McDonnell & Miller 157S-A Controller

The McDonnell & Miller 157S-A (Part Number 173702) is a heavy-duty combination low water cut-off and pump controller designed specifically for commercial and industrial steam boiler applications. Featuring a snap-acting SPDT switch, this unit ensures precise control over water levels and reliable burner cutout during low-water conditions. The 157S-A model includes alternate tappings, providing versatile installation options for equalizing pipes and blow-down valves. Built with a rugged water column-style body, it handles a maximum steam pressure of 150 psi (10.5 kg/cm²). This float-operated controller is engineered for high-pressure systems where maintaining a consistent water line is critical to preventing thermal shock and vessel damage. Contractors prefer the 157S series for its mechanical reliability and ease of field service, particularly in boiler feed pump control configurations. When replacing an existing 157S-A water level control, ensure the pipe tapping orientations match your specific manifold setup for optimal debris management and float buoyancy.

• Dual Functionality: Manages both pump operation and low-water burner cutout within a single float-driven housing.
• Snap-Action Switching: Reinforced SPDT switch prevents arcing and provides definitive electrical signals for pump cycles.
• Alternate Tappings: 1-inch NPT connections allow for flexible leveling and easier integration into existing steam headers.
• High Pressure Rating: Rated for systems up to 150 psi, making it suitable for demanding process steam environments.
• Debris Management: The integral water column design helps settle sediment away from the float mechanism, reducing maintenance intervals.

Common Questions

What is the difference between the 150S and 157S series?
The 157S-A utilizes a water column-style body with 1-inch NPT tappings, allowing it to be installed directly into the boiler's equalizing lines, whereas the 150S typically mounts via a flange or specialized tapping.

Can the 157S-A be used for both lead and lag pump control?
The 157S-A features a single snap-acting switch. For lead/lag or more complex sequencing, it is typically used as the primary safety and signal device integrated into a larger control panel or used in conjunction with secondary probes.

How does the 'A' designation affect installation?
The 'A' suffix indicates alternate tappings, which provide additional ports for gauge glass sets or tri-cocks, allowing for a more compact and centered installation on the boiler front.

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Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.