Pressure relief devices may be required between block valves where thermal expansion due to ambient temperature changes or heat tracing may cause damage. Pressure relief device may also be required in lower pressure side of a pressure reducing valves where the piping and / or equipment on the low pressure side are not suitable for the conditions that exist on the high pressure side.
All relief valves, except small thermal relief valves, shall be installed with the spring in the vertical position. All relief valves located from grade up to 5 meters above grade and thermal relief valves in pipe racks shall be considered accessible by portable ladder or movable equipment. All relief valves with a mass of more than 20 kg and above 5 meters must be accessible by ladder or permanent platform.
All other relief valves above 5 meters must be accessible from a permanent platform. Where ever feasible, relief valves should be located on plattforms which are also functional for other purposes. Relief valves may be located on the piping instead of on the equipment and shall be located at the lowest possible elevation that may be economically justified.
The size of the inlet line shall not be less than the size of the relief valve inlet. The length of the inlet piping shall be kept to a minimum. Wherever the inlet line size is higher than the safety valve inlet size, reducer shall be installed adjacent to inlet of safety valve.
PRV discharging Hydrocarbon vapors to atmosphere should be provided with outlet piping, at least 3 meters above any equipment within a horizontal distance of 15 meter or 4.5 meter above the roof of the nearest building whichever is higher. This riser shall be at least 30 meters away from any furnace regardless of the height of riser. PRV discharging steam to atmosphere shall have outlet piping in such a way that vapor clouds will not envelope any platform.
PRV in liquid service and discharging to atmosphere shall do at a visible and safe place. PRV discharging to closesd system shall be installed so that the discharge piping is free draining into the header. Blowdown headers shall be installed so that the piping slopes into the blowdown drum.
Valve sudden opening reactive forces shall not lead to any leakage at the flanged joints present in the system. To ascertain this the necessary calculations for checking leakage at the flanged joints shall be performed.
Reaction forces including both momentum & static pressure effects due to safety valve popping shall be ascertained in the connected piping according to API RP520 for systems discharging to atmosphere. The effect of these reactive forces on the piping supports and the anchors of the piping system shall be calculated to ascertain that the allowable limits at these locations are not exceeded.
The supporting structure shall be adequately designed so that when subjected to reaction forces the supporting elements connected to piping as well as the basic supporting structure i.e. platform members etc. are capable of withstanding them. System stresses in the inlet and outlet piping portions at safety valves also shall be kept within the allowable limits, inclusive of the distribution branching points in the inlet portion.
Adequate supporting must be provided to compensate for sudden opening of relief valves.
Minimum slope of the flare header shall be 1 :450 (per API 521). The bonnet vent of a relief valve with bellows shall be tubed free draining to a safe location. Relief valve discharging steam, air or other non-flammable vapour or gas directly to atmosphere shall be equipped with drain and shall be suitably piped to prevent accumulation of liquid at valve outlet.
Where a block valve is installed to permit removal of a relief valve during unit operation, it shall be located immediately below the relief valve and be car sealed open. Where a pressure relief valve discharges into a header and an outlet block valve is installed, this valve shall be car sealed open. Block valves around relief valves and in blowdown lines shall be positioned so that the block valve remains open if any component of the valve accidentally fails.
A 3/4″ bleeder valve shall be provided between any inlet block and pressure relief valve. The use of a bleed ring for this purpose should be considered.
Liquid-phase blowdown system piping connected to a closed system shall be self draining to the blowdown drum. Closed blowdown header shall be sloped towards the CBD drum to ensure free drainage. Blow down facilities / buried drum should be located at one corner of the plant farthest from furnace or any fired equipment and on the lee-ward side of the unit. Vent from Blow down facility shall be minimum 6 m above the highest equipment falling with in radius of 15 m from the vent.
Liquid-vapour phase relief valves shall discharge into the flare header at an angle 45 degrees in the direction of header flow, to minimise the effect of kinetic energy and to avoid accumulation of liquid. Pockets in the flare header and blowdown system shall be prohibited. Flare knock out drum for the process units should be located at battery limit of the unit.