| What would you do if you heard an unfamiliar sound coming from your water heater? If you’re like most people you’d make a mental note to keep an eye on it, but ignore it for the most part. Unfortunately, this less than proactive approach often results in water heater floods. As an engineer, I’m more likely than the general population to investigate the cause of the water heater’s sound and proactively seek a remedy before a real problem has a chance to develop.
The FDA’s Hazard Analysis Critical Control Point (HACCP) seeks to accomplish the same with regard to food production. As discussed last week with regard to HACCP Principle 1, those involved in designing food processing equipment must proactively analyze designs to identify potential food safety hazards. Now let’s see how common sense is once again employed through Principle 2, guiding design engineers to take control of situations where hazards have been identified through Principle 1. HACCP Principle 2: Identify critical control points. A critical control point (CCP) is a step in the design process at which a control can be most effectively introduced to prevent or eliminate hazards. In this context a “control” would be a design revision to eliminate hazards identified during the Principle 1 stage. We will once again use the two examples introduced in last week’s blog discussion on Principle 1. In our first example, hazard analysis revealed that food can accumulate in a food processing machine in areas where cleaning is difficult or impossible. This accumulation would eventually rot and fall into uncontaminated food passing through production lines. Design engineers would work to address this contamination hazard by identifying a CCP within the design process, that is, the best place where a preventative measure can be added to the machine setup to facilitate removal of the accumulation. At that CCP, measures can be taken to change the machine’s design. Perhaps all that is needed to correct the situation is to include easy to remove access covers. In our second example hazard analysis revealed that the metal tooling as designed for our food production machine was too fragile and would not withstand the repeated forces imposed on it by the mass production process. This design flaw presents a strong possibility that metal parts will break off and enter food on the line. To correct this situation, design engineers must once again identify the juncture within the design process at which a CCP is identified. There, a preventative measure can most effectively be introduced, enabling more robust metal to be used in the tooling. The previous two examples illustrate CCPs being utilized within the design process. CCPs can also be introduced outside the design process, as when they are identified during the course of training procedures involving the operation, cleaning, and general maintenance of equipment and production lines. And an excellent way of implementing this approach is to have design engineers collaborate with operating and maintenance staff. Working together, they are best able to identify key elements to be addressed and make note of them within written procedures. Now that we have identified some examples of CCPs within the design process, we can move on to HACCP Principle 3 and how it guides design engineers to establish critical limits for each CCP.
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Posts Tagged ‘water heater’
Food Manufacturing Challenges – HACCP Design Principle No. 2
Sunday, October 23rd, 2011
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. _____________________________________________ |
