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Maintenance of condensate discharge circuits

        

What control? Why?

The steam traps control and the regulation offers a considerable improvement of a power plant efficiency and security. A steam trap blocked in closed position can lead to water hammers. Water Hammer can cause pipelines to break, a dangerous situation to workers nearby.

However, the steam traps control is useless without the bypass tightness control by Acoustic Method (checking procedure by ΔT° upstream / downstream not efficient). A leaking bypass evacuates condensates and steam without distinction, what makes useless the replacement or the regulation of the associated steam trap.

D.R.S. Industries method allows controling the steam trap, his bypass and isolating valves as a complete set.

D.R.S. Industries acoustic method, validated and applied in the majority of plants, has the advantage to control the steam trap directly, as well as to see if it closes completely or if it is permanently opened.

The acoustic method is the most adapted to the steam traps control because it has no constraint bound to the thermal inertia provoked by steam trap openings and closures.
We can also make an additional thermographic analysis.


A steam trap is a device used to discharge condensate with a negligible consumption of live steam to save quality of steam required by the system. We know three major families of steam traps with different working modes:


• Mechanical or with float steam traps,
 opening and closing cycles depends to the condensates level or the density difference between condensates and steam.

STARTING:

In lower position, the float seals evacuation and then the cold thermostatic vent is open to let discharge air and incondensable on arrival of moving fluids.

The increase of the condensates level actuates the float to the top, thus condensates find a passage by evacuation opening.

WORKING:

The water level inside the steam trap depends to the condensate flow. The flow through the evacuation opening is thus proportional to condensates flow. So, the steam trap is regulating by a continuous rolling of the float on the seat.

Air can accumulate in the steam trap. The more the proportion of air in the air/steam mixture increases, the more the temperature of this mixture decreases compared to the saturation temperature at operating pressure. The thermostatic vent opens when this variation is at least equal to its value of adjustment. When it evacuates incondensable, the mixture temperature increases and the vent closes again.




• Thermostatic steam traps (bimetallic, bellows, dilatation) reacting to temperature variations of fluids.

STARTING: 

With starting, the cold bimetallic device is completely slackened. The valve is large open and allows air and incondensable to evacuate.

Cold condensates do not have any effect on the thermal switches. But the progressive increase of temperature towards the saturation temperature at operating pressure involves a progressive deformation of the bimetallic element. The traction generated on the stem tends to close the reversed valve.

WORKING:

When the bimetallic device exerts on the valve stem a traction higher than pressure push on the reversed check valve, the valve is closed again.

The valve closing causes a stagnation of condensates in the steam trap body and a water reserve upstream. The traction of the bimetallic device reduces by condensates cooling in the steam trap body. The opening force which the pressure exerts on the reversed valve becomes prevalent and the valve opens.
 



• Thermodynamic steam traps
 (with disc) have opening and closing cycles caused by the fluid pressure variations (fluid dynamics) or by the difference of energy between steam and condensates.

STARTING:

The disc stays on its seats, blocked in closed position by air locked up in the pressure room. The air acts thus like steam in working. The air evacuates and the disc opens up when condensates comes.

WORKING:

The open steam trap lets flow the condensates. Live steam under the disc increases the rate of flow between disc and seats and creates a depression which involves the closing.

The steam closed up inside the pressure room exerts a force higher than resultant of forces generated by fluids upstream and downstream. The disc is plated on the seats.

A heat exchange with outside causes a decrease of the steam pressure in the pressure room and the dic is raised. If condensates were accumulated upstream, the steam trap remains open. But live satured steam causes at once the closing of the disc and a new cycle. In particular cases, the expansion resulting from the high speed condensate flow generates an effect of steam flashing which can cause a prematured closing of the disc, before the total evacuation of the condensates



 The flashing

Part of the evacuated condensates is transformed into steam. Sometimes it can be confused with a live steam leak. However it is not because it normally happens in regular motion of the steam trap.

How can we explain it?

The saturation temperature upstream is higher than the saturation temperature downstream. The hot condensates evacuated by the steam traps know a brutal cooling and exchange calories with the ambient conditions. The condensates are transformed into steam by consuming the calories available.

In the case of a steam trap which discharge to atmosphere the phenomenon can lend to confusion because steam at atmospheric pressure occupies ±1700 times the volume occupied by liquid water.



 


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