E-Stop and Hydraulic Pumps

Is it better to shut down the hydraulic pump(s) or just make sure the solenoids can't be energized during an Emergency Stop?

I've seen it done both ways on similar equipment where I work, designed and installed by the same company!

Nothing worse than a stuck valve or busted coupling and everyone hitting E-Stops and nothing stopping. Or is there?

I appreciate any feedback, as always.

Thanks

E-stops should cut power to all the possible movement, including the pump. What if the problem that the operator E-stopped the machine for was the pump itself?

The only power we leave on during an E-stop is the 24V to any switches that may be on the machine or LED lights.

Also there is power wastage if pumps remains same.

Depressurize the accumulators too.

The accumulators store energy like capacitors. They must be discharged too.

It's hard to make a case that a machine is in a safer state when a hydraulic pump is running than when it's stopped.

bulletin blues said:

Is it better to shut down the hydraulic pump(s) or just make sure the solenoids can't be energized during an Emergency Stop?

I've seen it done both ways on similar equipment where I work, designed and installed by the same company!

Nothing worse than a stuck valve or busted coupling and everyone hitting E-Stops and nothing stopping. Or is there?

I appreciate any feedback, as always.
Thanks

Click to expand...

The only reason I can think of to not shut down pumps and all simultaneously with E stop is to provide pressure to some component to let it lower to zero energy state or to safest position. Once it was down "retracted" or whatever then the pump could shut off as demanded by E stop.

Dan Bentler

It IS possible to achieve the required safety by only the solenoids.
Just remember that it requires double safety elements and/or a selfchecking circuit in order to achieve a safety that is higher than class 0.

For true E-stops, cut the pump.

If the safety circuit shall be activated frequently (for inserting or removing work pieces in the dangerous zone for example, i.e. a safety stop - not an E-stop), then it may not be possible to start and stop the pump so frequently. In that case a safety circuit based on the solenoids is the solution. This is probably what you have seen.

I've seen valves with broken spools. I recommend stopping the pumps. Some hydraulic equipment may need two safety circuits. For example, an Emergency Stop circuit deprives everything, including the pumps of power and vents any stored energy. A second safety circuit deprives solenoid valves of power whenever safety shields are open or a light curatian is interrupted for changing parts, etc where stopping the pumps for only one or two seconds may be undesirable.

Additonally, if the hydraulic pumps and/or pump MCC are any distance from the equipment (such as in an eqipment room) I usually put high pressure ball valves with lockout plates in the hydraulic supply and return lines near the equipment.

edit to add: Depending on your ability to stop the equipment instantly and vent any stored energy, you might also consider locking safety shields closed whenever the equipment is running. On some equipment it is not possible to put the machine into a safe state faster than a determined operator can move.

I would imagine the true E-Stop would:

1. Remove power from the pump
2. By-pass/short the hydraulic fluid (route straight from pump back to tank)
3. close off the hydraulic circuit (everything stops dead)

Of course depending on the application there maybe some variance about the above to stop a safe position.

Peter Nachtwey said:

The accumulators store energy like capacitors. They must be discharged too.

Click to expand...

Are there accumulators in a hydraulic system ? I know of pneumatic accumulators, but not of hydraulic accumulators.
Maybe I can learn something.

There are hydraulic accumulators. The most common kind consists of a bottle with a diaphragm with oil on one side and a nitrogen precharge of a couple of thosuand PSI on the other side. Piston accumulators are also common.

http://en.wikipedia.org/wiki/Hydraulic_accumulator


One more source of sometimes overlooked energy is gravitational potential energy. Many hydraulic presses provide a counter balancing pressure to keep hydraulic pistons and attached loads from dropping due to gravity. Each piece of equipment needs to be evaluated for risk but most of the time any safety circuit should generally not vent the counter balancing pressure, you don't want to drop something on the operator. Gravitational potential energy needs to either be released in a safe manner or constrained so that it cannot cause harm.

There are accumulators in some hydraulic systems, they just aren't used in the same manner that pneumatic ones are.

We have one that is used to charge a certain volume of fluid to move a large cylinder a certain distance when discharged.

Completely agree with Peter IF there are accumulators they should be bled off also.

Hydraulic accumulators are commonly used in large hydraulic systems where the loads are of different sizes and or there may be multiple pumps in a lead lag setup. The accumulators store a "reserve" for larger loads to prevent system surges and to prevent short cycling the lag and standby pumps. There can also be "accumulators" in the return line to prevent pressure surge when a large load is bled down.

One thing we do not know is what the hydraulic system serves and what the industry is.

If it were a submarine with the safety demands on the hydraulic system I could comment further. Depth control was done with hydraulics also the hull valves were operated with hydralics. Hydraulics definitely impacted our ability to survive.

Dan Bentler

Agree with that releiving any accumulated energy is a must.
On our system, when you cut the pump, there is no stored hydraulic energy.

Never saw a hydraulic accumulator, but one cannot know everything.
So I did learn something.

There was a "slitting line" installed in 1969. All relay logic. I happened to notice the hydraulic pumps did not shut down when I hit the E-stop one day, while troubleshooting something completely unrelated. I looked at the original prints, and that is how it was designed. Last year, the entire line was "upgraded" to PLC and Digital Drives. The E-stops still don't shut down the hydraulic pumps. I figured they just recreated the original setup. Like I've said though, about half the equipment within the plant is one way or the other... so I get confused.

I'm not an engineer, and I hate to go changing things that I figure an engineer figured out for a reason. I believe the pumps should be shut down but I'm not sure why that wasn't incorprorated into the original design.

YES! There are some accumulators in the hydraulic system, maybe I should look for a "common thread" in our equipment.

Thanks for the replies so far, I didn't even think of the accumulators.