Fit For Pushing
Hi RPD,
This is the type of problem which can easily be assessed on site with a stopwatch, a set of scales, and a fit young apprentice.
1 - Push-pull on a handwheel. I would guess 10-14 kg
2 - How many revolutions per minute over a 5 minute period - depends on the force exerted. 60 revs at 15 kg, 120 revs at 10 kg would be a fair guess
3 - Wheel diameter - that's why mechanics use spanners and crowbars. If the force to move is large, then get a lever to help. This is the same as using a larger diameter wheel on your shaft.
You might well be the first to experiment with this type of time and motion study. Let us know what you learn from practical experience.
Regards - John.Rz
www.cospire.com / Engineering Tips ■
Re: Manual Valve Closure Times
Thanks John, I realise that it is possible to do the development work but I was trying to avoid re-inventing the wheel if possible.
Surely we are not the first people who have built larger and larger plants and found that larger valves take longer and longer to open manually? ■
Re: Manual Valve Closure Times
A lot of it comes down to cost and frequency of operation. Mostly it seems that for infrequent operation maintenance staff are expected to be strong!
I recently did a large cutoff gate and client downgraded the specification from motorised to handwheel operation. However I included the ability to put a windy gun (pneumatically operated wrench) on the handwheel shaft to make life a bit easier. ■
Knife Gate Valves Etc.
This makes me wish the knife gate valve that I was getting to know personally as a temporary laborer and was delegated/elected to opening and closing duty during a system test in a 63 year old (Combustion Engineering built)power plant was 60 inches in diameter rather than 12 inches.
I'll say one thing the screw on that thing was a long one!!!
Oh and I forgot it was under cooling system pressures :^) ■
Manual valve closure times
Does anyone use a standard or guideline for specifying when to move from Manually operated to Pneumatically operated valves? I have talked to some valve manufatcurers and they haven't been able to suggest a typical one.
The issue we have is that as our equipment gets larger, the isolation valves (a knife gate valve for example) we use get larger and valves that on a small plant would take 5 mins for an operator to close, can take 15 mins on the same duty on a large plant.
If the force required to turn a handwheel gets too great, manufacturers will move from a handwheel to a gearbox drive to reduce the torque required but this doesn't reduce the closure time.
What I think I am looking for is a:
1) Recommended load or loads that it is reasonable to expect an operator to exert at the periphery of a handwheel.
2) The maximum peripheral speed of the handwheel at this load or loads.
3) Should these recommendations change with diameter of handwheel?
Then, if a gearbox can't be selected to give the required closure time using the above figures, we would ask the valve manufacturer to provide an pneumatic actuator or similar.
I am surprised there isn't an industry norm for this but I haven't managed to find one yet. ■