01 August 2003

Pressure relief

Hybrid instruments overcome complex issues with simple solutions.

By Ravi Jethra

Power plants require a great deal of instrumentation, specifically pressure. Each power plant is unique and has varying needs based on the actual equipment and fuel used, regional regulations, and other factors. However, each has a basic minimum instrumentation package.

The distinct benefits of using instrumentation and control systems in a power plant are:

• Efficient use of labor, energy, and materials
• Immediate alert signal for malfunction
• Automatic shutdown to prevent major damage
• Minimizing the potential for human error

Power plants require pressure switches that are not only rugged and reliable but also have very high cycle life. Pressure switches appear on different equipment such as compressors, turbines, and pumps, and workers use them as safety components, alarms, or control elements. Typically a narrow dead band is common in safety applications, whereas a wider dead band is more common on control applications. Pressure switches tend to be electromechanical devices. In their most basic form these devices have seen use for almost two hundred years. Nowadays the trend is toward using solid-state devices.

In the early 1990s, all the major players launched smart transmitters on a grand scale. A smart transmitter is a microprocessor-based device with diagnostics capability and two-way communication using digital field protocols. These smart transmitters are now available with different protocols such as Highway Addressable Remote Transducer (HART), digitally enhanced (DE), Profibus, and recently Foundation Fieldbus. HART remains the de facto industry communication standard, although Profibus and Foundation Fieldbus are becoming increasingly popular. Irrespective of protocols and bus capabilities, smart field devices dominate power plant automation. Two-way communications, high accuracy, increased reliability (self-diagnostics), and ease of configuration are their primary drawing cards. The intelligent devices enhance basic process control system applications. In fact, more features for improved asset management come about every day, such as a transmitter that provides alerts for calibration and servicing of the instrument. However, intelligent devices can actually increase the complexity of some common applications, and, hence, are not necessarily the best solution.

The most common operational issues encountered with an electromechanical pressure switch or analog transmitter relate to:

• A minor adjustment in an electromechanical switch set point or in an analog transmitter span requires pulling the device off line and manually adjusting it.
• A need to know the process variable being measured with reference to the set points of the electromechanical switch or the span of analog and blind smart transmitters.
• The variances in fixed dead bands and the set point repeatability of electromechanical switches.
• Assuming an electromechanical switch or analog transmitter is performing its intended function properly until it is tested, and finding it with either corroded switch contacts or an indicator failure.

Hybrid instruments overcome these limitations of analog transmitters and electromechanical switches.

There are applications that require the sophisticated functionality of a smart transmitter, but there are times when you need more than just a pressure switch. For certain applications, hybrid pressure instruments that bridge the gap between simple electromechanical and analog instruments and fairly complex smart instruments are probably a better alternative. Exactly what are these "hybrid" devices, and how do they differ from smart instruments? The newest hybrids are microprocessor-based multifunction instruments that include common features such as:

• Local configuration (online)
• Local indication of process variable (up to 150% of upper range limit)
• Direct control (discrete output) via solid-state relays
• Analog output—for example, 4–20 mA
• Self-diagnostics
• Process minimum/maximum register

Power from coal A large part of the world's electric energy comes from coal-fired steam power plants. These plants convert the heat energy latent in coal into electrical energy using water (steam) as a conversion medium. Coal and air combine and burn to heat water into steam, which injects into a steam turbine. The turbine converts the heat energy to rotational mechanical energy. The mechanical energy then transfers to an electric generator that produces electricity for consumption.

The beginning of the twenty-first century saw the introduction of hybrids. Do not confuse hybrids with smart multivariable transmitters that monitor static pressure, differential pressure, and temperature in a single instrument. Manufacturers usually develop hybrid sensors as enhancements to traditional electromechanical switch and analog transmitter product lines, with an emphasis on niche markets and applications.

A multifunction pressure instrument is basically a switch and transmitter (loop-powered) in a compact housing with its own display (indicating switch transmitter) and Class I Division 2 rating for hazardous areas, which more than meets the requirements of a power plant. The instrument improves the integrity of the entire system through its self-diagnostics and manual circuit-test capabilities. This gives plant operators additional assurance that the device is functioning as required.

The PhotoMOS relays can wire directly to the local annunciator to alert the operator to an unusual pressure condition. The local display enables the operator to quickly understand the relationship between the pressure and the set points.

Intelligent settings and configuration routines alert the operator to invalid entries, and online functional testing of output circuits ensures the highest level of functional integrity. The PhotoMOS relays allow programmable dead bands. The instrument stores the minimum and maximum process values in RAM, starting at power up, and displays them on demand. The technology used allows very narrow or extremely wide dead bands and more repeatable set points for discrete outputs, improving the overall performance of the device and effectively reducing nuisance trips. Although the extent of diagnostics varies, most hybrid instruments improve overall system reliability through detection of basic internal hardware and software failures. Common alarm or direct control functions can occur without the addition of a current trip or programmable logic controller, lowering both the hardware and installation costs.

Hybrids require loop power (24 volts direct current), which is usually not an issue if an analog or smart transmitter is undergoing a retrofit. However, some of the greater benefits of a hybrid come about when upgrading from passive, electromechanical switches that provide direct control.

Although these units are intelligent devices, they do not get the "smart" moniker, because they do not use a common two-way digital communications protocol. Some hybrids do allow you to talk to a PC for configuration, but they use proprietary software.

Although they tend to respond faster than smart transmitters, hybrids typically have a slower response time to changes in the measured process variable than their electromechanical switch and analog transmitter cousins. Magnitudes can vary from 30 milliseconds to 150 milliseconds.

Some hybrids allow users to manually test outputs to ensure solid-state relays and analog loops are still functioning properly, further increasing the reliability of the instruments.

Applications

There are many generic applications throughout a power plant. Modern plants use a great deal of pumps, fans, valves, and other tools. Each of these typically requires an instrument for monitoring and an instrument for safety shutdowns or interlocks. Other areas that benefit from the simplicity and functionality of hybrids are:

• Lubrication systems (monitoring and low alarm)
• Pumps (variable speed drive control, monitoring, and shutdown)
• Boiler water/steam generation (monitoring and high/low alarm)
• Instruments for fire suppression systems
• Line pressure monitor and alarm for carbon dioxide distribution
• High-pressure shutdown alarm on pump valve
• Output pressure monitoring for pump performance
• Output pressure on boosters to monitor performance

Hydrostatic level application A single indicating switch transmitter will continually monitor the fluid level, provide input to a pump controller (S1), and provide an overflow alarm (S2).

Pumps typically use a multifunction pressure instrument to monitor input and output pressures. This is to avoid cavitating the pump and causing damage. Solid-state relays see use as a safety shutdown in case of loss of liquid pressure. Fans must also operate properly, and users must continually monitor them for blockage. The hybrid instrument (IST) can monitor fan performance and act as a safety shutdown device.

Lubrication systems: All rotating equipment has a lubrication system to avoid excess friction and damage. These systems typically include a reservoir for oil or another lubricant. An IST can monitor lube oil pressure.

Feedwater heaters: IST monitors water flowing from the hot well condenser. The feedwater heaters (both low and high pressure) use the IST, too.

Water treatment: Boiler water must be pure so contaminants will not build up on boiler tubes and turbine blades. Buildup on these surfaces can cause loss of efficiency and, in some cases, damage to equipment and personnel. Power plants draw their water from either a body of water (pond, lake, river) or an underground well. The pumps used in both cases use an IST to monitor pump performance and for control.

Generic pressure instrument (smart or analog transmitter) A generic pressure instrument (smart or analog transmitter) is hooked up to a DCS/PLC through I/O modules. The DCS/PLC performs the control functions required, including ON/OFF control.

Boiler management & control instrumentation

The water within the boiler is heated by burning coal, natural gas, or oil; by applying an electrical element; or possibly by a waste heat recovery system. Once the steam or hot water supply is available, you have to monitor boiler conditions to maintain efficient and safe operation. A typical boiler management system monitors gas pressure, oil pressure, oil temperature, water temperature, and steam or hot water supply pressure. Continuous monitoring of the steam or hot water supply pressure and high-pressure shutdown of the boiler are general provisions of these systems.

The multifunctional IST allows plant operators to control and monitor the pressures associated with normal boiler operation. A continuous 4–20 mA signal provides information to the control system, verifying that the supply pressure of the steam or hot water is adequate to meet the demands of the process. If not, the control system can modulate the burners as required. In addition to proportional control, the IST provides relay outputs that are suitable for high-pressure alarm and shutdown conditions. A relay wired directly to the local annunciation device as a high alarm alerts the operator to unusual pressure conditions requiring immediate attention. If operator intervention is inadequate, the second relay wired into the boiler management control system will initiate an automatic shutdown. A controlled shutdown will help minimize pressure vessel fatigue and reduce hazards to personnel.

Self-diagnostics and manual circuit-test capabilities further improve the system integrity and provide additional confidence that the IST is functioning as designed. To ensure normal operating temperatures will not damage the sensor, you can use a pigtail siphon to mount the instrument directly to the boiler outlet.

Generic pressure instrument (smart transmitter) A generic pressure instrument (smart transmitter) is hooked up through I/O modules of the DCS that also supply the required 24 volts direct current power. The digital output is fed to a trip amplifier that then turns a relay (ON/OFF) that is hooked to a PLC or safety shutdown system. The DCS is primarily used for process control and monitoring.

Steam turbine throttle valve position

Steam turbine generators are widely used within the power industry to produce electricity. In this process, the turbine converts the thermal energy of the steam into the mechanical energy of a rotating shaft, which then converts into electrical energy via the windings of the generator. The electrical output of the generator at any moment is proportional to the amount of steam feeding into the turbine. To adjust the output to meet demand, using one or more control valves is a common technique to "throttle" the amount of steam supplied to the high-pressure section of the turbine.

In addition to the pressure transmitter that controls the steam flow going into the turbine via the distributed control system (DCS), each control valve has some type of device that also informs the DCS of the valve stem position for verification. In some cases, this instrument is also a backup for the control system. If the primary steam pressure transmitter were to fail, the DCS could then use the valve position information to control the generator output until someone replaces the primary sensor. Some typical devices for monitoring valve stem position are potentiometers and linear variable displacement transducers (LVDTs). Because potentiometers are unreliable and LVDTs are costly for industrial applications, some companies may use a pressure transmitter to monitor the control signal sent to either a pneumatic or hydraulic valve actuator.

At one existing facility, the IST gave power plant instrument engineers and technicians the option to replace both a high-end smart transmitter used in analog mode and a mechanical pressure gauge.

Each of the two steam turbine generators within the facility uses a hydraulically actuated single throttling valve. The IST provides the plant operator with a linear output that relates to the required valve stem position and steam flow based on the hydraulic pressure of the control signal. The DCS can then compare the signal from the primary pressure transmitter to that of the IST to ensure the control valve supplies the appropriate amount of steam to the turbine.

Hybrid pressure instrument A hybrid instrument is connected through I/O modules. The instrument is powered up through the I/O modules of DCS/PLC, which uses the analog output for process monitoring and control. The solid state relays can be used to provide local ON/OFF control function or hooked up to an alarm annunciator.

Turbine and compressor controls

Customer: major power plant company in Iowa

• Process media: fuel oil
• Operating pressure range: 2-inch water column to 175 pounds per square inch
• Operating temperature: 80°F
• Ambient temperature range: –20 to 100°F

Power plant operators were using the old mercury-style electromechanical switches. They were also using gauges separately to display the pressure. They wanted to change the switch technology due to problems with handling mercury and the need for tighter dead bands. The analog output from IST was a bonus, because the engineer could now hook it up to the DCS for monitoring and control. P

Behind the byline

Ravi Jethra is a product manager at Static-O-Ring, Inc. (SOR) in Kansas. He holds a bachelor's degree in Instrumentation Engineering from Bombay University (India) and an MBA from Arizona State University. Jethra has more than thirteen years of experience in the industrial automation business and is a senior member of ISA. His e-mail is rjethra@sorinc.com.