Just a quick question regarding calculating the coverage runtime for difference id’s. On the od we ran passes with known intensity settings, to be able to determine the point at which >95% coverage was reached, from that we used the surface area & runtime to calculate the runtime for difference od’s, ie:

175mm dia @ 25mm/sec (coverage Check data) =

150mm dia @ 25 / 0.74 = 33mm / second.
200mm dia @ 25 / 1.30 = 19mm / second

My question is, would we be able to apply this philosophy to the id or would we use a different calculation? Would the formula differ if we were using 1, 2 and/or 3 orifice nozzles to achieve the said coverage?

Establishing time to achieve coverage (should be 98%, not 95%) and then using calculations for different parts is not only dangerous, it probably does not meet the requirements. You must demonstrate that you have achieved 98% coverage. From there you can do peening to "complete coverage" and in this case it would be just 98%. If you must do 150% coverage you would increase the exposure time by 50%. However, if you use a formula to calculate your exposure time for different size parts you may, or may not, have the correct (98% coverage). You should always perform trials to demonstrate you actually achieve the coverage required.
The coverage on the ID is another story. It will act a lot different then OD peeing due to ricochet (bouncing) media performing multiple impacts. You must also verify that you can achieve the required intensity within the ID.
I would not recommend that you rely upon formulas or calculations. Actual coverage inspection is always expected. You must be able to "Prove" you are correct with direct evidence.

It is always difficult to advise if one does not have all the facts. On the basis of the few available facts I must, for the very first time, disagree largely with Jack's analysis. 95% coverage could well be the optimum for the part concerned. 95% has the advantage of being measurable! Even if you were required to reach 98% then adding 50% to the measured times for 95% would achieve that objective in a controllable manner.

Your estimates on peening time versus OD are mathematically correct - surface area of a cylindrical surface being proportional to square of its diameter.

Estimates for ID peening times depend to a large extent on the ID value. Jack is correct to point out the ricochet effect.This is particularly important for small-diameter holes.If, however, you are dealing with large-diameter thin-walled tubes then predictions based on diameter/peening time ratios would be a reasonable guide.

One thing we do agree upon is that predictions must be tested by actual measurements.

Socrates: I agree with your disagreements. I jumped to a conclusion about degree of coverage desired. Usually people just ask for full coverage or complete coverage and this refers to the 98% rule. We have many examples of 80% coverage being completed in 20% of the time needed for 98% coverage and the results are quite satisfactory. Thanks for the correction.