1. Safety Devices and Equipment
Chapter 43
Safety Devices and Equipment
Personal Protective Equipment • Valves • Rupture Discs • Burner Control Systems • Alarm Systems • Hazardous Atmosphere Detectors

2. Personal protective equipment is used to protect individuals from workplace hazards.
A key part of a safety system is safe workers. Manu-facturing and chemical plants can be very dangerous environments. Some of the simplest and most important individual safety devices are the personal protective equipment that people wear while working. Personal protective equipment (PPE) is any clothing or device worn by a worker to prevent injury. See Figure All personal protective equipment must meet requirements specified in OSHA 29 CFR , applicable ANSI and MSHA standards, and other safety mandates. A list of typical protective equipment that is required is as follows:
• hard hats
• steel-toed safety shoes
• safety glasses with side shields
• chemical goggles
• face shields
• chemical or dust masks
• special outer clothing
• long-sleeved shirts
• special chemical boots
• appropriate gloves for the work being done
• fall protection when working at heights

3. A safety valve uses a huddling chamber to enable quick snap-open operation.
A safety valve is a gas- or vapor-service valve that opens very quickly when the inlet pressure exceeds the spring setpoint pressure. See Figure These valves are used in applications where the valve must open quickly to reduce dangerous conditions. The quick-opening action, or pop, is caused by the design of the valve disc. The force applied to the valve disc is the product of the pressure and the surface area of the disc. The disc has a special design that provides two surfaces.

4. A rupture pin safety valve can be used to open a valve at a set pressure or as an emergency shutdown valve.
A rupture pin safety valve is a safety valve with no spring; it is held closed by a pin or a thin rod. When the forces applied against the disc by the process pressure exceed the strength of the pin, the pin buckles and opens the valve. See Figure Since the valve cannot reseat once the pin buckles, this type of valve acts like a rupture disc. The advantage of the rupture pin safety valve is that it has a more precise pressure relief point than a standard rupture disc.

5. A safety relief valve uses a control ring to adjust the opening speed of the valve.
A safety relief valve is a valve that is designed so that it can be set to act as either a safety valve or a relief valve. Safety relief valves are usually used for unfired vessels such as compressed air receiver vessels. A safety relief valve has a control ring, sometimes called a blowdown ring, that is used to adjust the valve opening speed. The control ring adjusts the size of the huddling chamber. See Figure When the control ring is adjusted up toward the valve disc, the exiting vapors are restricted at this point, allowing pressure to build up in the huddling chamber.

6. Safety relief valves are made in standard sizes with standard codes to describe the nozzle.
The manufacturers of valves used to relieve pressure have agreed on a letter coding system to represent the various nozzle areas that are available. This letter code along with the valve inlet and outlet piping sizes identify a specific valve size. See Figure The piping sizes vary somewhat depending on the type of safety relief valve and the manufacturer.

7. A fuel safety shutoff valve is used to stop the fuel flow to a burner.
A fuel safety shutoff valve is a special spring-actuated valve used to stop the fuel flow to a burner system. See Figure Fuel safety shutoff valves are designed to be very reliable and to have no leakage. The internal valve trim is a sliding plate over a circular port.

8. A double block-and-bleed configuration is a safe way of protecting a fuel system from fuel accumulation in the lines.
The NFPA requires that fuel shutoff valve systems be installed in a double block-and-bleed configuration. A double block-and-bleed fuel shutoff is an arrangement with two fail-closed safety shutoff valves in series with a fail-open vent valve in a tee connection between the two. This valve arrangement is also called a double block-and-vent configuration. See Figure The valves are opened and closed together in a program-med sequence. The sequence is established using the open and closed limit switches in the individual shutoff valves of the system.

9. The use of a conventional metal rupture disc is a relatively inexpensive way to relieve pressure in a vessel.
A conventional metal rupture disc is a dished thin metal sheet, placed between a pair of holders, with the process pressure applied to the concave (hollow) side of the disc. See Figure The assembly is mounted between standard pipe flanges. The relieving pressure is determined by the metal thickness and tensile strength. A wide variety of materials are available for the discs. Frequent process pressures greater than 80% of the set pressure applied to the disc can cause premature failure.

10. A reverse buckling disc puts the disc metal under compression loading for a more precise rupture point.
A reverse buckling rupture disc is a dished thin metal sheet placed between a pair of holders, with the process pressure applied to the convex side and with the downstream holder having a set of pointed knife blades. See Figure When the disc starts to buckle from the pressure, it is pushed against the blades, which then cut the disc cleanly. Bulged rupture discs are weaker when the pressure is applied to the convex side, thus the disc material can be made a little thicker and the rupture point can be determined more precisely. In addition, the disc does not need a vacuum support because the convex side of the disc faces the pressure.

11. A composite rupture disc prevents debris and metal shards from blowing downstream from the disc assembly.
A composite rupture disc is a dished thin metal sheet, perforated to provide weak points and combined with a thin polymer sheet to prevent leakage. See Figure Lower rupture pressures can be obtained with a composite disc than with a conventional rupture disc, since the metal is weakened by the perforations. The perforations also control the breakage pattern to minimize loose pieces. A vacuum support is also available for these discs if they are used in vacuum service.

12. A graphite composite rupture disc is very resistant to corrosive chemicals.
A graphite composite rupture disc is a rupture disc composed of graphite powder in a polymer cement and is used where there are corrosive conditions. Additional corrosion protection can be provided by a TFE coating applied to the process side. See Figure Graphite rupture discs are also available to handle both pressure and vacuum relief in the same device. When graphite discs rupture, they release chunks of material downstream.

13. A burner control system is used to sequence the startup, operation, and shutdown of a boiler.
A burner control system is a specially designed solid-state electronics package that provides the sequenc-ing and safety logic for controlling industrial burners. See Figure A burner control system is de-signed to cover various levels of complexity and accept the signals from various types of flame detectors. A burner control system sequences and monitors safe startup, normal operation, and shutdown of the burner system. Burner control systems do not control the burning rate or duration. The burning rate and duration are controlled by other instrument systems. Burner control systems can be used with programmable logic controller (PLC) or safety PLC systems.

14. A UV flame detector is a safety device that allows a combustion operation to continue only when a flame is detected.
The most common industrial flame detectors are those based on the emission of ultraviolet (UV) radiation from flames. See Figure The combustion of carbon-based fuels emits enough UV radiation to be detected. It is necessary for the detector to actually “see” the flame, so the unit needs to be mounted close to and in direct line of sight of the flame. Sometimes the unit needs extra cooling, such as a water jacket, to protect the device from the heat.

15. Flame detectors use a variety of sensors to deter-mine the presence of a flame.
There are other sources of UV radiation that can also be detected by a flame detector. Care must be taken to have the detector aimed at a flame and not at any of these other sources. Some of these other sources of UV include hot refractory materials above 2300°F, bright incandescent light, lasers, and high-voltage coronas. In addition, smoke, dirt, dust, and oil mist can block the radiation and attenuate the signal, causing false alarms. There are other optical flame detectors based on visible light and infrared (IR) radiation. In addition, there are flame rods that must be in direct contact with the flame. See Figure

16. Annunciators combine audible and visual alarms into one system.
An annunciator is a solid-state or relay-based system for monitoring and alarming plant process operations. Annunciators are stand-alone devices that can work with any control system. An annunciator has back-lighted windows marked with a description of the type of alarm and provides an audible signal when there is an alarm condition. See Figure An alarm state typically results in a flashing light behind the appropri-ate window and an audible signal. Pressing a SILENCE pushbutton shuts off the audible signal and switches the flashing light to steady. The steady light remains lit until the alarm condition clears.

17. Annunciator operational sequences are the states of the lights and audible alarms during the various steps of an alarm sequence.
Standards have been developed that provide a description for all the light and audible signal actions used for alarms and subsequent alarms. See Figure Many of these standards are for the operational sequences. For example, sequence M-1-2 has an audible alarm and a flashing light upon an alarm condition. After the operator acknowledges the alarm, the audible alarm is silenced, but the flashing light continues. After the operator resets the flash, the audible alarm remains silenced and the flashing light changes to a steady light. When the process returns to normal, the light remains on until the operator resets to normal.

18. Upper and lower explosive limits in air vary considerably for different chemicals.
The lower explosive limit (LEL) is the lowest concen-tration of a combustible gas or vapor in air that can be ignited. The upper explosive limit (UEL) is the highest concentration of a combustible gas or vapor in air that can be ignited. The lower flammable limit (LFL) and upper flammable limit (UFL) are sometimes used in place of the LEL and UEL. Different combustible gases or vapors have different LELs and UELs. See Figure

19. A combustible gas sensor detects the presence of a flammable gas before the concentration becomes high enough to become a danger.
The sensors used in combustible gas detectors need to be able to respond to any combustible material. A catalytic combustible sensor is a common type of combustible gas sensor that uses a pair of resistors encased in ceramic beads, with the sensor element coated with a catalyst and the reference element coated with impervious glass. See Figure

20. A semiconductor combustible gas sensor measures changes in the resistance of the MOS coating to detect combustible gases.
Another common type of combustible gas detector is a sensor consisting of a semiconductor silicon material with a strip of metal oxide semiconductor (MOS) coating. Each end of the MOS coating has an electrode attached and the resistance of the coating is measured. The resistance of the MOS coating changes when the sensor is heated in the presence of combustible materials. See Figure The semiconductor sensors are very sensitive and are primarily used for low concentrations of combustible gases. Some sensors have heaters with a high demand for power and require large batteries or a wired power source.

21. A toxic gas detector uses replaceable sensor cartridges to detect different gases.
There are several designs of toxic gas detectors. A simple and relatively inexpensive toxic gas analyzer uses a sensing cell with an inert dispersal barrier or flame arrestor with an internal chemical sensitive to the desired hazardous gas or vapor. The toxic gas or vapor can migrate through the barrier and activate the cell. See Figure The cell is set for a specific concentration range of the toxic material and the output is proportional to the concentration. This design can be permanently disabled by exposure to high concentrations of the toxic material and then the cell must be replaced.