Change over valves

Wechsel-ventil, n. change-over valve, -ver-hkltnis, n. reciprocal relation reciprocal proportion. 

As the name implies, the sample is introduced into the mass spectrometer as a gas (Nier 1940). There are two types of sources, the classic viscous flow source and the continuous flow source. The viscous flow source typically consists of two identical inlet systems that are coupled to the mass spectrometer by a change-over valve, which allows rapid switching for comparison of isotope ratios measured for sample and standard gases. In the continuous flow source, samples gas is introduced as a bubble in a non-reactive carrier gas stream. 

Automatic gas flow change over valves with adjustable timers for PSA and de-oxo towers... 

Detectors can be fitted to indicate the quantity of product remaining in the pressure pot and thus predict when the product will be completely used up. For some systems an automatic change-over valve can be installed to switch supply from a second pressure pot thus maintaining supply of adhesive to the production line. 

Use separate control valves for heating/cooling Use same media for heating/cooling Design for rapid change over of services... 

The reaction mixture is heated and allowed to reflux, under atmospheric pressure at about 100°C. At this stage valve A is open and valve B is closed. Because the reaction is strongly exothermic initially it may be necessary to use cooling water in the jacket at this stage. The condensation reaction will take a number of hours, e.g. 2-4 hours, since under the acidic conditions the formation of phenol-alcohols is rather slow. When the resin separates from the aqueous phase and the resin reaches the requisite degree of condensation, as indicated by refractive index measurements, the valves are changed over (i.e. valve A is closed and valve B opened) and water present is distilled off. 

Improved emergency core cooling (ECC) reliability by reduction in number of valves, auto change-over to recirculation phase, sustained low pressure conditioning, four heat exchangers and four pumps... 

One day the automatic equipment broke down, and the foreman decided to change over to manual filling. He asked the drivers to check that the hand valves on the filling lines were shut, but he did not check himself. He then operated the switches that opened the automatic valves. Some of the hand valves were open. Gasoline and other products came out. overfilled the tanker (or splashed directly on the ground) and caught fire. Three men were killed. 11 injured, and the whole row of 18 filling points was destroyed. 

Figure 9.4 Solenoid valves, (a) Shut-off (Courtesy of Bailey Gill Products Ltd), (b) Change-over (Courtesy of Banco Controls Ltd)...

If the greater demand is for heating, the flows are reversed. The hot gas from the compressor goes directly to the solenoid valves, and so to each unit, to give up its heat, liquefy and pass back to the compressor unit, to be boiled off in the outdoor coil. Now if any unit wishes to cool, the solenoid valves on this item changes over and liquid will be drawn from the heater in the distributor, and pass as a gas, through the separator and back to the compressor. 

Where the switching of plant is purely manual, the plant instructions should specify the limits of control, and not leave these to the shift operator, who may not be sufficiently skilled to take the correct decisions. Standby plant is often fitted, and it is part of the operation discipline to change over machines to ensure that they get even wear and keep all sets in running order. All operation staff should be aware of the method of bringing standby plant into use in an emergency. Where refrigerant valves need to be opened or... 

Matsen (1985) pointed out a number of additional problem areas in scale-up such as consideration of particle size balances which change over time due to reaction, attrition and agglomeration. Erosion of cyclones, slide valves and other components due to abrasive particles are important design considerations for commercial units which may not be resolved in pilot plants. 

The control of combustion is based on regular readings of the resistance and its change over time. The gas valve opens with a heat load-dependent velocity, so the air excess ratio decreases. While the change exceeds a certain limit and the resis-... 

The time period 2L/a is known as the characteristic time of the pipeline. If a change of valve setting is made in a time less than the characteristic time, the adjustment is effectively instantaneous and the pressure rise is given by equation 10.40. In order for the valve adjustment to be effectively gradual, it must be made over a period of time that is long compared with the characteristic time 2L/a. 

An example of a causal factor chart for a relatively simple incident is shown in Figure 9-8. In this example, there are two redundant pumps, one of which is required to supply feed to a reactor downstream. The operator is requested to change-over operation from Pump A, which is running, to Pump B, which was previously shutdown. Instead of opening Pump B suction valve, the operator opens the wrong valve, causing the Reactor to trip on low flow detection. 

During valve change over, the closed valve should complete it s opening cycle before the open valve closes. 

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