Masoneilan 77-6 Air Lock-Up Valve for Pneumatic Control Valves
Overview
The Masoneilan 77-6 is an air lock-up valve from Baker Hughes, a small pneumatic accessory that sits in the output line between a controller or positioner and the diaphragm chamber of a control valve. Its job is to watch the signal pressure and to respond when that pressure drops below a preset value by closing the inlet path. As the name of this accessory indicates, the purpose is to lock up the air in the actuator side of the circuit so that the control valve is not driven by a failing supply. Baker Hughes publishes it as an instruction manual for the Masoneilan 77-6 Model air lock-up valve, and the manual states an estimated useful life of more than 25 years when installation and maintenance are correct.
Seven Star LLC is a trusted supplier for Masoneilan and supplies the Masoneilan 77-6 to refineries, gas plants and utilities in Oman and across the GCC. Because the set point depends on the signal range of the loop, our instrumentation team in Muscat asks for the controller range and the actuator type before confirming the unit, and provides the Baker Hughes manual so that installers can follow the published setting and maintenance procedure.
Operating Principle of the Masoneilan 77-6
The Masoneilan 77-6 lock-up valve is a spring-loaded diaphragm device. Signal pressure acts on a diaphragm sub-assembly against a range spring, and the diaphragm positions an inlet valve. While the signal is above the set pressure the inlet valve is open and air passes from the inlet to the outlet in the direction marked on the body. When the signal falls below the set pressure the spring prevails and the inlet valve closes, which isolates the actuator line from the inlet.
The factory setting is 20 psi for a 3-15 psi operating range and 35 psi for a 6-30 psi range. The manual describes how to set the valve if it ever needs adjustment: relieve the compression on the range spring, pipe an air supply with a gauge and regulator to the signal connection, raise the pressure to the desired set value and turn the adjusting screw down until no air can be felt at the outlet, at which point the valve is fully closed. The locknut is then tightened and the outlet and signal lines are reconnected.
Engineering Design of the Masoneilan 77-6
The Masoneilan 77-6 is built around a body and a removable body insert. The body insert carries the inlet valve sub-assembly, and the diaphragm sub-assembly is attached to the inlet valve stem by a locknut. A spring case, an upper spring button, the range spring and an adjusting screw complete the set-point mechanism, while a body cap with its own O-ring closes the top and is held by four body cap screws. Two O-rings seal the insert and an additional O-ring seals the inlet valve stem, and Baker Hughes advises lubricating that stem O-ring when the inlet valve is reassembled.
The installation rules are specific. The valve must be fitted in the controller output line as near to the control valve diaphragm chamber as possible, with as few connections as possible, and the air must flow in the direction shown by the INLET and OUTLET marks on the underside of the body. The signal pressure line must be taken upstream of the air supply regulator so that the lock-up valve responds effectively in the event of air failure. All lines should be cleaned before connection, and the valve is not to be installed in air lines above 250 psi (1.72 MPa, 17.2 bar).
Key Features
The following characteristics define how the Masoneilan 77-6 is specified and maintained:
- It is a simple spring and diaphragm design with a 16-item parts list, which keeps maintenance straightforward.
- Factory settings of 20 psi and 35 psi correspond to the 3-15 psi and 6-30 psi signal ranges.
- The set point can be changed on site with a gauge, a regulator and the adjusting screw, then secured with a locknut.
- A removable body insert allows the diaphragm sub-assembly and inlet valve to be withdrawn together for inspection.
- The inlet valve can be inspected for wear and replaced if it is roughed up or pitted.
- The diaphragm sub-assembly can be replaced if it is ruptured or damaged around its edge.
- The pressure limit of 250 psi is clearly stated and applies to the air line in which the valve is installed.
- Baker Hughes notes that stocking complete units can be more economical than replacing internal parts, given the labour involved.
Industrial Applications
A lock-up valve such as the Masoneilan 77-6 is chosen where the loss of instrument air or of the control signal must not send a pneumatic control valve to an uncontrolled position. The Masoneilan 77-6 belongs in the output line of loops that serve process units in refining, gas processing, power generation and water treatment, including facilities at Sohar, Duqm and Salalah. It is used with diaphragm actuators whose output pressure is generated by a positioner or controller.
In Oman and the GCC, Seven Star LLC supplies the Masoneilan 77-6 together with the actuators and instruments in the same loop, such as the Masoneilan Type 87/88 spring-diaphragm actuators and the Masoneilan SVI FF valve positioner. For booster or filter duties in the same line, the Masoneilan BR200 booster relay is a related accessory, and our engineers will help place each device in the right order in the pipework.
Technical Specifications
The table summarises the published data for the Masoneilan 77-6 from the Baker Hughes instruction manual.
| Parameter | Specification |
|---|---|
| Model | Masoneilan 77-6 Model air lock-up valve |
| Set pressure (factory) | 20 psi for 3-15 psi operation; 35 psi for 6-30 psi operation |
| Maximum air pressure | 250 psi (1.72 MPa, 17.2 bar); do not install in lines above this |
| Service medium | Industrial compressed air; consult Baker Hughes for other fluids |
| Installation point | Controller output line, close to the control valve diaphragm chamber |
| Flow direction | As marked INLET and OUTLET on the underside of the body |
| Signal connection | Taken upstream of the air supply regulator |
| Adjustment | Adjusting screw with locknut, against a range spring |
| Serviceable internal parts | Diaphragm S/A, inlet valve S/A, inlet valve O-ring, body insert O-rings, body cap O-ring |
| Parts list | 16 reference items, including four body cap screws |
| Estimated useful life | 25 years or more with annual inspection and routine maintenance |
| Warranty | One year from date of shipment under Baker Hughes terms |
Related Masoneilan accessories include the Masoneilan 78 air filter regulator, Masoneilan BR400 volume booster relay and Masoneilan 4411 I/P transducer.
Request a Quote for Masoneilan 77-6
To request pricing or technical support for the Masoneilan 77-6, send a formal RFQ to the Seven Star LLC instrumentation division with the signal range, the actuator it will protect and the supply pressure. We can confirm the set pressure, advise on availability and lead time and provide the Baker Hughes instruction manual from this page. In Oman and the GCC, Seven Star LLC delivers genuine products with manufacturer documentation and local engineering support.
- Instruction Manual Masoneilan 77-6 Air Lock-Up Valve Instruction Manual Download
Masoneilan control valves and actuator series
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Throttling duty on steam, hydrocarbons, gas and water is the daily work of these valves: feedwater and steam conditioning in power plants, separator and compressor loops in gas processing, high pressure drop letdown, cryogenic service in LNG and air separation, and general flow, pressure and level control in refineries and chemical plants.Ordering Masoneilan valves and spare parts: what we need
For spare parts the serial number is decisive, since the factory record behind it lists trim, materials and actuator. Send a clear nameplate photo with serial number, model number, size and pressure class. For a new valve add the process data: fluid, inlet and outlet pressure, temperature, minimum, normal and maximum flow, required leakage class, fail position, air supply and control signal.Browse Masoneilan products by category: Control Valves, Valve Actuator, Regulators, Valve Positioner, Level Transmitter.
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