BECHTEL CORPORATION POWER GLOBAL BUSINESS UNIT – CONTROL SYSTEMS ENGINEERING ENGINEERING DESIGN GUIDE THERMOWELLS FOR POWER PROJECT Reason 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 and design of thermowells for power plant applications. Lessons learned and improvements in thermowell design from past projects have been considered in the preparation of this design 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 guide discusses both the selection and installation of the thermowell. Additionally, the specification of isokinetic (constant velocity) sampling nozzles is covered in this guide. Isokinetic sampling is a condition wherein the velocity of the sample entering the port or the ports of the sample nozzles is at the same velocity as the velocity of the fluid in the main process pipe. This ensures the most representative sample is obtained for chemical analysis. These nozzles are very similar mechanically to a thermowell, and are typically procured as part of the thermowell purchase.
Page 1 of 20
TABLE OF CONTENTS
INTRODUCTION
1
1 .0
LFO IISG FTURES
2 .0
PURPOSE
3 .0
CODEASNSDTANDARDS
4.0A
3 4 4
PPLICATION
4
5 .0
STANDARTDHERMOWELLS
4
6 .0
ISOKINETN ICOZZLES
5
SAMPLCEALCULATION
5
7 .0
INSTALLATIODNETAILS
7
8 .0
STRESSANALYSISOFTHERMOWELLS
9 .0
MEASUREMENT CONSIDERATIONS
10.0
INSULATION
11.0
OTHERREQUIREMENTS
8
12.0
THERMOWELLFABRICATIONNOTES
8
13.0
THERMOWELLMATERIALSELECTION
9
14.0
REFERENCES
6.1
7 7 8
9
Electronic documents, once printed, are uncontrolled and may become outdated.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
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
Page 3 of 20
2.0 PURPOSE This design guide provides recommended thermowell design and installation details used on Bechtel designed piping and vessel temperature measurement, including the piping systems governed by ASME Boiler & Pressure Vessel Code, Section I, III and VIII and ASME B31.1 Power Piping. 3.0 CODES AND STANDARDS ASME Boiler & Pressure Vessel Code, Section I, III and VIII ASME B31.1 Power Piping ASME B16.5 Pipe Flanges and Flanged Fittings 4.0 APPLICATION This standard design should be applied to temperature measurement points in Bechtel designed piping and vessels and should be recommended for supplier package systems. For temperature measurements refer to Design Guide 3DG-J35G-00001. 5.0 STANDARD THERMOWELLS Thermowells purchased for installation in Bechtel designed piping and vessels are categorized into four types: ·
·
·
·
Machine Thread (“MT” Type) recommended for wall thicknesses more than 3/4inch; 1-inch NPT Threaded (“T” Type) used for line sizes 6 inches and smaller, with wall thicknesses less than or equal to 3/4-inch; 1-1/4 inch NPT Threaded (“T” Type) used for line sizes 8 inches and larger, with wall thicknesses less than or equal to 3/4-inch; and 3-inch 150# Flanged (“F” Type) used for lined piping, atmospheric tanks and vessels.
Typical drawings of these thermowells are shown in Figures 1 through 4. The ASME performance test code for steam turbines (PTC–6) calls for a minimum immersion length of 1/4 the line size or 3 inches, whichever is greater, however, 1/4 the line size is not always achievable since the well length is normally limited by stress considerations. Bechtel has used 3-inch immersion length successfully. The current design details are on standard drawings J8-000G-T0001 through –T0003 (-T1001 through –T1003 for metric).
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 4 of 20
6.0 ISOKINETIC NOZZLES Two types of isokinetic steam sample nozzles are recommended for Bechtel designed piping. One is “MT” Type for line wall thicknesses more than 3/4-inch and the other is Weld-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 be installed at locations where the velocity profile across the pipe can be determined. Ports shall be located such that each port samples from an equal fraction of the crosssectional area of the pipe being sampled. As the steam velocity varies across the pipe section, each port diameter must be sized to result in isokinetic sampling with the proper fraction of sample collected each port. Sample portsport shall be drilledThe cleanly using the standard drill size nearestfrom to the calculated standard diameter. port inlet 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 are not recommended as they may be subject to plugging. Total port area shall be determined to maintain isokinetic sampling in the nozzle ports at the desired sampling rate. Following calculation per ASTM D1066 may be followed: 1/2
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, 1r = (D1/2) x (1/N) 1/2 r2 = (D1/2) x (3/N) 1/2 r3 = (D1/2) x (5/N) Where: D1 = Pipe ID N == Total Totalport number ports a areaof = Af/F 2 A = Traverse area of Pipe = 0.7854 D1 F = Flow rate of fluid through pipe f = Flow rate of total sample extracted The number of sampling ports shall be four (4) for pipe ID 2 inches to 6 inches. The number 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.
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 5 of 20
STEAM SAMPLE NOZZLE Sample nozzle hole size for isokinetic flow Pipe inside diameter inches Pipe area square inches number of annuli needed area/#ofannuli+CC Center circle "CC" diam. ins
12 113.097 6 16.157 4.536
center next annuli center next annuli center next annuli center next annuli centernextannuli centernextannuli
Center of
Distance
Annulus
Total
Fraction of
Annulus
annuli
ID to center of annuli
OD ins
annulus area in^2
total area in^2
number includes circle
5.475 3.2625391 7.135 2.4324715 8.463 1.7682521 9.606 1.1967504 10.626 0.6870769 11.555 0.2225397
Steam flow rate lbs/hr Pressure psia Temperature °F Specific Volume Ft^3/lb Velocityf eet per second Sample flow cc/min. Sample flow rate lbs/hr Insertion depth of nozzle Number of sam ple nozzles Pipe diam. at sample nozzle tip By inspection # of holes By inspection fraction of area Considered steam flow lbs/hr Ratio steam flow to sample Covered area of pipe sq ins Area of each sample hole Diameter of each sam ple hole By inspection drill size Mimimum nozzle cente r bore ins
6.414 7.856 9.071 10.142 11.110 12.000
80.784 64.627 48.470 32.314 16.157 0.000
0.714 0.571 0.429 0.286 0.143 0.000
83460 82 513 6.9428083 204.93738 500 65.956739 4.5 2 3 5 0.714 59590.44 903.47766 80.751498 0.0089379 0.1066772 #36 (.1065) 0.337343
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 6 of 20
2 3 4 5 6 7
7.0 INSTALLATION DETAILS Refer to Figures 6 through 10 for installation of standard thermowells and isokinetic nozzles. It is recommended that Type “MT” thermowells and isokinetic nozzles should be 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, care must be taken to ensure the ports face upstream. For installation details onMT “ ” 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-inch and smaller, install an expander to increase the line size to 4 inches as shown in Figure 9 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 Design documents. Typical installations have been depicted in this guide for convenience. When feasible, the thermowells maybe shipped directly to the piping fabricator. Shop installation 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 to manufacture. 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 PTC19.3 methods. In-house verification of suspect applications is recommended to avoid iterating the design with the vendor, and to preclude later accidental installation of an unacceptable well. While vendors routinely perform vibration analysis (vortex shedding frequency), the pressure boundary verification is sometimes omitted. For high pressure applications (typical of supercritical plants), the pressure limit of the well must be verified. Confirm the vendor includes this calculation prior to award. Caution must be exercised regarding the location of thermowells. Normally, the average fluid velocity is entered into the data sheet, which is adequate for most installations. However, vibration inducedfatigue failures have occurred where the wells have been located in a region of locally high velocity flows. For example, the well must not be placed downstream of a butterfly valve, where the fluid stream past the disc has a higher than the average velocity (upon which the stress calculation is based). 9.0 MEASUREMENT CONS IDERATIONS Bechtel experience has been generally successful using a minimum 3-inch immersion from pipe inner surface. PTC 19.3 provides equations for estimating measurement error due to well conduction (“wall effect”), thermal radiation, low fluid velocity and 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 7 of 20
aerodynamic heating. These effects usually require consideration only for unusual applications. 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 the insulation. 11.0 OTHER REQUIREMENTS Refer to the attached flow chart (Figure 11), which explains the typical work process from thermowell selection to preparation of data sheet and the requisition process. Refer to GBU Design Guide 3DG-J21-G0008 “User Instructions for Instrument Data Sheets” for completion of data sheets. 12.0 THERMOWELL FABRICATION NOTES Thermowell dimension and well material shall be specified on data sheet and surface finish 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
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 8 of 20
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 test thermowells, provide 1/2-inch NPT brass plug with brass chain attached to head of well by non-welding technique. Bore drilling may be performed either by twist drill or gun drill. 13.0 THERMOWELL MATERIAL SELECTION Refer to Thermowell Fitting Material Selection Chart in Plant Design Standard 3DSP72G-00009. The thermowell shall be of the same material as the fittings provided on the pipe (e.g. threaded half coupling). 14.0 REFERENCES 1. Standard Practice for Sampling Steam ASTM D1066 – 97 (Re-approved 2001)
Page 9 of 20
1.375" HEX
1/2" NPT OR NPSM
3/4" WRENCH FLATS
LAGGING EXTENSION= T
1"
1" NPT
OVERALL LENGTH= L
1" DIA. SHANK INSERTION LENGTH= U
BORE= .260"
1/4"
Overall Length= U + T + 1.75"
3/4" DIA.
1" THREADED THERMOWELL TYPE T
FIGURE 1
Page 10 of 20
Page 11 of 20
Page 12 of 20
Page 13 of 20
1/2" SCH 160 x 6-1/2"END TO END NIPPLE OF SAME MATERIAL OR MACHINE END DOWN TO 0.840 O.D.
SOCKET DIAMETER AND DEPTH TO ANSI B-16.11 0.855± 0.010 DIA. 3.0" DEEP
DIA 1.5" ±0.015"
T=8"
CENTER BORE TO SPECIFIED ID ±0.015"
DRILL HOLES TO SPECIFIED SIZE
1" DIA.
U = 4 1/2 "
1/4"
WELD-IN ISOKINETIC NOZZLE
FIGURE 5
Page 14 of 20
SHOP INSTALLPROVIDE TEMPORARY PLUG AND TEMPORARY SHIPPING PROTECTION FOR THERMOWELL
SEAL WELD
1 3/4" UN-8 MACHINE THREAD
PIPE THICKNESS MORE THAN 3/4" 1/2" 1/8" 1/2"
MT TYPE THERMOWELL INSTALLATION
FIGURE 6
Page 15 of 20
Page 16 of 20
NPT THREADS
FULL PENETRATION WELD, GAS TUNGSTEN ARC WELDING (GTAW), NOT MANDATORY
.06
tc
tn
tn = NOMINAL THICKNESS OF HALF-COUPLING WALL tc = 0.7 tn OR 1/4 " WHICHEVER IS LESS
T TYPE THERMOWELL INSTALLATION DETAIL
FIGURE 8
Page 17 of 20
A F B G
D
C
E TYPE 1 PREFERRED ARRANGEMENT FOR USE ON 3" AND SMALLER LINES 1. VERTICAL LINES - CONCENTRIC 2. HORIZONTAL LINES - CONCENTRIC OR ECCENTRIC, INSTALL WELL IN HORIZONTAL PLANE.
DESCRIPTION OF PARTS: (A) STD THERMOWELL (B) HALF-COUPLING (C) ECCENTRIC OR CONCENTRIC REDUCER 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
THERMOWELL INSTALLATION FOR 3" AND SMALLER LINES
FIGURE 9
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 18 of 20
TYPE 2
C
D
B
FOR USE ON 2" AND SMALLER LINES DESCRIPTION OF PARTS: (A) STD THERMOWELL
F
(B) BUSHING, THREADED TO SUIT THERMOWELL OR LINE SIZE
A E
B
(C) THREADED TEE, MATERIAL AS PER PIPE CLASS SHEETS, SIZE 2" NPS (D) THREADED NIPPLE, USED IN CONJUNCTION WITH REQUIRED REDUCING COUPLING (E) REDUCING COUPLING (F) INSERTION LENGTH (U = 3" MAX)
THERMOWELL INSTALLATION FOR 2" AND SMALLER LINES
FIGURE 10
