipment 4 9.0

Flushing of Operating Systems 6 10.0 References 7

Appendix 1 8

Appendix. 2 9

-1-GUIDELINES FOR THE FLUSHING OF HYDRAULIC SYSTEMS

1 . 0 INTRODUCTION

The aim of flushing is to remove contamination from the inside of pipes and components which is inherent or may be introduced during system assembly. This is accomplished by passing fluid through the system usually at a velocity much higher than that during normal operation.

Flushing is a procedure which is vital to the satisfactory operation and life of a hydraulic system. Omission or curtailment of flushing will inevitably lead to rapid wear of components, malfunction and breakdowns.

The range of types of hydraulic system is very diverse, both in size and complexity. This document provides guidance applicable to all systems. Careful judgement must be exercised in modifying or supplementing the procedure outlined here to a particular installation.

It is the responsibility of the system designer to specify the required cleanliness standard and the checking procedure to be adopted.

For more information on the levels of cleanliness required for reliable operation, refer to table 3 in BFPA/P5 - Guidelines to Contamination Control in Hydraulic Fluid Power, and for information on possible methods to check the cleanliness levels, refer to BFPA/P54* - Guidelines for the Comparison of Particle Counters and Counting Systems for the Assessment of Solid Particles in Liquid.

2 . 0 DECISION BALANCE FOR FLUSHING

The amount of care and effort that is put into flushing has to be decided upon on an individual basis. For example, a sub-sea actuator system for North Sea oil is a simple hydraulic system, but because it has to be right first time every time, every effort is put into ensuring maximum cleanliness. On the other hand, a small company that occasionally produces a small economically priced power pack for a non-critical application may have only the most elementary precautions taken. Set out below are some general recommendations on the selection of the level of flushing required for various categories, each level implies that the lower ones are also covered where appropriate.

Typical Applications

Level 1 Very dirt tolerant systems w i th severe bu dget limitations (small power packs)

Level 2 Average systems where OEM has sufficient quality system to implement control within manufacture process Level 3 Average systems where

OEM has quality system to control suppliers

Operations Carried Out

Component suppliers asked to maximise cleanliness and ensure build of equipment is as clean as their facilities will allow.

Cleanliness standard instituted for machinery.

Component supplier required to meet cleanliness specification.

-2-Typical Applications Operations Carried Out

Level 4 Moderately sensitive systems and where high production rate requires minimum time for flushing (mobile plant, moderate size power packs)

Level 5 Systems with very sensitive components such as servo v a l v e s ( i n j e c t i o n moulding/tensile testing machines)

Level 6 Large systems requiring high levels of reliability (turbine controls, off-shore systems)

Flow through sensitive components avoided during start-up.

Sensitive components replaced with flushing blocks during flushing operation.

Sub-systems flushed individually and checked for cleanliness before total assembly of system.

Level 7 Absol u te required systems) perfor m ance (eg. satellite

Clean Room conditions required for components and systems.

3 . 0 CONDITION OF COMPONENTS

All components and hoses must be supplied ready for assembly into the system in pre cleaned condition, and all openings sealed. Temporary sealing devices, plugs, etc. should only be removed immediately prior to assembly. In addition, and where applicable, the component may be inhibited with a preservative oil. For more information on methods of assessing the cleanliness of components, refer to BFPA/P48

- Guidelines to the cleanliness of hydraulic fluid power components.

3 . 1 Coolers

These should be received from manufacturers in the clean condition, if not, contact the manufacturers.

3 . 2 Reservoirs

Unless supplied in the clean condition, the reservoir should be cleaned by grinding with mechanical grinders, steel wire brush or other suitable cleaning devices to remove rust, mill scale, welding splatter, greases, dirt and other foreign matter such as joint and gasket compounds. If adequate safety precautions are adhered to, petroleum solvents may be used for cleaning and degreasing reservoir surfaces. Toxic or highly flammable solvents such as carbon tetrachloride or benzene must not be used for cleaning. The surface must be wiped with clean lintless cloth (not waste). Putty or other suitable compounds may be used to pick up all loose dirt and metallic particles from corners and the base of the reservoir. After thorough inspection of all parts of the reservoir and re-cleaning where necessary, all openings should be sealed.

If the internal surfaces are to be left untreated, the reservoir should be coated with a rust preventative oil readily soluble in the flushing oil and then closed and sealed. Care should be taken to see that no water or foreign material is trapped in low pockets, inaccessible corners or horizontal surfaces.

In systems where the reservoir is integral with the machine, it should be cleaned and protected as above before installation.

4 . 0 CONDITION OF STEEL PIPES

Particular attention should be given to piping, which should be free from scale, rust, flux, etc. Piping not in the clean condition should either be rejected and replaced or cleaned before assembly. Thorough cleaning before assembly may not be possible in

the case of very large pipes so that these will require special attention during flushing to attain the required standard of cleanliness.

Before installation, and after all fabrication has been completed, all pipes and fittings, unless supplied in the clean condition should be treated as follows:

All accessible welds on pipes and fittings must be inspected for the presence of welding beads. If beads are found they should be ground off before the cleaning procedure is undertaken.

All fabricated steel pipes and flange fittings should be blown with steam or air to remove loose scale, welding beads, dirt or sand. They should then be immediately submerged and washed in a hot alkaline solution, prepared and used in accordance with recommended practices. During this operation all dirt, paint and grease is removed from the metal thereby preparing the pipe and fittings for pickling. The parts, with the exception of the valves, should then be pickled by any one of the standard pickling processes in order to remove mill scale from the metal. It is extremely important that all traces of pickling materials are removed by water washing or other suitable means so that subsequent contamination of the oil with the acidic products cannot occur.

After thoroughly drying the pipe with dry air, it should be coated with a suitable rust preventative oil which will be readily soluble in, and hence removed by, the flushing oil.

Finally, all the openings should be closed to prevent the entry of foreign material and excessive moisture.

5 . 0 SITE CONDITIONS AND STORAGE

In most cases, hydraulic systems are not assembled in clean room conditions. Care must, therefore, be exercised during system assembly to minimise the ingress of contamination, since in the majority of cases, site conditions cannot be controlled. It

is essential to avoid areas in the proximity of welding, grinding, air cleaning and any other contaminant generating activities. Piping and components awaiting assembly should be stored in a dry place or at least elevated and covered, with all ports and openings sealed.

6 . 0 FLUSHING FLUIDS

Although special flushing fluids, with good solvency power, inhibitors and de watering properties are available, the majority of flushing operations take place with the system hydraulic fluid for reasons of cost. If the intended system fluid has too high a viscosity to achieve the required turbulent flow, then a lower viscosity classification (BS 4231) of the same oil can be used.

Should special flushing fluids be specified, then it is essential to ensure that they are compatible with all the components in the system, particularly elastomers in seals and hoses. Thorough draining of flushing oil will be required with the additional possible requirement of a displacement oil to be charged into the system immediately after removal of the flushing oil. Delay may result in rusting of the newly cleaned smfaces.

7 . 0 MINIMUM FLUSHING PROCEDURES USING THE SYSTEM PUMP

Procedures needed will vary greatly and this section covers the minimum procedure required for simpler systems.

Ideally, flushing should be accomplished by passing fluid through the system at a velocity which is much higher than that occurring in normal operation. This requirement, however, necessitates the use of a separate pumping system. Additionally

some systems, particularly those using a fire-resistant fluid, may involve the use of a special flushing fluid which must be compatible with the fluid ultimately to be used. Recommended procedures for the use of separate pumping systems and flushing fluids are described in paragraphs 8.0 and 6.0 respectively.

If due precautions are taken, many systems can be adequately flushed using the system pump, and the procedures outlined below:

Fill the system with clean fluid, preferably by pumping it into the reservoir though a filter.

After starting the pump, ensure that it is properly primed and that the outlet pressure is low.

As the system fills, the level of fluid in the reservoir will drop, and filling should continue until the level of fluid in the reservoir is stable. This is particularly relevant when the system contains relatively large volumes in the pipework and cylinders.

The direction of flow through all parts of the system should be changed as often as possible, but extreme care must be exercised to ensure that damage is not caused by operating equipment out of sequence.

To ensure that the system is flushed as quickly and efficiently as possible, both fluid velocity and temperature should be as high as conveniently possible. All filter condition indicators should be monitored continuously, and any heavily loaded filter elements replaced before the element starts to bypass. At reasonable intervals fluid samples should be taken and the contaminant level measured, for example, after initial start-up and then after 10, 50 and 100 theoretical recirculations.

Flushing should continue until the sample indicates that the specified level of cleanliness has been achieved.

Any temporary equipment should now be removed and the complete system reconnected. When the system is re-started, additional fluid may be required to fill actuators that have been bridged out during the initial flushing.

Reconnecting components such as cylinders which have been bridged out during preliminary flushing can introduce further contamination into the system, so the flushing process should be continued until the specified fluid cleanliness level is again reached.

The equipment should subsequently be monitored in its working environment to verify that the filter element rating is satisfactory. If the required cleanliness level cannot be achieved in a reasonable period, it may indicate that the filters designed to maintain the general system cleanliness level need to be fitted with finer elements. In any case, it is prudent to change all replaceable elements at the end of a flushing period and examine, and if necessary clean any strainers. Finally, the system should be run again for a short period to remove any air which might have been introduced during the filter element change, and the reservoir fluid level re-checked.

8 . 0 USE OF A SPECIALIST FLUSHING EQUIPMENT 8 . 1 System Preparation

Only the simplest system can be flushed in one operation. In the majority of systems they should be divided into sections or loops, the sections being flushed in

turn. Pump stations and valve manifolds may be flushed separately from the piping system unless they have been supplied in the clean condition.

System components which may present a restriction to the flow or which may be damaged by the high flushing flow should be bypassed by suitable make-up pieces. These components, which may have been flushed on assembly, include reservoirs, pumps, motors, dead-end components such as cylinders, servo valves, flow regulators and other valves containing small orifices. Filters may be left in place but with their elements removed.

It is preferable that air bleeds should be fitted to all high points in the system to ensure filling of pipes and components.

Drain points should be fitted to every low point in the system and to dead-ends to assist in the removal of contaminants. Dead-ends should be avoided as far as possible or bridged with make-up pieces.

Flushing connections should be fitted to each loop in the system. The connections should be large enough to pass the flushing flow at a low pressure drop.

8 . 2 Flushing Equipment

The flushing rig should comprise pumps, reservoirs, heaters, coolers, filters, return line sampling points and hose connections.

The pumps, sized to cater for the required flow range, should incorporate adjustable relief valves to be set at a pressure above that required to pass the full pump flow through the system to be flushed. The required pump flow capacity is defined in the next section.

The reservoir should have a capacity at least equal to three times the pump delivery per minute and preferably of greater capacity than the system to be flushed.

The heaters/coolers should be capable of controlling the flushing fluid temperature. Where electrical heaters are used, it is recommended that the oil supplier be consulted to determine the heat input requirements. Failure to select the appropriate heating arrangements could result in damage to the oil.

The filters should incorporate clogging indicators, preferably incorporating an audible alarm. If the filter elements are not able to withstand the full system pressure as differential pressure, then it is desirable to have an automatic filter bypass valve to prevent the bursting of blocked elements. This valve should not commence to open before indication, either visual or, preferably, audible is given. The filter size should be selected to pass the specified pump delivery at a low pressure drop, depending upon type and should have a generous dirt-holding capacity. The degree of filtration should normally be of a finer grade than that to be used in the final system.

8 . 3 Flushing Velocity and Temperature

To ensure that the system is flushed as quickly and efficiently as possible, both fluid velocity and temperature should be as high as conveniently possible. A reasonable temperature limit for mineral oils is 60°C, and for water-in-oil emulsions and water-glycols it is 50°C. Higher temperatures are acceptable for silicone and some other synthetic hydraulic fluids.

The fluid velocity should exceed the rated velocity of the system and the flow condition should be in the turbulent region at all points in the system. Turbulent flow is considered to be attained when the flow in litres/min is greater than 0.189 ud where d = pipe bore (mm) and u = viscosity (cST). The derivation of this formula is given in Appendix 1.

8 . 4 Flushing Procedures

New fluid is often dirty by the standards required for hydraulic systems, therefore, with the system prepared as described in Section 4, the flushing rig should be filled through a fine filter (3µm absolute).

It is advantageous to re-circulate the fluid with the flushing rig until the cleanliness level matches or exceeds the required standard.

Having connected the flushing unit to the system, circulate the fluid and bleed the system to remove trapped air. Heat the fluid to the temperature given in paragraph 8.3.

As explained in paragraph 8.3, turbulent flow and high velocities are essential for effective flushing operations. These conditions may be enhanced by introducing vibration of the pipe work by mechanical or other means, for example, pneumatic hammers fitted with pipe saddles. This has the effect of loosening contaminant adhering to the pipe particularly at formed bends and weld areas. Techniques such as flow pulsing and gas injection are also sometimes used. With all of the above procedures, great care must be exercised in their application.

During the flushing operation, the cleanliness level should be monitored on a regular basis. The condition of the filter element should be monitored using the filter clogging indicator or differential pressure readings and the element changed as required.

It is an advantage to reverse the flow through the system by interchanging the supply and return lines to the flushing rig. Care must be exercised where low and high pressure lines are being flushed simultaneously to avoid over pressurisation of low pressure lines.

Continue the flushing operation until two consecutive samples within the required cleanliness level are obtained.

If the system comprises several loops, each individual loop must be flushed to the required cleanliness level before changeover to the next loop.

Cleanliness levels are established by assessment of fluid samples using qualitative and quantitative methods. For certification to international standards, a quantitative method must be adopted, preferably by particle counting.

When all individual loops have been flushed to the required cleanliness level, all temporary hoses, flanges and fittings should be removed and the system pipework and components reinstated.

After system reinstatement it is desirable, where possible, to install a temporary filter in the system for the commissioning period to safeguard the established cleanliness level.

Should the system require topping up after the flushing operation, it is essential that the fluid is transferred through a fine filter, as new oil may not be sufficiently clean.

8 . 5 System Final Oil

A full charge of new hydraulic oil should be installed in the system as soon as possible. This charge should be introduced through a fine filter. The oil should then be circulated and the equipment operated so that the cleaned surfaces will be properly protected.

9 . 0 FLUSHING OF OPERATING SYSTEMS

In the event of a major component failure, complete system overhaul or severe contamination by water or sea water, it may be necessary to conduct a flushing operation before re-commissioning the system.

Only by a complete inspection of the system and analysis of the oil can the necessity for flushing be determined.

Sea water contamination requires a very special procedure for handling. After removal of excess salt water, corrosion inhibitors specifically developed for this type of contamination must be used in the system. Careful cleaning procedures must be followed to obtain a satisfactory condition in the hydraulic system. Following this the whole of the flushing procedure should be followed.

10 . 0 REFERENCES BFPA/PS:1991 BFPAIP48 BFPA/P54* BS4231

*

Guidelines to Contamination Control in Hydraulic Fluid Power Guidelines to the Cleanliness of Hydraulic Fluid Power Components

Guidelines for the Comparison of Particle Counters and Counting Systems for the Assessment of Solid Particles in Liquids

APPENDIX 1

The flow condition in a pipe or hose can be assessed using Reynolds Number as follows:

where: Re

=

v

₌

=

'\)

=

The relationship between flow velocity, flow rate and internal diameter is given by: V = 21.2 Q

ud2

where: _Q

₌

hence,

Re

=

To have complete confidence that turbulent flow is occuring, Reynolds Number Re must be greater than 4000, ie.

2.12 x 104 _Q_ > 4000 ud

The flow rate required to achieve this condition can be determined as follows: Q > 0.189 ud

APPENDIX 2 EXAMPLES OF LEVELS

OF FLUSHING A 2 . 1 Level 1 Example

Flushing method for simple systems

Type of Equipment

Small individual hydraulic power pack design to service coal shute deflector cylinders in power station.

System is 120 litre capacity, ISO 46 grade mineral oil.

Pump is 30 litre/min, 150 bar maximum pressure, external gear type, valves are solenoid operated, 200 micron suction strainer, filler breather, 25 micron return line filter.

Company Operation

Small company employing five persons, main business is as the distributor of hydraulic components, repair service and builder of five to ten small power packs per year.

Cleanliness and Flushing Operations

1 The pump, valves, filters, hoses and fittings are purchased from component manufacturers and are normally received in capped condition, no extra cleaning being carried out.

ii The reservoir is fabricated on-site, and the area is curtained off from the assembly area to minimise contamination. Welds are ground by hand to remove slag and weld spatter. Prior to assembly the reservoir is vacuumed and wiped with a dry lint free rag, followed by a wipe with a clean oil soaked rag.

ill Pipe sections are cut, formed, de-burred and cleaned using an air gun purge.

1v Having assembled the system, the oil is transferred from the barrel to the system using a filling trolley with a 10 micron filter in the supply line.

v Prior to starting the system, the relief valve is wound completely open to allow the pump to start without any appreciable back pressure.

v1 After 20 minutes, the relief valve is closed to gradually increase system pressure to 150 bar; this, with leak and function tests, takes a further one hour 40 minutes.

vii The return line filter is fitted with a new filter element, and the unit is shipped to the customer.

A 2.2 Level 4 Example

Flushing method for high production systems

Type of Equipment

Backhoe excavator vehicle hydraulics.

System volume is 90 litres, ISO 32 grade mineral oil

Pump is internal gear 120 litres/minute, 220 bar maximum pressure.

System has ten cylinder functions controlled by a mixture of mechanical and solenoid valves, 120 µm suction strainer, 12 µm absolute return line filter, positive pressure reservoir filler cap and air blast heat exchanger.

Company Operations

The company produces 50 of the above model per day, (amongst other lower volume models) employs 2,000 people and has considerable quality control and quality assurance capabilities. Laboratory facilities exist with technicians skilled in contamination analysis. Company has total quality management structure (TQM).

Cleanliness and Flushing Operations

i All incoming components are purchased to a cleanliness specification that controls the number of particles allowed at various sizes. The control on reservoirs is tighter than for other components because it is the largest contributor of contaminant to the system as a whole. Nearly all component suppliers now have laboratory facilities to check the level of contamination achieved. Manufacturer carries out quality audits on all suppliers and checks cleanliness of components supplied on a statistical basis. All components are capped and sealed to prevent ingress of contaminant.

n Production plant is designed so that fabrication and machining operations occur in a separate part of the plant to where the assembly line is situated.

iii Assembly operators are trained in hygienic assembly of components. Caps and seals are only removed immediately before assembly.

IV The hydraulic system is complete four minutes before the end of the

production line. At this point, it is filled with hydraulic fluid, from a dispenser with a 6 µm absolute filter in the supply line, and the return line filter is fitted with a 6 µm absolute element.

v The engine is started with the hydraulic system in the open circuit position and the engine set to run at full speed. None of the system functions are operated in this critical first four minutes, to enable the system to purge the largest proportion of contaminants on to the return line filter, thus the most sensitive components (steering valve and pilot operated check valves) are exposed to flow only after the majority of the system has had this primary flush.

VI At the end of the production line, the steering wheel is fitted as the last

operation, and the vehicle is driven out of the factory for a 40 minute hot function test where all circuits are exercised to full cylinder travel for a minimum of ten operations each.

vn The 6 µm absolute return line filter element is left in the machine until the first service at 50 hours, when it is exchanged for the standard 12 µm absolute element.

-11-viii Machines are sampled and checked that they meet the contamination standard according to a statistical sampling programme.

A 2. 3 Level 6 Example

Flushing method for large offshore hydraulic piping systems

The following example details the necessary flushing requirements on a typical platform drillfloor hydraulic piping system on a module prior to float-out.

The system consists of two main piping assemblies:

the high pressure supply lines to the various drillfloor equipment packages the corresponding low pressure return lines from the above equipment. Flushing operations are necessary for a number of reasons:

the critical nature of the equipment is such that should any failure occur during normal operations, drilling and production activities on the platform are severely disrupted

the relative costs of repair or replacement of key components in the systems would be prohibitive due to the "inaccessibility" of the offshore installation. These costs can be as much as 2 or 3 times that of similar work conducted onshore.

Therefore, by conducting a comprehensive flushing programme, the potential for occurrence of such failures is subsequently reduced.

In order to achieve the optimum results from the flushing operation, certain criteria must be satisfied.

Referring to Figure 1, it is noticeable that in both the high pressure and return lines, there are a number of branch lines serving various equipment packages. As a single flushing flow path is required to control the conditions in the pipework, it will be necessary to address this system as a number of individual flushing operations for both the supply and return lines.

In this case, however, the line sizes are relatively close in diameter, i.e. supply 38 mm and return 50 mm. This will facilitate the utilisation of both lines during any single flushing operation.

It must be noted, however, that where the diameter of the lines vary greatly, e.g. supply 25 mm and return 75 mm, then the required minimum flow rate for the 75 mm line would be too great for the 25 mm line and would result in an unacceptable pressure drop. In this case, the lines should be kept separate.

Having established various flushing flow paths, attention must be turned to the flow/velocity requirements. Calculations for both the Reynolds Number and Velocity must be done and from these, the required flow rates determined.

In this type of piping system, it is not always possible to place the flushing rig in the ideal position, i.e. as close to the piping inlet as possible. It may be required to use long hose assemblies of up to 40 metres in length to connect the flushing rig to the piping system. In this case, it is important to include these hoses in the calculations as the pressure drop will be affected.

TEMP HOSE

-1'"'

1,0

_/

V'

u (

When the above criteria are met, the flushing operation can commence. Details of individual flow paths are shown in Figure 1. Where reverse flow flushing is adopted,

itmust be noted that if, as in this particular example, a pressure line is linked to a return line, the application of directing the flow up the return line, can lead to over pressurisation of that line. Therefore, reverse flow flushing can only be adopted where the line design pressures allow.

During the flushing operation, monitoring of the cleanliness level can be done as described in paragraph 8.4. When the required cleanliness level is achieved, the flushing operation is stopped and the system pipework reinstated.

In most applications, and where practicable, the fluid should be left in the line to prevent ingress of contamination prior to the commissioning of the complete system. However, in this case, i.e. offshore installation, weight is a critical factor and, therefore, it will be necessary to drain the system as much as possible. In

doing so, care must be taken when reinstating pipework. All open connections should be sealed positively with blank flanges, etc. and not with tape.

Due to the complexity of this type of system, it is imperative to adhere to the relevant calculation results, and that great care is taken in establishing the correct sequence of flow paths to achieve optimum results during flushing. If this is not done, the end result would be that the fluid in the pipework is cleaned to the required specification but the pipework itself is not. Consequently, the remaining contamination in the pipework may subsequently be released into the system during the commissioning perjod, causing component failure at a critical time resulting in very costly remedial work.