Reversibility check

Can the vehicles drive in and out without reversing Check separation distances from equipment, buildings, other activities, etc. 

To reversely check the kinetic model, the integral rate equation (for non-isothermal conditions) describing the As release during pyrolysis of CCA treated wood is used in combination with the measured temperature profiles T(t) in order to calculate the corresponding As content of the pyrolysis residues. The calculated arsenic content of the pyrolysis residues is compared with the experimental values labscale and TGA experiments) in the parity plot, presented in Figure 5. 

The obverse/reverse check in XPREP (see the file retl.prp) gives the same result and estimates the fractional contribution of the second domain as 0.16 ... 

Self-checking Monitors Reversal Check Fanlt Tolerance... 

In view of the complications which may be produced by surface hydration, hydroxylation and ageing, it is essential to check the reproducibility and reversibility of water isotherms if sound conclusions are to be... 

Instead of calculations, practical work can be done with scale models (33). In any case, calculations should be checked wherever possible by experimental methods. Using a Monte Carlo method, for example, on a shape that was not measured experimentaUy, the sample size in the computation was aUowed to degrade in such a way that the results of the computation were inaccurate (see Fig. 8) (30,31). Reversing the computation or augmenting the sample size as the calculation proceeds can reveal or eliminate this source of error. 

Check Valves Tlie pmpose of a check valve is to allow relatively unimpeded flow in the desired direction but to prevent flow in the reverse direction. Two common designs are swing-tvpe and lift-type check valves—the names of which denote the motion of the closure member. In the fonvard direction, flow forces overcome the weight of... 

Check for proper rotation and mounting of impeller. Reverse motor leads if necessary. 

Swing Check Valves These valves (Fig. 10-156) are used to prevent reversal of flow Normal design is for use only in horizontal lines, where the force of gravity on the disk is at a maximum at the start of closing and at a minimum at the end of closing. Unhke most other valves, check valves are more likely to leak at low pressure than at high pressure, since fluid pressure alone forces the disk to conform to the seat. For this reason elastomers are often mounted on the disk. Swing checkvalves are available with low-cost insert bodies. 

Closure at the instant of reversal of flow is most nearly attained in these valves. This timing of closure is not the whole solution to noise and shock at check valves. For example, if cessation of pressure at the inlet of a valve produces flashing of the decelerating stream downstream from the valve or if stoppage of flow is caused By a sudden closure of a valve some distance downstream from the check valve and the stoppage is followed by returning water hammer, slower closure may be necessary. For these applications, tilting-disk check valves are equipped with external dashpots. They are also available with low-cost insert Dodies. 

If a check valve is used in a system without an arrestor, excessive pressure may be exerted on the system when the reversing water column is violently stopped by the check valve. If a float-type air vent is located between the check valve and the closing valve, the float could easily be ruptured. 

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. 

Check Valve - A check valve is normally not an acceptable means of preventing overpressure by reverse flow from a high pressure source. However, where leakage throu a check valve would not result in overpressuring by more than 1.5 times design pressure, a check valve is acceptable. 

Connections to equipment are typically 50 mm and 80 mm for process vessels and exchangers, according to the size of the equipment. Each connection includes an accessible block valve. Double block valves are provided if required. A check valve should be included if overpressure or other hazard could result from reverse flow during simultaneous drainage from more than one vessel. Individual connections from the equipment are made into the top of the drain header. 

Some chemical reactions are reversible and, no matter how fast a reaction takes place, it cannot proceed beyond the point of chemical equilibrium in the reaction mixture at the specified temperature and pressure. Thus, for any given conditions, the principle of chemical equilibrium expressed as the equilibrium constant, K, determines how far the reaction can proceed if adequate time is allowed for equilibrium to be attained. Alternatively, the principle of chemical kinetics determines at what rate the reaction will proceed towards attaining the maximum. If the equilibrium constant K is very large, for all practical purposes the reaction is irreversible. In the case where a reaction is irreversible, it is unnecessary to calculate the equilibrium constant and check the position of equilibrium when high conversions are needed. 

The combined rod load should be checked anytime the gas loads are approaching the maximum rating of the compressor frame or anytime rod reversal is marginal or questionable. 

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 ... 

Check No No To restrict reversal of flow Isolation Swing check Valves only some cases Low /."-tp"... 

Remotely operated emergency isolation valves (see Section 7.2.1) should be fitted on filling lines. If the hose breaks for any reason, the flow can be stopped by pressing a button located at a safe distance. Reverse flow from the tank truck or car can be prevented by a check valve. 

Check valves and other reverse-flow prevention devices, if their failure can affect the safety of the plant. 

If a pump trips (or is shut down and not isolated), it can be driven backward by the pressure in the delivery line and damaged. Check valves are usually fitted to prevent reverse flow, but they sometimes fail. 

When the consequences of reverse flow are serious, then the check valve should be scheduled for regular inspection. The use of two, preferably of different types, in series, should be considered. The use of reverse rotation locks should also be considered. 

Pressure is released and returns are checked. If no returns are noticed, the packer by-pass is opened and excess cement reversed out. Washing off cement in front of perforations can be performed by releasing the packer and slowly lowering the work string during the reversing. 

It is essential for the safe operation of a non-return valve that it be regularly checked and be shown to be resistant to reverse pressures. To enable this check to be carried out, the non-return valve should be installed with a manual isolation valve at both inlet and outlet. 

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