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Technical

Valve Positioners Offer Improved Control Valve Performance

From time to time, we are re-posting well-received or particularly valuable articles that have previously run on VALVEMagazine.com so that those who might have missed them will be able to catch up on the best of the best. This article, “Valve Positioners Offer Improved Control Valve Performance initially ran on February 16, 2016.

By closely controlling the pressure and temperature of both heating and cooling mediums within heat exchange systems, the use of an automated control valve can help assure the efficient transfer of heat.


Putting Servo Valves Back to Work

Industries as varied as paper production, steelmaking and power generation rely on servo valves for precise motion control. A servo valve can last well above 1 billion cycles. But achieving that longevity only happens when selecting the right manufacturing techniques, mechanical designs and maintenance approach.

A Paper Chase

Paper mills, for example, use hydraulic pistons placed inside a roll to exert forces at each load zone to maintain a controlled profile across the complete width of the roll. The force exerted by each piston is controlled by a pQ (pressure and flow) servo valve that controls the position and pressure on these rolls as paper runs almost continuously through the machine shift after shift. If one spot on a roll causes a variation in pressure or position, the paper can slide or rip causing a stoppage that can shred the paper into confetti. We recently found the customized servo valves running a paper mill’s calendar rolls weren’t able to consistently control the pressure on its paper because routine repairs (by an un-authorized repair house) had not restored the valves to original condition.

Corrosion and Fouling: Is There a Solution?

According to a 1998 study released by the U.S. Federal Highway Administration, the direct costs associated with metallic corrosion in nearly every U.S. industry sector, from infrastructure and transportation to production and manufacturing was $276 billion—approximately 3.1% of the nation’s Gross Domestic Product (GDP). Bring those costs forward; in 2017 the GDP of the U.S. was $19.739 trillion, which means that last year, the direct costs of corrosion in the U.S. would be close to $600 billion.

According to that same 1998 study, the indirect cost of corrosion is conservatively estimated to be equal to or greater than the direct cost. If the indirect cost is also 3.1% of GDP, then the total cost of corrosion was $1.2 trillion in 2017. That is one of the largest single expenses in the U.S. economy, costing money in lost productivity, but also costing lives from dangerous failures.

Are Valves from Low-Cost Countries Getting Better

The last 25 years have seen standards created and implemented to increase the quality and repeatability of valves procured from low-cost manufacturing sources around the world. As part of the process, potential sellers into the U.S. market have reportedly spent millions of dollars improving facilities and processes.

Interior Coatings for Waterworks Valves

Since the 1990s, two types of epoxy coatings have been commonly specified and used for iron valves in the waterworks industry: fusion-bonded epoxy and liquid epoxy. Both coatings are based on thermo-set epoxy systems with similar corrosion resistance and are described in the American Water Works Association’s Standard AWWA C550, “Protective Interior Coatings for Valves and Hydrants.”

Fusion-bonded epoxy is applied to preheated components in powder form in an electrostatic or fluidized bed process followed by thermal curing. Liquid epoxy is a two-component mixed material that is applied by spray, brush or other methods and chemically cures after application. The purpose of this article is to explain the typical requirements that apply to these coatings and compare some of their properties.

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