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Hydrants made of cast iron with spheroidal graphite

Hydrants are water supply network shut-off devices that enable fire brigades, road maintenance companies and street cleaning companies, among others, to directly withdraw water from the public water supply network. Hydrants are also used to flush and ventilate pipe networks. A distinction is made between underground and pillar hydrants. Pillar hydrants are preferable for fire-fighting purposes, as they are ready for operation, easy to find and accessible at all times. In densely built-up areas and in narrow streets, underground hydrants marked with signs are used. The construction, handling, maintenance and operational safety of hydrants made of cast iron with spheroidal graphite must meet high requirements, because the large number of operators requires different qualifications.


Industrial hydrant for extinguishing water supply on industrial premises

Materials and coatings of hydrants

Valve shell parts of the hydrants are generally constructed with spheroidal graphite of cast iron and steel. Other materials are also permitted, e.g. aluminium upper parts are available. PUR (polyurethane) and EPDM (ethylene propylene diene monomer) are used as materials for shut-off elements. 

The epoxy coating of valves has meanwhile developed into the standard coating for all valves in the area of untreated water, drinking water and wastewater. In addition to the use of high quality epoxy lacquers, for valves the environmentally friendly, solventfree epoxy powder coating, also called (EP) coating has become particularly popular.

As an outstanding and durable corrosion protection, enamel has been established in the area of water supply for many years. In the meantime, enamel is also used as an exterior coating to ensure an integral, continuous coating. For several years now, a closed coating system “complete enamel” has been available for materials, production and testing technology, which has now found its way into practical applications in the area of transporting of raw water, drinking water and waste water.

Pillar hydrants

Pillar hydrants used in the public water supply system must meet the requirements and other national regulations where applicable. Pillar hydrants project above ground level. They consist of two parts: the bottom section of the hydrant which contains the main valve and is installed underground plus the top part of the hydrant which is generally flanged onto the bottom part at ground level. Pillar hydrants are equipped with a predetermined breaking point which is normally located in the connection flange between the top and bottom parts of the hydrant.

The majority of pillar hydrants are in nominal sizes DN 80 and DN 100, designed for an allowable component operating pressure of 16 bar. They have a vertical or horizontal inlet with a flanged, push-in or spigot end joint. The pipe covering usually varies between 1.25 m and 1.5 m. This ensures that, even with a minimum volume of residual water, the main valve cannot freeze up. The bottom part of hydrants is normally designed for a fixed depth of pipe cover. Pillar hydrants are used in different piping and pipe joint systems.


The upper part of a pillar hydrant without drop jacket with B outlets which can be shut off

Underground hydrants

Of course underground hydrants used in public water supply systems must meet the requirements and other national regulations. They will also be used in nominal sizes DN 80 and DN 100. They are usually housed in surface boxes in the road and can be operated from there. A standpipe is always required in order to take off water and this is connected to the locking claw. The main shut-off device is actuated by applying a hydrant key.

Underground hydrants consist of a one or two-part shell, also referred to as a jacket pipe or standpipe, the lower part of which houses the shut-off device. Underground hydrants can have single or double shut-off devices. The double shut-off version is usually a ball or cone design. The double shut-off version has the advantage that the shut-off device including its drive elements can be replaced in the surface box with the pipeline under full pressure. As underground hydrants are usually located in surface boxes there is the risk that, with insufficient maintenance and in unfavourable locations (road subsidence) road grit, stones or other small objects may get into the shell and damage the shut-off device. In order to minimise this risk, sealing flap and cover are used in the area of the locking claw. Underground hydrants are used in different piping and pipe joint systems.


DN 80 underground hydrant: double shut-off, opening against the direction of flow – fully enamelled

Possible fields of application for hydrants

  • taking off extinguishing water
  • ventilating pipelines
  • flushing piping networks, particularly in end sections for reasons of hygiene
  • producing temporary network connections
  • emergency water take-off
  • short-term water supply, e.g. for construction purposes, funfairs, etc.
  • bridging for emergency supplies
  • drainage of pipelines
  • leak detection


Author: Dr. Jürgen Rammelsberg

The article was slightly shortened by the editors. You can find the complete article with various illustrations as a PDF in the download area under Downloads Annual Issues EADIPS FGR.