First, let’s define what this article is all about.
Has your facility ever had a leaking flange? Just a tiny drip of liquid or a small release of gas?
What is the most common response to this leaky flange? Someone goes out and tightens the flange bolts to stop the leak.
This practice is done in the process industry hundreds of times a month in large facilities, and in many situations, no one ever thinks twice about it. They are stopping a leak, and this is a good thing – why would we stand in the way of a “good thing”?
What would you say if I told you there is a RAGAGEP for adjusting bolts or replacing bolts while the piping is IN SERVICE and PRESSURIZED?
One of my favorite support standards for piping is ASME PCC-2 (2008), and in Article 311, we are provided techniques to perform “hot bolting” and “half bolting” as defined by ASME.
Bolts in flanged joints that have been in service may be challenging to remove. Such difficulties at the start of shutdowns for plant maintenance can result in costly schedule interruptions or extend the duration of activities involving some hazards. Using “hot bolting” and/or “half bolting” removal procedures may be considered a possible solution. Here is how ASME PCC-2 defines these tasks:
| Hot bolting is the sequential removal and replacement of bolts on flanged joints while under reduced operating pressure (see para. 311-3.4). It is carried out one bolt at a time in a predetermined cross pattern sequence. Each replaced bolt is fully tensioned before the next one is removed. Hot bolting can be used to clean, lubricate, and reinstall existing bolts for pre-shutdown activities, replace corroded or damaged bolts, or upgrade the material specification or grade of bolts. NOTE: Hot bolting can also be used to check residual bolt stress after a period of operation, or to retighten loose bolts. Hot bolting for these purposes is beyond the scope of this Standard. Half bolting is the removal of every other bolt (so the flange is left with half the number of bolts) during plant depressurization, usually when the system is close to atmospheric pressure. NOTE: Half bolting is also sometimes referred to as skip bolting or odd bolting. Bolt as used herein is an all-inclusive term for any type of threaded fastener that can be used in a pressure boundary bolted flange joint assembly such as a bolt, stud, stud bolt, cap screw, etc. |
Although “hot bolting” and “half bolting” removal procedures can reduce plant downtime, they pose an INCREASE in risks to personnel performing the tasks.
When conducting “HOT BOLTING REMOVAL PROCEDURES”, the risks of fluid leakage are increased due to increased bolting stresses and relaxation of gasket compression, both potentially resulting in joint leakage or gasket segment blowout.
When conducting “HALF BOLTING REMOVAL PROCEDURES”, the risks of fluid leakage are increased due to reduced gasket compression and the possibility that the system may inadvertently be repressurized. For this reason, the system pressure at which HALF BOLTING is conducted MUST be well below design pressure and preferably at or near atmospheric pressure with local management controls in place to prevent depressurization.
The requirements and guidance for HOT BOLTING and HALF BOLTING REMOVAL provided in ASME PCC-2 are generic in nature and shall be combined with a job-specific procedure along with other appropriate plant safe work practices.
An engineering and risk analysis of the proposed HOT BOLTING or HALF BOLTING REMOVAL operation shall be carried out to establish that the operation can be performed safely; e.g., establish that no unacceptably high external loads and/or bending moments are acting on the joint that could cause leakage or gasket blowout during the operation. The analysis should consider, as a minimum, the following:
- contents of the piping or equipment
- design and operating pressures and temperatures
- possible upset conditions
- bending moment on joint
- position and functionality of piping supports
- position and type of expansion bellows
- maintenance history of the joint
- experience with hot or half bolting procedure application to similar joints
- the target torque used in joint assembly
- confined vs. unconfined gasket type NOTE: A spiral wound gasket with a centering ring is considered to be a confined gasket.
- through bolted vs. bolts studded unto tapped holes
- gasket material/thickness
- condition of flange
- condition of bolts; including consideration of whether the necessary bolt stress is sufficient or overstressed
- condition of nuts
Unless the engineering and risk analysis results establish that the consequences of either joint leakage or gasket blowout are acceptable, FLANGES SHOULD HAVE A MINIMUM OF EIGHT BOLTS. The 8-bolt Class 150 B16.5 flange is recognized as uniquely under-bolted and can only accommodate modest external bending moments, a consideration in the risk analysis. The MAXIMUM PRESSURE ALLOWED while conducting either operation shall take due consideration of
- the applicable pressure-coincident temperature rating for standard flanges (such as ASME 16.5 or ASME B16.47)
- the calculated maximum allowable pressure per the applicable construction code for nonstandard flanges
- the presence of external loads and/or bending moments on the flange and the role they play on the flange pressure carrying capability NOTE: It is recommended that the external axial loads and external bending moments acting across the flange be conservatively converted into equivalent pressure. This total equivalent pressure plus the design pressure of the system under consideration, becomes the pressure for risk analysis.
- the reduction of effective minimum gasket compressive stress between bolts associated with bolt removal; e.g., that sufficient loading remains on the gasket to ensure that it maintains a seal
Unless otherwise justified by an engineering and risk analysis, HOT BOLTING may be performed ONLY when the operating pressure is equal to or LESS THAN 50% OF MAXIMUM PRESSURE ALLOWED as determined in para. 311-3.3.
VERIFICATION that the pressure has been reduced and that REPRESSURIZATION IS NOT PRACTICABLE (note: “practicable” is NOT the same as “possible”) shall be obtained from operations personnel BEFORE the HOT BOLTING operation is started.
NOTE: “not practicable” means that sufficient controls are in place to ensure repressurization will not occur until the system is returned to a status when it is safe to do so.
Unless otherwise justified by an engineering and risk analysis, HALF BOLTING may be performed ONLY DURING DEPRESSURIZATION when the pressure is EQUAL TO OR LESS THAN 25% OF THE MAXIMUM ALLOWABLE PRESSURE or 50 psig, whichever is lower, AND when low pressure steam, air, or nitrogen is the purging media.
NOTE: If the fluid contents are such that there is low risk of injury associated with accidental contact, the need for purging may be reviewed for deletion.
VERIFICATION that the pressure has been reduced and that REPRESSURIZATION IS NOT PRACTICABLE (note: “practicable” is NOT the same as “possible”) shall be obtained from operations personnel BEFORE the HALF BOLTING REMOVAL operation is started.
Appropriate PPE shall be worn in case of leakage. The need for emergency standby and contingency planning shall also be considered.
Consideration shall be given to the accessibility of the area and that adequate escape routes are available should uncontrolled flange leakage occur.
The location and condition of the nearest upstream and downstream isolation valves shall be ascertained prior to commencing work. Operations personnel shall have a contingency plan for the isolation of the joint, which addresses the need to block in or depressurize the equipment rapidly should an uncontrollable leak occur. Emergency facilities such as safety showers and eye wash stations should be identified, and locations made known to personnel involved in the work.
If nitrogen is used as a purging medium, consideration shall be given the protection of personnel should a leak occur in a confined space.
A thorough inspection of the flange assembly should be conducted to verify the integrity of the flange and its bolting, with consideration given to the following:
- deterioration of the bolts and nuts, such as necking (corrosion thinning), worn threads, or cracking
- the ability of the gasket to be retightened and its blowout potential
- deterioration of the gasket, and the potential of gasket failure due to disturbance or retightening
- verification of sufficient thread engagement before half bolting of studded flanges, e.g., by ultrasonic measurement of stud length
If deterioration of bolts and nuts is found, consideration shall be given to the safety aspects of bolt removal and the increased load on the remaining bolts. An analysis of the minimum required bolt load for leak tightness and an assessment of the possible bolt stress may be required to ensure the bolts have sufficient load carrying capability.
To anticipate flange bolt relaxation, every bolt shall be checked for tightness PRIOR TO REMOVING THE FIRST BOLT. Bolt tightness may be checked by applying torque with either a hand wrench or a hydraulic torque wrench. Do NOT use hammer (slug) wrenches. When HOT BOLTING, the bolts shall be changed out ONE AT A TIME IN A PROPER TIGHTENING SEQUENCE (see ASME PCC-1, Table 4). The numbering system for the appropriate tightening sequence should be marked on the flange in a readily visible location before starting to remove any bolts.
Considerations Relative to In-Service vs. Pre-Shutdown
The provisions of ASME-PCC-2. 311-4.2.4 through 311-4.2.7, may be more applicable when the flange is intended to remain in service, as opposed to immediately prior to shutdown. This should be considered in the engineering and risk analysis.
As bolts and nuts are removed, the surface of the flanges where contact is made with the nuts should be wire or power brushed clean (remove protrusions, spot face if required). Proper lubrication with an APPROVED lubricant should be applied to the friction surfaces of the bolt assembly. This includes the threads and all bearing surfaces of the nuts except for the surface between the nut and the flange at the rear of the joint (on the opposite side of the flange from the torque wrench), where it is beneficial to allow friction to develop between the bearing surfaces.
Install bolts and nuts HAND TIGHT, with the marked ends of the bolts and nuts located on the same side of the joint and facing outward to facilitate inspection.
Before new bolts/nuts are installed, check to determine that the nuts run freely BY HAND past where they will come to rest after tightening (i.e. First Pass Torquing). If nuts will not turn freely by hand, check for cause and make necessary corrections/replacements. As each new bolt is installed, tighten the bolt using the target torque value. Continue this process for each bolt in the appropriate sequential pattern. If the adjusted torque value is substantially higher than that used for the first eight bolts, these bolts should be retorqued using the new torque value.
NOTE: Differential thermal expansion may cause a loss of bolt load during the cooldown of hot flanges (see ASME PCC-1, section 12). This should be considered in determining the hot reassembly bolt load.
In addition, consideration should be given to ALLOWING REPLACEMENT BOLTS TO REACH THEIR OPERATING TEMPERATURE PRIOR TO FINAL TIGHTENING.
After each tightening round, take measurements around the circumference of the gap between flanges with a vernier or caliper to VERIFY THAT THE FLANGES ARE BEING BROUGHT TOGETHER EVENLY (see ASME PCC1, para. 11.1). After the first pass torquing all bolts to the target torque value in an appropriate sequential order, one of the following actions shall be required, depending upon whether or not ultrasonic elongation control is used:
- Tighten all bolts again using the same torque value and following the sequential order from the first pass, followed by one or more passes using the target torque value in a rotational order.
- Take ultrasonic readings and adjust the torque value, if necessary. Then, if the torque value is changed as a result of the ultrasonic elongation measurements, retorque twice at the final torque value in the appropriate sequential order, followed by at least one pass in a rotational order, until the nuts will not turn at the target torque. If it is not necessary to change the target torque value, follow step (a) above.
Cutting of Bolts
Cold cutting of seized bolts and use of nut splitters is acceptable. Thermal cutting methods (i.e. cutting torch) should not be performed unless the engineering and risk analysis determines that it can be done safely. Of particular concern are lines containing contents that can generate a fire or release toxic materials.
Within the context of these requirements, the examination consists of an in-process VISUAL CHECK FOR GOOD WORKMANSHIP AND AN IN-SERVICE VISUAL CHECK FOR SIGNS OF LEAKAGE.
The markings on the bolts should be visually checked to verify that the correct material is being used. This may be supplemented with PMI if additional assurance of correct material is desired.
There are no formal pressure testing or leak testing requirements associated with this type of work.