Dp level measurement errors due to non-condensible gases 1

[Bjerke]

Non condensible gases may disturb dp level measurement in pressurizers or reactor vessels. The reason is accumulation of non-condensible gases in the dp condensate pot and dissolution of the gases in the reference leg. At pressure transients the gases might come out of solution and cause measurement errors.
The problem was acknowledged by NRC in the 90-ies. Im am interested in how utilities have handled the problem.

 

[vpl]

Bjerke

Most US utilities, insofar as I know, vent periodically.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[Bjerke]

Thank you Patricia,
My experience of a typical PWR pressurizer level pot made of a standard 2" Tee is that it will fill completely with hydrogen in less than a day. This will mean a lot of venting.
It is easy to calculate the transport of gases to the pot by multiplying the steam flow (typical 0,1-0,2 gram/s) by the hydrogen concentration in the steam (typical 25-100 ppm ). A 2" condensate pot completey filled with gas contains about 0.5 grams of hydrogen.
The accumulation is also easy to verify by measuring the surface temperature of the pot.

 

[vpl]

Bjerke

How are you getting steam into your pressurizer? It should be water-filled. Also the accumulation rate seems extremely high. Most places venting is monthly or quarterly.

The main problem is that the non-condensibles make the pressurizer level measurement inaccurate. I don't know of any way to calibrate the instrument that would accurately account for the non-condensibles.

You might want to check with your operators or review your emergency operating procedures to make sure the plant is not relying on the instrument(s) post-transient. It may mean all the difference between a nice, uncomplicated plant trip and one where you have a lot of downtime explaining why you were operating with non-calibrated instrumentation.


Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[Bjerke]

Patricia,
In a PWR the purpose of a Pressurizer is to allow for thermal expansion of the reactor coolant system and to control the pressure in the same system. The pressure is increased by electrically heat and boil the water in the pressurizer and decreased by spraying reactor coolant in the steam space of the Pressurizer to condense steam. Thus the water level in a pressurizer is about 25 % at zero power and about 60 % at full power to have a constant mass in the reactor coolant system at all power levels.

When hydrogen enters the steam space of the condesate pot it starts to disolve in the condensate and to transport down the reference leg by convection and diffusion. If the concentration of hydrogen is at the solubility level there will be a problem at all pressure transients as the hydrogen will come out of solution and disturb the level indication at pressure decreases. Thus hydrogen concentration in the condensate pot is unacceptable.

We have solved the problem in nothern Europe by installing condensate pots which do not accumulate non-condensable gases (Marketed by Westinghouse). I was interested how this is done in other parts of the world.

I am definitely sure of the gas filling time of a condensate pot as we have measured the filling time several times before we changed design.