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Fixed orifice steam traps
These are devices containing a hole of predetermined diameter to allow a calculated amount of condensate to flow under specific pressure conditions. In practice, condensate loads and steam pressures can vary considerably. For instance, start-up and running loads can differ considerably along with steam pressure which will change due to the actions of temperature controls. These varying conditions can result in the fixed orifice either holding back condensate in the process or passing live steam, which can affect plant performance and compromise safety.

Fixed orifices are often sized on running conditions, so that they hold back enough condensate and do not pass steam. If this is so, at start-up, they are undersized to a greater degree and the steam space stands a good chance of waterlogging.

The alternative is to size them so as not to waterlog during start-up. The hole is then effectively oversized for running conditions, and the device will pass steam. The size of hole is usually a compromise between the two conditions, such that, at some points in between, the hole is correctly sized.

Corrosion and service life of plant
Continual waterlogging significantly increases the risk of corrosion in the steam space. It is not unusual to find that after fitting fixed orifice traps, plant service life is reduced below that which may be expected with proper steam traps.

A proper steam trap should be able to achieve just sufficient capacity at all pressures and flowrates present in the application. It can then pass hot condensate without leaking steam under any condition. To achieve this, the size of the hole must vary in the trap. It must be large enough to meet the worst condition, and then have some means of reducing the effective orifice flow area when the capacity becomes too great. This exactly describes the operation of a steam trap.

Advantages of a fixed orifice trap Can be used successfully when pressures and loads are constant.

There are no moving parts

Disadvantages of a fixed orifice trap

If sized on running load, fixed orifice traps will waterlog on start-up, reducing plant performance over this period, increasing start-up times and the risk of corrosion.

If sized on start-up load, fixed orifice traps will waste steam when the plant is running, effectively
increasing running costs.

Fixed orifice traps often block with dirt due to the small size of orifice.

The cost of replacing a heat exchanger due to corrosion will be far higher than the cost of replacing the fixed orifice trap with a steam trap.

Note: Fixed orifice traps are not recommended for draining condensate from any application susceptible to varying load conditions.

Do They Save Energy Compared to Conventional Steam Traps?
The answer to the energy savings question really is "it depends." In order for a fixed orifice steam trap to be effective, the condensate load must be relatively constant, the orifice must be sized correctly, the steam should be clean, pH must be controlled, and the strainer placed in front of the orifice must be periodically blown down.

While vendors of fixed orifice steam traps claim both maintenance and energy cost savings benefits, such benefits are difficult to quantify. Fixed orifice traps continuously release condensate and a small amount of steam, while steam losses from conventional traps include cycling losses and losses due to the percentage of traps that have failed in an open or partially open position. Potential energy savings are related to the difference between two unknown values: steam losses given existing system operation with current maintenance practices, minus expected steam losses after the orifice traps are installed and in use.

For instance, in "real world" operation, facilities have populations of properly functioning and of "failed" traps. A facility may have 90 fully functioning traps and 10 that have failed closed, open, or partially open. The percentage of failed traps found is likely related to steam pressure, trap type, and frequency of trap inspection and repair.

A correct comparison of a base or conventional trap scenario versus a fixed orifice scenario would compare the energy losses from the 90 properly functioning and 10 failed traps versus the steam losses due to using 100 orifice traps. The orifice in fixed orifice traps is small compared to the orifice in a conventional mechanical trap. Steam losses from even a relatively small number of failed conventional traps are substantial. It is this failed trap steam loss value that must be compared with the losses due to the use of orifice traps.

Key variables necessary to conduct an analysis include: the percentage of failed traps, the orifice size for those traps failing open or partially open, the mean time between trap failures, and the steam trap inspection frequency associated with the facility's maintenance program. After the losses per failed trap are determined, the last two terms can be used to determine the time (in hours per year) that a failed trap would be expected to release live steam before the next inspection/repair cycle.

Fixed orifice traps would likely save energy for systems with a long inspection/maintenance interval and with large numbers of failed conventional traps. Conversely, fixed orifice traps could release more live steam than an existing system where the percentage of failed traps is low. (Note: manufacturers of fixed orifice traps contend that steam losses through their traps are less than those associated with brand new, properly functioning mechanical traps.)





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