Technology Update: Low-power solenoid valves
For point-to-point and bus-based configurations, new low-power solenoid valves offer economies for PLCs, DCSs, and other control devices and use of easier, less expensive, smaller gage wiring. They are the same price as the previous low-power generation. More savings accrue due to smaller footprints, easier reconfiguration, and greater reliability.
Low-power solenoid valves are widely used at process plants as pilot valves to open and close larger ball or butterfly valves, or on control valves (installed between positioner and actuator) for fail-safe air release if there’s a loss of power. The valves work by pressurizing or depressurizing associated actuators.
A new generation of even lower-power valves uses less power and is of interest to original equipment manufacturers (OEMs), valve assemblers, end-user engineers, or anyone who specifies solenoid valves for projects in refining, upstream oil and gas, chemicals, pharmaceuticals and life sciences, food and beverage, and power applications.
New low-power solenoid valves ease integration, improve reliability and decrease clogging, have improved communications for remote applications, and follow relevant industry standards.
What is low power for solenoid valves?
The first class of reliable mid-power (510 watts) solenoid valves was introduced in the 1960s. Many valves of this type are still used today, in less power-sensitive applications. But over succeeding decades, as electronic controls began spreading throughout the process plant, users and manufacturers realized the benefits of lower-power designs.
The first low-power solenoid valves, which debuted in the mid-1980s, were intrinsically safe designs. They reduced power draw to about 0.5 watt, like today’s most advanced models. The mid-80s valves were difficult to manufacture and featured low flow and low pressure ratings. Users had a choice of mid-power valves or intrinsically safe (IS) valves. The IS valves, although operating at 0.5 watts, required costly IS safety barriers in the installation.
IS valves in non-IS applications used less power compared to mid-power valves, but still faced limitations on flow and pressure.
By 1995, the performance level was improved by introduction of a higher flow, higher pressure rating cartridge type valve that delivered acceptable performance near a 1.5 watt rating.
Lately, efforts toward even greater efficiency and energy conservation—plus the popularity of bus networks and backup power schemes—have increased downward pressure on the power curve. Designs are again approaching the “magic” half-watt mark, with the newest generation of truly low-power solenoid valves rated at 0.5 to 0.75 watt. Designs vary.
Many OEM designers and end users have gravitated toward “integrated” solutions with a low-power solenoid valve built into a position indicator as one unit. This offers a cleaner package and eliminates the need to source the indicator and valve separately. Integrated solutions do have disadvantages, however. First, their “black box” nature makes them difficult to stock, troubleshoot, and maintain. If performance lags or fails, it’s often not apparent which component is at fault, or how repairs or adjustments should be made. Maintenance staff may simply discard the package and buy a new unit at the first sign of trouble, which is more expensive than troubleshooting and replacing one component.
With integrated solutions, valve selection falls to the OEM, who may just adopt valves designed for lighter duty uses, such as packaging lines. The eventual specifier or end user may have little or no say about the integrated valve. Will flow efficiency, orifice size, operating temperature, response times, pressure ratings, durability, and general reliability match the intended process application?
In addition, special valves usually mean special availability problems. Buyers of integrated solutions can’t just standardize on, stock, and easily replace a single part number valve with characteristics sufficient to accommodate all plant applications.
Designers or users forced to use integrated products must discuss the valve component with the vendor, asking:
- Was it a buyout item?
- Who was the manufacturer?
- What tests were conducted to ensure that the product would not fail in the field?
- What is the recommended maintenance schedule?
- What are its orifice size, pressure rating, reliability record, and other critical characteristics?
- How do these match the intended application?
OEMs and users report that integrated solutions and some nonintegrated, separately available valves can suffer from other reliability issues.
Clogging is a chief concern.
Most valves designed around a 1.5 watt or higher power rating easily comply with the ANSI/ISA 7.0.01 1996 quality standard for instrument air. This calls for clean, dry air or inert gas filtered to a particle size of 40 micrometers.
Many newer solenoid valves designed for lower power achieve this partially by limiting orifice size. Unfortunately, the smaller the orifice, the greater the chance of operational difficulty or failure due to the entrance of foreign matter into the valve: in other words, clogging.
Most models compensate with more complex filtration. Instead of the standard 40 micrometer variety, they use nonstandard 5 or 10 micrometer filtration. Special filters demand more complex air line installation, add substantially to system cost, and increase operating expense due to more frequent maintenance and shutdowns.
Clogging threatens long-term reliability and represents a serious threat to solenoid valve users. In many industries, larger process valves that small pilot valves control may remain in the same position (open or closed) for days, weeks, or even months. Yet when required, the valves must operate with unfailing reliability. If not, the consequences could be serious for process integrity, plant investment, or personnel safety.
Redesigning any valve to use less power can translate into a series of trade-offs, with at least a minimal “performance hit.” Trade-offs may involve decreases in factors such as orifice size and maximum allowable pressure. (For instance, buyers should confirm that a valve has a pressure rating high enough to avoid the need for add-on pressure regulators.)
Some valves, designed with generous parameters from the start, have negligible performance decreases. Lower power consumption means less heat, for longer life of coils and power supplies, which also means less strain on associated equipment. Since product lifecycles may roughly double with every 10 C decrease in operating temperature, these advantages add dependability.
Stand-alone low-power solenoid valves can meet almost any real-world process requirements with ensured reliability. Buyers should check with the manufacturer for specific application suitability.
Wiring savings beyond valve price
Traditional point-to-point wiring often incurs greater expenses for a valve’s installation (labor, cables and conduits, fittings, connectors, I/O, and plant real estate) than for the valve itself. Solenoid valves configured around the emerging half-watt standard can reduce these installation costs.
Their lower current draw eliminates the need for additional power isolation relays. They permit the use of smaller, less expensive wiring gauges. They also allow use of downsized, less costly power supplies.
In addition, power is a limiting factor in how many valves may be driven from one programmable logic controller (PLC) or distributed control system (DCS) output card. Using valves that require 0.5 watt to replace models drawing 1.5 watts, 1.8 watts, or more can double or triple each card’s valve carrying capacity, so fewer output cards are needed, also saving on the costs of associated power supplies and/or batteries.
In an older point-to-point arrangement using valves with conventional power limits, a single valve plus installation and equipment might cost $2,400. Low-powered valves on an optimized DeviceNet or ASI bus can cut materials and labor costs by close to 50% to near $1,200 for installation, valve, and accessories.
Greater efficiency and “fit” can also be achieved. For DeviceNet, ASI, and other bus network applications powered from the network, newer low-power valves may require only a third of the current of their predecessors, allowing more input and output devices on each bus segment.
Remote valve installations
Process control systems in remote locations, such as oil pipelines and remote gas extraction stations, can benefit from lower power usage.
Lower power drain of newer valves can allow the system to be specified with a smaller battery bank with the solar-power supply. Alternatively, designers may hold batteries to the same size but rely on decreased power consumption to optimize the system for longer operation without sunlight.
At extremely cold locations, standard low-power valves may require heat tracing or protection. This necessitates added power plus a larger, more costly battery and charging system. Instead, site designers should specify available low-power models rated for -40 F.
Less remote sites on the grid may deploy backup power arrangements (generators or batteries). These locations include pharmaceutical lines and critical petrochemical lines where the loss of costly consumables, feedstocks, or in-process materials due to line interruption would be especially serious.
Finally, supervisory control and data acquisition (SCADA) has enabled monitoring of distant locations via remote terminal units (RTUs), which communicate back to the production office, saving time and money by decreasing the need for on-site maintenance calls.
Industrial standards constantly evolve, and process industries worldwide are fast adopting the newer safety integrity level (SIL) requirements described in documents such as IEC 62061:2005 and IEC 61508. SILs provide uniform gages of risk factors so that designers may rationally allocate equipment purchase costs. Higher SIL numbers (from SIL1 through SIL4) indicate a lower probability of safety function failure on demand.
SIL3-capable solenoid valves are available, though not all manufacturers get all certifications and standard approvals, such as UL, FM, ATEX, IECEx, FM, and CSA.
When choosing low-power solenoid valves for process industry applications, consider such issues as orifice size and clogging potential, pressure rating and other physical characteristics, bus compatibility, backup power needs, and relevant industry standards. The newest generation of valves combines low power and reliable performance, for more applications than previous offerings.
- Fabio Okada is director, marketing; Jack Haller is engineering manager electronics and magnetic; and Manny Arceo is principal engineer, with Asco Numatics. Edited by Mark T. Hoske, CFE Media, Control Engineering, www.controleng.com.