Excess flow check valves

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

Single-unit tank cars are protected from excessive internal pressure by a spring-loaded pressure relief valve located on the manway cover plate. Excess flow-check valves are installed on the liquid eduction and vapor lines of many tank cars in compressed gas service. These excess flow valves are intended to contain the tank contents in transit, in a wreck environment if the valve is sheared off, or during other incidents when uncontrolled release of product may occur. 

Excess flow-check valves are installed in hoses and piping systems that carry compressed gases. In the normal open position of this device, flow is permitted in either direction, as shown in Fig. 8-21(a). 

Fig. 8-21(b) also shows an equalizing hole in the valve disk. If the excess flow-check valve closes suddenly due to a sudden opening of a shut-off valve in the system causing a high flow, the equalizing hole allows pressure on both sides of the disk to gradually equalize, thereby causing the excess flow valve to automatically reopen. 

In some cases, after careful evaluation, other valves may be considered for EBVs such as spring-loaded control valves that fail closed, back flow check valves (these are not normally considered reliable enough for EBVs by many engineers), and excess flow valves. Excess flow is the loss of material from the confined environment of a vessel or pipeline. Two approaches are available for the detection and valve action of excess flow valve systems (1) External, where excess flow is detected outside the valve itself, and (2) internal, which is within the valve unit and has limited applications. Excess flow conditions are detected more readily because of loss of resistance to flow than because of loss of pressure. All excess flow detection systems are based on product physical properties as well as flow rate. A change of products or process conditions may require a change in the excess flow detection system. For example, a number of excess flow valves are rated for low pressure and are made of materials not suitable for hydrocarbons. 

Too much flow through valve Both internal pressure valves fail open Operation Excessive chlorine flow to Tower Water Basin - high chlorine level in cooling water - potential for excessive corrosion in cooling water system Rotameter Daily testing of cooling water chemistry Relief valve on Pressure check valve outlet III None... 

Are the excess flow and/or back pressure check valves located inside the container or at a point outside as close as practicable to where line enters container ... 

Are loading/unloading lines fitted with backflow check or excess flow valves ... 

Fig. 1.2. Manifold for medium vacuum and inert gas. A low-temperature trap, on the right side of the figure, is used in the vacuum line to protect the pump from harmful vapors. When the apparatus is being filled with gas or purged with inert gas, the valve on the pressure release bubbler (which contains a check valve to prevent oil from backing up into the line) is opened to avoid excess pressure which would blow the apparatus apart. Often a mineral oil bubbler (not shown here) is connected in line with the inert-gas source to provide visual indication of the inert-gas flow.

Isolation devices, such as manual/automatic valves, check/excess-flow valves, dry-break couplings, which shut off flow if the coupling is parted, and positive displacement pumps, which stop flow when shut down ... 

In a loading line, an excess flow valve is not practical because it would close upon excess flow into the tank car but would not protect against loss of vapor out of the tank car. Protection can be better obtained by installing a backflow check valve in the loading line at the tank car end of any hose or swivel-type piping. 

Water Flow Alarms. Alarms have been enhanced to provide more reliability. This is accomplished when an excess pressure pump is put on the riser above the alarm check valve. This will prevent the clapper in the alarm check valve from opening on small surges. In addition, it will use a pressure switch that will trip an alarm even with the sprinkler control valve closed and no water flowing. These are often used where there are pumping supplies or a fluctuating city water supply. 

A check valve is installed on each tube inside the steam header, and a self-actuated isolating excess flow valve is placed inside the feed water header. 

The back pressure on the safety, valve due to the flow through the discharge line can be calculated using a suitable computer program (see Annex 4). However, the Omega method (see Annex 8). ban also be used to check that the back pressure is not excessive, if it is applicable and if the discharge piping from a safety valve is of constant diameter. 

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