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Table of Contents

Section Title Page

1. Purpose 2

2. Scope 2

3. Related Documents 2

4. Definitions 3

5. Temperature Elements 4

Table 1 Relative Merits of RTDs and Thermocouples 5 Table 2 Thermocouple Types 6 Table 3 Thermocouple Accuracy 8 Table 4 Thermocouple Cables 9 Table 5 Recommended Default U Dimensions for Welded and Threaded

26

Applications Outside Cold Boxes

Figure 1 RTD Wiring Options 11 Figure 2Typical Terminal Arrangement 15 Figure 3Thermowell Styles (Threaded Well) 16 Figure 4Threaded Well and Threadolet 18 Figure 5Threaded Well and Threaded Elbowlet 18 Figure 6DN20 Socket Weld Installation 19 Figure 7DN20 Socket Weld Elbow Installation 20

Figure 8Flanged Thermowell Installation 21 Figure 9A Threadolet Installation Dimensions 23 Figure 9B Weldolet Installation Dimensions 23 Figure 10Elbow Installation Nominal Dimensions 24 Figure 11Flanged Thermowell Installation 25 Figure 12Thermowell Extensions 27 Appendix A Fast Response Thermocouples for Reciprocating O2 Compressors 31

Authorization for this document is on file in the GEO Standards Department.

3PS15001, Rev. 3, Page 2 of 32 1. PURPOSE

1.1 This global engineering guideline provides background information and recommendations for the

selection and specification of temperature elements.

1.2This guideline is intended to encourage consistent and uniform good practice while meeting

performance and reliability requirements.

2. SCOPE

2.1 This guideline applies to temperature elements for both Air Products-owned and -operated plants

and for plants supplied to third parties.

2.2This guideline considers both RTD and thermocouple types of temperature elements. Other

temperature measurement methods (e.g., thermisters, optical) are excluded.

2.2.1This guideline specifically does not include details of very low temperature applications (liquid

hydrogen and liquid helium applications) for which silicon diode type temperature elements have

typically been specified.

2.3This guideline has been prepared specifically considering the requirements of:

?Air Separation Plants

?HyCO Plants

?Chemicals

The general principles described within this guideline may be applied to other plant types with

caution. Additional considerations may apply to other types of plants (e.g., ultra high purity plants). 2.3.1 This guideline is not intended for laboratory scale or other special applications.

2.3.2 Interested parties are invited to make proposals to broaden the scope of this document to cover

other processes at a future issue.

2.4 This guideline makes reference to existing Tier 4 Specifications, and provides some supporting

background information for these specifications.

3. RELATED DOCUMENTS

3.1 Air Products Engineering Documents

3MA27036 Mechanical Requirements for Distillation by Temperature Control for Packed and

Trayed Columns.

3PS00007 Selection and Installation of Transmitters

3PS00016 Material Selection for Control Valves and Instrumentation in Oxygen Service

3PS15002 Use of Thermowells in Pressurized Systems

?3PS45004 Oxygen Compressor Temperature Protection System

4WPS-TEMP01 Temperature Sensors and Thermowells for Cold Box Service

⊕4WPS-TEMP02 Resistance Type Temperature Detectors (RTDs) for Cryogenic and General Service 4WPS-TEMP03 Temperature Sensors and Thermowells for Oxygen Compressors - Fast

Response Type

4WPS-TEMP04 Temperature Sensors for Tray-Type Columns

4WPS-TEMP05 Temperature Sensors for Surface Type Measurements

4WPS-TEMP06 Temperature Sensors and Thermowells for General Service

4ECE-M25 Cold Box/Can Temperature Element Installation

APG Air Products PED Committee Guidelines (Intranet)

⊕Provided for background information only (not referenced in the text)

?To be published at a date subsequent to this issue date

3PS15001, Rev. 3, Page 3 of 32

3.2 American Society for Testing and Materials (ASTM)

E 230 Standard Specification and Temperature-Electromotive Force (EMF) Tables for

Standardized Thermocouples

⊕ E 608/E 608M Standard Specification for Mineral-Insulated, Metal-Sheathed Base-Metal

Thermocouples

⊕ E 1137 Standard Specification for Industrial Platinum Resistance Thermometers

3.3 Instrumentation, Systems, and Automation Society (ISA)

MC96.1 Temperature Measurement Thermocouples

3.3 International Electrotechnical Commission (IEC)

⊕IEC 60584-1 Thermocouples. Part 1: Reference Tables

IEC 60584-2 Thermocouples. Part 2: Tolerances

IEC 60751 Industrial Platinum Resistance Thermometer Sensors

3.4 British Standards (BS)

BS EN 60584-1 Thermocouples. Part 1: Reference Tables

⊕BS EN 60584-2 Thermocouples. Part 2: Tolerances

BS EN 60751 Industrial Platinum Resistance Thermometer Sensors

3.5 German Standards (DIN)

⊕DIN EN 60584-1 Thermocouples. Part 1: Reference Tables

⊕DIN EN 60584-2 Thermocouples. Part 2: Tolerances

3.6 National Institute of Standards and Technology (NIST)

⊕ITS-90 International Temperature Scale of 1990

4. DEFINITIONS

4.1 RTD means Resistance Temperature Detector (i.e., the actual sensing element in a PRT).

Operation is based on the principle of Resistance Thermometry, i.e., when a metal’s ele ctrical

resistance increases with increasing temperature. Metals that have shown a relative linearity and rangability for use in temperature measurement are Platinum and Nickel.

4.2 PRT means Platinum Resistance Thermometer (the whole assembly), i.e., a PRT uses a RTD. 4.3 Thermocouple Operation is based on the Seebeck Effect whereby a voltage is created along a

metal wire in the presence of a temperature gradient. A thermocouple uses a pair of dissimilar

metal wires joined at two points such that a net EMF is generated when the junctions are at

different temperatures. Standard combinations of dissimilar metals suitable for a range of

applications are given thermocouple type designators. See Table 2 for thermocouple types

currently used by Air Products.

4.4 Electromotive Force (EMF)– The term EMF is used throughout this document to describe the

voltages that are generated due to temperature gradients in thermocouple installation.

4.5 Cold Junction Compensation– The net EMF generated by a thermocouple installation depends

on the temperatures both at the measuring thermocouple junction (hot junction), and at the

reference junction (cold junction) [i.e., the junction between the thermocouple cable (or

compensating cable) and the copper cables or terminals] of the associated transmitter/control

system. Cold junction compensation is the process of compensating for errors that would

otherwise be introduced by changes in temperature at the reference junction. Historically, these

errors were eliminated by immersing the reference junction in an isothermal environment (e.g.,

melting ice). In contemporary industrial systems, the thermocouple cable (or compensating cable)

3PS15001, Rev. 3, Page 4 of 32 is connected directly to the associated transmitter/control system. The transmitter/control system compensates electronically for the actual temperature at the reference junction (i.e., at the

transmitter/control system’s terminals).

4.6The following dimensional definitions are used in this document:

ID dimension is the pipe internal diameter (which depends on pipe nominal bore size and wall

thickness or schedule). For example, see Figure 9A.

L Lagging extension length. See Figure 12 and paragraph 5.6.21.1.

N Nipple Extension length. See Figure 12 and paragraph 5.6.21.1.

OD dimension is the pipe external diameter (which depends on pipe nominal bore size only). For example, see Figure 9A.

s dimension is the setback from the pipe wall OD to the bottom of the fitting engagement. See

Figure 14 for an example.

T Thermowell Extension length. See Figure 12 and paragraph 5.6.21.1.

th dimension is the pipe wall thickness.

U dimension is the insertion length of the thermowell well measured from the tip of the well to the: ?Bottom of the thread for threaded wells

?Bottom of the weld section for welded wells

?Bottom of the flange for flanged wells

CAUTION: There is some variability in the letter codes used for dimensions both in other

Air Products standards and in industry. Care should be taken to clarify terminology when referring to dimensions by "letter."

5. TEMPERATURE ELEMENTS

5.1 Standards for Temperature Elements

5.1.1Section 3, above, illustrates that there are many national and international standards for

temperature elements.

5.1.2Air Products’ view is not to fa vor one standard over another (e.g., ASTM over IEC).

Consequently, temperature elements may be specified in accordance with any internationally

recognized standard.

5.1.3The European and IEC standards are typically identical to one another. However, there are some

significant differences between IEC and American Standards (e.g., claimed accuracies and

thermocouple cable colors).

5.1.4Consequently, it is necessary to specify thermocouples and compensating cable to a particular

standard to achieve consistent color coding. Particular standards are likely to be preferred for

certain projects to suit:

?Local conventions

?Client requirements

5.2 Element Type

5.2.1Because of the many types of temperature elements available, there may be several potential

solutions for any particular application.

3PS15001, Rev. 3, Page 5 of 32 5.2.2For Air Products to achieve benefits from standardization, this guideline recommends standard

element types for most applications.

5.2.3These standard element types should be specified whenever it is practical to do so. However,

other technically acceptable alternatives may be specified when there is an overriding reason to do so, such as:

?Modifications to an existing plant when a different element type is used as standard.

?When an existing product-based plant design is being repeated without re-engineering.

5.2.4 When no suitable precedent exists from which to select an element type, then information in the

following sections should be considered when making a selection.

5.2.5 The following table describes some of the relative merits of RTDs and thermocouples:

Table 1

Relative Merits of RTDs and Thermocouples

5.3 Thermocouples

5.3.1The following table describes the different standard thermocouple types currently in use at

Air Products:

Table 2

Thermocouple Types

Notes:

1. The above Element Temperature Limits refer to the limits of use for the thermocouple type

without taking into account construction issues such as wire diameter, insulation, etc., or

factors such as the anticipated environment or service life.

2. The ASTM E 230 recommended temperature range is based on:

?# 18 AWG dual thermocouple

?Sheath 0.25 inch OD magnesium oxide insulated

?Enclosed in thermowell

?Standard tolerance grade

?Inconel sheath for temperatures >870°C (1600°F). 316 SS sheath for lower

temperatures.

3. Other temperature ranges are available from alternative designs in accordance with ASTM.

5.3.2 A brief description of each of these thermocouple types follows:

5.3.3 Type K

5.3.3.1Low cost, base metal thermocouple. The traditional first choice of thermocouple for moderate

temperatures.

5.3.3.2Subject to thermoelectric instability with time and temperature:

?Long-term drift with exposure to high temperatures due to compositional changes caused by oxidation.

?Drift and failure at high temperatures due to "green rot" phenomenon, due to preferential oxidation of the chromium content within sealed sheaths.

?Temperature cycling hysteresis errors caused by magnetic and structural inhomogeneities.

?Errors caused by composition changes resulting from migration of the high vapor pressure elements (manganese and aluminum) from the negative to the positive wire.

5.3.3.3 Oxidation-induced errors are most prevalent above 800°C (1472°F). Temperature cycling errors

are prevalent in the range 250° to 600°C (482° to 1112°F).

5.3.3.4 Despite these disadvantages, Type K is still the most commonly used thermocouple type

throughout industry.

5.3.4 Type N

5.3.4.1Low cost, base metal thermocouple. Similar in price to Type K. A relatively new thermocouple

type (first standardized in 1986 in BS EN 60584.1 Part 8, and subsequently publicized in

IEC 584).

5.3.4.2Has the following advantages over Type K:

?Better oxidation resistance since the high levels of chromium and silicon in the Nicrosil conductor and of silicon and magnesium in the Nisil conductor act as a diffusion barrier.

?Temperature cyclic effects are reduced due to the high levels of chromium in the Nicrosil conductor and of silicon in the Nisil conductor.

?Migration effects are virtually eliminated since the conductors contain only traces of manganese and aluminum.

5.3.4.3 In summary, Type N is a much more stable alternative to Type K. It can also handle higher

temperatures than Type K, enabling it to be used as a low cost alternative to platinum-based

thermocouples in many applications, including all HyCO reformer high temperature applications.

5.3.5 Type S

5.3.5.1 Platinum metal thermocouple. In common with other platinum thermocouples:

?Suitable for very high temperature applications.

?More stable than base metal thermocouples.

?More expensive than base metal thermocouples.

?Generates a lower EMF than base metal thermocouples.

5.3.6 Type R

5.3.

6.1 Similar to Type S except:

?Suitable for even higher maximum temperatures.

?Improved stability.

?Slightly higher output.

5.3.

6.2 If a platinum metal thermocouple is required for a specific application, then Type R is generally

technically preferred over Type S.

5.3.7 Type T

5.3.7.1 Base metal thermocouple particularly well suited to cryogenic to warm temperatures. It is

unsuitable for higher temperatures since the copper arm would rapidly oxidize.

5.3.8 Thermocouple Accuracy

5.3.8.1 Suitable accuracy tolerance criteria for thermocouples are given in ASTM E230 and IEC 60584-2.

5.3.8.2 For example, tolerance data from IEC 60584-2 is as follows:

Thermocouple Accuracy

5.3.8.3 All thermocouple accuracies reflect the tolerance in the "as new" condition. Depending on the

thermocouple type, the thermocouple design, and the service conditions, the actual installed

accuracy will degrade with time to a greater or lesser extent.

5.3.8.4The cost difference between the different tolerances is generally very small. Consequently it is

recommended practice to specify Premium (ASTM) or Class 1 (IEC) as standard.

5.3.8.5 Thermocouple outputs are non-linear with temperature. Linearization of the temperature signal is

carried out within the associated transmitter or control system by using one of more complex

polynomial expression in order to match the published reference tables (e.g., IEC 584.1).

5.3.9 Thermocouple Extension and Compensating Cables

5.3.9.1 To minimize errors caused by unwanted EMFs generated in the cabling, thermocouple extension

or thermocouple compensating cable must be used between the thermocouple and transmitter or

control system.

5.3.9.2 Thermocouple extension cable has the same nominal composition as the thermocouple itself,

whereas thermocouple compensating cable is manufactured from different material which has similar thermoelectric characteristics to the associated thermocouple over a limited temperature range.

Thermocouple Cables

Note:

For the cable types, described in the above table:

?Suffix "X" denotes extension cable.

?Suffix "CA", "CB", or "C" denotes compensating cable.

5.3.9.3The use of shielded compensating cable is always recommended due to the low EMFs generated

and measured. The shield should be suitably grounded.

5.3.9.4Compensating cable is available in Class 1 and Class 2 grades (in accordance with both IEC

584.3 and ISA MC 96.1). For best accuracy, Class 1 compensating or extension cable should be specified.

5.3.9.5Data on temperature limits and associated tolerances for each cable type is available in IEC

584.3 and ISA MC 96.1. There is some variation in the tolerance figures between these

standards.

5.3.9.6For intrinsically safe circuits, the IEC color coding recommends using a blue sheath in place of

the colors listed above.

5.3.9.7There are several other country-specific color codes. For example:

?British to BS 1843

?German to DIN 43714

?French to NFC 42324

?Japanese to JIS C 1610-1981

3PS15001, Rev. 3, Page 10 of 32 ?Special high temperature cables should be specified when ambient temperatures may exceed the rating of normal cable (typical PVC-insulated cables are typically rated ~70°C

(~160°F), with high temperature versions rated ~90°C+ (~190°F+). Short sections of PTFE-

insulated cables on mineral-insulated cables (MIC) may be considered for particular hot

spots (e.g., within the penthouse section of HyCO reformer plants). Failure to address this

issue can result in reduced plant reliability as the insulation of the heat stressed cable

insulation can become brittle and fail during operation.

5.3.9.8The use of plain copper cable instead of extension or compensating cables is not

recommended since it introduces unnecessary measurement errors. The normal current

practice of carrying out the cold junction compensation inside the control system or transmitter

is based on the premise that extension or compensating cables are used. Changing to copper

cables remote from the control system or transmitter effectively moves the cold junction to the

point at which the cable material changes. Any temperature difference between the control

system or transmitter and the cold junction will result in a corresponding temperature

measurement error.

5.3.9.9 The use of plain copper terminals inside junction boxes or marshalling panels instead of

terminals made from thermocouple material is recommended for normal industrial

applications, a lthough terminals made from compensating materials are available for base

metal thermocouples. Reasons:

?Such terminals are less readily available and are more expensive than copper terminals.

?The error associated with using copper terminals can normally be neglected. Each end of the copper terminal introduces a thermocouple junction into the circuit. Provided that any

temperature gradient across the terminal is insignificant, the EMF generated by the pair of

junctions will cancel out, leaving no net effect.

5.3.10 Grounded/Ungrounded Tips

5.3.10.1Thermocouples are globally available with either grounded on ungrounded tips.

5.3.10.2Most common practice in U.S. is to use grounded tips, whereas in Europe tips are nearly

always ungrounded.

5.3.10.3It is recommended that unless technical considerations dictate otherwise, local practice be

followed in selecting either grounded or ungrounded tips.

5.3.10.4For reference, here are some technical considerations:

?Grounded tips are in intimate thermal contact with the sheath offering a faster response for the bare element. However, this makes little difference in practice since speed of response

is normally dominated by the heat transfer lag through the thermowell.

?Grounded tips could result in earth loops, voltage pickup, etc., resulting in measurement errors. This would only occur in practice if the associated transmitter or control system input

card did not provide galvanic isolation.

?Grounded tips are not permitted for IS loops in Europe, where 500V isolation is required in the field.

?Grounded tip element wiring requires the shield wire to be connected to ground in the element head (and isolated elsewhere).

5.4 Resistance Temperature Detectors (RTDs)

5.4.1Although a variety of RTD basic types are available, Air Products (and many others) have

standardized on the 100Ω Platinum Resistance Thermometer (PT100) type of RTD.

5.4.2Platinum is selected due to its characteristics of:

?Wide unreactive temperature range.

?Relatively high resistivity (factor of 6 greater than copper).

?Simple resistance vs. temperature characteristic.

3PS15001, Rev. 3, Page 11 of 32

5.4.3 Although the cost of platinum is high, the quantities used are sufficiently small that the platinum cost is an insignificant part of the cost of the complete assembly.

5.4.4

Commercially available PT100 RTDs use platinum that is doped with another metal to protect against contamination, and have a R (resistance value) of 100Ω at 0°C (32°F) , and a fundamental interval (R100 – R0) of 38.5Ω.

5.4.5

Number of Wires

5.4.5.1 Measuring the temperature at a RTD is only possible by passing a current through the RTD, and measuring the voltage dropped across the device.

5.4.5.2 To accurately measure the resistance of the RTD, it is necessary to compensate for the

resistance of the cabling between the RTD and the measuring device, which could be quite a long way away.

5.4.5.3

This can be achieved by wiring as a 2, 3, or 4 wire system.

Figure 1

RTD Wiring Options

5.4.5.4

2 wire – This method makes no attempt to compensate for cable resistance. Consequently, this is only suitable for very short cable runs. Although the associated error would be very small for installations using head-mounted transmitters, this method is rarely, if ever, used for industrial installations.

5.4.5.5

3 wire – With this method, the current flows through two of the three wires. The third wire is used for volt drop compensation. The voltage difference between the 3 wires is measured, and the volt drop at the RTD is calculated assuming that the volt drop in the two current-carrying wires is identical. This assumption is a reasonable one for most installations. However, measurement errors will arise in the event of cable connections that have a significant resistance, e.g., due to poor installation or corrosion. This method is Air Products’ default choice for wiring RTDs and has given good service over many years in many different environments.

Note: 3 wire RTDs are color coded with 2 red wires and 1 white wire. The 2 red wires are connected to the same side of the RTD.

5.4.5.6

4 wire – By completely separating the current-carrying wires from the voltage measuring wires, this method compensates for any possible mismatch in the resistance of the cabling system. This method is potentially the most accurate and reliable method and is specified by some clients. Duplex 4 wire RTD systems are impractical because of:

I

I

I I

I

I 2 wire

3 wire

4 wire

3PS15001, Rev. 3, Page 12 of 32 ?Reliability/ruggedness problems compared with other types, due to manufacturing difficulties.

?Being able to terminate all the wires in the head.

Note: 4 wire RTDs are typically color coded with 2 red wires and 2 white wires. The 2 red

wires are connected to one side of the RTD. The 2 white wires are connected to the other side.

5.4.6 RTD Accuracy

5.4.

6.1Suitable accuracy tolerance criteria for RTDs are given in IEC 60751 (BS EN 60751). Two

tolerance classes are offered as follows:

?class A: ±(0.15 + 0.002 |t|)°C

?class B: ±(0.30 + 0.005 |t|)°C

Where t is the actual temperature, in °C, of the platinum elements

Notes:

Class A applies only up to 650°C (1200°F) and does not apply to 2 wire devices.

The cost difference between Class A and B is generally very small. Consequently, it is

recommended practice to specify Class A as standard.

5.4.

6.2 RTD outputs are slightly non-linear with temperature. Linearization of the temperature signal is

typically carried out within the associated transmitter or control system by using equations of

the form:

For t > 0°C R t = R0 (1 + At + Bt2)

For t < 0°C R t = R0 (1 + At + Bt2 + Ct3 (t-100))

where (according to IEC 751):

A = 3.90803 x 10-3

B = -5.775 x 10-7

C = -4.183 x 10-12

5.4.7 Construction Methods

5.4.7.1Industrial RTDs are available manufactured by wire wound and film deposition technique.

5.4.7.2Air Products has a technical preference for wire wound PRTs for most applications, although

film PRTs are suitable for certain applications (e.g., ambient air temperature measurement). 5.4.8 Wire wound PRTs

5.4.8.1Modern designs of industrial wire wound RTDs can achieve good vibration resistance and high

stability. This is achieved by features such as:

?Use of metal doped platinum wire for robustness and contamination resistance.

?Winding and support techniques to fully support the wires while allowing reasonable freedom to expand and contract.

?Ceramic material has similar expansion characteristics to the platinum wire.

5.4.9 Thick and Thin Film PRTs

5.4.9.1Film PRTs are manufactured by depositing the platinum material on a suitable substrate.

5.4.9.2Thin film devices use vacuum semiconductor techniques to deposit the platinum whereas thick

film devices use screen-printing techniques.

3PS15001, Rev. 3, Page 13 of 32 5.4.9.3Film PRTs have advantages over wire-wound PRTs in areas of:

?Cost

?Fast thermal response due to low mass

?Vibration resistance

5.4.9.4However they have the following disadvantages:

?Less stable due to smaller amounts of platinum used

?Greater variability in characteristics

5.4.10 Element Type Selection Guide for Specific Applications

5.4.10.1 Cryogenic Applications (e.g., ASU, HyCO Cold Boxes) – Historically, both Type T

thermocouples and RTDs have been used for cryogenic applications. Since 1996, Air Products

has standardized on the use of RTDs for this service, since their accuracy is better than

thermocouples. Having selected RTDs for the cryogenic parts of the plant, RTDs become the

temperature element of choice throughout the plant (except for those few applications where

temperatures are too hot for RTDs, such as the Flare on HyCO Cold Boxes). There is no

technical reason why Type T thermocouples could not be used for cryogenic temperatures on a

specific project (e.g., to meet a client’s specification), provided that the associated reduced

accuracy is technically acceptable.

5.4.10.2 High Temperature Applications (e.g., HyCO Steam Methane Reformers) – The high

temperature sections of these plants are too hot for RTDs. Rather than mixing RTDs and

thermocouples, thermocouples are typically used throughout the plant.

Note:An optional exception to this would be to use RTDs for paymeters in order to give best

possible accuracy.

5.4.10.3Historically, a variety of thermocouples have been selected. For example:

?Type K for moderate temperatures

?Type K for high temperatures (on small reformers)

?Types R and S for high temperatures (on large reformers in Europe)

?Platinell II for high temperatures (on large reformers in US)

Literature searches indicate that Type K is unsuitable for high temperature applications. Plant

operating experience also confirms a history of measurement errors and failures, sometimes

after only a few month's operation at high temperature. Types R and S are technically suitable

for high temperatures – significantly higher than the highest temperatures measured on HyCO

plants. Their disadvantage is the cost of the element and associated cabling.

5.4.10.4Platinell II offers similar performance to Types R and S with the following disadvantages:

?More expensive than Types R and S, plus the additional cost of exotic compensating cable.

?Unknown outside the U.S.

?Requires special transmitters, since its EMF curve is not well known.

Platinell II has the specific advantage of being suitable for bare thermocouple applications in H2 atmospheres. However, this technical advantage is not usually relevant, even on H2 plants,

since the thermocouple is protected from the process fluids by its thermowell and sheath.

5.4.10.5Consequently, after some testing at the Martinez HyCO plant in 2003, a global decision was

made to standardize on Type N thermocouples for high temperatures. Type N gives the

following advantages:

?Low cost compared with platinum metal thermocouples (comparable to Type K).

?Suitable for the maximum temperatures expected on HyCO reformers.

?Globally available.

3PS15001, Rev. 3, Page 14 of 32 The HyCO PST’s present policy for reformer plants is to specify Type N thermocouples for high

temperature service and to retain the use of Type K thermocouples for lower temperature

applications.

This policy is consistent with the convention adopted on the Westlake Project (2004) where

Type N thermocouples were successfully used for high temperature service, typically >800o C

(1470o F), and Type K thermocouples used for lower temperature applications.

5.5 Element Construction Considerations

5.5.1 Sheath

5.5.1.1The most popular element protection style is the mineral insulated, metal sheathed type. This

consists of a seamless metal sheath enclosing compacted mineral insulant powder which

supports and electrically insulates the temperature element and wiring.

5.5.1.2The default standard sheath material for temperature elements is 316 stainless steel. The

manufacturer’s default materials are normally acceptable. For temperatures in excess of 800°C

(1470°F), the use of an Inconel sheath is recommended.

Note:Stainless steel sheaths must not be used with Inconel (or other high temperature alloy)

wells. Differential expansion of stainless sheaths inside Inconel wells have resulted in elements

becoming stuck at high temperatures, making it impossible to replace a failed element without

shutting down the process.

5.5.1.3The most popular sheath size is 1/4" (0.25") OD.

5.5.1.4The sheath’s OD must be a snug fit to the thermowell’s ID to ensure good thermal co ntact and

to minimize risk of damage in vibrating service.

5.5.1.5For example, 4WPS-TEMP01 and 4WPS-TEMP06 define the sheath and bore diameters as

0.25 and 0.26 inches respectively. Consequently, purchasing in accordance with detailed

specifications such as 4WPS-TEMP01 helps to avoid ambiguity, and gives a better chance that

spares can used across a wider range.

Note: 6 mm OD sheaths are in common use in Europe. A nominal 1/4" sheathed element will

not fit inside a nominal 6 mm ID well. Similarly a nominal 6 mm sheathed element will be a

loose fit inside a well designed for a 1/4" element. Both 1/4" and 6 mm sheaths are available in

Europe, whereas 6 mm is not readily available in U.S. Consequently, 1/4" sheaths are generally preferred for reasons of standardization across Air Products, although 6 mm sheaths remain

acceptable in order to meet local requirements.

5.5.2 Mineral Insulation Material

5.5.2.1Magnesium oxide mineral insulation is used in >90% of all metal sheathed temperature

elements manufactured today. Most of the balance use aluminum oxide.

5.5.2.2As an insulant, magnesium oxide has the disadvantage of being hygroscopic. Consequently,

any contamination by moisture, whether by poor manufacturing techniques or by damage to the sheath or seal during operation, can result in reduced element life due to the consequential:

?Reduced insulation performance.

?Increased migration effects (affecting thermocouples).

?Increased corrosion of the element wires.

5.5.2.3Non-hygroscopic insulation materials, e.g., MI-dry TM have recently been developed which claim

to improve thermocouple stability. However, such materials are not currently in common use.

Current normal best practice at Air Products is to accept MgO insulation, aiming to achieve

acceptable element life by specifying:

?Reputable manufacturers with a good track record.

?An appropriate element type, sheath, and well material for the service conditions.

3PS15001, Rev. 3, Page 15 of 32 5.5.3 Wires

5.5.3.1These are the wires connecting the sensing element to the terminals in the head. Thicker

gauge wires tend to be more reliable than thin wires at very high temperatures.

5.5.3.2The lead wires coming out of the sheath at the head need to be sealed in a potting compound

to seal and protect the insulation material inside the sheath. The potting compound must be

suitable for the most extreme temperatures that the temperature element’s head and terminals

may be exposed to during operation.

5.5.4 Terminals

5.5.4.1The element is normally supplied complete with a spring-loaded terminal block. An installed

spring compression of 1/2" is typically recommended.

Figure 2

Typical Terminal Arrangement

5.5.4.2The terminal block is spring loaded in order that the tip makes firm contact with the bottom of

the well.

5.6 Thermowells for Temperature Elements

5.6.1Thermowells are required for all pressurized systems. Alternative practices such as using a

compression fitting to seal a temperature element that is inserted directly into a pressurized

system are not permitted without approval of the Technology Manager Measurement & Control

Hardware or his designee.

5.6.2Two examples where wells are not required:

?Surface-mounted applications where the element is not exposed to process pressure (see 4WPS-TEMP05).

?Very fast response applications in accordance with 4WPS-TEMP03 (where the design has already been considered and approved).

5.6.3It is recommended that the element and well be shipped assembled. This can be made part of

the specification or purchase order. By having them shipped assembled, any errors in

component lengths or diameters should be identified before shipment.

5.6.4 Wake Frequency Calculations

5.6.4.1Wake Frequency Calculations or "Murdoch Calculations" calculate the resonant frequency of

the thermowell and compare it with the wake frequency of the vortices created as the process

fluid flows past the thermowell. In order to pass the test, in the worst case conditions (at the

highest velocity), the wake frequency must not be more than typically 80% o f the well’s

resonant frequency.

3PS15001, Rev. 3, Page 16 of 32 5.6.4.2These calculations were developed in response to a number of vortex-induced vibration failures

in industry.

5.6.4.3These calculations can be performed by the well manufacturer on demand. Air Products’ policy

is to request calculations for novel applications, and for those applications where wake

frequency induced vibration may be an issue.

5.6.4.4Indications for wake frequency calculations include:

?Long wells with relatively small diameters – reducing the resonant frequency.

?High temperatures - reducing the well’s stiffness and hence the resonant frequency.

?High velocities – increasing the wake frequency.

5.6.4.5If a problem with wake frequency is identified, the thermowell design must be changed to

increase the resonant frequency, e.g.:

?Reduce length

?Increase diameter

?Use tapered rather than parallel design well

?Use velocity collars (for flanged wells)

5.6.4.6Historical practice at Air Products has shown that wells with U dimension less than or equal to

6" in ASU services do not generally need calculations. Instances where calculations have

indicated potential problems have included flanged wells, wells with insertion length >6", and

high temperature applications.

5.6.5 Thermowell Styles – Thermowells are generally either parallel, tapered, or stepped. See

Figure 3.

Figure 3

Thermowell Styles (Threaded Well)

Parallel Tapered Stepped

5.6.5.1 Parallel Wells – The simplest well design is a parallel well, where the thermowell wall thickness

in unchanged througho ut the well’s length.Parallel wells are most suitable for thermowells with

relatively small process connections (e.g., 3/4" NPT or DN 20), where a stepped or tapered well could result in insufficient wall thickness at the well tip.

3PS15001, Rev. 3, Page 17 of 32 5.6.5.2Tapered Wells –For tapered wells, the thermowell’s wall thickness is reduce d along the length

of the well. This design results in:

?Improved thermal response due to reduced wall thickness at the tip.

?Increased natural frequency of oscillation due to the combination of maximum wall thickness (stiffness) at the base of the well and reduced mass at the tip. See Murdock Calculations

(below).

Note: If the process connections are small, this design is less suitable than parallel wells for

high pressures and temperatures due to the reduced thickness at the tip.

5.6.5.3 Stepped Wells – Stepped wells are similar to parallel wells except that the tip area has a

reduced diameter for optimal thermal response. The reduced wall thickness at the tip area

results in reduced pressure temperature rating compared with an equivalent parallel well. The

length of the reduced section must be long enough to ensure that the active part of the sensing

element is fully within the reduced section. This is unlikely to be an issue for a thermocouple,

which is effectively a point sensor. However, RTDs are active over the entire length of the

resistance element.

5.6.6 Thermowell Process Connections may be threaded, welded, or flanged.

5.6.7 Threaded Wells –Air Products’ default size for thermowell threads is DN20 (NPS 3/4). This

results in a typical maximum well OD of 19 mm (nominal 3/4"), which, for a tapered well, would

typically taper to 13 mm (nominal 1/2"). Air Products piping specifications typically discourage

the use of threaded thermowells on flammable and toxic services due to the difficulty of

ensuring a leak-tight seal with large [DN15 (NPS 1/2)] threads.

5.6.7.1There have been rare occasions when threaded thermowell installations have failed, resulting in

the well being ejected from the pipework under pressure:

?Threadolet fittings (purchased by Piping) although specified correctly did not have the correct DN20 (NPS 3/4) thread.

?The threaded pipework connection corroded over time, and failed during a 340 bar g (~5000 psig) pressure test.

5.6.7.2For reasons such as the above, certain clients do not accept threaded thermowells. Air Products’

current policy allows threaded wells only for temperature elements such as T/C and RTDs that

can be removed from their wells via bayonette type assemblies. Anytime an element can only be

removed by unscrewing a compression-type fitting, such as dial thermometers and filled bulb-type systems, tack welding or seal welding of the thermowell to the piping connection is required. This

is done to prevent accidental loosening of the process connection while only trying to remove the

element. Refer to 3PS15002 for additional requirements and more background on this subject.

Figure 4

Threaded Well and Threadolet

5.6.7.3For DN50 (NPS 2) pipes, threaded wells may also be installed in an elbow. This can eliminate

having to swage up to a larger diameter pipe to install the thermowell.

Figure 5

Threaded Well and Threaded Elbowlet

5.6.7.4Larger diameter screwed wells may be technically required for specific applications. However,

these are rarely specified by Air Products. See section 5.6.11 for PED considerations for

thermowells for applications in Europe.

5.6.8 Welded Wells are Air Products’ well of choice for cryogenic, and for toxic and flammable

services.

5.6.8.1Air Products’ default connection sizes for welded wells are:

?DN15 (1/2") for inside cold boxes (air plants and hazardous plants).

?DN20 (3/4") for applications outside cold boxes (whether cryogenic, warm or hot).

5.6.8.2The smaller DN15 size is considered mechanically adequate for inside cold boxes because of

the mechanical protection afforded by the cold box structure and packing.

Note:The weld section diameters for DN 15 and DN20 are 21.3 mm and 26.7 mm

respectively (matching the nominal OD of 1/2" and 3/4" piping).

5.6.8.3DN20 size wells are normally installed using a sockolet fitting as follows:

Figure 6

DN20 Socket Weld Installation

Note:Figure 6 identifies a radius (typically ~1/16") at the base of the well’s weld section. The

radius avoids having a sharp transition at this point which could concentrate stresses in the

event of vibration, etc.

5.6.8.4Figure 6 shows a DN20 well installed in a sockolet fitting. Note: T he well’s diameter must be

constrained to fit within the sockolet’s ID. The sockolet’s ID nominally should match the

schedule of the pipework. However, sockolets are not specifically designed for thermowells and are not manufactured to close tolerances.

5.6.8.5 A typical maximum well OD of 19 mm (3/4 in) should fit a Class 3000 ("Std Wall Thickness"

Bonney Forge) sockolet [e.g., a typical tapered well may have a 19 mm (3/4 in) OD tapering to

13 mm (1/2 in) at the tip]. Engineers are advised to talk to the Piping group to ensure suitable

sockolet dimensions.

5.6.8.6When Class 6000 sockolets are called up on the pipe specification, a DN20 sockolet’s ID will be

too small for this thermowell design. This occurs on a relatively small number of high pressure

pipe specs (e.g., WCG100). A non-standard approach is required for these wells. Either:

?Specify a smaller well OD to suit the reduced bore of the sockolet.

?Agree with Piping to increase the sockolet size to accommodate a non-standard well design.

3PS15001, Rev. 3, Page 20 of 32 5.6.8.7For installation in an elbow, a socket welded elbowlet is always used as in Figure 7:

Figure 7

DN20 Socket Weld Elbow Installation

5.6.8.8 For many years, Air Products has installed thermowells inside cold boxes without the use of

sockolets; however, recent inspections at cold box contractor shops have shown an inconsistent ability to obtain the full penetration weld required by the piping code. The use of backing rings are required but are very difficult to install in small and medium bore pipe. Therefore, the use of

sockolets is required for all coldbox installations. Direct welding is no longer permitted.

5.6.8.9Larger diameter welded wells may be technically required for specific applications. However,

these are rarely specified by Air Products. See section 5.6.11 for PED considerations for

thermowells for applications in Europe.

5.6.9Flanged Wells

5.6.9.1Flanged thermowells should be considered for the following applications:

?Corrosive or erosive service when the thermowell may need to be removed for inspection and possible replacement during the life of the plant.

?Very high temperature applications, when the well’s life may be less than the plant’s expected life.

?Certain high temperature applications when the use of flanged wells may avoid the need for special weld procedures for unusual combinations of dissimilar metals.

5.6.9.2 Flanged wells invariably result in longer element lengths than screwed or welded wells, due to the

offset between the interface piping flange and the process line.

5.6.9.3 As a result, flanged wells are more prone to wake frequency vibration problems than screwed or

welded wells. Thermowells flanged at 25 mm (1 in) are particularly prone to wake frequency

problems since they tend to be relatively long and thin.

5.6.9.4 Consequently, tapered wells with a minimum flange size of 37 mm (1.5 in) are normally

recommended for flanged thermowells.

5.6.9.5 For projects in Europe, flanged wells with process connections larger than 25 mm (1 in) must

comply with PED requirements. See section 5.6.11 for details.