Showing posts with label differential pressure. Show all posts
Showing posts with label differential pressure. Show all posts

Monday, April 9, 2018

What is a Pressure Transmitter?

Differential pressure transmitter
Differential pressure
transmitter (Yokogawa)
A pressure transmitter is a transducer that converts pressure into an electrical signal it outputs both analog and digital signals corresponding to the pressure. A pressure transmitter measures three phenomena: differential pressure; gauge pressure; and absolute pressure. The most common and useful industrial pressure measuring instrument is a differential pressure transmitter. This instrument senses the difference in pressure between two ports and produces an output signal with reference to a calibrated pressure range.

Industrial Applications of Pressure Transmitters

Pressure transmitters are commonly used to measure the pressure inside of industrial machinery or in industrial processes. They are used in various industries such as oil and gas, refining, chemical, pharmacy, and so on.

Pressure Transmitters in Industry

Pressure Transmitters in Industry
Pressure Transmitters in Industry
Pressure transmitters are widely used in industry to measure flow, level, and pressure. There are unlimited industrial applications. Oil and gas flow metering applications are found onshore, offshore and in subsea. It is also often used for monitoring filters in water and effluent treatment plants, monitoring sprinkler systems, and remote sensing of heating systems for steam or hot water. It can monitor pressure drops across valves and can be used to monitor pump control.

Differential Pressure for Flow Measurement

DP flow measurement is one of the most common applications for differential pressure transmitters by measuring the difference in fluid pressure. While the fluid flows through a pipe, it is possible to calculate the flow rate for differential pressure flow measurement. A primary and the secondary element are used. The primary element is designed to produce a difference in pressure as the flow increases. There are many different types of primary element, the most common being the orifice plate, Venturi flow nozzle, and pitot tube. The secondary element is a differential pressure transmitter. It is designed to measure the differential pressure produced by the primary element as accurately as possible. In particular it is important that the differential pressure measurement is not affected by changes in the fluid line pressure, temperature, or other properties such as ambient temperature. A good DP transmitter will ensure that the differential pressure is measured accurately regardless of other changing parameters and will reliably transmit a signal to represent the differential pressure. The DP flow transmitter output signal may also include square root extraction for flow calculation, although it is common for this function to be handled in a control system. In a typical control loop, the transmitter signal is fed to the controller whose output is used to regulate the flow rate through a control valve.

Differential Pressure for Flow Measurement
Differential Pressure for Flow Measurement


Differential Pressure for Level Measurement

Differential pressure transmitters can also be used for tank levels by measuring the pressure. The transmitter is installed at the bottom of the tank whose level is to be detected. In case of a sealed tank, a transmitter with capillaries measures a differential pressure between the upper side and the bottom side. The liquid inside the tank at the bottom creates pressure which is higher than the pressure at the top. The difference in these pressures can be used to calculate the level. In case of an open tank, the transmitter measures the differential pressure between the liquid inside the tank and the reference atmospheric pressure. In a typical control loop, the transmitter signal is fed to the controller whose output is used to regulate the tight level through a control valve.

Differential Pressure for Level Measurement
Differential Pressure for Level Measurement

Tuesday, March 27, 2018

Understanding Flow Sensing Technologies

When selecting a flow sensor, flow meter, or flow switch, one of the first considerations is always the process media: air, gas, steam or liquid. Some flow sensing technologies measure gas, some are better at liquids, some are best for a single media, such as steam, and others are good in multiple media. The industry’s major flow sensing technologies now available include:
Thermal
Thermal flow meters.
Depending on the process media and your application’s requirements, all of these technologies have their advantages/ disadvantages. By considering the process media to be measured, as well as your plant’s equipment and layout, environmental conditions, maintenance schedules, energy cost and ROI, you will be able to narrow the field to one or two best choices.

Coriolis
Coriolis
Coriolis (Mass): Coriolis flowmeters use the oscillating movement of two symmetric metal tubes that are made to vibrate from an internal driver coil.  When liquids or gases flow through the tubes, a phase shift occurs (like you see in the hose) and pickups measure the “twist” and then relate that value to the actual flow. In other words, the amount of twist is proportional to the mass flow rate of fluid passing through the tubes. The greater the twist, the larger the distance between, and the greater the flow.

Differential Pressure: The differential flow meter is the most common device for measuring fluid flow through pipes. Flow rates and pressure differential of fluids, such as gases vapors and liquids. The differential flow meter, whether Venturi tube, flow nozzle, or orifice plate style, is an in line instrument that is installed between two pipe flanges and measures the pressure drop across the flow restrictor and equates it to flow.

Magnetic
Magnetic
Electromagnetic: Magnetic flow meters, also called electromagnetic flow meters or "magmeters",operate on a very simple principal. An electrically conductive liquid moving through a magnetic field will generate a voltage that is related to the velocity of the liquid.

Positive Displacement: Provides a direct indication of actual volumetric flow rate. The fluid motion drives the mechanical assembly. As the fluid motion drives the positive displacement flowmeter assembly, its rotational, oscillating, or other regular movement is counted, often by electronic means using magnetic pickups on moving assembly. There are a number of different positive displacement flowmeter designs including oscillating piston, gear, nutating disk, rotary vane, and diaphragm.
Thermal
Thermal

Thermal (Mass): Measure flow by delivering heat into the flowing media and measuring the loss of heat between temperature measurement points. They are popular because they provide unrestricted flow, contain no moving parts, work well on large or small diameter pipes, provide accuracies over a wide range of flow rates, do not require temp/press compensation, and provide mass flow instead of volume.

Turbine
Turbine
Turbine: These types of flowmeters operate under the simple principle that the rotation of the turbine will be constant as the turbine is acted upon by a fluid passing through the flowmeter. The rotational velocity of the turbine is then interpreted as output, allowing for the operator to consistently monitor the flow rate of the process fluid. They are easy to maintain and reliable.
Ultrasonic
Ultrasonic

Ultrasonic: Measure, via sound waves, the velocity of liquid flowing through a pipe.  Doppler shift technology reflects ultrasonic beams off sonically reflective materials. The transit time method exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium. The use of ultrasonic flow technology is most used in the oil, nuclear, wastewater, pharmaceutical, food and beverage industries.
Variable Area
Variable Area

Variable Area: Measures flow rate by allowing the cross-sectional area the fluid travels through to vary, causing a measurable effect. Flow measurement is performed according to the float principle. Used to measure many different types of liquids and gases passing through closed piping.

Vortex Shedding
Vortex
Vortex Shedding: Refers to the phenomenon wherein flowing gas or liquid forms vortices around a solid obstruction placed in the flow path, which can be measured to calculate volumetric or mass flow. Measure the volumetric flow rate of steam, gas, and low viscosity liquids.

Contact Flow-Tech for any industrial or commercial flow application by calling 410-666-3200 in Maryland, or 804-752-3450 in Virginia. Visit https://flowtechonline.com.

Wednesday, January 31, 2018

Differential Pressure Level Detectors

Differential Pressure Level
Fig 1. Open Tank Differential Pressure Detector
Click for larger view
The differential pressure (∆P) detector method of liquid level measurement uses a ∆P detector connected to the bottom of the tank being monitored. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric). This comparison takes place in the ∆P detector. Figure 1 illustrates a typical differential pressure detector attached to an open tank.

The tank is open to the atmosphere; therefore, it is necessary to use only the high pressure (HP) connection on the ∆P transmitter. The low pressure (LP) side is vented to the atmosphere; therefore, the pressure differential is the hydrostatic head, or weight, of the liquid in the tank. The maximum level that can be measured by the ∆P transmitter is determined by the maximum height of liquid above the transmitter. The minimum level that can be measured is determined by the point where the transmitter is connected to the tank.

Differential Pressure Level
Fig. 2 Closed Tank, Dry Reference Leg
Click for larger view
Not all tanks or vessels are open to the atmosphere. Many are totally enclosed to prevent vapors or steam from escaping, or to allow pressurizing the contents of the tank. When measuring the level in a tank that is pressurized, or the level that can become pressurized by vapor pressure from the liquid, both the high pressure and low pressure sides of the ∆P transmitter must be connected (Figure 2).

Differential Pressure Level
Fig 3 Closed Tank, Wet Reference Leg
Click for larger view
The filled reference leg applies a hydrostatic pressure to the high pressure side of the transmitter, which is equal to the maximum level to be measured. The ∆P transmitter is exposed to equal pressure on the high and low pressure sides when the liquid level is at its maximum; therefore, the differential pressure is zero. As the tank level goes down, the pressure applied to the low pressure side goes down also, and the differential pressure increases. As a result, the differential pressure and the transmitter output are inversely proportional to the tank level.

Where the tank contains a condensible fluid, such as steam, a slightly different arrangement is used. In applications with condensible fluids, condensation is greatly increased in the reference leg. To compensate for this effect, the reference leg is filled with the same fluid as the tank. The liquid in the reference leg applies a hydrostatic head to the high pressure side of the transmitter, and the value of this level is constant as long as the reference leg is maintained full. If this pressure remains constant, any change in ∆P is due to a change on the low pressure side of the transmitter (Figure 3).

https://flowtechonline.com
410-666-3200 in Maryland
804-752-3450 in Virginia

Wednesday, December 20, 2017

Yokogawa Pressure eBook - A Basic Guide to Understanding Pressure

The impact of pressure on industrial processes would be difficult to understate. Pressure is an element of process control that can affect performance and safety. Understanding pressure concepts and how to effectively measure pressure within a process are key to any operator's success.

Yokogawa, a globally recognized leader in process measurement and control, has made available a handbook on pressure that covers a range of useful topics. The content starts with the very basic concepts and moves quickly to practical subjects related to process measurement and control.

The handbook will prove useful to readers at all levels of expertise. Share your process measurement challenges with application specialists, combining your process knowledge with their product application expertise to develop effective solutions.

Download your own copy of the Pressure Handbook here, or view online below.

Thursday, January 28, 2016

Advanced Differential Pressure Flowmeter Technology

McCrometer V-Cone
McCrometer V-Cone
The McCrometer V-Cone® flowmeter accurately measures flow over a wide range of Reynolds numbers, under all kinds of conditions and for a variety of fluids. It operates on the same physical principle as other differential pressure-type flowmeters, using the theorem of conservation of energy in fluid flow through a pipe.

The V-Cone’s remarkable performance characteristics, however, are the result of its unique design. It features a centrally-located cone inside the tube. The cone interacts with the fluid flow, reshaping the fluid’s velocity profile and creating a region of lower pressure immediately downstream of itself. The pressure difference, exhibited between the static line pressure and the low pressure created downstream of the cone, can be measured via two pressure sensing taps. One tap is placed slightly upstream of the cone, the other is located in the downstream face of the cone itself. The pressure difference can then be incorporated into a derivation of the Bernoulli equation to determine the fluid flow rate. The cone’s central position in the line optimizes the velocity profile of the  ow at the point of measurement, assuring highly accurate, reliable  ow measurement regardless of the condition of the  ow upstream of the meter.

The V-Cone is a differential pressure type flowmeter. Basic theories behind differential pressure type flowmeters have existed for over a century. The principal theory among these is Bernoulli’s theorem for the conservation of energy in a closed pipe. This states that for a constant  ow, the pressure in a pipe is inversely proportional to the square of the velocity in the pipe.

Simply, the pressure decreases as the velocity increases. For instance, as the fluid approaches the V-Cone meter, it will have a pressure of P1. As the fluid velocity increases at the constricted area of the V-Cone, the pressure drops to P2. Both P1 and P2 are measured at the V-Cone’s taps using a variety of differential pressure transducers. The Dp created by a V-Cone will increase and decrease exponentially with the flow velocity. As the constriction takes up more of the pipe cross-sectional area, more differential pressure will be created at the same flowrates.

Friday, December 4, 2015

Interesting Facts About Differential Pressure Cone Flow Meters

industrial differential pressure flow measurement device
Differential Pressure Cone Flow Meter
Courtesy McCrometer, Inc.
Requirements for measurement of flow exist throughout the industrial process control field. The applications are varied and vast. As a result, there are a number of technologies available for flow measurement and an even larger array of manufacturers providing devices and instrumentation that can be used to measure fluid flow.

Selecting the measurement technology that will provide appropriate performance for a process measurement application is an initial challenge for every process design. In order to accomplish this task, it follows that a well rounded understanding of the potentially positive or negative attributes for each methodology is necessary.

Differential pressure is one method of indirectly measuring fluid flow. It measures the change in pressure created as media flows past an obstruction in the fluid path, which, when combined with other information and calculation can be used to derive a measurement of mass flow. Like all measurement methods, there are applications where this one excels over others and some where it may not be as advantageous as alternate methods.

One manufacturer of differential pressure flow measurement devices is McCrometer. The company has been manufacturing DP flow measurement devices for over thirty years and has over 75,000 installations worldwide. In the company's own words, their flagship V-Cone product...
is an advanced differential pressure instrument, which is ideal for use with liquid, steam or gas media in rugged conditions where accuracy, low maintenance and cost are important.
Cutaway view of industrial cone flow meter
Cone Meter - Cutaway view
Courtesy McCrometer, Inc.
I have included below an interesting piece that provides, in brief form, some facts that will add to your knowledge of cone meters. Read the piece below. Contact a product specialist for any additional information you may need, or to discuss how this technology can make a positive impact on your industrial process measurement operations.