Continuous blowdown of the steam boiler based on the alkalinity of the boiler water
Ivan Tikhonov
Guidelines for maintaining the water-chemical mode of steam boilers prescribe continuous blowdown of boilers based on dry residue or salt content.
For example, RTM 108.030.114-77 (Stationary steam boilers of low and medium pressure. Organization of a water-chemical mode) gives the following formula for calculating the percentage of continuous blowdown based on the boiler steam capacity.
, % (1)
where,
Smw –makeup water salinity, mg/l (ppm)
Sbw – boiler water salinity, mg/l
Q – the share of makeup water in the feed water of a steam boiler
Q=1-К,
К – the share of condensate return.
The equation (1) assumes that the salinity of the returned condensate and the salinity of the steam produced are equal to zero.
This equation is used universally to calculate the fraction (percent) of steam boiler blowdown from steam production.
In fact, the calculation using this equation contains a significant error. It’s all about the method for determining the salinity of boiler water.
The salinity of the boiler water is determined on the bases of its electrical conductivity. In this case, the decisive role is played by the conversion factor of calculation of the electrical conductivity value into the salt content (n).
S=E*n, mg/l
where,
S – Salinity of water, mg/l
E – Electrical conductivity of water, µS/cm
The coefficient (n) used by standard conductometers ranges from 0.5 to 0.6. If it is assumed that all salt in the water is in the form of NaCl, then the conversion factor is assumed to be 0.5. Some manufacturers, realizing that most waters have a significant amount of bicarbonates, use a conversion factor of 0.6.
But for determining the salinity of boiler water, these conversion factors are fundamentally inadmissible.
In this case, it is much easier, more understandable to calculate the blowdown by the alkalinity value of the chemically treated and boiler water. In this case, the salinity values are simply replaced with the alkalinity values in the equation (1).
, % (2)
where,
Amw – total alkalinity of makeup water, mg-eq/l
Abw – total alkalinity of boiler water, mg-eq/l
In this case, an error in determining the share of the blowdown of the steam boiler and, accordingly, the loss of heat with blowdown water is practically excluded.
The equation (2) is valid because the following process takes place in the deaerator and boiler. An intensive distilling of carbon dioxide takes place and as a result, sodium bicarbonate passes into sodium carbonate, which in its turn hydrolyzes in water to form sodium hydroxide and the same sodium bicarbonate.
The process is illustrated by the following diagram.
2NaHCO3 <-> Na2CO3 + CO2
+
H2O <-> NaOH + NaHCO3
As a result, the more carbon dioxide is distilled off, the more caustic soda is formed.
A large amount of caustic soda forms in the boiler. In fact, about half of the salinity of boiler water is determined by caustic soda. The coefficient of conversion of electrical conductivity into salinity for caustic soda is 0.17. Accordingly, it is fundamentally incorrect to use a conversion factor of 0.5 (especially 0.6) to determine the salinity of boiler water based on its electrical conductivity.
In this case, the alkalinity taken in mg-eq/l does not give an error. Simply the equivalent amount of alkalinity in the feed water represented by bicarbonate converts into the equivalent amount of alkalinity in the boiler water represented by hydrate, carbonate and bicarbonate.
In fact, the amount of carbon dioxide removed from the water was replaced by an equivalent amount of hydrate. Due to this, the pH value of the boiler water increases.
Let’s consider an example of calculating the percentage of continuous blowdown of a steam boiler using the equations (1) and (2).
As the makeup water of the steam boiler we have the water of the following composition.
Cations (К):
Na – 4,0 mg-eq/l = 4,0*23= 92 mg/l
Anions (А):
НСО3 – 2,0 mg-eq/l = 2,0*61=122 mg/l
SO4 – 1,0 mg-eq/l = 1,0*96 = 96 mg/l
Cl – 1,0 mg-eq/l = 1,0*35,5 = 35,5 mg/l
Total: К=А = 4 mg-eq/l
Or
NaHCO3 – 2 mg-eq/l
Na2SO4 – 1 mg-eq/l
NaCl – 1 mg-eq/l.
The salinity of makeup water = 2*84+1*142+1*58,5=368,5 mg/l
For the sake of simplicity of the calculation, we assume that there is no condensate return. Accordingly Q = 1.
During the operation of the boiler, the boiler water was analyzed for alkalinity and electrical conductivity.
The alkalinity of boiler water for phenolphthalein (Aph) is 20 mg-eq/l, for methyl orange (Amo) is 2.0 mg-eq/l. Total alkalinity is 22.0 mg-eq/l.
The boiler water conductivity – 7798 μS/cm.
Let’s calculate the percentage of continuous blowdown by the equation (2).
%
In this case, the amount of continuous blowdown is 10% of the amount of steam produced by the boiler.
Let’s calculate the salinity of the boiler water using the conversion factor (n) equaling to 0.5.
Sbw = 7798*0,5=3899 mg/l
Next, we will calculate the percentage of continuous blowdown using the equation (1).
%
The difference between the two calculated values is 0.43%.
Let’s determine what the real value of the salinity the boiler water will have.
We calculate the coefficient of evaporation of the boiler water
Кe = Abw/(Amw*А) = 22/(2,0*1)=11
Thus, the feed water in the boiler is evaporated by 11 times. Accordingly, the concentration of cations and anions of strong acids increases 11 times.
Na2SO4 = 142*11=1562 mg/l
NaCl=58.5*11=643.5 mg/l
Total = 2205.5 mg/l
We calculate separately the salinity of hydrate and carbonate in the boiler water.
Putt simplistically, it can be assumed that the amount of hydrate in the boiler water is
ОН= Aph-Amo=20-2 = 18 mg-eq/l
Accordingly, the amount of carbonate is At – ОН= 22-18=4 mg-eq/l.
For the sake of simplicity of the calculation, we will assume that there is no bicarbonate in the boiler water.
(The calculation of hydrate and carbonate is presented in a very simplified form. More information can be found in [1])
We get,
NaОН=18*40=720 mg/l
Na2СО3 = 4*23+2*60=212 mg/l
We get the following salinity of boiler water:
Sbw = NaOH+Na2CO3+Na2SO4+NaCl = 720+212+1562+643.5= 3137,5 mg/l.
Let us calculate the proportion of continuous blowdown according to equation (1) using the value of the actual salinity of the boiler water
%
We get an unexpected result. The purge percentage has increased by 3%.
What percentage of purge is correct?
The percentage of blowdown obtained using the alkalinity of the boiler water is correct.
The fact is that in accordance with the above scheme for the decomposition of bicarbonate in the boiler, the salinity of the boiler water decreases due to the fact that sodium bicarbonate is replaced by caustic soda. In this case, the molar mass of caustic soda is 40, and sodium bicarbonate is 84. Accordingly, the salinity of boiler water, determined by sodium bicarbonate, becomes de 2 times lower after replacing bicarbonate with hydrate.
In this case, it is fair to assume that no decomposition of bicarbonates occurs in the boiler. Then, hypothetically, it can be imagined that the entire alkalinity of the boiler water is determined by sodium bicarbonate.
Then the “hypothetical” salinity of the boiler water will be equal to,
Sbw = NaHCO3+Na2SO4+NaCl = 22*84+11*142+11*58.5=4053.5 mg/l
Next, we will calculate the percentage of continuous blowdown
%
In this case, the percentage of continuous blowdown is the same as the percentage calculated using the alkalinity of the boiler water and feed water.
Next, we calculate the real electrical conductivity of the boiler water.
Let’s recalculate the salinity of the boiler water into electrical conductivity using the data [2,3].
Conversion factors (n) for a given salt content:
NaOH – 0,17
Na2CO3 – 0,55
NaSO4 – 0,65
NaCl – 0,5
Accordingly, we multiply each conversion factor by its share in the total salt content.
The share of:
NaOH (18+18)/88=0,41
Na2CO3 (4+4)/88=0,09
Na2SO4 (11+11)/88=0,25
NaCl (11+11)/88=0,25
Total: 0,41+0,09+0,25+0,25=1
We multiply the conversion factor (n) by the share
0,41*0,17+0,55*0,09+0,25*0,65+0,25*0,5=0,4
We obtain the coefficient of conversion of electrical conductivity into salinity for boiler water of a given composition equaling 0.4.
Accordingly, the value of the electrical conductivity of the boiler water in this case will be:
3137.5/0.4=7843 μS/cm.
The value obtained is quite close to the measured value (7798). This indicates the correctness of the calculation method.
If we use a conversion factor of 0.5 (included in most conductometers), then we get
3137,5/0,5=6275 μS/cm.
As we can see, the discrepancy is more than 1500 μS/cm.
Nevertheless, the use of this conversion factor (0.4) is inappropriate for calculating continuous blowdown, since it does not take into account the reduction in salinity due to evaporation of carbon dioxide with steam.
Using a conversion factor of 0.5 is incorrect in terms of the composition of the boiler water, but luckily it gives a more accurate calculation of the blowdown percentage because it takes into account the salt content of bicarbonate in the absence of its thermal decomposition.
It is obvious that it is wrong to control the flow rate of continuous blowdown by the salt content value.
It is necessary to control the flow rate of continuous blowdown by the value of the total alkalinity of the boiler water.
For this:
It is necessary to determine the optimal value of the boiler water alkalinity during the commissioning process at which the standard steam quality is ensured. In this case, the alkalinity value should not exceed the maximum value. For low-pressure steam boilers, this is typically 26 mg-eq/l.
Then, having determined the optimal value of the boiler water alkalinity, it is necessary to determine the flow rate of continuous blowdown of the boiler water according to equation (2).
In this case, the optimal value of alkalinity will correspond to the value of electrical conductivity. It is necessary to determine the conversion factor of electrical conductivity into salt content first by calculated and then by experiment.
If the conversion factors are the same, then the composition of the boiler water fully meets the requirements (excluding specific pollutants).
Then it is necessary to regulate the continuous boiler blowdown based on the conductivity value. In this case, there is no need to run a large number of tests for the alkalinity of the boiler water.
Many boiler manufacturers introduce a standard for the electrical conductivity of boiler water in addition to the standard for total alkalinity. In this case, more stringent requirements must be followed.
The standard value of the electrical conductivity of the boiler water should be no more than 6000 μS/cm and the alkalinity no more than 26 mg-eq/l. In this case, depending on the ionic composition of the initial water, one of the controlled parameters will be achieved earlier than the other.
In the overwhelming majority of cases, for surface waters, the boiler water electrical conductivity of 6000 μS/cm will be reached before the alkalinity value of 26.0 mg-eq/l. As a rule for surface hydrocarbonate waters a boiler water conductivity value of 6000 μS/cm will correspond to a total alkalinity value of 18-20 mg-eq/l.
In conclusion.
The calculation of the flow rate of continuous blowdown of a steam boiler based on alkalinity (using the equation (2)) allows you to determine simply and accurately the percentage of blowdown without using such a multipurpose and complex concept as water salinity.
The calculation of the water evaporation coefficient by alkalinity allows you to determine accurately the salinity of the boiler water and then the electrical conductivity and carry out automated blowdown of the steam boiler according to the value of the electrical conductivity of the boiler water with constant maintenance of the standard values of the salinity and alkalinity of the boiler water.
Sincerely yours,
Ivan Tikhonov
References
- Ivan Tikhonov, The influence of various forms of carbon dioxide in water on its pH value – https://tiwater.info/en/the-influence-of-various-forms-of-carbon-dioxide-in-water-on-its-ph-value/
- Methodological guidelines for the use of conductometric control for maintaining the water modes of power plants МУ 34-70-114-85 (РД 34-37-302)
- Ivan Tikhonov, The influence of ion composition of water on its electrical conductivity – https://tiwater.info/en/the-influence-of-ion-composition-of-water-on-its-electrical-conductivity
