Discover why your heat transfer system is performing poorly.



Most problems that occur in heat transfer systems can be avoided by adhering to proper design, operating and maintenance practices. This article will review design and operational problems that can lead to unnecessary downtime; methods of efficiently troubleshooting a heat transfer system; and common problems, including the probable causes and possible solutions (table 1).

Problems can be expected from systems utilizing:

1. Heaters with excessively high heat flux, which can subject the fluid to high film temperatures, causing thermal degradation. Thermal degradation leads to the formation of low boilers, which can cause pump cavitation, and high boilers, which can polymerize and deposit in the heat source and on heat transfer surfaces.

2. Systems with inadequate capability to completely drain used fluid or cleaning materials. Inadequately drained systems can experience cross-contamination, which ultimately shortens the life of any fresh fluid subsequently added.

3. Systems designed with inadequate venting capabilities. Inadequate venting allows pockets of vapors and light ends to accumulate in the system, leading to cavitation and equipment damage.

4. Systems that operate with the expansion tank at elevated temperatures, vented to the atmosphere. Venting a hot expansion tank to atmosphere exposes the hot system fluid to oxygen, causing oxidization and prematurely shortening fluid life.

5. A system that continues to operate even after the heat transfer fluid has been extensively oxidized or thermally degraded well beyond its condemning limits. The oxidized or degraded fluid forms carbonaceous sludge that will most likely lead to operating difficulties.

To effectively troubleshoot poorly performing heat transfer systems, follow these steps:

  • Clearly define the problem based on observations and accumulated information.

  • Review available historical system operation data and fluid condition analyses.

  • Identify and obtain any additional information and analysis that may be required.

  • Identify and list potential root causes and consider each.

  • Deduce the root cause based on the accumulated information.

  • Execute corrective action.

Once the problem is corrected, the necessary changes in system design or operating procedures should be implemented to prevent a reoccurrence.

Troubleshooting can be made easier if the system's operating parameters and the fluid's condition are monitored and analyzed routinely. In addition, the following equipment and materials should be available to facilitate troubleshooting:

  • Sufficient and properly located temperature and pressure gauges to monitor the fluid at the inlet and outlet of each heat-user and the heat source.

  • An up-to-date process and instrument drawing of the system.

  • Historical operating and fluid analysis data.

  • Equipment specification sheets and fabrication drawings.

  • Process and mechanical design information about the heat transfer system and users.

  • System operating data from the period prior to and during the problem. This can be compared with normal and historical data.

  • Fluid analysis during the problem.

  • Documentation of previous system problems.


Fluid Monitoring

A fluid monitoring program can alert users to the early warning signs of problems -- if the fluid is oxidized, thermally degraded or contaminated. A complete fluid monitoring program should involve regular analysis of the fluid's total acid number (TAN), viscosity, simulated distillation (GCD), flashpoint, solids, metals and water. This information will allow users to implement early corrective actions before the situation escalates into a problem that causes an emergency shutdown.

Should a problem occur, the accumulated data can be reviewed to determine if the problem is fluid or system related. Systems should be equipped with proper connections and sampling devices to facilitate direct fluid sampling from the hot circulating loop. If the sample is not taken from the hot circulating loop, it will not be representative of the system fluid, and the results will be meaningless.

Most often, system fluids are not analyzed until a problem occurs. Generally, the results indicate that the fluid has been oxidized extensively and the products of oxidation have polymerized and deposited throughout the system. A monitoring program allows key fluid properties such as TAN and viscosity to be tracked from startup. With enough data points, the rate of degradation can be determined, then extrapolated to project remaining fluid life. If a fluid is changed out just before its condemning limits are exceeded, the system only requires draining and fluid replacement, without the need for flushing or system cleaning. Continued use of an extensively oxidized fluid eventually will lead to carbonaceous deposits, necessitating system cleaning with an aromatic solvent or cleaning agent. Table 2 explains how to understand the results from a fluid analysis program.

Daily monitoring of the system's operating parameters, including the inlet and outlet temperatures and pressures at the heat source and users, also can provide a means for early detection of potential system problems.

Some Frequent Causes of Problems

Generally, most problems arise from improper operation of the heat transfer system. Six specific causes are most often at the root of the problem.

One common cause is using a heat transfer fluid well beyond its condemning limits. If the fluid's saturation level for products of oxidation is exceeded, continued use of the fluid leads to carbonaceous sludge deposits. Implementing a fluid monitoring program will identify the timing for fluid change-out and prevent the fluid from being used beyond these condemning limits.

A second common cause is subjecting the fluid to temperatures that exceed the recommended maximums. If the fluid's flow rate is inadequate, its residence time in the heat source may be excessive. Or, if the system is operating correctly but the fluid is being heated beyond its recommended maximum, it may be inappropriate for the application. Simulated distillation of a sample of the system fluid using the gas chromatographic distillation (ASTM D 2887) method can identify any increased amounts of low and high boilers, relative to fresh fluid.

Another source of problems is inadequate cleaning. If sludge and degraded fluid remain in the system after cleaning, they can cross-contaminate the fresh fluid and shorten its life. Prior to cleaning, the system fluid should be analyzed to identify the extent of cleaning required. Also, the new fluid should be analyzed within one to two days after startup to establish a baseline for the newly charged system fluid and identify any cross-contamination.

Another common cause is extraneous contamination. Most often, extraneous contamination occurs when the wrong product is added during system fluid top-off, or when fluid from the process side leaks into the heat transfer system. The fluid should be analyzed to determine the extent of contamination and its impact on the fluid's heat transfer properties such as viscosity and the potential for solids deposit. If an adverse impact on system performance is likely, the fluid should be replaced.

A fifth common cause of problems is water used during cleaning that is not removed during startup. When water is present in the system, rapidly heating the fluid to operating conditions can result in the expulsion of hot fluid from the expansion tank as the water expands during its conversion to steam. The proper procedure for system startup must include slow initial heating of the fluid to just above the boiling point of water. Then, the fluid should be circulated through the expansion tank until all water and vapors have been vented.

The final cause of problems is inadequate system or equipment design, or poor unit operation. To eliminate these causes, design and operating procedures must be changed to improve system operation and on-stream reliability.

Troubleshooting System Problems

If the following conditions develop, it is important to evaluate the possible cause and implement corrective action.

Insufficient Heat At User. If users on the system are unable to get the required amount of heat, possible causes may be:

Fouled Heat Transfer Surfaces. Fouled heat transfer surfaces at the user are caused by deposits of resinous carbonaceous material. This can develop if the fluid is thermally cracked to form heavy boilers. Another possible cause is fluid oxidation that leads to the products of oxidation polymerizing and depositing on the heat exchange surface.

Fluid analysis for TAN, viscosity, solids and GCD readings can quickly identify if the fluid has been oxidized or thermally degraded. An oxidized fluid will have a TAN greater than one and exhibit increased viscosity and solids. A thermally degraded fluid exhibits reduced viscosity and GCD 10% point, but increased 90% point and solids content.

Checking the historical operating conditions around the fouled equipment can act as a cross-check to confirm fouling. If fluid flow to the user is constant but the differential temperature across the user is reduced, this indicates that the transfer surface is fouled.

Low Fluid Flow. Low fluid flow, due to partially plugged lines or filters as well as pump-related problems, also can lead to insufficient heat at the user. Checking the pressure differential across the pertinent piping or equipment sections will identify any blockages.

Low Fluid Temperature from the Heat Source. Low fluid temperature from heat source often is caused by progressive fouling of the tubes or the heat source's electrical element. Other possible causes are reduced gas firing/watt setting or reduced fluid residence time in the heat source. Fouling in the heat source can be determined from a fluid analysis along with a review of the differential temperature across the source. The other possible causes can be identified by inspecting relevant temperature and flow controllers as well as monitoring devices.

If fouling is the cause, cleaning the heat source's heat exchange surface is the only way to restore system efficiency. If fluid oxidation has caused the fouling, the fluid must be changed. It is important to identify the cause of fouling: If the fluid is thermally degraded, for example, increasing the heat source's firing or watt setting to get more heat to the user will only compound the problem. Low fluid velocity or increased residence time in the heat source leads to heat transfer areas with high heat flux, and correspondingly high film temperatures, that result in thermal cracking and, ultimately, coke deposits.

Increased Viscosity. Caused by the continued use of an oxidized fluid, increased fluid viscosity also can result in insufficient heat to the user. An excessive buildup of the products of oxidation causes the fluid to thicken, and the higher viscosity fluid is less efficient at transferring heat. A 50% increase in a heat transfer fluid's viscosity results in approximately 20% reduction in the fluid's film coefficient. Inadvertently contaminating system fluid with a higher viscosity material also will increase its viscosity.

High Fluid Losses/Make-Up Rate. If the system constantly requires fluid additions, possible causes are:

Vapor Leaks from System. Heat transfer fluids that exhibit high vapor pressure (chemical aromatics, for example) tend to leak from connections and fittings. If the system's operating pressure is not set above the fluid's vapor pressure, fluid vapors will vent continuously from the expansion tank, requiring frequent top-offs to maintain fluid levels. Low vapor pressure fluids are preferred for liquid-phase systems because they can be operated with essentially no pressure and minimum venting of vapors.

Leaks from Fittings and Connections. At the system operating temperature, metal fittings and connections can expand, causing leaks. Another common cause of leaks is using a seal material that is incompatible with the system fluid. To prevent leaks, flanges and connections should be tightened while the system is at the operating temperature. If a systems is frequently shut down and restarted, it is more likely to leak. Where flanged connections are necessary, a 300 lb raised face flange should be used to improve sealing and minimize leaks. If the seals are incompatible with the fluid or (in the case of mechanical seals) if there is insufficient cooling/ flushing, leaks will occur frequently.

Thermal Degradation and Venting of Lighter Components. If the fluid being used is not thermally suited for the application, it will thermally degrade. The low boilers produced will be vented via the expansion tank. This problem is compounded if the expansion tank is operated at elevated temperatures.

Thermal stability at the bulk operating temperature is an important criterion to consider when selecting a heat transfer fluid. It also is important to ensure that the fluid's maximum film temperature is never exceeded -- otherwise, the fluid will degrade and foul heat transfer surfaces, reducing efficiency.

High Fluid Level in Expansion Tank. If the fluid level in the expansion tank is too high, some fluid droplets will become entrained with the vapors being vented. The fluid level in an expansion tank should be 50 to 75% when the system fluid is at its normal operating temperature. The expansion tank's operating pressure should be higher than the fluid's vapor pressure to minimize venting of fluid vapors.

Short Fluid Life. The main processes by which the life of a heat transfer fluid can be shortened are:

Oxidation. Oxidation is a result of the fluid coming in contact with atmospheric oxygen at elevated temperatures. The rate of fluid oxidation can be reduced by using a fluid with antioxidant additives and operating the expansion tank at atmospheric temperature with an inert gas blanket.

Thermal Degradation. Thermal degradation is the result of the fluid being subjected to excessively high temperatures, extended residence times in the heat source, or being unsuitable for the application. Flame impingement on heater tubes creates a localized hot spot with excessively high heat flux. This also causes thermal degradation.

Contamination. Contamination occurs when the wrong material is added as the top-off fluid, when the system is inadequately cleaned or when process fluid leaks into the heat transfer fluid.

Frequent Filter Plugging. Filter plugging reduces fluid flow and should be corrected. Possible causes are:

Polymerization. Frequent filter plugging indicates fluid polymerization and the buildup of resinous sludge. Most often, this occurs when a fluid has been used well beyond its condemning limits and has been extensively oxidized.

Fouling After Cleaning. Frequent filter plugging also can occur after an extensively fouled or coked system has been cleaned and new fluid added. Residual solid particles or sludge tend to accumulate on the filters during the initial operating period.

Unsaturated Components. Heat transfer fluids produced via the solvent refining/dewaxing approach may contain some unsaturated components and can form sludge deposits when thermally cracked or oxidized. Synthetic-based fluids are highly aromatic and can form carbonaceous solids when thermally cracked or oxidized. Fluids produced from hydroprocessed paraffinic base stocks that contain no unsaturated components are less likely to form deposits.

Increased Pressure Drop in the System. This can be caused by a restriction in the piping network, filter plugging or increased system fluid viscosity. If not addressed, it will lead to further problems.

Pump-Related Problems. Centrifugal pumps equipped with mechanical seals, water cooling on the bearings and seal flush system typically are used for fluid circulation in heat transfer systems. Possible problems include:

Cavitation/Vapor Locking. Cavitation will occur if the system pressure falls below the vapor pressure of the fluid being pumped, causing the fluid to vaporize and form pockets of vapor. Pockets of water that periodically come in contact with hot fluid also will create excessive system vapor.

The expansion tank should be installed at the system's highest point and should be piped upstream of the circulating pump to provide a positive suction head and allow venting of any vapors before they can cause pump cavitation. A dual-legged expansion tank is preferred because it allows the fluid's full flow to be diverted through the tank and allows more efficient venting of air, water vapors/steam and low boilers.

Leaks. Frequent leaks from a pump with a mechanical seal indicates inadequate cooling at the seal face. If the seal face temperature becomes excessively high, the fluid in contact with the seal face will thermally crack and leave a hard carbon deposit. The abrasive coke buildup will cause wear and eventually lead to fluid leaks. A constant flow of low-pressure steam at the seal face can prevent this problem.

Other Problems. Finally, other potential problems with a heat transfer system are:

Unable to Attain Desired Fluid Flow Rate.

  • Insufficient net positive suction head (NPSH), which is the minimum suction pressure required by the pump to prevent cavitation.

  • Plugged suction line.

  • Entrained air leak on pump suction.

  • Excessive low boilers in fluid to pump suction.

  • Impeller too small.

  • Increased fluid viscosity.

  • Pump speed too low.

Pump Running, But no Discharge.

  • Pump not properly primed.

  • Blockage in impeller or suction line.

  • Rotation is in wrong direction and motor needs to be reversed.

Pump Operates for Short Period, Then Loses Prime.

  • Air leak on pump suction.

  • Insufficient NPSH.

Excessive Noise or Vibrations.

  • Cavitation due to vapors or high viscosity fluid.

  • Diameter of suction piping too small.

  • Mechanical failure or misalignment.

  • Pump and/or piping not properly secured.

    Noise in Pipes. Most likely, noisy pipes are caused by water in the system at the operating temperature. Water can be removed by carefully circulating the hot system fluid through the expansion tank and allowing the water vapors to be vented slowly.



    Conclusion

    Good record-keeping of operational and maintenance data, which includes routine fluid analysis, regular monitoring of system operation parameters, visual observation of system components, and a record of previous problems and their solutions, are the keys to avoiding system problems, and to efficient troubleshooting when problems do occur.

    Heat transfer system maintenance frequently is ignored in the urgency to keep production at high capacity. However, the heat transfer system is the backbone of the production process and, as such, requires that operational and maintenance guidelines be developed and followed to prevent unscheduled and excessive downtime, loss of production and, ultimately, loss of revenue.

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