The safe placement of equipment with respect to a potential hazard is among the most effective (but least discussed) measures available for machine safeguarding. This technique is particularly relevant when attempting to comply with the rules related to eliminating trapped energy outlined in ISO 13849-1 in situations where exhaust time and the volume of compressed air in a machine are factors in enhancing safety.
The questions posed in a risk assessment, such as the severity of injury and the frequency of exposure, must be weighed as part of considering all aspects of a machine’s design.
The ISO 13855:2010 standard establishes the positioning of safeguards in relation to the approach speeds of parts of the human body.
The first questions typically to be considered are “How quickly can we bring this machine to a safe state?” and “How close to the hazard can the safety equipment be placed? It’s also important to consider the switching time for machine safety devices and exhaust rates in faulted conditions. Although the standard doesn’t mention exhaust flow specifically, it’s ultimately a critical aspect of bringing a pneumatic machine to a safe state.
A solution that combines products, switching time and flow rate considerations is necessary to ensure machine safety. However, the calculations necessary to create this solution can’t be completed until the safety products are identified because they all have differing flow rates and exhaust times.
“Faulted” flow rates are essentially worst-case scenario flow rates and are important considerations in calculating safe distances. When choosing components, it’s essential to know their exhaust times because these times will ultimately impact where machine guards should be placed.
Figure 1. Safety exhaust valves
The Safe Distance Formula
The ISO 13855 standard offers a rule-of-thumb formula for calculating minimum safe distance (for spacing purposes):
S = (K × T) + C
S = minimum distance (in mm) distance between the safeguard and the hazard zone necessary to prevent a person or part of a person reaching the hazard zone before the termination of the hazardous machine function
K = approach speed parameter (in mm/s) derived from data on approach speeds of body parts.
T = time interval (in seconds) overall system stopping time between the actuation of the sensing function and the termination of the hazardous machine function.
C = intrusion distance (in mm) speed which a part of a body (usually a hand) can move past the safeguard towards the hazard zone.
Safe distance time (T) is further defined in ANSI and OSHA standards:
T includes TS + TC + TR + TBM
where:
TS = stop time of equipment measured at the final control element
TC = response time of the control system
TR = response time of the presence-sensing device and its interface.
TBM = additional time for the brake monitor to compensate for variations in normal stopping time (where applicable).
Time to Consider T (time interval)
There are no standards to explain the correlation of exhausting and spacing, so machine designers typically must fall back on a commonsense approach to integrating safety exhaust valves into their machines.
The closest the standards come to explaining is to discuss “overall system stopping time.” Ideally, for the sake of safety, this time interval would include the time to exhaust, but this isn’t always practical. However, most of pneumatic safety exhaust valve suppliers can provide data for machines of various machine volumes, such as the example shown in Table 1.
Table 1. Exhaust Time – Normal and Faulted Conditions (s). The exhaust time data in this example refers only to the P33 Safety Exhaust Valve from Parker Hannifin; it is not relevant to safety exhaust valves from other manufacturers.
Note: A machine with 150L of trapped air at 6 bar requires 7.30 seconds to exhaust in faulted condition, so it may not be practical to assume full exhaust of a large machine. Instead, it’s best to weigh how to use this information to make the best decision for an application to ensure safety.
Optimizing Size for Both Switching Time and Exhaust Time
Before settling on a specific piece of equipment, it’s critical to evaluate its switching and exhaust times so that they can be included in the safe distance calculations whenever possible. In an ideal world, hazardous locations would be designed to use a small area of compressed air for rapid depressurization, but this isn’t always achievable when retrofitting a machine into an existing space.
It isn’t always possible to include exhaust time considering that it may take several seconds to exhaust larger machines. Here is where using sound judgment and relying on good engineering principles come into play. Once a safety valve is open to exhaust, it will start to reduce the pressure buildup in the machine rapidly, eventually returning the machine to a safe state. If it’s impossible to plan spacing based on exhaust time, other options are available to enhance machine safety, such as adding safe stopping devices like rod locks or clamps.
Safety Exhaust Valves and Safe Distance
Safety exhaust valves serve three primary functions. The first, obviously, is to exhaust compressed air rapidly. The second is to prevent restarting if a fault is detected. In the case of redundant designs, the third is to ensure both channels are operational or to fail safe/exhausted (even if no power is available).
Faulted Exhaust Time
To depressurize a machine as quickly as possible, choose a safety exhaust valve with the shortest possible faulted exhaust time. Let’s consider our previous example of a machine with a volume of 5.30 ft3 (150 L) operating at 90 psig or 6 bar. The P33 safety exhaust valve detailed in Table 1 will exhaust in faulted condition (worst case) in 7.30 seconds. Use the faulted exhaust time data from the manufacturer of a safety exhaust valve under consideration to weigh the depressurization situation fully.
Option 1: Employ secondary means
When the machine takes too long to exhaust, and an unsafe condition is still present, employ a secondary means of stopping, blocking or holding a load in place, such as clamping or rod locks.
Option 2: Double up
If the pneumatic air supply offers sufficient pressure to meet the safety exhaust valve’s operating pressure requirements, consider using two safety exhaust valves (as shown in Figure 2), an option that’s especially useful on large machines. Keep in mind that many safety exhaust valves need a minimum input pressure of 30 psig.
Figure 2. One option for faster depressurization of larger machines is the use of two safety exhaust valves instead of one
Follow the standards that provide greater detail on the height of machine guards and the basic safe distances required for parts of the body. Furthermore, stop-time measurement devices should be used to measure how long a machine takes to stop after a signal is given. Only then is it possible to establish the location of the safeguarding device (such as a light curtain) in relation to the hazard with the safety distance in mind. Calculating where to put safety input devices like gates or light curtains is a critical step in machine design. Determine machine guard spacing before building and testing the machine. It’s also important to check the stop-time device as part of the plant maintenance routine to ensure the machine’s stopping ability hasn’t been compromised by wear-and-tear or tampering.
Filed Under: Industry regulations
