When Does Zero Not Equal Zero?
Such incidents attract a lot of attention, but most leaks are well-hidden somewhere deep inside the equipment and piping that cover the grounds of a power or petrochemical plant. These valves gradually eat away at performance and profits, a problem that is particularly critical with severe service isolation valves (SSIV). These issues, however, are preventable by using properly designed valves that allow zero leakage. The problem is that zero leakage does not always mean zero leakage. The usual definition actually means acceptably slow leakage internally, with no visible external leakage. But by applying the best available technology and adopting new standards, zero can equal zero, both internally and externally.
SSIVs are isolation valves that are used in high energy conditions including elevated temperatures and/or elevated pressures. They are used for numerous applications throughout power plants. (See sidebar for “Uses of SSIVs in Power Generation.”) These temperatures and pressures exceed the normal operational limits of thermoplastic seals, so the seal needs to be made by the metal components of the valve. This requires precision manufacturing beyond the standards generally applied to valves that can use a flexible or compressible gasket to provide the seal. Further complicating the matter, the fluids involved may contain some solid content or abrasive materials that erode the seal surfaces, thereby producing leakage paths.
With SSIVs, the cost of the leakage is far greater than the cost of the valve. High temperatures and pressures coupled with erosive substances entrained in the fluid means that even minor leaks can grow into major ones. This results in unscheduled shutdowns and frequent equipment repair or replacement as well as wasted fuel/process liquids. To protect people from injury and equipment from damage, it is essential to achieve zero leakage with SSIVs.
Like other valves, leakage can be either internal or external. It isn’t difficult to discover external leaks: you can see the cloud of steam escaping, for instance. Internal leaks are entirely different. Take an isolation valve on a bypass line between the boiler and the steam turbine that redirects steam to the condenser. Any leakage though that SSIV lowers generator output while increasing fuel consumption. Since leakage is internal, it may only show up as a gradually increasing heat rate, requiring additional fuel expenditures and accompanying emissions remediation expenses to produce the same amount of electricity. The cost of replacing a faulty valve, on the other hand, is minimal compared to the lost output.
Keep in mind that there can be hundreds or thousands of SSIVs in a power plant. The overall losses are not from a single leaky valve, but the aggregate losses from each of them leaking a tiny amount. Together they add up to millions of Btu never reaching the turbines. Zero leakage SSIVs can typically improve a plant’s heat rate performance from 1% to 2%, to as high as 5% to 6%.
LOSS OF POWER AND PROFITS
To see how that impacts the bottom line, let us consider two scenarios, a 1,000 MW coal plant (annual fuel cost $150,000,000) and a 1,000 MW natural gas plant (annual fuel cost $300,000,000), each with 1,000 SSIVs. Those valves together, since they provide imperfect seals, cost the plant about 3% of its efficiency, adding $4,500,000 to the price tag for operating the coal plant and $9,000,000 to the gas plant. Given an average replacement cost of $4,500 per SSIV, it would cost $4,500,000 to replace all valves — a payback period of 12 months for coal and six months for natural gas. Following the 80/20 rule, if you replace 20% of the valves and eliminate 80% of the leakage, the replacement cost for 200 valves would be $900,000, and the payback time would be three months at the coal plant and 1.5 months at the natural gas plant. These figures do not include the additional benefits of increased output, reduced emissions, smaller amounts of downtime, and lower repair costs.
The losses cited above may seem extreme at first glance, but is validated by other well-established research in areas, such as leakage through an orifice in a pressurized pipeline, as well as heat and pressure losses in steam traps. Per the ANSI/FCI 70-2 leakage specification, a Class V valve should have a maximum seat leakage of 5 x 10-4 ml per minute of water per inch of seat diameter per psi differential (5 x 10-12 m3 per second of water per mm of seat diameter per bar differential). Buying Class V valves, therefore, would seem to eliminate the leakage losses. However, those standards apply only at the point of installation. Over time, the continuous steam leakage past the plug seat erodes the seal, causing steam cutting and wire drawing. What was once a Class V valve evolves into a Class IV, then Class III or Class II. To try to seal against the high pressures (a 2 in. ANSI 4500 globe valve is subject to up to 19,623 pounds of force), a hammer-blow handwheel is sometime used. This method uses up to ten times greater torque to drive the plug against the seat, but the method can damage the valve parts and won’t stop leakage through an eroded seal. Additional damage comes from vibration, flashing, cavitation, and internal erosion.
CREATING A NEW STANDARD
Just as utilities must apply best available control technologies (BACT) to eliminate excess emissions, so should they adopt best available isolation technology (BAIT) design features to eliminate the problem of erosion and gradually rising losses. Here, for example, are some of the elements that make up BAIT for ball valves.
• An integral seat – The integral seat is part of the valve body rather than a slip-in seat ring. A slip in provides a leak passage behind the seat, which doesn’t exist with an integral seat.
• High-strength belleville seat springs – Belleville springs are cone-shaped washers that apply a constant high thrust to create a mechanical preload on the ball and seat, and on the packing. By stacking several of the washers, you can increase the deflection while keeping the load on each washer constant. Use of Inconel 718 produces a high tensile strength (155,000 psi) with a high yield strength (125,000 psi) and high creep strength. This allows a seat spring compression of several hundred psi to fully position the ball against the seat, preventing ball misalignment or vibration and restricting particles from entering and damaging the seal.
• Full alignment/positioning of ball and seat
• RAM™ hardfacing – Rocket applied metallic (RAM) is a high-velocity oxygen fuel (HVOF) coating process that uses a hot, high-velocity gas jet to spray a coating of molten particles on to the ball and seat surfaces. Traditionally, disks and seats of carbon, alloy, or stainless steel are hardfaced by welding on an overlay of Stellite©, a cobalt-chromium allow with good wear and corrosion resistance properties. However, above 800ºF, Stellite becomes soft and subject to heavy wear and tear, and galling of the valve seating surfaces. RAM is both harder than Stellite and maintains its hardness at high temperatures. At room temperature, RAM 31 has a Rockwell C hardness of 72, vs. 39.8 for Stellite 6. At 1,400ºF, RAM has a Rockwell C hardness of 62, compared to just 8 for Stellite. RAM is also self-repairing in operation, so over 1,000,000 valve cycles are possible.
• Mate lapping of ball and seat – The ball and seat must be precisely mated to each other to form a perfect seal. This is accomplished by lapping the ball and integrals seat for several hours on a rotating fixture. The final step involves using a 3-micron diamond compound and moving the ball in a figure eight motion.
• Blowout-proof stem – a typical valve stem is externally inserted and uses a slip-on collar held in place by a pin. A blowout-proof stem is internally inserted and has an integral shoulder rather than a collar. Since it is internally retained, it is 100% blowout proof.
Adopting BAIT makes it possible to actually achieve absolute zero leakage on SSIV. Therefore, it is time for a new valve classification that goes beyond the FCI 70-2 Class VI standard. Class VII would be zero visible leakage for three minutes using the ValvTechnologies VQP-10 hydro test and VQP-10 gas test.
With this new standard, zero means zero, not something that we hope comes pretty close. TB
Kevin Hunt is president and owner of ValvTechnologies.
Uses of SSIV'S in Power Generation
• Above & Below Seat Drains
• Ash Handling
• Attemporator Spray Control
• Boiler Drains
• Boiler Feed Pump Isolation
• Continuous Boiler Blowdown
• Electronic Relief
• Feedwater Heater Drains
• Feedwater Isolation
• Instrument Isolation
• Main Steam Stop
• Seal Steam Regulators
• Sight / Gauge Glass Drains
• Soot Blower Regulators
• Startup Vents
• Steam Dump
• Turbine Bypass Systems
• Turbine Drain
Combined Cycle Power
• Gas Turbine Fuel Supply
• Fuel Gas Heat Exchanger
• BFP Recirc & Isolation
• BFP Discharge Isolation
• BFP Turbine Above & Below Seat Drains
• Main Steam Drains
• Main Steam Stop Before & After Seat Drains
• Main Steam Turbine Isolation, Double Block and Bleed
• Main Steam Attemporator
/ Superheat Spray Isolation
• Turbine Drains
• Extraction Steam Isolation
• LP Header Economizer Drains & Vents
• HP Header Ecomomizer Drains & Vents
• IP Steam Drum Drains & Vents
• Steam Drum Gauge / Sight Glass Isolation
• Superheater Header Drains & Vents
• HRSG Hot Reheat & Main Steam Isolation
• Electronic Relief Valve
• Main Steam Start-up Vent
• Main Steam Attemporator / Superheat Spray Isolation
• Boiler Feedwater
• Circulating Water System
• Component Cooling
• Condensate Extraction
• Condensate Cooling Water
• Emergency Feedwater
• Fire Protection System
• HP Safety Injection
• HP & LP Heater Drains
• Heat Exchanger Vent & Drains
• Main Steam System Isolation Drain & Vent
• Power Operated Relief Valve (PORV)
• Pressurizer Drain & Vent
• Rad Waste System
• Reactor Coolant Pump Drain & Vent
• Reactor Head Vents
• Reactor Water Cooling Vents & Drains
• Safety Injection System
• Secondary System Isolation Drain & Vent
• Service Water System Isolation
• Steam Generator System
• Turbine Bypass
• Turbine Drain & Vent