Possible System Failures: Intermittent (Spike) Temperatures

Darrell Peil

Darrell Peil is Vice-President of Marketing and Technical Sales for Aeroflex USA, Inc. (www.aeroflexusa.com), and a Regional Sales Manager serving the upper half of North America. Currently a member of ASHRAE and ASTM, he serves on the committees for ASTM C 534, C 1534, and C 1427. He is the current Chair of NIA’s Technical Information Committee and ASHRAE’s TC 1.8 Handbook Subcommittee. Mr. Peil has been associated with the building mechanical systems industry since 1982, holding various roles in the contracting, distribution, and manufacturing segments of the industry. During that time, he has served in several positions requiring in-depth work with business and product development, systems design, specifications, standards, and codes. He can be reached at dpeil@aeroflexusa.com.

June 1, 2016

Understanding Your System’s Limits

The term “spike temperature” tends to be brought up in the insulation industry when users are trying to discern whether a material can be used on a system that experiences temporary spikes in operating temperature that are outside the usual operating range of either the system, the insulation material, or both—intentionally or unintentionally. When creating or repairing an insulation system, you should choose an insulation material that has a continuous operating temperature that is compatible with your system’s highest spike temperature. Choosing an insulation material that can withstand only a brief interval at the spike temperature—rather than one that can operate without issue at that temperature—is risky as your system may rise to that temperature more frequently than the material can withstand or the temperature may stay at that spike level for a longer period of time than the material is approved for. Overall, using the “spike” or “intermittent” service temperature of an insulation material as the upper service temperature may not be the best method for selecting insulation, and can lead to premature failure of the mechanical insulation system, potentially leading to
negative repercussions for the system. Selecting an insulation material that has an upper continuous service temperature in accordance with any anticipated system intermittent or spike operating temperatures is the best method when considering service temperature as one insulation material selection criteria.

In order to avoid potential issues, it is advisable to choose an insulation material that has a continuous operating temperature—among other key attributes—that matches the expected insulated system operating temperatures, even if these temperatures are only experienced sporadically. Doing otherwise can lead to incorrect specification, selection, or misapplication of insulation materials for a given system and the operating parameters of that system.

Research Is Key

When insulation is incorrectly selected and installed, it opens up the mechanical system to potential damage, which can incur a significant expense. If a material has the intermittent service temperature listed on its data sheet, inspect it carefully to ascertain whether the upper or maximum service temperature limit is a continuous or an intermittent (spike) temperature. Some insulation products may indicate that they can be used at a certain upper temperature limit, but do not disclose that this use is intended only for sporadic or short intervals. Unfortunately, there is no universal standard for intermittent service temperatures, so manufacturers may represent it differently.

If a producer establishes a given temperature as an intermittent service temperature, a clear definition of the parameters for this value should be given to explain its proper use. This attribute will be specific to a given manufacturer and should not be ascribed to all products of a given material type. It is important to be aware of materials that publish the intermittent service temperature as the upper service limit, and may not divulge the information on how long the material can withstand such temperatures. In some cases, fine print may be used to qualify that the upper service temperature is an intermittent service temperature with specific limitations on how frequently or how long the material can take the excursion temperature before problems begin.

Some manufacturers have their product tested by third-party laboratories; a good way to determine if the service temperature is a continuous service temperature is to see if a standardized test method has been used to verify that limit. ASTM C411 is the primary test method used to verify the upper continuous service temperature of mechanical insulation, being intended for materials that have an upper continuous use limit of 200°F or greater.

System Spikes

The above begs the question of why or when a system would spike or operate at elevated temperatures outside the normal system temperature and possibly exceed the insulation service temperature. As an example, a process piping system that normally functions within the operating temperature range of the insulation may be periodically cleaned with higher temperature agents—delivered by a clean-in-place wash system—driving the system intermittent temperature and the insulation material’s required performance temperature appreciably above the insulation material’s published upper service temperature.

The same can happen with vessels that operate at lower temperatures and are infrequently shut down for a cleaning with hot cleaning agents. A refrigeration system, for example, can reverse itself and go to hot gas defrost, subjecting the insulation to an elevated temperature for a period of time. Hydronic solar systems may reach exceptionally hot stagnation temperatures in header piping that the designer may not plan for. Determine what these anticipated system excursions, intermittent, or spike temperatures will be and then use them to help you correctly select insulation material that can withstand and function at those temperatures. Knowingly selecting an insulation material that has an intermittent or spike temperature that is lower than or too close to the anticipated system spike temperature is risky and may lead to insulation system failure.

Unintentional system operating spikes can also occur. Systems can experience a “run-away” condition where they exceed the normal operating limits, such as a steam system in which the boiler limit controls fail. In this case, an insulation material that was properly specified based on the published service temperature and expected system operating temperature may unintentionally be subject to heat greater than the published temperature. These are unanticipated operating conditions and can be expected to cause insulation system failure if an exceptional safety margin was not designed into the insulation envelope. Some manufacturers have a safety allowance in the performance ratings of the products they produce, but do not publish this information so that the material will be used in a manner that allows it to constantly serve the end user over the system life—including operating at the upper service temperature. In the above instance, the safety margin of the material may not have been exceeded, and can be left in service, avoiding significant and unwarranted expense to replace an insulation system that can continue to provide lasting service. Each producer has their own limits for this property. In the event that an unexpected higher-than-normal-temperature operating condition has been experienced in an insulated system and there is a question surrounding serviceability of the insulation, the best route to follow to see if the insulation material is in serviceable condition or needs attention is to contact the material producer with the specifics of the unexpected operating condition and ask for an assessment.

As a best practice, you should determine the highest temperature (the spike temperature) that your system will run at (e.g., 498°F) and choose an insulation material that can operate continuously at the temperature (e.g., 500°F) so it will not break down or fail when your system reaches that temperature. If your system usually reaches a high of 300°F, but is occasionally cleaned at 500°F, then you should choose a material that can reach or exceed 500°F. Do not expect that the materials will endure the increased temperature for short intervals; they may fail and your system may need to be replaced. It is important to understand and design your insulation system to realistically meet its top operating temperatures in order to meet customer expectations, keep the system operating properly, and avoid unexpected delays, repairs, or downtime.

SIDEBAR

  • Intermittent or “Spike” Temperature for a System: A temporary rise in operating temperature that exceeds the usual operating range of the system, such as during a system cleaning
  • Upper Continuous Service Temperature for an Insulation Material: An insulation material that can be used on a system continuous at temperature range up to that temperature. Best practices are not to exceed that temperature