SPECIAL TOPIC : OIL & GAS
A shell model is a much better representation of a piping system than a beam model . Modern computer power and software packages now make shell modelling a realistic option for piping with large D / t ratios .
Vibration
Further improvements are expected and some predictions of future piping developments that will benefit LNG plants include :
· Increased use of shell modelling for large D / t ratios
· Advances in stainless steel welding
· More use of non-metallic piping
· Focus on dynamic loads due to fluid transients ( in progress with the development of ASME B31.3 D )
Figure 2 : Shell Model ( main ) and Beam Model ( inset )
P & IDs and isometrics usually show “ VE ” ( vented end ) to indicate the side of the valve on which the cavity relieves to , whereas the valve body marking often indicates directionality with an arrow where the base of the arrow is the vented end . This arrow can be misinterpreted to mean flow direction ( not always away from the vented end ) resulting in a valve being incorrectly installed . Valve body markings consistent with the design documents would reduce the chance of incorrect installation .
Globe Valves
Until 2013 there was no API standard for valves for sizes above DN100 . Normally , globe valves to BS 1873 conforming to ASME B16.34 were specified . The high gas pressures frequently encountered in LNG plants requires special consideration – the maximum differential pressure across the valve . A high differential pressure across a globe valve can lead to stem vibration and ultimately valve failure .
Table 1 : Beam ( B ) vs Shell ( S ) Model Nozzle Loads
Limiting the maximum differential pressure across globe valves to the lesser of 20 % of the upstream pressure or 1.4 MPa is one recommendation to avoid this , but this is often not practical due to the high pressures in LNG Plants . If this is not possible , then use of body or cage guided discs should be considered . Some manufacturers ’ globe valves can withstand a differential pressure equal to the maximum pressure rating of the valve , but frequently this information is not readily available . The first API standard for globe valves API Std 623 ( released in 2013 ) includes the “ design maximum pressure differential across the valve ” as an option to be specified by the purchaser .
Modelling for Stress Analysis
Beam type modelling is normally used to evaluate longitudinal stress compliance in B31.3 . Geometries such as tees and elbows are dealt with by the use of stress intensification factors ( SIFs ) and flexibility ( k ) factors . SIFs have their basis in work done in the 1950s . Recent work ( particularly ASME B31J ) has improved SIFs and flexibility factors . Both B31.3 ( Appendix D ) and B31J state that the validity of the stress intensification and flexibility factors have been demonstrated for D / t ratios up to 100 . Pipes with D / t ratios exceeding 100 can be found in LNG plants .
The limits of beam modelling are illustrated by the large bore piping system shown in Figure 2 : a DN1800 manifold connects via 10 DN600 nozzles to a heat exchanger vessel . The manifold is connected to a DN1800x 1650 tee and then to the DN1800 and DN1650 piping runs . This system was modelled two different ways : a beam model ( using standard B31.3 Appendix D flexibility factor of 1 on the tees ) and a shell model .
Table 1 shows the resulting forces and moments on the nozzles . The results from the beam model are effectively meaningless . Note the large discrepancy between minimum and maximum loads . The inherent flexibility of the shell model better resembles the reality that the load will be more equally shared between nozzles .
LNG plants contain many potential sources of piping vibration such as rotating equipment and high momentum / velocity internal fluid flow .
While the preferred solution is to eliminate potentially harmful vibration or reduce it to acceptable levels , the B31.3 piping code has historically been silent on quantitative assessment criteria for high cycle fatigue . Appendix W High-Cycle Fatigue Assessment of Piping Systems was introduced in the 2018 edition of B31.3 . This welcome addition to B31.3 is subject to Owner ’ s Approval . This should be viewed as another alternative rather than mandatory replacement for other methods such as ASME OM-3 or weld geometry fatigue curves .
ABOUT THE AUTHOR
There have been many changes over the past decade to piping codes and standards . A summary of notable changes is presented in Table 2 . This shows that the Codes and Standards committees , together with industry have actively responded to provide improved codes , standards and methods to engineers involved in LNG plant piping .
REFERENCE
- Proceedings of Robert Weyer – Amesk Perth , WA , Australia , PVP 2020 ASME pressure Vessels and Piping Division Conference , Minneapolis , MN , USA - ASME B31.3 Process Piping - Brittleness of materials by “ Naddir Patel and Gobind
Khiani - industry Code Committees and Colleagues authors participate .
Gobind ( Gobind N Khiani MEng PEng ) has served in engineering management roles for both operating and EPC companies and has received Fellowship in Engineering . He has a bachelor ’ s degree from the University of Pune in India and a Master of Engineering from the University of Calgary in Alberta , Canada . Currently he is Secretary of CPGCE , Vice Chairman of International Standards Organization , Volunteer at YPAC , GPS , API , PRCI , ASME , ISO and NACE representing Canada .
Table 2 : Notable Changes to Codes and Standards www . valve-world-americas . net • February 2025 | Valve World Americas 13