| How do they do it? Your teenager is busily doing his homework, the sound on their stereo speakers cranked up way past the point of your comfort. As the heavy bass beats against your eardrums, their sound waves continue to travel throughout the house, crashing into walls and uprooting small objects from their positions on shelves. Thankfully, controlling this uncomfortable sound level is relatively easy, as you scream out, “Turn that music down!”
Many dangerously loud sounds are not so easily controlled, as when they take place in industrial settings. Here, when complex machinery and manufacturing processes are at full tilt, one can’t just turn a single knob or pull a plug to gain relief. Controlling sound levels in factories, power plants, and construction sites is often a complex task, relegated to engineers with state of the art equipment meant to measure and assess sound exposures in order to devise a strategy to control them. Let’s take a look at a few of these control methods. For our example, we’ll consider the challenge faced by a fictitious company, Widget USA. Business has picked up, and they need to install an additional manufacturing line in their factory. Now widget manufacturing machinery is notoriously noisy, and management is thinking ahead about protecting widget line workers from potentially dangerous sound levels. Their manufacturing engineers dutifully keep this in mind while devising their requirements specification, a list of “must haves” routinely included in quotation requests to potential manufacturers bidding on the job. Of utmost concern is to limit the number of decibels (dB) that the new widget machine can produce. If a manufacturer under consideration is unable to meet these requirements, Widget USA will take their business elsewhere. This methodology essentially nips the excessive noise problem in the bud, eliminating the noise source before it is introduced into the factory, and this is by far the best way of dealing with our scenario. Well suppose things aren’t as neat for Widget USA. Their factory contains many existing manufacturing lines with old, noisy machines. Sure, they’d like to replace them with newer, quieter ones, but there’s a problem, and it’s one all-too familiar to most companies: the expense involved. How can they most effectively deal with this situation? Perhaps Widget USA can modify their existing machinery, or perhaps their overall noise reduction objectives can be achieved by simply replacing worn parts that have a tendency to vibrate. If this measure isn’t sufficient, perhaps sound barriers can be introduced. Whether these are installed around entire machines or parts thereof, they are often effective at absorbing excessive noise. Barriers such as these are made of materials like plastic foam and mass loaded vinyl (MLV) which serve to muffle sound waves. Yet another approach to noise containment is to provide workers in the vicinity of the machinery with sound-absorbing personal safety equipment, ear plugs and the like. If the noise present is loud enough, perhaps a wall, reminiscent of the type often built along stretches of populated highway can be erected. Yet another way to deal with undesirable sound levels is to divert the noise to a location not normally occupied by humans. This is the tact often taken with industrial boilers. Their highly pressurized steam expands so rapidly it can create a deafening roar, and engineers often design piping systems which stem from the safety valve on the boilers themselves up to the roof of the building housing them. In this way the steam and its accompanying noise is safely redirected outside, where only the birds passing overhead will be bothered by it. This wraps up our discussion on sound, its measurement and containment. Perhaps you’ve learned a trick or two to help alleviated unwanted sounds in your environment, whether it’s produced by your teen or your neighbor’s leaf blower. _____________________________________________
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Posts Tagged ‘safety valve’
Pressurized Containers – Industrial Overpressure Devices
Sunday, October 17th, 2010| Perhaps you went out on a drive to enjoy a nice summer day. As you ventured into uncharted territory, you might have ended up in an industrial area. There, you noticed factories, chemical plants, and oil refinery complexes, each surrounded by a huge system of pipes and tanks. You might have considered it to be an eyesore, but if you’re an artist and engineer like I am, you might look at it as a form of art, composed of interesting shapes, colors, and patterns. No matter how you look at it, you can bet that there are at least a few pressurized containers in there.
Last time we saw how something as seemingly harmless as a home water heater could become a dangerous missile if the pressure inside builds to the point where the tank ruptures. You can imagine what kind of explosive forces, steam, and chemicals would be unleashed into the surroundings if an industrial sized pressurized container failed due to overpressure. Let’s explore some other types of overpressure devices that are commonly used in industrial settings. One type of overpressure device is a safety valve. They are similar to a water heater relief valve, but they are generally used to relieve overpressure of gases and steam. How do they work? Basically, a safety valve is attached to the top of a pressurized container as shown in the cut away view in Figure 1 below.
Figure 1 – A Basic Safety Valve In The Closed Position A powerful spring in the valve body is designed to force down on the valve and keep it closed if there is normal pressure inside the container. Once the pressure begins to rise to an unsafe level, it pushes up against the valve and overcomes the force of the spring. The valve opens, as shown in Figure 2 below, and the contents of the pressurized container are safely vented out to an area that is normally unoccupied by people. In case you’re wondering, safety valves are commonly used on pressurized storage tanks and boilers.
Figure 2 – A Basic Safety Valve In The Open Position Another way to address the overpressure scenario is to employ a rupture disc. This is in fact a purposely constructed weak spot. It is intentionally built into a pressurized container and is designed so that it will fail when pressure starts to rise. In fact, this disc is designed to fail at a pressure point just below the pressure at which the container itself would fail. The disc is usually located within a vent pipe, which is in turn connected to the container. Should the disc rupture in an overpressure situation, the contents of the pressurized container will safely flow out of the vent pipe to a place normally unoccupied by people. The advantage of using a rupture disc is that they are made to safely release huge quantities of pressurized substances very quickly. The disadvantage in their usage is that they’re a one-time fix. That is, unlike relief or safety valves which may perform their function a multitude of times, a rupture disc is destroyed once it does its job. They are generally used in industrial settings where potential hazards are greater than at home, so once the rupture disc blows, the complete system generally undergoes a shut down so that the disc an be replaced before the pressurized container can be used again. Another option to pressure containment is the use of a fusible plug, usually constructed of a metal that will melt if the temperature within a pressurized container rises above a certain level. The metal plug melts, and excess pressure is vented through the aperture formed into a safe location. These are often used on locomotive boilers and compressed gas cylinders. Like rupture discs, fusible plugs are a one-time fix and must be replaced once they have done their job. Yet another option to pressure containment is to use a temperature limiting control. This category includes devices that monitor temperature and pressure within a pressurized container. If a dangerous situation should develop, the control system reacts, effectively reducing the pressure to prevent failure of the vessel. Automatic combustion control systems for boilers in electric utility power plants use temperature and pressure sensors to keep pressures within safe limits by regulating fuel and air input to the boiler. Next time we’ll cover the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), which establishes rules governing the design, fabrication, testing, inspection, and repair of boilers and other pressurized containers. _____________________________________________ |
