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.
Dealing with test results
Depending on the level of degradation, the system may need to be drained, flushed, and replenished with filtered or new fluid. Preserve the integrity of the fluid and the system with preventative action including filtration and isolation of the fluid from contacting the atmospheric air.
The traditional hot oil filtration method has been a strainer just upstream of the system pump. Strainers prevent particles to a minimum of 100 mesh 149 micron from entering the pump and eventually the system. By definition, strainers are designed to protect the piece of equipment, not the fluid. The strainer must be cleaned regularly to prevent restricted flow to the pump that eventually leads to cavitation. Cavitation causes mechanical seal failure or magnetic de-coupling.
Side-stream or full-flow filtration is another traditional method. For either arrangement, the filter consists of a housing with a perforated -90 to 250 micron pores- stainless steel basket to trap fine particles. For the side stream arrangement, the inlet of the filter is usually installed close to the pump discharge. The fluid passes through the filter and discharges to the suction side of the same pump or to a low-pressure return line.
These filtration configurations have limitations that include:
- reducing the rated process pump flow,
- inadequate differential pressure across the filter to maximize basket loading,
- temperature effects on the gasket materials for lid closure,
- impossibility of micron size particle removal, and difficulty to clean.
The cleaning process requires soaking the basket in a solvent or degreaser bath for a period of time and then flushing the basket using a high-pressure wash system then drying the basket thoroughly to prevent introducing moisture into the system.
The most effective filtration is the forced flow and side stream system incorporating a pump and filter designed for high temperature use. Such an approach continuously diverts approximately 5 to 10 percent of the process flow through a filter equipped with its own pump while the heating system is operating. The filter processes the total fluid charge at least 15 to 20 times in a 24-hour period.
Plumb the inlet of the filter pump to the process pump discharge piping to take advantage of the turbulence that keeps solids in suspension. After passing through the filter, the clean fluid either discharges to the same process line at a pressure higher than the system pressure or to the return pipeline downstream of the process pump. Figure 1 shows a diagram of a typical hot oil filtration system.
It is important to ensure that the material of construction of the components on the filtration system are suitable for high temperature use. Many filter housings use elastomer O-ring seals that are not safe for high temperature operation because they lose strength or dissolve in the heat transfer fluid. The temperature ratings on the components must exceed the application specifications.
Maintenance issues
When filtering heat transfer fluids for the first time, change filter elements frequently. The initial cartridge should be rated at 100 to 50 microns and then replaced to gradually reduce porosity to 25 microns. Filtering the fluid any more finely may remove additives from the oil and affect the performance of the fluid and system. Using combinations of particle removing elements increases the time between changeouts.
Depth filters using glass fiber wound filter cartridge elements have proven to be most effective. They withstand temperatures to 700 degrees F, have excellent dirt holding capacity, and are economical.
A typical filtration system should include a pump, a filter, controls, isolation valves, and safety accessories. The ideal system provides a safe and user-friendly system that is easy to incorporate in the heating system.
While heat transfer fluid providers recommend filtering the oil regularly, fluid users are generally at a loss to locate safe, user-friendly, and cost effective filter systems. For this reason, hot oil users are reluctant to incorporate proper filtration or are forced to improvise to meet their individual needs. A well-designed filtration system reduces downtime, increases productivity and system efficiency, and, most importantly, provides safety and longevity of both the hot oil system and its fluid.
A typical situation
A company's pilot plant experienced frequent wear in the heat transfer process pumps. Timers installed on the control panel revealed that the pumps needed replacement after 200 to 500 operating hours. The annual cost of replacement pumps, down time, lost productivity, maintenance labor and fluid was $40,000.
Analysis of the heat transfer fluid revealed coking in the oil that created abrasive particles causing the pumps to wear rapidly. After using a side stream filtration system for three years, records showed the process pumps lasting 10 times longer than before--over 3,500 hours--without maintenance.
In addition to the initial savings, incorporating the filtration system saved $22,000 per year in replacement parts and reduced power and fluid consumption. Other savings come from reduced downtime and increased productivity and manpower costs.