DEP 80.45.10.10-Gen.
January 2010
Page 10
2.2.4
Isolation valve operation control
It is vital to ensure that the relief device isolation valve and all other valves in the relief path
are in the proper position. The method for managing the valve positions SHALL [PS] be
consistent with the site's HSE-MS.
The preferred method for controlling this is to use valve interlocking systems on block
valves used on the inlet and outlet of relief valves, their spares and their open spool pieces
(in accordance with DEP 80.46.30.11-Gen.).
Alternatively, valve locks or car seals may be employed provided there are strict
administrative controls in place. With the valve locking method, the correct fully open or
fully closed position can be identified by having a locking system with two keys with
different colour codes, e.g. yellow and green. When the yellow key is in the key cabinet
mounted in the control room, it is known that the isolation valve is locked fully closed.
When using the valve locks/car seal methods, installed spare PRVs shall have the inlet
valve Locked Closed (LC) or Car Seal Closed (CSC) and the outlet valve Locked Open
(LO) or Car Seal Open (CSO). PRV bypass valves are normally closed (NC) and do not
required key locks or car seals. Emergency depressuring valves do not provide pressure
relief but, since they do play an important role, the isolation valves for these SHALL [PS] be
LO or CSO.
To indicate the proper operation of the upstream block valve, a vent connection shall be
provided between the upstream block valve and the relief valve.
For pipeline TERVs (which may be removed while the system is in operation), a single relief
valve with upstream and downstream isolation valves should be provided. Strict procedural
controls should ensure that the line is not shut in while the relief valve is out of service. The
block valves may be car sealed open rather than locked open, in a manner to be agreed by
the Principal and consistent with the site’s HSE-MS.
2.2.5
Balanced PRV bonnet venting
Balanced PRVs in very toxic service or balanced PRVs that discharge into a closed system
containing very toxic substances shall have the bonnet vent discharge to a safe location.
The amount of toxic gas leakage shall be estimated assuming complete failure of the
bellows with the flow limited by the annular area between the disk holder and the guide,
vent hole in the valve guide plate (if the vent hole is present), and vent opening in the valve
bonnet. Dispersion analysis shall demonstrate that the toxic gas levels are acceptable.
Providing a continuous purge at the relief valve outlet could reduce these levels. As an
alternative to performing dispersion analysis, the bonnet vent discharge can be located 8 m
(25 ft) away from the potential location of personnel. If the vent piping is installed it shall be
designed to prevent ingress of rain and shall have no pockets. For a balanced PRV in very
toxic service a balancing piston may also be considered.
2.3
PREVENTION OF MALFUNCTIONING OF RELIEF VALVES
2.3.1
Relief valves affected by hydrates and freezing
The Joule-Thomson effect, occurring across the relief valve when relieving, may lower the
temperature to within the hydrate or ice formation region. Due to the high velocities, there
will likely be no problem of relief valve blockage at relieving conditions. For additional
details, see (3.7).
Valve blockages could occur, however, due to small leaks across the relief valve seat. To
prevent this blockage, heat tracing/insulation SHALL [PS] be provided around the relief
valves that can be affected by hydrates and freezing.
If a relief device and/or its inlet/outlet piping requires heat tracing to ensure an open relief
path, then:
a. Steam tracing system shall have dual circuits (i.e., dual supply, tracers, and traps).