Taps and Valves

In this section only those taps and valves will be considered which are suitable for repeated use. Break-seals and seal-off points which can be considered as once-only valves are discussed in Section 3.2. Before discussing the different types of taps and valves there is a safety rule which applies to all taps which are operated by turning a key Always use two hands one hand to hold the tap barrel and the other to turn the key. 

1 Glass taps Glass taps should only be used at the pumping end of a vacuum system or where there is no likelihood of them coming into contact with liquids since, whatever grease is used, it will tend to be washed away 

PTFE is essentially inert, but very aggressive chemicals, such as SbFj, may cause deterioration of the sealing ability of PTFE over longer contact times. Therefore, to isolate reservoirs of aggressive chemicals for more than a few days it is better to use a glass break-seal. 

PTFE taps usually have two different types of seal as shown in Fig. 2.6. Seal E is produced by a vertical pressure of the key, whereas seal D is the result of a lateral pressure from the body of the key. The latter seal tends to be the weaker and the more temperature-sensitive, and this should be considered when planning the orientation of the tap in the system the better seal E should always be used to close the more permanent vacuum. 

Finally, it should be remembered that the glass parts of PTFE taps are made by moulding and not by blowing, so that they have some internal strain, and therefore the body of the tap must not be heated unevenly when it is fused to another piece of glass. 

As can be seen from Table 54, a miiiiber of glass taps and stop-cocks have been standardized. 

189 and 190 show taps for vacuuin work. Kinza has devised a three-way tap [12] having the unusual feature that if the plug is always turned in the same direction no mistakes in position can be made in vacuuin work (Fig. 244). Wolf [13] has published an excellent review of standardized and special-purpose taps. The latter include the Vestale rotary disk shown in Fig. 246. Straight-way taps, drain cocks, three-way and four-way taps are supplied for semi-technical purposes. 

Even with ordinary taps the plug should be protected against coming off (Fig. 217). As there is always a danger that the valve plug may be withdrawn accidentally when it is turned, it is advisable to provide the valve with a rubber disc or ring, or better still with a safety clip of steel wine (Fig. 246). 

Glass taps with greased ground surfaces, in particular with dimensions 40 cm, are unsuitable for semi-technical or pilot plants. In these cases valves are used instead of taps, especially since the combination of glass and Teflon has made possible many 

12551/67 Sheet 1 preliminary standard one-way tap cooks, provided with non-interchangeable, massive cocks 

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Gases. Few problems arise over homogeneity of gas mixtures where the storage vessel is not subjected to temperature or pressure variations. Difficulties may arise if precautions are not taken to clear valves, taps and connecting lines of any other gas prior to passage of the sample. Similarly care must be taken that no gaseous components will react with the sampling and analytical devices. 

The simplest, and oldest, form of seal is the packed gland, or stuffing box, Figure 5.12. Its applications range from sealing the stems of the water taps in every home, to proving the seal on industrial pumps, agitator and valve shafts. 

Ballcocks, faucets (taps), faucet cartridges, faucet underbodies, valve stems. .. Irrigation equipment and valves. .. 

Porcine hearts are obtained from a slaughterhouse. It is absolutely necessary to get the hearts within 5-10 min after killing the animals. The hearts have to be smooth (consistence comparable to liver) firmly contracted muscle should be discarded. Wash the hearts with running cold tap water and remove right ventricles, autrides, atria, fat, large vessels, tendons, and valves. Chop the left ventrides into pieces of about 1.5 cm and freeze immediately in liquid nitrogen. Store the tissue at -70 °C. 

They require a lot of space because of the need for glass-to-metal seals (Housekeeper seals) on each side of the tap and even more if small spirals are included. The purpose of these spirals is to absorb any movement which occurs as the tap is manipulated. This is particularly worthwhile for metal valves which often require more force to turn. 

Nevertheless, metal valves tend to be more reliable and robust than PTFE or glass taps and therefore last longer, and they are to be recommended when frequent use of a very corrosive material is expected. 

The flash gas from the top of F-l passed through a wet-test meter to a sample tap and was vented to the atmosphere. Condensed liquid product was collected through a valve at the bottom of F-7, weighed, and analyzed. 

As a purge medium, to prevent pressure taps and other instrument connections and relief valves, from plugging... 

The high-pressure water supply is employed for the operation of the ordinary filter pump , which finds so many applications in the laboratory. Several types of water-jet pumps of glass, plastic or metal construction are available from most laboratory suppliers. These are often fitted with a suitable non-return valve to prevent the apparatus being flooded as a result of fluctuating water pressure. Connection to the water tap in the case of the metal pump is by a direct screw-threaded joint with the glass or plastic models high-pressure tubing of suitable bore is wired to the tap and to the pump. 

Fig. 7.18 Mini-laparoscopy equipment (1.) trocar sleeve, 0 5.5 mm, with insufflation tap and magnetic-ball valve (2.) trocar, conical tip (i.) mini fibre laparoscope, 0 2 mm, angle of view 0°, working length 270 mm (4.) Veres cannula, 0 2.3 mm, with outer sheath, working length 270 mm. (autoclavable)...

For housing a petrol gas plant we need only a small room. The gas can be distributed to the practical tables using a 2" main gas pipe with further distributaries of 1/2" pipe. Each practical table is provided with gas taps and these taps should be of the rigid nozzle type and be fitted towards the back of the table. The gas tables be fitted in such a way that the point upwards and are at an angle of 45° from each other. If double benches are provided them taps should be fitted along the centre line of each bench. In most of the laboratories iron pipes are used but it would be preferable, in case of physics laboratory, if we use brass pipes. For controlling the supply of gas in addition to main control valve provision be made to control the supply of gas to each group of tables. These controls should be easily accessible to teacher and should not be easily accessible to students. 

Placement and orientation of the pressure taps need to be considered in the equipment design to allow for pressure measurements. The pressure taps should be positioned perpendicular to the flow of fluid, otherwise the velocity head of the fluid will alter the measurement. Placing ball valves between the pressure taps and measurement device is commonly done (especially when high pressures are involved) so that the pressure measurement device can be safely taken out of service for calibration or replacement. The pressure measurement device is preferably located near the pressure tap but is not essential. The diameter of the tubing connecting the pressure tap to the measurement device should be of adequate diameter to... 

Basic Supervisory Control and Data Acquisition Systems (SCADA) Pressure testing for steel and plastic pipelines Hot tapping and stopping Characteristics and properties of natural gas Excavation safety Compressor operation Pipeline purging Valve maintenance Cathodic protection... 

Open the orifice taps bypass valve. The indicated flow will then drop to zero, and the trip should be activated. 

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