aNamePEN no 1.Harsh Patel Prince Patel Ronak Trivedi Yusuf Khan Guided by: M. M. Chhatbar Subject: CPI-1

FLOW MEASUREMENT Industrial flow measurements include measuring of flow rate of solids, liquids and gases. There are two basic ways of measuring flow: one on volumetric basis and the other on weight basis. Flow can be measured in a variety of ways. Positive-displacement flow meters accumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow. Other flow measurement methods rely on forces produced by the flowing stream as it overcomes a known constriction, to indirectly calculate flow.

The most common principals for fluid flow metering are: Differential Pressure Flowmeters Velocity Flowmeters Positive Displacement Flowmeters Mass Flowmeters Open Channel Flowmeters

WHAT IS A DIFFERENTIAL- PRESSURE (ΔP) METER? Differential-pressure meters work on the principle of partially obstructing the flow in a pipe. This creates a difference in the static pressure between the upstream and downstream side of the device. This difference in the static pressure (referred to as the differential pressure) is measured and used to determine the flow rate. The popularity of differential-pressure flow meters is in part due to their simple design and low cost.

WHAT ARE THE DIFFERENT TYPES OF ΔP METER? WHAT ARE THE DIFFERENT TYPES OF ΔP METER? The most common types of differential pressure meter are: Orifice plates Venturi tubes Cone meters (e.g. V-cones) Nozzles Low loss meters (e.g. Dall tubes) Variable area meters Inlet flow meters Venturi cones Venturi nozzles Drag plates

ADVANTAGES AND DISADVANTAGES OF ΔP METERS There are a number of general advantages common to most Δp meters. These include: They are simple to make, containing no moving parts Their performance is well understood They are cheap – especially in larger pipes when compared with other meters They can be used in any orientation They can be used for most gases and liquids Some types do not require calibration for certain applications

The main disadvantages to Δp meters are: Rangeability is less than for most other types of flow meter Significant pressure losses may occur The output signal is non-linear with flow The discharge coefficient and accuracy may be affected by pipe layout or nature of flow They may suffer from ageing effects, e.g. the build-up of deposits or erosion of sharp edges

HOW A ΔP FLOW METERS WORKS HOW A ΔP FLOW METERS WORKS The concept of using the pressure drop caused by a fluid flowing through a restriction in a pipe as a measurement of flow rate dates back to the 18th Century, when it was discovered by Bernoulli.

The differential pressure principle: Manometer tubes measure the difference in static pressure upstream and downstream of the restriction. When a fluid flows through a restriction, it accelerates to a higher velocity (i.e. V2 > V1 ) to conserve the mass flow and, as a consequence of this, its static pressure drops. This differential pressure (Δp) is then a measure of the flow rate through the device. Using Bernoulli’s equation, and conservation of mass, it can be shown that the differential pressure generated is proportional to the square of the mass flow rate, Qm (kg/s).

COMMON TERMINOLOGY Beta (β) The diameter ratio or beta (sometimes referred to as the beta ratio) is the ratio between the diameter of the orifice or throat of device to that of the pipe. Discharge coefficient (C) The discharge coefficient is basically the ratio of the actual to the measured mass flow rate.

Turndown of a Δp meter The turndown of a meter is the ratio of the maximum to the minimum flowrate that can be accurately measured. Ideally a large a turndown ratio is desirable to measure a wide range of flow rates.

DIFFERENT TYPES OF COMMONLY USED ΔP METERS 1. Orifice plates Orifice plates are the most common type of Δp meter and are basically a machined metal plate with a hole. The plate has a sharp upstream edge and usually a bevelled edge downstream of the flow.

To allow the differential pressure to be measured, a set of pressure tappings are located upstream and downstream of the plate.

When fluid passes through the hole of an orifi The flow continues to contract and converges downstream of the plate with the point of maximum convergence (or minimum area) called the vena contracta ce plate the pressure drops suddenly. The fluid then expands and re-attaches to the pipe wall, and the velocity profile approaches that before the constriction. There is a relatively large net pressure loss across the orifice plate which is not recovered; this should be taken into account in choosing a meter as orifice plates are not suitable for applications where a large pressure drop is undesirable.

The face with the sharp edge must face the upstream flow as the sharp edge is needed to force the flow to detach from the plate and allow the flow to contract downstream of the plate and form the vena contracta.

Advantages: Low cost Ease of installation Availability of comprehensive standard (ISO ) No requirement for calibration - value of C from the standard Availability of different designs, e.g. for viscous fluids, bi-directional flows, suspended solids

Disadvantages: Low turndown (can be improved with dual range Δp cells) High pressure loss (35 to almost 100% of measured Δp depending on beta) Errors due to erosion / damage to upstream edges Errors due to high sensitivity to upstream installation (especially large beta devices)

2. Venturi tubes This type of meter has a gradual reduction in the pipe area, a parallel throat section and then a gradual expansion back to the full pipe diameter. The long expansion section (diffuser) enables an enhanced pressure recovery compared with that of an orifice plate, which is useful in some metering applications. Venturi meters are must less susceptible to damage than orifice plates owing to their robust and solid design. Venturi meters have a much lower pressure loss across the meter (approximately 10% of Δp); this means less energy is lost compared with that across an orifice plate with identical beta and pipe diameter.

Venturi tubes are more expensive than orifice plates owing to the increased machining necessary for their manufacture. In reality many Venturi tubes are calibrated to determine a value for C for applications where higher measurement accuracy is required especially in gas when the uncertainty of an uncalibrated Venturi tube is high.

Advantages: Low pressure drop (around 10% of Δp) Lower sensitivity to installation effects than orifice plates Less susceptibility to damage More suitable for gas flows with entrained liquid Comprehensive standards (ISO 5167)

Disadvantages: Low turndown (can be improved with dual range Δp cells) Greater cost to manufacture Greater susceptibility to “tapping errors” in high Reynolds number gas flows owing to the high velocity fluid passing the pressure tapping at the throat. Less experimental data than orifice plates

3. Cone meters Cone meters (e.g. V-cones) are proprietary meters and are essentially an inverted Venturi tube. Instead of a contraction in the pipe, the fluid flows around a central cone as shown in the following diagram.

Various designs are available and the downstream tapping can either be located in the base of the cone or machined through the wall of the meter body at the widest part of the cone, as shown below. The upstream pressure tapping is located before the cone.

Advantages: Lower sensitivity than orifice plates and Venturi tubes to installation effects Shorter installation lengths Less pressure loss than orifice plates Wafer (between flange) versions Effective for wet gas flow measurement applications Disadvantages: Lack of standards Not as much data available as “ISO 5167” meters Pressure loss higher than Venturi tubes

4. Flow nozzles Flow nozzles are mainly used in the electrical power generation industry. They have a curved entry and a cylindrical throat, but no divergent outlet section. Therefore, the discharge coefficient is similar to that of a Venturi tube, but the overall pressure loss is similar to that of an orifice plate of comparable size used at an equivalent flowrate and pressure difference.

One advantage of a nozzle over an orifice plate is that there is no sharp edge to erode, but they are more expensive to manufacture and are generally more difficult to install or remove from the pipe for maintenance purposes.

Common issues There a number of issues that can occur with the use of certain types of Δp meters. Some of these are discussed below. Static hole error Calibration issues Wear and tear Incorrect installation Maintenance

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