Problems:
Plugging the line as others have mentioned & this was long before P-91 came along. A whole lot easier to just close a valve.
Metengr mentioned a thermal concern. It was a long wait for the steam line to cool down so you could do the hydro.
Also you had to install a temporary valve for...
Back in the 1960's it was the passing fad not to use a boiler stop valve on unit systems.
However, I think the many resulting problems changed peoples' minds.
BACK IN 1960 IT WAS RECOGNIZED THERE COULD BE BURNER INSTABILITY WHEN STARTING UP ON NATURAL GAS. WHY? 2 OPINIONS BASED ON TESTS RUN BY ONE BOILER COMPANY IN MISSISSIPPI AND ANOTHER IN NEW MEXICO. PRODUCTS OF COMBUSTION IN THE AIR HEATER BASKETS DILUTING THE AIR FLOW TO THE BURNER OR MOISTURE...
Scrounging around a construction site gathering scrap lumber and then starting a "campfire" in the furnace was the fun part of being a boiler manufactuer's start up engineer.
Check out 2500 SOLVED PROBLEMS IN FLUID MECHANICS & HYDRAULICS
It is one of SCHAUM'S SOLVED OUTLINE SERIES
You can follow through & see how just about any kind of flow problem is solved.
Nil ductility? How cold was it during the hydro?
In one picture it looked like there might be snow on the ground & the man was wearing heavy clothing. dig1's post indicated the temperature was 12C.
This is 53.6F
I dug some old instructions for hydrostatic testing of boilers & external piping...
