Reverse flow valves

X3.5.5 When the reverse flow valve is turned there is a reversal of pressure conditions at the column ends that upsets the carrier gas flow. This flow should quickly return to the same flow rate and the base line level out. If it does not, the cause may be a leak in the carrier gas system, faulty flow regulator, or an unbalanced condition of the column or plumbing. 

Filter housings commonly incorporate protective devices, such as shut-off valves, check valves, reverse-flow valves and a bypass facility. Shut-off valves are usually incorporated in the filter head, automatically sealing the outlet and inlet lines when the filter housing is removed (for example, to replace an element). They also have the advantage of minimizing air entrainment during replacement of the filter element. It should be noted that shut-off valves, where fitted, are normally effective only at low system pressures (that is, not full system pressure unless specifically stated). 

Reverse-flow valves may be fitted to provide a bypass loop in the event of reverse-flow conditions, and thus eliminate any back-pressure on the element. With some elements reverse flow can result in damage. 

FIG. 8-69 Ecc( ntric plii valve shown in erosion-resistant reverse flow direction, Shaded components can he made of hard metal or ceramic materials, Cm 1 li(isy Fislier-R/iseuimini. ... 

Gas compressor anti-surge (GM-OFF) control circuit, comprising transmitters, computers and pneumatic control valve Reverse flow protection (on axnal compressors only) as supplementary protection device against surging, working independently of the control circuit Expander emergency stop valve with pneumatic actuator and solenoid valve... 

The oil system should be equipped with a main oil pump, a standby and, for eritieal maehines, an emergeney pump. Eaeh pump must have its own driver, and eheek valves must be installed on eaeh pump diseharge to prevent reverse flow through idle pumps. The pump eapaeity of the main and standby pumps should be 10-15% greater than maximum system usage. The pumps should be provided with different prime movers. 

Check Valve - A check valve is normally not an acceptable means of preventing overpressure by reverse flow from a high pressure source. However, where leakage throu a check valve would not result in overpressuring by more than 1.5 times design pressure, a check valve is acceptable. 

Connections to equipment are typically 50 mm and 80 mm for process vessels and exchangers, according to the size of the equipment. Each connection includes an accessible block valve. Double block valves are provided if required. A check valve should be included if overpressure or other hazard could result from reverse flow during simultaneous drainage from more than one vessel. Individual connections from the equipment are made into the top of the drain header. 

Many accidents occur because process materials flow in the wrong direction. Eor example, ethylene oxide and ammonia were reacted to make ethanolamine. Some ammonia flowed from the reactor in the opposite direction, along the ethylene oxide transfer line into the ethylene oxide tank, past several non-return valves and a positive displacement pump. It got past the pump through the relief valve, which discharged into the pump suction line. The ammonia reacted with 30m of ethylene oxide in the tank, which ruptured violently. The released ethylene oxide vapor exploded causing damage and destruction over a wide area [5]. A hazard and operability study might have disclosed the fact that reverse flow could occur. 

Remotely operated emergency isolation valves (see Section 7.2.1) should be fitted on filling lines. If the hose breaks for any reason, the flow can be stopped by pressing a button located at a safe distance. Reverse flow from the tank truck or car can be prevented by a check valve. 

Check valves and other reverse-flow prevention devices, if their failure can affect the safety of the plant. 

If a pump trips (or is shut down and not isolated), it can be driven backward by the pressure in the delivery line and damaged. Check valves are usually fitted to prevent reverse flow, but they sometimes fail. 

When the consequences of reverse flow are serious, then the check valve should be scheduled for regular inspection. The use of two, preferably of different types, in series, should be considered. The use of reverse rotation locks should also be considered. 

Soundness of non-return valve with regard to reverse flow ... 

Soundness of safety shut-off valves in the closed position for both forward and reverse flow. Note that this should be done with the valve in situ and without dismantling it, by means of a bubble leak detector, etc. The valve should not be dismantled to check for swarf on the seat, etc. 

Solenoid valves are used in refrigeration and air-conditioning systems for refrigerant lines, oil pressure pipes (to control oil return and capacity reducers), and water and compressed air lines. Four-port changeover valves (Figure 9Ab) are used to reverse flow in defrosting and heat pump circuits. A de-energized expansion valve will act as a solenoid valve. 

Non-return valves are used to prevent back-flow of fluid in a process line. They do not normally give an absolute shut-off of the reverse flow. A typical design is shown in Figure 5.5g. 

Reverse Flow Fall in line press, (compressor fails) high pressure at reactor NH3 in compressor — explosion hazard fit non-return valve (NRV1) hot wet acid gas-corrosion fit second valve (NRV4)... 

Figure 2.5. Analytical manifolds for the determination of phosphate by flow injection analysis (a) and reverse flow injection (b). The symbols S, M, and A are the seawater, mixed reagent, and the ascorbic acid solutions. The pump injection valve and detector are represented by P, I, and D, respectively. W = waste. From [177]... 

Note The isopentane feed valve is designed to fail closed on a power failure to prevent reverse flow from the kettles to the raw material storage tanks. This is the appropriate failure position for this valve. 

Figure 5.22 — Reversible flow-through fluorimetric sensor for the determination of potassium in human blood plasma based on the mechanism shown in Fig. 5.21.3. (A) Flow-cell containing the lipophilic membrane. (B) Flow injection conflguration. P pump IV injection valve W waste. For details, see text. (Reproduced from [86] with permission of Elsevier Science Publishers).

In many cases the process gas te be examined is so aggressive that the cathode would survive for only a short period of time. The AGM uses the property of laminar flow by way of which there is no reverse flow of any kind. Controlled with a separate AGM valve, a part of the working gas fed te the processes is introduced as purging gas", ahead of the pressure converter, to the TRANSPECTOR this sets up a flow toward the vacuum chamber. Thus process gas can reach the TRANSPECTOR only with the AGM valve closed. When the valve is open the TRANSPECTOR sees only pure working gas. Fig. 4.9 shows the AGM principle. 

Feed gas is supplied in pulses of up to about a second in length from a compressor and a surge tank. The pulse is controlled by a solenoid valve and a timer. During this feed pulse the exhaust solenoid valve is closed. Following the feed pulse both valves at the feed end are closed for about 0.5 to three seconds this period is called the delay. Finally the solenoid valve on the exhaust or purge line opens for a period of about five to 20 seconds. Since the pressure in this line is maintained below that in the feed line, a reverse flow of gas from the bed occurs. 

Check Valve Rverse Leakage (Pressure) Forward/ Reverse Flow Rate Dynamic Response... 

The primary system is intended to isolate the major equipment in order to prevent reverse flow and exhaust oxides in a gaseous state. It consists of a special restraint type valve (check) and two motorised shutdown valves, arranged in series. The system includes a low pressure detector on the main pipe that trips the sulphuric acid pumps, isolates the process and stops the helium flow into the reactor and heaters (emergency system EAS-200). 

The delivery is continuous. Inlet and outlet valves are not required, but a non-return, or check valve is normally installed on the pump outlet line to prevent reverse flow when the pump is switched off. The outlet may be blocked (for short periods) without damage to the pump. 

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