Accurate water analysis data is key to effectively maintain a steam boiler system for maximum efficiency. How boiler water samples are gathered can have a major impact upon both water treatment test results and operator safety. Water samples that are hot enough to flash (water changing to steam) should be cooled as they are collected.
Why Hot Boiler System Water Samples Need to be Cooled
Sample coolers are recommended to cool boiler system samples for two primary reasons.
- Safety: Handling hot boiler and condensate water samples increases the risk of burns.
- Accurate Test Results: Without proper sample cooling, the test results may not represent the actual conditions within a boiler system. Inaccurate test results can negatively impact the overall boiler water treatment program.
How Sample Cooling Impacts Test Results
- Pressure effects: The flash steam that results when hot pressurized water samples are collected will skew test results by causing the dissolved impurities in the remaining liquid to become more concentrated (see Table 1). For example, the concentration of dissolved solids and treatment chemistry in an uncooled boiler water sample collected at 100 psi will be about 13% greater than the actual levels inside the boiler.
Table 1 – Percent Sample Flashed Off at Atmospheric Pressure
- Temperature effects: Sample temperature can also impact test results due to the loss of volatiles such as neutralizing amines and carbon dioxide as hot water samples cool. In high purity condensate and feedwater samples, the loss of volatiles during sampling and cooling can cause the pH to be very different than the actual pH within the boiler water circuit. Using a sample cooler when collecting these samples can help ensure consistent pH test results for monitoring purposes. However, iron and copper levels can be a better indicator than pH for gauging corrosion protection with condensate systems.
- Test interferences: Sample cooling is also important to protect test meters from damage and in some cases, is required for accurate and representative test results. For example, hot water samples should typically be cooled to less than 160°F to both protect conductivity / pH meter probes from damage and enable accurate temperature compensation. When testing for oxygen scavenger residuals, proper cooling is important to avoid oxygen intrusion. Samples that are allowed to cool in open or sealed sample bottles with a head space can absorb oxygen causing the oxygen scavenger test result to be much less than the actual residual inside the boiler system. On the other hand, hot samples (> 120°F) cause sulfite test results to be higher than the actual residual due to degradation of the starch indicator that signals the test endpoint.
- Condensed steam samples: When monitoring boiler systems for compliance with FDA / CFIA neutralizing amine limits, a condensed steam sample must be collected for analysis using a sample cooler. Samples collected from condensate receivers or after steam traps will not provide representative amine test results. Note that the pH of a condensed steam sample can be much lower than the pH of condensate that forms in steam lines. The lower pH is due to the condensation of carbon dioxide gas that normally remains in the vapor phase until removed by venting at a steam trap.
What is a Sample Cooler?
Sample coolers are heat exchangers with cool water flowing on one side and hot boiler system water flowing on the other, with the purpose of cooling the hot boiler system water down to a safe and workable temperature. Often, the sample cooler is a shell and tube heat exchanger. (In a pinch, a more primitive method to cool a condensate sample is to install a copper or stainless steel coil in a 5-gallon bucket of water.)
How Cool Should the Sample Be?
To avoid safety issues, flashing, reagent degradation, and testing error, samples are recommended to be cooled to less than 120°F (50°C) during collection. Consult the test kit or testing equipment manufacturer’s specifications for more detailed temperature limitations.
Does Quality of Cooling Water Matter?
Depending upon the boiler operating pressure, the temperature of the boiler water could be hundreds of degrees (both Fahrenheit and Celsius). Using hard water to cool such hot temperatures will lead to hardness scale deposits forming on the cooling water side of the sample cooler. This will reduce the cooling efficiency and may eventually block the flow of the cooling water. For this reason, a softened cooling water is recommended.
Don’t Forget the Drain
When planning for and installing a sample cooler, one must not forget the waste streams generated while sampling. A drain will be required for both the cooling water and cooled boiler system water sample.
SAFETY TIP: To avoid burns and unsafe conditions, it is highly important to verify the cooling water is actually flowing within the sample cooler and to the drain. Turn on the cooling water flow FIRST. This will help ensure the water standing in the sample cooler does not reach unsafe temperatures that could cause harm when flow is turned on. Ensure all cooling water shut-off valves are open.
NOTE: It is typically recommended NOT to install a shut-off valve on the cooling water DISCHARGE line to eliminate a possible failure point preventing cooling water from flowing. The discharge is commonly sent to a drain at atmospheric pressure, so the valve is unnecessary to service the sample cooler.
Conclusions
Sampling hot steam boiler system water samples can pose safety risks that must be addressed. Hot water samples will flash off water and volatile components, which will impact the accuracy of test results. Lastly, hot water samples can both degrade test reagents and negatively impact sampling equipment, further impacting the reliability of test results. Installing and using sample coolers to sample hot boiler system water helps improve not just safety but overall water treatment control as well.