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HOSPITAL OXYGEN SYSTEMS


VIE simultaneously for peak demands for patient treatment demands exceeding the 3000 L/min throughput capacity of a singular VIE plant. However, this was deemed unsafe, as the pressure dynamics of two VIEs feeding into the same system were not fully understood by the medical gas provider, and no back-up system would be in place in the case of one of the VIEs failing. In summary, the control panel was identified as the O2


supply


bottleneck, with a maximum throughput of 3000 L/min.


O2


distribution network Pipelines transport the O2


gas from the


central supply source to outlets in wards, theatres, and the rest of the site. At all times, assuming no leakage, the pipe losses needed to be limited to achieve a pressure >3.6 bar at the outlets, below which alarms would otherwise be set off. At lower pressure levels, medical equipment is at risk of not being able to support the clinical requirements (although most medical equipment has safety mechanisms built in, maintaining safe operation even at pressures <3.0 bar). Pressure loss in a pipe network is dependent on multiple factors, but the most important parameters are pipe length, diameter, and the flow in the pipe. Addenbrooke’s was found to have two main distribution systems that separate immediately after the control panel. The ‘Main Ring’ supplies most of the hospital, including theatres, wards, the A&E department, laboratory facilities, etc., and the ‘ICU direct line’ directly supplies the ICU wards and other high dependency areas. This design allows for very stable pressure on the site distribution level, as the high flow rates (associated with high pressure losses) of the ICU wards are isolated, and therefore do not affect the broader hospital in terms of pipe pressure losses.


Distribution pipe network The distribution pipe network is documented in schematic drawings which contain all pipe connections, valves, and other important components of the system. However, being schematics rather than blueprints, they could not be used to determine pipe lengths (a critical variable in determining pipe losses). Mapping the pipeline system onto the actual floor layout to estimate lengths of the main pipe sections helped greatly, gaining clarity on the physical network. For individual pipe sections, the HTM 02-01 A – Appendix G: Pressure Loss Data provides a robust framework for pressure loss estimation. In cases where a pipe is made up of sections with different diameters, the pressure losses of each section can simply be added.


Since the pressure drop is related to gas flow, estimating pressure loss is difficult,


46 Health Estate Journal January 2021 Medical oxygen distribution configuration at Addenbrooke’s Hospital (simplified).


since a change in flow of a ward will influence the cumulative flow of other wards in the same cluster. Therefore, for a safe maximum capacity estimation, maximum O2


demand per patient and


‘worst case’ location of the demand (longest pipe length possible) have to be assumed, and the resulting maximum O2 flow can be assigned to that bottleneck. This approach produces the ‘minimal- maximum flow rate’ at the system level.


Virtual model built


Due to the complexity of the system, a virtual model of the pipeline network was built, using the software, FluidFlow v3.46, which allowed fast simulation of different demand scenarios, giving more realistic estimation of the maximum flow rates, as well as the ‘minimal-maximum’. Since it was not feasible to generate a 100% true replica of the real system, considering the many bends and turns in the pipeline, as well as the fact that some of the pipework is inaccessible for assessment, a 10% increased pipe length was added as a ‘security factor’ on every pipe branching of the main line.


To determine the pressure losses of the system under the hospital base load, the site maximum capacity of 3000 L/min was spread evenly throughout the system by assigning every ICU ward a demand of 450 L/min, and every combined ward cluster 150 L/min, with the remaining capacity distributed among the inpatient wards. With a VIE exit-pressure of 4.2 bar,


1600


1612 1492


1400


the largest pressure drop observed for the main ring was 0.19 bar to 4.01 bar outlet pressure at four wards, for the ICU direct line by 0.22 bar to 3.98 bar outlet pressure at the furthest ward. Scenarios were then run increasing the load on the highest pressure drop ward cluster to determine the overall pressure drop and effect on the other wards. This demonstrated that if the total cluster flow stays below 800 L/min and is well distributed, a maximum flow rate of 250 L/min of one ward can be achieved. It also showed the high interdependencies of the different system levels (ward clusters - ward) that can only be fully accounted for in a computer-aided model.


Establishment of system monitoring In ‘normal’ operation, hospitals built to HTM 02-01 specifications generally have no need for detailed O2


supply system


monitoring. However, the COVID-19 pandemic required a better understanding of the hospital’s O2


supply system. This is


because most COVID-19 treatments include O2


, and many hospitals’


‘reasonable worst case’ scenario plans include considerably more O2


supplier provides -consuming


devices operating in both ICU and general wards than intended by design. The hospital’s O2


access to its telemetry data on the VIE volume and pressure. This data initially appears as a cheap and appealing source to determine the supply flow rate. However, the original intention of the O2


ICU direct line ICU HDU J2&J3 Main ring Cluster C&D Rosie Hospital


Cluster F&G


Cluster A


1200


1102 1067


1000 800


n EPR flow rate (L/min) n Main and backup VIE telemetry flow rate (L/min)


600 16 Feb 1 Mar 15 Mar Date Data alignment of EPIC and VIE data. 29 Mar 12 Apr


659 545


Oxygen flow rate (L/min)


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