Control valve design

For any control valve design be sure to use one of the modem methods, such as that given here, that takes into account such things as control valve pressure recovery factors and gas transition to incompressible flow at critical pressure drop. 

In thermal solar power plants, high-temperature oil flows need to be controlled in hydrogen processes, very-low-temperature LH2 flows have to be controlled. For these reasons, the emphasis in this treatment will be on control valve designs that are suited for these applications and on phenomena (such as noise and cavitation) that are common in these applications. 

The noise sources in control valves include mechanical vibration (usually below 100 dBA) hydrodynamic noise caused by liquid turbulence, cavitation, or flashing (usually below 110 dBA) and aerodynamic noise (can reach 150 dBA). In control valve design, aerodynamic noise can be a major problem. Aerodynamic noise generation, in general, is a function of mass flow rate and the pressure ratio (p /pf) across the valve. The point at which sonic speed is reached in the valve vena contracta is a function of the valve design. 

Use control valves designed for small flows and industrial service with rugged shafts, tight machining tolerances, and minimum stick-slip. 

Although it has been common practice to specify the pressure loss in ordinary valves in terms of either equivalent length of straight pipe of the same size or velocity head loss, it is becoming more common to specify flow rate and pressure drop characteristics in the same terms as has been the practice for valves designed specifically for control service, namely, in terms of the valve coefficient, C. The flow coefficient of a valve is defined as the volume of Hquid at a specified density that flows through the fully opened valve with a unit pressure drop, eg, = 1 when 3.79 L/min (1 gal /min) pass through the valve... 

The valve cannot control if it is at either end of its travel. To ensure controUabiflty, a valve is generally chosen in such a way that at the maximum design flow rate the flow coefficient required is no more than 85% of the wide-open valve flow coefficient, and at the minimum anticipated flow rate requiring control, a flow coefficient of about 10% of the wide-open valve flow coefficient is required. Whenever practical, control valves are located at grade or at platforms, to assure adequate working space for servicing. 

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. 

Valve Application Technology Functional requirements and the properties of the controlled fluid determine which valve and actuator types are best for a specific apphcation. If demands are modest and no unique valve features are required, the valve-design style selection may be determined solely by cost. If so, general-purpose globe or angle valves provide exceptional value, especially in sizes less than 3-inch NFS and hence are very popular. Beyond type selection, there are many other valve specifications that must be determined properly in order to ultimately yield-improved process control. 

Rotaiy stem-valve designs are normally offered only in their naturally occurring characteristic, since it is difficult to appreciably alter this. If additional characterization is required, the positioner or controller may be charac terized. However, these approaches are less direct, since it is possible for device nonlinearity and dynamics to distort the compensation. 

Seal legs are frequently used in conjunction with solids-flow-control valves to equ ize pressures and to strip trapped or adsorbed gases from the sohds. The operation of a seal leg is shown schemati-caUy in Fig. 17-19. The sohds settle by gravity from the fluidized bed into the seal leg or standpipe. Seal and/or stripping gas is introduced near the bottom of the leg. This gas flows both upward and downward. Pressures indicated in the ihustratiou have no absolute value but are only relative. The legs are designed for either fluidized or settled solids. 

All drum filters (except the single-compartment filter) utilize a rotary-valve arrangement in the drum-axis support trunnion to facih-tate removal of filtrate and wash hquid and to allow introduction of air or gas for cake blowback if needed. The valve controls the relative duration of each cycle as well as providing dead portions of the cycle through the use of bridge blocks. A typical valve design is shown in Fig. 18-121. Internal piping manifolds connect the valve with various sections of the drum. 

Pneumatic and hydraulic vibrating conveyors have as their greatest asset ehmination of explosion hazards. If pressurized air, water, or oil is available, they can be extremely practical since their drive design is relatively simple and pressure-control valves can be used to vaiy capacity either manually or automatically. 

Design equipment to prevent excessively fast feed. Do not oversize pumps or control valves... 

Use separate control valves for heating/cooling Use same media for heating/cooling Design for rapid change over of services... 

Eail-Safe Design features which provide for the maintenance of safe operating conditions in the event of a malfunction of control devices or an interruption of an energy source (e.g., direction of failure of a control valve on loss of signal). A system is fail-safe if failure of a component, signal, or utility that would create a hazard initiates an action that maintains the system in a safe condition. 

In most units, the flue gas pressure is reduced to atmospheric pressure across an orifice chamber. The orifice chamber is a vessel containing a series of perforated plates designed to maintain a given back-pressure upstream of the regenerator pressure control valve. 

---