3DG-J35G-00002Thermowells for Power Project Test

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BECHTEL CORPORATION

POWER GLOBAL BUSINESS UNIT – CONTROL SYSTEMS ENGINEERING

ENGINEERING DESIGN GUIDETHERMOWELLS FOR POWER PROJECTReason for Revision: Issued for Incorporation of Lessons Learned

Prepared by:Checked by:Approved by:

INTRODUCTION

This design guide focuses on formulating a consistent approach to the selection anddesign of thermowells for power plant applications. Lessons learned and improvements in

thermowell design from past projects have been considered in the preparation of thisdesign guide. Thermowells that are not supplied by Bechtel, or are supplied as a part of mechanical supplier’s equipment, are generally not covered by this document. This guidediscusses both the selection and installation of the thermowell.

Additionally, the specification of isokinetic (constant velocity) sampling nozzles iscovered in this guide. Isokinetic sampling is a condition wherein the velocity of thesample entering the port or the ports of the sample nozzles is at the same velocity asthe velocity of the fluid in the main process pipe. This ensures the most representativesample is obtained for chemical analysis. These nozzles are very similar mechanicallyto a thermowell, and are typically procured as part of the thermowell purchase.



Electronic documents, once printed, are uncontrolled and may become outdated.Refer to the electronic documents in BecRef for current revisions.

Bechtel Confidential © Copyright Bechtel Corporation, 2002, 2007. All rights reserved.

3DG-J35G-00002-001 Page 2 of 20

TABLE OF CONTENTS

INTRODUCTION 1

1.0 LIST OF FIGURES 3

2.0 PURPOSE 4

3.0 CODES AND STANDARDS 4

4.0 APPLICATION 4

5.0 STANDARD THERMOWELLS 4

6.0 ISOKINETIC NOZZLES 5

6.1 SAMPLE CALCULATION 5

7.0 INSTALLATION DETAILS 7

8.0 STRESS ANALYSIS OF THERMOWELLS 7

9.0 MEASUREMENT CONSIDERATIONS 7

10.0 INSULATION 8

11.0 OTHER REQUIREMENTS 8

12.0 THERMOWELL FABRICATION NOTES 8

13.0 THERMOWELL MATERIAL SELECTION 9

14.0 REFERENCES 9



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1.0 LIST OF FIGURES

Figure 1 1” Threaded Thermowell Type T

Figure 2 1-1/4” Threaded Thermowell Type T

Figure 3 Machine Threaded Thermowell Type MT

Figure 4 Flanged Thermowell Type F

Figure 5 Weld-in Isokinetic Nozzle

Figure 6 MT Type Thermowell Installation

Figure 7 Thermowell Fittings Fabrication Details

Figure 8 T Type Thermowell Installation Detail

Figure 9 Thermowell Installation for 3” and Smaller Lines

Figure 10 Thermowell Installation for 2” and Smaller Lines

Figure 11 Typical Work Process Flow Diagram for Thermowells







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6.0 ISOKINETIC NOZZLES

Two types of isokinetic steam sample nozzles are recommended for Bechtel designedpiping. One is “MT” Type for line wall thicknesses more than 3/4-inch and the other isWeld-In type for line wall thicknesses equal to or less than 3/4-inch (refer to Figure 5).

Either type may be configured as multi-port isokinetic nozzles. These nozzles are to beinstalled at locations where the velocity profile across the pipe can be determined. Portsshall be located such that each port samples from an equal fraction of the cross-sectional area of the pipe being sampled. As the steam velocity varies across the pipesection, each port diameter must be sized to result in isokinetic sampling with theproper fraction of sample collected from each port. Sample ports shall be drilled cleanlyusing the standard drill size nearest to the calculated standard port diameter. The portinlet ends shall not be chamfered or rounded and the outlet ends shall be free of burrs.The smallest recommended port diameter is 1/8-inch. Port diameters less than this arenot recommended as they may be subject to plugging. Total port area shall bedetermined to maintain isokinetic sampling in the nozzle ports at the desired samplingrate. Following calculation per ASTM D1066 may be followed:

For determining port diameter, d = (a/0.7854N)1/2

For determining nozzle bore, b = (3a/1.5708)1/2

For determining Radii of port circles, r 1 = (D1/2) x (1/N)1/2

r 2 = (D1/2) x (3/N)1/2

r 3 = (D1/2) x (5/N)1/2

Where:

D1 = Pipe IDN = Total number of portsa = Total port area = Af/FA = Traverse area of Pipe = 0.7854 D1

2

F = Flow rate of fluid through pipef = Flow rate of total sample extracted

The number of sampling ports shall be four (4) for pipe ID 2 inches to 6 inches. Thenumber of sampling ports may be more for line sizes above 6 inches.

6.1 SAMPLE CALCULATION

Attachment on the following page is an example for determining the nozzle hole size for isokinetic samples. Note it is more accurate to use the center of equal area annuli to

determine port position.





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STEAM SAMPLE NOZZLE

Sample nozzle hole size for isokinetic flow

Pipe inside diameter inches 12Pipe area square inches 113.097

number of annuli needed 6

area /# of annuli + CC 16.157

Center circle "CC" diam. ins 4.536

Center of

annuli

Distance

ID to

center of

annuli

Annulus

OD ins

Total

annulus

area in^2

Fraction of

total area

in^2

Annulus

number

includes

circle

center next annuli 5.475 3.2625391 6.414 80.784 0.714



center next annuli 9.606 1.1967504 10.142 32.314 0.286center next annuli 10.626 0.6870769 11.110 16.157 0.143


Steam flow rate lbs/hr 83460

Pressure psia 82

Temperature °F 513

Specific Volume Ft^3/lb 6.9428083

Velocity feet per second 204.93738

Sample flow cc/min. 500Sample flow rate lbs/hr 65.956739

Insertion depth of nozzle 4.5

Number of sample nozzles 2

Pipe diam. at sample nozzle tip 3

By inspection # of holes 5

By inspection fraction of area 0.714

Considered steam flow lbs/hr 59590.44

Ratio steam flow to sample 903.47766

Covered area of pipe sq ins 80.751498

Area of each sample hole 0.0089379

Diameter of each sample hole 0.1066772

By inspection drill size #36 (.1065)

Mimimum nozzle center bore ins 0.337343





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7.0 INSTALLATION DETAILS

Refer to Figures 6 through 10 for installation of standard thermowells and isokineticnozzles. It is recommended that Type “MT” thermowells and isokinetic nozzles shouldbe installed in the pipe fabricator’s shop. Weld-in type isokinetic nozzles may be

installed in the field using half couplings. During installation of isokinetic nozzles, caremust be taken to ensure the ports face upstream. For installation details on “MT” and“T” type thermowells, refer to Figures 6 through 8. Figure 7 shows fabrication details for machine thread and NPT type thermowell fittings. For installation on line sizes 3-inchand smaller, install an expander to increase the line size to 4 inches as shown in Figure9 or install a tee at a 90 degree pipe bend as shown in Figure 10.

The location and installation details for thermowells and nozzles reside in Plant Designdocuments. Typical installations have been depicted in this guide for convenience.When feasible, the thermowells may be shipped directly to the piping fabricator. Shopinstallation by the fabricator is often more economical than field installation.

8.0 STRESS ANALYSIS OF THERMOWELLS

All thermowells must be analyzed and found to meet acceptable stress criteria prior tomanufacture. Current practice is to have the vendor provide the calculation of record.In-house assessment is also possible, normally using either the Brock or ASME PTC-19.3 methods. In-house verification of suspect applications is recommended to avoiditerating the design with the vendor, and to preclude later accidental installation of anunacceptable well.

While vendors routinely perform vibration analysis (vortex shedding frequency), thepressure boundary verification is sometimes omitted. For high pressure applications

(typical of supercritical plants), the pressure limit of the well must be verified. Confirmthe vendor includes this calculation prior to award.

Caution must be exercised regarding the location of thermowells. Normally, theaverage fluid velocity is entered into the data sheet, which is adequate for mostinstallations. However, vibration induced fatigue failures have occurred where the wellshave been located in a region of locally high velocity flows. For example, the well mustnot be placed downstream of a butterfly valve, where the fluid stream past the disc hasa higher than the average velocity (upon which the stress calculation is based).

9.0 MEASUREMENT CONSIDERATIONS

Bechtel experience has been generally successful using a minimum 3-inch immersionfrom pipe inner surface. PTC 19.3 provides equations for estimating measurementerror due to well conduction (“wall effect”), thermal radiation, low fluid velocity and





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aerodynamic heating. These effects usually require consideration only for unusualapplications.

10.0 INSULATION

Insulation thickness is project specific and is followed once released for project.Typically, the thermowell is dimensioned to extend about one inch beyond theinsulation.

11.0 OTHER REQUIREMENTS

Refer to the attached flow chart (Figure 11), which explains the typical work processfrom thermowell selection to preparation of data sheet and the requisition process.Refer to GBU Design Guide 3DG-J21-G0008 “User Instructions for Instrument DataSheets” for completion of data sheets.

12.0 THERMOWELL FABRICATION NOTES

Thermowell dimension and well material shall be specified on data sheet and surfacefinish shall be manufacturer’s standard. Tolerances are as follows:

Bore, “d” shall be 0.26 inch maximum to pass plug gauge of 0.254 inch OD.

Bore /OD eccentricity shall be +/- 10 percent of minimum wall thickness.

Bore depth shall be +/- 1/32 inch for depths to 30 inches.

Overall length, “L” shall be +/- 1/32 inch for lengths to 30 inches.

Insertion Length, “U” shall be +/- 1/16 inch for lengths to 12 inches and +/- 1/8 inch for lengths over 12 inches.

Head diameter shall be +/- 0.01 inch

Shank diameter shall be +/- 0.01 inch



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Tip diameter shall be +/- 1/32 inch

Tip thickness +/- 1/32 inch

Thermowells shall have the tag number and material on the well head. For testthermowells, provide 1/2-inch NPT brass plug with brass chain attached to head of wellby non-welding technique. Bore drilling may be performed either by twist drill or gundrill.

13.0 THERMOWELL MATERIAL SELECTION

Refer to Thermowell Fitting Material Selection Chart in Plant Design Standard 3DS-P72G-00009. The thermowell shall be of the same material as the fittings provided onthe pipe (e.g. threaded half coupling).

14.0 REFERENCES

1. Standard Practice for Sampling Steam ASTM D1066 – 97 (Re-approved 2001)



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1" THREADED THERMOWELL

TYPE T

1.375"

HEX

1/2" NPT

OR

NPSM

1" NPT

1" DIA.

SHANK

BORE= .260"

3/4" DIA.

1/4"

INSERTION

LENGTH= U

OVERALL

LENGTH= L

LAGGING

EXTENSION= T

3/4" WRENCH

FLATS

1"

Overall Length= U + T + 1.75"

FIGURE 1



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1/4"1"

DIA.

U = 4 1/2 "

T=8"

1/2" SCH 160 x 6-1/2"END TO END

NIPPLE OF SAME MATERIAL OR

MACHINE END DOWN TO 0.840 O.D.

DRILL HOLES

TO

SPECIFIED

SIZE

CENTER BORE TO

SPECIFIED ID ±0.015"

SOCKET DIAMETER AND DEPTH TO ANSI

B-16.11

0.855± 0.010 DIA.

3.0" DEEPDIA

1.5" ±0.015"

WELD-IN ISOKINETIC NOZZLE

FIGURE 5



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SHOP INSTALL-

PROVIDE TEMPORARY PLUG AND TEMPORASHIPPING PROTECTION FOR THERMOWELL

SEAL WELD

PIPE THICKNESS

MORE THAN 3/4"

1 3/4" UN-8 MACHINE THREAD

1/2"

1/2"

1/8"

MT TYPE THERMOWELL INSTALLATION

FIGURE 6



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NPT THREADS

FULL PENETRATION WELD, GAS TUNGSTEN.06

tn

tc

ARC WELDING (GTAW), NOT MANDATORY

tn = NOMINAL THICKNESS OF HALF-COUPLING WALL

tc = 0.7 tn OR 1/4 " WHICHEVER IS LESS

T TYPE THERMOWELL INSTALLATION DETAIL

FIGURE 8





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TYPE 1

FOR USE ON 3" AND SMALLER LINES

A

G

F

B

C

E

D

DESCRIPTION OF PARTS:

(A) STD THERMOWELL

(B) HALF-COUPLING

(C) ECCENTRIC OR CONCENTRICREDUCER 4" NPS TO RUN SIZE,BUTT WELDED, MATERIAL AS

PER PIPE CLASS SHEETS

(D) PIPE 4" NPS PER PIPE CLASS

(E) LINE PIPE, BUTT WELD

(F) SEAL WELD

(G) FULL PENETRATION WELD

PREFERRED ARRANGEMENT

1. VERTICAL LINES - CONCENTRIC

2. HORIZONTAL LINES - CONCENTRIC OR ECCENTRIC,

INSTALL WELL IN HORIZONTAL PLANE.

THERMOWELL INSTALLATION FOR 3" AND SMALLER LINES

FIGURE 9



BD

C

F

B

AE

TYPE 2

FOR USE ON 2" AND SMALLER LINES

DESCRIPTION OF PARTS:

(A) STD THERMOWELL

(B) BUSHING, THREADED TO SUIT

(C) THREADED TEE, MATERIAL ASPER PIPE CLASS SHEETS, SIZE2" NPS

(D) THREADED NIPPLE, USED IN

(E) REDUCING COUPLING

(F) INSERTION LENGTH (U = 3" MAX)

THERMOWELL OR LINE SIZE

CONJUNCTION WITH REQUIREDREDUCING COUPLING

THERMOWELL INSTALLATION FOR 2" AND SMALLER LINES

FIGURE 10

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3DG-J35G-00002Thermowells for Power Project Test