Pressure Measurement
Application Challenges
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Published 01/20/2015
PRESSURE MEASUREMENT APPLICATION CHALLENGES
In controlled environments, pressure measurement is relatively straightforward. However, in the mobile equipment industry and other hazardous areas, application variables can have a huge impact on pressure measurement accuracy. The consequences for pressure measurement errors
can be widespread and potentially dangerous. Fortunately, by anticipating and accommodating
challenges in pressure measurement common in hazardous environments, operators and engineers can prevent misreadings, damage to equipment or personnel injury, and costly downtime.
Any application located in a hazardous location, meaning there is media present that could cause
damage, such as an explosion, should be approved for the specific environment. Selecting a sensor with approval ratings appropriate to the type of hazardous environment for the application is
critical. Approval ratings can vary depending on local requirements, but are required in all hazardous locations. Pressure sensors that are HL rated include safety features such as “safe fails” that limit power to the sensor, to prevent sparking, or to protect the sensor from any physical damage.
This white paper addresses several common and sometimes difficult application challenges that need to be
considered when selecting a pressure sensor, as well as some suggestions to help avoid potential problems.
MEDIA: PURITY AND COMPOSITION CONSIDERATIONS
While pressure is a fairly straightforward concept, the media being measured can vary in numerous ways. In
an ideal situation, the media is pure, like air or water. However, in some oil and gas applications, the monitored media may have debris mixed in. If not handled properly, debris can clog the measuring cell on the
pressure sensor, causing the sensor to inaccurately measure the applied force. One easy way to avoid a misread
is to filter the media before it contacts the sensing element. In instances where filtering may not be practical because of logistics, time or expense, a better option is to find a sensing element that will prevent clogging, such as a flush-mounted diaphragm. This type of sensing element works the same as most other pressure sensors, but is not recessed inside of a process connector, reducing the risk for filling and clogging the
cavity. The application details will dictate which type of diaphragm is best suited for proper measurement.
If the media being measured is corrosive or hazardous, that, too, can impact the accuracy and performance of the pressure sensor. Corrosive media can destroy a sensor if it isn’t made
of the correct materials. Failure can take place immediately, or can occur gradually, resulting in unexpected inoperability and downtime. To avoid corrosion-induced sensor failure, operators and engineers should select a pressure sensor compatible with the media being measured.
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ELECTRICAL NOISE
Electrical noise is any type of electrical interference that can alter the representation of the pressure. Noise can be
caused by high-frequency devices located near the device, by wiring for the device or by stray voltage, as well as external, non-application sources. For best results, install sensors away from these potential problems. Another easy
way to ensure signal is sent properly is to have a physical shield around the wires for the signal. Typically, this is shield
is located inside of the cable transmitting the signal. As a final option, signal can be filtered by a conditioning device.
Depending on the application, an electrical ground loop could be created on the same plane as the electrical source. In applications where the sensor is on a different electrical plane than the area of measurement,
there is potential for the ground plane to be different, which can result in unwanted electrical noise. The
best way to avoid any grounding issues is to verify the power source and use proper grounding practices.
PRESSURE SPIKES
Spikes in pressure readings are commonly misunderstood as noticeable, excessive force exerted on the system
by the equipment being used. However, pressure transients in the measured media itself are the real issue because those changes are so brief that they aren’t as noticeable or may not even register on a sensor.
Pressure transients can be instantaneous and well in excess of the potential pressure for the system; for liquid
media, this is sometimes referred to as a water-hammer effect. These transient spikes can be caused by multiple factors, such as the sudden opening or closing of valves, pumps turning on or off, or an object suddenly
disrupting the media’s flow. This sudden shock can push the pressure to exceed the tolerance of the system’s
components, including the pressure sensor. It may also instantaneously change the direction of travel, resulting
in pressure many times higher than the original.
Pressure sensor manufacturers will designate the allowable amount of pressure a sensor can tolerate and still
function properly for both static and dynamic settings. High-end pressure ratings are typically stated because
pressure measurement is a dynamic state; this is sometimes referred to as allowable overpressure. When the
pressure stays within these specifications, the sensor continues to function normally and without issue. Once
the pressure crosses the threshold of allowable overpressure, a burst pressure rating can come into effect for the
sensor. The burst pressure rating is the maximum amount of pressure the sensor can handle before permanent
damage occurs. Since transient spikes are typically very high, they often well exceed the burst pressure ratings
for the sensor.
One of the best ways to avoid pressure spikes is to eliminate them before they become a problem. This can be
done by installing soft start systems for pump control. The soft start allows pressure to increase gradually instead of the instantaneous flow of media that comes with a transient surge. Soft start equipment is not the least
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expensive solution, but it will help prevent any damage to the system from pressure spikes.
Adding plumbing parts between the media and the pressure sensor is another option to prevent pressure
spikes. This can be done by adding a series of bends to the application’s piping. The extra turns the media has
to make inside the pipe help weaken the instantaneous high pressure peaks while maintaining a more constant
pressure. Depending on the application, this can be done by adding a single 90° bend, a very intricate series of
twists and turns, or anything in between. While this might be a bit cumbersome and add some upfront material
cost, a few simple fittings can dissipate the surges before they damage the sensor.
One of the most economical protections that can be added to a pressure sensor is a mechanical device called
a snubber. A snubber is placed between the process media and the pressure sensor to dissipate the pressure
spike. A good way to think of a snubber is in the same way as weight distribution. Think of an object sitting on a
table. If the table has just one leg, the weight is distributed directly to the area beneath the one leg. If the same
object is on a table with four legs, the weight is distributed evenly over the four legs to the area below. This same
idea works inside a snubber, when a very high pressure spike is distributed more evenly and in lower pressures
to the measuring element. One of the key components to a snubber is its internal filter type. In some designs,
this filter is metal mesh. With every open area that this mesh has, the single transient spike is broken apart and
is spread more evenly to the sensing element.
A final option to manage transient spikes is to add a stand pipe. Although this is a somewhat primitive solution,
it can still be effective in some applications. A stand pipe is a physical pipe added perpendicularly to the process
flow, typically in a vertical fashion. The top of this pipe can be capped but is typically vented, depending on the
application. The purpose is to fill the pipe, usually less than halfway, with a certain amount of the media. When
the pressure spikes occur, the level of the media inside the pipe will rise and absorb the shock. This process saves
the remainder of the system from experiencing the transient pressure.
If possible, try to avoid starting a system completely empty. An empty system that is suddenly filled with media
can cause those transient pressure spikes to occur as the system is filled. Also, reducing the number of plumbing
size changes can greatly decrease the opportunities for pressure spikes.
TEMPERATURE, CONDENSATION AND FREEZING
Temperature needs to be considered in two different ways when it comes to pressure sensing: the temperature
of the media, and the ambient temperature of the sensor environment. Understanding these concepts will help
prevent condensation and freezing that can affect sensors.
If the media temperature is an extreme condition, such as liquid nitrogen for cold temperatures or steam for hot,
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the sensor could stop measuring the pressure accurately or could even be destroyed if it was not manufactured
to meet those conditions. Pressure sensor manufacturers should always post the media temperature specifications for their units, as well as the specification for ambient temperature conditions.
The ambient temperature, or temperature present around the sensor, can affect the electronics inside the sensor and therefore the pressure sensing results. Like any electronic device, the components used to interpret and
represent the pressure have temperature limitations; outside of these limitations failure or damage can occur.
By obeying the temperature specifications of the sensor, the pressure can be measured accurately and effectively, and the life of the sensor will not be jeopardized.
For media temperature conditions outside of sensor manufacturer specifications, a common practice to avoid
potential problems is to add an isolator between the media and the pressure sensor. The isolator reduces the
thermal effect by isolating the media source and providing a heat sink before reaching the sensor. There are
multiple isolators available for numerous applications. Most sensor manufacturers will offer this service for their
products, but there are also companies that specialize in just the isolator.
Temperature consistency is critical for predicting the performance of a sensor in a hazardous location. Temperature fluctuation of both the media and the environment can contribute to the speed and impact of corrosion-induced pressure sensor failure by causing faster and varied reactions between the media and the exposed sensor
components. In scenarios where temperature fluctuation occurs, the exposed sensor components in contact
with the media eventually break down, causing the sensor to fail.
Ambient conditions can be harder to work around because they are always present. Sometimes, the best way
to avoid future issues is to move the sensor from the application to a safer location. This may require some additional hardware, plumbing or specialty devices to still measure the pressure accurately.
Temperature plays an important role in condensation, a potential challenge that is often overlooked because
the conditions have to be just right. Typically, this occurs when cold media is measured in warm ambient conditions, and the temperature differential causes moisture to build on the internal area of the sensor. Much like the
outside of a cold beverage on a hot day, the inside of a sensor can gather moisture buildup. Condensation in the
sensor can cause the electronics to operate outside of the normal function, or to even fail completely.
The first step to prevent damage from condensation is locate the sensor outside of the area with this type
of temperature differential. If this isn’t possible, isolators can separate the media from direct contact with the
sensor. Some pressure sensors are more conducive to this type of application, and will allow for the sensor to
function properly.
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Freezing is another temperature concern, not only when selecting a sensor, but also when installing the unit for
pressure measurement. Special care is needed to ensure a sensor will not be damaged in areas with both cold
temperatures and liquid media, especially if that media contains water. As the water molecules freeze, they expand. Since there is only so much room inside of the sensing area, any expansion beyond that space can cause
damage to the sensor.
A common application where freezing might occur is monitoring water pressure on an irrigation system. The
system is used for several months of the year, but is shut down for the winter. If not properly drained, the residual water inside the sensor could freeze and potentially damage the sensor beyond repair.
While some pressure sensors are designed to allow media to fully drain from the unit through larger openings,
the easiest step is to remove the sensor from the location and store it in a place within the sensor’s temperature
specifications. If this is not feasible because of application requirements, make sure to fully drain the sensor of
any media before freezing conditions are present. This may be done by removing the sensor, fully removing the
media and then placing the sensor back in to the application, or it may be done continuously if the sensor is
oriented properly. If possible, locate the sensor in a vertical position so it can drain by itself.
VIBRATION
Physical movements to sensors are usually tolerable until they become extreme. Because of the nature of pressure measurement, not all applications are as gentle on the sensors as others. Electronics are located inside the
sensor to measure and interpret pressure, and these internal movements need to be controlled and kept within
the specifications of the sensor. If vibrations cause these internal electronic components to change position, the
measurement will be incorrect and the sensor may be damaged beyond repair.
The best way to protect the sensor is to locate it in an area away from shocks and vibration. In applications
where the sensor must be close to the source of the measurement, flexible tubing can keep the sensor insulated from the vibration while allowing the pressure to transfer from the process connection to the sensor. Check
with the manufacturer for the specifications of the sensors to ensure the pressure sensor will work for any given
application.
CONCLUSION
Despite numerous environmental challenges, accurate pressure measurement is possible by anticipating and
mitigating potential conditions that may alter or disrupt results. By evaluating the specific material, temperature
and environmental factors in and around an application and implementing some of the suggestions provided,
operators and engineers can save frustration, downtime and money.
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