White smoke from seawater outlet

[KWKloeber]

I've had to use descalers on some heavy duty equipment with heavy duty build up.  Sometimes like 1/8" to 1/4" thick.

The key to doing it properly (meaning most effectively and quickly) is to keep a fresh boundary layer against the scale.  If the acid sits stagnant, the boundary layer reacts and pH rises, and looses effectiveness.

Picture washing your boat. You don't soak it and expect all the dirt to disappear. You need fresh water flowing.  Same principle.

I used to set up two ways, a closed circulation with a small pump or letting whatever I was descaling drain into a sump and suck it up with a hose and hand spray it back onto whatever I was descaling.

So unless you have a huge reservoir (like a radiator shop has to dip and drain an dip again, the acid is best being circulated, which can be done on a closed system with a cheap inline 12v rubber impeller pump, or an open system having the Hx inlet draining into a bucket and a cheap pump picking it up and back to the Hx outlet. 

What pros do to know when to stop is check pH of the descaler exiting the equipment.  When it rises and levels off, there is no more scale left to react with the acid.
OR, in extreme cases of scale, if the pH is too high it needs to be renewed because it's spent and isn't reacting any longer.

[Noah]

Ed- I drained first by removing pencil zinc. I also took off end caps CAREFULLY "poked" opened/cleared any of the holes that appeared restricted, using a scratch awl.
Pic below is before: