Backflow check valves

When loading a car, it may be desirable to use a backflow check valve in the loading line to prevent the loss of vapors or liquid from the tank car in the event of a system breakdown. Such a check valve would be installed at the tank car end of any hose or swivel-type piping. 

Simple flow controllers use the differential pressure at an adjustable restrictor the plus and minus sides are on the two sides of a spring-supported diaphragm. These controllers are used as backflow (check) valves or as isolation valves, which shut off if the flow is reversed or too high. Level controllers with floats are in condensate separators often coupled with simple temperature controllers as in Figure 12.28. These devices contain an electrical jump switch and operate therefore as two-point On-Off controllers. Controllers without auxiliary energy are used, where it is not essential to have high control accuracy and when one must do without the auxiliary power for safety reasons. 

Backflow check valves should be installed on all handheld torch sets and at the regulator to minimize possible fires or explosions. 

Backflow due to high backpressure 2. Same 2. Install check valve JFL 2/93... 

Water. Water mains should be connected to plant fire mains at two or more poiats, so that a sufficient water supply can be deHvered ia case of emergency. The plant loop and its branches should be adequately valved so that a break can be isolated without affecting a principal part of the system. If there is any question of maintaining adequate pressure, suitable booster pumps should be iastaHed. Any connection made to potable water for process water or cooling water must be made ia such a manner that there can be no backflow of possibly contaminated water check valves alone are not sufficient. The municipal supply should faH freely iato a tank from which the water is pumped for process purposes, or commercially available and approved backflow preventers should be used. 

Check valves must be used on each pump discharge to prevent backflow through the idle pump. On those systems where a flooded suction is not furnished, an orificed bypass around the appropriate check valve should be furnished to keep the suction line of the non-flooded suction... 

Check valves are required in the piping system at any point where backflow of gas after a shutdown has the ability to restart the compressor, running it backwards or, for that matter, even in the normal direction. Reverse rotation is totally bad, as many components of the various compressor types are not designed for reverse rotation, and there is some possibility, generally remote, that the compressor could reach a destructive over speed. Forward rotation is bad primarily because the intent was to stop the compressor, and it is now operating out of control. This is a problem, particularly if the shutdown was caused by a compressor failure indication, and the need to stop was to prevent further damage. In this mode, it is unlikely that the compressor can attain an overspeed condition. An application with a high potential for backflow is the parallel operation of two or more compressors. 

A mechanic was instructed to fit a steam supply at a gauge pressure of 200 psi to a process line in order to clear a blockage. By mistake, a steam supply at a gauge pressure of 40 psi was connected. Neither supply was labeled the 40 prsi supply was not fitted with a check valve resulting in backflow of process material into the steam supply line. Later the. steam supply caught fire when used to disperse a small leak. 

Backflow Check V alve Flow Safety Valve ... 

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 ... 

Figure 18-4. The check valve was relied on to prevent backflow, and the isolation valves were not used. A spillage was inevitable.

Although not 100% reliable, check valves are usually installed in most piping systems to prevent backflow in... 

A reciprocating piston pump is a pump that utilizes a piston in a cylinder to pull and push the liquid mobile phase from the mobile phase reservoir through the HPLC system. Two check valves (backflow preventers) are in-line to help force the liquid in only one direction. See Figure 13.4. 

After another review of the practical constraints and technical considerations, it was concluded that an eductor vacuum-enhanced recovery system would be feasible and still be cost-effective and efficient. In all, 11 6-in.-diameter wells extending to a depth of 28 ft were installed at strategic locations in the area where product seeps were observed. Each well was serviced by a high-pressure supply and a low-pressure return line. A basic domestic-type deep-well eductor was installed in each well, attached to a drop pipe that extended to 25 ft below the surface. A check valve on the drop pipe prevented backflow into the well during service. The top of the casing... 

Are loading/unloading lines fitted with backflow check or excess flow valves ... 

This company s approach was somewhat different than the approach described in the previous incident. The polymer plant implemented a procedure to require force-loaded check valves for backflow prevention on any temporary connection to piping that contained flammables. The check valve had to be rated for the highest design process pressure of either fluid. [8]... 

Hiac ABS-2 automatic bottle sampler is a pressure sampling device used for batch analysis of volatile or viscous fluids. The ABS-2 delivers liquid to the sensor at flow rates of 10 to 200 ml min T A check valve in the sample introduction line eliminates backflow and minimizes sample cross contamination, and an automatic drain mechanism allows unattended sample analysis of multiple runs. The sampler has a built-in pressure/vacuum chamber that subjects samples to pressures up to 60 psi, accommodating viscosities up to 80 centistokes. In addition, vacuum levels of 23 psi are used to degas samples which contain entrained air. 

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. 

As with the single check valve, the effectiveness of double check valves depends entirely on their sealing integrity. Should the seals develop leaks, undetected backflow could occur. For this reason, double check valves should only be used where backflow could be objectionable—but not hazardous—and should never be used where backflow could cause a potential health hazard or other serious condition. 

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. 

In reduced-pressure backflow preventers (Fig. 7), a zone is provided between the check valves, in which a relief valve continually and automatically maintains pressure at a level lower than supply pressure. Because fluids w ill not flow from a low-pressure zone to a higher-pressure zone, these devices are able to provide excel-... 

Nonreturn valves are used to prevent backflow of fluid in a process line. They do not normally give an absolute shut-off of the reverse flow. A typical design is shown in Figure 5.9g. Since swing-type check valves depend on gravity to close the valve, care must be taken to orient the valve properly when locating and installing it. 

A Check Valve (or similar device) must be installed in the oxygen line immediately upstream of the diffuser per NFPA 86 (sect. 4-4.3.8). The purpose of the check valve is to prevent backflow of any gases into the oxygen piping. 

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