- Automation & ControlView all 1,652 products
- AC Drives
- Industrial Communications
- PLC & I/O Cards
- Vibration Monitoring
- Automation Accessories
- Controllers & Data Acquisition
- CCTV & Security
- Safety Switches & Guard Locking
- Encoders
- Industrial Computers
- Servo Drives
- Isolator & Barrier
- Motor Soft Starters
- HMI Displays
- Pneumatic Cylinders
- Automation Software
- Machine Vision
- Robotics
Brand: Samson
Samson Type 3709 Pneumatic Lock-Up Valve (Fail-in-Place)
Description
Type 3709 pneumatic lock-up valve for fail-in-place control valves
The Samson Type 3709 is a pneumatic lock-up valve that protects a control valve from drifting when the instrument air supply drops. It watches the supply pressure, and if that pressure falls below an adjusted value or disappears, it shuts off the signal pressure line to the actuator. The actuator is then held in its last position, so the valve fails in place. Seven Star LLC supplies the Type 3709 in Oman and the GCC as an independent supplier based in Muscat.
The 3709 is built as a family: small versions with KVS about 0.2 mount directly on a positioner or sit in the tubing, while larger versions with an integral booster reach KVS 4.3 for big actuators. Body materials are aluminum or stainless steel, and the switching point is changed with one screw.
Type 3709 versions and what the numbers mean
The two-digit suffix after 3709 selects the mounting arrangement and flow capacity. The data sheet lists these versions.
| Version | Mounting and flow |
|---|---|
| 3709-01 | Direct attachment to a positioner; G or NPT ¼; KVS 0.2; for linear actuators and VDI/VDE 3845 rotary actuators |
| 3709-02 | Installed in the signal line in any position; G or NPT ¼; KVS 0.2 |
| 3709-04 | With booster, installed in any position; G or NPT ½; KVS 4.3 |
| 3709-05 | With booster, input hooked up as required; mounts on single-acting rotary actuators per VDI/VDE 3845; ¼ thread; KVS 2.0 |
| 3709-06 | With booster, rotary mounting per VDI/VDE 3845; ½ thread; KVS 4.3 (on request) |
| 3709-07 | With booster, NAMUR ¼ interface, for mounting a solenoid valve per VDI/VDE 3845; KVS 2.0 |
| 3709-08 / -09 / -10 | On request: NAMUR ½ or ¼ interfaces and sandwich-style versions for VDI/VDE 3845, KVS 4.3 or 2.0 |
| 3709-12 | With booster, double-acting, installed in the signal line in any position; ½ thread; KVS 4.3 |
| 3709-13 | With booster, attachment to double-acting rotary actuators per VDI/VDE 3845; KVS 4.3 |
Type 3709 technical specifications
| Parameter | Specification |
|---|---|
| Manufacturer / part number | SAMSON AKTIENGESELLSCHAFT / Type 3709 |
| Function | Shuts off the signal pressure line below an adjusted supply pressure or on total loss of supply air |
| Max. supply pressure | 12 bar (-01, -02); 6 bar (-04 and above) |
| Max. output pressure | 6 bar |
| KVS | Approx. 0.2 (-01, -02); approx. 2.0 (-05, -07, -09); approx. 4.3 (-04, -06, -08, -10, -12, -13) |
| Set point range | 0.5 to 6 bar (-01, -02); 1.5 to 6 bar (booster versions), continuously adjustable |
| Switching accuracy | About 0.2 bar at 2 bar, 0.3 bar at 4 bar, 0.4 bar at 6 bar set point |
| Ambient temperature | -25 to +80 °C, -40 to +80 °C or -45 to +80 °C (code field); extended range on request |
| Connections | Input and output G/NPT ¼ or ½, or NAMUR ¼ / ½ interfaces; supply air G/NPT ¼ |
| Weight, aluminum | 0.4 kg (-01/-02) up to 2.6 kg (-06) |
| Weight, stainless steel | 0.4 kg (-01/-02) up to 6.1 kg (-06) |
| Materials | Aluminum 3.3547 / 3.2315 or stainless steel 1.4404 body; VMQ diaphragm; POM plugs in booster versions; PA 66 cap, stainless cap as spare |
How the 3709 locks the actuator
Supply air acts on a diaphragm, and the force it creates is balanced by an adjustable spring. While the supply pressure stays above the set value, the diaphragm force wins and the input and output are connected, so the positioner signal flows freely to the actuator. If the supply drops, the spring pushes the plug into the seat and the pressure trapped in the actuator is held.
In the booster versions the diaphragm opens a double-seat system that releases a control pressure to a booster plug, allowing high air flow to the actuator. When the spring wins, the signal line is blocked and the control pressure is vented. The double-acting versions -12 and -13 route the control pressure through a capillary tube to both spring systems, so both plugs open together.
Using the 3709 with positioners, boosters and solenoid valves
The Samson application sketches show three arrangements: a positioner with a lock-up valve in the output line, the same with a volume booster ahead of the lock-up valve for fast actuators, and a loop with a solenoid valve for a safety function, in which the pneumatic lock-up valve has priority over the solenoid valve. For fast actuators, the Type 3755 volume booster is installed upstream of the lock-up valve. Positioners that accept direct attachment of the 3709-01 include Type 3766, 3767, Type 3730 versions, the Type 3731 family and the TROVIS SAFE variants listed on the data sheet.
The NAMUR-interface versions (-07 to -10) can also take a Type 3967 solenoid valve according to VDI/VDE 3845.
Selection checklist for the Type 3709
- Actuator: single-acting or double-acting, linear or rotary, to pick -01/-02, -04 to -10, -12 or -13.
- Air capacity: KVS 0.2 for small actuators; 2.0 or 4.3 where a booster is needed.
- Mounting: positioner direct attachment, tube hook-up, VDI/VDE 3845 or NAMUR interface.
- Thread: G or NPT, ¼ or ½, or without thread for NAMUR.
- Set point range: 0.5 to 6 bar or 1.5 to 6 bar, and the supply pressure margin above it.
- Ambient: choose the temperature code field for the site.
- Body: aluminum or stainless steel; also consider the paint-compatibility code if painting is planned.
Setting, testing and maintaining the 3709
Set the switching point with the adjustment screw so it lies safely below the normal supply pressure and above the air level at which the actuator can no longer hold the valve position. Lock the screw with the lock nut. To test, slowly reduce the supply pressure while watching the actuator pressure, and confirm that it holds at the set value. Vent plugs for the booster versions are available as accessories in G ¼ and ½ NPT sizes, along with replacement caps and O-rings for the NAMUR interfaces. Keep the vent free of paint and debris.
Typical 3709 applications in Oman and the GCC
Lock-up valves are used where an unplanned air failure must not move a process valve, for example in utility air headers on desalination plants, feed and cooling water control in power stations, and gas treating or export lines in oil and gas facilities. Holding position may be preferred to full-open or full-closed travel when sudden valve movement could upset the process. In hot Gulf conditions the temperature code and stainless steel body are worth selecting for outdoor, coastal installations.
Information to send with an RFQ for the 3709
- Version number (-01, -02, -04 and so on) or the full article code
- Actuator type, size, single or double acting and stroking time
- Normal supply pressure and the pressure at which the valve should lock
- Thread or interface required
- Body material and ambient temperature
- Quantity and whether vent plugs or spares are needed
Documents and quotation
The Samson datasheet for the Type 3709 is attached on this page and lists every version, dimension and the complete article code. Send the 3709 version, actuator details and supply pressure to Seven Star LLC for a quotation in Oman and the GCC.
Other Models in the Samson Positioner Accessories Range
Seven Star LLC supplies models from the Samson Positioner Accessories range in Oman and the GCC. Compare the related models below and request a quote for the one that matches your application.
| Model | Category | |
|---|---|---|
| Samson Type 3755 Pneumatic Volume Booster, KVS 2.5 | Instrument Valves & Fittings | View details |
| Samson Type 3709 Pneumatic Lock-Up Valve (Fail-in-Place) | Instrument Valves & Fittings | You are viewing this model |
Datasheets & Documents
- Datasheet Samson Type 3709 Datasheet Download
Reviews (0)
About brand
SAMSON is a German manufacturer of control valves, actuators, positioners and self-operated regulators. Hermann Sandvoss founded the company in Düsseldorf on 1 April 1907, and its first important product was a thermostatic valve that controlled temperature without external power. After moves to Mannheim, SAMSON settled in Frankfurt am Main in 1916, where SAMSON AG is still headquartered.
Seven Star LLC supplies Samson control valves, pneumatic actuators and positioners to process plants in Oman, including the types listed on this page and their replacement units.
Samson company profile
| Founded | 1 April 1907, Düsseldorf, by Hermann Sandvoss, as VULCAN Technische Apparate Baugesellschaft; products sold under the SAMSON name |
|---|---|
| Headquarters | Frankfurt am Main, Germany, since 1916 |
| Legal form | SAMSON AG |
| First product | A self-operated thermostatic valve |
| Group brands | SAMSON together with PFEIFFER, LEUSCH, VETEC and AIR TORQUE |
| Product families | Globe valves such as Type 3241, rotary and butterfly valves, 3271 and 3277 pneumatic actuators, 3730 and 3731 positioners, self-operated pressure and temperature regulators |
Milestones
- 1907: Hermann Sandvoss founds the company in Düsseldorf.
- 1909: It is re-established as SAMSON Apparatebau GmbH.
- 1916: After a period in Mannheim, the company moves to Frankfurt am Main.
- 1975: The name becomes SAMSON Mess- und Regeltechnik.
Samson valve, actuator and positioner types
Samson builds a control valve from modules. The Type 3241 globe valve is the general-service body of Series 240, and the Type 3251 is the Series 250 body for higher pressure ratings. Either one is combined with a Type 3271 pneumatic diaphragm actuator or a Type 3277, whose yoke allows the positioner to be attached directly with a concealed travel linkage.
Among the positioners listed are the Type 3725 and the Type 3730 family: the 3730-1 as the basic electropneumatic version, sold in its current generation as TROVIS 3730-1, and the 3730-3 with HART communication and valve diagnostics. Samson also makes PROFIBUS PA and FOUNDATION fieldbus variants in the same family, plus self-operated pressure and temperature regulators.
Where Samson valves are used
Samson control valves throttle steam, gas and liquids in chemical and petrochemical units, refineries, gas processing, power stations, water and desalination plants, and food production. Many reach site as part of a skid or package, so the plant instrument team often meets them first when a positioner or actuator needs replacing. We supply these items to end users and contractors throughout the GCC.
Samson products we list
Ordering Samson parts: what we need
Every Samson device has a nameplate with the type, a configuration ID (Var-ID) and a serial number. A photo of that plate is the surest route to an exact match. Without it, please send:
- Valve: type, DN or NPS, pressure rating, body and trim material, Kvs or Cv, plug and seat style, packing and bonnet type
- Actuator: type, effective area in cm², bench range, fail action and travel
- Positioner: full type code including Ex protection, communication and options such as limit switches or position transmitter
We check the code against Samson documentation and provide the manufacturer's data sheet on request.
Facts checked against SAMSON's corporate chronicle and press material, October 2026.
Browse Samson products by category: Control Valves, Valve Positioner, Valve Actuator.
FAQ
Questions about the Samson Type 3709 Pneumatic Lock-Up Valve
Answers from our engineering team on installation, compatibility and integration.
01What does the Type 3709 lock-up valve do?
It blocks the signal pressure line when the supply air falls below an adjusted value or fails completely, so a pneumatic actuator stays in its last position instead of drifting to its fail-safe position.
02What is the difference between 3709-01 and the booster versions?
Types -01 and -02 are small, KVS about 0.2. Types -04 to -13 add a booster section and reach KVS up to 4.3 for large actuators. -01 is for direct attachment to a positioner and -02 for hooking up anywhere in the line.
03What switching point range can be set on the 3709?
The adjustment range is 0.5 to 6 bar for the -01 and -02 and 1.5 to 6 bar for the booster versions. The data sheet gives a switching accuracy of about 0.2 bar at a 2 bar set point, 0.3 bar at 4 bar and 0.4 bar at 6 bar.
04Which positioners can the 3709-01 be fitted to directly?
The data sheet lists Type 3766 and 3767, Type 3730 versions, TROVIS 3730-1 and 3730-3, TROVIS SAFE 3730-6, Type 3731-3 and 3731-5 and TROVIS SAFE 3731-3.
05What body materials are available for the 3709?
Aluminum or stainless steel 1.4404. Weights range from 0.4 kg for the small types to 2.6 kg in aluminum and 6.1 kg in stainless steel for the larger booster versions.
Reviews
There are no reviews yet.