Valves, check

Available in wafer design for mounting between flanges. 

Easier to cut out in sandy. service than full-opening swing. 

Does not have a tendency to slam shut on flow reversal. 

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Backflow due to high backpressure 2. Same 2. Install check valve JFL 2/93... 

Most small Hquid helium containers are unpressurized heat leak slowly bods away the Hquid, and the vapor is vented to the atmosphere. To prevent plugging of the vent lines with solidified air, check valves of some sort are included in the vent system. Containers used for air transportation are equipped with automatic venting valves that maintain a constant absolute pressure with the helium container in order to prevent Hquid flash losses at the lower pressures of flight altitudes and to prevent the inhalation of air as the pressure increases during the aircraft s descent. Improved super insulation has removed the need for Hquid nitrogen shielding from almost all small containers. 

Fig. 6. Homogenizer used as a timing pump for HTST pasteurization. Details of bypass, relief lines, equalizer, and check valves are not included (10).

Water. Water mains should be connected to plant fire mains at two or more poiats, so that a sufficient water supply can be deHvered ia case of emergency. The plant loop and its branches should be adequately valved so that a break can be isolated without affecting a principal part of the system. If there is any question of maintaining adequate pressure, suitable booster pumps should be iastaHed. Any connection made to potable water for process water or cooling water must be made ia such a manner that there can be no backflow of possibly contaminated water check valves alone are not sufficient. The municipal supply should faH freely iato a tank from which the water is pumped for process purposes, or commercially available and approved backflow preventers should be used. 

Storage and Handling. The acid should never be allowed to stand in a line completely sealed between two closed valves or check valves. Excessive pressure caused by thermal expansion of the Hquid can cause leaks or pipe mptures. AH lubricants and packing materials in contact with chlorosulfuric acid must be chemically resistant to the acid. Elanged connections are recommended over screwed fittings and flange guards should be used. 

Reciprocating pumps are those most commonly used in high performance Ic. The single-piston type usually has inlet and outlet check valves with some mechanism such as variable stroke frequency to minimize the effect of pump pulsations. Dual-piston pumps operate with the pistons 180° out of phase to minimize pulsations. For this system to work optimally, the piston units must be identical. 

Gate valve Globe valve Check valve Angle valve... 

Software packages are commercially available for simulation of hydrauhc transients. These may be used to analyze piping systems to reveal unsatisfactoi y behavior, and they aUow the assessment of design changes such as increases in pipe-wall thickness, changes in valve actuation, and addition of check valves, surge tanks, and pulsation dampeners. 

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... 

Unhke the control valve, the centrifugal pump has poor or nonexistent shutoff capabihty. A flow check valve or an automated on/off valve may be reqiiired to achieve shutoff reqiiirements. This requirement may be met by automating an existing isolation valve in retrofit apphcations. 

Spring is broken in the spring-type check valve. 

Check valve flapper pin is worn, and flapper will not swing open. 

Overhaul check valve restore proper clearance to pin and flapper bore. 

Install a positive-pressure steam (from nmuiug pump) to fill the suction line from the block valve through the check valve. 

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. 

Tilting-Disk Check Valves These valves (Fig. 10-160) may be installed in a horizontal line or in hnes in which the flow is vertically upward. The pivot point is located so that the distribution of pressure in the fluid handled speeds the closing but arrests slamming. Compared with swing checkvalves of the same size, pressure drop is less at low velocities but greater at high velocities. 

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. 

Virtually all steel components contacting coolant were attacked. A check valve from a spray system and a hanger bracket were both severely pitted and corroded. The mill components were corroded after an upset that permitted hydrochloric acid to mix with the mill coolant. The pH was at or below 2 during the upset, which continued for at least several weeks. 

Figure 7.19 Pits on a carbon steel check valve. Note how pits intersect to form areas of jagged metal loss. A steel prohe tip is in the photo. (Magnification 7.5x.)...

Where used, include check valves in mechanical integrity program... 

Provide check valve (s) in feed line (secondary control)... 

Install check valve in distillate discharge line... 

Provide excess flow check valve upstream and check valve downstream of hose... 

Internal Re-circulation Operation / Design / Maintenance Internal Re-circulation results from something in the discharge side of the pump strangled valve, clogged filter, over-pressurized header, check-valve installed backwards. 

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