In the world of process safety, we live by our chosen and adopted Recognized and Generally Accepted Good Engineering Practice(s) (RAGAGEP) and when our HHC/EHS is Chlorine (Cl2) we turn to the Chlorine Institute and their “Pamphlets” for our needs. And one of their most-used pamphlets is Pamphlet 6 – Piping Systems for Dry Chlorine. Note: The Chlorine Institute makes most of their Pamphlets available for FREE. In Pamphlet 6 the Chlorine Institute established six (6) service classes for Dry Chlorine piping. The end-user can easily use the table in Chapter 2 to determine which service class they need to build their chlorine piping to.
| Service Class | Physical State | Design Pressure | Design Temperature |
| Class I | Gas ONLY | Vacuum to 150 PSIG | -20°F to 300°F |
| Class II | Gas ONLY | Vacuum to 150 PSIG | -50°F to 300°F |
| Class III | Gas ONLY | Vacuum to 150 PSIG | -150°F to 300°F |
| Class IV | Gas or Liquid | Vacuum to 300 PSIG | -20°F to 300°F |
| Class V | Gas or Liquid | Vacuum to 300 PSIG | -50°F to 300°F |
| Class VI | Gas or Liquid | Vacuum to 300 PSIG | -150°F to 300°F |
Here is the one “catch” in the whole exercise and one of the reasons I love the Chlorine Institute Pamphlets; the footnote to the table above states:
Piping classes corresponding to the fluid state “gas or liquid” are to be used for all liquid-only lines and gas lines where the POSSIBILITY of liquid entry exists or where there is the POSSIBILITY that gas in a line may liquefy.
So way back in the early 1990’s I was taught by one of the very best piping engineerings that when it comes to Chlorine, the ONLY time you would ever consider using Service Class I-III is when you are running off a 150-pound compressed gas cylinder UNDER ambient temperature conditions that would NOT allow the gas to condense. When a 1-ton cylinder is our delivery method and we have liquid chlorine available (as a liquified pressurized gas), service classes I-III are OUT OF THE DISCUSSION! Now that is just how I was taught to read codes/standards and design my chlorine piping system, but that is not how everyone else does it; although it is my expectation when auditing a Cl2 process or designing one!
Most 1-ton processes are built to a service class IV or V, depending on where the process is located in the country and wether the process is outside vs. inside. You noticed that the Design Pressures did not vary much when dealing with Class I-III for GAS ONLY or IV-VI for Gas or Liquid, but the Design Temperatures will be the most likely deciding factor as they range from -20°F to -150°F for the SAFE LOWER DESIGN TEMP.
It is ABSOLUTELY CRITICAL that the proper service class be established as the rest of the design will be based on our service class. In the next article I will discuss the joinment opition we have for our Chlorine piping based on the service class of our piping.