PWM Control
BURKERT 20024873 (Type 8691) is a pneumatic control head configured for decentralised on/off automation of ELEMENT Type 21xx process valves. It features an integrated double-acting (5/2-way) pilot control valve system, an internal contact-free analogue inductive displacement sensor, and a cable gland connection interface with screw terminals. Valve end positions are registered and stored automatically during start-up using an integrated Teach function. The design couples directly to ELEMENT actuators without external pilot air tubing by channeling operating air through internal air ducts.
Status visualization is provided through high-performance colored LEDs on the upper perimeter of the housing, allowing operators to identify valve operating conditions at a glance. Built with a polyphenylene sulfide (PPS) base, a stainless steel body sleeve, and an impact-resistant polycarbonate (PC) cover, the housing withstands wash-down regimes in hygienic processing environments found across food, beverage, and pharmaceutical manufacturing.
Technical Specifications
| Parameter | Specification |
|---|---|
| Control Head Type | 8691 (Revision 3) |
| Article Number | 20024873 |
| Actuating Function | Double-acting (5/2-way pilot valve) |
| Valve Diameter / Orifice | 3 mm |
| Pilot Air Flow Rate (QNn) | 250 l/min (aeration and ventilation, 7 to 6 bar abs) |
| Supply Pressure | 3.0 to 7.0 bar (must exceed actuator minimum requirement by 0.5 to 1.0 bar) |
| Control Medium | Neutral gases, air according to ISO 8573-1 |
| Dust Specification | Class 7 (particle size < 40 µm, particle density < 10 mg/m³) |
| Moisture Content | Class 3 (pressure dew point ≤ -20 °C or 10 °C below lowest operating temp) |
| Oil Specification | Class X (< 25 mg/m³) |
| Pilot Air Port | G 1/8 internal thread, stainless steel |
| Position Sensor | Teachable inductive analogue linear sensor (self-adjusting PNP) |
| Stroke Range | 2.5 to 45 mm (valve spindle) |
| Operating Voltage | 24 V DC ± 25% (range: 18 to 30 V DC, UL: NEC Class 2) |
| Residual Ripple | 10% |
| Power Consumption | 2 W (standby/base), 5 W (maximum load on digital output) |
| Digital Output Rating | Max. 100 mA per output |
| Electrical Connection | M16 × 1.5 cable gland (cable dia 5 to 10 mm) with screw terminals for 0.14 to 1.5 mm² |
| Ambient Temperature | -10 °C to +55 °C |
| Ingress Protection | IP65 / IP67 according to EN 60529, 4X according to NEMA 250 |
| Operating Altitude | Up to 2000 m above sea level |
| Relative Humidity | Max. 90% at +55 °C (non-condensing) |
| Conformity & Approvals | cULus Listed (Certificate E238179, UL 61010-1), CE compliant |
Material Specifications
The external components of the 8691 control head are manufactured from chemically resistant materials suited for hygienic wash-down cleaning procedures:
| Item | Component | Material |
|---|---|---|
| 1 | Transparent Cover | Polycarbonate (PC) |
| 2 | Cover Seal | EPDM |
| 3 | Body Casing | Stainless steel |
| 4 | Casing Seal | EPDM (external), NBR (internal) |
| 5 | Push-in / Threaded Port G 1/8 | POM / Stainless steel |
| 6 | Basic Housing | Polyphenylene sulfide (PPS) |
| 7 | Fastening Screws | Stainless steel |
| 8 | Electrical Connector Area | Stainless steel / Polyamide |
Hygienic Protection Mechanism
During routine operation and wash-down cleaning, the enclosure's IP65/IP67 integrity is reinforced by positive internal pressure generated by pilot exhaust. When integrated with ELEMENT actuators, the system vents pilot exhaust to aerate the actuator spring chamber, preventing outside atmospheric air and moisture from entering the actuator spring cylinder.
Dimensions and Mounting on ELEMENT Valves
Type 8691 mounts directly onto the top cover of ELEMENT Type 21xx pneumatic actuators. Article 20024873 supports direct integration on ELEMENT actuator sizes ø 70 mm, ø 90 mm, and ø 130 mm (universal mounting kit part no. 679917).
When installing the unit on a valve with a welded body, complete pipe welding before mounting the control head to protect plastic guides and electronic sensor assemblies from excessive thermal distortion.
Electrical and Pneumatic Connections
Terminal Block Pin Assignment (M16×1.5 Cable Gland)
Article 20024873 provides terminal block connections through an M16 × 1.5 cable bushing, accepting conductor cross sections from 0.14 mm² to 1.5 mm²:
| Terminal Mark | Signal Direction | Description |
|---|---|---|
| DO2+ | Output | Digital output active when actuator is in activated end position (max. 100 mA) |
| DO1+ | Output | Digital output active when actuator is in deactivated end position (max. 100 mA) |
| 24 V+ | Power Input | Operating voltage +24 V DC (18 to 30 V DC) |
| 24 V- | Power Input | Operating voltage GND (0 V) |
| DI+ | Control Input | Digital control input valve (+24 V DC) |
| DI- | Control Input | Digital control input valve (GND) |
Service Interface (Service-büS)
The control head contains an internal micro-USB service port accessible when removing the stainless steel body casing. It connects to a PC via the USB-büS interface set (accessory part no. 772551) to configure parameters, adjust LED colors, and retrieve diagnostic data using Bürkert Communicator software.
Pneumatic Integration
Control air is supplied through a G 1/8 stainless steel threaded port. Supply line air must be filtered, non-condensing, and free of oil mist according to ISO 8573-1 specifications. The pilot air feed line requires an operating pressure 0.5 to 1.0 bar higher than the actuator switching pressure.
Fastening Torque Limits
Do not overtighten housing components. Lateral casing fastening screws must be tightened to a torque of max. 1.5 Nm to prevent casing thread damage and preserve IP65/IP67 sealing.
Commissioning and Teach Function
The double-acting pilot system on article 20024873 supports automatic Teach-in calibration:
- Ensure the control head is securely mounted to the ELEMENT actuator, supply pressure (3 to 7 bar) is active, and 24 V DC power is applied.
- Unscrew the outer body casing counterclockwise from the basic housing using hook wrench 674077.
- Press and hold the Teach button on the circuit board for approximately 5 seconds until the red status LED starts blinking.
- Release the button within 5 seconds. The unit automatically strokes the double-acting actuator to detect both mechanical travel stops, measures opening and closing times, and records switching points.
- When the red status LED extinguishes, calibration is complete and verified. Reinstall the casing and tighten to the physical stop.
Valve Combinations and Ordering Structure
Type 8691 combines with pneumatic ELEMENT process valves to form factory-tested automated control units:
- Type 2100: Angle seat valve ELEMENT series
- Type 2101: Globe valve ELEMENT series
- Type 2103: 2/2-way diaphragm valve with stainless steel actuator
- Type 2104 / 2105: T-valve and tank-bottom diaphragm valves
- Type 2106: 3/2-way seat valve ELEMENT series
Ordering Variants with Integrated Pilot Air Duct
| Article No. | Control Function | Actuation Type | Electrical Connection | Approval |
|---|---|---|---|---|
| 20024873 | Double-acting | Pilot valve (5/2-way) | Cable gland M16x1.5 | Standard / cULus |
| 20024840 | Single-acting | Pilot valve (3/2-way) | Cable gland M16x1.5 | Standard / cULus |
| 20024817 | Position sensor only | None (external pilot) | Cable gland M16x1.5 | Standard / cULus |
| 20024875 | Double-acting | Pilot valve (5/2-way) | M12 circular plug, 8-pin | Standard / cULus |
| 20024842 | Single-acting | Pilot valve (3/2-way) | M12 circular plug, 8-pin | Standard / cULus |
| 20024886 | Double-acting | Pilot valve (5/2-way) | Cable gland M16x1.5 | ATEX II Cat. 3G/D, IECEx, CCC |
Maintenance and Filter Service
Type 8691 features an exchangeable air intake filter (0.1 mm mesh size) located at the pneumatic inlet connector to protect internal pilot valves from particulate fouling.
To clean or replace the intake filter:
- Depressurize the pneumatic system and verify lines are vented.
- Depress the push-in connector collet ring and pull the filter cartridge outward.
- Clean the mesh element or insert a new filter element.
- Inspect the internal sealing O-ring and push the filter fully back into place until securely locked.
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