Category Archives: Pressure Measurement

pressure sensor

Engine Air Filter Pressure Drop

Diesel and gasoline engines require a lot of air as part of the internal combustion process.  Anyone who has lifted the hood on their automobile knows that the air coming into a vehicle engine is filtered to keep out road dust and grit from entering the engine cylinders.  All vehicles have an air filter for this purpose and these are commonly made of a paper filtration material, are housed in a plastic or metal can, and can be opened easily for filter replacement.

Military vehicles also have an engine air filter but this is a much more critical component – off road operations tend to involve much more dust and debris than normal driving and should the filter become completely clogged, the engine will stop.  And you don’t want a stalled engine in the middle of a combat operation.

One way to avoid filtration problems is to measure the pressure drop across the air filter at all times during engine operation.  The best design of an engine air filter is one that allows a lot of air to pass with a minimum loss of pressure, and a lot of effort goes into filter design that can provide a large surface area in a small space – even an ordinary air filter contains dozens of folds to achieve this.  But the danger of a filter clogging is too great and the best method of determining how well a filter is operating is to directly measure the pressure drop across it.

The pressure drop across a working air filter is often very small – on the order of a few hundred Pascals, especially at low engine RPM – so a pressure sensor for this purpose must be sensitive to very low pressures.  In addition, the pressure transducer must be rugged enough to function well under the hood of a vehicle where shock, vibration and high ambient temperatures are common.

Validyne has supplied a variant of the P55 pressure transmitter that has been designed for use in military vehicles to measure air filter pressure drop.  The P55 pressure transmitter electronics has been secured with epoxy and a special housing, connector and custom mounting plate provide a sturdy attachment to the air filter enclosure.  The 4-20 mA signal is used by the engine control system to alert the driver when the air filter starts to clog.  This allows for preventative maintenance and a way to avoid stalling the engine in challenging conditions.

Contact an application engineer at sales@validyne.com or 818-886-8488 for more information on similar applications.

flame instability

Pressure Transmitter to Detect Burner Flame Instability

Large-scale industrial installations such as refineries and chemical plants often have large heaters that are used in the processing of fluids and gases.  Such heaters must now be designed to minimize NOX emissions to meet Federal clean air requirements and much research has gone into finding ways to make the burning process more efficient.

The technology to reduce emissions in large-scale heaters is complex, but for gas-fired heaters part of the solution is to run the natural gas feeding the burners at a reduced pressure to allow for more complete combustion.  But running lower burner gas pressures increases  the risk of a flame-out – and restarting a large heater could cost hours of downtime.

A large oil company that was retrofitting low NOX burners into their heaters decided to design a system that would detect the conditions present in the combustion chamber just before a flame-out.  When a burner flame becomes unstable it wobbles – much like a candle flickers just before going out.  Because the burners are enclosed, the air pressure inside the heater will exhibit a characteristic oscillatory behavior just as the flame begins to fail.  The air pressures inside the heater are low – just a few inches of water – and this pressure will vary at a characteristic frequency of a few Hz just before flame-out.

Validyne developed a version of the DR800 that is sensitive enough to detect the low heater pressures, yet responsive enough to capture the pressure waveform prior to a flame-out condition.  The electronics of the DR800 were modified to pass frequencies up to 50 Hz and the damping circuit was bypassed.  The modified DR800 is capable of passing low pressure variations up to 10 Hz with no distortion.  The DR800 is FM approved as an intrinsically safe device for Class I, Div 2, Groups B, C and D hazardous locations and is ideal for this application in oil refineries.

Pressure Transmitter

The customer developed a signal processing algorithm that detected a pending flame-out condition in their burner from the character of the DR800 signal.   This involves analyzing the  signal for the right combination of frequency and amplitude of air pressure that occurs prior to a flame-out.   When this situation is detected, a warning light is displayed on the operators control panel and action can be taken to avoid a flame-out and time consuming restart.

The major refinery operators – Exxon, Chevron, Union, etc – will be able to develop a detection algorithm suitable for each individual heater configuration.   Another potential market is the burner manufacturers such as Zeeco, John Zink and others, who are purveying low NOX systems.

 

 

relief valve

ASME Pressure Relief Valve Testing

Many processes involve the use of high pressure steam, water or air.  Piping systems carrying these fluids must be protected from over-pressures that could cause damage or injury.  A pressure relief valve is a device that opens to vent any pressure higher than the relief valve’s operating set point.  The water heater in your house, for example, has a pressure relief valve set to open at a pressure that is lower than the burst pressure of the heater tank.  That way if pressure inside the tank exceeds the relief valve’s set point pressure, the valve will open and vent the pressure before the tank is damaged – you get a wet floor but you don’t have to replace the heater tank.

Pressure relief valves come in all sizes and pressures and these are critical parts of a high pressure piping system carrying steam in an industrial plants, refineries, power plants, etc.  The ASME has established criteria for the size and set point pressures for relief valves operating in industrial systems.  Additionally, these valve are tested on a regular basis to insure that they open at the correct pressure and do not impede the flow of fluid as the pressure is vented.  The vales are tested at their operating pressures and temperatures, and the opening pressure and pressure drop through the valve as it vents must be measured.

There are testing laboratories that are used to test industrial pressure relief valves by simulating the operating conditions for water, air and steam.  One customer of Validyne has a test lab capable of generating up to 10,000 lbs. per hour of steam at 300 psig, air flows to 3500 SCFM at 500 psig and water flow rates of 500 gpm at 300 psig.  Pressure relief valves are tested depending on their operating conditions, and the valves are instrumented to verify correct operation at their set point pressure.

The Validyne product used to make relief valve measurements is the DP15 pressure transducers.  One transducer is used to measure the pressure upstream of the relief valve, a second DP15 measures the downstream pressure.  These transducers are 300 or 500 psi, depending on the test.    A third DP15 measures the pressure drop across the relief valve when it is flowing and this transducer is typically 100 In H2O full scale.  The DP15s are used because they can be mounted remotely from the control station.  A large steam relief valve, for example, is connected to piping with runs of 25 and 30 feet.  The DP15 can be mounted at the measurement point and the cable to the demodulator can be up to 50 feet with no compromise in calibration.

The pressure transducers are connected to Validyne CD23 demodulator with digital display.  The CD23 features large LED displays that are helpful for the operator to see while opening and closing large control valves during the test.  The display can be given directly in PSIG and the CD23 provides an analog output proportional to pressure that can be connected to a LabVIEW computer to record the pressures during the test. Alternatively the pressure sensors can also be connected to the USB2250 DAQ.

The Validyne CD23s and DP15s have given many years of service in this difficult environment and this reliability, plus the ability to interface to a data acquisition system make it a great solution for relief valve testing.

Pressure Sensors and External Carrier Demodulators

The most popular Validyne pressure transducers are the P55/P61/P365 series.  These all include a pressure sensor, carrier demodulator electronics package, a high level output signal, temperature and linearity correction as well as a compact form factor.  There are applications, however, where a better solution might be to separate the pressure sensor from the electronics, with the two connected by a cable.  This article describes when this approach makes the most sense.

Validyne offers the sensors and electronics package from the P55/P61 available as stand-alone components.  The DP15 series of pressure sensors is identical to that used in the P55 and P61, while the DP360 and DP363 are high pressure variants the same as used in the P365 and P368.  The CD16 standard analog output electronics or the CD17 USB-based electronics can be used with any of these sensors, and standard cables are available in a variety of different lengths to connect the two.

Pressure Sensors

When should a sensor be separated from the electronics?  The biggest reason to do this is to allow convenient re-ranging of the pressure sensor.  The full scale pressure range of Validyne sensors can be changed by replacing the sensing diaphragm.  There are 23 different full scale ranges available for the DP15, for example, and these run from a few inches of water to 3200 psi. Changing the diaphragm is straightforward; the connector and four body bolts must be removed to gain access to the sensing diaphragm, and the DP15 sensor makes this easy, requiring just a torque wrench and a vise.  With a little practice, the diaphragm in a DP15 can be replaced and re-calibrated with the CD16 or CD17 electronics in about 20 minutes.  The DP360 and DP363 high pressure sensors are similar in construction and also lend themselves to straightforward diaphragm replacement. Frequent re-ranging of the full scale of a Validyne transducer is common in laboratory situations where pressure measurements vary widely from day to day.  Test labs and university labs are typical places where a separate sensor and electronics package are used to best advantage.

Another reason for separating the pressure sensor from the electronics is to conserve space or limit the weight at the measurement point.  In tight locations, such as aircraft compartments or in submersible vehicles, the pressure connection may be in a relatively inaccessible space and the smaller footprint of the DP15 sensor, might fit better than the full P55.  If mass or weight is important, the sensor will be lighter than the full transducer and this will relieve any stress on the piping connections in areas where shock and vibration are a consideration.

It is important to realize that separating the sensor from the electronics will compromise the temperature correction as the temperature sensor is located on the electronics package and not at the pressure sensor.  A pressure sensor such as a DP15 used with a remote electronics such as the CD16 will be most effective in applications having a stable temperature environment.

 

pressure sensor

Pressure Sensor Accuracy

When using a pressure sensor to measure pressure, one of the first questions that comes up is accuracy: How closely does the value reported by the pressure sensor come to the actual pressure?  We use the term accuracy, but what we really want to know is the error of the pressure measurement.  This article will describe standard specifications and methods for calculating sensor accuracy.

The definition of sensor accuracy actually defines error, and the two terms are used more or less interchangeably:  The accuracy of the sensor is the simply the difference between the pressure reported by the sensor and the actual pressure, expressed as a percent of the sensor full scale.  For example, if a pressure sensor with a full scale range of 100 psi reports a pressure of 76 psi – and the actual pressure is 75 psi, then the error is 1 psi, and when we divide this by the full scale and express it as a percentage, we say that accuracy (or error) of the sensor is 1%.  Most industrial sensors are better than that, with specified accuracies of +/-0.25% or +/-0.1% of full scale (FS).  So the error of a 100 psi FS sensor with an accuracy of +/-0.1% FS will not exceed +0.1 psi or -0.1 psi – at any point in the measurement range of the sensor.

Sensor accuracy is comprised of two error modes: Non-linearity and Hysteresis.  Ideally, pressure sensors are perfectly linear – the output signal or reading is directly proportional to the applied pressure.  Because sensors are mechanical devices, however, they are not perfectly linear, and this error is called non-linearity.  Non-linearity is determined by the five-point calibration method.  A pressure standard (a device know to be at least five times more accurate that the pressure sensor) is used to apply pressures at 0%, 50%, 100%, 50% and 0% of the sensor full scale.  A best-fit straight line is fitted to these points, and the maximum deviation of any of the five points from the value predicted by the best-fit line, is defined as the non-linearity error.

Hysteresis is simply the difference in the value of the readings at the same pressure along these five calibration points, up-scale and down-scale.  Hysteresis can be measured from the readings at the 50% points and at 0% FS pressure.  The greatest difference in any of these is defined as the hysteresis.

The accuracy of the sensor is defined to be the sum of the non-linearity error and the hysteresis error.

Note that the sensor accuracy calculation is pretty much the worst case error that can be determined from the calibration data, and it may not occur at every point along the pressure sensor FS range.  But this is considered to be a conservative method for calculating the error present at any pressure over the range of the sensor.

For differential pressure sensors – those having a plus and minus full scale range, the same techniques are used to calculate accuracy with the addition of -50%, -100%, -50% and 0% calibration points required (9 all together).

Spreadsheets that automatically calculate non-linearity, hysteresis and accuracy are available from Validyne for gage, absolute and differential pressure sensors.  Factory calibration sheets showing these errors are shipped with most sensor models.