What are recommended practices for cleaning instruments, in dusty industries ?

1st you've to be sure that you are selecting the right instrument protection or isolation in order to save instruments from been damaged due to dusty, humidity & hot zone.

The right procedure is to use dry air ( There is kind of blowers for such of this activity available in market (Like Makita Blower, Bosh Blower...etc) 

It depends on the type of Instrument you are using. Using dry compressed air is mostly used method but it is considered dangerous because of the fact that the dirt/dust will be again recirculating in the air and can even cause injury. In some countries it is prohibited by law. Better to use sweeping compounds, or vacuum cleaners equipped with filters. Ultrasonic cleaners are used in hospital instruments.

IN DUSTY INDUSTRIES INSTRUMENTS TO BE COVERED PROPERLY AND ENSURE THAT THE INSTRUMENT IS WORKING PROPERLY.

MAKE THE SCHEDULE AND CLEAN THE INSTRUMENTS (PREVENTIVE MAINTENANCE) 

Totally depend on instrument how sensitive it which kind of it is For Example i work in Water wast Treatment plant some instrument which are waste water they get dirty so we Use DI (distilled) water to clean some are sensitive so we only use Compress air and then rubber wipers are also fitted in them which clean them automatically after some time .... and we have to kill germs on them some time we use wet wipes also

Is depeand on the instrument it self from housing , montning and other. I have not seen any ISA standard speak about "how to clean". I think the best practice to check the manfactring manual. On other hand, choosing the right instrument with considering the environment where will installed is very important. You can check the rating choose ip65 or ip67 from IEC 60529. This will make sure the instrument can handel such Dusty environment

Identify the instrument. Observe the location in terms of the exhaust and ventilation. Follow and maintain a regular house-keeping schedule.