Product category:
Level and tank contents instrumentation
News Release from: Emerson Process Management - Mobrey Measurement | Subject: Displacer transmitters
Edited by the Processingtalk Editorial
Team on 14 February 2005
Displacer transmitters in the
hydrocarbon industry
The displacer transmitter for liquid level measurement is based on Archimedes principle, that the buoyancy force exerted on a body immersed in a liquid is equal to the weight of the liquid displaced
Archimedes' principle states that the buoyancy force exerted on a body immersed in a liquid is equal to the weight of the liquid displaced This is the principle on which the displacer transmitter for liquid level measurement is based
This article was originally published on Processingtalk on 22 Apr 2003 at 8.00am (UK)
Related stories
Flow monitoring system saves Beckton STW GBP20k
Innovative use of a new differential level monitoring system from Solartron Mobrey has saved Thames Water thousands of pounds on a flow meter refurbishment project at Beckton Sewage Treatment Works
Boiler controls to be launched in Europe
Solartron Mobrey launches the industry-leading Erab liquid level measurement systems for boiler control across Europe: the range offers high-reliability and long-term measurement stability
If the cross sectional area of the displacer and the density of the liquid are constant, then a change in level brings about a corresponding change in the apparent weight of the displacer.
Displacer transmitters have provided highly reliable level measurement in difficult hydrocarbon applications for many years.
The measurement technology is simple, reliable, accurate and adaptable to a wide range of needs, including the measurement of an interface between two immiscible liquids.
Further reading
Tankered domestic waste load monitoring system
Solartron Mobrey, leading manufacturer of measurement and control sensors, with Meteor Communications Europe, provides an innovative water industry solution for measuring tankered domestic waste
Helping water companies meet MCERTS Certification
Bestobell Service, the service arm of Solartron Mobrey, has a new MCERTS package to help water companies meet the Environment Agency's new legislation regarding the self-monitoring of effluent flow
Successful steam measurement leads to new service
Bristol-based Fulton Boilers, renowned both for its products and its ability to provide solutions for every type of industrial application, turned to Solarton Mobrey to measure accurate steam flows
Importantly for hydrocarbon applications it can be used at very high temperatures and pressures, when most other technologies fail.
There are two types of displacer transmitter in common use today; torque tube and spring operated.
Both have a cylindrical displacer element of a length corresponding to the range of the level measurement required and weighted to sink in the liquid being measured.
In both the maximum change in effective weight of the displacer element is equivalent to the weight of the liquid displaced when the displacer is completely submerged in the liquid.
It is important to also take into account the effect of the upper fluid which, even if a vapour, will have an effect on the buoyancy force, particularly if the vapour space is at a high pressure.
The difference between the two types of displacer transmitters centres around the mechanics of transmitting the displacer movement because of the buoyancy force from the wetside of the instrument to the dryside where it can be translated into an electronic, or in earlier designs, a pneumatic, signal proportional to the liquid level change.
With a torque tube design, the displacer element is suspended on a knife edge hanger at the end of a cantilever arm, the other end of which is welded to the torque tube.
The torque-tube is a hollow tube welded at one end to the instrument flange which is put in torsion by the weight of the displacer element on the cantilever arm.
A rod, welded to the torque tube at one end but free at its other end, sits inside the torque tube and is thus caused to rotate axially as the torque tube rotates.
When the displacer rises or falls, the corresponding angular displacement of the torque rod is linearly proportional to the displacer movement and therefore to the liquid level.
The knife-edge bearing support minimises friction and a limit stop on the torque arm is used to prevent accidental over-stressing of the torque tube.
With regular maintenance, this type of design is proven to measure reliably.
There is a huge installed base in the hydrocarbon and other industries and the technology is well understood.
It is suitable for use in very high pressures - up to about 17Mpa/170bar (2,465psi) and in process temperatures from 200C (-328F) to more than 450C (842F).
However, it is a bulky instrument which can be awkward to install.
As a mechanical device with a critical knife edge bearing, it requires constant, careful maintenance to ensure continued accuracy.
And finally, because the design relies on a welded pressure joint at the flange end of the torque tube, regular inspection for signs of fatigue or corrosion is essential.
The newer spring-operated displacement transmitter is a more elegant design that overcomes many of the problems associated with torque-tube devices.
Just as reliable as the torque-tube, it is smaller, lighter and more robust.
In a spring-operated instrument, the change in apparent weight of the displacer is transmitted directly, through a spring or coil from which the displacer weight is hung.
When the displacer rises or falls with changing liquid level the spring will relax or extend accordingly as dictated by the formula: Spring extension (contraction) = Force/Spring Rate.
A core piece is located on top of a rod attached to the spring and is thus caused to rise or fall inside the pressure tube.
A precision linear variable differential transformer (LVDT) is situated outside the pressure tube, totally isolated from the process pressure and vapour.
Movement of the core within the fields of the LVDT causes an imbalance which the instrument electronics detects and is able to convert into a signal proportional to the liquid level.
It should be understood that the spring is a heavy duty coil made from typically three or four mm (0.125ins) gauge specially selected alloy wire.
The coil is always selected such that it is operating at about 10% of its yield stress, ensuring maximum sensitivity to changes in the force on it, without the possibility of over-stressing.
Mechanical stops prevent over extension or coil bound operation.
The best instruments on the market are those with coils made from Nimonic, a nickel alloy which gives the spring a perfectly linear expansion over the full operating temperature range of the instrument giving highly accurate level measurement.
Key advantages of the spring operated transmitter are that it has a much smaller mounting envelope than a torque-tube, it is lighter and much easier to install and does not have critical welds under stress.
However, the operating range is not quite as wide; spring-operated devices are typically suitable for use in pressures up to about 25Mpa/250bar (3,600psi) and in process temperatures from 260C (-436F) to about 300C (572F), although specifications do vary between manufacturers.
Whichever transmitter technology is chosen for the application, the size and weight of the displacer is crucial, since it determines the relationship between the change in the apparent weight and the liquid level.
The optimum displacer diameter for any one application depends on the density of the process liquids, the process operating conditions, and the level measurement span.
It is important at the ordering stage to give the manufacturer the correct data so that the instrument can be sized correctly and be calibrated to give the correct level reading at the process operating conditions.
The inclusion of powerful microprocessor electronics and digital communications in modern displacer transmitters does however give the user the facility to trim, re-calibrate or re-range the instrument very easily on site.
As mechanical devices, displacer transmitters have traditionally needed regular maintenance, cleaning and checking of the calibration.
If you are thinking of investing in this sort of instrumentation then it is worth looking into this aspect thoroughly, since some instruments need significantly more work than others.
The best spring-operated displacement transmitters offer very stable operation with long maintenance intervals, while the maintenance investment required with some torque-tube instruments may be considerably higher.
Might TDR radar be an attractive alternative? In the last two to three years, time domain reflectometer (TDR) radar has been put forward as an alternative to mechanical displacer transmitters for level measurement in difficult applications.
TDR radar makes its measurement by sending a radar signal down a guide rod or wire and monitoring the time taken for a portion of the transmitted microwave energy to be reflected from the liquid/air interface.
The position of the liquid level surface is identified because the change in dielectric which occurs in the transmission line at that point causes reflections.
The time it takes for the reflections to get back to the receiver provides an indication of the distance between the transmitter and the surface of the liquid in the tank.
With no moving parts, this technology is attractive in clean non-viscous liquids because it requires much less maintenance than mechanical devices.
It is starting to be used in hydrocarbon applications and is said to be capable of operating in conditions up to 200C (392F) and 34Mpa/345bar (5,000psi) but because it has not yet established a track record its long term reliability in difficult level measurement applications is not yet proven.
Displacement transmitter technology is a tried and trusted method of level measurement for high temperature and high pressure environments.
The evolution from torque-tube to spring-operated instruments and the addition of high accuracy LVDTs, sophisticated electronics and digital communication options has made significant gains in functionality and operability in the field.
The introduction of TDR radar as a non-mechanical alternative suitable for all but the harshest applications offers the possibility of reliable measurement with much lower cost of ownership.
Although it is starting to make an impact on the market, TDR radar has a long way to go to catch up with the huge installed base of displacer level transmitters in demanding hydrocarbon processing applications on and off-shore.
• Emerson Process Management - Mobrey Measurement: contact details and other news
• Email this article to a colleague
• Register for the free Processingtalk email newsletter
• Processingtalk Home Page
Search the Pro-Talk network of sites
