Flowmeters enable energy management
Building owners, manufacturers and institutions must seek ways to make their facilities more energy efficient with less environmental impact.
In today’s business climate, keeping energy costs in line has never been more important. Energy has become a hot topic for businesses, governments and consumers. For many, finding new and better ways to reduce energy usage and increase energy efficiency is a high priority. As a result, energy management is receiving a lot of emphasis.
Measurement is fundamental to energy management because to control it, you first must measure it. For example, building and campus managers invoice tenants for electricity, water, natural gas, boiler water for heating and chilled water for air conditioning and cooling. These utilities must be measured, and their use accumulated with respect to time to produce accurate billing.
This article focuses on flow measurement technologies as they apply primarily to natural gas. Topics involving steam and water are included for comparison and/or context as needed. Flowmeter types are discussed as well, although the list is not exhaustive.
Fundamentals of flow
Mass flow rate can be calculated from the density of the substance being measured, the cross-sectional area through which the substance is flowing and the substance’s velocity relative to the area of interest. This is reasonably straightforward if the density is constant. Flow measurement is a quantification of an amount of fluid or gas passing through a pipe or a duct. Flow can be quantified as a volumetric flow rate such as gal/min, or a mass flow rate such as lb/hr. The rate is always some unit of measure with respect to some unit of time.
The density of some fluids may change when subjected to changes in temperature, pressure or composition. Some fluids also may have combined phases or entrained gas bubbles. Varying densities make mass flow measurement more complex — but more so with gases than with liquids.
There are many technologies and designs for measuring flow, but not every flow measurement technology lends itself to energy monitoring applications. For a flow measurement technology to be a good candidate for use in energy monitoring applications, it must be cost-effective, noninvasive and easy and inexpensive to install. The technology must also have reliability and accuracy appropriate for the application.
In manufacturing, flow measurement is essential to operate a process plant that manufactures products from raw materials or adds value to a product. The demand for accurate flow measurement instruments is driven by business demands or environmental restrictions such as a need for tighter process controls leading to reduced emissions and increased efficiency.
Each flow sensing technology and each flowmeter has advantages and disadvantages. Flow sensing technologies in widely used flowmeter devices on the market include:
- Differential pressure (DP)
- Positive displacement
- Coriolis (or mass flow)
DP devices include orifices, venturi tubes, flow nozzles, pitot tubes, averaging pitot tubes and V-cone. Positive displacement devices include rotary lobe, diaphragm and turbine.
Note that there are many more flowmeter types on the market. However, devices intended primarily for fluids are not covered in this article.
DP flowmeter operation requires placing a flow-restricting device such as an orifice plate, flow nozzle, pitot tube, or venturi tube in the fluid, gas or steam flow path, and measuring the pressure differential across it. Orifice plates are the most widely used restriction and typically create the most pressure loss (see Figure 1). The rate of steam flow varies with the square root of the pressure drop across this restriction. A DP transmitter measures the drop in pressure across the restriction.
Although orifice-type DP flowmeters typically have a low initial cost, no moving parts, and can be installed inexpensively, they have a high-pressure loss, low rangeability, and lose accuracy over time. Orifice plates can eventually wear enough to affect flowmeter calibration.
Many flow measurement instruments installed today are based on DP-sensing technologies. DP transmitters account for around half of all flow measurement transmitters shipped annually. The improvements in measurement range, accuracy, repeatability and the ability to make multiple measurements, provided by intelligent multivariable pressure transmitters have added life to this mature but still viable technology.
Positive displacement flowmeters
Positive displacement meters measure flow by mechanically displacing a moving part in the meter, then counting the number of displacements per unit of time. Each count represents a volumetric amount of fluid. At each count, the fluid passes through the meter. The energy required to drive the moving parts of the meter is supplied by the pressure of the fluid being metered.
Rotary lobe flowmeters. A rotary lobe flowmeter directly measures the actual volume of fluid that passes through it at the actual operating pressure. Two counter-rotating lobed impellers rotate within a casing. Gas or liquid flowing through the meter drives the impellers, which trap a known volume of gas in the interspace. The flow volume is proportional to the speed of rotation.
The volume is measured by counting revolutions and multiplying the count by the known volume displaced with each revolution. Counting can be accomplished mechanically or electronically. The most common arrangement is to embed magnets in the lobes. These are sensed by proximity switch pickups mounted in the meter casing. Switches driven by the magnets activate local digital displays or are input to an integral electronic system for processing, correction and remote communication.
Rotary lobe meters are used mainly for natural gas billing for commercial and industrial customers when loads exceed 1,000 cubic feet per hour (CFH). They are used mainly for gases and liquids in industry and in oil and gas production. They also have been used in industrial submetering applications for energy management. The main reasons for their use are high accuracy and ruggedness. ANSI/ASC B 109.3 approves these meters for custody transfer.
Rotary lobe flowmeter advantages include:
- High accuracy.
- Low cost due to large production quantities.
- Very low turn-down ratio.
- Able to meter fluids with low Reynolds Number.
- Reliable and rugged. Low maintenance costs.
Other factors include:
- The volume flow must be corrected to standard conditions when operating at fluid temperatures and pressures outside the calibration range. This function is done automatically when the meter is used as a billing meter.
- A bypass with manual isolation valves is recommended in case of mechanical failure.
Diaphragm flowmeters. A diaphragm flowmeter comprises an outside casing charged with natural gas (or other gas) and houses two gas bellows (diaphragms). Each diaphragm has a slide valve that alternately opens and closes the inlet valve and the outlet valve to each diaphragm. As gas flows into the meter, the bellows fill up and empty alternately by means of slide valves, which act as counters. As each valve closes and then opens, one known volume of gas is counted. A mechanical linkage connects each valve to a counter that registers a count and a display that shows it on a dial. The gas is discharged into the meter casing and to the meter outlet. The two sets of bellows and valves operate alternately to provide a continuous gas flow. The mechanism is driven by gas pressure on the bellows and the result is a small pressure drop across the meter.
Diaphragm meters employed in billing applications for natural gas are installed with a pressure regulator upstream to maintain constant pressure. This eliminates the need for a pressure correction transmitter. Temperature variations can cause error. However, in practice, the temperature changes are minor when gas is supplied via underground piping.
Diaphragm flowmeter advantages include:
- Inexpensive and accurate.
- Can be used for small energy management projects.
- Pulse output and communications capabilities are available.
Diaphragm flowmeter disadvantages include:
- Should be used with clean gases only.
- Large, heavy and expensive in large sizes.
Turbine flowmeters. Turbine flowmeters are available in a wide range of sizes, types and prices. They are used in many applications in industry and for billing purposes by water and gas utilities. Utility-type turbines for metering gas and water are approved for custody transfer. Turbine flowmeters are approved by AGA report 7. They are normally supplied with corrected mass flow reading electronic outputs and communication capability. A prominent use of turbine meters is metering and billing large volume customers (see Figure 2).
Industrial turbine flowmeters are also available as inline and insertion types. Insertion turbine flowmeters are available in a wide range of sizes and materials from ½ inch to 36 inches and larger. Because of their ability to meter large pipe flows, they are often used for stack gas monitoring. They cover a wide range of applications in industry for metering steam, water, air flow and other gases.
Industrial turbine flowmeters directly measure the velocity of the fluid or gas stream, QV. Together with the velocity measurement, the mass flow can be calculated with P and T inputs. A flow computer or multi-variable smart transmitter at the meter is required.
A rotor with attached blades is suspended in the fluid or gas stream on free running bearings. The rotational speed (RPM) of the rotor is directly proportional to the fluid or gas velocity. There are several ways of counting rotations of turbines including from magnetic and electrical transducers embedded in the rotor or blades. These transducers produce a weak pulse or sine wave electronic output, which is converted electronically to a flow velocity QV, then to a 4-20 mA output signal from the transmitter. The mass flow is calculated by multiplying the volumetric flow by the fluid density. Mass flow equals flow velocity times density times pipe area.
Qm = QV * r * A
A corrected mass flow requires additional inputs including pressure and temperature.
Inline industrial turbine flowmeters are suited to energy management applications because they are available in a wide range of diameters and are lower in cost.
Industrial, inline turbine flowmeter advantages include:
- Good accuracy and repeatability.
- Wide size range.
- Minimal upstream straight lengths of pipe required.
- May be low cost.
- Low installation cost, insertion type.
Industrial, inline turbine flowmeter disadvantages include:
- High installation cost, in-line type.
- Moving parts-susceptible to contaminants.
Coriolis flowmeter technology provides direct mass flow measurement that is independent of changes in pressure, temperature, density and fluid viscosity. Coriolis flowmeters come in a variety of configurations including U-tube, twin U-tube, bent tube and straight tube designs, all of which operate on the following principle:
- The fluid or gas to be measured enters one end of the flow tube, which is subjected to an external oscillating drive, an electromechanical oscillator that vibrates at the natural frequency of the tube (around 10 kHz) depending on tube design and fluid or gas density. Flow tubes are commonly made from 316 stainless steel or other metals such as titanium depending on the application.
- As fluid or gas streams through the flow tube, it is subjected to an upward or downward force caused by the external oscillation driver. The Coriolis Force opposes the driving force, pushing upward on the tube from the inflow side and downward on the outflow side, causing the tube to twist. The twisting motion is also an oscillation.
- Sensors located on each leg of the flow tube detect the position and frequency of the tube as it twists. There is a phase shift between the two sensors as mass flow changes. This phase shift is directly proportional to the mass flow of the fluid or gas.
- The Coriolis meter is equipped with a microprocessor-based signal processing system that interprets the raw signals from the sensors and the frequency shift being measured, converting these to signals that can be used by standard instrument and data systems such as 4-20 mA, pulse and voltage outputs.
The Coriolis flowmeter has a wide range of applications. The first applications were in the petroleum and chemical industries measuring many kinds of liquids, mainly for process control purposes. Because of its high accuracy, it is also used for custody transfer in the oil and gas industry, AGA report 11. Coriolis flowmeters have been used to measure high viscosity fluids, slurries and mixed phase fluids such as oil and water, water with bubbles, pulverized coal in air, etc.
Compatible fluids include crude oil, cryogenic liquids, polymers, asphalt, fuel oil, paints, nitric acid, phosphoric acid, molten sulfur, sodium hydroxide and tar sands. Food products include beer, fruit juice, milk, pie fillings and peanut butter.
More recently, design improvements and increased precision have made Coriolis flowmeters a candidate for metering gases. They are capable of metering industrial gases and have been used for customer transfer of natural gas for large users (see Figure 3). They also can be applied to low temperature gaseous or liquid oxygen, nitrogen and CO2.
Coriolis mass flowmeter advantages include:
- Direct mass flow reading, no correction for pressure or temperature.
- High turndown ratio: 20:1 is standard, 100:1 as required.
- No need for straight pipe runs upstream and downstream.
- Capable of metering multiple liquids, gases, slurries and mixed-phase flow.
- Stainless flow tubes are standard.
- Capable of metering cryogenic gases, some models.
- Very high accuracy and repeatability.
- Low maintenance.
- Custody transfer approvals.
Coriolis mass flowmeter disadvantages include:
- High cost compared to most other meters.
- Moderate PPL, except straight-through Coriolis.
Vortex flowmeter operation is based on a principle called the “von Karman effect.” The von Karman effect makes use of fluid passing a non-streamlined or “bluff” body placed in the flow stream generates vortices that are shed from the rear of the body. These vortices can be detected, counted and displayed. The frequency of the vortices is proportional to the flow rate of the fluid or gas. The shedding frequency and the fluid velocity have a near-linear relationship under ideal conditions.
Typical vortex flowmeter applications include direct steam measurement, both at the boiler and point of use, and natural gas measurements for boiler fuel flow. Steam is the most difficult fluid to measure because of the high pressures and temperatures involved, and because measurement parameters vary according to steam type. Vortex flowmeters are preferred for steam flow measurement because of their ability to tolerate these high process pressures and temperatures.
Vortex flowmeters are very reliable. They also have wide rangeability, which means they can measure steam flow at varying velocities. Other advantages of vortex meters include ease of installation, low maintenance, moderate installed costs and good accuracy. However, they are somewhat sensitive to vibration and inlet flow.
Multivariable vortex flowmeters include pressure and temperature sensors. In addition to flow rate, temperature and pressure, the flowmeter uses these sensors to determine volumetric flow, fluid density and mass flow. Multivariable vortex flowmeters perform the necessary mass flow compensation without a separate flow computer.
– This article appeared in the Gas Technology supplement.