Author Archives: admin

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.

 

 

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.

water main break

Measuring Water Main Leakage

If a large water main is leaking, how would you know? What part of the city consumes the most water, and when? These are questions often asked of the city water engineer. In order to give an accurate answer, direct measurement of the flow in a large water main is often the best way to verify metering station data. How can the flow in a water main be observed directly?

Fluid flow in a water main can be measured using a pitot tube and differential pressure transducer. Water mains are usually accessible from underground vaults in city streets. A pressure tap is made, while the main is still under pressure, and a pitot tube inserted into the water flow. A cross-sectional survey is made by traversing the pitot tube across the ID of the water main pipe. The differential pressure is recorded at each point across the survey. Calculations are later made to convert the differential pressure readings into water velocity, and the flow rate calculated from the velocity profile.

Water in large mains may be moving very slowly, so the differential pressure developed across the pitot tube will also be small. The pressure transducer must be capable of measuring low differential pressures while operating at the static pressure of the water main. The analog output of the pressure transducer can be sent to a portable data logger for long-term studies.

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.