Stretching the Limits - COVID-19 vs. Medical Gas Systems
The current COVID-19 pandemic has many hospitals frantically trying to dedicate areas in their existing buildings where COVID-19 patients can safely be treated. This includes using existing medical air and oxygen systems to feed patient ventilators that have higher flow rates than a standard bed side outlet. The increased ventilator requirement is stretching existing systems to their maximum and sometimes beyond.
Pipe sizes for medical gas systems in a modern hospital are determined by multiple factors. Estimated maximum flow rate, allowable pressure drops, length of run of individual pipe sizes, fittings and valves all come into play. For this topic, only oxygen and medical air are being discussed as these are the primary medical gases used for ventilators.
The typical basis of design for oxygen and medical air is to provide a flow rate of 10 liters per minute of oxygen and 20 liters per minute of medical air per bedside outlet. The starting pressure at the source piece of equipment should start at 55 PSI and the accumulated pressure drop across the system should not exceed 5 PSI…so there is 50 PSI at the most remote outlet in the system. Due to all outlets not normally flowing simultaneously, a diversity factor is normally implemented to optimize pipe sizes as much as possible while also allowing the system to properly function at the anticipated peak demand. The diversity factor is based on the number of beds being fed as well as the area of the hospital (use) being fed. Diversity factors are not used for operating rooms whereas standard patient rooms can have a diversity factor as low as 10%.
With the current need to feed as many ventilators as possible, existing systems are being stretched to a point to where they may be needed to operate beyond how the original design was intended to perform. Ventilators often use as much as 50 liters per minute of both oxygen and medical air. In some extreme cases the numbers can be even higher depending on the severity of the COVID-19 patient need. The COVID-19 crisis also necessitates a diversity factor not be used as all ventilators could be running simultaneously. The exact flow rate of any ventilator needs to be confirmed with the manufacturer ,but 50 liters per minute is an appropriate number for use as a basis of design when the flow rate of the particular ventilators can’t be readily confirmed.
The capacities of existing medical gas piping will vary from design to design based on the size, length and flow through the piping. The table below can be used to estimate the capacity of branch piping for oxygen and medical air systems. The sizes of branch piping will typically vary from ½” to 1 ¼”.
When moving beyond the zone branch piping and into the piping mains, the pipe sizes grow as does the available capacities of oxygen and medical air. The table below can be used to estimate the capacity of main piping for oxygen and medical air systems. The sizes of main piping typically varies from 1 ½” to 3”.
The above numbers are based on 100-foot runs of each size pipe with the pressure drop of approximately 1 PSI per one hundred feet. As the length of each size pipe increase so will the pressure drop. In turn the number of ventilators that can operate simultaneously will decrease. The designer needs to do a full analysis of the entire system to fully understand the pressure drops within the system as well as the availability of gases. The designer should also understand that within the hospital there are still procedures in other areas of the hospital (that may not pertain to the COVID-19 patients) also using oxygen and medical air so those flow rates must also be accounted for in their whole system analysis.
There are also other concerns that need to be vetted prior to proposing to feed ventilators from existing medical gas systems for the COVID-19 crisis. The amount of available oxygen in bulk or in cylinders, the ability of the oxygen supplier to replenish the bulk oxygen system, the capabilities of the oxygen vaporizer and regulator capacity to meet the increased demand, the end pressure at the most remote outlet and the capacities of medical air that the compressors can provide are also vital to properly transition to COVID-19 use.
In these uncertain times existing systems may be pushed beyond capacity but with thorough system analysis, teams working together with the hospital and suppliers will make it through this pandemic and come out stronger and smarter on the other side.
Plumbing Engineering Managers