Fisher 377 Pressure-Sensing Trip Valve

In process control systems, when supply pressure drops below a safe threshold, the actuator must respond immediately-either fully open, fully closed, or locked in its current position. Traditionally, achieving these three failure modes required three separate switching valves, resulting in complex piping, higher costs, and dispersed spare parts inventory.

Fisher 377 compresses all three functions into a single valve body. By changing only the external hook-up connections, you can switch between fail-up, fail-down, and lock-in-last-position modes without replacing any hardware. The immediate benefits include reduced procurement costs, lower spare parts requirements, and significantly simplified pneumatic piping.

Adjustable trip point is another key feature. Users can set the specific actuation threshold on the valve according to actual supply pressure conditions on-site, rather than relying on fixed setpoints. When supply pressure falls below the trip point, the trip valve triggers the actuator into the preset failure mode; when pressure rises above the trip point, the valve automatically resets and the system returns to normal operation.

From a construction standpoint, Fisher 377 features large diaphragm areas and very few moving parts. In the field, this translates to fewer wear points and longer maintenance intervals. For continuous-duty equipment, that means lower unplanned downtime risk.

The 377 series trip valves are compatible with Fisher 480, 585C, 685, 1061, and other actuators. Mounting options include bracket mounting, yoke mounting, or top-mounting on a manifold【4†L5】. Typical applications include:

· Supply pressure protection for critical process loops – ensuring valves move in the intended direction when instrument air pressure fluctuates.

· Auxiliary actuation for emergency shutdown (ESD) systems – acting as a pneumatic logic element to complete safety actions under trigger conditions.

· Equipment protection at remote or unattended sites – automatic reset reduces the need for manual intervention.

Q: What is the relationship between 377, 377U, and 377L?
A: 377 is the base model. 377U and 377L are variant configurations within the same series, with identical core functionality and valve body construction, differing only in interface orientation and mounting bracket layout.

Q: Does switching failure modes require disassembling the valve?
A: No. Switching between the three failure modes is accomplished through external piping changes only-no disassembly or internal parts replacement is required.

Q: How is the trip point adjusted?
A: The valve body includes an adjustable mechanism that allows users to manually set the trip threshold based on actual on-site supply pressure requirements.

Q: Can the 377 be used in corrosive environments?
A: The 377 is designed as a standard pneumatic component. If corrosive gases are present in the environment, material compatibility should be evaluated and protective measures considered. Consult your Emerson sales engineer for guidance.

Call to Action

To configure a Fisher 377 trip valve for your actuator, or to request selection support and a quotation, please provide the following information:

· Actuator model

· Normal system supply pressure range

· Required failure mode (fail-up / fail-down / lock-in-last-position)

· Available mounting space and interface orientation

We will assist with sizing calculations and piping scheme design.

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