pressure measurement in the oil and gas industry

pressure measurement in the oil and gas industry,

pressure measurement in the oil and gas industry,focusing on the different types of devices used for sensing operating pressures and generating output signals. Pressure is defined as the force exerted per unit area of surface, and in processing plants, hydrocarbon gases and liquids are handled in pipes and vessels.

Various types of pressure are considered, including absolute pressure scale reference points, gauge pressure scale, and vacuum scale. Absolute pressure scale starts from a zero reference point representing the full vacuum and extends through atmospheric pressure to the highest limit of measurable pressure. Gauge pressure scale starts from zero reference point representing the local atmospheric pressure and extends to a chosen limit applicable to the specific process system.

EURO CALTECH uses two main pressure units: Imperial (British and American) units and S.I. (System International). Imperial units are the pound force (lbf) divided by the inch square (in2) (pounds per square inch), often abbreviated to PSI. S.I. units are the Newton (N) per square meter (N/m2), a very small unit of force, and liquid column height is expressed in terms of inches water column (in Wc) and millimeters water column (mm Wc).

In summary, pressure measurement is crucial for the proper functioning of oil and gas production systems, safety considerations, and monitoring of equipment and piping. The manual provides an understanding of the different types of pressure devices and their applications in the oil and gas industry

Pressure conversions are essential for measuring process pressure, and a table below provides examples of different pressures. Bourdon tubes are the most common type of pressure sensors, consisting of metal tubes with a flattened circular cross section bent into a C-shape, Spiral, or Helix. When pressure is applied through the open end, the increased pressure causes the flattened cross section to become more circulars and the shape to straighten, moving the closed end.

There are three common types of bourdon tubes: C-type bourdon, helical bourdon, and spiral bourdon. C-type bourdon tubes are simple, accurate, and have good repeatability but are bulky and highly subject to damage from over-ranging. Helical bourdon tubes are used for ranges as low as 0 - 200 psig (0 - 1300 kPa) up to 0 - 6000 psig (0 - 40,000 kPa). Spiral bourdon tubes are used for both very low ranges and very high ranges.

Bellows sensors are axially flexible, cylindrical enclosures with folded sides that extend axially when pressure is applied through an opening. Bellows gauges with under/over range protection rotate a pointer by a mechanical linkage, and the movement of the bellows is posed by the spring action of the bellows material, the pressure surrounding the bellows, and usually, the force of an external spring or another bellows.

Absolute pressure gauges are used to measure absolute pressure ranges as low as 0 -100 mm Hg and gauge pressure ranges as low as 0 -5 inches H O (0 -125 mm H 0). Bellows elements can measure absolute pressure, gauge pressure, and other pressures.

In summary, pressure conversions are crucial for measuring process pressure, and various types of sensors are available to suit different pressure ranges.

Diaphragm sensors are thin, flexible, flat or corrugated disks held in place to be axially flexible. When pressure is applied to one side of the diaphragm, it will deflect, and the force opposing the pressure is the sum of the spring constant of the diaphragm, the pressure on the opposing side of the diaphragm, and the spring constant of any opposing spring. The sensitivity of a diaphragm increases as the diameter increases. The axial movement of the diaphragm can rotate a pointer or actuate a controller or transmitter by attaching the free end to a mechanical linkage. There are two types of diaphragm elements: elastic and limp.

Elastic diaphragms use the stiffness of the diaphragm to oppose the pressure applied, and they come in two different configurations: single and capsular. Single diaphragms are either flat or with concentric corrugations, while capsular diaphragms consist of two diaphragms welded together at their perimeters. Capsules can be either convex or nested, and can be mounted in multiples to give more deflection for a given pressure. Evacuated capsules are used for absolute pressure reference, and single diaphragms for very sensitive measurements.

Resonant-wire sensors are used in electronic pressure transmitters, where the resonant frequency of a vibrating wire is a function of the length, the square root of the tension, and the mass of the wire. In resonant-wire pressure transmitters, a wire or ribbon under tension is located in the field of a permanent magnet, and the tension on the wire is proportional to the pressure. An electrical signal with a frequency proportional to the square Kroot of the tension will be generated, which is converted to a 4-20 mA transmitter output.

Strain-gauge pressure sensors are used in most brands of electronic pressure transmitters, and when metallic conductors or semiconductors are subjected to mechanical strain, a change in resistance will occur. This resistance is then electrically converted into a 4-20 mA signal proportional to the pressure. Most strain elements in current use are semiconductor type.

Capacitance pressure sensors operate on the principle that the change in capacitance resulting from the movement of an elastic element is proportional to the pressure applied to the elastic element. The elastic element usually is a stainless steel diaphragm, but other materials are available if stainless steel is not suitable for the process fluid. A high-frequency oscillator is controlled by the sensing element, and changes in pressure deflect the diagram and the resultant change in capacitance changes the oscillator frequency. The variation in oscillator frequency is converted to a 4-20 mA signal proportional to the pressure.

 Spring-loaded piston sensors are used for both pneumatic and electric pressure switches. Companies who manufacture devices for only pneumatic use their products as pressure sensors or pressure pilots. Heavy-duty pressure sensors, such as stick pilots, are manufactured so that they can serve as either a high-pressure sensor or low-pressure sensor as required. The terms high-pressure pilot and low-pressure pilot refer to the way the sensor is connected rather than being two different devices.

In certain applications, the pressure sensor may not remain functional for a reasonable amount of time. Devices described in the following sections can be used to protect the pressure sensor. Diaphragm seals are used to isolate the pressure sensor from the process fluid, such as toxic, corrosive, dirty, solidifying at ambient temperature, or extremely cold and freezing the instrument.

The diaphragm seal is a thin, flexible disk that separates the pressure sensor from the process media. It is composed of three main components: the top housing, bottom housing, and diaphragm. Siphons are metal, tubular devices used to isolate hot-process media from the pressure sensor. They can be filled with high-boiling-point liquid or process condensate, acting as a barrier to heat contained in hot gases or steam. These devices also act as pulsation dampeners.

 Throttling devices are commonly used to dampen high-frequency pressure fluctuations by putting a restriction in the inlet to the pressure sensor. Throttling screws are a special screw that comes in various orifice sizes and are inserted into a tapped hole in the base of the pressure sensor to provide a flow restriction. Pressure snubbers are compact fittings with a porous element that restrict velocity and filters the fluid. Pulsation dampeners, also known as pressure snubbers, do not have a filtering element but do not have a filtering element.

 Pulsation dampeners have several designs, such as bar-stock fittings or two fittings screwed together. They force the piston up and restrict the flow from the large chamber by closing the outlet of the chamber. Pressure-limiting valves protect the pressure sensor from overpressure by blocking the process fluid at a preset limit. There are several designs of pressure-limiting valves, such as a common design with a piston passing around the piston and out to the pressure sensor.

 Manometers work on the principle of balancing an unknown pressure against a known pressure produced by a column of liquid in a vertical or inclined tube. The typical pressure range covered by manometers is from absolute zero pressure to approximately 1.5 bar, depending on the length of the tube and the liquid used within the manometer. It is important to use the correct relative density of liquid in a manometer to avoid incorrect readings.

 The U tube manometer is a simple and accurate method for measuring low pressures, with its advantages including simplicity, accuracy, and repeatability. It must be mounted vertically and has an adjustable scale graduated from a center zero line. The applied pressure equals the sum of the tow scale readings.

The U tube manometer has several disadvantages, such as being too long and cumbersome, and must be carefully positioned. If mercury is used as the liquid, care must be taken when reading the manometer, as the meniscus of mercury is convex compared to other liquids. The reading should always be taken at its center, considering the dangers of using mercury in fragile glass containers and proper precautions in case of spillage.

A single limb or well-manometer is essentially a U tube manometer with one larger limb and is widely used for convenience. Inclined manometers offer greater sensitivity and are used for measuring very low pressures like drafts in furnaces, chimneys, or ventilation ducts. They enable small pressure differentials to be measured more conveniently and accurately than the U tube or well type.

The inclined manometer is a modification of the well manometer, with the single leg sloped at a small angle above the horizontal, producing a larger movement and easier reading of the length of liquid. To avoid parallax errors, ensure your eye is in line with the meniscus. Other sources of error when using a manometer include the effect variation in local gravity and the effect of temperature.

Pneumatic pressure transmitters are used to measure pressure in various applications, such as monitoring and controlling atmospheric pressure. They can be either suppressed zero (above atmospheric pressure) or elevated zero (below atmospheric pressure). The use of pneumatic transmitters is decreasing, but some manufacturers still produce them for the replacement market and some new installations are still being made. Electronic transmitters with 4-20 mA outputs are the most common type. 

Pneumatic transmitters are available in various ranges, and the range and span are two different parameters. The span is the actual pressure range to be measured after the transmitter has been adjusted, while the range is the pressure range within which the span can be adjusted. Most transmitters have two adjustments, zero and span. 

Differential pressure transmitters, also known as DP cells, provide a pneumatic or electronic output for use in remote indication panels or as input signals to control loops. Typical ranges include 0.2 to 1.0 Bar or 3 to 15 psi, and 4 to 20 mA. 

Pneumatic DP cells use a diaphragm deflected by applied differential pressure to separate the HP and LP chambers of a DP cell. A force bar at the top moves a flapper closer or further away from the nozzle, resulting in a change in the output pressure proportional to the applied pressure difference. Electronic DP cells provide a higher level of accuracy than their pneumatic counterparts.

Two sensor systems have gained popularity: capacitance type and resonant (vibrating) wire type. Capacitance type sensors use a movable diaphragm fixed between two capacitance plates, which changes the capacitance between the plates as differential pressure is applied. Resonant wire type sensors use a pre-tensioned wire suspended in a magnetic field, forcing it to oscillate at its natural frequency when differential pressure is applied.

 Data flow for an electronic pressure transmitter can be summarized in four major steps: applying pressure, measuring a change in pressure, conditioning the sensor signal for various parameters, and converting the conditioned signal to an appropriate analogue output.

 Pressure controllers are devices that sense process pressure and develop an output to control a device to regulate that pressure. They can be indicating or blind, with the indicating controller having a mechanism for direct reading of the process pressure, while the blind controller requires an adjacent pressure gauge or other device. The set point is usually marked on the indicator, making adjustment easier. Indicating controllers are slightly more expensive than blind controllers but have a moderate cost difference if a pressure gauge can be eliminated.

 The output of a pressure controller can be electric or pneumatic, but is typically pneumatic for field-mounted controllers. The control action needed for pressure control is proportional plus integral, or P and I, which varies the output in proportion to the difference between the measured pressure and the set pressure. The integral or reset action gradually increases the correction until the measured pressure is returned to the set point.

 Auto/manual switches are common options for pressure controllers, allowing the output of a manual regulator to be directed to the end element instead of the controller's automatic output. These transfers can be bump-less or manual balance. Pressure controllers can be surface, panel, pipe-stand, or yoke mounted. Surface-mounted controllers are fastened to a wall or vertical surface, panel-mounted, pipe-stand mounted, and yoke mounted.

 Electric pressure switches sense pressure and open or close an electrical switch element at a set pressure to signal another electrical device. They come in various styles, including gauge pressure, absolute pressure switches, vacuum switches, and vacuum switches.