Estrés Laboral Tabla 12

The pipework system should be designed to safely and efficiently transport liquid fuels and fuel vapours. Joints between pipe, bends and fittings which can be mechanically dismantled should not be buried, but should remain visible and accessible in a containment chamber.

The system should be designed to allow for leak tightness testing and to ensure it can be made safe by draining and purging and testing when required.

Pipework has to be fit for the application for which it is to be used in terms of:

− structural strength relative to internal pressures and external loading;

− robustness for handling and installation;

− reliability of assembly;

− full internal compatibility with the fuels and fuel vapours to be carried, and

− external compatibility with the materials to which it might reasonably be exposed in the ground and in chambers during its service life.

Pipework certified in accordance with the requirements of EN 14125 Thermoplastic and flexible metal pipework for underground installation at petrol filling stations can be taken as fit for use with EN 228 and EN 590. Suitability for fuels containing higher alcohol or fatty acid methyl ester (FAME) content should be certified by the system supplier.

8.5.2.2 Suction systems

Suction systems draw fuel from the tank by means of a pump located in the dispenser housing. A non-return valve should be provided in a suction pipe system at the connection point to the dispenser, above the leak plate, to ensure that the suction pipe remains primed whilst the dispenser is at rest. It is important that non-return valves are not installed in any other location in a suction system. Where for system design purposes an intermediate or tank lid check valve has to be used fuel will be retained under a positive hydrostatic pressure and it should be considered as a pressure pipe and provided with leak containment

It is good practice to install a lock down valve fitting at the tank lid and an access plug at the connection to the dispenser to allow subsequent testing of the lines without the necessity for disconnection of pipework or fittings.

The system design should aim to minimise the vertical lift from the tank bottom to the dispenser inlet and the friction head losses of the pipe system layout

All internal suction pipes should terminate 35 mm or more above the bottom of the tank internal fill pipe so a liquid seal is maintained. The bottoms of internal suction lines may be fitted with suitable deflection devices to minimise the uplift of sediment and reduce the introduction of vapour into the suction lines.

8.5.2.3 Pressure systems

Pressure systems use submerged turbine pumps to generate flow and maintain a positive pressure in the pipework between the tank and the dispensers. Multiple dispensers are commonly linked to a single pressure fuel pipe. Pressure pipe systems are also used with remote pumping units with pumped or gravity feeds from an above-ground tank.

Pressure lines should be installed with a means of containing any release of vehicle fuel due to a leak in the line and be equipped with a suitable system to detect such a failure.

The leak detection system should be able to isolate the power supply to any pumping unit and give immediate warning to the operator.

Valves should be installed to facilitate hydraulic isolation of each individual dispenser and sited immediately adjacent to each pumping unit. Where a gravity feed system is installed a valve arrangement should be provided to allow the isolation of the storage tank from pipework.

Means should be provided for draining all pipework, ideally back to the storage tank, and to allow subsequent testing of the lines without the necessity for disconnection of pipework or fittings.

Table 8.2 Principal differences between suction and pressure systems

Suction system Pressure system

Vehicle fuel is drawn through the line by atmospheric pressure as a result of the partial vacuum created within the line by a low pressure pump located within the dispenser.

Vehicle fuel is pumped along the line to the dispenser under pressure created by a high pressure pump located either within the storage tank or between the storage tank and the dispenser.

Should there be a leak in the line, the non-return valve located under the dispenser allows fuel (or water) to drain back towards the tank. Depending on the position and severity of the breach in the pipework there should be limited loss of fuel to the environment. Water ingress or repeated difficulty in pump start-up are key symptoms of a suction line failure.

Should there be a leak in the line fuel will be forced out under pressure resulting in considerable loss in a short time. For this reason lines should have secondary containment and be equipped with an impact valve at ground level.

More pipework is necessary as each pump generally needs a dedicated suction line from the tank.

Less pipework is necessary as lines can be spurred off a single main feed for each grade of fuel.

A separate pumping unit is required for each grade at each dispenser.

Failure of a pump puts only that dispenser out of use.

Fewer pump units are required as the pump in the tank can supply a number of dispensers.

Failure of a pump isolates that storage tank and every dispenser fed from it.

Suction pumps are generally less reliable in operation than submerged pressure pumps and require more maintenance.

Pressure pumps submerged in fuel are generally more reliable than suction pumps, require less maintenance and have a longer life.

The pressure line into each dispenser should be fitted with a double poppet impact valve with fusible link at ground level and an isolating valve immediately beneath it so that outflow of fuel under pressure is prevented in the event of impact to, or fire at, the dispenser.

The inlet port of the pumping unit should be located at least 150 mm above the bottom of the tank in order to ensure a liquid seal is maintained. Some means of stopping take off from the tank while the fuel level is still above this inlet is desirable as pump life may be significantly reduced if it is allowed to run dry.

Where above-ground storage tanks are installed the pipework from the tank to the remote pump should have secondary containment outside the tank bund with suitable leak monitoring. The use of double-skin or secondarily contained pipework is considered good practice. The pipework within the tank bund does not require separate secondary containment.

Each remote pump should be fitted with an emergency impact safety cut-off valve fitted to its discharge side incorporating a fusible link designed to activate on severe impact or fire exposure.

Internal pipework within the storage tank should be installed as for suction systems so as to maintain a liquid seal and to minimise the uplift of sediment.

It is important for designers and operators to understand that mechanical or electronic leak detection systems on a pressure line will only operate when the submersible pump is not running. Therefore on a busy site where the pump is running for long periods the detection systems may not detect a leak and other leak detection systems should be used (see section 11).

8.5.2.4 Vent and vapour recovery systems

A venting system is an essential element of the fuel installation that allows for the displacement of vapour (either to atmosphere or back to the road tanker) during the unloading process and the ingress of air into the tank when fuel is dispensed. The system should be designed and installed to prevent any build-up of excessive pressures exceeding the design limits of the tank. Normally, atmospheric vents will be at least 50 mm nominal pipe size.

Vent pipes should be fitted as near as possible to the highest point of any installed tank or compartment. The open ends should be constructed so as to discharge upwards in the open air. Petrol tank vents should be fitted with a flame arrester approved to EN ISO 16852, integrated into the P/V valve (end of line flame arrester) or fitted separately behind the P/V valve (inline flame arrester), that will not jeopardise the tank's ability to breathe.

Pipework used for tank vents, vapour return lines in the Stage 1b vapour recovery system or vapour recovery pipes as part of a Stage 2 vapour recovery system should be fully compatible with the fuels to be carried. Pipes should allow for falls to the tank or an accessible trap to allow for the drainage of condensation which will be a combination of fuel vapour and water vapour from air drawn into the vent system. Vent and vapour recovery pipework should be periodically tested to ensure no loss of vapour to the environment or the ingress of water to the system.

Above-ground vent and vent manifolds must be fire resistant and generally fabricated as a steel assembly. Jointing materials and gaskets used in the assembly must be fully compatible with the fuels and fuel blends to be carried in the system.

For further guidance on venting systems for the control of vapour emissions see section 10.

8.5.2.5 Offset or remote fill pipes

Pipes for gravity filling of tanks must fall directly to the tank to avoid any sections containing trapped fuel. Fuel will be under a positive pressure during unloading and will be lost from the pipe in the event of any failure. Fill pipes must be periodically tested to minimise the risk of significant loss to the environment or the ingress of water to the system. In high risk locations fill pipes should be contained (e.g. double-skin) and monitored.

For details of offset fill assemblies and tank internal fill assemblies see 8.5.4.1 Pumped fill pipes must be double-skin as they will retain fuel between deliveries. A positive isolation valve should be provided at the connection to the pump and a non-return valve at the connection to the tank.

8.5.2.6 Siphons

A siphon is used where it is required to interconnect two or more tanks so that they operate as a single unit. Such installations will result in different operational procedures when filling and discharging these tanks. When installed with suction systems it may be necessary to provide a separate priming arrangement on the siphon.

When siphoning systems are used in conjunction with a pressure system they should comply with the pump manufacturer's specification, which will include the provision of an automatic priming arrangement.

Siphon lines within tanks should extend to the same level as the bottom of the fill pipe.

Siphons will transfer fuel between tanks as soon as a differential head is present.

When intending to fill both tanks in a siphon connected system these should be done simultaneously to ensure available ullage in the connected tank is not reduced by the action of the siphon. Valves should be installed in siphon lines to allow for the isolation of any interconnected tanks, when simultaneous filling is not used.

Siphon pipework should be designed as suction pipes.

For further information see CFOA Information note: Petrol filling stations - siphon systems.

8.5.3 Selecting appropriate pipework material