Introduction 

Despite unprecedented inflation and energy crisis the global demand is expected to rise putting severe pressure on  manufacturers to keep costs low while ramping up capacity. The political economy of energy security was built on  redundancies in the past. “Now, we see increased "stress" levels (variance of demand), more infrastructure complexities  while becoming increasingly non redundant. The market will face a market shake out; those increasing optimal operations  by employing best in class technologies and mastering the mindset likely prevailing over time” says Sahand Hagi, MD Vague  Ventures. The need to innovate at scale has never been more dire and the challenges for process industry are further  intensified with the strict safety compliance protocols. As a pioneer in engineering with their experience & domain  expertise, Linde has successfully delivered extremely complex projects including some of the largest chemical processing  facilities in the world to date. Ensuring safety of piping when transporting material & energy across the whole plant is a  critical aspect of guaranteeing integrity of assets and personnel. Securing fluid system networks against phenomena like  pressure surge during sudden events (planned downtimes or emergencies) poses a tough challenge for plant designers.  Such systems can be extremely complex comprising more than 10,000 interacting hydraulic components with multiple  recirculating closed loops which makes them very difficult to analyse without a reliable tool like Simcenter Flomaster offered  by Siemens Digital Industries Software as part of the Xcelerator offering. In this paper, we will highlight safety critical aspects  of designing such systems, relevance of certain details that are often overlooked by designers and how Linde’s rigorous  workflows ensure that their plants are ready to handle any foreseeable eventuality with robustness. As plants of the future  get more complex and bigger – sometimes by factors as large as 4 in terms of production capacities – we will elaborate on a  collaborative approach led to innovative solutions that strongly reduced computational times, thereby demonstrating  readiness to cope with complex challenges in shorter time cycles. 

Safety at scale in chemical processing plants/refineries: Overlooked Details 

Many designs ignore the risks of fluid pressure related problems in process plants, since pipes are shorter and thinner in  comparison to midstream pipelines. This has time and again led to some very unfortunate events across the globe. Although  these incidents are not as frequently reported they do occur. About 5 incidents are known within 10 years at Frankfurt Höchst Industrial Park (damages, partly with release of substances). One such incident occurred where the pipe partially left the rack bridge. 

Figure 1. Example of a fluid distribution system model illustrating the scale and complexity of the problem

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Linde’s commitment to safety has resulted in a very rigorous design process that assesses the system’s performance under  all foreseeable risk scenarios even for highly complex systems like the one shown in figure 1. This led to a sophisticated  approach that starts with creating an “as designed” digital twin model followed by series of steady state analyses to obtain  the “as operated” state and thereby carrying out thorough accuracy check while ruling out human errors. This establishes  confidence in the model and allows various transient scenarios to be analysed for hydrodynamic force computations in  adequate depth within reasonable time. The approach applies to all sorts of fluid systems that are critical to maintain plant  operations. The rigorous approach taken by Linde captures certain key design aspects that are often ignored in favour of 

speeding up delivery times by most designers but may have severe losses of accuracy, thereby making the analyses  questionable and most importantly, compromising safe operability of the plant.  

Since pressure surge is a momentum-based phenomenon - any discrepancy in fluid velocity estimation will inevitably lead to  unreliable results. In addition to that, having losses for multiple fittings adequately captured is also essential to accurately  predict the line-packing effect. In addition to hydrostatic elevation based losses, the friction losses contribute significantly to  back pressure build up and cause changes to wave patterns which in turn might lead to different interference patterns  resulting in significantly different max. / min. pressure peaks and their frequencies in the overall system. 

Starting with an “as operated” state for more reliable hydrodynamic force calculations 

The age-old adage of “well begun, is half done” applies quite aptly to modelling and simulation. A typical approach is to run  transients without disturbances for long enough to achieve the right initial state. Such an approach has two major  disadvantages. First, it consumes a high volume of computational resources before delivering any valuable insights and  second, it often requires multiple runs sometimes with an optimizer/controllers in the loop simply to tune specific  parameters for obtaining the desired initial state. As we will discuss in later sections, computation speed is going to be  critical factor for next generation plants so such an approach is neither pragmatic nor scalable. 

Figure 2: Example demonstrating a zoomed-in view of plant section where operational flow requirements/measurements  need to be replicated for a reliable model

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On the other hand, an integrated solver with embedded flow balancing option offered by Simcenter Flomaster allows Linde  to quickly replicate operational states without necessarily consuming computational resources for time-stepping thus,  providing a very efficient workflow to address this complex problem. Most fitting components offered are well documented  with loss data from the book “Internal Flow Systems” by D.S. Miller and therefore provide reliable estimate of fitting losses.  As no changes to the model are needed switch between steady and dynamic analysis, models can be precisely initialized to  an “as operated” state which emulates the network behaviour just before a disturbance is triggered and sets them up for  reliable transient calculations.  

It then leaves just a click of a button to launch a transient calculation with the confidence that no details will be missed. The  calculations solve for mass, momentum, energy/enthalpy conservation depending on the type of fluid and deliver the time  profiles for key operating variables like pressures, flows and temperatures throughout the network. The results further  deliver the hydrodynamic force loads on various components like pipes, fittings, valves etc. An example of resulting  hydrodynamic forces on the piping is presented in figure 2 - left. As can be observed in figure, even small disturbance  generated forces of the order of 5000N on the system. Therefore, considering the impact of such forces on the overall  structural layout is essential for a safe and robust design that either mitigates or sustains such loads. Thanks to collaboration  between Linde engineers and product development, a further automation of specific workflows for Linde was implemented.  Thus, enabling seamless export of the hydraulic forces resulting from transient simulation and further mapping on to  specialized structural analyses software provided by other vendors as shown in figure 3-right.