From the Coast Guard Auxiliary:
This VE TIPS addresses the practice of examining fire extinguishers during vessel safety checks, and specifically, the practice of shaking or tapping fire extinguishers. The Vessel Safety Check Manual, Item #5: Fire Extinguishers, Vessel Safety Check Techniques (pg.18), second bullet indicates: "Check dry chemical extinguishers by holding the fire extinguisher inverted to its normally stored position and solidly hitting the base of the extinguisher with the palm of the hand several times".
Recent articles in "For Safety's Sake" (Vol 3‐2010, pg.3 and Vol 1‐2010, pg.4) discuss the practice of shaking or inverting and tapping the bottom of a dry chemical type fire extinguisher to feel the powder moving inside and free up any caked material. The articles point out that there is no official guidance requiring or recommending this practice and, in fact, it is not recommended by National Fire Protection Association (NFPA). The bottom line is that there is no need to shake or strike a modern dry chemical extinguisher that has been USCG approved. To do so actually risks damage to the shell of the extinguisher or the pressure gauge, and has no long term effects on the loosening of "packed" dry chemical. Additionally, this procedure could actually end up clogging the pickup tube, resulting in the extinguisher becoming inoperable. Therefore, the practice of shaking or tapping an inverted dry chemical type fire extinguisher is NO longer recommended and should NOT be used.
Additional information regarding caking/packing of dry chemical extinguishers can be found at the following link:
To summarize this ANSUL Technical bulletin, there is a distinction between caking and packing as related to dry chemical powder. Caking of powder into lumps or chunks is caused by moisture and could render an extinguisher inoperable. Packing, on the other hand, of dry powder in a cylinder that is oriented vertically is normal and expected. However, the design of modern dry chemical extinguishers that are USCG marine‐approved takes into account that powder will pack and the amount of propellant used is sufficient to overcome the packing. It is not necessary to strike the extinguisher with the palm to manually loosen the powder, which will only pack again during normal boat operations. The key checks to make during the VSC are for USCG approval, that the pressure is within the proper range, and that there is no evidence of leaking powder or damage to the integrity of the cylinder. If the pressure is maintained, there should be no way moisture can enter the cylinder after the manufacturing process and thus no danger of caking of the powder.
The National V‐Department recognizes the need to update the Vessel Safety Check Manual and the changes discussed above are on the "to do" list. In the interim, the guidance provided in the "For Safety Sake" newsletters should be followed.