Backflow protection

The storage system has remotely operated emergency block valves on all transfer lines into and out of the tank. The storage area has the required drainage to direct a spill away from the tank. Backflow protection has been installed and is tested to prevent backflow into the transfer line and storage. 

Used to restrict reversal of flow, check valves should not be considered as positive shut-off valves when flow is reversed, since the seating element is always in the flow stream and subject to erosion (Figures 15-9 to 15-13). A section of a line should not be considered isolated if the only batrier to flow is a check valve. On the other hand, because they do restrict backflow to very low levels, check valves installed in appropriate locations can protect equipment and minimize damage in case of a leak in the upstream line. Some of the advantages and disadvantages of the various check valve configurations are as follows ... 

Many of these enclosures are designed to meet certain standards. For example, the American Society of Sanitary Engineers (ASSE) has developed Standard 1060, Performance Requirements for Outdoor Enclosures for Backflow Prevention Assemblies. If an enclosure will be used to house a backflow preventer, this standard specifies the acceptable construction materials for the enclosure, as well as the performance requirements that the enclosure should meet, including specifications for freeze protection, drainage, air inlets, access for maintenance, and hinge requirements. ASSE 1060 also states that the enclosure should be lockable to enhance security. 

Backflow presents its most serious problems, of course, when human health may be affected. For example, where a single water-supply system furnishes both potable water and process water, great care must be taken to protect potable water from possible contamina-... 

Vacuum breakers (Fig. 3), of both atmospheric and pressure type, also are used to control backflow. They are at best a partial solution, offering some protection against backsiphonage but none against backpressure. 

Another method for controlling both backsiphonage and backpressure is a single check valve (Fig. 4). In theory, a check valve should provide adequate protection. It is, however, entirely dependent on the leakproof integrity of its seals and, as such, is not acceptable for use in situations where backflow into potable-water supplies could include hazardous, noxious or otherwise undesirable materials. 

A more effective method of protecting against back-flow is the double check valve (Fig. 5). Should upstream pressure drop below a certain level, the two check valves close to protect against backflow. When pressure is restored, the valves open. 

Reduced-pressure (RP) devices take over where double check valves leave off. Like the latter, they make it possible to transmit line pressure across a cross-connection, but they are designed to provide a greater degree of protection against backflow where hazardous dowm-stream liquids are involved. 

Protection Flood doors. Flood shields over doors and windows. Provide manually operated backflow valves on floor drains and... 

If there is a central vacuum system, it does not serve areas outside of the facility. The vacuum system has in-line HEPA filters placed as near as practicable to each use point or service connection. Filters are installed to permit in-place decontamination and replacement. Other liquid and gas services for the facility are protected by devices that prevent backflow. 

When utility lines are connected to process lines or equipment, there is a danger of backflow of process material into the utility system and a danger of overpressurization of one system by the other. All connections of this kind require careful study during design and hazard analysis. Many plants review similar connections as a group and develop standard details. This standardization can include formal requirements for the number of layers of protection for each group. 

Cylinders should be protected fi-om physical damage and stored in a cool, dry, well-ventilated area. The temperature in the storage area should not exceed 125°F (51.7 C). The valve protection cap should remain in place unless the cylinder is secured and has the valve outlet connected through appropriate piping to the use point. Do not heat the cylinder to increase the discharge rate. Use a check valve or trap in the discharge line to prevent hazardous backflow into the cylinder. 

Tanks with dedicated import lines, whether these enter at the top or the base can be protected against backflow from the tank by the provision of non-return valves. Lines that enter at the top of the tank and deliver via a dip leg may in some cases be adequately protected by the provision of a siphon break to prevent the tank contents flowing back out via the feed line. 

Prevent backflow into cylinders keep systems simple or add positive protection where necessary. 

The procedures must also cover the source of the gas. If a compressor is used to boost tire gas to filling pressure, its startup, operation, and shutdown must be covered. Backflow prevention must be designed into the system, along with protection against overpressure at the source or tire cylinder. 

Direct flooding from liquid releases as well as backflow flooding of safety systems through drains is a potential problem with implications that are broad in nature. This problem applies only to older plants (see also IH4, Adequacy of the mitigation of the secondary effects of fire and fire protection systems on plant safety). 

In a loading line, an excess flow valve is not practical because it would close upon excess flow into the tank car but would not protect against loss of vapor out of the tank car. Protection can be better obtained by installing a backflow check valve in the loading line at the tank car end of any hose or swivel-type piping. 

Since mechanical valves provide a physical barrier to the backflow of combustion products through the combustor inlet during the positive-pressure phase of the pulse combustion cycle, the unidirectional flow is the fundamental feature of valved pulse combustors. There are, however, certain problems associated with the design of mechanical valves, such as minimizing valve inertia, protection from corrosion, and resistance to material fatigue due to thermal stress. These specific problems are of major importance in heavy-duty pulse combustors operated at large pressure amplitudes (Kentfield, 1993). 

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