Seal Modification of Tee Ball Valve in Primary Reformer Oil Gun

Literature Overview

The paper by Liang Zhiyong from Qilu Petrochemical Company's Second Fertilizer Plant, published in Large Nitrogen Fertilizer (2002, Vol. 25, No. 2), documents a practical engineering solution to a recurring sealing failure problem in tee ball valves used in the oil gun system of a primary reformer furnace. The modification involves the use of X-resin modified materials for valve packing and the design of trapezoidal grooves with fluororubber (FKM) O-ring seals between the valve body and bonnet. The modified valve passed field testing under operating conditions and demonstrated reliable sealing performance, extended service life, and significant economic benefits.

Problem Analysis

Operating Environment

The primary reformer furnace in a natural gas-based ammonia or methanol production facility operates under extreme conditions:

The tee ball valve in the oil gun system is subjected to:

• High temperature: The oil gun operates at elevated temperatures due to proximity to the reformer furnace tubes.
• Thermal cycling: Repeated start-up and shutdown cycles cause thermal expansion and contraction of the valve components.
• Hydrogen attack: The hydrogen-rich atmosphere can cause hydrogen embrittlement of carbon steel components and degradation of elastomeric seals.
• Pressure cycling: The valve must maintain a tight seal under fluctuating pressure conditions.

Failure Mechanism

The original sealing failure was attributed to several contributing factors:

1. Thermal degradation of packing material: The original packing material (likely graphite or PTFE-based) could not withstand the sustained high-temperature exposure, leading to hardening, cracking, and loss of sealing capability.
2. Thermal mismatch: Differential thermal expansion between the valve body, bonnet, and stem created gaps at the sealing interfaces during operation.
3. Chemical attack: The hydrogen-rich atmosphere caused swelling and degradation of the original seal materials.
4. Mechanical wear: The ball valve's rotation mechanism caused progressive wear of the sealing surfaces over time.

Modification Design

X-Resin Modified Packing Material

The X-resin modified material is a specialized sealing compound designed for high-temperature and chemically aggressive environments. Key characteristics include:

The X-resin modification enhances the base material's resistance to thermal degradation and chemical attack, while maintaining the mechanical properties required for effective sealing under compression.

Trapezoidal Groove Design

The modification includes the machining of trapezoidal grooves between the valve body and bonnet, into which FKM (fluororubber) O-rings are installed:

• Groove geometry: The trapezoidal profile provides better O-ring retention and alignment compared to simple rectangular grooves, reducing the risk of O-ring extrusion under pressure.
• O-ring material: FKM (Viton) offers excellent resistance to hydrocarbons, hydrogen, and elevated temperatures (up to approximately 200-250°C for standard grades, or up to 300°C for specialty grades).
• Sealing principle: The O-ring provides a primary seal between the body and bonnet, while the X-resin packing provides a secondary seal around the valve stem.

Multi-Barrier Sealing Strategy

The modification implements a multi-barrier sealing approach:

1. Primary seal: X-resin modified packing around the valve stem prevents leakage along the stem.
2. Secondary seal: FKM O-rings in trapezoidal grooves seal the body-bonnet joint.
3. Pressure relief: The multi-barrier design ensures that even if one seal degrades, the remaining seals provide backup protection.

Failure Mode and Effects Analysis (FMEA)

Applying FMEA methodology to the original and modified valve designs:

Where S = Severity, O = Occurrence, D = Detection, RPN = Risk Priority Number.

Testing and Verification

Field Testing Protocol

The modified valve was subjected to the following testing sequence:

1. Bench testing: The valve was tested on a test rig at representative operating pressure and temperature to verify basic sealing performance.
2. Thermal cycling test: The valve was subjected to multiple thermal cycles between ambient temperature and operating temperature to simulate start-up and shutdown conditions.
3. Field installation: The modified valve was installed in the primary reformer oil gun system and operated under actual process conditions.
4. Performance monitoring: Leak detection methods (visual inspection, soap bubble test, or electronic leak detectors) were used to monitor sealing performance over time.

Acceptance Criteria

Economic Analysis

The modification yielded significant economic benefits:

• Reduced maintenance frequency: Extended service life between packing replacements reduced labor and material costs.
• Reduced unplanned shutdowns: Reliable sealing eliminated production losses from valve-related leaks.
• Material cost savings: The X-resin and FKM materials, while more expensive than the original packing, provided a better cost-benefit ratio over the extended service life.
• Safety improvement: Elimination of hydrocarbon leaks reduced fire and environmental hazards.

Engineering Practice Implications

This case study illustrates several important principles for valve maintenance and modification in process industries:

• Root cause analysis is essential: The successful modification was based on a thorough understanding of the failure mechanisms, not just a superficial replacement of the failed component.
• Material selection must match the operating environment: The choice of X-resin and FKM materials was directly informed by the specific temperature, pressure, and chemical conditions of the application.
• Multi-barrier sealing provides redundancy: The combination of packing and O-ring seals creates a defense-in-depth strategy that improves reliability.
• Field validation is critical: Laboratory testing alone is insufficient; the modified valve must be validated under actual operating conditions to confirm its effectiveness.

Key Questions and Reflections

A key question is the long-term durability of the FKM O-rings at the body-bonnet joint. While FKM offers excellent chemical resistance, its long-term performance at elevated temperatures (particularly above 200°C) depends on the specific grade and the actual temperature profile at the sealing interface. If the body-bonnet joint is exposed to temperatures exceeding the O-ring's maximum rating, thermal degradation will eventually occur.

Another consideration is the compatibility of the X-resin packing with the valve stem material. If the stem is made of carbon steel or a low-alloy steel, the X-resin packing must not promote corrosion or galling at the stem-packing interface. Surface treatment of the stem (e.g., hard chrome plating or nitriding) may be necessary to ensure long-term compatibility.

Study Insights

This case study is a valuable example of practical engineering problem-solving in the process industry. The modification of a tee ball valve seal through careful material selection and design optimization demonstrates that even small components can have a significant impact on plant reliability, safety, and economics. The approach taken—analyzing the failure mechanisms, selecting appropriate materials, designing a multi-barrier sealing system, and validating the solution in the field—is a template that can be applied to similar problems in other process industries.

The key lesson is that effective maintenance and modification require a deep understanding of the operating environment, failure mechanisms, and material properties. A superficial approach that merely replaces the failed component without addressing the underlying causes is likely to result in repeated failures. The systematic approach demonstrated in this case study—combining failure analysis, material science, mechanical design, and field validation—provides a robust framework for solving similar engineering challenges.