Pressure Relief

As the pressure in the tank increases, it eventually reaches the point where it is greater than the pallet weight acting against it. When this point is reached, the pallet will lift allowing vapour to escape. The more pressure that is applied the greater the pallet lift and therefore the greater capacity of the valve.

Fig1 shows a positive pressure equal to or less than the set pressure of the valve with both pressure and vacuum pallets closed.  Fig 2 shows how the positive pressure exceeds the set pressure and lifts the pressure pallet and forces the vacuum pallet closed.

Fig.1        

Fig. 2  

Vacuum Relief

The same applies when vacuum relief is required. When the tank is pulling a vacuum and the vacuum reaches the point where it is greater than the pallet weight against it, it will lift the pallet allowing air to enter through the valve into the tank.

Fig 3 shows atmoshpheric pressure acting against the vacuum pallet which remains fully seated. When the vacuum within the tank exceeds the set vacuum of the pallet, Fig 4 indicates the point at which vacuum pallet lift is achieved. The pressure pallet remains closed.

Fig. 3              

Fig. 4      

Emergency Relief

When a vessel is subject to an external fire source or dramatic heat increase, an emergency relief valve (manway) is required. These need to have much greater capacities than the relief valve, but operate in the same way.  As the pressure in the tank increases, it eventually reaches the point where it is greater than the pallet (cover) weight acting against it.  When this point is reached the pallet will lift allowing tvapour to escape. The more pressure that is applied the greater the pallet lift and therefore the greater the capacity of the valve.

Fig 5 shows a positive pressure equal to or less than the set pressure of the manway with the cover (pallet) remaining closed. Fig 6 shows how the positive pressure exceeds the set pressure and lifts the cover (pallet).

Fig. 5        

Fig. 6