Maximum allowable reaction force in a waterhose after a restriction

[presso]

We use railcars for storage and transport that are cleaned with water hoses. In the process of improving cleaning operations, we are now using a 1.5” diameter hose, instead of 2" before, coming out 2" diameter piping. However, operators now complain that the reaction force is too intense.

We are wondering if standards are available regarding the maximum reaction force allowed for one person using a waterhose. After various investigations through the web and local firefighting services, we have 2 values : 200 lbf or 890 N (source : local firefighting) and 60 lbf or 266 N (source :

http://www.firetactics.com/NOZZLE-REACTION.htm)

for one person. Calculation methods are probably different. For the second value, it depends on the type of nozzle used, however we do not use nozzles, we only have a restriction changing hose diameter from 2” to 1.5”. If such standards exist, how is the reaction force calculated?

I calculated the reaction force considering my control volume as the volume is the smaller diameter section and, with 1 indicating upstream the restriction and 2, downstream the restriction, I came up with the following formula :
F = m(v2-v1) + P1A2, m the mass flowrate, v the velocity, P the pressure and A the surface area.

However I obtained values much higher than our 2 standards (meaning the operatior would have to hold 120 kgs roughly!), which does not make sense, considering the handling of the hose is difficult and unsafe but is doable even by a small person (the toes are lifting however at start-up!).

Did anyone have any knowledge or experience of such calculations?

Thank you!

 

[dicksewerrat]

I guess I wouldn't worry about the calculation. If it is very uncomfortable for the cleaning crew, lower the pressure. If you insist on the higher pressures that you have now, the crew will not do as good of a job. If and when you realize that the crew knows more about cleaning than you do, and ask for input, prodution will go up with less pressure. Trust the help once in a while. They do it all day, you walk by once a week.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[presso]

That's exactly what we want to do : we plan on installing an orifice upstream. However, instead of going trial and error by testing different orifice sizes for all pressure conditions and for different operators, we are looking for a quantitative criteria.