Keep heat-transfer-system repairs uneventful

A good plan is the key to avoiding risks in the maintenance of systems using high-temperature organic fluids.

By Conrad E. Gamble, Solutia Inc.

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Traditional heat transfer fluids (HTFs) and systems have been around for many decades. While much has been learned and written about the safe operation of these systems, less has been shared about the proper approach to making uneventful repairs. The safe execution of repairs depends upon effective planning to protect against potential hazards. This article focuses on mitigating potential HTF-related hazards that could be encountered when repairing high-temperature organic HTF systems (those normally operating above 500°F). It is not intended to supersede any fluid-specific risk information available from the manufacturer, nor process or equipment-specific risk details associated with individual processes involved.

Every process has unique hazards associated with the chemical(s) being handled. Effective job planning and execution takes into consideration the hazards of both the processes and HTFs involved. The risks associated with HTFs can be divided into three primary areas: fire, human exposure and environmental exposure. The single, best source for fluid safety information is the material safety data sheet (MSDS) provided by the manufacturer.

 

Table 1. HTF chemistries and fire properties

HTF type

Fire properties (ASTM D92 & D445)

Flash point, °F

Fire point, °F

AIT, °F

DP/DPO Eutectic

235-255

245-265

1,100-1,150

Alkylbenzenes

350-375

410-425

650-710

Mineral oils

340-445

380-500

640-715

Dibenzyltoluenes

390-420

445-475

840-880

Diaryl alkyls

245-380

255-310+

725-805



Fire potential is typically assessed using flash point, fire point and autoignition temperature (AIT) data (Table 1). Most of the commonly used HTFs operating from 500°F to 750°F, including diphenyl/diphenyl oxide (DP/DPO) eutectic fluid and the others listed in the table, have hydrocarbon chemistries. As such, they are typically classified as Class IIIB combustible liquids capable of igniting under the right conditions (fuel/heat/oxygen).

Depending upon the system design, operating environment, and fluid age and maintenance, some fire properties might become depressed below the values shown, which can increase fire risk. With awareness of these conditions, proper job planning can interrupt the hazard mechanisms and greatly reduce or eliminate the fire risk. A safe approach to fire prevention should include minimizing spark potential, inerting, conducting work while equipment is near ambient temperature, and insuring the area is cleared of hydrocarbon residues before beginning hot work.

Limiting the potential for human exposure to chemicals should be at the forefront of job planning considerations. Specific chemistries of HTFs will help define certain personal protective equipment (PPE) requirements, materials selection for fluid handling, etc. Review of the MSDS and technical literature will aid in the proper selection of PPE to avoid permeation of gloves or protective aprons, and could prevent failure of polymer components and other equipment resulting in a loss of containment.

The most common HTF-related injury is thermal burns. Whenever possible the system should be allowed to cool sufficiently to prevent burns from hot fluids or equipment contact when hands-on work begins. An additional benefit of cooling the fluids is reduced vapor pressure, thereby lowering potential exposure to fluid vapors.

Should non-routine material transfers be required, selection of hose materials, gaskets, containers, and O-rings/seals should all conform to manufacturer guidelines to avoid unexpected leaks, releases and physical contact. Ideally, job plans will incorporate these considerations, plus knowledge from industrial hygiene monitoring, to best determine the right combination of splash, face, eye, thermal burn, and/or respiratory protection required for each situation.

Exposure to the environment can also be safely managed by proper planning. Chemicals should be responsibly handled, but certain fluids might have regulatory restrictions that place stricter emphasis on environmental protection. This information should be provided in the MSDS.

During repairs, the bulk of the fluid should be kept properly isolated within the system designed for its containment, if possible. Any fluids that require removal by pumping, draining, blowing or other means should be transferred using equipment and materials fully compatible with the fluid chemistry. Any doubts about materials compatibility should be first resolved through discussion with a competent person knowledgeable of the fluid.

Additional preventive measures to protect the environment might include temporary dikes/curbs, drain plugs and absorbent media/pigs/socks. For fluids with high crystallizing points (e.g., DP/DPO), ensure that the piping is cleared of standing liquid to avoid possible failures due to expansion effects upon freezing. This can be done by draining low points, blowing lines with inert gas, and carefully opening low point flanges, if necessary. Successful management of this component of the job will help prevent unnecessary clean-up work, disposal of fluid and recovery materials, and allow faster completion of the job, thereby minimizing downtime.

Opportunities for reducing risks of fire, and risks to people and the environment are summarized in Table 2.

 

Table 2. Ways to reduce risk

Fire

Human exposure

Environment

Clean oily residues from area.

Cool to < 140°F.

Provide for containment.

Proper preparation for hot work.

Don’t reuse gaskets.

Keep fluid in the system.

Complete isolation of work area from process.

Ensure drain containers are water-free.

Provide for proper reuse or disposal.

Remove saturated insulation from area.

Consult with MSDS about PPE.

Consult MSDS for handling and disposal guidance.

Ensure fire watch, extinguisher and water hose are present.

Practice good industrial hygiene.

Be aware of reportable

quantities.

Retest for safe conditions after break periods in work.

Drain into compatible containers (hose, buckets).

Return all plugs, flange blinds, etc., after completion.

Minimize spark generation and monitor atmosphere with explosion meter.

Repair with compatible or like-for-like parts.

Fully depressurize equipment prior to opening system.

Develop and execute the plan
When developing a repair plan for a heat transfer system, there are seven steps to follow.
Identify the needs for repair: Many processes run with a high on-stream time. In such cases, unplanned shutdowns generally occur only for unavoidable repairs that will define the primary intent of the job scope. However, the downtime may enable taking care of other items. Keep a list of secondary items that don’t require immediate correction so they can be addressed at the first available opportunity. These secondary items might include insulation repairs, valve stem packing repairs, gasket replacement, equipment inspections, instrument calibrations and interlock checks.

Downtime should be used as effectively as possible, so materials should be acquired in advance to maximize the amount of work that can be completed.

Assess the root cause of failure: Root cause assessment should be made for each failure to determine and correct its cause. This helps ensure a repair is made only once for a given item. Several common failure modes and possible corrective measures are given in Table 3.

 

Table 3. Some common failures and remedies

Problem

Typical Causes

Possible Corrections

Flange leaks

 

Poor piping flexibility

Improve piping flexibility.

Poor gasket compression

Ensure flanges, nuts and bolts meet gasket requirements. Ref. ASME B31.3 code

Poor flange alignment

Correct alignment. Ref. ASME B31.3 code.

Ensure adequate piping supports are provided.

Over-pressure

Identify and correct source of excessive pressure

 Pump seal leakage

Fluid contains solids

Provide fluid filtration; seal flush.

“Dry” start-up

Ensure pump is flooded at start-up.

Fluid oxidation
at seal

Consider seal quench and cooling.

High vibration

Do hot alignment to reduce vibrations at operating temperature.

 Valve stem leaks

Defective
packing

Replace packing; tighten packing.

Consider high-temperature,
graphite-based packings.

Consider bevel packing with anti-extrusion end-rings.

Heat exchanger tube leaks

Weakened connection of tube to tube sheet

Minimize thermal cycles and severity.

Consider welded, not rolled, connections.

 Expansion tank
corrosion

High moisture content

Provide better moisture removal by allowing full circulation through the expansion tank while venting to a safe location.

High acidity of fluid

Identify source of acidity and correct, then replace fluid. Consider system flush to avoid residual acidity. Fill systems with new fluid from the lowest point with the expansion tank vent open, to allow best removal of air. Then, consider a dry inert gas “blanket” in the expansion tank to protect the fluid during operation.



Develop the job plan: A job plan is formed by combining the primary repair item(s), selected items from the secondary list, and any corrective actions from root cause of failure assessments. All job plans should consider the basic facets of work place safety to protect against the hazards discussed earlier. The plans should list each task in sequence, along with materials and manpower requirements. Any unique needs for a specific site or process should be added to these basic considerations.
Prepare for work activities: Preparation involves four steps: shutdown, HTF management, lock out/tag out (LOTO) and isolation, and clean out.

Typically, it is the responsibility of operations personnel to safely shut down the process unit, including the HTF system, and properly prepare it for repair. The piping and equipment in the repair area should be emptied of process liquids and HTFs by pumping, draining, blowing with inert gas, or other means. Proper job planning can protect the fluid and even allow for its safe reuse. The two key factors that compromise fluid condition are oxidation and moisture contamination.

Hot fluid can oxidize when exposed to air. Draining the HTF when its temperature is below 250°F should adequately protect it from oxidation. Draining it into clean, dry containers for storage might allow it to be safely reused. Otherwise, the HTF should be properly disposed of. Check with your fluid supplier or site management about options.

When draining hot fluids, receiving containers must be free of moisture to prevent violent splattering and increased burn potential. Thermal burn prevention requires that fluids are cooled to at least 140°F. If the bulk of the HTF can be kept within the system and isolated from work activities, it should be suitable for reuse.

The job plan should include an approved list of those items requiring LOTO, including all rotating or energized equipment. The job coordinator should ensure the safe isolation of energy in the work area prior to initiating the process entry step and should always keep in mind that energy can be stored in various forms, including electrical, thermal, hydraulic or mechanical tension/compression.

Using the process and instrumentation diagrams (P&IDs), all valves must be identified for closure and securing with a lock/tag to prevent fluid flow into the work area. In a typical heat transfer system, this might include main pumps discharge valves, filter bypass loops, branch isolation valves, heat-tracing supply valves, fuel supply to heaters, and possibly the valves on the expansion tank piping. If vessel entry is required at the expansion tank, it might also be necessary to isolate the inert-gas supply.

All connecting branches of piping around the equipment where work is to be performed must be isolated. Electrical energy isolation requirements might include breakers for electrical heating elements or heat tracing, and starters for circulation and other pumps and motors. A good practice is to try starting equipment with start/stop switches to ensure each is de-energized, especially in older plants where power might come from more than one source.

If work is to be performed on a section of the plant while the remainder is in operation, great care should be taken to ensure critical safety interlocks are not compromised, such as via isolation of over-pressure protection devices or safety interlock instrumentation. It might also be necessary to insert blanks into certain flanges. The safest time to do this is when the system is down since blank installation and removal requires process-entry permitting and depends upon effective performance of block valves. Consideration should also be given to provision for thermal expansion or contraction of the liquid in the system.

If spark-producing work is to be performed, the affected piping might require cleaning by steam or solvent, and/or purging with inert gas. Check with the fluid manufacturer for a suggested cleaning procedure. Solvents must be compatible with the HTF and system components, and could be hazardous.

Before beginning any hot work, consider whether there might be residual vapors in the system. It might be preferable to remove sections of piping at flange connections to allow more controlled cleaning and ventilation.

Review safety provisions: Hydrocarbon vapors above their flash point can ignite if in concentrations within their combustible range in air while an ignition source is present. Depending on the amount of combustible vapors in the area, ignition might result in a short-lived flash fire or a continuous burn. Cooling the system will reduce ignition potential, as well as vapor concentrations to which workers might be exposed. Ignition sources can include electrically powered equipment, extension cords, cutting torches/flames and static discharges, as well as sparks from the impact of steel tools.

If work is required near or above the fluid’s flash point, non-powered tools such as hammers, screwdrivers and wrenches should be used where possible. Tools should be coated or made of brass to prevent sparking. Electrically powered equipment should be used only after obtaining an acceptable reading from an explosion meter, and when properly grounded. Static discharges can be prevented by attaching grounding cables.

An explosion meter can be used to help prevent fluid ignition. Most companies have policies that do not allow work to proceed if any flammable vapors are detected. Detection of such vapors might require further decontamination of the piping and/or equipment. Repeat testing during the execution of the job is a good practice, particularly before resuming work after extended breaks in activities.

Getting to work: Using a band saw, as opposed to a cutting torch, for the first cut into piping  can reduce sparking and temperature. After the piping is severed, the opposing pipe ends at the cut should be separated. If flanges are to be welded at the cut, balloon plugs can be inserted as far into the piping as possible for isolation from residual organic liquid and vapors. The inner walls can then be wiped down with cleaning solvents to remove residues before grinding and welding commences using standard work procedures. Of course, the balloon plugs should be removed when finished.

If flanges are not to be installed, clean the piping section by first steaming thoroughly, and then inerting with a dry gas prior to any cutting, grinding and welding. If a portion of the piping can be removed or separated, this might facilitate effective cleaning of residual HTF from inside piping surfaces before proceeding. Normal work procedures can then be used to prepare and weld the piping. Take care to remove all debris from the piping. The sidebar gives several references for piping and equipment repairs.

At the conclusion of all work, the unit should be prepared for normal operation, which includes removal of blanks, locks and tags, and returning valves into their normal operating positions; an appropriate inspection should follow. It is the responsibility of facility management to ensure the piping is safe for service based upon proper inspection and pressure testing. ASME B31.3 provides guidance on inspection and examination of chemical plant piping. Insulation should be left off any affected flanged connections until after start-up to allow inspection for leakage.

Return to service and start up: An effective shutdown is followed by an effective startup. During startup, the HTF system is filled to normal operating levels from the lowest point possible. Using standard operating procedures, establish circulation through the system; full flow through the expansion tank will facilitate removal of residual air and moisture from the system.

Gradually heat the system; use caution when above 212°F since any water in the system will begin to vaporize. A prudent approach is to heat the system assuming excessive moisture is present. Continue circulating fluid through the expansion tank with the vent open to allow moisture to escape to a safe and acceptable location. Holding the temperature steady while venting will prevent sudden and potentially violent reactions to vaporizing water. After effectively removing the moisture, proceed with the standard heat-up sequence for the heater.

Operations personnel should periodically walk along the piping pathways looking for leaks, paying special attention to areas where work was performed. Leaking flanges on high-temperature systems might require a shutdown so repairs can be made at cooler temperatures. A majority of flange leaks can be attributed to stresses induced from thermal cycling. It is quite difficult to achieve a leak-free flanged connection if flange faces are under severe tension or have faces out of square due to thermal expansion of the piping. After verifying the exposed piping flanges are free of leaks, insulation can be re-applied.

The secret to success
The keys to uneventful repairs of high-temperature heat-transfer systems involve safely shutting down a process using standard procedures, then developing the proper approach to manage the risks that are present. Once the risks are effectively addressed, routine repair techniques can then be applied. Using the input from a network of safety professionals, industry standards and fluid specialists, an effective and successful approach to the task is within reach.

Conrad E. Gamble, P.E., is marketing technical service principal for Solutia Inc., St. Louis, Mo. E-mail him at cegamb@solutia.com.

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