NPS 32 Class 900 Metal Seated Butterfly Valve for Salalah Refinery, Oman

Tags:metal seated butterfly valve, high pressure butterfly valve, FLOWKS valve, Salalah Refinery, process gas isolation

FLOWKS NPS 32 Class 900 Metal Seated Butterfly Valve Solution for Salalah Refinery, Oman

Project Overview

Salalah Refinery in southern Oman operates under conditions that test equipment to its limits — sustained high ambient temperatures, corrosive coastal atmosphere, and process streams at the edge of design capability. When the refinery needed a reliable isolation valve on a critical process gas line, the engineering team had to decide what valve architecture could handle large-bore, high-pressure gas service without the operational headaches of heavier alternatives.

The original specification referenced Metso valves. FLOWKS was selected to replace the incumbent supplier across the board, meaning the replacement had to match or exceed the original in pressure containment, sealing integrity, cycle life, and actuation reliability. The valve carries line number MOV-20102 and sits on the same pipeline as MOV-20101, an electric actuated gate valve of the same size class. That proximity shaped several engineering decisions.

Service Conditions

The valve handles process gas at a design pressure of 150 barG. Design temperature spans from -29°C to 425°C — a range wide enough to rule out elastomer-lined designs entirely and push selection toward all-metal construction.

Process gas at refinery scale is unforgiving. Any seat leakage means product loss and potential safety exposure. The specification called for FCI 70-2 Class VI shutoff, the tightest class the standard defines. Achieving Class VI on a valve this size with metal-to-metal seating required careful attention to seat geometry and surface treatment — not just selecting a hard material and hoping for the best.

The valve operates in on-off isolation duty. It does not throttle. When it closes, the refinery needs certainty that the downstream line is isolated. The engineering behind making that binary behavior reliable at NPS 32 and Class 900 is anything but simple.

Valve Design and Material Selection

Body Construction

The valve body is manufactured from ASTM A216 Gr. WCB cast carbon steel. WCB is a workhorse material in refinery service — it handles the pressure rating, it is weldable for repair and modification, and it performs predictably across the specified temperature range. For a body that needs to contain 150 barG at 425°C, WCB offers the right combination of strength, ductility, and code acceptance under ASME B16.5.

The original specification called for raised face flanges. FLOWKS changed this to RTJ (Ring Type Joint) connections. At Class 900 and 150 barG, raised face flanges depend on gasket compression that can relax over thermal cycles. RTJ joints seal through metal-to-metal contact between the ring gasket and the groove machined into the flange faces. The ring deforms plastically under bolt load, filling groove irregularities and maintaining seal integrity through thermal transients. For gas service at these pressures, RTJ is the conservative choice.

Disc

The disc is ASTM A351 Gr. CF8M — cast austenitic stainless steel with molybdenum for pitting resistance. Selecting CF8M for the disc while using WCB for the body is deliberate. The body provides structural strength and pressure containment. The disc faces the process medium directly and needs corrosion resistance that carbon steel cannot provide, particularly in a gas stream that may carry trace sulfur compounds or chlorides at elevated temperature.

Seat Design

The seat is Stellite hardfaced. Stellite — a cobalt-chromium alloy — is applied as a weld overlay to the seating surface and then machined to final geometry. The reason for choosing Stellite over other hardfacing options comes down to galling resistance and dimensional stability at temperature. At 425°C, softer overlays can deform under the seating load of a large disc, and repeated cycling work-hardens the contact zone until it cracks. Stellite maintains hardness at elevated temperature and resists the adhesive wear that occurs when a metal disc contacts a metal seat under high load. This is what makes metal seated butterfly valve performance viable in this pressure class — the seat material has to survive thousands of open-close cycles without losing sealing geometry.

Stem

The valve stem is forged from ASTM SA564 Gr. 630 — a precipitation-hardening martensitic stainless steel commonly known as 17-4 PH. This grade offers yield strength well above what austenitic stainless steels can deliver, which matters because the stem transmits both the actuator torque and the disc seating force. At NPS 32, the torques involved are substantial. A stem that twists under load loses positioning accuracy and can eventually fatigue at stress concentration points. 17-4 PH at the H1150M condition provides the strength margin needed for reliable long-term operation.

Packing

Live-loaded graphite packing was specified. Live-loading uses Belleville washers to maintain constant compressive load on the packing set, compensating for thermal expansion and contraction that would otherwise relax the seal. Across a range this wide, the packing stack experiences significant dimensional change between cold startup and hot operation. Without live-loading, operators would need to re-torque the gland periodically, and between adjustments, leakage could occur. Graphite packing also handles fire exposure better than PTFE-based alternatives — a real consideration on a process gas line.

Actuator System

[Pneumatic Actuation](https://flowks.com/products/pneumatic-actuators)

The valve is equipped with a pneumatic actuator operating on a 4-8 barG instrument air supply. The choice of pneumatics over electric actuation for this valve was driven by several factors specific to this installation.

Pneumatic actuators deliver high torque in a compact envelope. At NPS 32 Class 900, the seating torque required to achieve Class VI shutoff against full differential pressure is significant. An electric actuator sized for that torque would be substantially larger and heavier, which creates mounting and structural support challenges on a pipeline already carrying a heavy gate valve at the adjacent position.

Pneumatic actuators also fail predictably. The specification called for fail-in-last-position (lock-up) with an option for fail-close, depending on the safety logic configuration. Achieving fail-close with a pneumatic spring-return actuator is mechanically simple — the spring provides the closing force, and the instrument air opposes it. Electric actuators require battery backup or a separate UPS to achieve the same result, adding complexity and maintenance burden.

The actuator is paired with a manual hydraulic pump as a secondary means of operation. If instrument air is lost or the control system is down for maintenance, an operator can cycle the valve by hand using the hydraulic pump. This redundancy is standard practice in refinery isolation service — the valve must be operable under all conditions, not just normal conditions.

Instrumentation and Accessories

The valve's control package includes a comprehensive set of accessories, each serving a specific function:

APL510 Limit Switch* — Provides position feedback to the control system, confirming open and closed positions. On an isolation valve, position indication is not optional; the control system needs to know the valve's actual state, not just the command that was sent.

SMC AW60 Filter Regulator* — Conditions the instrument air before it reaches the actuator. Particulate or moisture in the air supply degrades actuator seals and solenoid valves over time. The AW60 is sized for the flow requirements of this actuator, ensuring adequate air delivery during stroking.

SMC VFE3130 Solenoid Valve* — Controls the air path to the actuator based on the electrical signal from the control system. The VFE3130 is a 3-position, 5-port solenoid that enables the fail-lock function by trapping air in the actuator when de-energized.

SMC VPA4150 Air Control Valve* — Manages the pilot air logic for the actuator's directional control. This valve ensures smooth, controlled transitions between open and closed positions rather than abrupt slam-closing, which could damage the seat or generate pressure transients in the line.

SMC AS600 Flow Control Valve* — Regulates the speed of actuator stroke. Opening and closing times are adjustable within the 5-70 second range, allowing the commissioning team to tune the valve's response to the pipeline's hydraulic characteristics. Too fast, and the valve generates surge pressures. Too slow, and the isolation function is compromised during emergency shutdown.

Air Reservoir* — Provides a local volume of stored instrument air to ensure the valve can complete at least one full stroke even if the main air supply is interrupted. This is particularly important for the fail-close mode, where the actuator must overcome pipeline pressure and seat friction to reach the closed position.

FLOWKS standardized on SMC pneumatics for large-bore actuated valves because the components are field-proven, globally available, and interchangeable with existing refinery spares inventory. When a maintenance technician at Salalah needs to replace a solenoid during a turnaround, availability matters more than theoretical specification differences between brands.

Valve Characteristics and Flow Capacity

The valve has a rated CV of 20,383 with an equal percentage flow characteristic. These two numbers define the valve's hydraulic behavior and deserve explanation.

A CV of 20,383 means the valve presents minimal resistance to flow when fully open — exactly what an isolation valve should do. The full-bore disc design ensures the process gas passes with minimal pressure drop. In a refinery where every fraction of a bar of pressure loss costs compression energy, low resistance through isolation valves matters economically.

The equal percentage characteristic matters even for an on-off valve. As the disc approaches the closed position, this geometry ensures the final degrees of rotation produce a controlled reduction in flow, reducing hydraulic shock at seating. This protects the Stellite surface and extends seat life.

The valve achieves FLOWKS valve performance standards across the full temperature range. Full-open torque, seat leakage, and cycle life were all validated against the specification requirements before the valve was released for shipment.

Key Engineering Decisions

Why a Butterfly Valve Instead of a Gate Valve

The adjacent line valve, MOV-20101, is an NPS 32 Class 900 electric actuated gate valve. It would be reasonable to ask why MOV-20102 was not specified the same way. The answer lies in the specific requirements of the isolation duty at this location.

Gate valves are excellent isolation devices — straight-through flow path, low pressure drop, reliable shutoff. But at NPS 32, a Class 900 gate valve is an extremely heavy assembly. The body, bonnet, wedge, and stem require substantial structural support, specialized lifting equipment for maintenance, and a large installation footprint. The electric actuator on MOV-20101 adds further weight.

A high pressure butterfly valve achieves the same isolation function with a fraction of the installed weight and envelope. The disc rotates 90 degrees to open or close — there is no rising stem, no bonnet, no wedge to lift out of the flow path. This means faster stroking, simpler actuator sizing, and lower overall system mass. On a pipeline where the structural supports were already carrying the weight of an NPS 32 Class 900 gate valve, specifying a butterfly valve for the second isolation point reduced the total load on the pipe rack.

The trade-off is seat leakage. A gate valve's wedge generates high linear seating force that typically outperforms a butterfly valve's rotary contact. But FLOWKS's Stellite-faced seat design, combined with precision machining of the CF8M disc seating surface, brought leakage performance to FCI 70-2 Class VI — no visible leakage. At that point, the butterfly valve matches the gate valve's isolation capability while offering advantages in weight, speed, and maintainability.

Why Pneumatic Instead of Electric

MOV-20101 next door uses an electric actuator. MOV-20102 uses pneumatic. This was not inconsistency — it was deliberate system design.

The gate valve on MOV-20101 is a rising-stem design that requires multi-turn operation. Electric multi-turn actuators are well suited to this: they provide precise positioning, high torque output, and can hold position without continuous power. The trade-off is stroke time — electric actuators on large gate valves are slow.

The butterfly valve on MOV-20102 needs quarter-turn operation with defined speed control. Pneumatic actuators excel at quarter-turn duty. They stroke fast when needed, they can be tuned with flow control valves to manage stroking speed, and they provide fail-safe action through spring return or trapped-air logic without requiring batteries. The 5-70 second adjustable stroke time range gives the commissioning team flexibility to match the valve's closing speed to the pipeline's surge tolerance.

Using different actuator technologies on adjacent valves is standard practice in refinery design. Each valve's function dictates the actuator. The DCS interfaces with both through standard discrete and analog signals — actuator type is transparent to the control system.

Testing and Inspection

Every FLOWKS valve for refinery service undergoes structured testing before shipment. For this valve, the sequence included hydrostatic body and seat testing at Class 900 pressures, functional cycling across the full travel range, and seat leakage verification against FCI 70-2 Class VI criteria.

The RTJ flange grooves were inspected using dye penetrant examination to verify that no casting defects or machining anomalies were present in the sealing surfaces. RTJ integrity depends entirely on the geometric precision of the groove — any defect that prevents the ring gasket from seating properly will leak under pressure.

The actuator assembly was tested as a complete unit. Valve and actuator were mounted together with the full control package — solenoid, filter regulator, air control valve, flow control valve, limit switches — wired and pneumatically connected for functional testing. Stroke times were measured at both ends of the adjustable range. The manual hydraulic pump was exercised to confirm it could drive a full cycle without instrument air.

The air reservoir was pressure-tested and verified to deliver sufficient volume for a complete stroke under simulated supply-loss conditions.

Documentation included material traceability for all pressure-retaining components, certified test reports for Stellite hardfacing chemistry and hardness, and a complete actuator sizing calculation package showing safety margins.

Project Delivery

FLOWKS executed this project as a direct replacement for the originally specified Metso valves. The changeover required that every interface — flange dimensions, face-to-face, actuator mounting, control connections — matched the original installation. ASME B16.5 compliance on the flanges ensured the valve would bolt up to the existing pipeline without modification.

The valve was delivered with a comprehensive data package including as-built drawings, material certificates, test reports, operating and maintenance manuals, and a recommended spare parts list. The list identified components most likely to need replacement during the first maintenance cycle — packing set, solenoid valve, filter regulator element — so the refinery could procure spares before the valve went into service.

For Salalah Refinery, the result is an isolation valve that handles the full design envelope — 150 barG, -29°C to 425°C, process gas — with the operational simplicity of a quarter-turn butterfly design. The pneumatic actuation system provides the speed and fail-safe behavior the safety logic demands, and the manual hydraulic override ensures operability under all conditions.

FLOWKS Manufacturing Range

FLOWKS manufactures a comprehensive range of industrial valves for oil and gas, petrochemical, power, and water applications:

Ball Valves* — Floating and trunnion-mounted designs, soft seated and metal seated, in sizes from NPS 2 to NPS 56 and pressure classes up to Class 2500.

Control Valves* — Globe and angle body configurations with pneumatic or electric actuators, characterized trims for modulating service across a wide range of process conditions.

Gate Valves* — Wedge and flexible wedge designs in cast and forged body constructions, suitable for isolation service in refinery and pipeline applications.

Globe Valves* — Straight pattern, angle pattern, and Y-pattern bodies for throttling and isolation duties.

Check Valves* — Swing check, lift check, tilting disc, and dual plate designs for flow direction prevention in process and utility systems.

Strainers* — Y-type and T-type basket strainers for pipeline debris removal and equipment protection.

[Knife Gate Valves](https://flowks.com/products/knife-gate-valves)* — Unidirectional isolation valves for slurry, powder, and viscous media applications.

Plug Valves* — Lubricated and non-lubricated designs for isolation and diversion service in oil and gas pipelines.

All FLOWKS valves are manufactured under ISO 9001 quality management systems, with testing and certification in accordance with international standards including API, ASME, and EN specifications.

**FLOWKS — How Quality Lasts!