Single Seat vs Cage Guided Globe Control Valve

Only by fully understanding single seat and cage guided globe control valves can they be used correctly. This article helps you understand them better.

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Single Seat Globe Control Valve

Definition

Single Seat Globe Control Valve, the trim is interchangeable in the body. It's content to various technics liquid process by replacing the different trim assembled.

Application

The single-seat globe control valve is the furthest common control valve type, a simple structure, is used for the strict sealed close situation. A single-seat globe control valve can be controlled the maximum flow rate by changing the size of the plug and seat ring. setting fluid characteristics by changing the curved surface. They are applicable in a wide scope of liquid services as well as steam services.

Structure

Among different types of valve bodies, single-seated valves have trim structures with the easiest collocation, for example, angle valves. As a result of high-pressure fluid is generally held the load to add the whole area of the bottom seat, made the single plug to be endured huge unbalanced force from fluid, relative to the control valve with balanced trim, the allowable pressure drops are much less for the single-seated valve on the process control.

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Cage Guided Control Valve

Definition

Cage Guided Control Valve, cage structure, straight stroke control valve. the trim is interchangeable in the body Its content to various technics liquid processes by replacing the different trim assembled.is designed focused on high-pressure drops, effectively reduce the effects of cavitation and the noise in the high-pressure drops services. favorable Interchangeability, be reached different flow characteristics by replacing different cage, to get much higher precision of control. Can be reduced noise, dispelled the effects of cavitation by replacing trim.

Application

Cage Guided control valve size is in common use for 1-12, an actuator for standard diaphragm type. By metal seat, the suitable temperature range is -196 +538, and by soft seat, the suitable temperature range is -45 +200. It's content to most industrial process control.

Structure

The most excellent cage guided structure control valve is balance characteristic, for plug balance ports function, fluid pressure acted on static unbalance force of plug, it reduced unbalance force, then the valve can be configured actuator smaller than conventional single seat valve.

The most simple structure of cage guided control valve is twin-seat, the twin-seat structure is applicable to situations that requirement of leakage is not high. The cage-guided single-seat control valve has a sliding seal between plugs top with cage. by seals function, dispelling the leak that upriver fluid entered the low-pressure downriver system. So it have the tightly closed characteristic as conventional single seat valve, moreover have balance characteristic.

Advanced control schemes can't produce optimum results unless the control valves operate properly. Instrument technicians must understand these final control elements as well as their diagnostic software to ensure the valves in the plant operate as the system designers intended.

Renewed interest in the performance of control valves is emerging, partly as a result of numerous plant audits that indicate roughly one-third of installed control valves are operating at substandard levels. Even though properly operating control valves are essential to overall plant efficiency and product quality, maintenance personnel frequently don't recognize the signs of poor performance. The basics of control valve design and operation must be well understood for end-users to reap the benefits of improved valve operation.

Basic types of control valves

The most common and versatile types of control valves are sliding-stem globe and angle valves (see Figure 1). Their popularity derives from rugged construction and the many options available that make them suitable for a variety of process applications, including severe service. For example, sliding stem valves typically are available with options that satisfy a range of requirements for ANSI Class pressure-temperature ratings, shutoff capability, size, temperature compatibility and flow characteristics.

Figure 3. Cage-guided valve.

Selection and sizing

Control valve selection is based on the process fluid to be handled and a number of performance objectives. Required sizing parameters include specific gravity, pressures at the valve inlet and outlet, pressure drop across the valve, fluid temperature at the valve inlet, flow rate and vapor pressure. Other vital information includes the desired response time, process gain characteristics and the potential for cavitation or flashing.

Achieving complete valve shutoff is important in many applications to prevent leakage that either could contaminate a process fluid or result in product loss. Tight shutoff also prevents erosion damage that could occur if a high-velocity stream leaked across seating surfaces.

Many control valves are oversized as a result of inaccurate information and safety margins added by each individual or group that participates in the sizing procedure. Oversized valves are a problem for three reasons.

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First, the valve operation may become unstable because it never opens very far from the fully closed position. Process gain is generally high when the valve is throttling near its seat. The combined valve and process gains may be too high to maintain stable operation at low lifts. Second, excessive seat wear may result from high velocity flows between the closure member and the seating surface. Third, the design flow characteristic may not be achieved, resulting in controller tuning problems.

Valve manufacturers and vendors usually use specialists in fluid thermodynamics who can provide system designers with state-of-the-art solutions to unusual sizing situations.

Figure 4. Eccentric rotary plug valve.

Actuation

A properly selected and sized control valve can deliver optimum performance only when the plug, disk, ball or ball segment positions itself properly in response to the control signal. Closure member positioning is a function of actuator performance and the instrumentation that provides loading pressure to the actuator. There are three relevant factors to consider.

Force at the closed position: For globe and angle valves, the actuator must provide sufficient force (or thrust) to achieve the specified ANSI Class shutoff. For rotary-shaft valves, the actuator must provide sufficient torque to move the closure member into and out of the seat.

Actuator stiffness: To maintain valve plug stability, the actuator must offer sufficient resistance to fluid buffeting forces by means of a mechanical spring or air spring effect.

Fail mode: This defines the position to which the closure member moves if the compressed air supply is lost.

Some controversy exists over the relative merits and limitations of piston actuators versus spring-and-diaphragm actuators. Both devices are useful, and each has a place in process control.

Spring-and-diaphragm actuators are simple and reliable, and they can be used in most control applications. One of their major advantages is the built-in spring fail action (inherent fail mode) that provides full shutoff force in the event supply pressure is lost.

Positioners

A valve positioner ensures the actual valve stem position matches that which the control signal is trying to achieve. Even the best control valves can't achieve peak performance without a high-performance positioner. In a competitive production environment, positioners have achieved the status of essential automation instrumentation.

They're frequently used to overcome high valve friction and reduce the resulting deadband and hysteresis, which provides more accurate control. A positioner must be used with a double-acting piston actuator (with or without springs) to provide throttling control.

Positioners typically are used to increase actuator force in the travel stop positions. The extra force is especially useful for rotary valves because of their substantial seal friction.

During the past several years, considerable interest has developed over "smart" field-mounted instruments and the protocols that allow communication among them, the control system host and other devices on the control network. Smart field-mounted instruments deliver a number of useful features that give technicians the ability to configure, calibrate and troubleshoot instruments and control valves from remote locations. Advanced diagnostic features allow users to implement predictive maintenance procedures by scanning large numbers of devices automatically and identifying those that most need service. Systems that support smart devices make possible advances in asset management, generation of work orders, alerts and alarms in control systems, and in the not too distant future, instant messaging of valve alerts and failures to maintenance department pagers and cell phones.

The overlooked technology

Process engineers have long looked upon control valves as low-tech "iron" that simply obeys instructions. Yet control valves are actually among the most complex field devices, required to perform rigorous tasks repeatedly, often under severe service conditions. If they're not properly selected, installed and maintained, control valves can cause drastic reductions in process efficiency.

Where a concerted effort has been made to understand the performance of these final control elements, increases in throughput and profitability are typical. Points to remember include:

• Process variability decreases when control valves are properly installed, calibrated and maintained.
• Ensuring control valves respond quickly and accurately to control signals reduces process variability.
• Minimizing process variability improves plant performance.

How can you take advantage of such benefits? The answer is education. Process engineers and instrument technicians who truly understand control valves, including recent advances in valve and instrumentation technology, are able to install and calibrate new or rebuilt valves correctly, troubleshoot problems more effectively and make beneficial adjustments without removing valves from operation. Well-trained personnel implement less costly maintenance programs that result in high efficiency with a minimum of unexpected shutdowns. They do it through careful monitoring of the condition of every control valve in the plant and selecting for immediate repair or replacement only those valves whose early failure or loss of performance is a distinct possibility.

Steve Hagen, Senior Instructor at Emerson Learning Solutions, Marshalltown, Iowa, can be reached at http://www.emersonprocess.com/education and (641) 754-.

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