Valve Requirements

The failure mode of ESDVs for gas processing areas should always fail in the closed position, since this is the only mechanism to resolve gas fed fires or prevent explosive vapor buildups. The valves should be provided with an automatic fail close device such as an actuator with spring return specification. 

ESD valves should lock in the fail safe mode once activated and be manually reset once it has been confirmed the emergency has passed or has been resolved. 

The emergency isolation devices should be arranged so that they can be fully function tested without affecting the process operation. This entails that a frill flow bypass should be installed at each emergency isolation valve. These bypass installations should be locked closed when not in service for functional testing the ESDV. 

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Installation of a shut-off valve in the expander exhaust line prevents baekflow during expander isolation (Figure 6-37). Although the need for this valve may not be immediately obvious, it ean be very bene-fieial if the expander is operated for an extended period in the windmilling mode. Often this type of operation is not antieipated during the initial evaluation of valve requirements, but beeomes neeessary at some later date. For example, it is useful when it is deemed prudent... 

The strict requirements placed on the quality of the regenerator pressure control system necessitate complex control strategies that can only be achieved using modern freely programmable control systems, while the short actuating times of the control valves require controllers with ultrafast response. 

Case, R.C., a Method to Determine Exchanger Relief Valve Requirements, Proceedings, American Petroleum Institute, Division of Refining, Vol. 50, New York, 1970, p. 1082. 

Pressure Rehef Valve Specification - Standard calculation procedures are applied to determine the size of the pressure rehef valve required for the maximum reheving rate, together with other information necessary to specify the valve. 

Number of Valves Required - Normally a manufacturer s standard PR valve with Drifdce area equal to or larger than the calculated requirement is specified. In some cases, e.g., large reheving rates or to prevent chattering, two or more valves are necessary. Likewise, if there is an appreciable difference between the calculated orifice size and the available size, multiple PR valves are desirable to more nearly match the available area to the required orifice area. The column for "spares" indicates the requirement, if any, for spare PR valves installed on the equipment. Normally, this applies only in the case of refinery preference or local regulations, but is required in many European countries. 

Instrument Failure assume instrument control valves freeze or fail in open position (or closed, which ever is worse), determine capacity for relief based on flows, temperatures, or pressures possible under these circumstances. The judicious selection of instrument failure sequence may eliminate or greatly reduce relief valve requirements. 

Safety Valve Required to Prelect Reducing Station Discharge Pressure in Cose of Valve Failure. SV-I is Set at Slightly Above Downstream Pressure of Reducing Station,and Protects All Equipment Operating at this Pressure on Steam Header. 

K = liquid correction factor for variable back pressures from Figure 7-28. Applies to balanced seal valves only. Conventional valves require no correction. 

The number and quality of the safety shut-off valves required is specified in the relevant Standards and Codes of Practice. These are usually electrically operated valves conforming to class 1 or class 2 as defined in BS 5963, Specification for electrically-operated automatic gas shutoff valves (1981). This specifies forward pressures and reverse pressures that the valve must withstand in the closed position together with closing times. 

For start gas supplies two safety shut-off valves are needed on all but the smallest burners. For automatic burners the main gas safety shut-off valve requirements are ... 

There are four basic valve designs used in these compressors finger, channel, leaf, and annular ring. Within each class, there may be variations in design, depending upon operating speed and size of valve required. 

Halocarbon systems are almost invariably controlled by mechanical or electronic thermostatic expansion valves, requiring a superheat signal to operate the control. The superheating of the suction gas into the compressor will cause it to expand, resulting in a lower mass flow for a given swept volume. Reduction of the superheat setting of the expansion valve will therefore result in better use of the compressor. The limit will be reached when there is insufficient signal to work the expansion valve. 

Control valves require at least 10 psi drop for good control. 

By themselves, valves cannot control a process. Manual valves require an operator to position them to control a process variable. Valves that must be operated remotely and automatically require special devices to move them. These devices are called actuators. Actuators may be pneumatic, hydraulic, or electric solenoids or motors. 

A disadvantage of injection via rotary valves is that while the volume of the sample that is injected is very small, the procedure for filling the loop of the valve requires a high volume of sample to flow through. This may reduce application of this technique in cases where there is only a limited quantity of sample available. 

Large and medium size piping and valves require rubber or rubber lined construction whereas smaller ones are constructed with mild steel, concrete or stainless steel of suitable grade. 

Ball valves require a certain low pressure-difference before being opened, in order to avoid shock pressures in the piping system. 

In addition to sensors, Si can be used at (lie subminialure scale for the production of tiny pipe, nozzles, and valves required by automatic control systems. Thus, the weight and bulk of future control systems may be reduced by several orders of magnitude. 

It is convenient to store frequently used solvents in containers directly attached to the vacuum line. The valve requirements are even more stringent than for gas storage, because the attack of stopcock grease by organic solvent vapors is a certainty. Therefore, Teflon-stemmed valves are generally called for. A good solvent storage container may be constructed from a distillation flask. As shown in Fig. 9.19, the solvent may be fractionally distilled directly into the sidearm of... 

Organic fluid vaporizer safety valve requirements (Dowtherm Service, PVG-12)... 

Next try a 2-in valve size with 1.3 x 11.05 ft/s = 14.37 ft/s. Also increase the valve-required AP to 25 psi. This derives a more reasonable 2-in valve size Cv. 

Conventional solid-state actuators and valves such as peristaltic pumps and solenoid valves require external power and complex fabrication schemes which limit their use in many microfluidic applications. Of particular interest at the moment is the development of low-cost, efficient, polymer-based actuators and valves for sample handling in microfluidics systems. 

Rotary Teflon valves have greater initial costs than comparable vacuum glass stopcocks. However, due to their lack of maintenance needs, as well as not requiring any stopcock grease, their long-term expense can be considerably less than that for standard vacuum stopcocks. However, because Teflon stopcocks or valves require a limited temperature environment, all glass stopcocks will always remain in demand. 

A chamber in service for a 10-yr period in which rebuilt valves were present was found to be saturated with chlorides and phosphates. The chloride caused the corrosion of bolts to such an extent that the chamber was leaking. The chamber housed 24-inch and 16-inch valves along with air drain-out. All the bolts on the valves required replacement. Furthermore, the aluminum ladder rungs were completely corroded. 

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