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The Impact of Reynolds Number on Differential Pressure Flow Measurement

By
22. January 2024

How to select a suitable thermowell

Differential pressure (DP) flow measurement is a widely employed technique in industrial applications for accurately quantifying fluid flow rates. One crucial factor that significantly influences the accuracy of DP flow measurements is the Reynolds number. The Reynolds number is a dimensionless parameter that characterizes the flow regime and helps predict the transition between laminar and turbulent flow. In the realm of flow measurement, understanding the implications of Reynolds number is vital for obtaining reliable and precise results.

Advantages of Orifice Plates

What is Reynolds Number?

The Reynolds number (Re) is a dimensionless quantity used to predict the flow patterns in different fluid flow situations. It is defined as the ratio of inertial forces to viscous forces within a fluid. Mathematically, Re is expressed as:

 

Where:

  • ρ is the fluid density
  • V is the fluid velocity
  • D is the characteristic dimension (e.g., pipe diameter)
  • μ is the dynamic viscosity of the fluid

Reynolds Number and Flow Regiments

The Reynolds number is a critical parameter for predicting the flow regime. There are 3 types of flow regimes; Laminar, transitional and turbulent. Laminar flow occurs when the Reynolds number fall below 2000. Above 4000, the flow is considered turbulent. Values in between is called transitional flow.

In laminar flow, the fluid moves smoothly and predictably, with well-defined streamlines. The fluid velocity close to the wall is close to zero and at the center of the pipe there are not retaining forces. This means the fluid is flowing in different velocities across the flow profile. Laminar flow is seen at either low velocity or high viscosity fluids.

On the other hand, turbulent flow is characterized by chaotic, irregular fluid motion. This results in most of the fluid is moving at the same velocity.

The transition between laminar and turbulent flow is influenced by the Reynolds number.

Impact on DP Flow Measurement

95% of DP flow measurement devices, such as orifice plates and venturis are designed for turbulent flow. The turbulent flow creates an even pressure distribution across the measurement device as opposed to laminar flows. This is evident when looking at discharge coefficient (Cd) over a wide range of Reynolds numbers. Cd is a dimensionless factor that relates the actual flow rate to the theoretically predicted flow rate. In laminar flows, Cd is variable. In turbulent flows, however, Cd is constant. Illustrated in the figure below. The higher the Reynolds number, the more reliable the flow measurement will be.

Conclusion

The Reynolds number can be helpful to understand how the flow acts, which is crucial for obtaining accurate and reliable results. Engineers must carefully consider the flow regime and choose appropriate measurement devices and calibration procedures to ensure the accuracy of flow rate measurements across varying Reynolds numbers. By appreciating the complexities introduced by different flow regimes, engineers can enhance the precision and reliability of DP flow measurements in diverse industrial applications.

Choosing the Right Flow Meter: Orifice Plates vs Averaging Pitot Tubes

By
6. October 2023

How to select a suitable thermowell

Differential pressure (DP) flow measurement is a crucial process in various industries, from oil and gas to carbon capture storage and water treatment etc.. Two common devices used for DP flow measurement are orifice plates and averaging Pitot tubes. Both have their advantages and are suited for different applications. In this article, we will explore the advantages of each and compare them to help you make an informed decision.

Advantages of Orifice Plates

1) Simplicity and Reliability

Orifice plates are known for their simplicity. They consist of a plate with a precisely drilled hole to create the differential pressure. This simplicity makes them reliable and easy to maintain and replace. They have no moving parts, removing the risk of mechanical failure.

2) Cost-effective

Orifice plates are cost-effective compared to many other flow measurement devices. The initial investment is relatively low, making them a favorable option for projects with budget constraints.

3) Wide Range of Applications

Orifice plates can be used in a wide range of applications, from clean liquids to gases with solids. They are versatile and suitable for both low and high-pressure systems.

4) Well-Documented Standards

Orifice plate design and installation follow well-documented industry standards, ensuring accuracy and repeatability in measurements.

Advantages of Averaging Pitot Tubes

1) Multiple Bores

They provide a precise measurement of flow velocity by averaging the pressures at multiple points within a pipe, reducing the impact of turbulence.

2) Low Permanent Pressure Loss

Averaging Pitot tubes have lower permanent pressure loss compared to orifice plates. This means they are not as affected by the pressure and flow rate, making them efficient for long-term operation.

3) Easy Installation

Averaging Pitot tubes can be installed in existing pipe works without adding flanges. This significantly lowers the total cost a flow meter installation.

4) Minimal Maintenance

These devices have no moving parts, reducing the need for frequent maintenance. Their robust design allows them to withstand harsh conditions. A purge unit can be added if the process consists of solids that can cause blockage.

Comparison of Orifice Plates and Averaging Pitot Tubes

Accuracy

If the focus is high accuracy, then the orifice plate should be the preferred choice. The averaging Pitot tube requires a quite long straight pipe run to be able to deliver the same high accuracy. However, the repeatability of the Pitot tube is quite high, so the accuracy is reliable and constant.

Cost

Orifice plates as a standalone product are generally more cost-effective than averaging Pitot tubes, however looking at the full installation the orifice plate requires orifice flanges meaning the total cost of a flow measurement can surpass the cost of the Pitot Tube

Pressure Loss

Averaging Pitot tubes have a much lower permanent pressure loss.

Versatility

Both Orifice plates and Pitot tubes are versatile and suitable for a wide range of applications. When pipe sizes become larger than 500-750 mm, Pitot tubes scalability takes the advantage.

Maintenance

Both devices are low-maintenance due to their lack of moving parts. The installation of the Pitot tube makes it extremely easy to replace.

Conclusion

In summary, orifice plates and averaging Pitot tubes are both valuable tools for differential pressure flow measurement, each with its own set of advantages. Orifice plates are cost-effective, reliable, and versatile, making them a solid choice for many applications. On the other hand, averaging Pitot tubes offer lower installation cost, lower pressure loss, and is better great for larger pipe sizes

The choice between these two devices ultimately depends on your specific application, budget, and accuracy requirements. Consider the nature of your fluid, the level of accuracy needed, and the long-term operating costs to make an informed decision. Regardless of your choice, proper installation and maintenance are essential to ensure accurate and reliable flow measurements.

What You Need to Know About Classical Venturi Tubes

By
16. August 2023

How to select a suitable thermowell

EMCO Controls manufactured its first venturi tube in 1987, which was also the first delivery to an international customer. The Venturi tube therefore paved the way for our international endeavours and has been a core product ever since. In this article, we will offer you a surface-level understanding of this important piece of flow instrumentation. 

Definition

A venturi tube is a short pipe with an inner constriction that can be used to measure fluid. The Venturi tube is designed by and named after the 18th–19th-century Italian physicist Giovanni Battista Venturi, who noted the effects that constricted channels have on fluid flow. Based on his observations, he designed a device with a narrow throat in the middle, causing a phenomenon known as the Venturi effect. 

The Venturi effect states that the pressure will drop when a fluid passes through the tube and enters the narrow throat, which is the opposite of what one would immediately think. This relationship between pressure and velocity can be used to mix certain substances such as gasses or fluid with air at a continuous rate.

The natural design of the Venturi makes it suitable for applications requiring a low permanent pressure drop. 

Applications

The Venturi effect is widely used in industry and everyday products to mix different substances at controlled rates. Here are some examples of where it is used:

  • in barbecues, gas stoves, bunsen burners and airbrushes
  • the choke on a car engine
  • the air pump on a fish tank
  • atomizers that disperse perfume or spray paint
  • foam firefighting extinguishers nozzles
  • sand blasters – draw sand in and mix it with air

Types

There are two different types of Venturi tubes which each have their own specific advantages. The 2 types differ in such a way that the body of the Venturi tube can either be welded from rolled sheet or machined from solid bar. This results in a difference in accuracy, with the machined process ensuring a more precise internal diameter which gives it an accuracy of 1% as opposed to 1,5% for welded sheet Venturies. 

The machined Venturi tube allows us to make a Venturi body with large wall thickness and is well-suited for: 

  • High pressure applications i.e., high pressure steam from boiler process or high-pressure gas applications i.e. LNG process.
  • Producing quite small Venturi tubes i.e. 2” to 4”, where fabrication processes are less time consuming and therefore more cost effective.

The welded sheet Venturi is welded together in sections thus making it suitable for larger Venturi diameters in low pressure applications. Welded sheet Venturi tubes are ideal for:

  • air flow intakes i.e. primary and secondary air intakes in boiler applications.
  • flue gas or exhaust gas going to chimney/stack
  • low pressure water application i.e. feed water process
  • low pressure steam applications i.e. industrial applications

Related Products

Classical Venturi Tube Machined

Graphic model of machined venturi tube
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Classical Venturi Tube Welded Sheet

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The Importance of Restriction Plates and How They Differ From Orifice Plates

By
25. July 2023

The Importance of Restriction Plates and How They Differ From Orifice Plates

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There are many important factors to look at when selecting and sizing restriction plates. Although there are highly advanced control valves available in the market, there is a remarkable usage of restriction plates in the piping industry. A restriction plate is mainly used to control the flow of the fluid or to achieve pressure restriction. Based on the requirements, the restriction plate should be sized for critical or pre-critical conditions.

In general, pressure control restriction plates are sized by considering the maximum pressure drop lesser than the critical pressure. Moreover, the flow-controlling restriction plates are sized for critical pressure drops. The thickness of the plate is calculated as per the R.W. Miller handbook and sizing is conducted as per ISO 5167-2.

The restriction plate should be selected based on the flow rate and required pressure drop. The following are the types of restriction plates:

Restriction plates, as well as orifice plates, are both used to create a pressure drop. Both of these are based on Bernoulli’s principle, which states that when the pressure drops across the restriction plate, it is directly proportional to the volumetric flow rate that passes through the orifice plate. Furthermore, restriction plates and orifice plates are similar in structure. 

 

So what is the difference between a restriction plate and an orifice plate? A restriction plate works on the same principle as an orifice plate, but it provides a different purpose. The main difference is their usage. A restriction plate is used for overcoming pressure in a pipe by raising fluid velocity, while an orifice plate is used for measuring flow rate. The restriction plate is also thicker. When the fluid passes through the thick plate, energy is lost in heat and friction, resulting in significant pressure drop.

 

 

Gas Calibration of Flow Meters

By
14. April 2023

How to select a suitable thermowell

Calibration of a flow meter

Our flow meters are generally designed and constructed according to ISO 5167, which describes in detail the geometry, construction, requirements, and calculations of various types of flow meters.

What is calibration of a flow meter?

It is an empirical method of finding the discharge coefficient1. The calibration institute sets up our meter in their pipeline, where they have a way of knowing the exact flow, usually by the way of one or more calibrated coriolis meters. The standard only has a few requirements regarding calibration. The main one is that the meter must be calibrated to the full range of operational Reynolds numbers2.

Why should a flow meter be calibrated?

There are a few reasons to calibrate flowmeters.

  1. To verify the accuracy of the meters. This is often a client requirement, especially in the offshore industry.
  2. To improve on the accuracy of the meter. Depending on the type of meter, the accuracy can vary from 0,5 to 5%, according to ISO 5167. Clients often have strict requirements on the accuracy of the flow meter, ranging anywhere from 5% to 1%. When calibrated, depending on the institute and exact conditions of the calibration, this can be improved all the way down to 0,2%.
  3. To confirm function and accuracy of meters which are beyond the scope of the standard. The standard has set limitations in terms of size, beta, and Reynolds number. These are often exceeded, especially for a lot of the larger or high-pressure meters which are produced.

What requirements are needed of a Calibration Facility?

All the calibration facilities used by EMCO Controls are certified. This, as a bare minimum, means they are accredited to ISO17025, which is the requirements for the competence of testing and calibration laboratories. This is certified by both DANAK and ILAC MRA. They often have numerous additional certifications as well.

Why gas calibration?

Generally, meters are calibrated on the phase medium as the operational use, even though this is not a requirement. This is usually for practical reasons; a meter meant for gas will usually require extremely high flow of water to reach the same Reynolds numbers. As most of the meters, EMCO products are meant for hydrocarbon gas applications, the logical solution is a gas calibration. This choice is only helped by the fact that we have one of the most capable calibration institutes in the world, FORCE Technology, just a short drive away, and with the opening of their MEGA Loop in May 2023, they will double both their flow and size capacity.  

1 What is a discharge coefficient?

It is a corrective factor, which tells how close to the theoretical that the meter performs. For example, a venturi has a discharge coefficient of 0,995, where as a cone meter has one of 0,82. This is due to the difference in geometries and the placement of the pressure tappings.

2 What is Reynolds number?

Reynolds number is essentially an indicator of the behaviour of the flow. In very rudimentary terms, it describes if the flow is laminar or turbulent. However, a requirement of the standard is that all flow is turbulent, in other words, a Reynolds number of over 5,000 to 200,000, depending on the type of flow meter. Reynolds number is calculated as:

where Re is the Reynolds number, ρ is the fluid density, V is the velocity in the pipeline, D is the pipe inner diameter, and μ is the dynamic viscosity of the fluid.

Basics of Primary Elements for Flow Measurement

By
6. March 2023

How to select a suitable thermowell

Our primary elements measure the flow of liquids, gases and steam according to the differential pressure principle. The primary elements are widely used in many industries including power stations and other thermal installations, chemical, and petro‑chemical, offshore and water industry. In this article, we are covering the basics of primary flow elements for flow measurement. 

Principle of Measurement

Flow measurement according to the differential pressure principle is based upon the law of energy balance developed by Bernoulli: the sum of dynamic and static energy remains constant in a circular pipe where the fluid is fully contained. The fluid must be in one phase and changes of flow shall be slowly i.e. without pulsation.

A restriction (an orifice plate or similar) in a pipe will change the combination of energy. The velocity in the throat will increase and consequently, the pressure will decrease. The pressure before and after the restriction is measured and the applied differential pressure is an expression of flow velocity. Between the differential pressure DP and flow Q there is a square root relationship which is expressed in the following formula:

The formula can be reduced to:

In the constant K, many factors are contained such as the geometrical shape of the restriction, the pressure tapping, ratio of diameters and the condition of fluid characterised by the pressure, temperature, viscosity, density and other factors known by experience.

Calculation and Manufacturing Standard

Primary elements should be calculated and constructed according to international standards. The most common standards used by EMCO Controls are:

  • ISO 5167 “Measurement of fluid flow by means of orifice plates, nozzles, venturi tubes, Cone Meters and Wedge Meters inserted in circular cross section conduits running full”
  • ISO/TR 15377 
  • The American standards ASME MFC-3M, ASME MFC-14M 
  • The German standard DIN 1952 “Durchflussmessung mit Blenden, Düsen und Venturirohren in voll durchstromten Rohren mit Kreisquerschnitt DIN 19205‑19215, VDI/VDE 2040/41” 

Upon request, primary elements are manufactured according to other American standards such as L.K. Spink, AGA No. 3, R.W. Miller : Flow measurement Engineering Handbook and Shell Flow meter Engineering Handbook.

Accuracy

Equipment for flow measurement according to the principle of differential pressure consists of two elements:

  1. The primary element
  2. The differential pressure transmitter and signal conditioning electronics elements.

The overall accuracy of the flow meter depends on the accuracy of the individual parts plus the use of the correct physical values, i.e. pressure, temperature, density and for gasses expansion factors. The accuracy is calculated according to ISO 5168.

Basic Tolerance and Additional Tolerance

The basic tolerance depends on the construction of the individual primary element (orifice plate, venturi, nozzle). For an orifice plate, the basic tolerance varies with the diameter ratio “β” which is equal to d/D. Additionally, the basic tolerance is 0,5% for “β” while it is less than 0,6, and equal to “β” at greater diameter ratio. For venturi nozzles, the tolerance is (1,2 + 1,5 x b4 )%. For classical venturi tubes, the tolerance is 1% for the machined type, 1,5%, the welded sheet type and 0,7% for the “as cast” type. These values are based on the ISO standard, whereas the ASME standard states more conservative figures. In addition to the tolerances mentioned above, the manufacturing and the installation tolerances are not taken into account.

During manufacturing of the primary elements, special care is taken of the bore of the orifice plates – the sharp edge and the surface of the upstream side of the plate. Special jigs are to be used to ensure narrow tolerances. In the case that the sharp edge is rounded slightly, the discharge coefficient C will change resulting in an error in flow reading.

When installing a primary element in a pipe run, the minimum requirement for the straight pipe run up and downstream must be taken into consideration. Upstream disturbances influence the symmetrical flow profile and lead to incorrect flow measurement. Examples of disturbances are 90° elbow, two 90° elbows in two planes, control valves and thermowells.

The most common used instrument for measuring the differential pressure is the  differential pressure transmitter. Due to the square root relationship between differential pressure and flow, it is very important to choose an instrument which is very accurate, especially when a wider rangeability is required. In some cases, the use of only one transmitter is not sufficient when a wide rangeability and high accuracy is required. By using two transmitters or more, the range is divided and each transmitter covers a fraction of the total range. Additionally, the signal conditioning equipment switches between the transmitters according to the actual flow rate.

The multivariable transmitter offers a big step forward for flow measurement using primary elements. This transmitter combines 4 instruments into 1 unit. The transmitter not only measure the differential pressure, but also the static pressure and can convert the signal from an external temperature sensor. The transmitter also has a flow computer for calculation of mass flow. The flow computer performs a dynamic correction on constants. With a total accuracy (orifice plate and transmitter) of 1 %, a rangeability of approximately 10 : 1 is achievable. The accuracy of venturis and nozzles is inferior to orifice plates.

Permanent Pressure Loss

The previous mentioned transformation of energy in a restriction from pressure to velocity and back to pressure results in a permanent pressure loss. The pressure loss depends on the chosen primary element, and b value.

The permanent pressure loss for orifice plates is between 35% and 80% of the calibrated span of the transmitter. Thanks to the square root relationship between differential pressure and flow, the drop in the permanent pressure loss is also the square root to the measured differential pressure. For instance, at 2/3 flow, the pressure loss is reduced to half.

The divergent in a classical venturi tube and short venturi nozzle results in a higher pressure recovery without turbulence and a considerably lower pressure loss. The pressure loss is 7‑10% for classical venturi tubes and 10-15% for venturi nozzles of the calibrated span.

Correction for Physical and Mechanical Factors

The calculation of the bore “d” is normally based upon ISO 5167 and ASME. In order to use the discharge coefficients C mentioned in the standards, the physical properties of the fluid must be constant. If this is not the case, the discharge coefficient C must be corrected. This is easily done with modern micro‑processor technology in the multivariable transmitter, the flow computer or in the main process control computer.

The need for correction depends on the required accuracy plus the magnitude of the variations. Most common is flow correction for gas and steam flow. The change in gas temperature from 20 to 26 degrees Celsius, which is equal to 2% of the absolute temperature, results in an error of 1%. More rarely, it is necessary to correct for specific gravity of a liquid.

 

Viscosity

A fluid can flow in a pipe run in two forms: laminar and turbulent. By laminar flow, the velocity profile forms a parabola whereas by turbulent flow, the profile has a “flat front” almost covering the whole cross section area of the pipe.

The velocity profile depends on the dimension less Reynolds Number which is a relationship between flow velocity V, inner pipe diameter D and kinematic viscosity γ.

At low Reynolds No., Re less than 2300, the flow is laminar in the pipe. Orifice plates are following the ISO 5167 standard when Reynolds No. is higher than 5000 at max. flow. This gives a margin of safety for turbulent flow through the restriction at reduced flow.

For viscous liquids (low Reynolds No.) other discharge coefficients and shapes of the orifice plates must be used. If high accuracy is required, it is recommended to calibrate at service condition. In the chart below, the range of the different primary elements can be seen.

For correct calculation of the bore “d”, calculated from a computer programme, it is recommended to use our questionnaires. The standards refer to the primary elements with inner pipe diameter greater than 50 mm and less than 1200 mm. For primary elements with inner pipe diameter less than 50 mm, a wet calibration must be foreseen if high accuracy is required. In many modern process plants with high pressures and high velocities, the calculated Reynolds No. is higher than specified in the standards. However, the standards say in the appendix that this may only lead to a slight increase in inaccuracy.

Installation of Primary Elements

The primary element must be mounted in a straight pipe run of the same size. It is crucial for correct flow measurement that the straight pipe run up- and downstream are following the standards. The length of straight pipe run depends on two things: the diameter ratio and the disturbance upstream.The requirement is at least 10 times the upstream inner diameter and 4 times downstream of the primary element. Worst case is 90° elbows in 2 plans plus a high “β” which requires 80 times inner pipe diameter of straight pipe run – although 20‑30 times is the most common requirement.

If the required straight pipe run is not available, a flow straightener can be used. Different types are available each with their own advantages and disadvantages. The most common is the “Bundle of Tubes” which usually consists of 19 tubes fastened together and to the main pipe. The length of the tubes depends on D ‑ the inner diameter of the pipe. The inner roughness of the pipe has to be small in order to not influence the flow profile. This consideration is mainly necessary in the smaller sizes. Additional considerations for installation include:

  • Installation of a primary element must be as far away as possible from any pulsation.
  • For liquid flow measurement, the liquid shall run full in the pipe.
  • The connecting tubes between the pressure tubes and the differential pressure transmitter shall always be mounted with a fall.
In air and gas flow measuring, the best place to mount the transmitter is above the primary element allowing any condensation to run backwards into the main pipe. In liquid flow measuring, it is best to mount the transmitter below the primary element enabling any air or gas to escape into the main pipe. Finally, for steam flow measuring it is recommended to use condensing chambers placed in the same horizontal height. The differential pressure transmitter is mounted below the primary element.

The condensing chambers should be half filled with water. The pressure of the water column on the plus and minus side of the transmitter is the same and has consequently no influence on the accuracy. The “+” side of the orifice plate is connected to the “+” side of the DP-transmitter and the two “-” sides are connected. The impulse lines must be installed with a slope to let captured air escape. The impulse lines should not be less than 12 mm and in a material suitable for the service condition. The primary element is normally supplied with single or double isolating valves.

As you can see, there are many considerations when choosing primary elements for measuring flow. Our job is to translate many years of accummulated expertise into precise and useful guidance, ensuring optimal application of our flow measurement solutions. We achieve this best through dialogue with our customers. You can take the next step here.

 

How to Select a Suitable Thermowell

By
1. February 2023

How to select a suitable thermowell

In many pressurised processes it is often not possible or advisable to insert the temperature instrument directly into the media being measured. A thermowell protects the temperature instrument, permits the instrument to be exchanged or calibrated, and keeps dangerous or expensive media within the process installation. To make a proper selection of a thermowell one must also take into account the thermal lag (response time), the sensing accuracy as well as the service condition, i.e. pressure, temperature, velocity, corrosiveness and so on. To help you make the correct selection, we will highlight the different configurable elements of the thermowell.

Instrument Connection

The thermowell is provided with a 1/2″ NPSM internal thread connection. This female connection may be used with NPT or NPSM mating threads. The NPSM thread gives better quality in mechanical connection without seizing or galling.

Shank Configuration

For general purpose, where additional strength is not required, the straight shank or stem is used. The shank diameter is 20 mm (3/4″). The last 63.5 mm (2 1/2″) of the shank steps down to 13 mm (1/2″) to provide faster response. Tapered thermowells are designed for use in high velocities, where extra strength is required. Special attention shall be given to the vibration effects caused by the fluid passing the thermowell. 

The fluid will form a wake, known as a “Von Karman Trail”. The wake has a specific frequency, which is a function of the diameter of the thermowell and the fluid velocity. If the wake frequency coincides with the natural frequency of the thermowell, the well will vibrate to destruction.

Bore

The thermowells are internally gundrilled to a diameter suitable to match the stem or bulb of the temperature sensing instrument. The standard bore diameters are 7 and 10 mm, but other bore diameters are available i.e. according to SAMA standard: 1/4″ and 3/8″ nominal bore (0,260″ and 0,385″).

Insertion Length

The insertion length, often called “U”, is the portion of the thermowell, which is inserted into the process line (not necessarily into the fluid). The length of the insertion is determined by the installation. The tip of the well shall be immersed into the fluid, where a representative temperature is ruling and long enough to accommodate the length of temperature sensing devise of the instrument. The insertion lengths are 63.5 mm (2 1/2″) to 572 mm (22 1/2″).

Instrument Insertion Length

The length of the instrument insertion is standardised to accommodate standard temperature instruments or determined by the instrument, where these are not according to a standard. The lengths are 102 mm (4″) to 610 mm (24″).

Lagging Extension

Thermowells with lagging extension are used in pipe systems and on vessels, where these are insulated. The length of the lagging is determined by the thickness of the insulation. The standard lagging extension is 89 mm (3″) except for thermowells with insertion length of 63.5 mm (2.5″), where the lagging extension is 60 mm (2″). Other lagging extensions are available on request with multiple of 89 mm (3″) to provide for standard instrument insertion length.

Process Connection

The thermowells are provided with 4 main methods of process connection: thread, flange, welding, and clamp. Threaded thermowells are the most commonly used wells, because of the low cost and the ease of installation. Threaded thermowells are not recommended for pressure above 70 bar (1000 psi). The standard thread sizes are 3/4″ and 1″ NPT. 

Flanged thermowells are used, when the pipe spec. calls for flange executions and at elevated pressure. They are supplied with flanges according to B 16.5 with raised face or ring type joint for pressure classes from 150 – 2500 lbs. The flanged thermowell may either have the flange welded on the shank (stem), or the flange and shank may be forged as an integrated unit for use in severe service conditions. Flange sizes are : 1″, 1 1/2″, and 2″. 

Weld-in thermowells are used at extreme pressure and temperature service conditions, and when ASME codes require welded connections. The weld-in type thermowell is a low cost item, but the disadvantage is that the well is not removable for inspection or replacement. The standard weld-in size is 38 mm (1 1/2″), 26.9 mm (3/4″ nominal pipe size) or 33.4 mm (1″ nominal pipe size).

Materials

The thermowells are normally manufactured in stainless steel AISI 316 (L, Ti) or materials to customers’ requirements and service conditions.

EMCO Controls Anticipates a 10-fold Increase in Sales of Float Level Switches by 2025

By
9. June 2022

How to select a suitable thermowell

Danish instrumentation company Emco Controls has carved a solid niche for itself in supplying the shipbuilding industry with float level switches and anticipates a 10-fold increase in sales of the product over the next 3 years. It is already selling 10 times as many as it did just four years ago and is satisfied with the current level, though declines to specify number of switches sold.

According to Mads Lisberg, CEO of the company based near Copenhagen, the surge in sales is due to a good design. He says: “We believe Denmark is known for good design and when you do a good design we’re following that same tradition that Denmark is known for.”

He adds that the company had enjoyed modest sales of float level switches for about 30 years to shipbuilders and the process industry. Float level switches are used to monitor liquid levels in containers and sound an alarm when a certain level is reached, either high or low. They can also sound when a pump is starting or stopping.

Emco says its sales received a boost four years ago when it was approached by Norwegian company Odim, which expressed an interest in selling the switches to the shipping industry. This led to a new design and construction. Today, it has buoyant sales and is hungry for new markets. Emco was founded in 1966 by Enevoldsen og Mogensen and is owned by Lisberg, who bought the firm in 1984.

Lisberg says the company listed five demands it wanted met for the new design. The switch had to be lightweight, modern, highly anti-corrosive, production costs had to be low, and finally the construction had to be flexible enough to accommodate customer wishes.

The switches are still made at EMCO’s factory in Hillerød near Copenhagen in line with ISO 9001 standards. While shipping is a major segment, with clients such as shipyards and product agents, other sectors for the company include power stations, water treatment plants, as well as the pharmaceutical and chemical industries.

Emco has supplied switches for AP Moller-Maersk’s eight container vessels built at the Odense Steel Shipyard in Denmark. “We think there’s a big market for us and for a product that is well designed and does something specific…We’re going after yards, shipowners and firms that supply marine equipment. It is particularly seeking to tie up with Far Eastern yards that are building Maersk vessels.

It also makes and sells flow, level and temperature instrumentation devices.

What You Need to Know about Cavitation When Using Restriction Plates

By
9. June 2022

How to select a suitable thermowell

Cause and effect of cavitation in your system

Flow measurement of liquid, gas and steam according to the differential pressure principle has been recognised principle for many years using orifice plates, venturi tubes and flow nozzles. A restriction in the pipeline creates a pressure drop when the fluid flows. The pressure drop is determined by the velocity of the fluid.

The method is thoroughly described in many standards, practices and books. Orifice plates can also be used for pressure control and flow limitation. This special use is not covered by international standards and is only scarcely mentioned in literature.

Restriction Orifice Plates

For a restriction orifice plate the flow quantity through the orifice will vary with the pressure before and after the orifice plate.

The restriction orifice plate is mostly applied in non-critical flow limitation. The use of the expression “non-critical” must not be compared to the word “critical” as in critical flow devices. Non-critical is here to be understood as non-complicated with less requirements for the accuracy.

This could for instance be after a control valve in order to divide the required pressure loss into two elements. This is done to reduce the noise in liquids and to avoid cavitation. It is advisable to avoid cavitation in order to protect the process elements before the restriction orifice plate in blow-down systems.

Liquid flow

If the pressure in Vena Contracta (because of high velocity) decreases to the vaporisation pressure of the liquid, a cavitation zone is created. This cavitation zone will act as a flow limitation.

Cavitation

Cavitation occurs frequently in liquid flow restrictions where large pressure drops exist. According to the Equation of Bernoulli the liquid flow velocity increases when the bore area is reduced causing a pressure drop to occur. The lowest static pressure is reached at highest velocity in Vena Contracta. If the lowest static pressure is lower than the vapour pressure, steam bubbles will form.

After the liquid has passed through the restriction, the velocity decreases, and the static pressure increases. This leads to a collapse of the formed vapour bubbles. The collapse of the bubbles generates noise and will cause the plate as well as the pipe to erode. It is therefore important to avoid a full cavitation through a liquid flow restriction. Consequently, to avoid incipient or full cavitation it is important that the differential pressure does not create a static pressure in Vena Contracta that is lower than the vapour pressure.

When the steam bubbles collapse caused by the increased pressure, chock waives are formed resulting in extremely high static pressures lasting for an ultra-short period of time. The internal part of the pipe downstream will be damaged due to this in combination with the erosion caused by the local heating of the inner pipe. Cavitation generates loud noise which sounds like pebbles running in the pipeline.

How to combat cavitation

Cavitation can be eliminated in different ways depending on the severity of the cavitation. A multistage single hole or multi hole depressurizing unit is reducing the total required pressure drop in stages. The number of stages depends on the pressure drop, the plate cavitation factor FL and the selected hole design (single or multi). The distance between each plate must not be less than D, preferably 1,5 – 2 X D

A differential pressure of ΔP as indicated below, should be used:

ΔP < FL(P1 – Pv)

  • FL = Critical flow factor or cavitation factor
  • P1 = Upstream pressure
  • Pv = Vapor pressure

The FL factor depends on hole design – single hole, multi hole, cylindrical or conical holes. The FL value is between 0,5 and 0,7


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