Pneumatic Movement on start up - SMC technote

UKB

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Aug 2014
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UK
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Hi
I grabbed this image

Screenshot-2021-02-18-121358.jpg


from this Tech Note
https://www.smc.eu/portal_ssl/webpages/00_local/cee/services/safety_folder/pdf_securitybrochure_en.pdf


I was hoping someone might be able to explain the operation to me in the following scenario...
Scenario:
1) Following the activation of a safety function the cylinder controlled by 2V1 is in the fully extended position.
2) The safety system is then reset.
3) Upon operation of the reset button a free flowing path to the annulus side of the cylinder is made through valve 2V1. The cylinder moves back into its fully retracted state upon operation of the Safety reset button.

-Question
The 13849/1 regs state in section 5.2.2 that



The manual reset function shall
— be provided through a separate and manually operated device within the SRP/CS,
— only be achieved if all safety functions and safeguards are operative,
not initiate motion or a hazardous situation by itself,
— be by deliberate action,
— enable the control system for accepting a separate start command,
— only be accepted by disengaging the actuator from its energized (on) position.



So I guess my question is what's happening? Have I misinterpreted the diagram? or Have I misinterpreted the regs? Or both?

thanks,

Btw, I have sent this question to festo and SMC -but I expect ill get a better quality answer, faster, from this forum!
 
1) Following the activation of a safety function the cylinder controlled by 2V1 is in the fully extended position.
I guess that scenario is only possible if 2V1 is commanded to change state to the retracted position, and then immediately the air supply is cut via the safet valves so the cylinder does not have time to move.
When the safety system is reset, the 2V1 valve remains in the same position and the cylinder moves.

I think that could indeed be a safety issue.


When there are dangerous movements controlled by pneumatic cylinders, we always use 3-position, 2-solenoid valves. So when the solenoids are not activated, the valve stays in the middle position where the pressure is vented.
 
When there are dangerous movements controlled by pneumatic cylinders, we always use 3-position, 2-solenoid valves. So when the solenoids are not activated, the valve stays in the middle position where the pressure is vented.
+1. I've just had this exact discussion with a client who ordered 5/2 way valves and had to send them back and replace them with 5/3 way valves for this exact reason.

One other consideration - there are two types of 5/3 way valves, centre blocking or centre exhaust. That is, the middle position that is in play when neither solenoid is on can either block all ports, or exhaust all ports. If your pneumatics are moving vertically, you will need centre blocking, otherwise when you turn the solenoids off, the pneumatic actuator will fall under their own weight. However, this means that when you press an e/stop, dump the air, and turn off all the solenoid coils, you still have two forms of stored energy present - pneumatic pressure in the cylinder, and gravitational energy from the raised actuator. This will need a very careful risk assessment and potentially other control measures to ensure that people are not exposed to risk.
 
One other consideration - there are two types of 5/3 way valves, centre blocking or centre exhaust. That is, the middle position that is in play when neither solenoid is on can either block all ports, or exhaust all ports. If your pneumatics are moving vertically, you will need centre blocking,
The 'other' measures you mention later can negate the need for blocking the centre venting port.
otherwise when you turn the solenoids off, the pneumatic actuator will fall under their own weight. However, this means that when you press an e/stop, dump the air, and turn off all the solenoid coils, you still have two forms of stored energy present - pneumatic pressure in the cylinder, and gravitational energy from the raised actuator. This will need a very careful risk assessment and potentially other control measures to ensure that people are not exposed to risk.
We prefer that in a maintenance situation all energy is removed before accessing the moving parts.
If there is the risk of potential energy build up (gravitational, or spring force for example), then this risk must can be safeguarded by for example locking pins.
We have had this discussion recently where another department actually wanted to hold a load in position by trapped pneumatic air pressure. I had them change it to an inherently safe design.
 
I want to explain that by trapping air in a cylinder in a situation when a maintenance person can reasonably expect there to be no pressure presents several risks.
1. By removing a hose from a cylinder, the trapped air will be unexpectedly vented, and may cause harm to the eyes of the person performing the maintenance.
2. If the trapped air pressure is used to keep a suspended load in place, by removing a hose from a cylinder, an unexpected movement of both the actuating mechanism and the load may occur, causing serious harm or death to a person getting in the way of the dangerous movement or the load.

(and if anyone is curious, that is the kind of language I use when writing risk assessments).
 
We had a similar problem, many of our machines had heads that were controlled by very large cylinders the operation was from top to bottom, When an emergency stop or guard was broken there was a delay (form the safety circuit PILZ), this delay enabled two locking cylinders to release (i.e. No air spring open) to shoot into the heads to keep them in place, after the delay the air was released from most of the system apart from the head cylinders. We were not totally happy with this, although the lock bolts were more than enough to hold the heads even under air pressure, we modified them so that even the heads would loose the air, the guards had lock bolts etc. so that these would not open until the air had been dumped, it also meant should the lock bolts on the head not operate and locate the head would fall under gravity when the air was dumped, this way it removed a number of potentially dangerous scenario's for example the possible pressure on the head, the delay before being able to open the guard & the possibility of the lock pins not locating would prevent any air being present once the guards are open, no air pressure for maintenance to worry about, the head would naturally drop (if lock pins not located) before the guard could be opened.
 
Completely agree with both your posts Jesper. I've had several projects for a large multinational where they used locking guards, and a dual-channel safety device for positive feedback that the load was fully lowered, which had to be activated before the guarded area can be opened. That's 100% the best practice way to do it.

But on some other projects I've had the client perform a risk assessment and come up with a way to deem it "safe" without such measures. They've noted things like the relatively small weight of the load (lesser potential for harm), the infrequency of access (lesser opportunity for harm), the fact that access without LOTO is permitted only for clearing blockages and not for maintenance (administrative controls), and used a specific type of cylinder and/or fittings that apparently have some kind of "blown hose" protection, which reportedly self-seals a cylinder and keeps the air inside if a sudden loss of pressure is detected on the ports (engineering controls). Personally I'd be somewhat uneasy about this, but hey, it's not my name on the risk assessment, and if their processes reduce the risk to an acceptable level then best of luck to them!
 

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