A control valve can be used to control pressure, pressure drop, flow and temperature in any system where there is a fluid flowing through a pipe that needs to be controlled. Control valves are commonly referred to as “final control elements,” which convert a signal from the process controller into the desired adjustment to the process. A control valve (with its associated actuator) does this by adjusting the size of its flow passage, based on the signal received.
Many parameters help determine which control valve to select for a given application. The choice, often complicated, can become even more so for control valves destined to handle severe flow conditions.
In a recent webinar, Stephen O’Neill, control valve sales manager at DFT, Inc., offered background and guidelines for selecting control valves for severe service.
WHAT KIND OF SERVICE IS SEVERE?
Severe service conditions occur in many different industries and applications, including:
- Food and beverage
Severe service includes Class 2500 and 4500, handling water, gas, steam or slurries, O’Neill says. He gives the example of boiler feed water with an inlet pressure of 2650 psig (18,271 kpa) and a pressure drop of 2580 psi (17,788 kpa).
Severe service conditions may include any or all of the following, O’Neill says:
- High pressures, exceeding 1500 psig (10,342 kpa)
- High pressure drops, up to 3000 psig (20,684 kpa)
- High cycling service
- High temperatures, up to 1900°F (1038°C)
- Low temperatures, down to -425°F (-254°C)
- Highly erosive flow (slurry)
Such conditions are very hard on equipment and these applications require unique and specialized control valves.
TYPES OF CONTROL VALVES
O’Neill discussed four styles of valves used as control valves (Figure 1). Each offers different characteristics with regard to Cv (coefficient of flow), pressure recovery, modulating control and shutoff. (Cv is the flow rate in gallons (3.8 l) of 60°F (16°C) water that will pass through a valve in one minute at a one-pound (7 kpa) pressure drop.)
Globe style control valves are used in applications suitable for low Cv, low pressure recovery, fine modulating control and a range of shutoffs, depending on conditions.
Ball style control valves typically provide high Cv, high-pressure recovery, coarse modulating control and tight shutoff.
Butterfly-style control valves offer high Cv, high-pressure recovery, coarse modulating control and a range of shutoffs, depending on design.
The straight through venturi type, offers high Cv and high-pressure recovery, fine modulating control and tight shutoff.
In choosing among valve types, O’Neill recommends looking at the pressure recovery needed versus the precision of control needed. For example, a globe valve, with its tortuous flow path, shows a lower Cv and pressure recovery, but it offers fine flow modulation, while a ball valve offers high Cv and pressure recovery, but only coarse flow modulation.
THE CHALLENGE OF HIGH-PRESSURE APPLICATIONS
High-pressure applications come with their own set of challenges. These include both withstanding high pressure and managing large pressure drops. Valves for high-pressure applications need greater material thickness in valve body, bonnet and flanges to contain the pressure and meet or exceed the relevant standards (ANSI B16.34 Valves—Flanged, Threaded, and Welding End and ANSI B16.5 Pipe Flanges and Flanged Fittings). Valve bodies can be machined from forged stock that has higher material integrity than that typically found in castings and is therefore able to hold higher pressures.
HIGH TEMPERATURE APPLICATIONS REQUIRE SPECIAL MATERIALS
Applications with temperatures over 800°F (427°C) qualify as “high temperature.” Because metals generally lose yield strength at elevated temperatures, special high-temperature materials are needed. These include chromium-molybdenum (“chrome-moly”) alloy steels and stainless steels. A carbon steel used in mild- or medium-service valves, for example A1105, might have a maximum working pressure of 6170 psig (42,541 kpa) at 800°F (427°C), says O’Neill. F22 chrome moly, by comparison, would withstand a maximum working pressure of 7610 psig (52,180 kpa) at the same temperature.
At temperatures over 1200°F (649°C) many manufacturers provide valves made from 316 stainless steel, he says. A recent trend in the power industry is using F22 chrome moly valves for water or steam at high temperature.
Another adaptation for high-temperature applications is to provide an extended yoke structure to keep the control valve’s actuator far enough from the hot valve and piping to protect it from heat damage.
PROBLEM: FLASHING AND CAVITATION
While flashing and cavitation can occur in any system, they can make severe service even more challenging for a valve.
Flashing occurs where the pressure in a flow falls below the vapor pressure of the liquid and vapor bubbles form in the liquid. This condition will typically occur at the point in the flow stream where the flow area is at a minimum, the velocity is at a maximum and the pressure is at a minimum, O’Neill explains.
Cavitation is the phenomenon that occurs when the pressure in the flow increases to be greater than the liquid’s vapor pressure. When the pressure recovers to this level, the vapor bubbles implode. Where the bubbles are at or near a surface, such as the valve body or trim, they deform and implode, as shown in Figure 2, producing intense jets of fluid that impinge on the surface. These jets can produce severe damage to metal surfaces (Figure 3).
A SOLUTION TO CAVITATION DAMAGE: KEEP CAVITATION AWAY FROM SURFACES
In the straight-through venturi valve design, O’Neill explains, the highest velocities, and hence lowest pressures, occur near the center of the flow path. Thus, if vapor bubbles are formed they are far from any surfaces of valve body or trim and when they collapse, the resulting fluid jets do not impinge on any surface.
Another approach to preventing cavitation damage is the addition of anti-cavitation trim elements, an option offered by many valve manufacturers. For example, such a trim may direct the flow in a globe valve so that any cavitation that happens is distant from the valve surfaces. Alternatively, another type of trim solution gradually reduces pressure through a series of channels and expansion areas, which can prevent cavitation damage and may prevent cavitation altogether.
PROBLEM: EROSION BY SLURRY FLOWS
A slurry is defined as a suspension of solids in liquid. This type of flow mixture is present in many applications, O’Neill says. Examples include lime slurries in power plants, oil sand slurries, and coal or cement slurries. Slurries may be very abrasive and tend to cause severe erosion in mechanical equipment, he says. Many of them behave as liquid sandpaper.
In valves, the slurry can affect the whole flow path, all areas where the slurry contacts any surface, including trim and valve body.
A SOLUTION TO BOTH CAVITATION AND EROSION DAMAGE: HARD MATERIALS
Where valve body or trim are subject to physical damage by cavitation or the abrasive action of a slurry, the affected parts can be made from hard material or protected with hardened coatings. Special trim packages of hard materials can be effective in reducing the damage, but at an additional cost. The valve body can be made from a hard material, but also at some expense.
In some cases, such as the straight-through venturi control valve, vulnerable areas of the valve body can be protected with replaceable inserts, bushings or sleeves made of a hard material such as Stellite.
MIND THE MAINTENANCE
All valves, especially severe service control valves, benefit from periodic maintenance, O’Neill says. He recommends working with the end user to establish a manageable, scheduled preventive maintenance program, or PM, and always refer to the manufacturer’s installation and maintenance manual.
Most plants undergo an annual outage to address maintenance and repair for their equipment. Severe service applications present unusual challenges, O’Neill reiterated. For any equipment, but especially for these severe service control valves, always conduct the indicated maintenance, even if the equipment has not developed any issues.
When given the necessary attention and maintenance, carefully sized and selected control valves can operate effectively and keep the systems they serve up and running, even under severe conditions.