Tag Archives: pressure measurement

Petroleum Core Testing

How much oil will come out of an oil field? Is it worth developing once it is discovered? These are not trivial questions if you must drill and complete your wells in deep water offshore or on the remote North Slope of Alaska. All of the major oil companies maintain core testing labs whose job it is to evaluate the oil producing potential of a given field based on the evaluation of oil cores. An oil core is a sample of the oil-bearing rock as obtained from exploratory drilling. The idea is to subject the recovered core to down-hole temperatures and pressures, then measure the flow of fluids through it. Oil producing rock typically has the density and porosity of cement, so the study of fluid flow in these materials requires the ability to measure small differential pressures (just a few psi) at very high static pressures (several thousand psi).

The oil core is prepared by fitting it into a special jacket that is heated to down-hole temperature. A special high-pressure pump forces brine through the core. The static pressures around the core are typically 5000 to 10,000 psig.

Core Testing

The core has sealing packers placed along its length at regular intervals. A Validyne variable reluctance pressure transducer like the P365 is plumbed between the packed-off sections so that the pressure drop through the core rock as a function of flow rate can be measured. Validyne offers several models of transducer that have a full scale of little as 5 psi of differential pressure, while both ports of the transducer are at a static pressure of 10,000 psig. A carrier demodulator displays the differential pressure digitally in engineering units. The relationship between flow and pressure drop is a measure of the permeability of the oil producing formation, and this can be used to determine the amount of oil that can ultimately be brought into the well bore from the surrounding rock.

Reducing Glove Box Filter Costs

Glove Box Filter

Handling radioactive materials must be done in glove box. A glove box is a clear plastic enclosure with rubber gloves attached to the sides so that an operator can handle material inside the glove box, but air and radioactive dust are not allowed to escape. The air pressure inside the glove box must be controlled so that it is always less than the ambient atmosphere. A series of risers, ducts, fans and filters connected to the glove box keeps the pressure inside the glove box lower than the outside atmosphere so that no dust can escape.

If the airflow velocity up the riser from the glove box is too high, radioactive dust is carried into the exhaust system, and these particles are trapped by a series of special filters. When a filter needs to be changed, it is very expensive to dispose of the dirty filter because it is charged with radioactive dust and dirt. The key to reducing filter disposal costs is to limit the exhaust airflow velocity so that dust is not carried out of the glove box and into the filters, while at the same time insuring that the pressure inside the glove box is always less than the ambient atmosphere.

Measurement of the air velocity in the glove box risers is accomplished with a pitot tube and a sensitive differential pressure transducer. As air velocity increases, the differential pressure across the pitot tube increases. The very low velocity needed to exhaust air from the glove box, without raising dust translates into a very low pressure drop across the pitot tube. This pressure drop is as little as 0.02 In H2O under normal operating conditions. A very sensitive differential pressure sensor, combined with a high level DC signal output, is needed to provide the air velocity signal to the system controller.

A special version of the Validyne P532 provides the low pressure measurement required for such a control system and these are used at a major North American nuclear fuel rod processing facility.

What is the Difference Between Line Pressure and Overpressure?

The over pressure and maximum line pressure specifications for differential transducers are often confused. This application note will describe the differences and give examples.

Over Pressure:

For all differential pressure transducers, over pressure is defined as the maximum differential pressure the transducer can withstand without compromising subsequent measurements. This is determined by the Validyne pressure range code of the transducer . The range code is given in the transducer model number.

For example, a transducer having a -36 range code will measure from 0 to +/-5 psi, differential
pressure. If the sensor is exposed to up to 10 psi differential, no damage will result and the transducer can make subsequent measurements of 5 psi and below accurately.

Some further examples:

-42 will tolerate a maximum differential pressure of 40 psi
-20 will tolerate a maximum differential pressure of 7 In H2O
-56 will tolerate a maximum differential pressure of 1000 psi

Note that the maximum pressure that the P55 or DP15 sensor can contain is 4000 psig! So the -64 range, with a full scale of 3200 psi differential will likely leak once the over pressure reaches 4000 psig – somewhat less than twice the full scale range of the transducer.

Line Pressure:
The line pressure specification is the maximum pressure that can be applied to both ports at the same time. The maximum line pressure for the P55D, for example, is 3200 psig, and this is the maximum pressure that can be applied to both ports simultaneously. It is often necessary to measure small differential pressures at high line pressures – as in measuring the pressure drop across a high-pressure filter. The filter may operate at 1000 psig, but have less than a 5 psi differential pressure drop across it. A P55D with range code -36 could be used to measure the actual pressure drop across the filter because the + port (upstream side of the filter) might have 1005 psig and the – port (downstream side) 1000 psig. There is a difference of 5 psid but the common-mode line pressure is 1000 psig.

There is a slight error that occurs as a function of line pressure – the zero output will shift as much as 1% per 1000 psig of line pressure. This can be corrected using a 3-valve manifold so that the differential pressure can be equalized across the sensor while the full line pressure is applied to both ports. Turning the Zero adjustment will re-zero the output signal at the operating line pressure.

For any differential transducer operating at high line pressures, care must be taken not to expose one side of the transducer to full line pressure while the other side is at atmospheric pressure – this will result in severe over pressure and require repair.

 

Pressure Transducer

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.

Connecting the USB2250 Data Acquisition System to a String Pot

A string pot is a displacement measuring sensor that can conveniently measure displacements of a few inches to a few feet. The string pot is something like an ordinary tape measure but instead of a ruler there is a cable attached to a spring-wound potentiometer so that the distance the cable moves changes the position of the pot wiper. Simply mount the string pot securely and connect the cable to the moving part to be measured. When the potentiometer is powered, the output of the string pot is a voltage proportional to the displacement of the cable.

String pot construction is shown below along with a typical enclosed unit.

String Pot

 

 

 

 

String Pot

 

 

 

 

 

Electrically, the string pot is a simple potentiometer and it can be powered by the USB2250 and the signal received as a single-ended DC voltage. The connection diagram is shown below:


Data Acquisition

The +5 V power is supplied by the USB2250 terminal block. The position of the wiper changes the voltage at the A-In terminal from 0 to +5 Vdc, depending on the position of the wiper.

 

A simple scale factor is all that is needed to convert the voltage into a reading in inches. For example if a string pot has a 7 inch displacement, the signal will be 0 to +5 V from 0 to 7 inches. The scale factor for Easy Sense to convert the voltage signal into a reading in inches is 7 inches/5 Vdc = 1.4