The management of process plants requires understanding and applying the fundamental tenets of safe design, construction and operation of facilities. This overview summarizes best practices that have been developed to ensure the safest possible use of DP:DPO (diphenyl–diphenyl oxide) eutectic heat-transfer fluids. A review of the concepts necessary for effectively maintaining a leak-tight heat-transfer-fluid (HTF) system is provided.
HTF system requirements
A “typical” HTF system is composed of an energy source, such as fired heaters or waste-heat recovery systems, as well as pumps to force the fluid flow, an expansion tank to accomodate the volume expansion of the fluid and one or more heat consumers.
High-temperature heat-transfer systems are usually closed systems, so a release of fluid can only occur in the case of accidents or malfunctions. As with all HTF systems, the design must accommodate the volume expansion of the heating fluid. The extensive network of piping, instruments, and vessels, combined with elevated vapor pressures, can increase the potential for leaks from systems if adequate design and maintenance measures are not incorporated.
Leak sources
Primary sources of leaks are flanged connections, flexible connectors or rotary joints, and pump seals. When insufficient flexibility is provided in piping networks, the resulting force applied to the flange pair can reduce gasket compression and lead to leakage. Stainless-steel flexible hoses were extensively installed on the early concentrating solar power (CSP) systems. Experience in the mode of failure in such hoses has found that the hoses develop small cracks. A properly installed flex hose will avoid torsion and misalignment, and maximize its usable life. When flexible hose is used in place of rigid pipe, consult manufacturers’ recommendations of inspection and replacement frequency.
Rotary joints have the advantage of greater wall thickness, which give rise to a more robust joint, yet maintaining the flexibility to support the demands of mobile piping. For high-temperature service, the joints will typically require periodic injection of graphite-based packing to maintain leak-free performance in a fire-resistant design.
Standard centrifugal pumps will typically use mechanical seals. While some seepage is not uncommon, problems can arise that can reduce seal life and lead to more significant leakage (see “Pump and pump seals” section).
When DP:DPO eutectic fluids freeze, the material contracts in volume by over 6%. If the product then melts between frozen plugs of product or other mechanical boundaries, tremendous pressure can result. The pressure can lead to release of the product through the weakest constraint. When thawing a frozen section of piping or equipment, it is very important to accommodate the expansion in volume into unobstructed piping or equipment.
Component selection
Because the HTF is significantly above its normal boiling point at process temperatures, adequate overpressure protection in the form of pressure relief valves (PRVs) and rupture disks must be provided with adequate capacity for relief between selected points of isolation.
Piping, flanges and gaskets. Seamless carbon steel has been shown to be an appropriate selection for piping with organic HTFs in plants up to their maximum bulk-operating temperatures. Fully welded construction in piping is preferable to threaded fittings, which should be avoided. Graphite-based, paste thread sealants have demonstrated marginal success with threaded connections, in cases where they cannot be avoided, such as in instrument connections, pump-casing drain plugs and others.
With any metallurgy employed, the linear expansion and contraction of the piping must be accounted for through the use of expansion loops and flexible connection members. Pipe supports should be generously spaced to prevent sagging. Where flanges are necessary, Class 300 and Class 600 ring-joint flanges or raised-face flanges are commonly used. When using raised-face flanges, gaskets should have a metal ring for blowout resistance, and graphite-filler with 316 stainless-steel spiral windings for fire-resistance. A key necessity for flanged connections is to ensure that adequate and uniform sealing compression (seating stress) is provided on the gasket faces.
Pumps and pump seals. Pumps in high-temperature service can have double mechanical seals, or can be of a sealless design. Excessive temperatures at mechanical seal faces can vaporize the HTF, resulting in an absence of lubrication and mechanical damage to seal-face materials. In order to avoid excessive temperatures (which can create particles that can erode the seal face and result in leakage), cooling of the stuffing box and seal gland is important to maintain lower temperatures and also improve lubricity of the HTF.
Valves. Valves in DP:DPO service may include forged or cast steel, or stainless steel bodies, balls, plugs and disks. Bellows seal designs can provide physical barriers for reduced emissions and leaks. Soft-seat materials should be avoided, since they can burn out in case of fire, potentially adding to the complexity of the HTF release. Small valves with welded end-connections should be considered to reduce potential leak points, and larger valves should be considered with flanged end connections. Relief valves can use engineered installations of rupture disks beneath.
Leak detection
Human senses. When there is leakage, even above its normal boiling point, the vapor emitted can quickly condense to form a visible, near-white mist cloud. For small leaks it may be possible to observe liquid droplets present at the source of the leak, such as valve stems, pump seals, flanges, and so on, as well as on the ground.
Also, the odor threshold is 9 parts per billion (ppb) for DP:DPO in air, making detection by odor possible without exceeding established airborne exposure limits for either component. In indoor areas, the airborne HTF may not dissipate readily, preventing easy tracing back to the leak point. In outdoor environments, the odor is more closely localized around the leak point due to the more rapid dissipation in air.
Process level indications. Today’s installations will have installed instrumentation for the routine measurement of liquid HTF levels. Changes in liquid level in vessels can be a somewhat crude, but important component of detecting material loss.
Specialized instrumentation. Instruments using the principal of photoionization are well suited for use in all kinds of process plants. These instruments can readily detect concentrations as low as the ppb range. For realtime personnel exposure monitoring, the ppb sensitivity is appropriate to quantify exposure. For maintenance needs, parts-per-million (ppm) sensitivity is sufficient to determine orders of magnitude of identified leak sources so that repairs can be scheduled on a prioritized basis. The instruments are available as hand-held units or as fixed-mount, continuous area monitoring stations.
While not comprehensive, the above guidance can help develop practices enabling a safer system design, reduce leaks and lower make-up fluid costs.
References
1. Gamble, C.E., Schopf, M., “Heat Transfer Fluid Leaks: Break the Fire Triangle,” Chem. Eng., Dec. 2010, pp. 26–33.
2. Solutia Inc., “Systems Design Data,” Pub. No. 7239193 version C, Therminol Heat Transfer Fluids, Solutia Inc.
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