Pressure Points

Pressure measuring devices are everywhere, but do you really understand them? Peter Welander points out what you need to know.

Measuring pressure is one of the most basic instrumentation functions in any industry. From an oil refinery to a bulldozer, measuring the pressure of compressed air, hydraulic fluid, process liquids, steam, or countless other media is a daily occurrence and critical to all manner of control. As a result, pressure measurement devices are everywhere, and there are countless varieties of options. Reviewing technologies in general terms can yield better applications, although there likely will be exceptions to anyspecific point.

Pressure, also called compressive stress in mechanical terms, has peculiar characteristics thataffect approaches to measure it:

• It is usually quantified in units of force per unit of area; and,
• It exists in both static and moving fluids;
• Fluid pressure is always measured as a differential to something else.

Two types of pressure measurements can be divided into three categories:
• Absolute pressure is measured against an absolute vacuum, discounting entirely the effects of atmospheric pressure. This measure is used primarily for research or design, but there are some applications where an absolute reading is useful in a process context. Since it isn’t practical to pull a full vacuum inside a sensor housing, sensors typically adjust a gage reading using either a fixed correction factor or more sophisticated units use a measured barometric pressure.
• Differential pressure is the pressure in one area or vessel measured against another. The reading is the difference between the two and does not account for the pressure of either side relative to atmosphere or a vacuum.
• Gage pressure, which is a type of differential pressure, is the pressure in one area or vessel measured against atmosphere. This is the most common practice.

The two most common units of pressure measurement are psi and bar. While the former is primarily still used in the U.S., the latter metric unit is becoming more common. Bar has largely replaced pascals and kilopascals since the numbers are more convenient. Many other units of measure exist, but their uses generally are relegated to specialized applications. Both psi and bar use a suffix “a” or “g” to indicate absolute or gage. When there is no suffix,gage is assumed.

Differential pressure measurements do not specify absolute or gage, since both sides of the measurement are compared directly. The reading is the difference and has no indication of the magnitude of the two sides. If the differential pressure between two tanks is 50 psi, the tanks could be 10 psi and 60 psi, or 5,000 psi and 5,050 psi. There is no way to determine pressure relative to atmosphere without anothersensor. Differential pressures often use a suffix “d.”

Traditionally, mechanical gages with curved bourdon tubes were standard, and these are still available in many configurations. As with other forms of instrumentation, however, electronic versionshave gained popularity.

Accuracy, range and safety
Electronic pressure instruments and mechanical gages express accuracy in the same way, specifying an error factor as a percentage of range. For example, a good quality 0-500 psi gage could offer accuracy of±0.5% of span. That means it has an error band of5 psi (±2.5 psi) at any point on the scale. Electronictransducers and transmitters usually use the sameaccuracy specification. Because range is linked toaccuracy, it is important to choose a device with arange as close as possible to the actual operatinglevel while allowing for potential pressure surgesor spikes. In other words, if your process runs at 75psi, it is better to use a 0-100 psi gage rather than a0-500 psi gage, even if they have the same accuracyrating. Poor ranging is frequently cited as the mostcommon mistake when selecting and applyingpressure devices. Cost-minded purchasers chooseone pressure instrument for the sake of minimizinginventory and then try to make it work in a widerange of applications. Accuracy suffers as a result.

Some transmitters can have their range adjusted electronically. For example, a unit designed to measure 0-500 psi can be electronically adjusted to read 0-300. This helps spread out the relevant reading area on the 4-20 mA signal, but does not actually increase accuracy. The turn-down ratio willbe the same as the full 0-500 scale in most cases.

“There are $2 or $3 sensors that can go in your automobile tires and feed back to a panel on your dashboard. You don’t need a lot of accuracy or fast response to say that your right front tire is low,” says Tom Reid, product manager, GE-Druck. “The same thing could apply if you just want a ballpark reading of the pressure inside a tank. As long as the mechanism and housing will hold up, that may be all that matters. It all depends on how critical the process is.”

Selecting a pressure instrument
Once you have determined the measurement requirements for a specific application, you can choose an appropriate device based on other performance attributes beyond range and accuracy.

Such considerations include:
Material: Instruments specify materials of wetted parts, meaning those that touch process fluid. There is a wide range of choices to accommodate aggressive fluids or gases. Housings also present choices, since a corrosive atmosphere in a plant can attack from the outside. Exotic materials are expensive, so choose wisely.

Internal configuration: Many sensors have internal recesses that fill with process fluid while in operation. If the fluid is benign and a small trapped amount won’t harm the product, its presence is permitted. However, some critical applications do not allow this. Sensors are available with flush diaphragms and sealed interiors to prevent infiltration of fluids and maintain minimal disruption of process flow. An isolating diaphragm also may be available as an accessory.

Housing: The safety needs of a wastewater plant differ from an oil refinery. Many options can comply with environments that demand explosion proof or intrinsically safe instrumentation. Moreover, many companies have established clear policies on what gets used and where. On the other hand, if an explosive environment isn’t a concern, the device variety available is much larger. Also, the bulkiness of the unit will depend mostly on the ancillary electronic equipment needed to communicate. A simple transducer with only a 4-20 mA output pigtail can be very compact, whereas a smart transmitter with a fieldbus connection will be larger to accommodate additional circuitry.

Mounting connection: Instruments usually have a pipe thread inlet ranging from 1/8 to 1/2 in. NPT or BSPT. However, there are additional choices for more specialized applications, including sanitary tri-clamps and other flange options. Differential pressure devices often use manifolds tosimplify connections.

Communication: Most transducers transmit data using analog 4-20 mA signals. If an application uses this method, additional signal conditioning may be required to ensure reliable transmission. Transmitters also communicate via fieldbus, wireless, or HART protocols.

Sensing technology: There are about 10 technologies and variations for converting a pressure into a scalable electronic signal, but none is universal. Device manufacturers tend to employ one or two technologies following a combination of performance attributes and commercial applicability, doing their best to optimize performance and minimize drawbacks. Product literature often doesn’t even mention the technologies used. “The most critical thing is having enough experience with a given sensor technology,” says Allen Hood, product manager with ABB Instrumentation. “The standard used to be capacitance, but now there are multiple sensor technologies available because they’re optimized for the specific range. The capabilities of microprocessors in smart transmitters compensate for sensor weaknesses and allow manufacturers to concentrate on other things.”

Mounting, add-ons, maintenance
Proper mounting can be as important as selecting the right device. Pressureinstruments are usually mounted with ashutoff device, especially in continuousprocesses. That way the devices can becalibrated, repaired, or replaced while theprocess runs. In situations where operationis intermittent, this is not as critical, and adevice can be mounted directly in the flow.A section of pipe or tubing to the sensor iscalled an impulse line and helps in situationswhere there isn’t enough room or access forthe transmitter housing. However, impulselines should be used carefully:

• Keep them as short as possible;
• If the process fluid is a liquid, make sure all air is bled out;
• If you need the readout in a more convenient place, extend a cable, not the impulse line; and,
• In high temperature situations, especially steam, ensure the impulse line can act as a siphon.

Accessories can simplify mounting, or protect the device. Here are someexamples.:

Block and bleed valve – A valve mounted on the impulse line that allows pressure to be released from the sensor once the processconnection is closed.

Snubber – A device to retard flow from process to sensor, primarily to suppress pulsations in an effort to extend sensor life. When properly applied, the reading is correct without harming the sensordiaphragm.

Diaphragm seal – A diaphragm mounted below the sensor to transmit pressure without allowing process fluid to infiltrate the sensor. When used, the sensor has to be filled with an inert fluid, usually a siliconeoil, to transmit the system pressure.

Overpressure protector – A spring loaded valve that closes in an overpressure situation before the sensor is harmed, used wheremajor spikes are a possibility.

Manifold – A device to simplify complex piping to a differential pressure gage, usually incorporating internal valves to facilitate cutoff, pressure relief, and equalizationfunctions.

Maintenance and calibration requirements vary tremendously, based on the individual instrument and process needs. A quality device in an application that is of secondary importance could operate for years without attention. On the other hand, in a critical application where precision is paramount, transmitters will need periodic calibration. However, high quality transducers and transmitters receive very precise calibration at the factory, which is well beyond the capabilities of most in-plant maintenance departments. Often, attempts to improve performance only manages to make it worse. Devices from reputable manufacturers that drift or need constant attention have most likely been damaged, or are mounted in a way that hinders theability to transmit reliable data.

Smart diagnostics
The term smart instrument describes a device capable of more functions than just sending a single process variable, but how does that apply to a pressure sensor? Whatelse is there to measure?

“Whenever a failure or an event occurs that requires intervention by an operator or maintenance personnel, there is often a need to understand the cause and prevent the situation from happening again,” says Paul Schmeling, senior marketing manager, Emerson Process Management. “As instruments are the ‘eyes and ears’ into the plant’s operation, there is a need to add functionality to provide greater insight into the history of the measurement or process.”

Some smart functions relate to the process and duplicate what a DCS might do, but if the instrument exists on its own, such functions can be very important. While manufacturers’ offerings differ, here are afew examples:

• Measuring and logging over-pressure events;
• Counting alarm and error signals;
• Analysis of system “noise” changes signaling coming upsets;
• Process fluid and ambient temperature measurements;
• Self correction of the pressure value to compensate for temperature changes;
• Self diagnostics of internal electronic functions and hardware;
• Functionality of alarm settings; and,
• Calibration history and schedule.

The transmitter can communicate this information using a protocol like HART or a sophisticated fieldbus. Communication can be continuous, if attached to a larger system, or via a hand-held device at given intervals.

Ultimately the best advice for selecting a pressure measuring device or any type of instrumentation, is to know the application. Knowledge of precision, communication,and diagnostics translates readily into specifications. Otherwise, pressure sensor selection could miss the mark and you’ll lose a critical element of process control.

Instrumentation nomenclature The terms sensor, transmitter, and transducer describe different things, but their use is not always consistent and can be confusing. Here are some key, generally accepted definitions: Sensor – The most basic device with a diaphragm and electronic components to create a raw signal. It is not sufficient to operate usefully on its own and sold primarily only to OEM’s. Transducer – A sensor with basic electronic support to amplify and translate the raw signal into a useful format, such as 4-20 mA. A simple transducer may be all that is required if an analog process variable is sufficient for an application. Transmitter – A sensor with more sophisticated electronic support capable of communicating the process variable in analog or digital format, either hard wired or via fieldbus. Smart transmitter – A transmitter with additional diagnostic and supplemental variable measurement capabilities. HART-enabled transmitters are a common example. Instrument – A less specific term that can encompass transducers, transmitters, and other devices.