Flow Measurement & Control


Modular control valve for a host of applications

May 2000 Flow Measurement & Control

The makers of the BR 33 rotary plug valve have recently developed this valve further; the result is the Foxflex control valve system.

The components of this control valve system are:

* One-piece valve body either flanged in DIN or ANSI ratings and dimensions or as wafer type (IEC takeout dimension), made of carbon steel, stainless steel or special alloys.

* Pneumatic rolling diaphragm actuator with adjustable 'full signal' valve opening angle.

* Fully integrated positioner system, consisting of:

- Positioner (pneumatic or I/P retrofitable)

- Limit switches.

- Position transmitter.

- Solenoid valve.

* Supply air filter and reducer.

* Valve.

The one-piece valve housing of the Foxflex (with no static seals except pipe flanges and shaft packing) minimises risk of external leakages and offers full compliance with modern safety concepts. This makes the Foxflex a good device to handle dangerous or toxic process fluids.

The circular-symmetric housing enables the valve body to cope with high pipe forces without leakage or loss of function.

The Foxflex is available in flanged style, in DIN and ANSI ratings and dimensions. Takeout dimensions are the same as that of reciprocating globe style control valves, permitting exchangeability between Foxflex and globe valves without the need for spacers. Flanges (in ring joint design), with true ANSI FTF dimensions, are within the delivery program.

Foxflex can be delivered in wafer style with IEC FTF dimensions. This provides interchangeability with other popular eccentric rotary valves. The ratings are ANSI Class 150 and 300 (ANSI Class 150 not available with 1" [25,4 mm] and 1 1/2” [51 mm] wafer style valves) and in DIN PN 10–40.

Material options include carbon steel A 216 WCB, stainless steel (A 351 CF8M) and many special alloys such as Hastelloy, Titanium, high temperature steels or even Zirconium.

The closing member of the Foxflex, consisting of shaft, curved arc and separate plug, assures optimal control performance, easy maintenance and low cost of ownership.

The separate plug (not on valves DN 25 /1") enables simple change between all plug materials, starting from 316 stainless steel, and going via stellited trims up to trims made of tungsten carbide or solid ceramic.

Since normally only the spherical plug surface shows erosion, separation of arc and plug result in low spare parts prices and reduces maintenance costs. The curved arc enables free flow passage at 90° of valve opening - a desirable characteristic.

The arc and the shaft are connected with a conical pin. This means that the shaft is blowout safe permitting the actuator to be dismounted from the valve even when the valve is under system pressure. The roller-furnished shaft is a guarantee for extremely good tightness and long lifetime of the packing box.

Stem sealing

The universal Foxflex system is also based on the stuffing box system, modular and suitable for all applications. The standard stuffing box (adjustable), which can be equipped with PTFE, PTFE graphite compound or pure graphite packing rings, corresponding to process fluid and temperature. The system encloses an adjustable double stuffing box (with or without control connection 1/4" NPT) and leads to the stuffing box type-certified according TA-Luft (German clean air act) designed to handle extremely toxic or dangerous process fluids and gases.

The 'TA-Luft' stuffing box is maintenance-free and can operate up to 200°C with the control connector. The secondary seal system and the helium tightness measured at more than 100 000 full stroke cycles is approved by authorities as a 100% replacement of all bellows seal systems.

All stuffing box systems described above are fully exchangeable; bonnet extensions and special stems are not required.

Special designs

Since Foxflex is designed as a modular system, there are many special solutions based on the standard design. Some of these specific solutions are mentioned below.

The first solution is designed to handle and to withstand abrasive fluids, such as slurries, suspensions and dust. This would be useful in flue gas desulphuration plants or the control of lime and limestone slurry, etc. In this design, the plug and the seat ring are made of solid ceramic and the valve is built in the flow-to-close direction. This makes fluid velocity small in the valve housing and the arc and shaft area, since energy conversion takes place in the valve outlet. That area is protected against erosion with a full ceramic outlet liner. The shaft guides are protected against damage by O-ring seals.

Another application would be where fluids tend to crystalise. The one-piece valve body makes the Foxflex a good choice for a steam jacket. The steam jacket is in contact with the valve body at the pipe flanges and on the actuator flange as well as the packing area, so there are no thermal bridges. One application example for a Foxflex with steam jacket would be where molten sulphur has to pass through a valve.

Actuator

The diaphragm actuator Series 99 is specially developed for use with the Foxflex valve, offering optimal control performance. The torque characteristic is optimised for eccentric rotary valves, and the actuator sizes 122, 240 and 779 cm2 guarantee economic actuator selection on all valve sizes up to DN 300 (12").

The dimensions of the rolling diaphragm and the mechanics of the actuator are based on a 600 kPa air supply pressure. This, together with the epoxi-coating of the actuator, result in a long expected lifetime under hard industrial service conditions. The modular system of the actuator permits in situ changing of failed components and has a field-retrofitable handwheel unit.

An external coupling between actuator shaft and valve shaft enables easy dismounting of actuator and valve subassembly, the design permits four different (in quarter turn steps) actuator mounting positions.

The Foxflex actuator system is in compliance with the Health and Safety Act and the European Machinery Requirements, this also includes the safe release of actuator spring precompression without any special tools. The unique double-lever system enables the selection of different valve opening angles 30°, 45°, 60° and 90° (at full actuator signal). This corresponds to 45%, 75%, 100% or 120% of nominal rated Cv. (Nominal rated Cv is the rated Cv at 60° valve opening) The selection of the rated Cv is performed by means of an adjusting bolt.

With only two standard seat rings on each valve size - eight different Cv's ratings are obtained, with a significantly reduced spare part storage.

When changing the Cv-setpoint, the closed position of the valve and the signal range of actuator and positioner will not change (the positioner feedback is linked to the primary lever, which is connected to the rolling diaphragm).

Thanks to the inherent rangeability of a rotary plug valve and since the reduction of the opening angle, (as viewed mathematically) is nothing but a projection of the given signal to a smaller rotary angle, without reduction of positioning accuracy, the control valve Cv-ratio will not be reduced when reducing rated Cv.

Positioner system

The built-in positioner system, consisting of positioner and limit switch unit/position feedback transmitter makes the Foxflex a real control valve system that is capable of fulfilling all industrial requirements.

The integration of valve equipment into the actuator housing gives optimal protection and smallest possible outline dimensions. The steady rinsing of the actuator with instrument air reduces condensate, pollution and corrosion to a minimum.

The modular system of positioner, limit switch unit and position feedback transmitter, the possibility of changing from pneumatic to electric signal and the unique concept of the positioner as an insert unit (quick exchange system) enables easy service.

Since the positioner startpoint and range can be adjusted without opening the actuator housing, Foxflex complies with all safety requirements.

The Foxboro range of control valves, including the Foxflex control valve, is distributed and supported in South Africa by BTR Flow Control SA, the Jetpark-based, wholly owned Invensys.




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