Management of Tightening of Flanged
Fasteners at Plants
1. Introduction
Let me express my congratulations on the 90th
anniversary of the establishment of Nippon Valqua
Industries, Ltd. and on the publication of the 90thanniversary issue. I greatly respect everyone who has
supported the sealing industry.
Skills to prevent the fluid within flanged fasteners
from leaking are very important for stable and safe
plant operation. In addition, high reliability is requiredin response to the lengthening period of continuous
plant operation, environmental problems, and emission
regulations regarding leakage. To ensure reliability, itis necessary steadily to build skills and techniques, as

1. Designing of fasteners
・Gasket selection
・Designing according to a leakagevolume standard
2. Setting of the initial fastening conditions
・Tightening force set by taking
operational conditions into account・Tightening methods
3. Tightening techniques
・Evaluation and improvement oftightening techniques
4. Maintenance and management during operation
・Understanding of behavior during operation
・Life cycle（decrease of tightening force,
deterioration of materials, and other problems）
Figure1

Concept of fastener management

shown in Figure1. Especially, regarding sheet gaskets,for asbestos joint sheets, non-asbestos joint sheets,

asbestos materials, and so fluororesin sheet gaskets

which contain organic fibers including aramid fibers

became the norm. So, we needed to take into accountand inorganic fibers, were considered the primary

the operational behavior of fluororesin sheet gaskets

candidate. However, non-asbestos joint sheets containto set the tightening force for them. Robust tightening

non-asbestos fibers, which are stronger than asbestos

operation and management for such settings alsofibers and have weaker fiber-to-fiber interwinding

became important.

power than asbestos fibers. In asbestos fiber joint-

This report describes the evaluation of alternatives heet gas ket s , t hin and long as bes t os fibers

gaskets to asbestos, the results of collaborative studies

themselves were the strength members of a gasket,

to clarify the behavior of fluororesin gaskets during

whereas in a non-asbestos joint sheet, rubber as aoperation, and Mitsubishi Chemical Corporation'sbinder served as the strength member. Therefore,

certification system to ensure proper tightening

functional decline due to hardening of the rubber wasmanagement.

a concern for gaskets used at temperatures above theheatproof temperature of rubber. To address this2. Gasket Selection

concern, we decided to conduct evaluation tests

Regarding sheet gaskets, asbestos fiber joint-sheet

regarding the temperatures of non-asbestos joint

gaskets have mainly been used for their excellent

sheets.

heat resistance, sealing properties, ease of handling,

Figure2 illustrates the test line. We set up test piping

and cost performance.

in a plant steamline with an inner pressure of 1.3 MPa

In 2006, an ordinance amending the Industrial Safety

and a temperature of 190ºC and then conducted aand Health Law Enforcement Ordinance was issued.

6-month loading test. During the course of the study,

The ordinance banned the manufacture and use ofno macroscopic external leakage was found. However,asbestos and other substances, with an exception forin an opening process after the test, cracks developed

some products. Therefore, asbestos gaskets needed towhen a non-asbestos joint-sheet gasket was removed

be replaced with non-asbestos gaskets. As substitutes

from the surface of a flange sheet as shown in24

Summer 2017

restrictions on asbestos led to greater use of non-No.33

Management of Tightening of Flanged Fasteners at Plants

sheet gaskets cannot be used for a long time at high

3. Tests to confirm the behavior offluororesin gaskets

temperatures and that additional tightening cannot beFluororesin sheet gaskets had been used as gaskets

applied to those gaskets. So, we concluded that non-with excellent chemical resistance before the non-asbestos joint-sheet gaskets cannot be a substitute.

asbestos trend caught on. When fluororesin-specificAlso, in this test, other types of gaskets wereflow causes a reduction of tightening force, we haveassembled simultaneously. Foamed-carbon gaskets

applied additional tightening to solve the problem.

stuck to the surface of a flange sheet and were not

However, the possibility and timing of additional

easy to handle, and so we concluded that foamed-tightening depend on empirical rules. For proper

carbon gaskets could not be a substitute either. On

maintenance and management, we needed tothe other hand, no major macroscopic problems

understand the behavior of gaskets and devise proper

occurred with fluororesin gaskets, making thesecountermeasures. Therefore, we conducted variousgaskets the primary candidate.

tests to understand their behavior.

Figure3. This result showed that non-asbestos joint-Mounting of test-use gaskets

3-1）Test equipment
Figure4 illustrates the test procedures, and Table 1
shows the test conditions1）. A strain gauge attached tofastening bolts was used to measure bolt axial tension,Summer 2017

and the contact pressure of the gasket was calculated
from the contact area with the gasket. Tightening wasconducted according to JIS B 2251（2008）to achieve agasket contact pressure of 25 MPa2）（hereafter, bolt
tightening was conducted according to the samestandard）. No. GF300 gaskets were used.

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Steam trap
Figure2 Outline of the test line

Figure4

Illustration of the stress-mitigation equipment

Table1Figure325

Removed gasket

Conditions for the stress-mitigation test

Test-use dimension

JIS 10K 50A RF

Test-use temperature

200ºC（24-hour cycle）Gasket

V/NO.GF300

FlangeMaterial: SS400
Roughness of sealing surface:Rz = 14.4 μm（ave.）Bolt（loaded with astrain gauge）

Dimensions: M16 × 4 bolts
Material: SS400

Contribution3-2）Heat cycle-stress mitigation test
Gaskets were heated at 200ºC in an electric furnace,3-3）Confirmation of the effects of additionaltightening
Usually, as a countermeasure against reduced contact

were measured over time. Heating was conductedpressure, additional tightening is applied. Regarding

under the following three conditions: room temperature;

the timing of performing this additional tightening, thecontinuous heating at a consistent temperature offollowing two conditions are taken: application at high200ºC; and application of a heat cycle alternating

temperatures after the start of operation; andbetween 200ºC and room temperature. In this test,

application at room temperature during operational

one heat cycle was 24 hours with 2 hours ofsuspension for periodic maintenance and othertemperature rise and approximately 3 hours ofreasons.

temperature fall.

Figure6 shows the test results. Additional tightening

Figure5 shows the test results. The rate of contactincreased the residual contact pressure, and thepressure reduction with temperature load was more

effects of contact-pressure retention were confirmed.

reduced than that at room temperature. In the heat-Regarding the timing, additional tightening at higher

cycle case, a great reduction in contact pressure

temperatures resulted in greater residual contact

during the temperature-fall period was confirmed.

pressure than at room temperature. Since the stiffness

Although the contact pressure of the gaskets

of gaskets decreased at high temperatures, tightening

increased during re-heating, it did not reach thewas conducted while flowing, which resulted incontact pressure before temperature reduction. After

weaker flow after additional tightening. Such

periodic maintenance and before the start ofweakening was supposed to reduce stress mitigation.

operation, airtight tests are conducted. When the fall

In the case of additional tightening under decreasedin temperature greatly reduces the contact pressure,stiffness, crushing strength is also expected tocountermeasures to restore the contact pressure

decrease, so the tightening force must be carefully

should be taken to confirm the absence of leakage andconsidered. Although additional tightening at room

then to prepare for the start of operation. Also, intemperature resulted in weaker effects than that atareas where frequent heat cycles develop, the contact

high temperatures, it is considered an adequate

pressure of gaskets decreases gradually, so the same

countermeasure in pre-operation airtight tests.

countermeasure to restore the contact pressure
should be taken.

Maintained at 200ºC
Additional tightening
at high temperature
Additional tightening
at room temperature

Roomtemperature
Maintainedat 200ºC
Heat cycle

Figure5

Results from the heat cycle-stress mitigation test

Figure6

Results of verification testing of additionaltightening

26

Summer 2017and changes of the contact pressure of the gaskets

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Management of Tightening of Flanged Fasteners at Plants

4. In-house certification system

including new employees who had no on-site

Ideally, a flanged fastener's behavior is clarified,experience and skilled operators who had 30 years oftightening is quantitatively managed for all

experience participated in the training. Regarding

procedures, and tightening is performed in a controlled

their post-training impression,“understood very well”manner. However, due to limitations of managing

and“understood”accounted for over 90%. In addition,

tools, measurement cost, and limited work period, we

the training received good feedback including

select the target. So, most flanged fasteners are“visualization of changes in bolt axial tension madetightened based on the operator's skills and experience.

the training easy to understand”. So, the training

In the past, assembling while misunderstanding aappeared to be very effective. We are now studying

gasket's specifications, inadequate tightening, andhow to utilize the mobile training system anduneven clamping caused malfunctions. To address

incorporate it into our certification system. In

these problems, Mitsubishi Chemical Corporation

Mizushima's case, the number of trainees was 40.

introduced and is utilizing a certification system for

Therefore, we need to study the curriculum contents

chief gasket operators. Table 2 shows the contents ofand training using multiple training devices to enable

the certification training. Chief gasket operators who

hundreds of trainees to participate during periodic

have passed the test instruct, mentor, and supervisemaintenance.

operators and conduct the final confirmation ofassembling operations. In this way, they have

Summer 2017

responsibility for gasket assembling. Gasketassembling cannot be conducted without the presenceof a chief gasket operator. To date, approximately
3000 operators have been certified as chief gasket
operators. This system appears to be effective forpreventing malfunctions. All certification training

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sessions are provided in a classroom, which isexpected to enhance knowledge, but skills are alsoacquired through on-site experience. Nonetheless, only
on-site experience causes the problem that although

Table2

Contents of the training session

１．Responsibilities of a gasket operator
２．Training to prevent mistakes in gasket mounting
・Gasket types and their handling・Pressure rating and flange specifications
・Bolt specifications at each plant・Points to confirm regarding core tightening anduneven clamping
・Tightening methods (circling tightening)・Inspection and record submission
3．Handling of asbestos gaskets
4．Previous trouble cases
5．Verification testing

operators understand differences in tightening
conditions occurring when bolt axial tension isreduced by elastic interaction and when a different
tightening attitude is applied, the timing andmechanism of the effects of bolt axial tension andtorque cannot be visualized numerically and othermeasures remained as a problem. Therefore, part ofour mobile training system was introduced in the
Mizushima Plant and a practice training session washeld for operators. Figure7 shows a picture of thepractice training session. The training session includes
the following four contents: evaluation of flangetightening skills, torque-sensing training, understanding
the importance of bolt maintenance, and recognition of
crushing-prone gaskets. A total of 43 operators
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Figure7

Training session

Contribution

5. Conclusion

systems, as well as training sessions utilizing ourDue to the retirement of old skilled operators and themobile training system. Through such training media,

trend of younger operators to leave work, there has

we will strive to maintain the levels of our operators'

been a declining number of operators specialized inskills. We will also introduce new systems in theflange-tightening operation and a decrease in the levelpursuit of more reliable leakage-preventionof techniques and skills. A system for conducting

technologies.

proper tightening management without depending on
skilled workers is necessary. For example, sealing-6. Reference

operation management systems enable automatic1）H ajime Nonogaki, Masayuki Yamabe, Riichi

tightening tools to conduct tightening according toMorimoto. Stress-Mitigation Characteristics ofprogrammed procedures, enabling even novices toGasket Fasteners, Piping Engineering, Vol. 52, No. 7,

systematically apply tightening force at ap. 28; 2010predetermined torque value.

2）JIS B 2251“Flange Joint Assembly Guidelines”,

This report introduced our education and certification（2008）

Riichi Morimoto
Summer 2017

Mitsubishi Chemical Corporation
Mizushima Plant
Mechanical Group 2
Maintenance and Engineering Department

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