eds 07-4110 grid and primary transformer retrospective oil...

Document Number: EDS 07-4110

Version: 5.0

Date: 08/12/2017

ENGINEERING DESIGN STANDARD

EDS 07-4110

GRID AND PRIMARY TRANSFORMER RETROSPECTIVE OIL CONTAINMENT

Network(s): EPN, LPN, SPN

Summary: This standard outlines the guidance for the design and construction of retrospective bunds around existing grid and primary transformers.

Author: Uriel Arias Date: 08/12/2017

Approver: Paul Williams Date: 15/12/2017

This document forms part of the Company’s Integrated Business System and its requirements are mandatory throughout UK Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset Management. If you have any queries about this document please contact the author or owner of the current issue.

Applicable To

UK Power Networks External

☒ Asset Management ☐ G81 Website

☒ Capital Programme ☐ UK Power Networks Services

☐ Connections ☐ Contractors

☒ Health & Safety ☐ ICPs/IDNOs

☐ Legal ☐ Meter Operators

☐ Network Operations

☐ Procurement

☐ Strategy & Regulation

☐ Technical Training

Grid and Primary Transformer Retrospective Oil Containment


Version: 5.0

Date: 08/12/2017

© UK Power Networks 2017 All rights reserved 2 of 16

Revision Record

Version 5.0 Review Date 15/12/2022

Date 15/12/2017 Author Uriel Arias

Why has the document been updated: Periodic review.

What has changed:

Narrative and document layout rationalised.

Reference to EAS 07-0000 introduced (Section 1).

Oil containment programme removed.

Document renumbered from EDS 07-0110 and title revised


Date 20/04/2015 Author Lee Strachan

Why has the document been updated: Minor version update (Re AIRLine contact number change)

What has changed: AIRLine contact number changed



Why has the document been updated: Update

What has changed: Drawings EDS 07-0110-001 and 002 updated. Section 1 – Reference to EDS 07-0110-003 removed, email address introduced. Section 4.1 – re-worded to allow for different shingle depths depending on site condition, Section 4.2.1 – Reference to Ultracoat removed and replace by Adelline 400, Section 5.1 – reference to Aquasentry removed, reference to 110V and 230V sump pumps added, Section 5.3.1 – Construction products regulation 1991 – Declaration of conformity reference added for allowed manufactures, New section 5.3.3 added. Appendices updated to conform with template, Appendix B – example figures added and 150mm freeboard note added, Appendix C - Reference to Ultracoat removed and replace by Adelline 400.



Bunding system specifications reviewed and updated, alternative bunding and drainage solutions included. – new products and approaches included


Date 28/09/2011 Author P. Linkeshwaran

Document combined with the Engineering Specification ES 07-0002 and renamed Engineering Design Standard. Bunding priority lists reprioritised and updated.


Date 17/02/2010 Author Mark Dunk

Original



Version: 5.0

Date: 08/12/2017


Contents

1 Introduction ............................................................................................................. 4

2 Scope ....................................................................................................................... 4

3 Glossary and Abbreviations ................................................................................... 4

4 Existing Ground Contamination ............................................................................. 5

4.1 Responsibilities ......................................................................................................... 5

4.2 Pollution Control Measures ........................................................................................ 5

5 Quality of the Effluent from the Bund .................................................................... 6

6 Secondary Oil Containment and Pollutions Control Measures ............................ 6

7 Bund Design ............................................................................................................ 7

7.1 Assessment of Risk and Bunding Requirement ......................................................... 7

7.2 Volume Capacity ....................................................................................................... 7

7.3 Operational Clearance ............................................................................................... 7

7.4 Fabric of the Bund ..................................................................................................... 7

8 Storm Water and Drainage .................................................................................... 10

8.1 Oil Discriminating Pumps......................................................................................... 10

8.2 Oil Interceptor .......................................................................................................... 10

8.3 Alternative Drainage Approaches ............................................................................ 10

9 Quality of Works .................................................................................................... 12

9.1 Water Test ............................................................................................................... 12

9.2 Integrity Inspection .................................................................................................. 12

10 References ............................................................................................................. 13

10.1 UK Power Networks Standards ............................................................................... 13

10.2 National and International Standards ....................................................................... 13

11 Dependent Documents.......................................................................................... 13

Appendix A – Examples of Combined Bund Solutions .................................................. 14

Appendix B – Discharge Consent Flow Chart ................................................................. 15

Appendix C – Drawings Record ....................................................................................... 16

Figures

Figure 7-1 – Plastic Bund Details .......................................................................................... 8

Figure 8-1 – Bund Dewatering Unit ..................................................................................... 11



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1 Introduction

This standard details the requirements for the construction of secondary oil containment structures around existing grid and primary transformers and associated plant (auxiliary/earthing transformers, oil filled NER/NEX etc.).

Approved products associated with this standard are listed in EAS 07-0000.

2 Scope

This standard applies to all existing grid and primary transformers requiring the provision secondary oil containment structures across UK Power Networks regions.

3 Glossary and Abbreviations

Term Definition

AONB Area of Outstanding Natural Beauty

Bund Secondary oil containment structure attached to the plinth of the transformer. Typically made up of reinforced concrete slabs and wall designed to contain the oil in the system plus residual rainwater.

DC Discharge Consent

GRP Glass Reinforced Plastic

ORG Oil Reactive Geo-membranes

RDC Residual Current Device

SCADA System Control and Data Acquisition

Secondary oil containment

Containment of oil in order to prevent pollution of soil and water. The structure of the secondary oil containment is independent from the structure of the oil container or transformer.

SPZ Source Protection Zones

SSSI Site of Special Scientific Interest

TEC Trade Effluent Certificate

WC Water Company (Local Utility)



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4 Existing Ground Contamination

4.1 Responsibilities

For non-bunded transformers, failed or obsolete oil containment structures, UK Power Networks is committed to assess the risk of pollution to ground and surface water systems, the ground strata in general and to third party land including SSSIs (Site of Special Scientific Interest) and AONBs (Area of Outstanding Natural Beauty).

This commitment is based on the understanding of the duty of care on importers, producers, carriers, keepers, treaters or disposers of controlled waste to prevent unauthorized or harmful activities (Environmental Protection Act 2008).

UK Power Networks acknowledges its responsibility to remediate the impacts of substances, which have escaped into other land or watercourse, as defined in the Environment Act 1995.

4.2 Pollution Control Measures

4.2.1 Repair of Equipment

The repair of leaking equipment shall always be considered as the first course of action when addressing the mitigation and control of pollution wherever it is operationally safe and practical to do so.

Generally, repairs shall be carried out in conjunction with control measures. These shall aim to prevent or reduce further oil leakages into the ground.

4.2.2 Control of Existing Pollution

Bunding or replacing a leaking transformer may prevent further oil contamination but it does not remove or control the oil already present in the ground which may still migrate to watercourses or ground water systems beyond the distances quoted in the Control of Pollution (Oil Storage) (England) Regulations 2001.

As part of the oil pollution control works a site assessment of latent pollution shall be carried out in accordance with HSS 01 016 in order to assess the degree and spread of the oil pollution present on site and the likelihood to affect nearby watercourses and ground beyond the site boundary. HSS 01 016 shall be used to assess and mitigate the existing pollution risks.

4.2.3 Transformer Replacement

Transformer replacement is part of a long-term replacement strategy and is often dependant on load growth in the area as well as the general reliability of the transformer in service. Consequently, the process of transformer replacement is not examined in this document but reference is made, as it is a mean of mitigating the oil leakage.



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5 Quality of the Effluent from the Bund

The Environment Agency requires a consent in place for companies that ‘discharge sewage or trade effluent directly into surface water, such as rivers, streams, canals, groundwater or the sea’ (Water Resources Act 1991 and Water Framework directive 2000/60/EC).

The storm water management solutions presented in this document shall be specified to prevent the release of oil into the surrounding ground or watercourse. The effluent discharged should contain less than 5mg/litre of hydrocarbons and therefore it can be considered as ‘uncontaminated surface water’, thus negating the need for a discharge consent.

If the discharge is into a surface water sewer, the local water company shall be consulted in case a Trade Effluent Certificate is required.

According to BS EN 858, the quality of the discharge shall not exceed 5mg/litre of oil under standard test conditions. However, this concentration limit only applies under standard test conditions and it should not be expected that separators will always perform to this limit. For some environmentally sensitive areas, 5mg/litre may be too high.

The flowchart in Appendix B provides direction as to which, if any, licence is required.

6 Secondary Oil Containment and Pollutions Control Measures

Options for site decontamination, equipment repair and the provision of permanent bunding shall be investigated prior to developing a retro-bunding scheme. The most suitable and effective combination of mitigation options shall be employed in order to eliminate the risk of oil pollution to ground water, watercourses and ground strata.

The design of bunds and control measures shall take into account:

Environmental risk: site proximity to an environmentally sensitive area. Potential risk (High, Medium or Low) to ground water sources or surface water (e.g. rivers and streams). The greater the risk the more priority shall be given to provide a bund to the transformer(s).

Asset strategy: design solutions shall match the strategic plans for the site and the transformers. Future works required on transformers (e.g. re-gasketing) may drive design decisions. Major substation projects shall look to incorporate retro-bunding of existing transformers.

Transformer oil fill rate and transformer health index: Transformers with a history of regular topping up of oil or where maintenance records show that large volumes of oil have been used in keeping the transformer oil to the correct level shall not make use of short lifespan bunding solutions.

Surrounding structures: given space constraints in some substations, walls from adjacent substation buildings may be used to create the structure for the containment of oil.

Existing cables: no main cable shall be covered with or cast into concrete. In planning and constructing of a bund, it is essential that cable disturbance is kept to a minimum and future access is allowed.



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7 Bund Design

7.1 Assessment of Risk and Bunding Requirement

Secondary oil containment should be provided for containers with more than 201 litres of oil stored that pose “significant risk” to the environment.

While transformers are not classified as oil storage containers, whilst in operation leaking transformers can pose a significant risk to ground strata, ground water and watercourses. For this reason, it is necessary to consider all transformers with a potential to contaminate.

Secondary oil containment shall be provided for transformers located at less than 10m away from any inland freshwater or coastal waters, or less than 50m away from a from a well or borehole otherwise known as a Source Protection Zones (SPZ).

7.2 Volume Capacity

Bunds are required for each individual transformer, or for a group of these. The total capacity of the bund shall be greater than 110% of the oil contained in the largest transformer in the system or 25% of the aggregate total capacity of the transformers, whichever is the greater.

In the calculation of the volume of the secondary oil containment it has to be taken into account that the bund will have a layer of 40mm aggregate material (PD6682-1 Table C1 20/40 GC85/20 to BS EN 12620:2002). This material shall be deep enough to provide a regular/walkable surface and sufficient volume capacity to keep in the voids residual and accidental oil spillages.

A 300mm depth is the standard recommendation, but this is subject to site conditions. For calculation purposes, one cubic metre of 40mm single sized shingle retains 300 litres of oil.

A minimum height of 150mm above the level of aggregate material is recommended for the bund wall to allow for rainfall and the use of firefighting foam.

7.3 Operational Clearance

It is essential to ensure that by constructing the bund walls, the minimum clearance to overhead bars and equipment is kept when operatives stand on top of the wall.

Where clearances are compromised, the top of the wall shall be provided with means to prevent personnel standing on the wall (e.g. steep top chamfer).

7.4 Fabric of the Bund

The transformer shall be situated within a secondary containment system with impermeable base and walls. Retrospective bunds shall be designed in accordance with the Control of Pollution (Oil Storage) (England) Regulations 2001 and the Environment Agency guidelines.

7.4.1 Reinforced Concrete Bunds

Secondary containment areas are typically constructed in reinforced concrete, because it is cost-effective and provides good structural strength.

In order to ensure the durability of the concrete seals and the general stability of the bund, where the design of the bund makes use of the foundations of an existing structure, all new concrete work shall be dowelled to and cast against the existing structure.



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7.4.2 Alternative Bunding Solutions

Alternative bunding solutions introduced in this section shall be considered where:

Existing foundations and structures can be used as part of the new secondary oil containment.

Where excessive shallow cabling is present.

Where existing structures requires materials flexibility to provide a cost-efficient solution.

When defining alternative solutions or the combination of these, there shall be no compromise in fire resistance and durability bund structure or the quality of the effluent discharged.

7.4.2.1 Oil Resistant Coating Applied on Existing Slabs and Walls

Oil resistant flexible coating spray or hand applied poly-urea shall be used to repair/upgrade existing walls, when one or more of the following is found on site:

The existing walls can withstand the hydrostatic pressure of the volume to be contained.

Walls are affected by minor cracking only (less than 4mm crack width); therefore, the containment capacity can be safely achieved with the application of the poly-urea.

Existing volume capacity is insufficient but there are surrounding walls that can form the new bund.

The warrantied lifespan of the coating should be no less than twenty years. Anti-slip surfaces along the designated walkways shall be allowed for.

7.4.2.2 Plastic Bunds

Sites with shallow cables can make the excavation work too onerous and risky. For this type of condition, plastic bunds are considered an efficient alternative to concrete. An advantage of the plastic bunds is that given its flexible nature, the preparation of the ground can be limited simply to the clearing of the area receiving the bund.

As shown in Figure 7-1, the poor fire resistance of the plastic is compensated by the addition of a proprietary fire board inside concrete caps along the bund walls.

The sump is formed with a 600mm diameter pipe at a convenient location on site.

Figure 7-1 – Plastic Bund Details



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7.4.2.3 Proprietary Bund Walls

Where a hard base is available or can be efficiently formed (e.g. by adding concrete precast slabs). There are standard options for the use of proprietary walls:

Plastic or composite dwarf columns fixed to the floor slab and jointed together with horizontal panels of the same material.

L-shape GRP panels fixed along the floor slab. Walls are fixed directly on to the concrete slab or masonry.

Proprietary bund walls can be formed with materials of low conductivity (e.g. GRP or plastic); they can be designed with demountable sections for planned or emergency maintenance. Appendix A shows some examples of proprietary bund walls.

Proprietary bund walls can be used to form the secondary oil containment in conjunction with an approved bund sealant (see Section 7.4.2.1) in conjunction with Oil Reactive Geo-membranes to create an oil impervious base for the bund (see Section 8.3.2). The selection of the type of proprietary wall or combination of alternatives shall be decided according to the site conditions.



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8 Storm Water and Drainage

8.1 Oil Discriminating Pumps

All bunds shall be equipped with an oil discriminating pump, located in the sump, to remove rainwater. The pump shall discharge at minimum flow rate of 2.1l/s over a 3m head. 110V AC pumps have been historically installed on safety grounds. However, given the controlled environment in substations, 230V pumps are permitted providing a 30mA residual current device (RCD) is fitted and a double-pole isolator is installed close to the pump for maintenance.

The pump control box shall be located in the relay or control room. Alarms shall be connected to SCADA system. The pump discharge shall go into an oil separator sized for the pump’s discharge rate.

8.2 Oil Interceptor

A certified Class 1 oil interceptor shall be installed below ground as it affords better protection to the interceptor and simplifies the drainage requirements.

When the effluent has passed through the oil interceptor, the discharge can go into a local soakaway or watercourse (see Section 5); approval from the Water Authority/Environmental Agency shall be sought.

8.3 Alternative Drainage Approaches

8.3.1 Treatment of Storm Water inside the Bund

A simple way to treat the storm water in bunds is by implementing an automatic bund dewatering system to treat the oil-contaminated rainwater before it leaves the bund; the system consists of an oil discriminating pump discharging into a proprietary oil interceptor unit inside the bund.

The unit comprises of a full retention oil separator designed to remove small amounts of hydrocarbons. This unit negates the need for an external oil interceptor and the associated drainage pipework.

An acceptable bund dewatering system shall be:

Manufactured to BS/EN 858.

Certified as a Class 1 oil interceptor

Provided with a ‘Declaration of Conformity’ stating the name and address of the organisation taking responsibility for the product, the product specifications, the safety directives the product complies with and details of relevant standards used.

A typical automatic bund-dewatering unit is shown in Figure 8-1, note that it is placed at ground level and its location is flexible within the bund wall.

Being located within the bund and above ground facilitates installation; inspection and maintenance during the operational live of the unit.



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Figure 8-1 – Bund Dewatering Unit

8.3.2 Oil Reactive Geo-membranes (ORG)

ORG are to be used at schemes where it is known that the environmental risk is ‘low’, the Health Index of the transformer is above 2 and the oil replacement rate is less than 20 litres/year.

ORG are formed with a polyethylene (HDPE) layer that contains windows of a polymer membrane impervious to hydrocarbons but that allows water to run through. For ORG to work, it is essential that the ground has enough porosity so the water can disperse away from the bund by gravity; in addition, the water table should be known to be well below the bottom of the bund.

Only where straight forward bund layouts are achievable (minimum corners and bending) ORG are recommended.

If the transformer are known to be leaking ORG shall not be used.

8.3.3 Oil Filtering Cartridges

A soil investigation and confirmation of the environmental sensitivity of site are required prior to allowing the use of oil filtering cartridges. Oil filtering cartridges are permitted providing that all the following conditions are satisfied:

The site is not within an environmentally sensitive area.

The transformer is in a good asset condition (health index above 2).

There is not record of oil replacements in the last two years.

The water table of the site is confirmed to be well below the drain’s invert level.

The percolation rate of the soil permits for the discharge to soak away naturally into the ground.

An alarmed indicator of the cartridge saturation shall be connected to the SCADA system.



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9 Quality of Works

9.1 Water Test

A water test for reinforced concrete bunds will consist of filling with water above kicker level but below the top of the plinth. The bund is considered to have passed the test if the water depth loss is no more than 1mm/hour over a continuous six-hour period. A test compliance certificate is to be produced on successful completion of the test.

For alternative bund solutions, a test method shall be agreed with the project designer before the bund is built. The test results shall be the same or better than expected from a reinforced concrete bund -1mm/hour level drop.

9.2 Integrity Inspection

Following completion, every bund shall be inspected for integrity and assessed during each major substation inspection as detailed in EMS 10-6501.



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10 References

10.1 UK Power Networks Standards

EMS 10-6501 Inspection and Maintenance of Civil Assets

HSS 01 016 Working on Contaminated Land

10.2 National and International Standards

Control of Pollution (Oil Storage) (England) Regulations 2001

Environment Act 1995

Environmental Protection Act 1990

PD6682-1, Aggregates for Concrete – Guidance on the use of BS EN 12620

Water Framework directive 2000/60/EC

Water Resources Act 1991

11 Dependent Documents

This document is referenced in the following documents, any of which may be affected by updates.

EMS 10-6501 Inspection and Maintenance of Civil Assets

HSS 01 016 Working on Contaminated Land



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Appendix A – Examples of Combined Bund Solutions

Recycled plastic bund walls combined with existing walls.

Bund created using GRP upstands bolted to and existing concrete based covered with spray-applied poly-urea.

ORG combined with composite bund wall.



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Appendix B – Discharge Consent Flow Chart

Do we have a drainage plan?

Yes No

Arrange drainage survey

Does the plan show any discharges to Surface water sewer, watercourse or soakaway?

Yes No

Does it all go to foul sewer?

Yes No

TEC required

contact WC.

Further investigation needed.

Foul Surface

Does the plan show any kind of treatment?

Yes No

DC required.

Contact EA

Is there a cesspit

shown?

Yes No

No DC required

Further investigation required.

Is there an interceptor or other treatment method shown on the plan?

Yes No

No DC. Contact Env team for guidance

No DC, but Further investigation

needed

Does this include contaminated site

runoff?

No Yes

Uncontaminated surface water does not require DC. May require TEC if to surface water sewer. Contact WC for guidance

Is this from the foul or surface water system?



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Appendix C – Drawings Record

Drawing Reference

Title Latest Version

EDS 07-4110.01 Reinforced Concrete Retro-bund A

EDS 07-4110.02 Retro-bund Drainage A