Background
In 2005, EPRI Program 65 (Steam Turbines, Generators, and Balance of Plant) began working on a maintenance guide for turbine bolting. Members of the Turbine-Generator Program have increased concerns for high-temperature bolting in the utility industry. With the age of the steam concerns for high-temperature bolting in the utility industry. With the age of the steam turbines, replacement of bolting in high-temperature applications has occurred and will continue to occur as the number of thermal cycles on the bolting continues to accumulate. It is important to know the condition of the existing bolting and to anticipate replacement of bolting that does not meet the criteria for continued service.
Bolted joints are used in power generating plants to allow access to specific components for maintenance purposes. Steam turbines are contained in casings that facilitate power generation during operation but that must be opened for periodic examination of the turbine. The performance of the bolting used to maintain these joints is important to the reliable operation of both nuclear and fossil power plants. Appropriate manufacturing and maintenance practices have been developed to ensure that joints are leak-free during operation. In general, these practices involve careful control of the composition and the fabrication methods used to make the fasteners as well as specification of the level of tightening applied.
At high temperatures, creep within bolts results in time-dependent stress relaxation of the fastener load as the elastic strain introduced by the initial tightening of the bolts is progressively converted to creep strain. This relaxation reduces the effective load on the joints. The final joint load depends on the following factors:
The initial load at temperature
The thermal expansion and creep properties of the flange and bolting materials
The operating times and temperature
At the end of an operating cycle, it is the final reduced load that must continue to exceed the steam loading to prevent leakage from a joint.
In practice, the initial load on an individual bolt at the operating temperature cannot be measured easily. Bolts are normally tightened to a prescribed strain. The load at operating temperature is estimated from the tensile properties of the bolt and the flange materials. The estimation includes allowances for any differences in thermal expansion between the two materials. Matching of the thermal coefficients of expansion is an important consideration in joint design. Too large a mismatch could lead to slackening of the joint if the bolt had a higher expansion coefficient than the flange. Excessive tightness can occur if the flange had a higher expansion coefficient than the bolt.
The strain that the bolts are initially tightened to depends on the properties of the bolting, the flange materials, and the service requirements. The usual strain of 0.15% represents a compromise between a high load (to maintain joint tightness) and excessive plastic strain accumulation during service. In designing a joint for relaxation at operating temperature, it is normally assumed that the flange remains rigid, that the strain distribution along the bolt is uniform, and that the entire bolt operates at the maximum temperature within the joint. These assumptions lead to an overestimation of the amount of stress relaxation and, in terms of the final load on the joint, to a conservative design.
The temperature distribution along the bolt depends on the temperature of the steam and the degree of outer covering present. In well lagged joints, the metal temperature is assumed to be the same as the steam temperature, but in poorly lagged joints, considerable temperature gradients can exist along the bolts. Temperature gradients can lead to nonconservatism in the design.
The temperature distribution along the bolt depends on the temperature of the steam and the degree of outer covering present. In well lagged joints, the metal temperature is assumed to be the same as the steam temperature, but in poorly lagged joints, considerable temperature gradients can exist along the bolts. Temperature gradients can lead to nonconservatism in the design.
However, the actual life achieved will depend on the details of the tightening procedure, the operating conditions, the quality of the original alloy composition, and the fabrication procedures. Although the highest operating lives will be obtained with careful checks of all the important variables, in practice, there will always be some uncertainty regarding actual performance. In many cases, bolts will be subjected to some nondestructive inspections during scheduled maintenance outages to check for evidence of distress.
Replacement of bolting in high-temperature applications is normally based on an overall engineering judgment of serviceability, the manufacturer’s recommendations, and the results of service inspections. With the increasing age of the steam turbines, bolt replacements will continue to occur as the number of operating hours and the thermal cycles on the bolting continue to accumulate.
Basic Material Lives of Common Bolting Materials
Metal Temperature
698–752°F
(370–400°C) 754 –849°F
(401–454°C) 851–905°F
(455–485°C) 907–959°F
(486–515°C) 961–1000°F
(516–538°C) 1002–1040°F
(539–560°C)
Bolting
Materials Operating Hours
Chromium
Molybdenum
Vanadium
Steel, Type B
350,000 350,000 300,000 250,000 200,000 175,000
Chromium
Molybdenum
Vanadium
Steel, Type A
350,000 300,000 200,000 150,000 N/A N/A
Chromium
Molybdenum
Steel
300,000 250,000 150,000 N/A N/A N/A
12%
Chromium
Steel
350,000 350,000 300,000 250,000 200,000 175,000
Precipitation
Strengthened
Stainless
Steel
350,000 350,000 300,000 250,000 250,000 200,000
Nickel-Based
350,000 350,000 350,000 350,000 350,000 300,000
The present guideline provides a summary of the key information involved with:
Approaches that will maximize bolt life
Condition assessment of existing bolting to prevent unplanned outages
Anticipation of replacing bolting that does not meet the criteria for continued service
Report Structure
This document has been produced to describe key aspects of the design, material selection, maintenance, and performance assessment of bolting used in steam turbine components. The report covers bolts, studs, and nuts in the following component applications:
High-pressure (HP) outer and inner shell
Intermediate-pressure (IP) outer and inner shell
Low-pressure (LP) outer and inner shell
Nozzle attachment
Diaphragm hold down
Main stop/throttle valves
Control/governor valves
Reheat stop valves
Intercept valves
Main/reheat steam lead bolting
Coupling bolts
The alloys covered are 1% chromium type, 12% chromium type, precipitation strengthened stainless steels (for example, ASTM A286), and super-alloys (including alloy 80, and alloy 718). Information has been obtained from a very broad range of sources including:
Standards and specifications covering engineering design issues and materials
American codes covering design and materials, for example, ASTM 193/194, 437, 540, 962
European codes covering general information on joints, for example, EN 1591 and EN 1515- 1 and -2, “Flanges and Their Joints, Bolting, Selection of Bolting,” and material specifications, for example, BS 4882 and DIN 17240
Manufacturers’ specifications regarding material, installation, maintenance, and component replacement criteria
Review of publications and papers including journal publications and conference proceedings
EPRI programs, including review of the following documents:
Good Bolting Practices: A Reference Manual for Nuclear Power Plant Maintenance
Personnel, Volume 1: Large Bolt Manual. NP-5067-V1.
1-4
l, Volume 1: Large Bolt Manual. NP-5067-V1.
– Good Bolting Practices: A Reference Manual for Nuclear Power Plant Maintenance
Personnel, Volume 2: Small Bolts and Threaded Fasteners. NP-5067-V2.
– Degradation and Failure of Bolting in Nuclear Power Plants, Volume 1 and 2. NP-5769-
V1 and NP-5769-V2.
– Fracture Toughness Characterization of Type 410 Stainless Steel. NP-5511.
– New Materials for Advanced Steam Turbines. TR-100979-V5
– Technical Basis for the Ultrasonic Examination of Studs and Bolts. TR-104997
– Relaxation and Creep of NiCr Bolting Alloys for Application in Steam Turbines of Coal-
Fired Power Plants. TR-104846
– Technical Basis for the Ultrasonic Examination of Studs and Bolts. TR-104997
Introduction
– Bolted Joint Maintenance and Applications Guide. TR-104213.
– High-Temperature Bolting Life Prediction and Life Assessment. TR-113529
– High-Temperature Bolting Life Prediction and Life Assessment. TR-113529
– Performance Characterization of Bolt Torquing Techniques: Sealing Technology and
Plant Leakage Reduction Series. 1003150
– Materials Reliability Program: Materials Handbook for Nuclear Plant Pressure
Boundary Applications. 1012039
! Utility engineers have provided the following support:
– Information about fasteners used and in-service experience
– Information from failure analysis/cause analysis investigations that can be included to illustrate the section describing examples of damage and/or case studies
– Information regarding procedures for installation, removal, and the associated equipment
– Information regarding inspection methodologies and approaches used for scheduling maintenance and inspection
– Information regarding criteria for replacement (For example, is an in-house method used or are replacements scheduled based on OEM guidelines?)
– Background regarding alloys and alloy performance
– Background regarding safety
In addition, a utility survey was distributed to obtain detailed information regarding utility experiences. In specific cases, follow-up contact allowed greater detail to be obtained.
The present guideline summarizes information from these various information sources. In each
case, the original reference is included so that the original document can be reviewed. The following sections have been developed:
! Glossary: providing background on engineering and metallurgical terminology
! Technical description
! Tooling and procedures
! Failure modes
! Material selection
! Inspection, assessment, and maintenance planning
In addition, information is included in a number of different appendices. These appendices provide the following:
! Bolting subject index table (an aid used for finding interrelated bolting topics within the guide)
! Shaft coupling bolt considerations
! Maintenance procedures associated with bolt tightening
Maintenance procedures associated with the removal of failed or seized bolts
! Bolting information bulletin summaries
! Summaries of selected properties specified by relevant ASTM documents.
! Maintenance procedures associated with the assessment of bolt life
! Procedures for using the ultrasonic assessment of bolts to identify and characterize evidence of damage development
! Resources for bolting vendors/suppliers (for example, proprietary bolting, tensioning devices, lubricants, service companies, etc.)
! Questionnaires to fossil and nuclear plants
! Turbine valve bolting information
! Turbine bolting procurement guide
Concluding Remarks
In contrast to many plant applications, bolting used in turbine applications is often required to be slackened and retightened after periods of service to allow for essential maintenance of the component. In this report, only a few essentials of the design aspects are covered to provide background to the case studies selected.
To prevent steam leakage of a joint, it is necessary that the load applied by the tensile loads within the bolts at all times exceeds the steam load on the flange. The actual number of the bolts, their cross-sectional area, and distribution within a joint are dictated by the load required, the properties of the bolts, the operating conditions of time and temperature, and geometrical considerations.
Joint lives are normally equated in design to an idealized situation in which overhauls, and therefore joint dismantling, takes place at fixed intervals of 30,000 hours and no re-tightening occurs between overhauls. It is then assumed that the joints are reassembled using the same bolts and that up to six re-tightenings may be required during the life of a turbine. The creep relaxation of a bolt re-used in this way must continue to meet the design requirement so that the residual stress on the bolt does not cause the applied load on the flange to fall below the steam load. In practice, a joint may experience many more re-tightenings than six if the source of the operating periods is significantly less than 30,000 hours. For continued use of the bolts, it would be necessary to assess the effects on joint integrity.
Possibly, the most important material property for high-temperature bolts is the stress relaxation strength of the bolt material. The relaxation strength is usually taken as the relaxed stress after a fixed time, nominally 10,000–30,000 hours, as a function of an initial strain level of 0.2%.
Analysis of the stress relaxation data was beyond the scope of this project, but there are numerous sources of relaxation properties available.
Repair the threaded hole if the tap wobble exceeds 20 mils (0.51 mm) or (10 mils/in) (0.254 mm/25.4 mm). A maximum of three holes in the main steam stop valve disk can have this 20 mil (0.51 mm) wobble. The following options should be considered for repair:
! Repair using welding.
! Use oversized studs (It is important to remember that the joint now has different size studs, which can add to a supply parts issue).
! Install a threaded insert in the holes that need repair.
Threaded Inserts
Power-plant turbine shop repairs to damaged threads can be performed by inserting a coil type or pinned mild steel repair assembly. This historical method, dating back 50 years, provided good performance for the 1950s. It requires drilling with a special drill and then hand-taping the drilled hole. A special insertion tool (different for each size) is used next, followed by placing the coil/insert into the tapped hole, then using another special tool to break off the “tang,” and finally, putting chemical “locking agents” on the bolt to finish the repair. This method requires a high level of training, as well as a large amount of tooling, to accomplish the repair.
A recent development offers an improvement on this technique [4-4]. The Gardsert technology reduces this form of repair to two simple steps. First, drill a hole with a standard fractional bit, and second, insert the self-taping insert either manually with a bolt or drive the insert in with a pneumatic impact wrench. The repair has been shown to be effective on all steels under 50Rc, and the same insert can be used for all types of metals. The pullout on steels is in excess of
165,000 psi (1137 MPa) tensile, making it stronger than a Grade 8 fastener. As shown in laboratory testing has revealed that under tensile loading, failure occurred in the fastener rather than in the insert.
Tensile Testing of a Hole Repaired Using a Gardsert Insert Showing That Failure Occurred in the Bolt Rather Than the Insert [4-4]
Assembly Procedures
The performance of a bolted joint is critically dependent on the assembly procedure. If the joint is not properly assembled, it will not perform as intended. Many variables affect the performance of a joint. Examples of these variables include smoothness and lubricity of all surfaces, condition of the parts (for example, rust, tool marks, defects, etc.), hardness of the parts, calibration of the tools used on the parts, accessibility of the bolts, and the environment in which the mechanics operate.
The following are guidelines for bolting assembly procedures:
! Be consistent. Do not magnify the variables that affect joint performance with inconsistent assembly procedures. Whenever possible, use the same tools in the same way and in the same sequence for each assembly.
! Train the bolting personnel. Explain why good work practices are important. Warn the technicians of problems that will be encountered if procedures are not followed. Training improves bolting results.
! Supervise the work, especially on critical joints.
! Keep tools in good repair. Tool repairs waste time and are counterproductive. Periodically calibrating and rebuilding the tools ensures that they perform as required.
