1236 Single and multicircuit protection valve

Purpose Circuit protection valves are incorporated in the brake charging system to provide an independent method of charging a number of reservoirs to their operating minimum. Where there is a failure in one of the reservoir circuits, causing loss of air, they will isolate the affected circuit so that the remaining circuits continue to function.

Single element protection valve (Fig. 12.12(a)) When the compressor is charging, air pressure is delivered to the supply port where it increases until it is able to unseat the non-return disc valve against the closing force of the setting spring. Air will now pass between the valve disc and its seat before it enters the delivery port passage on its way to the reservoir. A larger area of the disc valve is now exposed to air pressure which forces the disc valve and piston to move further back against the already compressed setting spring. As the charging pressure in the reservoir increases, the air thrust on the disc and piston face also rises until it eventually pushes back the valve to its fully open position.

When the air pressure in the reservoir reaches its predetermined maximum, the governor or unloader valve cuts out the compressor. The light return spring around the valve stem, together with air pressure surrounding the disc, now closes the non-return valve, thereby preventing air escaping back through the valve. Under these conditions, the trapped air pressure keeps the disc valve on its seat and holds the setting spring and piston in the loaded position, away from the neck of the valve stem. As air is consumed from the reservoir, its pressure drops so that the compressor is signalled to cut in again (restarting pumping). The pressure on the compressor side of the non-return valve then builds up and opens the valve, enabling the reservoir to recharge.

Fig. 12.12 Quadruple circuit protection valve

Should the air pressure in one of the reservoir systems drop roughly 2.1 bar or more, the setting spring stiffness overcomes the air pressure acting on the piston so that it moves against the disc valve to close the inlet passage. The existing air pressure stored in the reservoir will still impose a thrust against the piston, but because the valve face area exposed to the charge pressure is reduced by the annular seat area and is therefore much smaller, a pressure increase of up to 1.75 bar may be required to re-open the valve.

A total loss of air from one reservoir will automatically cause the setting spring of the respective protection valve to close the piston against the nonreturn valve.

Multi-element protection valve (Fig. 12.12) Multielement protection valves are available in triple and quadruple element form. Each element contains the cap, piston, setting spring and non-return valve, similar to the single element protection valve.

Charging air from the compressor enters the supply port of the multi-element protection valve, increasing the pressure on the inlet face of the first and second valve element and controlling the delivery to the front and rear service reservoirs respectively. When the predetermined setting pressure is reached, both element non-return valves open, permitting air to pass through the valve to charge both service reservoirs.

The protection valves open and close according to the governor or unloader valve cutting in or cutting out the pumping operation of the compressor.

Internal passages within the multi-element valve body, protected by two non-return valves, connect the delivery from the first and second valve elements to the inlet of the third and fourth valve elements, which control the delivery to the secondary/park and the trailer reservoir supplies respectively. Delivery to the third and fourth valve elements is fed from the reservoir connected to the first and second valve element through passages within the body.

The additional check valves located in the body of the multi-protection valve act as a safeguard against cross-leakage between the front and rear service reservoirs. Failure of the front reservoir or circuit still permits the rear service reservoir to supply the third and fourth element valve. Alternatively, if the rear service reservoir should fail, the front service reservoir can cope adequately with delivering air charge to the third and fourth reservoir.

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