OXYGEN SYSTEMS
Clamp-on flowmeters help manage increased demand
Since the start of the COVID-19 global pandemic, NHS Trusts have experienced significant concerns around oxygen system deficiencies, especially with the increased use of high flow oxygen devices such as ventilators, and the significant increase in ICU beds across NHS hospitals. Here, Andy Hammond, MD at Flexim Instruments UK, part of a privately-owned German manufacturer of non-invasive clamp-on flow metering, discusses the importance of achieving actual dynamic flow measurement at various delivery and take-off points in the piped oxygen delivery system, emphasising that the resulting real-time data can be used to help ensure efficient bed management and patient care by clinical staff.
The unprecedented COVID-19 oxygen demand has led to a genuine concern that existing oxygen systems are vulnerable to bottlenecks in supply pipework, or, worse still, overloading the VIE without advance warning, a situation with potential risk to life. Vacuum Insulated Evaporators (VIEs) driven at maximum delivery capacity of the evaporators and control panel are often operating at the edge of specification limits. Systems designed for ‘normal’ oxygen demand rates have quite simply insufficient capability to accommodate high, continuous flows, leading to overload, system freeze-up – resulting in evaporator inefficiency, and consequent oxygen delivery capacity limitations. Fortunately, oxygen capacity at most hospitals has largely been confirmed as at least adequate, with other, inherent, system design problems – such as inadequately sized (too small bore) copper pipework, and distribution system leakage and waste (excessive boil off), being responsible for the systems’ inability to support the number of increased patient ventilators with sufficient oxygen. Most conventional cryogenic oxygen systems comprise at least one vacuum insulated evaporator (VIE) storing cryogenic liquid oxygen at –196˚C, plus possibly a secondary VIE or other reserve sources of liquid oxygen. These VIEs convert liquid oxygen into gas by absorption of atmospheric heat, the pressure of the system dropping from ~14 bar to the hospital system delivery pressure at typically 4 bar. Flow rates vary from 1,500 L/min on the oldest systems, to as much as 6,000 L/min on the newest dual VIE systems. Any changes or modifications to the VIE oxygen delivery system can only be made in consultation and agreement with the gas supplier, as all VIE system equipment is under their ownership. Likewise, any modifications requiring penetration or modification to
A Flexim FLUXUS G721CA transmitter and transducers mounted onto copper pipe under protective stainless steel shroud.
the oxygen delivery pipework around the hospital can only be allowed with the approval of a pharmacist, following satisfactory results from subsequent gas quality testing.
Management and refilling The management and refilling of these VIE systems is generally based upon the pressure of liquid oxygen within the VIE pressure vessel delivering indicative average consumption data. VIE manufacturer systems have been developed with the intent solely to provide sufficient advance warning of the need for refilling by tankers brought onto site for the purpose before oxygen supplies are fully depleted. However, these systems were designed for pre-
COVID-19 levels of oxygen demand, delivering capacity data in terms of ‘days of supply at typical demand’, and were not designed around the soaring daily demand seen during COVID-19 patient peaks, and increased beds within those ICU wards. Systems designed for VIE pressure vessel replenishment generally only provide a measurement frequency of every hour at most, which is generally then only uploaded once a day to an online portal, and thus incapable of providing the data granularity required to understand which hospital wards and departments are creating most demand, or which are at greatest threat of supply deficiencies. The systems were simply not designed in the past with the ability to manage the current increased oxygen
January 2021 Health Estate Journal 55
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