Think about filtering your heat transfer fluids

It just might be the fastest way to increase efficiency and reduce maintenance and downtime.

By Zak Shums

PlantServices.com

Do you use circulating hot oils or heat transfer fluids to provide indirect processes heating of reactor vessels, tanks, molds, calendars, extruders and heat exchangers? In daily use, these fluids see temperatures from 300 to 750 degrees F for heating applications in the chemical, plastics, rubber, petrochemical, pharmaceutical, pulp and paper, fiber and food industries.

Properly maintaining the sys tem controls and retards heat transfer fluid degradation. Some factors that contribute to degradation are exposure to the oxygen in the air, low velocity of the fluid through the heating chamber and piping, and improper heater selection. Another factor is operating the system beyond the manufacturer's recommended maximum temperature that degrades the oil to produce by-products like sludge and coke. Other contaminants may be circulating in the system including pipe slag, mill scale, dirt, and dust accumulated in the system during the original installation or during maintenance. As the amount of contaminants in the system increase, the fluid undergoes property changes that affect the heat transfer capability of the overall system.

If left unchecked, the contaminants cause problems like:

• wear of rotating components such as pump impellers, gears and shafts, mechanical seals, and valve stems;
• reduced capability of heaters and heat exchangers caused by fouling of heat transfer surfaces with coke and sludge;
• increased viscosity attributable to solids build up; and
• increased energy consumption from longer heat up time at the process

Hire a laboratory to analyze the heat transfer fluid at least once a year as part of your maintenance practice. Depending on the system usage, use quarterly or monthly analyses to determine the condition of the fluid and compare the lab results with specifications from the material safety data sheet for the oil.

The analysis should include properties like specific gravity, total acid number, viscosity, insoluble and flash point of the fluid

A specific gravity greater than that of new liquid means other materials are present in the sample. This indicates the presence of low- or high-boilers and contamination.

Moisture has low solubility in most heat transfer liquids except glycol-based formulations. The presence of water causes volatility problems and two-phase flow vapor and liquid that leads to pump cavitation and excessive pressurization, especially during system startup.

Total acid number is also known as the neutralization number. This acid/base titration detects strong and weak acids in the fluid that generally are associated with open vented expansion tank operation. The heat transfer liquid oxidizes or degrades to produce weak acids. Acids break molecular structures and form insoluble solids that accelerate mechanical deterioration of seals, valves, and pumps. The initial total acid number ranges from 0.00 to 0.01 and the maximum value in used fluid should not exceed 0.50

Insolubles indicate the amount of inorganic contaminants such as pipe slag, sand, construction debris, and coke carried by the fluid. High amounts of coke indicate thermal degradation. Insoluble solid levels of 50 milligrams per 100 milliliters or more indicate problems in the fluid and system.

High and low boilers are important because, when heated to high temperatures, certain molecular bonds begin to break or thermally degrade. Some new materials that form, called low boilers, have lower molecular weight and a lower boiling point than the original fluid. Other compounds from thermal degradation polymerize into higher molecular weight and higher boiling point molecules called high boilers. High- and low-boilers decrease heat transfer efficiency and thermal stability.

Two chemical composition tests performed for cross reference spot the presence of high or low boilers. The first is atmospheric or vacuum distillation in which the test fluid is completely distilled. This test accurately detects high boilers. The second test is a simulated distillation at a specific final temperature using gas chromatography, a method that is extremely accurate in detecting low boilers, especially aromatics up to the initial boiling point temperature of the fluid.

Viscosity refers to the fluid's flow characteristics per unit time and indicates thermal degradation. Viscosity changes affect the overall heat transfer capability of the fluid.

A gas chromatography scan gives the signature of the degradation components (high- and low-boilers) and often detects contaminants. When cross referenced with the atmospheric boiling range test, it confirms the levels of high- and low-boilers as well as presence or absence of outside contaminants.

Flash point-Cleveland Open Cup-test provides a means of detecting fire or flash point of liquid. Low boilers reduce the flash point. A low flash point indicates the presence of thermal degradation products or outside contaminants.

The primary tests are moisture by ASTM D1744 (Karl Fisher), percentage of high/low boilers by ASTM D86 or ASTM D1160 and ASTM D2887, and insolubles by ASTM 893 Modified.

Analysis of the fluid provides a snapshot of the condition of the sample. Because you need a representative of the fluid circulating in the system, it is critical to take a live sample of the fluid. Inaccurate analysis is the result of taking the sample from leaking fluid drips or from drums of used fluid.

Collect the sample while the system is at operating temperature. Take utmost care while drawing the sample. Wear protective clothing, including heat resistant gloves, face shield, and eye protection. Open the sample valve slowly to prevent splashing, collect the fluid in a clean metal container never glass or plastic that will not react with the used fluid. Draw the sample from close to the discharge of the process pump where the turbulence is maximum. After collecting the sample, seal the container immediately to prevent introducing environment contaminants.