McDonnell & Miller 51-2-HD Control

The McDonnell & Miller 51-2-HD is a heavy-duty head mechanism only (Part Number 135100) designed specifically for field replacement on Series 51-2 mechanical water feeders. This unit is engineered for low pressure steam and hot water boiler applications, supporting systems with a maximum vessel pressure of 35 PSI. As a specialized low water cut-off (LWCO) component, it features a snap-acting SPDT switch that provides simultaneous feed control and burner cutout, ensuring the boiler maintains a safe operating water level even under fluctuating loads.

Built for durability in commercial and industrial environments, the 51-2-HD utilizes a proportional feed action to prevent the rapid cycling of the water supply, which reduces thermal shock to the heat exchanger. The quick-change cartridge valve allows for streamlined maintenance without breaking pipe connections, a critical feature for technicians managing uptime. This mechanism is compatible with boilers exceeding 5,000 sq. ft. of capacity and operates with a maximum water supply pressure of 150 PSI. The integrated No. 2 Switch facilitates both the electrical cutout of the burner circuit and the activation of an optional alarm or signal, ensuring compliance with safety protocols for boiler water level control.

• Enhanced System Safety: The snap-acting SPDT switch provides a reliable electrical break for the burner and an auxiliary contact for alarms, preventing dry-fire conditions.
• Efficient Serviceability: Features a quick-change replaceable cartridge valve and strainer, allowing technicians to clear debris or replace internal components rapidly.
• High Capacity Performance: Optimized for large-scale low pressure steam boilers with a cold water feed, ensuring consistent proportional feed action.
• Rugged Construction: Designed with a monel bellows to resist corrosion and handle the demanding environments of industrial boiler rooms.

Common Questions

Can the 51-2-HD be used to upgrade a standard Model 51?
Yes, the 51-2-HD head mechanism can be used to add low water cut-off functionality to an existing Model 51 water feeder that currently lacks an electrical switch.

What is the maximum water temperature this unit can handle?
The mechanism is rated for a maximum inlet water temperature of 120°F (49°C) to ensure the integrity of the internal valve seals and bellows.

How do I test the LWCO function during a service call?
You should perform a blow-down procedure while the burner is firing. As the float drops, the No. 2 switch should break the circuit to the burner before the water level disappears from the gauge glass.

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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.