Farm Waste Structures: Guidance on Construction, Repair and Maintenance

Above-ground circular steel slurry stores

Note No: CGN 004, July 2000
Prepared by ADAS National Building Design Team. Funded by The Department for Environment, Food & Rural Affairs.


Note: Ladder is normally removed as a safety precaution

cgn004_fig1.jpg (27088 bytes)

Selection

  • Steel tank installation is a specialist job; it is advisable to use a manufacturer/supplier who can provide a complete installation service.
  • Before ordering check the specification, especially the long-term maintenance requirement for protective coatings and joint seals.
  • A durable vitreous enamel is commonly used to protect the steel. Other methods include galvanised steel and epoxy coatings. Slurry may corrode galvanised steel so an additional factory applied protective coating has to be used.
  • Select a supplier/installer who will provide certification that the completed installation conforms to the Control of Pollution Regulations (See Reference 1).

Site Suitability

  • Consult the Environment Agency and the Local Planning Authority when selecting a site.
  • Avoid sites in a flood plain or sites with a high water table.
  • Avoid improperly filled ground or other unsuitable ground conditions such as peat soils or ground liable to subsidence.
  • Locate at least 10 metres from a water-course (See References 1 & 2)
  • Suitable sites will be on firm and level, non-organic sub-strata capable of bearing the full weight of the tank and its contents without adverse effect.
  • The appointed contractor should take responsibility for properly preparing the site during the construction if a formal contract has been entered into for an installation to be built in accordance with the Control of Pollution Regulations.
  • Ensure that you have provided sufficient information about the proposed site location to contractors prior to tender so that they can allow for the appropriate conditions in their tender.
  • Check before placing an order, or signing a contract, that the site conditions are acceptable to the contractor and that these have been allowed for in the tender.

Specification

  • Allow for freeboard and rainfall when calculating the size (See References 1 & 2).
  • Decide whether you are likely to require increased capacity in future. Increasing the height will require stronger, more expensive base rings and possibly a stronger foundation.
  • Ensure that the store is designed to conform to BS 5502 requirements particularly Parts 50 and 22 (See Reference 3).
  • Check that durability life is at least 20 years given proper maintenance.
  • Enamelled steel plate should conform to BS 3830 using VE grade steel.
  • Examine fixing details including bolts and corrosion protection.
  • Check specified torque for correctly tightening bolts. Over-tightened bolts can cause damage to protective coatings.
  • The base of the store is usually constructed of in-situ concrete. Refer to Leaflet Number CGN 003 for more information on in-situ concrete construction (See References 4 & 5).
  • Outlet pipes should be watertight, capable of easy cleaning and be fitted with two in-line locking valves at least 1 metre apart.
  • Access ladders, inspection platforms and warning notices must be fitted as required by the Control of Pollution Regulations and the Health & Safety Executive ( See References 1, 2, 3 & 6).

Construction

  • An experienced building contractor should be employed to construct the store and ancillary works.
  • Advice on building contracts is given in the associated leaflet No CGN 100.

Repair and Maintenance

Vitreous enamel coated steel plate provides excellent corrosion resistance against slurry and provided it is correctly specified, properly constructed and adequately maintained, it should have an extremely long life. Any damage to the protective coating will cause rapid corrosion especially where the unprotected steel is in contact with slurry. Several major pollution incidents have occurred as a result of failure by rapid corrosion so it is important that any defects are quickly rectified.

Areas particularly susceptible to corrosion include the following:-

  • Corrosion of sheet edges due to failure of sealant.
  • Flaking of enamel coating around bolts often results from over-tightened bolts, lack of fit or overstressing.
  • Damage from mechanical impact.
  • Abrasion often occurring on internal faces and in pipework.

Both internal and external surfaces should be washed down and inspected in detail, preferably annually, but at least every two years. Special safety precautions should be observed, particularly against the presence of toxic gases when working in confined spaces. Check concrete bases and seals, repair any defects.

All parts of the tank should be open to observation at all times. No trees, bushes or shrubs should be allowed to grow around the perimeter.

Deteriorated bolts and fixings will require attention and possible replacement.

Maintenance includes repair and replacement of sealant between panels. The tank supplier or manufacturer should be consulted on correct procedures and suitable sealants.

cgn004_fig2.jpg (9895 bytes)
A possible repair procedure for localised damage or hole in a steel panel.

The supplier or manufacturer should be consulted on buckled or damaged plates and about the repair of such defects.

Reception Pits and Slurry Channels

  • The provisions of the Regulations apply to all reception pits and slurry channels.
  • Structural materials include in-situ concrete, pre-cast concrete and glass fibre reinforced plastic.
  • They must be capable of withstanding the loads calculated as shown in Paragraph 5 of BS 5502 Part 50 and be designed to prevent the ingress of groundwater and prevent flotation.
  • Reception pits should hold at least two days slurry production, be impermeable and constructed to last 20 years with proper maintenance.

References

  1. The Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) Regulations 1991 (as amended).
  2. The Code of Good Agricultural Practice for the Protection of Water.
  3. British Standard 5502 Part 50 : Design construction and use of storage tanks and reception pits for livestock slurry.
  4. British Standard 8110: Structural use of concrete.
  5. British Standard 8007: Code of Practice for design of concrete structures for retaining aqueous liquids.
  6. Health and Safety Executive: Construction (Design and Management) Regulations.


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