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PH Measurement

Wednesday, July 04, 2018   By: Vangel [2265] 0 Stars
How do you measure the pH ? With electrode or calculated from EC and CC measurements?
At our gas fired combined cycle power plant it seems that calculated pH at condensate, LP Drum and HP Drum we have only ammonia, we cant calculate pH for IP drum where TSP is added is more stable and reliable than electrode measurements. We measure EC and CC online and then calculate the pH according to VGB -S-006-00-2012-09-EN standard.

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Wednesday, July 04, 2018   By: MW [2231] 1 Stars
We use electrode measurements here 160mw biomass plant. All other plants I have visited here in UK of varying size and fuel sources have also used pH electrode

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Saturday, July 07, 2018   By: Ross Burns [390] 5 Stars
This is not an easy question, but a good one. Presently, there are 3-methods of pH measurement instruments. All of them require programing calculations, but the simplest needed some calculation too as some point when developing it. Just know there is no-direct measurement method to determine pH, for they all have to be determined based on conductivity.

Of the 3-conductivity pH instruments, none are not based on type of Boiler or even pressure steam drums like LP, IP and HP steam drum. However, what is a decison to use which on is based on continuing operating cost and installation costs as well as the decision for the need for accuracy, which is likely to come from one of the three TSP tri-sodium-phosphates water treatment programs as well as for continuous readings like steam sampling.

Most steam plants that are stable such as moderate degree of daily swing loads probably does not need continuous pH monitoring, except of perhaps steam sampling, but there maybe some special cases too.

The simplest pH instrument is a handheld and called a Colorimetric slide. This works off of titration. It is accurate for many applications, but not used much today,

The most common is glass electrode instrument and it is accurate enough for your HRSG or as Mr.MW MEGA-Watt said his BioMass high-pressure boilers.

The most advanced is the a cation -acid- based conductivity instrument. It can be in-stream that is to say shunted to the main steam. It is very expensive but very-sensitive after some ion conditioning in the instrument. It is so sensitive that it accommodates the super-cleanliness waters being sampled.

Allow me to basically explain the inherited problem of super clean, relatively speaking, rankine cycle waters. The problem is pH will swing dramatically and rapidly in real world without a buffer to dampen the the OH hydroxyl ion. This is because when there is little alkaline conditioning to buffer the pH because the constant chemical reactions of boiler or condensate waters produce less alkaline conditions. that is why we like to see sufficient alkalinity in most boilers. So, when using the cation conductivity instrument to determine pH you are seeing the hard swings of ups and downs in the pH, but probably just swinging within a tolerance, but very-accurate and if you are running your TSP on the edge then you will need it. But this maybe too accurate for some water treatment specialists, since it can be meaningless too.

When using a glass electrode conductivity probe you tend to get a spot reading that is practical in most applications.

Seems to me you are asking a real good question, which I did not answer directly, but it may give better understanding of the issues of determining and applying pH instruments.

Ross

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Saturday, July 07, 2018   By: Jim Watts [764] 5 Stars
In steam cycle power plant it is not useful to have a PH number that is not accurate so the methods used are intended to get accuracy.
A PH meter is accurate. Its only problem is that it needs to be calibrated and carefully operated if used for different samples. On line instruments are no different to any other QMI.
The PH meter works by measuring the voltage produced between electrodes and is basically just checking how good a battery you can make with the solution.
The reading varies with the Hydrogen ion concentration of the solution and the less ions there are the more sensitive the result and the more likely poor lab technique will give an error.

This is where inferred PH comes in.

Inferred PH avoids the PH meter problem by not measuring the PH at all.
It is possible to calculate the PH if you know the number of Hydrogen Ions in the water.
A simple way to know the Total number of Ions is to measure the conductivity but it only works if there is only one type of Ion and you know its Conductivity to TDS ratio.
With very pure water dosed only with Ammonia you are very close to knowing this except there may be contaminants. A trick is to also measure the Cation Conductivity which causes the Ammonia to be exchanged for H20 so that the conductivity represents the non Ammonia TDS. You subtract the cation TDS from the Total TDS and you then use to Calculate PH.
This method cannot be used when the Non Ammonia Ions vary to much in their conductivity to TDS ratio as the inacuracy is worse than measuring PH with a meter.

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