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Industry Headlines

MSS Revises Standards for Instrument Valves, Sealing of Rising Stem Valves

Tuesday, 19 September 2017  |  Chris Guy

The Manufacturers Standardization Society (MSS) is pleased to announce the publication of the following revised Standard Practices:

  • MSS SP-99-2016a, In...

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Industry Headlines

MSS Revises Standards for Instrument Valves, Sealing of Rising Stem Valves

1 HOUR AGO

The Manufacturers Standardization Society (MSS) is pleased to announce the publication of the following revised Standard Practices:

  • MSS SP-99-2016a, Instrument Valves (replaces 2016 edition)
  • MSS SP-105-2016a, Instrument Valves for Code Applications (replaces 2016 edition)
  • MSS SP-120-2017, Flexible Grap...

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Spirax Sarco Celebrates 4 Years Without a Lost-Time Accident

6 HOURS AGO

Spirax Sarco recently reached a safety milestone – 2.4 million man-hours, or 4 years without a lost-time accident, which means 288 employees did not miss work due to an injury suffered on the job. This significant accomplishment was celebrated  on August 10th with a fun lunch.

When asked why...

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Gulf Coast Oil & Gas Operations Returning to Normal

6 HOURS AGO

According to the IEA’s latest Oil Market Report , disruption to local oil markets in the U.S. Gulf Coast is easing on a daily basis and its impact on global markets is likely to be relatively short-lived . Given the severity of the storm, it was inevitable that the normal output and distribution o...

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Cause of 2016 ExxonMobil Refinery Fire Found

8 HOURS AGO

The U.S. Chemical Safety Board released a safety bulletin on the November 22, 2016 fire that severely burned four workers at the ExxonMobil refinery in Baton Rouge, LA.

The fire occurred during maintenance activities when operators inadvertently removed bolts that secured a piece of pressure-containing...

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Harvey Caused Steep Drop in U.S. Industrial Output

1 DAY AGO

Industrial production declined 0.9% in August following six consecutive monthly gains. Hurricane Harvey is estimated to have reduced the rate of change in total output by roughly 0.75%. The index for manufacturing decreased 0.3%; storm-related effects appear to have reduced the rate of change in facto...

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Empire State Manufacturing Shows Strong Growth in September

1 DAY AGO

Business activity continued to grow strongly in New York State, according to firms responding to the September 2017 Empire State Manufacturing Survey . The headline general business conditions index held steady at 24.4. The new orders index rose four points to 24.9 and the shipments index climbed four...

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Ball Valves in Power Plants

vmwnt12_nuclear_plantCan metal-seated ball valves provide effective long term, economical solutions for critical applications in steam-generating power plants? The answer lies in knowing the design limitations of the valve, in correctly identifying the application requirements and in proper installation techniques.

During the last 20 years, advances in machining, tooling, measuring and coating technology have led to ball valve designs that provide positive sealing solutions for the power industry. These new “power plant ball valves” can withstand temperatures and pressures that make them a viable solution for the industry.


THE HISTORY

Before these new designs, creating true and matching spherical sealing surfaces was limited so that one surface—the ball—typically was metal and highly polished. The second sealing surface—the seat—generally was a fluoropolymer or elastomeric compound. This combination works well in lower temperatures and pressures, and still is widely used today. However, it is not suited for the extremes seen in steam power generating stations.

vmwnt12_Figure-1Figure 1. Typical Y-pattern globe valveInstead, traditionally rising-stem globe valves have been the work horses for stop and isolation service in steam-generating power plants (Figure 1). These valves have massive stems, plugs and seats. They are top-entry valves and often have welded or pressure seal bonnets.

The sealing areas of the plug and seat are matched conical surfaces, which are well-suited for the high pressures, temperatures and velocities found in vent, drain and continuous process services. Operation of the valves is intuitive: If the stem is in the “up” position, the valve is open; if the stem is in the “down” position, the valve is closed.


METAL-SEATED BALL VALVES

Computer numerically controlled (CNC) machines with special tooling can now produce virtually perfect spherical metal-to-metal sealing surfaces—both concave, and convex for ball and seat—with almost infinite repeatability. Highly qualified and specialized coating contractors also can provide many varieties of flame-sprayed carbide coating compounds applied with exacting precision in laboratory conditions by CNC robotic machines. These coatings carry a toughness and hardness far exceeding that of the cobalt-based weld overlays traditionally used in rising stem globe valves, and the final coated surface exactly mirrors the machined contours of the underlying part. These technologies have lead to the emergence of the power plant ball valve, capable of withstanding temperatures in excess of 1050° F (566° C) and ­pressures in excess of 3000 psig.


CONSTRUCTION FEATURES

Many power plant ball valve manufacturers are machining valve bodies from forged bar stock materials with massive wall thicknesses far exceeding that of rising stem valves. Several use the same valve body design on 1.5-inch and smaller sizes for all pressures classes—so that an ASME Class 1500 valve may actually have an ASME Class 4500 body. This standardization reduces raw material inventory, streamlines machining, shortens production times and allows for virtually unlimited alloy material selection. Usage is restricted by the pressure and temperature rating stated on the original equipment manufacturer (OEM) tag and relevant American Society of Mechanical Engineers (ASME) and American National Standards Institute (ANSI) standards.

vmwnt12_Figure-2Figure 2. Two-piece end-entry design

 

vmwnt12_Figure-3Figure 3. One-piece end-entry design

vmwnt12_Figure-4Figure 4. Top-entry designValve manufacturers produce a ­variety of power plant ball valve body designs, including two-piece end entry (Figure 2), one-piece end entry (Figure 3) and bolted bonnet top entry (Figure 4). Different seat designs are produced: integral seat, pressed-in seat, welded seat and locked seat. All designs incorporate a downstream sealing seat, with an upstream ring used as a guiding, balancing and bearing surface.

vmwnt12_Figure-5Figure 5. Live-loading methodThese designs all include a large cross-section conical load spring (Belleville washer) behind the upstream bearing ring. The designs feature live-loaded stem packing, although several methods provide this live-loading (Figure 5). Some use one large central Belleville spring, while others use four, equally spaced spring sets. Packing design varies among manufacturers—some use a single die-formed ring with wire-braided backup while others use multiple die-formed rings, and still others use top- and bottom-braided rings with internal die-formed rings.

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