Differential Pressure Flowmeter-FAQ 

 

Introduction

 

The Differential pressure flowmeter is the technology for measuring the volumetric flow of liquids, gases, and steams. Differential pressure flow meters have laminar plates, nozzle, or venture tubes that measure the pressure loss of fluids. We manufacture high-quality Differential pressure flowmeters at BSCT.

We will discuss the common FAQs regarding Differential pressure flowmeters in this guide. You can use that information while you are planning to purchase and use this transmitter. Hopefully, you will get the answers.

 

What is Differential Pressure Flowmeter?

Differential pressure Flowmeters are the most common types of flow meters. When there is a high pressure, high temperature, or large diameter, you can use these flowmeters. These flowmeters are popular in chemical, oil, gas, and power industries.

The concept of using the pressure drop as a measurement of flow rates dates back to the 18^th century. This technology uses Bernoulli’s equation for calculating the flow of medium in a Differential pressure flowmeter. This flowmeter is accurate, authentic and its measuring principle is standardized in ISO 5167. This standard guarantees that the Differential pressure flowmeter produces reliable results.

 

How does a Differential pressure flowmeter work?

Differential pressure flowmeter measures the flow of fluid in the pipe by using Bernoulli’s principle. This principle states that the sum of all energies – static, kinetic, and potential – remains the same throughout the pipe. This flow measurement system consists of a flow element and a flow transmitter. These flowmeters are not suitable in situations with small diameters, high viscous liquids, and low flow.

Differential pressure flowmeter consists of plate, nozzle, and tube to create the artificial constriction. The transmitter then measures the pressure loss of fluids that passes that constriction. The pressure drops which is directly proportional to the square of the flow rate. The higher the pressure drop, the higher will be the flow rate.

 

How to use a Differential pressure flowmeter?

Differential pressure flowmeters measure the flow of liquids, gases, and vapors, such as water, industrial gases, chemicals, and steam. You have to be careful while using a Differential pressure flowmeter for liquids of high viscosity. Erosion can arise in high viscous liquids such as hydrocarbons when the Reynolds number is low.

You can use them for clean fluids. For measuring the flow of corrosive liquid, you can use proper attention to materials of construction. Dirty fluid can plug the piping and give an incorrect measurement. In these applications, you can use diaphragm seals.

What are the applications of Differential pressure flowmeters?

Differential pressure flowmeters are a popular choice for measuring the flow of liquids, gases, and vapors in many industries. Some industries that use Differential pressure flowmeters include mineral processing, mining, pulp and paper, petroleum, pharmaceutical, water, and wastewater.

Other flow meter technologies are popular in many industries. However, Differential pressure flowmeters are still preferable due to long last user familiarity with the technology. These are hugely popular. According to an estimate, 40% of the industrial flow meters are Differential pressure flowmeters.

What are the application cautions for Differential pressure flowmeters?

In the Differential pressure flowmeter, there is a non-linear relationship between flow rate and differential pressure. It causes the inaccuracy of flow measurement in the lower portion of the flow range. Another area of concern is the plugging of the impulse piping.

For measuring liquid, impulse piping should be oriented and sloped so that it does not collect gas. Similarly, for gas service, do the orientation of impulse piping to remain full of gas and do not collect liquid. In vapor service, vapor may be allowed to condense in the piping to protect the transmitter from heat.

What is the role of calibration in a Differential pressure flowmeter?

When there is the accumulation of liquid or gas in the impulse piping, it can affect the calibration. Also, it degrades the accuracy of measurement. For the successful application of the technology, degradation is essential.

When a differential pressure transmitter is removed for calibration, it exposes multiple problems that can affect the measurement. Generally, you can perform calibration in a Differential pressure flowmeter in-situ, and you should make the provisions in the design phase.

What are the benefits of a Differential pressure flowmeter?

Differential pressure flowmeter is a widely used flow measuring device for low viscous fluid in a pipeline. It gives an accurate measurement at reasonable costs. Some of the benefits of a Differential pressure flowmeter are as follows.

• These flow meters are simple to use, reliable, flexible, and long-term usage.
• They consist of various materials. You can use them in many types of gases and liquids.
• They are suitable for low and high temperatures.
• They are easy to maintain, with a low cost of ownership.
• It has a wide application range and easy production of standard parts.
• You can use them for different flow rates and pipe sizes.

Disadvantages of Differential pressure flowmeter?

Differential pressure flowmeter offers several advantages for its users. However, there are some disadvantages of this flowmeter as well. These disadvantages include:

• They have a relatively high-pressure drop.
• The flow rate rangeability is low because of the relationship between flow rate and pressure head.
• It cannot measure high viscous liquid and low flow rate.
• The accuracy of the measurement is dependent on the characteristics of the fluid as temperature, specific gravity, etc.
• It can cause freezing at the connecting piping.
• You can block the pressure extraction easily. It requires high fluid cleanliness and high maintenance.
• Differential pressure flowmeters may suffer from aging effects—for instance, the build-up of deposits or erosion.

What are the components of a Differential pressure flowmeter?

Differential pressure flowmeter consists of two components, primary devices, and secondary devices. The purpose of the primary device is to restrict the flow in the pipe and create differential pressure. They include an orifice plate, flow nozzle, pitot tubes, elbow tap, flume, etc.

The purpose of the secondary device is to measure the differential pressure and provide a readout or signal for transmission to the control system and include manometers, bellow meters, ring balance meters, force balance meters, etc.

The purpose of the primary device is to check the compatibility with a specific fluid or application. The secondary device is for readout or transmission signal.

Types of Differential pressure flowmeter?

• Integral Orifice plate DP flow transmitter

Integral Orifice plate DP flow transmitter uses the orifice plate as the primary element when differential pressure has to be mounted on the orifice assembly. The secondary element of this Differential pressure flowmeter is the differential pressure transmitter. The integral orifice flowmeter reduces leak points to over 50% and improves the accuracy up to 0.5% of the volumetric flow rate.

• Annubar DP flow transmitter

Annubar DP flow transmitter contains multiple pressure tapings to ‘average’ the flow. There are numerous tapping ports on the opposite side of the bar which measure the pressure. It creates a differential pressure that is proportional to the square of the velocity of the fluid. When fluid passes through the annubar sensor, it creates a low-velocity profile which makes low pressure downstream.

• Multivariable DP flow transmitter

A multivariable DP flow transmitter is a Differential pressure flowmeter that can measure several independent variables, including differential pressure, temperature, static pressure, etc. You can use these values to compensate for changes in velocity or other parameters.

• V-cone DP flow transmitter

V-cone DP flow transmitter is an advanced Differential pressure flowmeter that is ideal for measuring liquid, gas, or steam in rugged conditions. It is an innovative invention that takes differential pressure flow measurement to another level. They have better accuracy, readability, flexibility, and less maintenance.

• Pitot tube DP Flow Transmitter

Pitot tube DP flow transmitter consists of a pitot tube and an annular tube. You can use this Pitot tube in various applications for measuring gas, steam, and liquid. They are best for high-velocity air and gas systems. It has extremely sensitive differential pressure measurement, which allows it over a wide range. This Differential pressure flowmeter is simple, inexpensive, reliable, and suited for high temperatures and pressures.

• Flow Nozzle DP flow transmitter

The flow nozzle is the type of Differential pressure flowmeter that consists of a smooth convergent section leading to the nozzle. These nozzles are primary elements in differential pressure flowmeters. These nozzles generate a pressure drop to calculate the flow rate. As fluid passes through the nozzle, the velocity of fluid increases while the pressure drops. The difference in pressure before and after is measured using differential pressure transmitters, also known as secondary elements.

What is the working principle of a Differential pressure flowmeter?

In the differential pressure principle, manometer tubes measure the difference between static pressure upstream and downstream of the restriction. The fluid accelerates to a higher velocity when the fluid passes through the restriction.

The higher velocity results in decreasing in the static pressure. The measure of the flow rate is the differential pressure (Δ). The higher the differential pressure, the more will be the flow rate. You can drive the relationship between flow rate and differential pressure using Bernoulli’s equation.

How is the Differential pressure flowmeter calculated?

As the fluid passes from the tube, it accelerates. This causes the line pressure to drop at the point of constriction. The following equation gives the volumetric flow through the pipe:

Q = KA (ΔP/P)

Q = Volumetric flow

K = discharge coefficient

A = cross sectional area of pipe opening

ΔP = Differential pressure

P = density of flowing fluid

The discharge coefficient is influenced by Reynolds number and beta ratio. To measure the flow rate, the flow transmitter calculates the differential pressure. The d/p cell connects the upstream and downstream sides of the flow sensor.

What is meant by calibration of Differential pressure flowmeter?

Differential pressure flowmeter is popular in various industrial and domestic applications. Liquid and gases have different properties, so this affects their flow rate as well. There are different types of flow meters, and each requires a complex calibration using a master meter.

Calibrating an instrument involves checking the output of the flowmeter to given inputs at several points. The master meter is the standard calibrated transmitter that uses a comparison point for an operating transmitter calibration. These calibration methods use international standards.

How to calibrate a Differential pressure flowmeter?

The calibration of the Differential pressure flowmeter starts by connecting the flowmeter and standard calibrator in series. You need to run both at the same time and under the same pressure. There are five checkpoints in which the calibration is done. Mark 5 points as five checkpoints, i.e. 10%, 30%, 50%, 70%, and 90%.

Calculate the input from standard and operating flowmeter. Compare the readings of both devices. If the results show the difference in readings, then there is a correction factor. Tune the Differential pressure flowmeter till there is zero correction factor. Repeat the same procedure for five checkpoints and tune the flowmeter accordingly.

What are the standard calibration methods in Differential pressure flowmeters?

When you are calibrating the Differential pressure flowmeters, three standard calibration practices are used, which include:

• Zero trim
• Sensor trim
• Analog output trim

When the Differential pressure flowmeter is in usage, issues like mounting problems may arise. You can use the zero trim procedure for the correction of any zero error. In the sensor trim procedure, first, you have to verify the calibration of the sensor at the upper and lower trim points. Sensor trim should be performed on the differential pressure sensor, and if sensor outputs are within the specifications, no sensor trim is required.

In analog output trim, you need to adjust the current output by doing a loop test. The test measures the current meter to the loop readout meter. If both do not correspond, carry out an analog output trim procedure.

What is the rangeability of the Differential pressure flowmeter?

The performance of the Differential pressure flowmeter installation depends on the flow element’s precision and the accuracy of the DP cell. You can get the flow precision in the percentage of actual reading (AR) terms, whereas the accuracy of the DP cell is the percentage of calibrated span (CS). The percentage of DP cells is ±0.2% of the calibrated span.

The Differential pressure flowmeter is limited to use within a 3:1 and 4:1 range. The rangeability of the flowmeter can be increased by operating various differential pressure flowmeters in parallel runs. You can also stack two or more flowmeters onto the same element, one for 1-10% and the other for 10-100%.

What is the accuracy of the Differential pressure flowmeter?

The accuracy of the Differential pressure flowmeter is usually ±0.1% CS, and the errors arise due to rangeability and linearity. Potential errors are exempted due to temperature, pressure, humidity, radio frequency interference, and power supply variation.

If you include these errors, then the accuracy of the transmitter is stated as ±0.2% CS. Differential pressure flowmeter ranges between two calibrated spans. You can get orifice installations with 1% AR inaccuracy over a 10:1 flow range.

How to do the installation and maintenance of a Differential pressure flowmeter?

With the flow measurement in the Differential pressure flowmeter, you also measure the process temperature and pressure. You should not install the pressure flowmeter in the process pipe but connect it to the lead line. The installation of thermowell for pressure measurement needs to be ten diameters downstream of the flow element.

You need to install straight pipe runs for upstream and downstream of the differential pressure meter. Flow straighteners such as a tube or internal tabs need to install on upstream of the flow element. Adaptors or couplings will cover the connections on the pressure holes.

What are various installation methods for Differential pressure flowmeters?

When you are installing the Differential pressure flowmeter, you can use three different techniques.

• Direct pipe installation is a method that is simple and requires fewer materials.
• Flange installation is the technique more common in the measurement of liquids. In this method, you use the static pressure of the liquid for measuring the liquid level.
• Bracket installation is the most popular, adopted, and convenient way of installing a Differential pressure flowmeter. It is dustproof, rainproof, and maintenance-free for five years.

While installing the Differential pressure flowmeter, ensure that the differential pressure on positive and negative pressure sides is appropriate.

What are the operating regulations of Differential pressure flowmeters?

It would be best to use the handheld communicator for range adjustment and accuracy of the Differential pressure flowmeter. Once you adjust the range, you should do the calibration of the flowmeter. For calibration, first, open the balance valve. First, close the low-pressure inlet valve and then close the high-pressure inlet valve. Afterward, open the high-low pressure valve.

Adjust the pressure to its maximum range. Check whether the data range is consistent with the range of the indoor computer. Remove the calibration instrument and slowly close the balance valve. If there is disassembly in the Differential pressure flowmeter, turn off the flowmeter and re-correct it.

What are the elements of a Differential pressure flowmeter?

There are three essential elements of a Differential pressure flowmeter. The primary element is inside the pipe that maintains the change in pressure. The transmitter reads the pressure change and converts it into readable numbers.

The third element is the structure that connects the primary element and the pressure created by the transmitter. This structure includes tubing, impulse lines, valves, and other mechanical tools.

What are the Reynolds numbers for Differential pressure flowmeter measurement?

Osborne Reynolds is famous for his study of flow. You can use Reynolds numbers for predicting turbulence. It helps you in finding out how fluid behaves in different scales. A Reynolds number is the ratio of inertial forces to viscous forces within the fluid. These numbers can find out the range and applicability of the Differential pressure flowmeter.

The Reynolds number, Re, is expressed as:

Re=VDρ/µ

Where V stands for velocity of the fluid, D stands for diameter, ρ is density, and µ is the absolute velocity. Reynolds number denotes the ratio of inertial forces to viscous forces. At low Reynolds number, viscous forces predominate, and inertial forces have little effect. At high Reynolds number, viscous forces have little effect, and inertial forces prevail.

What are the common errors in Differential pressure flowmeters?

In any Differential pressure flowmeter system, there are three sources of error, DP transmitter errors, pressure and temperature variation errors, and primary element errors. For the best working of the flowmeter, you need to avoid these errors.

There can be accuracy issues with the flowmeter even if you have a well-installed and well-maintained flowmeter. In outdoor applications, the temperature may vary more than fifty degrees Fahrenheit from calibration temperature. These variations can affect accuracy. Also, in some applications, flow error may rise in gas and steam applications.

How to reduce pressure loss in a Differential pressure flowmeter?

Different sources lead to pressure loss in the Differential pressure flowmeter. These sources could be valves, pipe friction, measurement devices, etc. When it comes to measurement devices, more flow meters will lead to more pressure loss. When designing the system, you need to consider permanent pressure loss (PPL), as it is crucial in sizing the pump and compressor to deliver the desired pressure.

By minimizing the PPL in the system, you can increase the size of the pump, compressor, or boiler, which will result in accuracy. This saving in PPL loss will help you save dollars which can add to the company’s profit.

Conclusion

The Differential pressure flowmeter is among the most common technologies for measuring flow. It has a long history of having accuracy and reliability. Through this technology, you can get the compensated flow, pressure, and temperature readings. They are easy to install and increase overall productivity.

We have over 20 years of experience in manufacturing high-quality and affordable Differential pressure flowmeters in Instruthink. We can supply you with Differential pressure flowmeters within a week and assist you in providing technical support. If you have any queries, you can contact us at sales@instruthink.com.