1. The shot flow rate in a direct pressure peening system using a fixed orifice or a "grit valve" can vary +/- 10% over a 30 day period depending upon the condition of the shot, humidity, hose and nozzle wear. AMS 2432 is the only spec that requires the tolerance to be within 10%.
The shot flow rate can be measured by an inductive sensor (Electronics Inc.) located above the mixing tee. It can also be measured by a capacitive sensor (Tealgate Ltd.)located in the blast hose. These are like adding a "speedometer" to a car. You can also add a "cruise control" to actively regulate the shot flow rate by using a magnetic valve (MagnaValve) to actually control the shot flow rate and maintain a 1% flow accuracy.
2. Several tests indicate that the peening intensity does not vary significantly when the nozzle is between 2" and 6" from the surface to be peened. However, you should always prove how your system acts at those extremes.
3. The peening intensity would only be affected if sufficient rust should obstruct the shot flow and starve the nozzle. Shot should be continuously screened to preclude rust and other contaminents from interferring with proper operation.
4. Generally you should maintain air pressure +/- 5psi, flow rate by +/- 10%, distance between 2" and 6". Again, you should always verify how your system performs (peening intensity and coverage) at the extremes of each of these conditions.
The shot size will be specified by screening size requirements of whatever specification you must perform to. For instance, AMS 2430 and AMS 2432 require the use of shot specified in AMS 2431. The MIL-S-13165C (now AMS-S-13165) has its own screening requirements. A new (soon to be released) specification from SAE J2441 will use the media listed in HS-84 (also referred to as the "J" specifications).
