Sizing Medical Gas Piping
Editor's Note: The tables and figures referenced within this article could not be reproduced online. Please see the print edition for these figures.
This article will discuss only the methods and criteria required for sizing medical gas piping distribution systems. Nothing downstream of the main shut-off valve of the facility, such as tanks, source arrangements, and equipment or system configurations such as valve locations, alarms, etc., will be presented.
The information provided is strictly the author's own and was developed after years of engineering for many hospital facilities, using the primary standard for the installation of medical gas systems, NFPA 99. The standard, however, does not provide any guidance for sizing.
This article will cover medical/surgical compressed air, high-pressure instrument compressed air, laboratory compressed air, dental compressed air, oxygen, nitrous oxide, carbon dioxide, nitrogen, surgical/medical vacuum, waste anesthesia gas disposal (WAGD), laboratory vacuum, vacuum pump exhaust, and finally, dental vacuum piping.
General Design Criteria
The typical design criteria are the same for most of the compressed gases except for instrument air. Vacuum is also considered a medical gas, and the same type of criteria is necessary. Note that the information presented here is for sizing purposes only.
All of the piping networks are sized using the following four items: total connected flow rate, the diversity factor, the allowable friction loss, and the equivalent length of piping. The equivalent length is found by using the actual measured run and adding 50% of the measured run to account for fittings. The total is the equivalent length.
Dividing the equivalent run of pipe (in hundreds of feet) by the allowable system loss will establish the allowable friction loss per 100 feet of pipe. As an example, 5 psig is allowed for total system loss with an equivalent length of 400 feet. Therefore, 4 divided into 5 equals 1.25 psi friction loss per 100 feet.
The final delivery pressure for all systems, except for instrument air and vacuum, is 50 psig. With a generally allowable friction loss of 10%, the source should be arranged to deliver 55 psig in order for the pressure loss through the piping to be 5 psig. For instrument air, the delivery pressure is higher, usually around 200 psig. The delivery pressure will vary based on the type of pneumatic instruments used, but will never exceed 200 psig because of the standards that are used for these systems. This means a system loss of 20 psig. The modern instruments developed use less pressure than the older instruments. It will be some time before a complete changeover will occur due to the cost involved. All vacuum systems should deliver 15" if mercury, with the source set to deliver 20" mercury. Any pressure adjustment shall be made at the point of use.
Oversizing some portions of the piping system will allow for future changes or expansion, while the cost of adding another pipe or replacing a smaller pipe with a larger one will be many times the cost of larger sizing during the initial construction. Good practice is to make the smallest size branch and drops to individual outlets for the compressed gases 1/2", sub-mains should be a minimum of 3/4" in size and main size no less than 1". This is to allow for future expansion and renovations without replacing piping. NFPA 99 requires that the minimum size of all mains and branches be 1/2" nominal size. The minimum size of vacuum piping shall be 3/4", with 1/2" drops to individual inlets permitted.
Medical/Surgical Compressed Air
The medical/surgical (low-pressure compressed air) system must be a dedicated system. It is not permitted to be used for any other purpose.
Table 1 gives the actual common usage in the medical facility and the diversity factor to adjust the total connected load to a level that approximates probable usage. Add all the outlets together to calculate a total connected load. Using the appropriate diversity factor, calculate the maximum adjusted flow rate for the entire branch, sub-main, main or project.
In Table 2, enter the adjusted scfm (standard cubic feet per minute) on one side and use the allowable friction loss to find the friction loss that most closely meets the allowable figure. Select the smallest correct size. If the exact figure for friction loss is not found, use the smallest size based on the scfm.
Instrument air is a higher pressure system because of the large variety of pneumatic tools used. The largest flow rate (generally between 6 and 15 scfm) will be used in facilities that do orthopedic, thoracic and neurosurgical procedures. The connected load is a matter of the scfm used by the appropriate tool(s) and the pressure required that the tools use. The type of tools used must be found from the facility. A 100% diversity factor is used. Use Table 3 for sizing purposes. 175 psig is the most often-used figure.
Laboratory Compressed Air
The basis of sizing shall be Figure 1, which is direct reading. Enter the chart with the number of connected outlets and read the adjusted scfm. With the friction loss calculated, size the system using Table 2.
Dental Compressed Air
Dental tools are available that use both high- and low-pressure air. High-pressure tools such as drills use a pressure of 50 psig (345 kPa) and a flow rate of 2 scfm. Low-pressure handpieces used for cleaning and by hygienists use 30 psig (210 kPa) and a flow rate of 3 scfm. Auxiliary outlets use 1 scfm. For diversity factors for dental compressed air, refer to Table 4.Use Table 2 to size the piping network.
Use Table 5, using the flow rate in lpm and a diversity factor for the outlet or equipment used. Size the piping system using Table 2, entering it with the adjusted scfm on one side and using the allowable friction loss to find the friction loss that most closely meets the allowable figure. Select the smallest correct size. If the exact figure for friction loss is not found, use the smallest size based on the scfm.
See Table 6 for room or outlet usage in lpm and a diversity factor to be used. Size the piping system using Table 7.
The use of carbon dioxide is very limited. When used, it is normally supplied from cylinders and not from centrally piped systems. The connected load and usage shall be confirmed from the facility. Diversity shall be 100% to be on the safe side. A central system is sized using Table 7.The difference in size between nitrous oxide and carbon dioxide falls well within acceptable limits.
There is no general consensus of opinion as to the quantity of nitrogen that might be used over an extended period of time in a typical facility, because of the constantly changing requirements of tools using nitrogen, the desire of medical staff to use specific instruments and the degree of use for inhalation therapy, if any. The largest flow rate (generally between 6 and 15 scfm) will be used in facilities that do orthopedic, thoracic and neurosurgical procedures. Size piping for this system using Table 8.
Medical Surgical Vacuum
Each individual station inlet, except WAGD (waste anesthesia gas disposal), must provide a minimum flow rate for proper functioning of connected equipment under design conditions. The various inlets are separated into usage groups based on the expected usage. Group A is heavy usage; Group B is a lesser usage. For a separation of various areas into suggested use groups to find the actual cfm, flow rate for various station and service inlets, and diversity group, refer to Table 9.For convenience, Table 10 has been prepared to give a direct reading for the actual adjusted scfm for both A and B types.
For sizing the piping system, use Table 11. Using the cfm from inlet types A and B added together and finding the allowable friction loss similar to that previously explainedusing a 5" vacuum loss, select the pipe size using the lowest friction loss in the table based on the adjusted scfm.
Waste Anesthesia Gas Disposal (WAGD)
WAGD means Waste Anesthesia Gas Disposal and is used to remove the waste anesthesia gas from any anesthetic location. In the past, this has caused problems for facility personnel after they breathed it in.
The vacuum pressure is the same as the surgical/medical system and uses a 5" vacuum loss for the piping system. The usage of each inlet is 1 scfm and uses a 100% diversity factor. After adding the total scfm from the system, use Table 11 for sizing.
This system is intended only for laboratories within a health care facility, but it can actually be used for laboratories of all types. Use 1 scfm for each inlet and see Figure 2. Figure 2 is a direct reading figure that uses the number of inlets and the (weighted) adjusted sfcm.
Vacuum Pump Exhaust
The exhaust piping for vacuum pumps are sized using the total scfm for the system (both lead and lag pumps) and the equivalent length of run.
The number of inlets shall be the same as dental chairs.
The level of vacuum varies depending on the usage as follows:
- Dental surgery-12 to 17 inches of mercury.
- General dentistry-8 to 12 inches of mercury.
- Laboratory-5 to 9 inches of mercury.
Experience has shown that a level of 10 to 12 inches of mercury has proven satisfactory at chairs for small dental practices or clinics.
Commonly used dental instruments use the following flow rate:
- Saliva ejector-2 to 3 scfm, depending on tip size.
- High volume ejector-5 to 10 scfm, depending on tip size.
- Hygienist-5 scfm.
- Dental chair-10 scfm.
- One dentist and one hygienist-15 scfm.
- Dental laboratory-20 scfm; 30 scfm for a fishmouth when grinding.
For clinics, use the following flow rates that include diversity:
- 2 chairs-15 scfm.
- 4 chairs-22 scfm.
- 5 chairs-30 scfm.
- 8 chairs-44 scfm.
The diversity factor varies as follows:
- 1 and 2 chairs-100%
- 3 and 4 chairs-75%
- 5 to 10 chairs-60%
Calculate the friction loss if there is much of a piping system. If it is under 25 feet, ignore it. Use Figure 3 for pipe sizing because there is a good chance liquids will be mixed with vacuum. For the chart, use the friction loss for the project and the scfm. Use the larger pipe size when the point falls between lines.