I can't imagine that steam purging would be more expensive than a chemical cleaning, but I don't know the economics of your process. When we have performed tube replacements, we like to do a series of steam blows, i. e. bring the boiler up to pressure (but isolated from the generator and/or other boilers) and then quickly open a valve to vent the steam off to the atmosphere. We clamp a soft metal square rod across the vent to measure the particles that are coming out with the steam. After each blow, we turn the rod to a new side and repeat the blow. When there are no more particle marks on the rod, we deem the boiler is clean. This process takes about a day, but it may depend on the capacity and operating pressures of your boilers.
We have chemically cleaned our boilers, and that process took several days to complete. I'm not sure of what the cost was, but considering the use of the outside contractors and the down time, it was probably far more than a simple steam blow.
