The following post is Part II in a two-part series considering the selection of valves and flowmeters and piping requirements for integrated rotating equipment systems.
There is a growing demand for integrated systems that include rotating equipment — such as a pump, a compressor, gas turbine, etc. — and all of the auxiliaries — such valves, instrumentation, and piping — in one or a few skids.
Machinery and rotating equipment packages require sophisticated instruments, valves, and control systems, and special consideration should be given when selecting these component solutions. For example, many instruments and valves can be susceptible to damage, abnormal wear, or malfunction if mounted in a location where they are subject to vibration or pulsation from the larger system. If any part of the flow instrumentation, control valves, or actuation equipment may be subject to vibration or pulsation in a machinery package, the affected instruments should receive special attention, such as vibration-free supports or other provisions.
Here we consider the selection of flowmeters and piping requirements commonly employed for integrated rotating equipment systems.
Flow can be measured in a variety of ways, such as differential-pressure flowmeters, variable-area flowmeters, magnetic flowmeters, turbine flowmeters, positive-displacement flowmeters, vortex flowmeters, and others.
Differential-pressure flowmeters measure flow inferentially from the differential pressure caused by flow through a primary element. Flow is proportional to the square-root of the differential pressure produced. To maintain accuracy at low-flow readings, a range greater than 4-to-1 is not recommended. This differential is sensed by diaphragms, bellows, or manometers.
Differential pressure is the most commonly used method of flow measurement. The sharp- (square-) edged concentric orifice plate is one of the most frequently used elements because of its low cost and adaptability and the availability of established coefficients. For most services, orifice plates are made of corrosion-resistant materials; often type 304 or 316 stainless steels. Eccentric orifices or segmental plates should be used for very dirty fluids or wet gases. Quadrant orifices should be used for viscous liquids.
Advantages of orifice plates include good repeatability, ease of installation, use of one transmitter regardless of pipe size, low cost, the wide variety of types and materials available, and the relative ease with which they can be changed. Limitations of orifice plates include their susceptibility to damage by foreign materials entrained in the fluid and to erosion. Straight runs of upstream and down-stream piping are required for an orifice plate.
Venturi and flow tubes are used with differential-pressure flowmeters where high capacity and minimum head loss are critical factors. Their advantages are good repeatability and low permanent loss. The limitations of Venturi- and flow tube-based differential-pressure flowmeters include relatively high cost and the size and weight of the installation, which may require additional support. They are usually the most expensive differential-pressure producer. Straight runs of upstream and downstream piping are also required for a Venturi or flow tube, similar to other types of differential-pressure flow measurement devices.
Differential-pressure transmitters of the diaphragm type are extensively used in different units and packages. The transmission signal may be either pneumatic or electronic, in most cases, electronic. Because of their low displacement, these instruments can generally be used without a seal or condensate pot. Line mounting is preferred if the location is accessible and has minimum vibration. Gas meters are mounted above the line to allow any condensate to drain back. Liquid meters are mounted below the line to prevent gas or vapor from being trapped in the sensing lines, which could cause errors from unequal static heads.
Another type is the bellows flowmeter. In a bellows meter, the bellows is opposed by a calibrated spring system and is filled to prevent rupturing when the bellows is over-pressured and to provide pulsation damping. Bellows meters can be line-mounted or remotely mounted at grade, or on platforms with adequate support.
Variable-area flowmeters (rotameters) work on the principle that a float within a vertical tapered tube will assume a position that is a function of the flowrate through the tube from the bottom. The float should have a density greater than that of the measured fluid. The annular area through which the flow should pass is the difference between the internal area of the taper tube at the point of balance and the area of the float head, since the internal area of the tube increases constantly and is continuously variable from bottom to top, whereas the float head area remains constant. At a constant differential pressure, flow is directly proportional to area.
Advantages of variable-area meters include wide flow range (for example, l0-to-1), linear (nearly linear) transmitter output, and minimal effect of gas compressibility. Limitations of variable-area meters include their lack of availability in all materials, viscosity ceiling limits, which are provided by manufacturers and should be observed, and the need for vertical installation.
Turbine flowmeters measure volumetric fluid flow with a pulse train output, the frequency of which is picked up magnetically from a rotor located in the flowstream and is (approximately) linearly related to flowrate. Turbine meters are used where their high accuracy and rangeability are required. Outputs from turbine flowmeters are suitable for control or recording applications, and they are also ideally suited for batch control applications. Compensation for nonlinearities due to viscosity is also available. Accuracy of 0.5 percent of rate with a repeatability of 0.2 percent or better is normal. Rangeability usually varies depending on meter design, fluid viscosity, density, and meter size. A high flowrate for a given line size is obtainable. Designs for very low flow-rates are also available. Turbine meters are available for a wide range of temperature and pressure ratings, and specially designed meters can accommodate bidirectional flow.
Limitations of turbine flowmeters include their susceptibility to wear or damage if the process stream is dirty or non-lubricating. They are also susceptible to damage from over-speed and pulsing flow. They require maintenance and may require return to the manufacturer for recalibration after a bearing change or other maintenance. Their rangeability is affected by high viscosity and low density. Their cost is relatively high, and they require strainers.
Positive-displacement flowmeters measure flow by mechanically trapping successive volumetric segments of the liquid passing through the meter body. The basic types of positive-displacement meters are oscillating piston, fluted rotor, rotary (lobed impeller and sliding vane), and oval-shaped gear. Positive-displacement meters measure flow by mechanically trapping successive volumetric segments of the liquid passing through the meter. The number of segments is converted to shaft rotation. A gear train and calibrator convert shaft rotation to the appropriate volumetric units. Temperature compensators are available to correct the output as the fluid temperature changes. Pulse generators are available to provide pulse outputs for meter proving or remote readout.
Positive-displacement meters are used because of their excellent repeatability over wide flow ranges. They can be used for heavy or viscous fluids. Positive-displacement meter support accuracies as high as 0.2 percent of actual flow. Ensuring high accuracy requires some form of meter proving. Typical repeatabilities are 0.1 percent, and rangeability of 10-to-1 is achievable.
Advantages of positive-displacement meters include excellent rangeability and accuracy, particularly with heavy or viscous fluids. They come in a range of sizes. Disadvantages include the susceptibility to mechanical wear. They are not interchangeable, and should be supplied to match the service. They require filter or strainer, and their installation requires special considerations.
Vortex flowmeters use an obstruction in the flowing stream to generate a vortex train of high- and low-pressure areas. A vortex train is generated when a bluff-body obstruction is placed in a liquid or gas stream. This train of high- and low-pressure areas can be measured by sensors on the body or the pipe wall. The frequency of pressure changes is usually linear to the velocity of the fluid stream. Since flow in any pipe is a function of cross-sectional area and velocity, a direct relationship exists between frequency and flowrate. Vortex meters are used in applications that require wide rangeability and accuracy. Vortex have an accuracy of 1 percent of rate, and they are available in a wide range of sizes. Vortex flowmeter limitations include a limited range of construction materials, and they are generally not suitable for slurries or high-viscosity liquids.
Coriolis mass flowmeters measure mass units directly. Fluid flow through a tube vibrating at its natural frequency produces a Coriolis force. The resulting tube deflections are measured and signaled proportionally to the generated mass flow. A Coriolis flowmeter can be used with liquids, including liquids with limited amounts of entrained gas, and slurries. A Coriolis flowmeter can also be used with dry gases and super-heated steam if the fluid’s density is high enough to operate the unit properly.
Although Coriolis meters are nonintrusive, in some designs, the flow path through the meter is circuitous. In addition, the flow is generally separated into two tubes that are much smaller in cross-sectional area than is the inlet process piping. For this reason, it is relatively easy for any secondary phase to build up in a Coriolis flowmeter that has not been carefully installed. The pressure loss can be substantially higher than other types of nonintrusive elements, and cavitations and flashing can be problems with volatile fluids. Improper installation can result in start-up problems, so installation should be strictly in accordance with the manufacturer’s recommendations. Pressure containment enclosures are available when required.
Coriolis flowmeters are not affected by distortion of the velocity profile and do not usually require metering runs. Although Coriolis flowmeters generally cost much more than other types, they measure mass flowrate without the need for additional elements. The applications for these flowmeters have been limited to difficult fluids or applications in which their accuracy justifies the higher cost.
Pulsation in machinery package
The measurement of pulsating flow, such as flows in a reciprocating compressor or pumps, is difficult and it should preferably be avoided if possible. Head-type flow-meters and instruments with mechanical movements, such as positive-displacement meters and turbines, should not be used in pulsating-flow applications. Too often, the flow measurement is not dependable, and the pulsing may contribute to premature wear of the mechanical components.
Amin Almasi is a senior rotating machine consultant in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical), IMechE (CEng MIMechE), holds bachelor’s and master’s degrees in mechanical engineering, and is a registered professional engineer in Queensland. He specializes in rotating machines, including centrifugal, screw, and reciprocating compressors, gas turbines, steam turbines, engines, pumps, subsea, offshore rotating machines, LNG units, condition monitoring, and reliability. Mr. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 100 papers and articles dealing with rotating equipment, condition monitoring, offshore, subsea, and reliability. He can be reached at email@example.com.