Part 1 of a two-part series on medical gas.

During pressure testing, if nitrogen is left pressurized
against an existing closed valve of a lower PSIG of oxygen, the nitrogen will seep into the pure-oxygen system.

The NFPA 99C publication covers the installation, performance, and maintenance of a facility’s medical gas and vacuum systems. Its primary purpose is to prevent fire and explosions caused by the vast array of piping, fittings, and supply equipment installed in your facility.

Since January 2000, the NFPA 99C codebook has targeted cleanliness of systems and overall system usage. This ensures a safer clinical environment and improved patient outcomes.

The NFPA 99C publications cover a vast number of topics. When managing a medical gas piping system, a biomedical engineer might overlook some of the following five problem areas. (An additional five areas will be covered in part 2 of this article.)

1) Oxygen Concentration Versus Patient Saturation
The purity of your oxygen system directly affects patient outcomes. The NFPA 99C requires all oxygen outlets to be 99% pure or greater. This can be a challenge if your facility is constantly being renovated or remodeled.

Since a certified plumber must use nitrogen (national formulary grade) to both pressure-check and flow a constant purge while fitting the system together, he is required to drop the oxygen piping affected to an oxygen concentration of less than 1% by monitoring the flow and concentration. Afterward, the facility’s verifier has a long way to go to remove all nitrogen and re-establish the oxygen concentration to an allowable percentage at 50 pounds per square inch gauge (PSIG). Critical care areas such as emergency/trauma rooms, pediatric intensive care units (ICUs), ICU/critical care unit wings, nurseries, and operating room suites should be spot-checked daily with a mini oxygen analyzer to verify that no nitrogen has tainted these systems while renovation is under way.

Pressure-testing of a new outlet might also cause the oxygen concentration to not be up to standard. During pressure-testing, nitrogen may be pressurized against an existing closed valve in which the source side of the closed valve has 50 PSIG pure oxygen; the new connection to the new outlets may be pressure-tested with nitrogen (NF) to 125 PSIG. If this much nitrogen is left pushing against the existing oxygen, the higher pressure of nitrogen will seep into the existing pure-oxygen system through the closed valve. An effective way to remedy this is to purge each outlet in order of nearest to the nitrogen pocket to farthest with an adapter without any therapy flowmeter on it. Then, a full 7.5 cubic feet per minute of oxygen will escape the pipe and draw the nitrogen out with it.

2) Medical Air Quality and How It Destroys Respiratory and Anesthesia Equipment
Your facility’s medical air outlets are dedicated to respiratory usage only and cannot, by code, be connected to laboratory air or instrument air for endoscopic workrooms. All medical air outlets must be free of any contaminants such as hydrocarbons like methane; halogens; particulates; carbon monoxide; and, of course, humidity. For reference limits of the above contaminants, see page 40 in the NFPA 99C 2002 edition, and page 65 in the 2005 edition.

All medical air outlets must not exceed a dew point of 39°F to prevent rainout (condensation). If a medical air pipe runs through any untempered space, such as hidden in your ceilings and interstitials, and the air temperature drops 18 degrees below the 39°F maximum, 100% rainout will occur. This is temperature dew point, which when mixed with pressure dew point, can wreak havoc on your ventilators, blenders, anesthesia machines, and other sensitive respiratory-delivery systems.

Most medical gas equipment manufacturers have opted for desiccant medical air dryers to drop the temperature dew point to a super-dry -40°F. These dryers are usually prepiped with all required valves, fillings, and monitors to facilitate an easier, code-compliant installation. If your facility has the old-style refrigerant dryers, it is critical that a calibrated dew-point alarm be working to monitor the efficiency of the system’s output. This alarm output is required to be a dedicated signal at both master alarm panels in your facility. A whole book could be written on the maintenance of the medical air refrigerant dryers, but to be brief, a monthly preventive maintenance program must be in place.

3) The Master Alarm and Its Complex Wiring Scheme
An entire three pages of the NFPA 99C 2002 edition are focused on the requirements of the master alarm and its wiring scheme. This is determined by which type of source medical gas equipment is currently used. Annex “A” table A.5.1.9.2. on pages 79–81 of the 2002 edition describe these alarm conditions.

The following is a list of the most commonly missed conditions reported to both master alarms, the engineer’s office, and the PBX switchboard:

1. medical air dew point high;
2. oxygen main supply less than 1-day supply;
3. oxygen reserve supply less than a 1-day supply;
4. oxygen reserve supply pressure low;
5. dedicated waste anesthetic gas disposal (WAGD) pumps main line vacuum low;
6. vacuum systems local alarm (Note: This would indicate, most commonly, lag pumps and high temperature exhaust.); and
7. medical air systems local alarm. (Note: This would be a common signal for lag compressor, carbon monoxide level high, high water level for liquid-seal-type compressors, or high-temperature discharge.)

Even if all of these alarm conditions are present at both master-alarm panels, the alarm points must be tested yearly for a positive indication. In rural areas or industrial cities, a neighboring paper mill or refinery can produce a plume of high-carbon-monoxide or halogen-laden smoke. Your facility’s medical air intake on the rooftop can, with the right wind conditions, bring various contaminants into your system. Also, I have seen medical-air-system intakes located in mechanical rooms near sewer drains that were exuding methane gas; that contaminated air was then being compressed for patient usage.

4) Bulk Oxygen System Record Keeping
It is important to inspect and record the levels in both primary and secondary supply vessels on a daily basis. If you fail to do so, the hospital may be left in a hazardous situation. For example, if the primary vessel’s 125-175 PSIG output comes within 10 PSIG of the setting of the secondary vessel’s set point to activate, the reserve-supply vessel might kick in and begin supplying your facility.

If the alarm switch for “secondary-vessel contents low” exists and is set correctly and wired to the master panels, a bulk inspection would show that the primary vessel was still at an acceptable level. If this problem occurs and the secondary vessel is drained, it must be corrected by the facility’s bulk supplier, who must readjust all regulators and refill the secondary tank. This condition happens with both high-pressure cylinder reserves and cryogenic reserves.

5) EOSC and an Emergency Plan for Its Usage
The emergency oxygen service connection (EOSC) is a connection required by code located on the exterior of your hospital. It should be capable of feeding the building’s oxygen system in case of catastrophic failure of your primary and secondary vessels. A concrete pad adjacent to the EOSC is required to park a trailer with a vessel large enough to supply your facility. Piping associated with this emergency inlet can be found in NFPA 99C (on page 72 of the 2002 edition and on page 140 in the 2005 edition). An emergency 24-hour contact number to your oxygen supplier and clear access to this connection is recommended. If the EOSC is at a remote location, the buried piping should not be in the same vicinity or trench as the primary feed because future construction excavation work could damage it.

Michael Cohen is director of Med Gas Co, Minneapolis, and an ASSE 6020 Certified Medical Gas Inspector and ASSE 6030 Certified Medical Gas Verifier.