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Condenser Vacuum

Most of the material in this topic is general among all steam cycle power plants, not just nuclear.
The main condenser is designed to remove the excess (waste) heat after all possible work has been extracted
from the steam. The condenser is kept at a vacuum of 25-30 in. of Hg for several reasons. To name a few, the
first is the larger the delta-P over the turbine, the more work you can extract from the steam. The second is to
create an environment that draws steam in before it starts to get "wet" on the turbines, as water on turbine
blades causes significant erosion. A third reason for the vacuum is to ensure that any gasses or other things
which may form in the steam can be sucked through the vacuum system and be properly filtered and removed.
During operation the condenser vacuum is measured and used as an indirect indication of the cooling capacity
of the condenser. Low vacuum generally means lower cooling capacities, and larger vacuums means better
capacities, and ultimately, efficiency of the plant. The condenser vacuum is maintained through a combination
of the vacuum system and the cooling water system. The cooling water either goes back to a lake, or through
the big cooling towers as shown in the following picture:

(Cooling Towers)
The vacuum system consists of mechanical pumps and steam jet pumps. The mechanical pumps are used to
draw the initial vacuum, and are used during very low power operation until sufficient steam is available to
operate the steam jet pumps. The steam jet pumps work by creating a very high velocity steam flow region that
"entrains" or "entraps" air around it, creating a suction. Jet pumps have no moving parts. This in turn draws a
very strong vacuum. In a BWR, due to the potential buildup of hydrogen in the main steam system, the
mechanical pump is not used at power conditions. The steam jet pumps are used though, as with no moving
parts, there is no heat which could spark an explosion. The steam jet pumps then pass the steam/air mixture
through filtration systems, charcoal beds, hold up lines, hydrogen removal/separation, and ultimately out the
plant's main stack.
Condenser vacuum is a very important operating parameter. Should the vacuum start to degrade, it could
indicate a loss of heat sink, which could cause destruction of the condenser internals. To protect itself, the
condenser has 4 signals it will send to the rest of the plant. The first signal, at around 21", shuts off the main
turbine. This eliminates the primary source of heat to the condenser. If the reactor power is high enough this
turbine trip will also shutdown the reactor. The second signal, around 18", trips the main steam powered feed
pumps. The main feed system for the reactor uses steam as its energy source and discharges that steam into
the condenser, so this is the second largest heat load for the condenser. If the condenser situation does not
improve, around 8" the main steam valves will close, which prevents any steam from even entering the turbine

it usually also requires the use of safety valves to help depressurize the reactor. between the steam and and cooling water tubes that have a film of condensate. The NRC classifies these scenarios as "complicated trips" and they usually receive augmented inspections. all turbine related valves. Therefore. What things can cause loss of vacuum? Loss of lake/river cooling water can do this. Therefore. BWRs tend to be capable of this more than PWRs. Abstract Vacuum applies to steam turbines because the pressure of saturated vapour above available cooling water is well below 1 bar (typically 17 mbar at 15°C ambient). the non-condensate (air) can be removed from the condensatesteam circuit by pulling and maintaining a vacuum in the steam side. it can continue to handle the main feed pump supply steam. the condensate can be used as boiler feed. In cases where there is a slow reduction. in these large and many-jointed vacuum vessels attached to the exhausts of turbines. So the condenser has such a vacuum and takes exhausted steam from the turbine and gives condensate for feeding back to the boiler. Continuously running air pumps are provided to keep condensers as free of air as possible. Condenser is very important as it is the normal heat sink for the plant and reactor. In general. as wasteful collisions between the two kinds of molecules develop a thermal barrier. Condenser vacuum tends to change very rapidly. One example is if a single cooling water pump fails. Another way the vacuum can fail is if there is a failure in the vacuum system. will automatically close and seal shut. but more on that in a later post. the ventilation of condensers. and a lot of times the operators don't have much action other than preemptively scramming the reactor. Secondly. greater will be the enthalpy drop of steam. as PWRs have to rely on boron levels. and thus any additional partial pressure of air hinders condensation. you now have to relay on your important-to-safety isolation cooling systems to remove decay heat and restore coolant levels. a rapid downpower may save the unit. but if the turbine and reactor shut down.system. their condensing ability and hence their thermal performance is . while a BWR can simply change its core flow and make a rapid change. By neglect of some simple vacuum physics in a few air extraction systems. the efficiency of the steampower plant can be increased as greater the vacuum in the system. for some in-leakage from the atmosphere is to be expected. and around 7". Why must a vacuum be maintained in the steam condenser? Answers: By maintaining a vacuum in the steam condenser. as an ultimate protection to the condenser. in case the main steam valves did not close properly. the condenser may not be able to handle 100% rated power. including the bypass valves. and if you "lose" the ability to use the condenser as a heat sink. more work will be available per kg of steam condensing.

and their benefits demonstrated.spoilt. On the shell side.” The removal of air and other noncondensable gases from the condenser shell side is required for proper heat transfer from steam to cooling water in the condenser and. The drains should be located on the boiler side of the throttle valve. in turn. with very serious results What three methods are used to restore casing surfaces that are excessively eroded? Answers: Metal-spraying. the “holding mode. air and other noncondensable gases would severely reduce the heat transfer in the condenser. the vacuum equipment is used for two purposes: rapid evacuation of air to reduce condenser pressure before steam turbine start. These shortcomings are examined and used to explain the anomalous response of a turbine to a copious supply of cold winter cooling water. thus.” or for continuous removal of air and associated water vapor from the main condenser when the steam turbine is in operation. This. Water in any pocket in a turbine casing may cause a slug of water to be carried over in the turbine during operation. the primary use of vacuum systems is to remove air and other noncondensable gases from the shell side of the condenser in order to maintain design heat transfer and thus design vacuum. to maintain high vacuum in the condenser. The steam supplied to a turbine should be reasonably dry at all times. . one often-overlooked cause is an inadequately sized vacuum system. In a power plant. in the laboratory and in a power station. This article provides plant designers with an understanding of published design standards for air-venting equipment used in condenser shell side applications and the vacuum system sizing methodology used by equipment suppliers. and the plant would require a condenser with significantly more surface area for the same thermal load. A separator of the receiver type and having ample drains should be installed in the steam supply line near the turbine. Some simple modifications are suggested. In addition it is necessary to drain all portions of the turbine casing where water from condensation may collect. STEAM SEPARATOR AND DRAINS It is unsafe to permit water to enter the steam passages of a turbine. the “hogging mode. If holding condenser vacuum is a persistent problem. Welding. Without a vacuum system. The primary application of vacuum systems in the power generation industry is for the evacuation of air and other noncondensable gases from the shell side and the waterbox side of a steam surface condenser. enhances the amount of energy extracted from the steam exhausted from the steam turbine and increases the plant’s energy production.

Gland[edit] A gland is a general type of stuffing box. machine or bulkhead facilitates assembly and prevents liquid or gas ingress. .Insertion of filler strips or patch plates. a cable gland or fitting that connects a flexible electrical conduit to an enclosure. The most common example is in the head of a tap (faucet) where the gland is usually packed with string which has been soaked in tallow or similar grease. Other types of sealed connections without moving parts are also sometimes called glands. for example. The linear seal around the piston rod of a double acting steam piston is also known as a gland. Likewise the shaft of a handpump or wind pump is sealed with a gland where the shaft exits the borehole. The gland nut allows the packing material to be compressed to form a watertight seal and prevent water leaking up the shaft when the tap is turned on. The manufacturer should be consulted on the metallurgy involved so that the best method can be selected. used to seal a rotating or reciprocating shaft against a fluid. The gland at the rotating shaft of a centrifugal pump may be packed in a similar way and graphite grease used to accommodate continuous operation. particularly in marine applications.