Valves trapped liquids

Other Start-up Shut down Relief system Power/service failure Corrosion/erosion Materials of construction T oxicity/asphyxia Maintenance (provision of isolation, etc.) Double valves (trapped liquids) Valve access Provision for testing instruments, trips, alarms Fire Static electricity Noise Thermal radiation Ionising radiation Sampling Spares Anything else ... 

Any valves isolating the column, including those that need to be closed while blinds or slip plates are being fitted, must be locked shut with a padlock and chain or a similar device (210). It is a good practice to post a "Danger workers in column notice at the isolation. Blinds and slip plates should be installed as close to the column as possible, and on the column side of any isolation valves, in order to avoid trapping liquid between the blind and the column (210, 275). Liquid trapping in the bonnet of an isolation valve, which was blinded on its other side, has been reported to have caused at least one fatal explosion (275). 

Sealed Capsule Expansion Microvalves In 1987, Zdeblick and Angell [3] first proposed the concept of the sealed capsule expansion (SCE) valve, which utilized the thermal expansion of a trapped liquid to actuate the valve instead of the buckling of a beam or membrane. Later, a family of sealed capsule expansion valves trademarked as the FluistorTM was developed by Redwood Microsystems. Generally, in an SCE microvalve, there is a small cavity with trapped liquid which has a low boiling point and/or a high expansion coefficient. When the liquid is heated, it expands and closes the valve. When it is cooled, it shrinks and opens the valve. 

Plug valves have the same major characteristics as ball valves but are generally harder to operate. Their design involves a balance between turning torque and shutoff capability. They usually have reduced bores and so higher flow resistance than ball valves. They also can trap liquid when closed, and the precautions listed above for ball valves apply to plug valves as well. 

The IC relies on the difference of densities between the IC hot leg and cold leg fluid to initiate and maintain a gravity-driven circulalion. Under station blackout conditions the reactor can be depressurized and cooled by first closing the main steam and feed-water isolation valves, followed by opening the IC coimection valve. The liquid column normally trapped by the coimection valve is allowed to flow into the reactor inlet. As fliis drains into the reactor, the IC heat exchanger tubing will be exposed to steam from the reactor outlet, allowing heat transfer to the reserve water pool. Further steam produced by the reactor due to the decay heat will sustain the circulation. 

Valves used in hydrocarbon liquid service should not trap liquids in their bodies. The liquid could expand during a fire and overpressure the valve body. Standard gate valves are normally not a problem because excessive pressure can be relieved by minor leakage past the valve seat. However, valves such as the double-block and bleed type will trap liquid. When these valves are used in liquid service, body relief valves are needed. 

Liquid traps in safety valve release lines. 

The ASME code requires every pressure vessel that can be blocked in to have a relief valve to alleviate pressure build up due to thermal expan sion of trapped gases or liquids. In addition, the American Petroleum Institute Recommended Practice (API RP) 14C, Analysis, Design, Installation and Testing of Basic Surface Safety Systems on Offshore Production Platforms, recommends that relief valves be installed at vari ous locations in the production system and API RP 520, Design and Installation of Pressure Relieving Systems in Refineries, recommends various conditions for sizing relief valves. 

Pilot-operated valves have the advantage of allowing operations n the set point v/ith no leakage, and the set position is not affected by ba pressure. However, they will not function if the pilot fails. If the sens line fills with hydrates or solids, the valve will open at 25% over pressure trapped above the disc (usually the normal operating pre the vessel). For this reason they should be used with care in dirty vice and liquid service. They are used extensively offshore where all platform relief valves are tied into a single header because up to 5 back-pressure will not affect the valve capacity. 

To avoid injuries of this sort, we should use protective hoods or helmets when breaking joints on lines likely to contain corrosive liquids trapped under pressure, either because the pressure cannot be blown off through a valve or because lines may contain solid deposits. 

Water hammer (also known as hydraulic shock) occurs in two distinct ways when the flow of liquid in a pipeline is suddenly stopped, for example, by quickly closing a valve [13], and when slugs of liquid in a gas line are set into motion by movement of gas or condensation of vapor. The latter occurs when condensate is allowed to accumulate in a steam main, because the traps are too few or out of order or in the wrong place. High-pressure mains have been ruptured, as in the following incident. 

Liquid trapped between valves will produee a large rise in pressure as it warms up. 

The incident also shows the importance of placing spades as near as possible to the equipment that is to be isolated, particularly when a vessel is to be entered. Valves should not be left between a slip-plate and the vessel, as liquid can then be trapped between the valve and the slip-plate and enter the vessel if the valve leaks or is opened. 

When 5.00 mL of ether has been delivered by the syringe pump, the pump is shut off The reactor is allowed to run an additional 15 min before the fluorine and the mercury arcs ate shut off. The preaerosol furnace, the evaporator heater unit, and the coolant pump are shut off. Once the system approaches ambient conditions, all the helium carriers are shut off and the product trap valves are closed The product trap and its Dewar flask filled with liquid nitrogen are removed to the vacuum line where the trap is evacuated... 

Piping should also be arranged to allow Uie shortest, most direct route without congestion. They should be installed to prevent Uie trapping of liquids and should contain adequate valving and provisions for contracUon and expansion. Joints must be properly welded and secure. 

The procedure can be made more automatic by using a temperature-sensitive liquid expansion steam trap in place of the manual valve. Such an arrangement is a compromise between supervised start-up and the automatic start-up discussed below. 

The liquid-ring or liquid-piston compressor is shown in Figure 36.8. It has a rotor with multiple forward-turned blades that rotate about a central cone that contains inlet and discharge ports. Liquid is trapped between adjacent blades, which drive the liquid around the inside of an elliptical casing. As the rotor turns, the liquid face moves in and out of this space due to the casing shape, creating a liquid piston. Porting in the central cone is built-in and fixed and there are no valves. 

On a single-evaporator flooded system, a float valve can be fitted which will pass any drained liquid from the condenser direct to the evaporator. The action is the same as that of a steam trap. The float chamber is at condenser pressure and the control is termed a high-pressure float (Figure 8.3). 

There have been several experimental studies of two-phase flow in which the holdup has been measured, either directly or indirectly. The direct method of measurement involves suddenly isolating a section of the pipe by means of quick-acting valves and then determining the quantity of liquid trapped.00111 Such methods are cumbersome and are subject to errors arising from the fact that the valves cannot operate instantaneously. Typical of the indirect methods is that in which the pipe cross-section is scanned by y-rays and the hold-up is determined from the extent of their attenuation.02 13141... 

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