Andy,

We have a boiler that is essentially a "firebox" firetube boiler with mud legs.  When we drain this boiler for the year there is about 1/2" of water in the bottom of the mud legs we cannot drain.

This boiler has undergone numerous "R" stamp repairs to the mud legs and I would like to prevent this for our customer in the future, so we are washing it out and then drying it out as best we can. 

In many cases, we put a shop light in the water side and the heat from the bulb keeps it dry.  In this case, we can't fit one inside the mud legs. 

Do you have any advice regarding keeping the mud legs dry?  We are going to try draining it initially with a vacuum attached to a long piece of conduit we can push down the leg to get most of the water.

Any ideas?

Try to get as much water out as possible with the vacuum. The dry layup chemical known as the "Boiler Lizard" will protect metals in both the water and vapor phase. Put the Boiler Lizard in the boiler like usual, but then sprinkle some of the material over the areas that are still wet. This will protect the metals that are in contact with water.

Finally, seal the water-side of the boiler up. You need to ensure that there are no air leak, or the solid lay-up chemical will exit the boiler via sublimation. (Sublimation is the natural coversion of a solid, directly to a gas. Whereas, evaportation the the natural conversion of a liquid to a gas.)  You should be able to open the boiler at a later date and see that there is still some solid lay-up material left in the Boiler Lizard. If no material is seen, then either not enough chemical was added initially, or there was a leak and the material was able to exit the boiler.

In a properly sealed boiler, equilibrium between the solid and gaseous phases is reached and the sublimation is greatly slowed or even halted.

Are there other, less expensive methods of keeping areas dry that can't be dealt with using heat from lights or ventilation?

For example, do water treatment companies typically sell some sort of basic desiccant product?

If the boiler is dry, (No standing water) and all you want to do is keep it free of condensation/humidity, then you could use a desiccant such as Drierite. You place the Drierite in a plastic pan and insert into the vessel/boiler. I have seen where children’s plastic sleds have been filled with dessiccant and inserted into the steam and mud-drums of boilers. The Drierite is blue when good, and pink when exhausted. Also, with this product you need to seal the vessel/boiler after inserting the chemical.

Drierite is the most commonly used desiccant, but it is a poor choice if there is any free standing water left in the boiler. The new VCI technology (volatile corrosion inhibitors) have pretty much displaced the use of desiccants altogether. I would choose a VCI over a desiccant in almost all situations.

I think a cheap quick way to get water out of your boiler drip legs is to put a "wick" or wet rag into the drip leg and the water will run out through the rag.  It may be damp, but any standing water will be eliminated.  

I have found using "Boiler Lizards" are cumbersome and take alot of extra time and effort not to mention the fact they are VERY expensive.

Check your stop valves too.  The boiler should dry up all by itself if these valves are holding.