FA1D56 Booster Pump Main Cover Hex Nut FA1D56-03-14 Common Q&A

In the maintenance and overhaul of the FA1D56 booster pump, the FA1D56-03-14 Main Cover Hex Nut, though seemingly trivial, is the critical fastener that ensures the pump body's sealing integrity and structural stability. Many power plant techs encounter issues related to material selection, complementary parts, and replacement standards during selection and installation. Below, we address these high-frequency questions one by one to help you avoid common operational pitfalls.

I. Why Choose 35CrMoA Material? What is the Advantage Over Standard Carbon Steel Nuts in High-Temperature Conditions?

The core reason for choosing 35CrMoA alloy structural steel for the FA1D56-03-14 nut is to adapt to the booster pump's high-temperature and high-pressure conditions. Compared to standard carbon steel nuts, the advantages are threefold:

• "Hardcore" Mechanical Properties: The booster pump's main cover needs to withstand the pressure of the internal sealing chamber, and the nut must endure sufficient tensile and yield strength. 35CrMoA offers a tensile strength of ≥985 MPa and a yield strength of ≥835 MPa, perfectly meeting the maintenance manual requirement of "tensile strength no less than 950 MPa, yield strength no less than 800 MPa." In contrast, standard carbon steel nuts (like Q235 material) typically have a tensile strength of only 375-500 MPa. Under long-term stress, they easily "stretch out," leading to enlarged sealing gaps in the main cover.
• High-Temperature Stability Resists "Attenuation": During pump operation, medium circulation causes the temperature near the main cover to rise, potentially reaching 150-200°C long-term (approaching 300°C in some conditions). 35CrMoA maintains stable strength below 500°C; it won't "soften" under high heat like standard carbon steel. One plant tried using standard nuts and found the torque decayed by 30% after six months of operation, resulting in minor main cover leakage. After switching to 35CrMoA, the torque remained within the standard range even after 18 months of running.
• "Longer-Lasting" Durability: 35CrMoA has better fatigue resistance than standard carbon steel. Pressure fluctuations during pump start-up and shut-down repeatedly stress the nut. Standard carbon steel is prone to "fatigue cracks," while 35CrMoA reduces this risk. Its service life is 2-3 times that of standard carbon steel nuts, reducing the hassle of frequent replacements.

II. When Replacing the FA1D56-03-14 Nut During Overhaul, Which Parts Must Be Used as a Set? Can I Substitute Other Washer Models?

The FA1D56-03-14 nut is not a "solo operator"; it must be used with three types of complementary parts, and substituting non-original parts is not recommended, as detailed below:

1. Three Mandatory Complementary Part Types and Their Functions

• Type 1: Sealing Washers: You must use the FA1D56-03-21 (Copper Washer) and FA1D56-03-24 (Standard Washer). The copper washer fills microscopic gaps between the main cover and the pump body, while the standard washer protects the contact surface between the nut and the main cover from being scratched during tightening.
• Type 2: Locking Washer: The FA1D56A-01-11 Locking Washer is necessary to prevent the nut from "coming loose" due to pump vibration. Especially during booster pump operation, shaft system vibration causes slight main cover movement, and without the locking washer, torque loss is likely.
• Type 3: Coordinating Components: These include the LTJ-8B1D-FA1D56 Mechanical Seal and the FA1D56-03-03 Seal Cooling Chamber. Tightening the nut fixes the position of these components, ensuring stable pressure within the sealing chamber.

2. Why Can't I Use Other Models as Substitutes?

The main risk is that "poor compatibility" leads to seal failure: for example, using a copper washer that isn't FA1D56-03-21 might have the wrong thickness—the original is 3mm; if you use a 4mm one, the nut won't tighten sufficiently, and the main cover won't be adequately compressed. If you use a standard rubber washer, it will age and deform under high temperatures, leading to even worse leakage. Another example is the locking washer: the FA1D56A-01-11 is a "single-tab stop type" that precisely fits the nut's hex size. Substituting other models might not lock tightly, negating the anti-loosening function. One plant tried to take a shortcut by using a generic washer; the main cover leaked after just 3 months of operation. Upon disassembly, the washer was found to be crushed, ultimately increasing the overhaul costs.

III. Is Replacing the FA1D56-03-14 Nut Mandatory When Dismantling the Main Cover During Overhaul? What Torque Should I Use for Re-tightening During Routine Leak Inspection?

These two questions require looking at the specific scenario, but the core principle is: "Follow the standards, avoid wishful thinking."

1. Mandatory Replacement During Major Overhaul: Don't Skimp on the Cost

Regardless of how "new" the old nut looks, you must replace it with a new FA1D56-03-14 nut after dismantling the main cover during a major overhaul. This is because the nut undergoes "plastic deformation" when tightened. When reinstalled, the thread fit will degrade. Furthermore, even though 35CrMoA is durable, it develops fatigue marks after long-term stress—these may be invisible to the naked eye but could cause sudden failure during the next operating cycle. One plant reused old nuts during a major overhaul, and 10 months later, the main cover started leaking. Disassembly revealed fine cracks in the nut threads. In the end, they not only had to replace the nuts but also re-treat the sealing surface, incurring double the cost. Replacing with new nuts as per maintenance standards reduces subsequent failures.

2. Routine Tightening Torque: Follow the Manual, Not Your Feel

If routine inspection reveals minor main cover leakage and insufficient nut torque is confirmed, you must not "tighten by muscle." You need to use a torque wrench according to the standards in the FA1D56 booster pump maintenance manual. Typically, for the corresponding specification (e.g., M24) 35CrMoA nut, the standard torque is 280-320 N·m (always refer to your plant's specific equipment manual, as minor variations between manufacturers are possible).

Two points of caution here:

• First, clean the threads of the nut and bolt first; they must be free of oil or impurities, which would affect torque accuracy.
• Second, use a "two-step tightening" process: tighten to 150 N·m first, pause for 10 seconds, and then tighten to the final standard value to prevent "falsely high" torque readings. One tech tightened by feel, exceeding 400 N·m, which snapped the bolt and damaged the main cover sealing surface—a costly mistake.

Summary

The selection, complementation, and replacement of the FA1D56-03-14 nut must follow the standards—choose the correct 35CrMoA material to handle high temperatures, use the correct original complementary parts, don't skimp on replacing with new ones during a major overhaul, and adhere to torque standards during routine tightening. This ensures the main cover seal of the booster pump remains stable, reducing leakage issues. If you are unsure during spare parts procurement or installation, we recommend seeking technical data from a professional supplier to prevent a small component from affecting the operation of the entire pump set.

HKCYT-2025-10-29