Pros & Cons of Liquid Vessels
NFPA 99 outlines three manifold design alternatives for Category I and II facilities. Two of the manifold alternatives incorporated liquid vessels. The intent of this article is to point out the pros and cons of liquid vessels to help with your manifold decision making and trouble-shooting.
|Lower gas price vs cylinders||Normal Evaporization Rate (N.E.R.) up to 3% per day|
|Lower demurrage costs vs cylinders||Limited vaporization rate|
|Lower cylinder change-out cost||Higher installation cost vs cylinder system|
|System requires less floor/wall space||Mechanical problems with the vessels (pressure building regulators, liquid & pressure gauges, vacuum seals, relief valves)|
|Requires greater operator knowledge|
|Loud noise when the vessels vent|
The ideal application for a liquid manifold system would entail a constant (24/7) and sufficient usage with little variance in peak or valley flow rates where the operator understands the mechanics of the liquid vessels and the manifold. Yes, despite the higher installation cost, medical facilities can realize a significant savings with a liquid manifold system due to the lower; gas price, demurrage and cylinder change -out labor costs vs a comparable cylinder manifold system. In this ideal scenario; the bank of liquid vessels in use would be depleted in 5 – 7 days, the peak flow rate would not exceed the vaporization capacity of the bank and the valley flow rate would be minimal in duration and always exceed the economizer circuit flow capacity. The problems which will arise when these ideal conditions are not maintained detract from the facilities savings and possibly create dysfunction in the manifold operation. Here are some examples: if there is insufficient gas usage the liquid vessels will vent to atmosphere up to 3% of their contents per day (your money savings ‘literally disappears into thin air’), if the change-over cycle is prolonged (liquid x liquid model LLU system shown at left) – the reserve bank which becomes the service bank will be significantly depleted (via the economizer circuit) and create an uneven changeover cycle (i.e. – 12 days to deplete the left bank but only 3 days to deplete the right bank) making re-ordering/deliveries a challenge.
Unfortunately, ideal scenarios seldom exist. Applications where cylinder systems should be installed would include; intermittent gas use, insufficient gas use and high variance in the peak or valley flow rates. For example – installing a liquid x liquid system for Nitrogen (N2) which is only used intermittently by the facility would not be a good application for this system because the use is intermittent and the peak flow rate would exceed the system capacity (in typical multiple bone tool applications). Another incorrect application for a liquid x liquid system would be Oxygen in an outpatient surgery center. In most facilities of this nature, this system would utilize an extremely low and intermittent flow. A high percentage of the gas would be vented and at the time of changeover, most likely both banks would be empty due to gas being used thru the economizer circuit.
The hybrid or preferential logic manifold (model PLU shown at right) is a great alternative for facilities that meet the criteria outlined above to warrant a liquid manifold installation, but don’t use enough gas to deplete a bank every 5 – 7 days. The PLU provides most of the cost savings associated with liquid x liquid systems.The only small drawback is the reserve bank must be changed out at a pre-determined pressure (wasting gas) when it no longer contains sufficient gas to provide one days average use.
What conclusions can we make from all of this information? Before deciding on a manifold system, arm yourself with all of the information necessary to make an informed decision; peak & valley flow rates, overall gas use, gas prices in both cylinder and liquid forms, system costs and installation costs. Ask an expert to assist in analyzing and recommending the system that best meets your needs.