Technical information

Transcription

1 Technical information Reuse of screws, bolts and nuts Screws, bolts and nuts are frequently reused for cost reasons, often from a lack of knowledge about the resulting hazards. The operators (employers) are responsible for proper and hazardfree operation of systems. The requirements result from the Ordinance on Industrial Safety and Health (BetrSichV). Tools, devices, machines and systems (functional units) are work equipment that must be provided by operators. Safe operation of a system depends on the interactions of functional units (see 2 of BetrSichV). According to 3 of BetrSichV, operators have to make hazard assessments according to 5 of the German Occupational Safety and Health Act (ArbSchG) and establish the essential measures for safe provision and use of work equipment. The provided work equipment must be suitable for the conditions at the workplace and ensure safety and health protection with use as intended (see 4 of BetrSichV). According to 10 of BetrSichV, operators must ensure that qualified persons test the proper assembly and safe function of work equipment whose safety depends on assembly. Point 4.2.2, Operation, use, maintenance and repair, of the technical rule on operating safety TRBS stipulates in that worn, fissured or bent screws, cracked or otherwise damaged nuts must not be used again but replaced instead. Next to ASME PCC and 2013, there is additional important information on the reuse of screws provided in Appendix N in section N-3 Guidelines : Quote (source: the licensed German translation of ASME PCC on the assembly of standardized steel flange connections (ISBN-13: , PP Publico Publications, Quote (a) When using bolts and nuts of common grade for fasteners up to M30 (11 8 in.) diameter, the use of new bolts and nuts is recommended when bolt-load control methods such as torque or tension are deemed necessary. For larger diameters, it is recommended that the cost of cleaning, deburring, and reconditioning be compared to the replacement cost and considered in the assessment of critical issues of the assembly. (b) Strong consideration should be given to replacing bolts of any size should it be found that they have been abused or nonlubricated during previous assemblies. (c) Thread dies generally do not yield a highly cleaned reconditioned surface; therefore, turning bolt threads in a lathe is the preferred method to recondition costly fasteners. Although preferred, this process will remove thread material and tolerance limits specified in ASME B1.1 must be maintained. (d) Nuts are not generally reconditioned. End of quote In the procedural instruction on the assembly of flange connections according to ASME PCC , the reuse, especially that of large screws, is not recommended; reference is made to the hazard potential. VAT ID no. DE Page 1 of 6

2 The generally possible reuse of screws, bolts and nuts is considered in the following. For reuse, the following points must be observed (source: Martin Rüedy, Installation safety Antirust protection Surface treatment quality Durability Efficient production-assembly costs It is still common practice to clean screws. If this cannot be done with wire brushes (no steel wire brushes for stainless steel screws), then they are rethread time and again with tap and dies which in itself already indicates great damage and the need to scrap those items. Rethreading is not permissible because it can involve intervention with the mass of the thread and thus the strength of the component. It entails some problems for the use of used screws, bolts and nuts (reuse). While a system of regular tests including regular crack tests, depending on use, is provided for according to the BGR rules for e.g. chains in order to safely assess further use, no such system exists for screws, bolts and nuts. The assessment of reusability is for the most part made by assembly staff. Here, the question arises as to sufficient competency, qualification and the actually possible assessment of the capacity for reuse. The currently published DIN EN : on the qualification of assembly personnel does not include the assessment of the reusability of screws, bolts and nuts. According to TRBS and , it must be assumed that pipes and devices which are pressurized always have a high hazard potential. This can be reduced with proper assembly, which is not firmly guaranteed though if insufficiently tested screws, bolts and nuts are reused. VDI and -2 divide the application areas into categories (Tab.1). Tab.1: Reuse depending on the hazard class according to VDI Hazard class Description Reuse Category A High risk assessment Danger to life, limb and the environment only after a representative and complete random sample test 1) Category B Medium risk assessment Malfunction / system downtime not recommended Category C Low risk assessment Noncritical possible 1) A reuse without these finding can be rated as negligent; an individual test is not possible because of the required destructive tests Tab. 1: Reuse depending on the hazard class according to VDI VAT ID no. DE Page 2 of 6

3 There is damage to be assessed which can generally not be performed on site. The following table (Tab.2) illustrates the damage and the options for assessment. Tab. 2: Damage, test, assessment of reusability, feasibility Damage Damage symptoms Test can be carried out on site by qualified person Screw loosened 1) Loose, easy to disassemble Determination during dismantling yes Visible mechanical damage, deformation Grooves, kinks, bent, overexpanded Visual testing, nut can be screwed onto entire thread yes Severe cracks Visual testing yes Cracks Fine cracks Ultrasonic test Magnetic particle test no Corrosion Visual testing yes Changed surface structure Roughness, changed friction coefficient Friction coefficient measurement no Changed durability Yield strength change e.g. from relaxation Change in creep strength from use at high temperatures Destructive test (bar impact test; tensile test), if values from certificates are available or an unused screw from the same batch is available for comparison no Hydrogen embrittlement 1) In the fact sheet Maintenance of highly stressed screw connections, the Swiss professional association SUVA recommends not reusing the loosened screw and the two screws to the left and the right thereof Tab.2: Damage, test, assessment of reusability, feasibility It is very obvious that a truly correct, clear assessment of reusability is impossible. This might also explain why this is not addressed in DIN EN and why assemblers are not trained accordingly. You can start from the premise that the use of new screws is generally assumed. Reuse without non-destructive testing of used screws on pressure devices cannot be recommended for safety-related reasons. VAT ID no. DE Page 3 of 6

4 Lannewehr + Thomsen GmbH & Co. KG A key problem is the change in friction coefficients with repeated screw connections (Fig.1 and Fig.2). Torque for friction coefficient calculation: 80 Nm Total torque [Nm] Pre-stressing force [kn] The Figure shows the analysis of a repeat screw connection. In screw cases 1 to 3, the friction values are still close together. After the 4th screw case, it is shown to be clearly increased. With the reuse of screws, with rapidly rising friction values and thus strong pre-stressing force losses must thus be expected. Source: MicroGleit Fig.1: Change in friction coefficients with repeated screw connections Torque for friction coefficient calculation: 80 Nm Total torque [Nm] Pre-stressing force [kn] In the same test with another screw paste, the values for the pre-stressing force losses were clearly higher! For a flange connection, this means a higher, unplanned leakage rate!!! Source: MicroGleit Fig.2: Change in friction coefficients with repeated screw connections Managing directors: Gerd Lannewehr, Peter Thomsen VAT ID no. DE Schwachhauser Heerstraße Bremen, Germany Page 4 of 6 Telephone Fax

5 If you look at the reasons for system failures (Fig.3), you will find that these are frequently caused by failing sealing connections. Insufficiently applied bolt force frequently causes the failure of sealing connections. This naturally includes the errors that are caused by poor, reused screws. Reasons for system failures Sealing connections * Bearings Static connections Other Hydraulics Couplings * Sealing connections probably static and dynamic Fig.3: Failing sealing connections are the main cause of system failures Failing sealing connections cause considerable costs and damage, for the most part including environmental damage. If you look at the required effort and the thus required costs for the reuse of screws Cleaning approx. 3 to 5 Visual testing approx. 2 to 3 Non-destructive test approx. 40 to 50 Test of the surface structure approx. 25 to 30 and pay attention to durability and undetectable damage, you will very quickly find that the use of new screws is clearly more favorable in nearly all cases than the reuse of used screws. Please note that all operators act at their own risk and individual danger and cannot delegate this to contractors. They must develop clear rules for handling used components, incl. e.g. gaskets, and monitor compliance. Screws, bolts and nuts are safety-relevant and should be specified and treated appropriately. You can find more information on screws, flanges, gaskets, and sealing systems and their assembly in our sealing vade mecum (ISBN-13: , PP Publico Publications, in the licensed translation of the ASME PCC on the assembly of standardized steel flange connections (ISBN-13: , PP Publico Publications, and in our manual Technical information on sealing connections ( Our latest book Ten steps toward an optimal, permanently technically tight sealing connection (ISBN-13: ) was published by PP Publico Publications. VAT ID no. DE Page 5 of 6

6 Please visit our website for more interesting information on different subjects. I will be happy to provide personal technical consulting, even on short notice. Best regards from Bremen Peter Thomsen Legal disclaimer: The contents of the technical standards are in part quoted, in part reproduced in the words used in the standards; the comments and interpretations are based on longstanding experience, serve as a decision aid and do not substantiate any claim for warranty. Peter Thomsen / flangevalid Version dated VAT ID no. DE Page 6 of 6