Tag Archives: pressure transducers

landfill gas

Low Pressure Transducers to Monitor Landfill Gas

A new source of natural gas is the collection of methane that accumulates in landfills.  Methane accumulation is the result of the natural decay of organic materials that are part of the landfill.  Conventional natural gas wells are thousands of feet deep and produce gas at very high pressures.  The methane gas in landfills occurs at much lower pressures – from just a few inches of water to a few psi.

It is important to measure the pressures that can be sustained by the methane gas in a landfill, and this is done by installing several shallow wells and recording the pressure over time.  The Validyne P55 series of pressure transducers are ideal for this application because of their sensitivity to very low pressures, compatibility with methane gases and a 4-20 mA power/signal cabling that requires just two wires and can be run over very long distances to a central data collection point.  The Validyne low pressure transmitters are available in full scale ranges as low as 0 to 3.5 InH2O for a 4-20 mA signal, with an accuracy of 0.25% FS.  The 410 steel wetted parts and Buna-N o-ring material is compatible with methane gas and many other hydrocarbon fluids.  The P55 is rugged and compact, and is available in a weatherproof NEMA 4 enclosure.

P532 Pressure Transmitter

If after testing a landfill is found to be able to sustain methane gas production, collection and transmission facilities are built on-site to bring the landfill gas into the wider natural gas delivery network.  For permanent installations the Validyne DR800 pressure transmitter is often used as this provides even lower full scale ranges – as low as 0.25 InH20 for a 4-20 mA signal.  The DR800 pressure transmitter also has a NEMA 4 enclosure with conduit connections and a junction box for signal and power wiring.  The selectable damping feature smooths out small variations in the signal to provide for better pressure control.  The DR800 is also available with a Factory Mutual Intrinsically Safe rating for use in hazardous locations.

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.

Resolution and Frequency Response in Pressure Transducers

Resolution and frequency response in pressure sensors and pressure transducers are two performance parameters that are important, but often misunderstood.  This article will describe how each of these parameters relates to Validyne pressure transducers and pressure sensors.

Resolution

Resolution of a pressure transducer is defined as the smallest change in pressure that can be detected by the transducer.  Validyne pressure transducer are analog devices and the resolution, in theory, is infinite.  As a practical matter, however, the resolution of the analog signal from the pressure transducer electronics is a function of the signal to noise ratio.  All analog signals contain noise and the various carrier demodulator circuits used with Validyne variable reluctance sensors have somewhat different specifications for the noise level, depending on the demodulation scheme employed and the output filtering used.  In general, the noise level of the carrier demodulator signal will be 0.05% or less of the pressure transducer full scale.  So the smallest pressure change that can be detected from the Validyne pressure transducer signal will be less than 0.05% of the maximum pressure range of the pressure transducer.

Frequency Response

The frequency response of a pressure transducer is a measure of how quickly the pressure transducer can respond to changes in pressure.  There are two ways to define this: response time and flat frequency response.  Response time – sometimes called the sensor time constant – is the time, in seconds, required for a sensor signal to change from 0 to 63.2% of the full scale when the pressure sensor is exposed to an instantaneous full scale pressure change.  Response time is often used for slower pressure transducers that respond to pressure changes as a first-order system.  Knowing the time constant of the pressure transducer allows the user to calculate how the sensor signal will change in response to different applied pressure signatures during operation.

For faster pressure transducers – such as Validyne pressure transducers – the flat frequency parameter is a more accurate way to describe the pressure transducer frequency response.  Flat frequency is the maximum frequency, in Hz,  that the pressure sensor can pass into its signal without distortion.  This depends on the geometry and construction of the pressure sensor, the plumbing leading up to the pressure sensor, the fluid media and the output filtering of the carrier demodulator.

Validyne has tested the standard DP15 family of pressure sensors or the P55 family of pressure transducers types for flat response and this has been found to be  80 Hz in air when the varying pressure source is close-coupled to the sensor port.  That means that the pressure sensor is capable of allowing pressure changes of up to 80 times per second to pass, without distortion when the pressure transducer is close-coupled.

Many times, however, the pressure transducer is connected by a length of tubing to the source of the pressure variations, and this degrades the flat frequency response, as shown in the table below:

Tubing Length, FT            Flat Response, Hz

0                                         80

0.5                                      50

1.0                                      36

2.0                                      25

3.0                                      20

4.0                                      12

5.0                                        8

The output filtering of the carrier demodulator will also affect the system response, but in general most will pass the 80 Hz sensor response frequency.  Those carrier demodulators models that feature selectable low-pass filtering, however, may provide for lower settings that will filter out these frequencies, even when the pressure sensor is capable of passing them into the signal.

The flat frequency response of the pressure transducer/plumbing system will change by ratio of the speed of sound in air to the speed of sound in a liquid.  For water this ratio is about 4X, so the maximum frequency response of the pressure transducer in liquids will be greater than 300 Hz.  For a CD15 demodulator and DP15 pressure sensor, the output filtering will allow frequencies of up to 1 KHz to pass, and this is the fastest system we offer.  The P55 electronics in the P55 pressure transducer, however, has a low-pass cut-off frequency of 250 Hz and so may attenuate very fast pressure changes in liquids.

The response time can be roughly related to the flat frequency response for the purposes of comparing the performance of various sensor types.  Since response time is a measure of how long it takes for the pressure transducer signal to rise from 0 to 62.2% of full scale, the rise time can be assumed to be not more than one quarter of one complete cycle of the pressure sensor maximum flat frequency.  This is simply the reciprocal of 4 times the maximum flat frequency.  Thus the 80 Hz flat response would be 1/(4 * 80) or about 3.2 msec.

Pressure Measurement in Engine Test Cells

Does synthetic oil reduce bearing wear? Do gasoline additives really improve combustion? These questions are investigated in a special laboratory known as an engine test cells. There are many pressure measurements needed on an engine: oil pressure, exhaust pressure, coolant pressures and any number of emission pressure measurements. Pressure transducers must be protected from the ambient environment of the test cell and still provide useful signals to a data acquisition system.

An engine test cell is a noisy, dirty and cramped room containing a running engine and everything needed to conduct the test and make the required measurements. The engine must have a sturdy mounting and a dynamometer to simulate loads. There must be piping to remove the exhaust gases and to bring in clean air. There must be adequate ventilation so that heat exchanged by the radiator can be removed from the test cell.

Pressure transducers used inside the engine test cell are mounted in a large protective enclosure that usually hangs from the wall or the ceiling. Sometimes a portable enclosure is used that has wheels so the sensors and transducers can be moved for different engine configurations.There are often several dozen transducers inside the enclosure and all of the power and signal wiring must be brought out to the data acquisition system outside the test cell. The plumbing for the transducers is a series of hoses that are connected to the outside of the enclosure and run to various points on the engine.

Engine Test Cell

Because the enclosure holding the transducers is tightly packed, the ideal transducer must be compact and easy to install. The cramped conditions in the typical test cell also require that the transducer be highly reliable; changing out a pressure transducer during a long-term engine test will inevitably create gaps in the test data. A high-level output from the transducer reduces cost for external signal conditioning for the data acquisition system.

The Validyne P55 has been successfully used in test cell environments for many years and provides the rugged stability needed for this challenging environment.  New versions of the P55 with a CAN Bus interface are now able to connect directly into the data stream from the engine’s processors.