Throttling

Block Valves are used to shutoff separate systems. The term is loosely used and may be understood to mean any valve used for shutoff service rather than for throttling. 

A Bleed Valve is a small valve used to draw-off liquids. 

A Double Block and Bleed means two block valves with a bleed valve located between them. Bleed-off valving for pressure instrumentation on hot lines is provided with a double block and bleed valve. These valves and the bleed piping for hot services should not be less than 1/2 in. or 3/4 in. in size. They are arranged so that a rod may be driven up the valve and fitting. Many refiners still believe in the tried-and-true method of using double block and bleed valves for bubble-tight service. 

Block valves and drains have been installed in HF alkylation units in the water piping on the 

A through-conduit type gate valve has proven very satisfactory in coking services where other valves have given trouble. This type of valve is one in which there is no body cavity exposed to line fluids in the open or closed position. A 4-inch, 

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Hall, D.W. and L.M. Gibbs (1976), Carburetor deposits are clean throttle bodies enough . SAE paper No. 76-0752, Automobile Engineering Meeting, Dearborn, Ml. 

If high wellhead pressures are available over long periods, cooling can be achieved by expanding gas through a valve, a process known as Joule Thomson (JT) throttling. The valve is normally used in combination with a liquid gas separator and a heat exchanger, and inhibition measures must be taken to avoid hydrate formation. The whole process is often termed low temperature separation (LTS). 

Fig. 7. Control of fan performance with inlet vane control. SoHd lines marked A and N show normal performance without vanes (vanes wide open). As vanes are progressively closed, static and power curves are modified as indicated by dashed lines. Intersection ( - ) of the system resistance curve with these reduced pressure curves at points B, C, D, and E shows how imparting more spin to the inlet air reduces flow. Projecting points A to E vertically downward to the corresponding power curve locates fan power points A through E7 Power savings achieved over throttling control can be estimated by projecting points B through E vertically downward to the A power curve and comparing the value with that from the proper reduced power curve. To...

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

In most utibty boilers, steam pressure regulation is achieved by the throttling of turbine control values where steam generated by the boiler is admitted into the steam turbine. Some modem steam generators have been designed to operate at pressures above the critical point where the phase change between Hquid and vapor does not occur. 

V ri ble Frecjuency Drives. An important energy by-product of soHd-state electronics is the relatively low cost variable speed drive. These electronic devices adjust the frequency of current to control motor speed such that a pump can be controlled direcdy to deUver the right flow without the need for a control valve and its inherent pressure drop. Eigure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throttle control valve system, and at half load, it uses only about 25% as much power. 

A control valve consists ot a valve, an actuator, and possiblv one or more valve-control devices. The valves discussed in this section are applicable to throttling control (i.e, vviiere tlovv through the valve is regulated to anv desired amount betvv een maximum and minimum limits). Other valves such as check, isolation, and relict valves are addressed in the next subsection. As defined, control valves are automatic control devices that modify the tliiid tlovv rate as specified bv the controller. 

Multi-Port This term refers to any vaWc or manifold of s ah es with more than one inlet or outlet. For throttling control, the three-way body is used for blending (two inlets, one outlet) or as a dh ertor (one inlet, two outlets), A three-way S ah e is most commonly a special globelike body with special trim that allows flow both o cr and under the plug. Two rotaiw s ah es and a pipe tee can also be used. Special three-, four-, and fh e-way ball- ah e designs are used for switching applications. 

As most throttling control valves are still operated bv pneumatic actuators, the control-valve device descriptions that follow relate primarily to devices that are used with pneumatic actuators. The function of hydraulic and electrical coimteiparts are very similar. Specific details on a particular valve-control device are available from the vendor of the device. 

On springless actuators where the actuator is not usable for throttling control without position feedback,... 

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