Limit switch

Various accessories can be suppHed along with the control valves for special situations. Positioners ensure that the valve stem is accurately positioned following small or slowly changing control signals or where unbalanced valve forces exist. Boosters, which are actually pneumatic amplifiers, can increase the speed of response or provide adequate force in high pressure appHcations. Limit switches are sometimes included to provide remote verification that the valve stem has actually moved to a particular position. 

Loop two or more elements where a signal passes between them for measurement and/or control, eg, sensor -i- transmitter -i- control + valve device two or more elements combined for a discrete state condition, eg, block valve -i- solenoid valve -i- actuator -I- limit switches... 

Discrete Device States Discrete devices such as two-position valves can be driven to either of two possible states. Such devices can be optionally outfitted with limit switches that indicate the state of the device. For two-position valves, the following combinations are possible ... 

Open. The valve has been commanded to open, and the limit switch inputs are consistent with the open state. 

Transition. This is a temporaiy state that is only possible after the valve has been commanded to change state. The limit switch inputs are not consistent with the commanded state, but the transition time has not expired. 

Discrete alarms can be defined on discrete inputs, limit switch inputs from on/off actuators and so on. For discrete alarms, the following possibilities exist ... 

Limit Switches and Stem-Position Transmitters Travel-limit switches, position switches, and valve-position transmitters are devices that, when mounted on the valve, actuator, damper, louver, or other throtthng element, detect the component s relative position. The switches are used to operate alarms, signal hghts, relays, solenoid valves, or discrete inputs into the control system. The valve-position transmitter generates a 4-20-mA output that is proportional to the position of the valve. 

Provide limit switches to verify physical valve positions... 

The 2NO contacts of the limit switch are wired in series with the closing coil of the interrupter (Figure 13.45). As soon as the spring is fully charged these contacts change over to NC and the closing coil circuit gets ready to clo.se. 

LS Limit switch with spring charging G Generator... 

Control switches Limit switch failure Valve binding, in ability to move valve Test and inspection... 

LINE STARTER ENCLOSURE WITH LOAD LIMIT SWITCH... 

Unauthorized interference with computer hardware is usually difficult, but interference with peripheral equipment may be more serious than on a traditional plant, as the computer will not know that interference has occurred. For example, the leads on a limit switch on a valve were interchanged to carry out some tests. The plant was on manual control at the time but was switched back to computer control before the leads were restored to their coirect positions. The computer thought the valve was open (when it was shut) and elected to close it. It actually opened it, releasing flammable material [2]. 

Closed-position indicators on the valves usually satisfy a system check . These show that the valves are nominally in the closed position but do not actually prove that there is no leakage. A proving system can be met by various systems which prove the soundness of the upstream and downstream safety shut-off valves by sequential pressurization, evacuation, mechanical over-travel with limit switches and a normally open vent valve, or other means. 

All interlocks checked for correct setting and operation. These will include thermostats, pressurestats, limit switches, pressure switches, process interlocks, etc. ... 

When coolers or condensers are shut-down but remain full of water, the amount of current required to maintain satisfactory cathodic protection is considerably reduced. If the current is not reduced over-protection occurs and excessive amounts of chlorine can be generated which would tend to accumulate in the upper section of the water boxes causing considerable corrosion, not only to the water boxes, but also possibly to the tubes. To ensure against this a stand-by condition should be included on the control panel which effectively reduces the current required under shut-down conditions. This control is effected by a limit switch fitted to the outlet valve of the condenser or cooler concerned. It is impossible to determine exact requirements for the protection of circulating water systems in advance and it is normal to adjust the current to provide protection during commissioning. 

Most electric motor actuators are equipped with limit switches, torque limiters, or both. Limit switches de-energize the electric motor when the valve has reached a specific position. Torque... 

Equipped with limit switches and/or torque limiters, the electric motor actuator has the capability of 0-100% control and has not only a motor but also a manual handwheel, and a clutch and gearbox assembly. 

Controller Functions Limit switches, Auto-Zero, Peak-to-Peak, Minimum, Maximum, Average, Storage of 3 configurations (calibrations) ... 

It is now an industrial standard to use positive identification of the valve position on all important safety valves — limit switches that are tripped when the valve is open or shut. In addition, all valve vent lines are now covered with bug screens to prevent blockage. 

Add fail-safe block valves with a positive indication of the valve position (limit switches). 

An indoor enclosure for pilot devices such as limit switches, selector switches, foot switches, push buttons, and pilot lights for protection against lint, dust seepage, external condensation, and spraying of water, oil or coolant... 

The minimum bandwidth inventory boundary is set for all planning periods. Exceptionally, the minimum bandwidth inventory can be reduced down to 0 in a specific period controlled by the binary parameter <7 " , V / ,/ e I12, teT defined as minimum bandwidth inventory limit switch. The minimum bandwidth inventory includes the safety stock to ensure delivery capability, physical line fill inventory in the resources during production, as well as minimum unavoidable working level inventory occurring due to the operating mode. 

Inventory boundary constraints have to combine absolute and range boundaries considering limit switch parameters and relaxation variables. The absolute minimum and maximum inventory boundaries are defined in the following constraint. 

Lower inventory boundary is 0, if the minimum inventory limit switch <7 is 1. Upper inventory boundary is equal to the maximum bandwidth inventory, if the maximum inventory limit switch 0, tax is 0, otherwise the maximum absolute inventory quantity is applied as upper limit. The constraint is relaxed with A1" " of A a , if no feasible solution exists due to hard inventory constraints. 

Inventory range boundaries got a very similar structure also considering limit switch variables and relaxation. Inventory ranges are applied to the distribution demand xDfM, V / , / e I12, t e T -1, the transfer point has to fulfil in the following period. This ensures sufficient built up of inventory for the distribution demand in the next period. 

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