Re: Steam Purity - What happens to the solids?

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Posted by Jeff on June 30, 2000 at 18:53:52:

In Reply to: Steam Purity - What happens to the solids? posted by Buddy on June 23, 2000 at 21:01:02:

: Got a couple of questions and I hope the gurus who frequent this site can shed a little light
: on the subject. If this question is overly basic, please cut a beginner some slack.

: We have two low pressure HRSG's (150psi) that produce superheated steam (250 degC) to
: drive a 2MW steam turbine. The conductivity in steam drums are on the average 1000 micro-mhos/cm.

:Since we don't have a device to measure steam purity at the inlet to the turbine, I am guessing it is in
: the neighborhood of 0.3 micro-mhos/cm (based on manufacturers specs).

: Question #1: What happens to all the solids? How do they reduce from 1000 to 0.3.
: Question #2: Does anyone know of an easy way to calculate steam purity?

First, I think that mhos and Siemens are numerically equivalent. There is no conversion factor.
Secondly, You can't assume that the conductivity at the turbine inlet is anywhere near the manufacture's spec. For all we know the conductivity is 1000 micromhos/cm at the turbine inlet.
Since I'm not sure where you actually took the steam sample from the steam generator I would have to ask you to think about the quality of the sample you took. At 1000 micromhos/cm conductivity, the steam purity is grossly poor. It's 1/3 of what is recommended for the boiler water! Steam entering the header, excluding any chemicals added like amines, should be very pure. Either you took a poor sample (eg bad location or contamination) or you have carryover, priming or foaming into the steam space that is ultimately being sent to the turbine. Another question to ask is are you injecting any volatile chemicals into the steam generators or the steam header that can account for this conducitivity?
The ASTM writes a standard for steam sampling or you can ask you chemical rep. for assistance in taking the sample. I would look along the steam header leading up to the turbine to find a place that you could pull a sample from. You should have at least one trap you could get a sample from, or try the turbine drains or even the condensate fron the turbine. The one thing that I would be aware of is that any sample taken after the inlet to the turbine may be lower than one taken from the exit of the steam generator due to solids plating out on the turbine blading like silica.
I'm not sure what you mean by "calculating" steam purity since once you have a sample all you need to do is measure the condutivity with a meter.

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