MEDICAL GAS SYSTEMS
discussed with clinicians, APs (authorised person) and anaesthetists prior to starting, and it is essential to work with the AP to ensure every source of supply is completely full, from the VIE (vacuum insulated evaporator) to the stand-by manifolds. It is inevitable that a site-wide test will result in a significant amount of oxygen usage. Once this has been established, the engineers can conduct a total site flow test under controlled conditions, with both the team and the site APs continually monitoring and recording the flows and pressures to 3.5 bar. Each engineer is in constant contact with the test control engineer, with all point of supply VIEs also continuously monitored. In the hospitals tested previously, the
initial required rates ranged from 60 litres per minute (L/min) at each outlet point for CPAP patients, down to 15 L/min for step-down patients. As treatment techniques became more refined, flow rate requirements reduced down to a maximum of 30 L/min for each outlet. However, it remains essential to continue to test for a worst case scenario of 60 L/min.
Using digital test gauges and metered leaks capable of adapting to different ports, engineers can monitor and record the pressure as the flow increases. They should also take into account the fact that some wards and intensive care units will be occupied and unavailable for testing. With the cooperation of the AP and department managers, the team can then ascertain the actual flow rates in use within those departments and include them in their calculations. The NHS guidance also suggests using
MEC flow and pressure drop testers to use as a ‘dial up’ variable orifice for loading the oxygen system. However, in our experience this is not the optimal method for site-wide testing. Instead, sets of calibrated Amal jets and carriers should be used to match the design flows and test flows required.
To mitigate this risk, we advise hospitals to avoid using departments or wards on radial branch lines towards the end of the run. Instead, we recommend ensuring patients are located in proximity to larger diameter mains. As well as identifying wards capable
of passing high flow rates, this testing approach can also help hospitals understand how many patients they can realistically supply without being overwhelmed. If a ward is capable of drawing a total
of 300 L/min, it follows that its maximum occupancy would be five patients requiring the highest volumes of oxygen.
This way, it is possible to build a detailed picture of which wards and areas can deliver the best support to COVID patients. For some hospitals, the results add an additional layer of confidence in their system design and contingency plans.
Mitigating the risk of radial oxygen flow systems One of the consequences of the varying flow rates delivered in radial branch systems is that hospitals are forced to relocate COVID-19 patients to areas where pressure remains consistently high while the system is experiencing a high load. Yet while these designated areas may
have been able to pass the 60 L/min standard when initially designed, the pressure would still drop during a simulation of patients overflowing to other areas. This is not a problem of wards running
out of oxygen – the VIEs generally hold up very well. It is the system itself that was not designed to pass that flow.
Over the course of the pandemic, healthcare engineering company MIG Medical Installations has conducted more than 20 oxygen flow capacity tests for 12 hospitals throughout the UK. MIG engineers found that many hospital estate teams were facing impediments ranging from system bottlenecks to pipes of insufficient diameter
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Increasing flow capacity Where VIEs were operating at capacity, many hospitals increased pressure as an interim measure. Often a radial system with several VIEs will include a manifold, which backs up the system if the pressure drops too far. Yet as with every pipeline, the size of the pipework dictates the volume that can be delivered safely. One method of increasing flow capacity with a rising main is to put another linked main into that block. So when there is a high level of need, additional flow capacity can be generated simply by opening up a valve. Depending on test results, it is also
possible to add more resilience with additional linked branches to create a mini ring main effect. With more branches closer to the source of supply, pressure can be more consistently distributed throughout the system. The NHS Nightingale hospitals were built with ring main systems for exactly this reason, and newer medical gas pipelines usually follow the same design.
The importance of realistic stress simulations Significant modifications to the oxygen pipeline’s infrastructure can sometimes be the only answer to ensuring the right flow capacity throughout the hospitals. One hospital tested had a system designed to deliver 2500 L/min of oxygen. By simulating the stress across the
entire site and comparing it with initial calculations, it was established that a full occupancy of COVID-19 patients could require a 40 per cent increase in flow capacity up to 3500 L/min. This was reported back to the hospital’s estates team, along with a summary of the areas that needed upgrading to get the most from the system. The hospital Trust was then able to
upgrade its oxygen VIE capacity and modify the oxygen supply pipeline mains to specific risers within the hospital. While results will differ depending on a huge range of factors, this does not diminish the importance of benchmarking hospitals’ oxygen pipeline performance.
IFHE DIGEST 2022
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