Switch over valves

Regenerative heat exchangers of both the fixed-bed and moving-bed types (67) have been considered for MHD use. The more recent efforts have focused on the fixed-bed type (68), which operates intermittently through recycling. A complete preheater subsystem for a plant requites several regenerators with switch-over valves to deflver a continuous supply of preheated air. The outlet temperature of the air then varies between a maximum and a minimum value during the preheat cycle. 

More experience in application of olfactometric technics and working out the guideline showed us a basic disadvantage of the TO 4 type the panelist shouldn t be able to see the setting of the dilution level. Therefore the TO 4 type got some modifications. The mask was turned to the other side of the equipment and the TO 4 got an additional throttle valve and a flow selector, fig.l. That means, the panel leader is able to switch over from odourless air to the adjusted dilution and to a mixture of both which is twice as much diluted as the adjusted dilution. 

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. 

Figure 5.15.C shows the auxiliary manifold used for application of this sensor to the determination of hydrogen peroxide. The sample is injected into an appropriate buffer that is merged with a stream of substrate. The mixture then reaches the biosensor, where the enzymatic reaction and retention of the product formed take place. On switching the valve, the eluting stream flushes the reaction product retained in the biosensor, which is thus made ready for the next sample. In this way, H2O2 can be determined at the parts-per-billion level over a wide linear concentration range with excellent reproducibility.

There are, however, specially designed switch-over systems on the market that respect the API recommendation of a maximum pressure drop of 3% and that are designed to have a smooth flow path. This needs careful checking with the manufacturer, as the 3% is considered over the valve only and does not take into account any additional pipe riser runs (Figure 6.9). 

Earlier in this book, we cautioned against using the type of changeover valves or inlet isolation valves related to inlet pressure drops. The main factor here is the Cv (valve flow coefficient) value of the changeover device or block valve used upstream of the SRV. If individual foil-bore valves are used, the same principle applies as for changeover valves. Note once more, however, that if individual valves are used upstream of an SRV, they must be interlocked if used in a switch-over system or locked open if used in an individual isolation... 

The analysis is started with the guard column and separation column in series. After about 3 min, the valves, controlled by the detector programme, switch over such that the eluent flows past the guard column, direct to the separation column. The columns are switched at a point when the components to be determined, but not the interfering matrix, have reached the separation column. After about 35... 

This patented valving system provides switch-over from high pressure recirculation to impingement, and vice versa, without interruption of the flow streams preventing related pressure changes in the material supply lines. 

After the adsorber has been steamed, the outlet valve is switched over. The steamed adsorber is then dried with solvent-free air taken from the laden adsorber. A feature of this process is that the steamed adsorber is always dried with solvent-free air. An advantage of this two-stage process is that the laden adsorber can be loaded beyond the breakthrough capacity since the second adsorber is unladen and can therefore accept any excess solvent. 

Recuperators with high alumina bricks Alter the temperature of the exit air starts falling below a certain predetermined value, the unit is taken up for passing hot gases again. The hot gas and cold air streams are thus passed alternately by operating butterfly valves in the ducts which are switched over manually or automatically (can be by timers) as per temperature patterns desired. 

More recently electrically actuated valves have become available and can be integrated into the Microscal CALDOS PC based FMC control and analysis system. This allows the user to leave the FMC unattended for the duration of the entire experiment, as there is no need to return to the instrument and switch the valves over manually... 

Measurements based on switching over the three-way valve (placed outside the hot reactor) and accumulating soUds in a vessel through the certain time period. No continuous measurements possible... 

In the suction and pressure lines, simple ball valves are provided as manual shut-off valves (AA06, AA08, AAll and AA14). Driven valves or non-return valves are not required for the automatic switch-over of the pumps, as the pumps con-... 

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. 

With the application of injection-process regulation, the mould internal pressure is first measured and compared with a nominal value. If there is a deviation, a hydraulic valve that applies pressure to the injection cylinder is actuated. It is thus possible to follow the nominal value precisely and independently of negative factors. The switch over from injection to holding pressure also occurs as a function of internal pressure. Thus, no pressure spikes can occur since the switch over takes place when a specified threshold value is reached. 

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. 

Closed-position indicators on the valves usually satisfy a system check . These show that the valves are nominally in the closed position but do not actually prove that there is no leakage. A proving system can be met by various systems which prove the soundness of the upstream and downstream safety shut-off valves by sequential pressurization, evacuation, mechanical over-travel with limit switches and a normally open vent valve, or other means. 

When coolers or condensers are shut-down but remain full of water, the amount of current required to maintain satisfactory cathodic protection is considerably reduced. If the current is not reduced over-protection occurs and excessive amounts of chlorine can be generated which would tend to accumulate in the upper section of the water boxes causing considerable corrosion, not only to the water boxes, but also possibly to the tubes. To ensure against this a stand-by condition should be included on the control panel which effectively reduces the current required under shut-down conditions. This control is effected by a limit switch fitted to the outlet valve of the condenser or cooler concerned. It is impossible to determine exact requirements for the protection of circulating water systems in advance and it is normal to adjust the current to provide protection during commissioning. 

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