Valve non-return

The high-pressure water supply service is employed for the operation of the ordinary filter pump, which finds so many applications in the laboratory. A typical all metal filter pump is illustrated in Fig. 11, 21, 1. It is an advantage to have a non-return valve fitted in the side arm to prevent sucking back if the water is turned off or if the water pressure is suddenly reduced. Theoretically, an efficient filter pump should reduce the pressure in a system to a value equal to the vapour pressure of the water at the temperature of the water of the supply mains. In practice this pressure is rarely attained (it is usually 4 10 mm. higher) because of the leakage of air into the apparatus and the higher temperature of the laboratory. The vapour pressures of water at 5°, 10°, 15°, 20° and 25° are respectively 6-5, 9-2,12-8, 17 5 and 23 8 mm. respectively. It is evident that the vacuum obtained with a water pump will vary considerably with the temperature of the water and therefore with the season of the year in any case a really good vacuum cannot be produced by a filter pump. 

This is provided to prevent reverse rotation of the pump in the event of a power failure or a deliberate shutdown due to backflow of liquid from the rising mains (pipelines). This is located immediately after the last pump stage casing/discharge outlet to prevent the shaft from rotating in the reverse direction. The provision of a non-return valve also ensures that the pump always starts in a shut-off condition, when the power requirement is at a minimum. 

Non-return valve with pneumatic actuator and solenoid valve... 

Many accidents occur because process materials flow in the wrong direction. Eor example, ethylene oxide and ammonia were reacted to make ethanolamine. Some ammonia flowed from the reactor in the opposite direction, along the ethylene oxide transfer line into the ethylene oxide tank, past several non-return valves and a positive displacement pump. It got past the pump through the relief valve, which discharged into the pump suction line. The ammonia reacted with 30m of ethylene oxide in the tank, which ruptured violently. The released ethylene oxide vapor exploded causing damage and destruction over a wide area [5]. A hazard and operability study might have disclosed the fact that reverse flow could occur. 

In larger vertical storage tanks containing residual fuels, bottom inlet filling may sometimes be used. A non-return valve must be fitted in the filling line as close to the tank as possible. The entry position of the filling pipe into the tank in relation to the position of the draw-off connection must be carefully selected to avoid air entrainment into the oil-handling system. 

Competent personnel in conjunction with the manufacturer s maintenance instructions improve safe operation of all plant by means of regular maintenance. Re-commissioning in a safe manner should always follow this. It is particularly important that valves such as safety shut-off valves, non-return valves, etc. are regularly checked for soundness. 

Constant-pressure governors are required at various stages of the gas supply within the user s premises from the first pressure reduction from distribution pressure to supply pressure at the meter installation to the appliance governor. The complexity and design of the governor installation depends on pressure, throughput, duty, etc. In addition to governors, pressure-relief, slam-shut and non-return valves may be relevant in some installations. 

It is important to note that the requirements of this standard are specific to gas, and include a low-pressure differential under forward-flow conditions. It is unlikely that a non-return valve suitable for compressed air, etc. will be acceptable for gas. 

In the particular case of oxygen, valves shall be resistant to exposure for up to 12 h at all pressures up to 2 bar at 20° C. Requirements for non-return valves for oxygen are also discussed in the British Gas publication Guidance Notes on the Use of Oxygen in Industrial Gas Fired Plant and Working Flame Burners (lM/1). 

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. 

Soundness of non-return valve with regard to reverse flow ... 

The period between such tests will depend on the maintenance schedule for the plant, which should also include the in-situ soundness testing of non-return valves and safety shut-off valves by bubble leak detectors, etc. 

Dissimilar pumps may be used. As Figure 32.47 shows, the resulting combined curve indicates that pump 2 will not begin to deliver until the specific energy from pump 1 drops below its shut-valve energy rise, therefore if the non-return valve for pump 2 does not seat, pump 1 can cause it to turbine. 

Where two or more compressors feed into a single air line, the discharge line from each compressor shall be fitted with a non-return valve and isolation valve at the furthest point from the compressor or outlet, just prior... 

Non-return valves used in compressor delivery lines must be designed to withstand the pressure, temperature and pulsations of compressed air. 

Where a refrigeration system is to provide cooling in summer and heating in winter, the design of the system will need to be a compromise of the two functions. In particular, the expansion valve cannot work in both directions, so two valves are required, with the necessary extra piping and non-return valves (see Figure 30.1). 

If a non return valve is not incorporated in the delivery or suction line, the liquid will run back into the suction tank as soon as the pump stops. 

Oscillations occur because the liquid column requires more than the equilibrium quantity of air to produce the initial acceleration. It therefore becomes over-accelerated and excess of liquid enters the limb with the result that it is retarded and then subsequently has to be accelerated again. During the period of retardation some liquid may actually run backwards this can readily be prevented, however, by incorporating a non-return valve in the submergence limb. 

All LC pumps require the use of a non-return valve and so the function of non-retum valves will now be described. 

Non-return valves are essential in the operation of pumps. It is important to ensure that when the pump compresses the solvent, it leaves at the exit port and is completely arrested at the inlet port. Conversely when the pump draws fresh solvent into the cylinder, the non-retum valves must allow solvent to flow through the inlet valve, but flow-back from the other side of the exit valve must be... 

With careful design and exacting construction these types of valves can be extremely efficient. In practice, to ensure absolute minimum back flow through the valve, a single non-return valve assembly usually contains two non-return ball valves connected in series as shown in figure 6. 

All respiratory protective systems should be stored in clean, dry conditions but be readily accessible. They should be inspected and cleaned regularly, with particular attention to facepiece seals, non-return valves, harnesses etc. Issue on a personal basis is essential for regular use otherwise the equipment should be returned to a central position. Records are required of location, date of issue, estimated duration of use of canisters etc. 

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