Trip valves

Trip Valves The trip valve is part of a system that is used where a specific valve action (i.e., fail up, Fail down, or lock in last position) is required when pneumatic supply pressure to the control valve falls befow a preset level. Trip systems are used primarily on springless piston ac tuators requiring fail-open or fail-closed acrion. An air storage or Volume tank and a check v ve are used with the trip valve to provide power to stroke the valve when supply pressure is lost. Trip valves are designed with hysteresis around the trip point to avoid instabihty when the trip pressure and the reset pressure settings are too close to the same value. 

The trip valve is provided as a second line of defense in case of overspeed The trip valve is frequently equipped with a trip-actuating solenoid which can be operated by push button, by low od pressure, or by some other process upset. When the speed control functions as described above, the trip will not be actuated by load dump. 

Sticking of valve stems is common if solids are present in the steam. The steam mast be without sohds. (Note comments later under loss of power.) It is important that units operating on a steady load for long periods be checked for sticking stems at regular intervals. The records show that in several cases deposits have caused the stem of both the governor valve and the trip valve to stick when there was a loss of load. The effect of the loss of load was destructive overspeed. 

Various protective instruments are used to provide a shutdown signal (to a fast-acting trip valve at the expander inlet) that senses various things, such as overspeed, lubricant pressure, bearing temperature, lubricant temperature, shaft runout, icing, lubricant level, thrustbearing load, and process variables such as sensitive temperatures, levels, pressures, etc. However, too many safety shutdown devices may lead to excessive nuisance shutdowns. 

An expander emergeney trip valve, eapable of elosing in less than one-half seeond, must be installed elose to the expander inlet flange. A 60-80 mesh sereen, differential pressure monitor, and shutdown must be installed between the trip valve and the expander inlet. Additionally, it is highly reeommended that a 40-60 mesh sereen be installed upstream of the eompressor inlet for use during the initial startup period. 

First-out annuneiators are highly desirable and should provide suffieient points to aeeommodate the speeified alarm and shutdown funetions. Standard features should inelude aeknowledge/silenee, lamp/test, and both audible and visible deviees for general alarm indieation. A dry auxiliary eontaet from the shutdown eireuit must be provided for expander inlet trip valve aetuation. Additional sets of eontaets are useful for other alarm requirements. 

An emergency trip of the expander or generator requires fast closing of the trip valve and inlet control valve. An emergency trip also must not adversely affect the differential pressure between the reactor and regenerator. 

A speed controller synchronizes the expander/generator with the electricity supply grid, and actuates the inlet trip valve to its fully open position. 

At the rated duty point, the differential pressure eontroller is aetive. The inlet eontrol valve and trip valve are eompletely open. The main bypass valve is eompletely elosed and the small bypass valve eontrols the differential pressure. Approximately 96%-98% of the flue gas flows through the expander, with the rest passing through the small bypass valve, orifiee ehamber, and double slide valve to the expander outlet to rejoin the main flue gas flow. 

Investigation showed that the pressure drop through the trip valve—a globe valve—was so high that the valve eould not elose against it. There was a flow eontrol valve in series with the trip valve (Figure 14-1), and the trip nonnally elosed this valve as well. However, this valve failed in the open position—this was the reason for the high temperature in the reactor—and the full upstream pressure was applied to the trip valve. 

Figure 14-1. When the control valve was open, the pressure prevented the trip valve from closing.

The pressure drop in the pipeline should be measured, and if it gets too low, a trip valve should be closed automatically. A very reliable, duplicated system may be necessary [16]. 

Overspeed trip Releases trip valve when speed exceeds 115% of maximum rating Stainless steel... 

Trip valve Shuts off steam on overspeed Steel, stainless, and nickel iron... 

A governor valve controls the flow of steam to the turbine. This valve is actuated by the governor, which is operated by the speed of the machine. When the speed exceeds the set value of the governor, a trip-valve is actuated to completely shut off the steam supply. The trip valve may... 

Steam Admission Volve(s) Governor Controlled Single, Emergency Trip Valve Combine d with Throttle Valve ... 

Devices mounted on the control valve that interface various forms of input signals, monitor and transmit valve position, or modify valve response are valve control devices. In some applications, several auxiliary devices are used together on the same control valve. For example, mounted on the control valve, one may find a current-to-pressure transducer, a valve positioner, a volume booster relay, a solenoid valve, a trip valve, a limit switch, a process controller, and/or a stem position transmitter. Figure 8-80 shows a valve positioner mounted on tne yoke leg of a spring and diaphragm actuator. 

Payout Period, Years Hrs/Yr Trip Valve Q Integral (Zl Seperate ... 

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