The operation, advantages and limitations of different types of steam flowmeter, including orifice plate, variable area and vortex shedding devices.
Types of Steam Flowmeter
The principle of operation for a ‘transit-time’ ultrasonic flowmeter is based on measuring the time it takes for ultrasound pulses to pass between two transducers attached to the pipe of the fluid being monitored. (Figure 4.3.18). Each transducer alternatively fires pulses of ultrasound where the time it takes for each pulse to reach the other transducer is affected by the velocity of the fluid flowing through the pipe. By knowing this information, flow velocity can be calculated, leading to the volumetric and mass flowrates of the fluid being monitored. This is covered in more detail in Module 4.2 – Principles of Flowmetering.
A typical application for ultrasonic flowmeters is energy monitoring, where Resistant Temperature Detectors (RTD’s) form part of the transducers’ assembly. The RTD’s measure the temperature of the flowing liquid, allowing the rate of energy flowing through the pipe to be calculated, using the equation below:
One of the greatest benefits of an ultrasonic flowmeter is that the transducers or RTD’s are externally mounted. This means that there is no invasive installation requiring pipeline genetration or pipeline shutdowns. In addition, with no moving parts or components in the flow being measured, there are no issues concerning corrosion and erosion, so minimising maintenance requirements. Any maintenance that is required can be carried out without the need to shutdown the pipeline.
Ultrasonic flowmeters are best suited to monitoring liquids, such as measuring condensate return. The fluid passing through the pipe being measured must be single phase, in other words, the line must be flooded. Ultrasonic flowmeters cannot accurately measure a mixture of water and steam or air, for example.
Advantages of ultrasonic flowmeters:
• Quick and simple installation, requiring no plant downtime, as all components are externally mounted.
• Bidirectional flow measurement.
• Highly accurate (up to 1% of flowrate).
• Can be used to measure energy flow.
• Fluid conductivity not an issue.
• Corrosive fluids not an issue.
• A turndown of 30:1 is achievable with the correct installation.
• Cost of unit is independent of pipeline size, making it commercially attractive for larger pipelines.
Disadvantages of ultrasonic flowmeters:
• For single phase liquids only.
• 10-30D straight pipeline lengths required.
• Not as accurate as in-line flowmeters.
• Unreliable if there is more than 5% gas or vapour in the pipeline.
Typical applications for ultrasonic flowmeters:
• Liquid flowmetering: As with all liquids, care must be taken to remove air and gases prior to them being metered. If the unit is used for condensate flowmetering it is important that the line is flooded and no live or flash steam is present.
• Energy monitoring for heating and cooling applications.
Every ultrasonic flowmeter operates within minimum and maximum signal strengths to provide accurate measurement readings. If the signal strength is too weak the flowmeter will not detect flow and if the signal strength goes beyond the maximum signal strength specified the pipeline will become ‘flooded’ and the signal received will result in inaccurate flow measurement. For optimum results the signal strength should be in the range specified by the manufacture.
Vortex shedding flowmeters
These flowmeters utilise the fact that when a non-streamlined or ‘bluff’ body is placed in a fluid flow, regular vortices are shed from the rear of the body. These vortices can be detected, counted and displayed. Over a range of flows, the rate of vortex shedding is proportional to the flowrate, and this allows the velocity to be measured.
The bluff body causes a blockage around which the fluid has to flow. By forcing the fluid to flow around it, the body induces a change in the fluid direction and thus velocity. The fluid which is nearest to the body experiences friction from the body surface and slows down. Because of the area reduction between the bluff body and the pipe diameter, the fluid further away from the body is forced to accelerate to pass the necessary volume of fluid through the reduced space. Once the fluid has passed the bluff body, it strives to fill the space produced behind it, which in turn causes a rotational motion in the fluid creating a spinning vortex.
The fluid velocity produced by the restriction is not constant on both sides of the bluff body. As the velocity increases on one side it decreases on the other. This also applies to the pressure.
On the high velocity side the pressure is low, and on the low velocity side the pressure is high.
As pressure attempts to redistribute itself, the high pressure region moving towards the low pressure region, the pressure regions change places and vortices of different strengths are produced on alternate sides of the body.
The shedding frequency and the fluid velocity have a near-linear relationship when the correct conditions are met.
The frequency of shedding is proportional to the Strouhal number (Sr), the flow velocity, and the inverse of the bluff body diameter. These factors are summarised in Equation 4.3.3.
The Strouhal number is determined experimentally and generally remains constant for a wide range of Reynolds numbers;which indicates that the shedding frequency will remain unaffected by a change in fluid density, and that it is directly proportional to the velocity for any given bluff body diameter. For example:
Then the volume flowrate qv
in a pipeline can be calculated as shown in Equation 4.3.4:
Advantages of vortex shedding flowmeters:
• Reasonable turndown (providing high velocities and high pressure drops are acceptable).
• No moving parts.
• Little resistance to flow.
Disadvantages of vortex shedding flowmeters:
• At low flows, pulses are not generated and the flowmeter can read low or even zero.
• Maximum flowrates are often quoted at velocities of 80 or 100 m/s, which would give severe problems in steam systems, especially if the steam is wet and/or dirty. Lower velocities found in steam pipes will reduce the capacity of vortex flowmeters.
• Vibration can cause errors in accuracy.
• Correct installation is critical as a protruding gasket or weld beads can cause vortices to form, leading to inaccuracy.
• Long, clear lengths of upstream pipework must be provided, as for orifice plate flowmeters.
Typical applications for vortex shedding flowmeters:
• Direct steam measurements at both boiler and point of use locations.
• Natural gas measurements for boiler fuel flow.