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Safeguarding Refractory Installation: 12 Vital Steps to a Flawless Dry-Out

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Dan Szynal, VP of Engineering & Technical Services, Plibrico
Dan Szynal, VP of Engineering & Technical Services, Plibrico

Installing new refractory materials is a necessary furnace maintenance practice which needs to be done periodically. But extended downtime and installation errors can be a major financial and operational headache. In this article, Dan Szynal, VP of Engineering & Technical Services, Plibrico, gives 12 factors which will ensure that the refractory installation is successful.

At 700°F, steam can exert 3,000 psi pressure.
At 700°F, steam can exert 3,000 psi pressure.

During an initial dry-out, the powerful effects of superheated steam can cause explosive, devastating consequences to freshly cured refractory material. To that end, removing moisture from castable and precast shapes is a serious pursuit. The production pressures to minimize downtime can lead to shortcuts and rushed dry-out procedures. Usually, these sidesteps have the opposite effect, quickly compounding delays and costs by causing thermal damage to the linings and potentially incurring personal injury.

Dry-outs fail due to imprecise management of water extraction from refractories. At the boiling point of water, the pressure of steam is less than 1 psi. However, at 700°F, saturated steam reaches 3,000 psi, and possesses enough energy to disintegrate the most resilient refractories. Too much heat, rapid ramp-ups, vapor lock, poor curing, and surplus water can contribute to potentially hazardous situations.

Here are the 12 preventive factors to manage for dry-out safety and success:

1. Hot spots and flame impingement. Ensure that your burner flame is centered accurately. The direction of flame in the vessel must promote equal heating of all the refractory surfaces. A flame that impinges on a single area of the surface will quickly create a hot spot, forcing an unequal expansion of water vapor in that area and resulting in thermal spalling.

Thermocouples need to be monitored at both hot and cold areas to measure temperature consistency.
Thermocouples need to be monitored at both hot and cold areas to measure temperature consistency.

2. Temperature spikes. Insulation is ill-advised. Attempting to cover green castable with an insulating blanket can lead to destructive temperature spiking when the blanket is removed, breaks, or falls off. At a wall surface temperature of only 550°F, the removal of insulation exposes the surface to an extreme temperature shift which will activate unequal steam expansion and pressure.

3. Thermocouple placement and monitoring. Pay attention to the locations and readings of your TCs. Watching only the coldest location will allow the hottest area of your vessel to heat too quickly in the dry-out schedule. Conversely, monitoring only the hottest area will allow the colder area to retain more water than specified. This will lead to failure later in the schedule or during hold periods. At 700°F, steam can exert 3,000 psi pressure.

4. Air temperature vs. surface temperature. Thermocouples should report surface temperature. Air temperatures are typically 50°F to 100°F hotter, thus misreporting schedule impact. The initial hold period is typically designed to melt burn-out fibers. That creates important permeability. If the actual load temperature is lower than specified, permeability is not created, leading to failure in the next ramp-up period.

Pre-cast refractory requires longer bake-out schedules to release all water vapor.
Pre-cast refractory requires longer bake-out schedules to release all water vapor.

5. Field vs. precast dry-out schedule. A field dry-out schedule is specified for single-sided heating. It precipitates a dual water migration, first (stage 1) towards the heat as the path of least resistance, but then reversing course (stage 2) and moving away from the heat, escaping towards the furnace shell. Field dry-outs are faster schedules than precast, where the pieces are heated from all sides simultaneously. The precast water migrates to the center of the piece, and that takes longer to escape. By misapplying the faster field dry-out to precast, there is a greater risk of water retention, which will ultimately lead to spalling, even at temperatures of 550°F or less.

6. Venting and air circulation. Proper venting is required to rid the furnace of water vapor during dry-out. Without vents and free air circulation, the steam is forced to exit via the furnace shell, which takes longer than the schedule would provide. Water will be retained closer to the shell side, increasing the likelihood for disintegration as temperature and steam pressure rise.

7. Surface coating. An impermeable coating on the refractory surface will prevent the stage 1 escape of water. Slowly, this water will be forced to move to its second exit, the furnace shell. This delay prepares the still-saturated refractory for failure at the next heat ramp-up.

8. Clear obstruction from weep holes. As stage 2 water migration occurs, it will escape to the furnace shell. There should be adequate weep hole capacity, cleared of obstructions which will allow the water to exit the furnace shell. These provide a release valve for buildup of steam pressure. Thermocouples need to be monitored at both hot and cold areas to measure temperature consistency. Pre-cast refractory requires longer bake-out schedules to release all water vapor.

9. Cold weather curing. In the curing process, simple hydrates form needle-like morphology. These structures promote permeability, and water/steam can more easily migrate through the refractory to escape. Curing in below-freezing temperatures alters the hydrates to be less permeable, thus trapping the water, even during dry-out and creating an inherent risk. As well, cold weather curing slows the required strengthening process, leading to a weaker refractory and likely spall. We have had a thermal operator tell us about a below-freezing cure that went badly: The water in the castable actually froze in place. When the dry-out was initiated, the castable melted and fell to the floor, where it subsequently cured and dried.

10. Cutting short cure time. Recommended dry-out schedules always assume a 24-hour equivalent curing time at moderate temperatures. By cutting short the cure time, water is retained, and strength is reduced. For example, a conventional castable requires 24 hours cure time; high cement/low moisture castable needs at least 16 hours. Adherence to product cure time specifications ensures optimum strength and a successful dry-out.

11. Free water removal without consideration. The goal of curing and dry-out is to create permeability in the refractory at lower temperatures (300°F) to enable water to escape. By quickly ramping up dry-out temperatures for the sake of time, permeability is diminished. At higher temperatures, (+500°F) steam pressure rises aggressively. Again, refractory composition drives curing and dry-out schedules, and as a rule, the faster temperatures rise beyond specification, the higher the risk of failure.

Pre-cast shapes spall at 550°F.
Pre-cast shapes spall at 550°F.

12. Refractory strength as a function of water content. A simple 1% excess of water will reduce refractory strength by as much as 20%. Overwatering by 1.5% cuts strength 25% to 40%. The implications are profound: the refractory will not withstand the steam pressures in dry-out, and worse yet, there is more water that must be extracted. A successful dry-out can be jeopardized by the slightest variance in water composition.

Conclusion

Meticulous care in refractory installation is the foundation to successful furnace operation. While no one looks forward to non-productive downtime, close adherence to product specifications, cure times, and dry-out schedules will ensure a more profitable return to operations. Managing the water issues in refractory composition is job one.

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Applying “Thru-Process” Temperature Surveying To Meet the TUS Challenges of CQI-9

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Dr. Steve Offley, a.k.a. “Dr. O”
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In the modern automotive manufacturing industry, CQI-9 HTSA (AIAG) has become a key part of driving process and product quality in heat treatment applications. The standard has a broad scope and covers many different aspects of common heat treatment processes (see Process Tables A-H in the standard) and monitoring requirements used. A critical part of the standard is the requirement to perform a temperature uniformity surveys (TUS) in order to validate the temperature uniformity of the qualified work zones and operating temperature ranges of furnaces or ovens used. In this Heat Treat Product Spotlight, Dr. Steve Offley, a.k.a. “Dr. O”, Product Marketing Manager with PhoenixTM, discusses the challenges of performing a TUS on continuous furnace types and one possible solution his company offers.

CQI-9 Heat Treat System Assessment

A critical part of the CQI-9 HTSA (AIAG) standard is the requirement to perform temperature uniformity surveys (TUSs). The TUS is performed to validate the temperature uniformity characteristics of the qualified work zones and operating temperature ranges of furnaces or ovens used. (See Figure 1.)

Fig 1: Schematic showing TUS principle. Thermocouple measurement from the field test instrument, of the furnace’s actual operational temperature, against a setpoint to check that it is within tolerance. Setpoints and tolerances are defined in CQI-9 Process Tables A-H to match each heat treat process.

The “Thru-Process” TUS Principle

Traditionally, TUSs are performed by using a field test instrument (chart recorder or static data logger) external to the furnace with thermocouples trailing into the furnace heating chamber. This technique has many limitations, especially when the product transfer is continuous such as in a pusher or conveyor-type furnace. The trailing thermocouple method is often labor-intensive, potentially unsafe, and can create compromises to the TUS data being collected (e.g., number of measurement points possible, thermocouple damage, and physical snagging of the thermocouple in the furnace).

Fig 2: PhoenixTM thermal barrier being loaded into a batch furnace with a survey frame as part of the TUS process.

The “Thru-Process” TUS principle overcomes the problems of trailing thermocouples as the multi-channel data logger (field test instrument) travels into and through the heat treat process protected by a thermal barrier (Figure 2). The short thermocouples are fixed to the TUS frame. Temperature data is then transmitted live to a monitoring PC running TUS analysis software, via a 2-way RF telemetry link.

Data Logger Options

To comply with CQI-9, field test equipment needs to be calibrated every 12 months minimum, against a primary or secondary standard. The data logger accuracy needs to be a minimum +/-0.6 °C (+/-1.0 °F) or +/-0.1% (TABLE 3.2.1).

Fig 3: PhoenixTM PTM1220 20 Channel IP67 data logger comes calibrated to UKAS ISO/IEC17025 as an option with an onboard calibration data file allowing direct data logger correction factors to be applied automatically to TUS data.

The data logger shown in Figure 3 has been designed specifically to meet the CQI-9 TUS requirements offering a +/- (0.5°F (0.3°C) accuracy (K & N). Models ranging from 6 to 20 channels can be provided with a variety of noble and base metal thermocouple options (types K, N, R, S, B) to suit measurement temperature and accuracy demands (AMS2750E and CQI-9).

Mixed thermocouple inputs can be provided to support the process specific requirements and also allow the use of the data logger to perform system accuracy testing (SAT) to complement the TUS.

Innovative Thermal Barrier Design

Fig 4: “Octagonal” thermal barrier fitted to product/survey tray.

CQI-9 covers a wide range of thermal heat treatment processes and as such the thermal protection for the data logger will vary significantly. A comprehensive range of thermal barrier solutions can be provided to meet specific process temperature requirements and space limitations. Figure 4 shows a unique octagonal thermal barrier designed to fit within the boundaries of the product tray/survey frame used to perform a TUS using the “plane method” (See “Thermocouple Measurement Positions (TUS)” below in this article.). The design ensures maximum thermal performance within the confines of a restricted product tray/basket.

Live Radio TUS Communication

Fig 5: Schematic of LwMesh 2-way RF Telemetry communication link from data logger TUS measurement back to an external computer.

The data logger is available with a unique 2-way wireless RF system option allowing live monitoring of temperatures as the system travels through the furnace. Analysis of process data at each TUS level can be done live allowing full efficient control of the TUS process. Furthermore, if necessary, by using the RF system, it is possible to communicate with the logger installed in the barrier to reset/download at any point pre-, during, and post-TUS. In many processes, there will be locations where it is physically impossible to transmit a strong RF signal. With conventional systems, this results in process data gaps. For the system shown in Figure 2, this is prevented using a unique fully automatic “catch up” feature.

Any data that is missed will be sent when the RF signal is re-established, guaranteeing 100% data transfer.

Thermocouple Options (TUS)

In accordance with the CQI-9 standard (Tables 3.1.3 / 3.1.5), thermocouples supplied with the data logger, whether expendable or nonexpendable, meet the specification requirements of accuracy +/-2.0°F (+/-1.1°C) or 0.4%. Calibration certificates can be offered to allow the creation of thermocouple correction factor files to be generated and automatically applied to the TUS data within the PhoenixTM Thermal View Survey Software. Care must be taken by the operator to ensure that usage of thermocouples complies with the recommended TUS life expectancies and repeat calibration frequencies. Before first use, thermocouples must be calibrated with a working temperature range interval not greater than 250°F (150°C). Replacement or recalibration of noble metal (B, R or S) thermocouples is required every 2 years. For non-expendable base metal (K, N, J, E), thermocouples replacement should be after 180 uses <1796°F (980°C) or 90 uses >1796°F (980°C). For expendable base metal (K, N, J, E), thermocouples replacement should be after 15 uses <1796°F (980°C) or 1 use >1796°F (980°C). Note that base metal thermocouples should not be recalibrated.

Thermocouple Measurement Positions (TUS)

To perform the TUS survey, a TUS frame needs to be constructed to locate the thermocouples over the standard work zone to match the form of the furnace. The TUS may be performed in either an empty furnace in which case thermocouples should be securely fixed as shown in Figure 6. A heat sink (thermal mass fixed to thermocouple tip) can be used to create a thermal load to match the normal product heating characteristics. Alternatively, the thermocouples should be buried in the load/filled product basket. See Figure 6 to see schematics of TUS Frames for a box and cylindrical batch furnace with CQI-9-quoted number of thermocouples required to match void volume (Volumetric Method Table 3.4.1).

Fig 6: TUS Thermocouple Test Rigs. Required number of thermocouples: 1) Work Volume < 0.1 m³ (3 ft³) = 5; 2) Work Volume 0.1 to 8.5 m³ (3 to 300 ft³) = 9; 3) Work Volume > 8.5 m³ one thermocouple for every 3 m³ (105 ft³). (Click on the images for larger display.)

Fig. 7.1, 7.2. PhoenixTM system showing 9 Point TUS survey rig and Thermal View Software TUS frame library file showing as part of TUS report exactly where thermocouples are positioned. (Click on the images for larger display.)

 

For continuous conveyorized furnaces, it is recommended that an alternative thermocouple test rig is employed called the “plane method”. Since the system travels through the furnace it is only necessary to monitor the temperature uniformity over a 2-dimensional plane/slice of the furnace (Figure 8). The required number and location of thermocouples are shown in Table 1 (CQI-9 Table 3.4.2).

(Click on the images for larger display.)

Table 1: Required thermocouples and locations for differing work zones (Plane Method)

(1) 2 Thermocouples within 50 mm work zone corners 1 Thermocouple center. (2) 4 Thermocouples within 50 mm work zone corners. Rest symmetrically distributed.

“Thru-Process” Temperature Uniformity Survey (TUS) Data Analysis and Reporting

Operating the PhoenixTM System with RF Telemetry, TUS data is transferred from the furnace directly back to the monitoring PC where, at each survey level, temperature stabilization and temperature overshoot can be monitored live, with thermocouple and logger correction factors applied. The Thermal View Survey software generates TUS reports which comply with the requirements of AMS2750E/CQI-9 standards.

As defined in CQI-9 (Section 3.4) for furnace with an operating temperature range ≤ 305°F (170°C), one setpoint temperature (TUS level) within the operating temperature range is required. If the operating temperature of the qualified work zone is greater than 305°F (170°C), then the minimum and maximum temperatures of the operating temperatures range shall be tested.

The TUS levels can be automatically set up in the TUS analysis software. Figure 9 shows both the TUS level file and TUS levels applied against the TUS survey trace.

Fig. 9.1
Fig. 9.2

Fig 9.1, 9.2 PhoenixTM Thermal View Survey Software showing TUS Level set-up and application to TUS trace.

Within CQI-9, there is a very prescriptive list of what should be contained in the TUS report (Section 3.4.9).

To comply with all said requirements, the software package provides a comprehensive reporting package as shown below.

Fig 10.1, 10.2, 10.3.  TUS Report showing a TUS profile at three set survey temperatures (graphical and numerical data). The probe map shows exactly where each thermocouple is located and easy trace identification. A detailed TUS report is generated, meeting full CQI-9 reporting requirements. (Click on the images for larger display.)

Overview

The PhoenixTM Thru-Process TUS System provides a versatile solution for performing product temperature profiling and furnace surveying in industrial heat treatment meeting all TUS requirements of CQI-9 within the automotive manufacturing industry, providing the means to understand, control, optimize and certify the heat treat process.

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Jason Schulze on AMS2750E: Initial and Periodic Temperature Uniformity Surveys

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This is the seventh in a series of articles by AMS2750 expert, Jason Schulze (Conrad Kacsik).  Click here to see a listing of all of Jason’s articles on Heat Treat Today. In this article, Jason advances the discussion of initial and periodic TUS requirements. Please submit your AMS2750 questions for Jason to editor@heattreattoday.com.

Introduction

Any technician who has performed a Temperature Uniformity Survey (TUS) understands that the assembly, use, and placement of thermocouples is imperative to the success of the TUS.

As we move through the requirements of Temperature Uniformity Surveys, we will examine the requirements that apply to TUS thermocouples.

Initial Temperature Uniformity Surveys

Before we get started, let’s take a look at how AMS2750E describes :

An initial TUS shall be performed to measure the temperature uniformity and establish the acceptable work zone and qualified operating temperature range(s). Periodic TUS shall be performed thereafter in accordance with the interval shown in Table 8 or 9. ~ AMS2750E page 23, paragraph 3.5.1

Most companies, whether purchasing a new furnace or used one, know what they would like the acceptable work zone size and qualified operating range to be. I emphasize “would like” because what we would like our furnaces to be capable of is not always what they are able to do. We would like to use every square meter of our furnace control zone in an effort to maximize capacity and, of course, maximize profit on each cycle we process. We would like our furnaces to operate at the very limits of what the furnace manufacturer states it can do.  Unfortunately, these items don’t always exist once the furnace is subjected to an initial Temperature Uniformity Survey per AMS270E.

An initial TUS is used to tell us what our furnaces can do based on pre-determined parameters. Normally, these parameters should be flowed down to our furnace manufacturers, and prior to shipping, these parameters are compared to what the furnace can actually attain making the furnace conformative and ready for shipment. I strongly recommend this whenever purchasing a new or used furnace.

Initial temperature uniformity testing requirements are as follows;

  1. Initial survey temperatures shall be the minimum and maximum temperatures of the qualified operating temperature range(s).
  2. Additional temperatures shall be added as required to ensure that no two adjacent survey temperatures are greater than 600 °F (335 °C) apart.

These requirements are simple and straight forward. One could argue that I may be oversimplifying the requirements of an initial TUS, but let’s not forget, these are merely the requirements, not the conditions, under which an initial TUS must be performed. Let’s look at an example that would conform to the stated requirements.

Example

A furnace (in this case, it is irrelevant what type of furnace or what it is used for) processes production hardware from 900°F to 2200°F. Based on the requirements of AMS2750E, the initial TUS would start by testing at 900°F and the last temperature tested would be 2200°F. The supplier would need to select temperatures between 900°F and 2200°F to ensure that there is no more than a 600°F gap between each adjacent temperature. Figure 1 is an example of temperatures that could be selected.

 

Figure 1

 

We’ve covered the requirements of an initial TUS; we will now address the conditions when an initial TUS is required. Initial TUSs are required when a) the furnace is installed (new or used) and b) when any modifications are made that can alter the temperature uniformity characteristics. You could dispute this by stating if a TUS fails (and the furnace is then repaired to be put back in service), if the qualified work zone is expanded, if a thicker control thermocouple is installed, etc. a new initial TUS is required. I would agree, but these would all fall under “B”.

Periodic Temperature Uniformity Surveys

Periodic TUSs are performed for single operating ranges greater than 600°F. In this case, the temperatures are selected must be 300°F from the minimum- and 300°F from the maximum-qualified operating range. If there is a gap of greater than 600°F, additional temperatures must be selected so there is no gap greater than 600°F. Using the example above, we could select temperatures as stated in Figure 2 below.

 

Figure 2

 

It is required that at least once each calendar year the minimum and maximum temperatures of the qualified operating range (in our example, it would be 900°F and 2200°F) are tested. Some suppliers may choose to perform an initial TUS once per year to ensure they capture the minimum and maximum.

Initial and Periodic Test Frequency

Tables 8 and 9 within AMS2750E describe the TUS frequency which is based both on furnace Class and Instrumentation Type. As an example, if our furnace referenced previously was identified as a Class 3 (±15°F), Type A instrumentation, the initial survey frequency would be quarterly. After two successful consecutive surveys, the frequency of testing could then be extended to being done annually.

It is important to recognize the difference between initial and periodic TUS temperatures and initial and periodic TUS frequency. Let’s use our example to expand on this. The supplier would perform a TUS using initial temperatures shown in Figure 1. If the TUS passes, the supplier would then, three months later, perform a TUS using the temperatures shown in Figure 2. This would then count as two successful consecutive TUSs. The next TUS could then be performed annually using the temperatures stated in Figure 2.

Conclusion

Understanding initial and periodic TUS requirements is imperative to ensure conformance to AMS2750E and Nadcap. In the next installment, we will discuss TUS data collection, relocation of hot and cold thermocouples, as well as quality requirements.

Submit Your Questions

Please feel free to submit your questions, and I will answer appropriately in future articles. Send your questions to editor@heattreattoday.com.

 

 

 

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Jason Schulze on AMS2750E: Examining Requirements That Apply to TUS Thermocouples

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This is the sixth in a series of articles by AMS2750 expert, Jason Schulze (Conrad Kacsik).  Click here to see a listing of all of Jason’s articles on Heat Treat Today. In this article, Jason advances the discussion of TUSs with an examination of requirements that apply to TUS thermocouples. Please submit your AMS2750 questions for Jason to editor@heattreattoday.com.

Introduction

Any technician who has performed a temperature uniformity survey understands that the assembly, use, and placement of thermocouples are imperative to the success of the TUS.

As we move through the requirements of Temperature Uniformity Surveys, in this installment we examine the requirements which apply to TUS thermocouples.

TUS Thermocouples Re-Use, Quantity, and Arrangement Requirements

TUS Thermocouple Re-Use Requirements

AMS2750E, paragraph 3.1.3, can be difficult to understand at times. To start, it’s important to understand the difference between expendable and nonexpendable thermocouples.

Expendable Thermocouples:

“Thermocouples made of fabric or plastic covered wire. The wire is provided in coils or on spools. Insulation usually consists of glass braid or ceramic fiber cloth on each conductor plus glass braid overall.”

Nonexpendable Thermocouples:

“Thermocouples that are not covered with fabric or plastic insulations. One type consists of ceramic insulators over bare thermocouple wire, sometimes inserted in a tube for stability and protection. A second type consists of a combination of thermocouple wires, mineral insulation, and a protecting metal sheath compacted into a small diameter. The thermocouple thus constructed is protected, flexible and, within the temperature limits of the sheath material, may be used many times without insulation breakdown. This type of thermocouple, conforming to ASTM E 608, is available under many trade names.”

Once these definitions are understood, we focus on paragraphs 3.1.3.3, 3.1.3.4, and 3.1.3.5 carefully to ensure you apply the correct usage allowance to the correct thermocouples.

Paragraph 3.1.3.3:

“Expendable test sensors may be reused if ‘U’ in the following formula does not exceed 30. A ‘use’ for test thermocouples is defined as one cycle of heating and cooling the thermocouple (2.2.77). U = Number of uses below 1200 °F (650 °C) + 2 times number of uses from 1200 °F (650 °C) to 1800 °F (980 °C). Expendable base metal test thermocouples shall be limited to a single use above 1800 °F (980 °C).”

Notice the paragraph begins with the term “expendable test sensors.” This prohibits the U-formula from governing the replacement frequency of nonexpendable test sensors as well as expendable sensors which are not used as a test sensor.

Paragraph 3.1.3.4:

“Any base metal TUS thermocouple that is (1) used exclusively under 1200 °F (650 °C), (2) identified, and (3) preserved/protected from damage (i.e., crimping, excessive moisture contact, corrosion, etc.) between tests or remains installed on a rack that is protected between tests,) shall be limited to no more than 90 uses or 3 years, whichever comes first and may be reused subject only to the limitations of 3.1.3.1 to 3.1.3.2.”

This paragraph begins with “Any base metal TUS thermocouple.” This would apply to any base metal thermocouple (i.e. Type K, Type N, etc.) used for a TUS, whether expendable or nonexpendable.

Paragraph 3.1.3.5:

“Nonexpendable base metal TUS thermocouples reinstalled for each TUS through ports in the furnace, used in the same location and depth of insertion for each TUS and used exclusively under 1200 °F (650 °C) shall be limited to no more than 90 uses or 3 years, whichever comes first and may be reused subject only to the limitations of 3.1.3.1 to 3.1.3.2.”

This paragraph is very specific regarding its application. For this paragraph to apply, the supplier would need to be using a) nonexpendable thermocouples that are b) base metal, which are c) reinstalled through ports in the furnace and used (non-resident) d) at the same location and e) depth of insertion.

Suppliers interpreting the usage requirements of test thermocouples should pay close attention to Figure #1 in AMS2750E. Figure #1 lays out the usage requirements of AMS2750E in an easy-to-read format that can be used as a quick reference.

Figure 1, AMS2750E

 

TUS Thermocouple Quantity Requirement

AMS2750E, page 27, paragraph 3.5.13.1, states that the number of TUS thermocouples shall be in accordance with Table 11. The top 2 lines reflect the most widely used. (See Figure 2.)

Figure 2

The amount of test sensors is based on the cubic foot of the qualified work zone. This should not be mistaken for the cubic foot of the heating area in the furnace, or control zone, as the full heating area is not always the size of the qualified work zone.

Table 11 begins by categorizing the options as “Workspace Volume Less Than.” Once your qualified work zone is established, you will need to apply that to the table to determine how many TUS thermocouples will be needed. As an example, if your qualified work zone is 562 cubic feet, you would need a minimum of 19 test thermocouples distributed throughout the qualified work zone during the TUS.

TUS Thermocouple Placement Requirement

Thermocouple placement is described in AMS2750E paragraphs 3.5.13.2.1 and 3.5.13.2.2. Paragraph 3.5.13.2.1 relates to the thermocouple placement for qualified work zone volumes that are less than 3 cubic feet. Typically, this would apply to small air furnaces or laboratory furnaces used for testing, although could very well apply to smaller atmosphere or vacuum furnaces. Each paragraph describes the requirements for a rectangular qualified work zone and cylindrical qualified work zones.

Paragraph 3.5.13.2.1

“For furnace work zone volumes less than 3 cubic feet (0.085 m3), four TUS sensors shall be located at the four corners and one at the center. If the furnace work zone volume is cylindrically shaped, four TUS sensors shall be located 90 degrees apart at the periphery and one shall be located at the center. In both cases, all TUS sensors shall be located to best represent the qualified work zone.”

To better describe the requirement within this section, I’ve included a diagram of the requirement for both rectangular and cylindrical qualified work zones.

 

 

 

The location is a requirement, although the numbering sequence identified in these diagrams is optional and the supplier has the freedom to number the locations as they see fit.

Paragraph 3.5.13.2.2

“For furnace work zone volumes greater than 3 cubic feet (0.085 m3), eight TUS sensors shall be located at the corners and one shall be located in the center. If the work zone volume is cylindrically shaped, three TUS sensors shall be located on the periphery of each end, 120 degrees apart. One of the remaining TUS sensors shall be located at the center; the other two shall be located to best represent the qualified work zone. For furnace work zone volumes greater than 225 cubic feet (6.4 m3), the additional TUS sensors required by Table 11 shall be uniformly distributed to best represent the qualified work zone. When radiant heat from the periphery of the work zone is used to heat the product, the additional sensors shall be uniformly distributed at the periphery of the work zone.”

Again, the diagrams to the right better describe the requirements within paragraph 3.5.13.2.2.

Conclusion

Now that the TUS thermocouple requirements have been established, we will move on to the requirements of initial and periodic TUS requirements in the next article.

Submit Your Questions

Please feel free to submit your questions, and I will answer appropriately in future articles. Send your questions to editor@heattreattoday.com.

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Troubleshooting Thermocouples: Detecting Small Issues Before They Become Big Problems

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Source: GeoCorp, Inc.

 

Heat treating thermocouples are thermal processing sensors that allow equipment operators to accurately measure and maintain the right temperatures for softening, hardening, and material modification. A faulty thermocouple can result in inaccurate measurements, so it’s critical for quality management personnel to determine when and why thermocouple failure has occurred. GeoCorp, Inc., has provided a series of short articles that walk operators through assessment and troubleshooting of thermocouples that go bad.

Sometimes the smallest issues can lead to big problems. When your business relies on crucial temperature readings from your thermocouples, it can pay to pay identify individual factors that may have cause part failure. ~ James LaFollette, GeoCorp, Inc.

When troubleshooting thermocouple failure, the authors, James LaFollette and John Ochenas, recommend reviewing three key issues: system wiring, probes, and wire and junction. In addition, the following could cause misreadings:

  • metal fatigue
  • oxidation
  • contamination
  • poor installation
Read more:

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Jason Schulze on Understanding AMS 2750E — Alternate SAT

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Heat Treat Today Original ContentJason Schulze, Conrad Kacsik Instruments, Inc.

This is the third in a series of articles by AMS 2750 expert, Jason Schulze. Please submit your AMS 2750 questions for Jason to Doug@HeatTreatToday.com.

Introduction

Of all the changes made to AMS2750 through the years, the Alternate Systems Accuracy Test (ALT SAT) is arguably the one that has had the largest impact within the heat treat industry. The requirements for the ALT SAT, as presented in AMS2750E, make up just 0.008% of the specification as a whole; yet these requirements account for an inordinate amount of time spent on discussion and debate.

Below, we’ll discuss the requirements of the ALT SAT as they are presented in both AMS2750E, and in the Nadcap Pyrometry Guide.

ALT SAT Applicability

Prior to revision E of AMS2750, a load thermocouple that was single-use, or which was replaced more often than the applicable SAT frequency, did not require an SAT of any kind. During the time period when Revision D was in effect, the Alternate SAT did not exist. This meant that if you used a load thermocouple and had a documented single-use statement or replacement schedule, which ensured the usage did not exceed the applicable SAT frequency within your internal procedures, that particular load sensor was not subject to the SAT requirements of AMS2750D.

AMS2750D page 14, paragraph 3.4.1.2

3.4.1.2 An SAT is not required for sensors whose only function is over-temperature control, load sensors that are limited to a single use (one furnace load/cycle), sensors not used for acceptance as part of production heat treatment, or load sensors whose replacement frequency is shorter than the SAT frequency. See 3.1.8.4 and 3.1.8.5.

When AMS2750E replaced AMS2750D, the ALT SAT was introduced. In addition to the ALT SAT, paragraphs 3.4.4 through 3.4.4.3 were also inserted:

AMS2750E pg 19, para 3.4.4

3.4.4 The SAT can be accomplished using any one of 3 methods:

3.4.4.1 Perform an SAT following the requirments in 3.4.5

3.4.4.2 Alternate SAT process defined in 3.4.6

3.4.4.3 SAT Waiver process, as described in 3.4.7

By stating that the SAT “…can be accomplished using any one of 3 methods”, this section has often been misinterpreted to mean that a supplier may simply choose which type of SAT they wish to implement. This is not the case.

An ALT SAT must be performed on any thermocouple that is either

  1. single use, or
  2. replaced more often that the applicable SAT frequency.

Throughout the industry, these two items typically apply to load thermocouples. As an example, let’s assume that a non-expendable load thermocouple is used in a furnace that is designated as a Type A, Class 5 furnace. This would put the standard SAT frequency at quarterly (no SAT extension & parts-furnace). If the non-expendable load thermocouple that was used had a documented replacement frequency of monthly, the ALT SAT requirements would apply to this particular load thermocouple.

In the example above, a supplier could not accomplish the SAT “…using any one of the 3 methods” – the ALT SAT requirements would be required for that particular load thermocouple system and would need to be accounted for in the supplier’s internal pyrometry procedure.

ALT SAT Requirements

The ALT SAT requirements can be split up into a single main requirement and two sub-requirements which suppliers may choose to implement.  The main requirement is:

  • Calibration of instruments at the point at which the sensor is connected.

This means that, wherever the thermocouple is connected directly, instrument calibration must take place at this point. Let’s look at a vacuum furnace as an example.

Vacuum Furnace showing Location A and Location B for an Alternate SAT
Vacuum Furnace showing Location A and Location B for an Alternate SAT (photo courtesy: PVT Inc.)

Location A indicates where load thermocouples will be plugged in directly. Location B is where the extension wire from inside the furnace travels to the outside of the furnace and then on to the recording instruments. Location A is where the calibration of the recording instrument must take place per the ALT SAT requirements. This requirement in no way changes the standard requirements for instrument calibrations as they are presented in AMS2750E; it only specifies exactly where the instrument calibration must take place within the furnace sensor system. Your internal pyrometry procedure must state that this is a requirement.

The next paragraphs, 3.4.6.1.1 & 3.4.6.1.2, are where the supplier must read and understand both paragraphs in order to make a choice regarding which option best suits their furnace set-up and production. Let’s break both paragraphs down.

Option Number 1

3.4.6.1.1 - Establish appropriate calibration limits for sensors which when combined with the calibration of the instrument/lead wire and connector, will meet the SAT requirements of Table 6 or 7, as appropriate.

There are several ways to go about conforming to this paragraph. Keep in mind, that when choosing an option you are dealing with 2 variables; the error of the instrument which records the thermocouples in question and the error of the thermocouples themselves.

a) This option relieves you of one of the variables stated above. When calibrating your instruments which the thermocouples are plugged in to, ensure there is absolutely no error at all. Adjustments (offsets) may need to be made to accomplish this. This means that, if you do not permit offsets currently, you will either need to account for them in your procedures or choose option “b” below. Once you’ve established that your instrument has no error, you restrict the error of the thermocouples you purchase not to exceed the appropriate SAT difference stated in Table 6 or 7.

As an example, let’s assume you have a vacuum furnace that uses 2 load thermocouples which are single use only. The furnace is classified as a Type A, Class 2 furnace – this means the Maximum SAT difference is ±3°F or 0.3% of the reading.  You would ensure that the recording instrument for those 2 channels recording the load temperature have no error. Then, order load thermocouples which have an error of ±3°F or 0.3% of the reading, or less.

b) This option is most attractive to those who do not wish to allow offsets within their heat treat operation. To accomplish this, you compare the error of the specific channels of the instrument the thermocouples in question plug into, to the error of the thermocouples themselves. The resulting value cannot exceed the maximum error permitted for the appropriate furnace class. Internal pyrometry procedures specifically state how thermocouple wire will be received and the ALT SAT calculation accomplished prior to releasing the thermocouple wire to production. There are two variables that must be verified in this option. Anytime one of these two variables change, the calculation must be obtained. The Nadcap Pyrometry Reference Guide requires that this calculation be evaluated at the instrument (chart recorder) calibration points (min, max & middle 1/3rd.)

Overview of a Calculation – Single Temperature
Overview of a Calculation – Single Temperature

For Your Consideration

There has been some confusion in the industry that the ALT SAT process, specifically Option B above, must be accomplished at the furnace. This misunderstanding includes suppliers using a Field Test Instrument to simulate the min, max and middle 1/3rd of the instrument calibration temperatures in an effort to obtain the error of the instrument channels in question. This amounts to nothing more than an additional instrument calibration; one could simply obtain the error from the current instrument calibration instead of performing extra work at the furnace.

Option A and B above would be performed as a desk operation; none of the tasks would be performed at the actual furnace.

Conclusion

The ALT SAT process has been successfully implemented by many suppliers in the Aerospace Industry; both Nadcap approved and non-Nadcap. As with any AMS2750E process, detailed procedures and training are key to executing the ALT SAT process.

Submit Your Questions

Please feel free to submit your questions and I will answer appropriately in future articles. Submit your questions by sending an email to doug@heattreattoday.com.

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