Chlorine Piping design specs and joinment

A few months ago we discussed the first step in establishing our Dry Chlorine piping “service class” and in that article, I stressed how critically important that step was, as the rest of our design will be based on the chosen “service class”.  In this article, we will see that I was not joking!  As a reminder, here are our service classes:

Now we have to establish our field joinment methods based on which service class we have established.  A couple of key points from the Cl2 Institute Pamphlet 6:

(emphasis by me)

• For service temperatures ABOVE -20°F (-29°C), fine grain carbon steel pipe such as ASTM A587 can be cold bent, but other carbon steel pipe should be normalized after bending. To ensure rigidity and protection from outside abuse, pipe of ¾” nominal size or larger should be used. Nonferrous and nonmetallic tubing and fittings of appropriate size may sometimes be used (Sections 7 & 9). Cast iron or malleable iron fittings and general-purpose valves are not recommended for chlorine service. Ductile iron per ASTM A395 may be used for valves and strainers designed for chlorine gas service (Class I) only.
• Temperatures BELOW -20°F (-29°C) may be encountered in chlorine systems, and below this temperature some steels become brittle. ASTM A333 carbon
  steel and alloy steel pipe and corresponding fittings are recommended, with grades specific to the expected low temperature. Stainless steels of the 300
  series have useful properties for low-temperature service, but can fail due to CHLORIDE-STRESS CORROSION-CRACKING in the presence of moisture, particularly at ambient or elevated temperatures.
• Butt-welded and flanged joints are recommended for all sizes. Screwed and socket-welded joints are alternatives for nominal pipe sizes up through 1½” in
  some classes (Section 3).
• When thermally or electrically applied coatings are applied to fasteners for corrosion protection, the coating process should be reviewed to ensure
  against the possibility of HYDROGEN EMBRITTLEMENT or altered mechanical properties (detempering).
• For service temperatures between -20°F (-29°C) and 0°F (-18°C) where THERMAL SHOCK OR LIQUID HAMMER IS ANTICIPATED, the use of impact-tested steels or the use of Class V material is advisable.
• There may be situations where the user contemplates chlorine service outside the process conditions defined in this pamphlet. In those cases, the user is advised to review each aspect of the service class in this pamphlet which is closest in process conditions to the contemplated use, individually determine which aspects of the recommendations are acceptable, and engineer revisions to those portions which are not acceptable.

 Before going further in this discussion, I want to point out that some companies will consider Chlorine as a “lethal service” chemical and others will not.  It is 100% up to the end-user to make this determination and this exercise is SEPERATE from the “service class” determination.  The “service class” is based on the physical state, design pressures, and design temperatures.  The “lethal service” evaluation is based on the criteria found in ASME B31.3.  Within that standard, there are three fluid classifications. They are “D – Normal Fluid Service”, and “M – Lethal Service”.  Please see my other articles on this topic:

Typically, chlorine piping systems have been designed for “D – Normal Fluid Service”, but elements of Category M design have appeal to users. The Institute position relative to Category M (Appendix B) design is as follows:

The owner is responsible for determining fluid class

• A single exposure to a very small quantity of chlorine does not cause irreversible harm.
• Current design practices have been adequate to prevent significant releases and several elements of Category M design would eliminate use of currently
  proven equipment.
• It is good practice for users to develop site-specific chlorine piping specifications that use the recommendations outlined herein as a base and consider including elements of Category M requirements (such as NDT examination) that enhance reliability at their sites.

So the Pamphlet 6 does not require the user to classify their piping as “lethal service”, but there are some aspects of “lethal service” design that can raise the level of safety of our chlorine piping.  I can say this about my experiences with chlorine:

• 2 of 4 plants where I worked classified their chlorine process as “lethal service” for both pressure vessels and piping
• about 1/4 of my clients consider their process as “lethal service”

So when it comes to joining up our piping connections, Pamphlet 6 gives us THREE (3) options BASED on our “service class”:

1. Threaded Construction – Classes I and IV
2. Socket-welded Construction – Classes I, II, IV, and V
3. Butt-welded Construction – Classes I, II, III, IV, V, and VI

The Institute has long recommended all the above construction types for chlorine service but as time has passed it has become clear that the PREFERRED METHOD of construction for all chlorine service pipe is BUTT-WELDED CONSTRUCTION. Based on the ability to fully inspect all welds and insure high integrity connections through raised face flanges, this construction type is preferred over threaded connections which can corrode and leak over time due to the small threads that routinely get mauled or socket-welded connections in which a full penetration weld cannot be made, which is, by default, a weaker weld.