Greetings!
Our topic for this month is atmospheric relief valves. Graham fabricates atmospheric relief valves called Viking Relief valves. This article will
cover the uses of atmospheric relief valves, alternates for atmospheric relief valves, and types of atmospheric relief valves. It is also going to cover the material of construction and internal parts.
Turbine exhaust condensers are meant to operate under vacuum. However, upset conditions may occur where the pressure rises above the condenser mechanical design pressure. Atmospheric relief valves are designed to avoid this potentially hazardous situation.
The Graham Viking Relief valve or atmospheric relief valve is utilized on turbine exhaust surface condensers to relieve all the steam which is known to enter the condenser
under the worst conditions. The Viking valve is sized to pass the rated maximum capacity of steam at a pressure not exceeding 10 PSIG, as outlined by Heat Exchange Institute (HEI) Standards for Steam Surface Condensers. The Viking valve is a weighted disc or a “
dead weight”
valve, which means there is no spring or set point like other relief valves. In operation, the valve remains tight under vacuum and opens automatically above atmospheric pressure. The valve will then automatically reseal back to the original position when the vacuum is restored. Therefore, there is no adjustment or replacement required after a venting occurrence.
The Viking valve includes two water connections. Each valve includes an external water seal connection for the water seal on top of the internal weighted disk, as required by HEI. There is also a gauge glass supplied that will help to visually observe the water seal level. There is an overflow connection provided for adequate drainage and maintaining a seal height. In operation, the water seal insures proper sealing against air in-leakage. The seal water flow rate should be supplied at a minimum of 0.25 gal/min. This should allow a small amount of overflow and the same amount of overflow should always be present when the system is in operation.
The Viking valve is equipped with a manual hand wheel to allow the valve to be checked and maintained
when off-line. Please note the manual hand wheel and lifting mechanism are not intended for use as a vacuum
breaker. If manual operation of the hand wheel is attempted while the inlet side of the valve is under a vacuum, then this will cause damage to the
valve and expose plant personnel to possible hazardous conditions.
When installing the Viking valve the pressure drop in the piping at the inlet and outlet should be reviewed to insure the accumulated pressure of 10 PSIG plus the piping pressure drop does not exceed the design pressure of the condenser being protected.
Some more key items when installing the Viking valve is to insure the valve is installed in the correct position, make sure the hand wheel is free to operate without obstruction from other piping or equipment, insure a water seal is piped to the valve at a minimum of 0.25 gal/min, insure the overflow connection is piped to a drain point, and that it is installed for accessible inspection. The way the Viking valves are fabricated, the outlet body of the valve can be rotated around the bolt circle for piping flexibility.
WARNING:
Exhaust piping from the Viking relief valve will admit live steam and must be routed to a safe area where personnel cannot be affected.
When installing in the Viking valve there should be no external reactions from piping, etc. The relief valves can be installed directly on the condenser, they may be placed on the turbine exhaust hood, or in the turbine exhaust piping.
During plant shutdowns, the condenser should be allowed to reach atmospheric pressure. The Viking relief valve seal water can then be shut off and the valve disk raised to drain the valve. When restarting the Viking valve the valve disk should be lowered and the seal water flow resumed.
