Understanding Chemical Dosing

Chemical dosing can be thought of as a routine operation performed with a dosing pump. However, there are many nuances to correctly delivering the right chemical in the designed quantity to water.

Chemical dosing is carried out for many purposes, here are a few:

• Dosing disinfectants into water to eliminate the activity of harmful microorganisms present in water. • Dosing special chemicals into water to prevent or delay crystallization of minerals such as calcium, magnesium and silicate that crystallize in steam boiler and cooling water systems and damage the system. • Dosing "coagulants" into water to agglomerate very small-sized solids present in water for effective filtration. • Mixing chemicals into water to decrease or increase pH level. • Dosing corrosion inhibitor chemicals into water against metal corrosion. • Dosing one chemical to another in a chemical process to produce the designed product. In this article, we will not discuss the chemicals being dosed, but will address important points regarding proper chemical dosing.

When we say "proper chemical dosing", the following points emerge:

1. Compatibility of dosing equipment with the chemical to be dosed, 2. Ensuring the correct chemical is being dosed, 3. At what concentration the chemical will be prepared before dosing, 4. In case of multiple chemical dosing, compatibility of chemicals with each other and the risk of creating unwanted chemical reactions, 5. Ensuring proper mixing of the dosed chemical into water, 6. Risk of chemical causing corrosion in the pipe at the dosing point, 7. Most suitable dosing location, 8. Adjustment sensitivity of chemical dosing pumps, 9. Risk of deviation in the designed chemical dosing quantity. Let us examine the topics we summarized above one by one.

1. Compatibility of Dosing Equipment with the Chemical to be Dosed:

Some chemicals such as sulfuric acid are so aggressive that they damage certain types of metals and plastics. Therefore, a dosing pump and dosing hose manufactured from materials compatible with the chemical to be dosed should be selected. For example, we witnessed such an incident: We installed a dosing system equipped with sulfuric acid-resistant materials at an industrial facility. However, due to an operational error, the dosing hose broke; the customer, unfamiliar with sulfuric acid, purchased an ordinary hose and connected it to the dosing pump. Since this new hose was not resistant to sulfuric acid, it was perforated in several places within a short time and many materials in that area suffered corrosion from the acid spraying into the environment.

2. Ensuring the Correct Chemical is Being Dosed:

It is essential to ensure that the "correct chemical" is placed into the dosing system reservoir. We have witnessed errors in this regard many times. As seen in Image 1, if at a facility two separate chemical reservoirs and two separate dosing pumps are mounted side by side and pumping chemicals to the same process line, and especially if one of these chemicals is dosed to water under pH control, when the wrong chemical is filled into the chemical reservoirs, since the pH level of the water passing through the pipe will not change, the dosing pump will operate at maximum flow rate without stopping and this error will result in many problems at the facility. This bad example we provided here is not a scenario; we have encountered such problems many times. To prevent such problems from occurring, personnel must be trained repeatedly at every opportunity. [caption id="attachment_156548" align="aligncenter"] Image 1. Two Different Chemical Dosing to Water System[/caption] [caption id="attachment_156551" align="aligncenter"] Image 2. Koflo Brand Static Mixer[/caption]

3. At What Concentration the Chemical Will Be Prepared Before Dosing:

If diluted chemical dosing is to be performed, the dilution process must also be carried out with great precision. Dilution performed without measurement yields poor results. For example, when it is desired to dose 10 g/m³ chemical to a process water of 3–4 m³/hour flow rate, dosing must be done at a very low rate of about 30–40 g/hour and since there is no pump that doses at such a low flow rate, the chemical to be dosed is diluted by, for example, 10% and dosed to water, which means dosing at 300–400 g/hour flow rate. If the 10% chemical solution is not prepared correctly, under-dosing or over-dosing occurs, which creates problems in the facility and process. We have witnessed the following errors being made while diluting in the chemical reservoir several times: The operator, who should prepare a 10% chemical solution with 90% water ratio, first places the chemical in a chemical drum, should fill water to a certain level with a hose into the drum; but during this time the mobile phone rings and during the conversation the drum fills and overflows, so the chemical ratio in the drum becomes perhaps 1% instead of 10%! 4. In Case of Multiple Chemical Dosing, Compatibility of Chemicals with Each Other and the Risk of Creating Unwanted Chemical Reactions: Not all industrial facilities have an expert who knows chemistry very well. For this reason, operators can sometimes make major errors. Here is an example: While two different chemicals are being dosed to water with two separate dosing pumps, when one dosing pump breaks down, the operator may think it clever to fill both different chemicals into the same tank and dose to water with a single dosing pump. This experiment can sometimes produce very frightening results, because when certain chemicals are placed in the same container, they can generate high heat and spray hot and corrosive liquids into the surroundings; or one chemical can completely neutralize the effect of another.

5. Ensuring Proper Mixing of the Dosed Chemical into Water:

It is desired that the dosed chemical mixes well into water. However, in some cases the point where chemical is dosed to water may be very close to the point where water is used, for this reason the chemical only mixes into a portion of the water. In situations similar to this, the operator, believing that dosing is sufficient, cannot understand why they are not obtaining the desired result in the process and complains about the quality of the chemical. If there are many elbows in the pipe system between the point where chemical is dosed to water and the point where water enters the process, successful mixing of water with chemical can occur. If there are not many elbows in the pipe system, mounting a "Static Mixer" shown in Image 3 on the pipe solves the problem of good chemical mixing into water. Cartridge filters and high-pressure pumps found in reverse osmosis systems also serve as mixers. The "Antiscalant" chemical dosed into water to prevent crystals forming on reverse osmosis membranes can be considered to mix very well into water as it passes through the protective cartridge filter and high-pressure pump before reverse osmosis. Piston dosing pumps, due to the slow movement of the piston, deliver the dosed chemical to the water passing through the pipe at a low rate and continuously, therefore the distribution of chemical into water is more successful. However, diaphragm dosing pumps, when operating at low flow rates, deliver chemical to the water passing through the pipe "intermittently", so chemical/water mixing may not be as desired. However, dosing "intermittent" chemical with a diaphragm dosing pump to a water pipe feeding a water reservoir creates no problem, because water/chemical mixing occurs inside the water reservoir. [caption id="attachment_156549" align="aligncenter"] Image 3. Piston Dosing Pump[/caption]

6. Risk of Chemical Causing Corrosion in the Pipe at the Dosing Point:

If chemical is being dosed to water through a small ½" diameter liquid inlet created by welding it onto a water pipe, the chemical will come into direct contact with the pipe material and if the dosed chemical is an acid and the pipe is carbon steel, metal corrosion problems will definitely occur at the dosing point. To prevent this problem, it is correct to design the dosing point so that the chemical dosing is directed into the center of the pipe.

7. Most Suitable Dosing Location:

Dosing should absolutely not be done at the pump intake, because there is a vacuum at the pump intake and at a vacuum point the dosing pump cannot be adjusted to the desired dosing quantity. If the chemical dosing point to water is a point where water is pressurized, the dosing adjustment can be made correctly. The water pressure in the water pipe where chemical is to be dosed may be higher than the pressure of the dosing pump, therefore, pressure should be considered when selecting the dosing pump.

8. Adjustment Sensitivity of Chemical Dosing Pumps:

Various types of dosing pumps are available on the market and their flow adjustments are made in different ways. To ensure the flow adjustment of the dosing pump is done correctly, for example, the dosing pump is operated for a period such as 15 minutes and the dosed liquid is collected in a container, thus the "initial flow check" of the dosing adjustment is performed. The second and correct check will be based on the result of the chemical measurement performed on a water sample taken from the water downstream from the dosing point. Because at the dosing point, the pressure in the water pipe affects the flow rate of the dosing pump, therefore "initial dosing flow check" is not sufficient. [caption id="attachment_156550" align="aligncenter"] Image 4. Precision Controlled Chlorine Dosing System[/caption]

9. Risk of Deviation in the Designed Chemical Dosing Quantity:

The following deviations can occur in chemical dosing: • The dosing pump fuse blows and no chemical is dosed until this electrical problem is noticed and corrected. • During the night shift, the sleeping technical officer does not fill the chemical being dosed into the dosing pump reservoir. • Because it was purchased as "cheap", the alternative brand chemical does not have the same concentration and since the dosing pump adjustment is not redone, problems occur in the facility's process.

Solutions to Prevent the Deficiencies We Described Above:

A. Equipment is available on the market that measures the presence of chemicals whose results in water are measurable, such as chlorine, acid, and caustic, and based on the result, reduces or increases dosing; through these devices we can be sure that such chemicals are truly being dosed. However, dosing of chemicals such as coagulants and antiscalants, which are not easily measured in water, is at risk. B. For proper dosing of chemicals whose measurement in water is not easily performed, the method is to use an "intelligent dosing pump" that precisely measures the dosing quantity and provides information to the PLC that controls the system. These dosing pumps, when the designed quantity of chemical is not dosed, through the signal given by the dosing pump, the PLC control can stop the entire system and sound an alarm. We can summarize the dosing topic as follows: Chemical dosing should not be taken lightly; the chemical dosing process has many "nuances" and carries many risks both for the operator and for the facility. Therefore, operators should be trained very frequently on chemical dosing and warning notices should be posted at dosing points. Reference: Burkut Company has been designing and installing process water systems for industrial facilities since 1984 and all of these systems contain chemical dosing equipment. Our article on "Chemical Dosing" has been compiled thanks to the experiences of our customers and ourselves in the systems we have installed and serviced over the last 39 years.   Enis Burkut Senior Engineer General Manager Burkut Water Technology Inc.