Block valves are usually provided upstream (and downstream if discharging into a closed system) of pressure relief valves where necessary to permit on stream isolation and maintenance of the PR devices without interrupting process unit operations.
Where block valves are used, the installation should conform to the requirements of Section VIII of the ASME Boiler and Pressure Vessel Code when protecting an unfired pressure vessel. These valves are typically installed so that they are car sealed open (CSO). Mechanical interlocks or key systems are usually required for spared PR valve installations to help ensure continuous safety protection of the system.
Generally, the most difficult and important feature associated with sizing relief valve discharge lines and headers is the determination of the maximum probable flow. The flow is based on the number of valves which may discharge simultaneously owing to a fire or to abnormal process conditions. To do this, the layout of the unit must be considered along with many possible abnormal operation conditions.
The permissible back pressure must also be determined. Generally, the back pressure should not exceed 10 percent of the set pressure for unbalanced safety valves. Balanced pressure-relief valves will operate satisfactorily at higher back pressures (approximately 30 percent of the set pressure), and consequently their
use will sometimes result in a more economical relieving system.
Pressure relief valve discharge piping should be sized so that any back pressure that may exist or develop will not reduce the capacity of the pressure relief valve below that required to protect the equipment. Regardless, the discharge piping for each pressure relief valve should not be smaller than the nominal pipe size of the pressure relief valve outlet.
For gas, vapor, or flashing liquid service the inlet piping pressure drop at design flow should not exceed 3 percent of the safety relief valve set pressure. Nor should the inlet piping to a pressure valve be smaller than the valve inlet nominal pipe size. The inlet piping includes all piping between the protected equipment and the
inlet flange of the valve. Excessive pressure drop in the inlet piping will cause valve chatter (extremely rapid opening and closing of the valve) which may lower the valve capacity and damage valve seating surfaces.
Pressure relief valves should be located so that the inlet piping is short and direct and self-draining with no pockets. However, on installations where pressure pulsations or turbulence are likely to effect the pressure relief valve (e.g., discharge side of reciprocating compressors and pumps), it may be desirable to locate the valve farther from the source in a more stable pressure region. The differential between operating and valve set pressures is also important when the operating pressure is not steady. A large differential will tend to reduce valve maintenance costs.
On certain vessels, pressure relief valve leakage and consequent premature shutting down of the process unit can be anticipated. For getting details on various types of Valves, checkout Gotstock. These vessels should be provided with a sufficient number of pressure relief valves (and accompanying block valves) so that in the event of pressure relief valve leakage it will be possible to shut off any one defective valve and replace it while the vessel is in service and still retain full calculated relieving capacity.
Pressure Outlet Piping Discharge to Atmosphere
Outlet piping for pressure relief valves discharging flammable vapors directly to the atmosphere should normally be equipped with steam and drain connections controlled from grade, as shown by Fig. 1.
Snuffing steam to pressure-relief atmospheric vent
Outlet piping from pressure relief valves should be equipped with drains or otherwise suitably piped to prevent accumulation of liquids at the valve outlet. Pressure relief valve outlet piping for water or other liquids should be self-draining.
Separate pressure relief valve lines should be provided for each valve discharging directly to the atmosphere. On towers, the pressure-relief valve vent piping should be extended at least 10 ft (3 m) above the nearest working platform within a radius of 40 ft (12 m). Outlet piping should be arranged so that the pressure-relief valve discharge will not impinge on any equipment.