Spring-operated and torque tube displacer transmitters for the hydrocarbon processing industry

Displacer transmitter technology 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. 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, and accurate; adaptable to a wide range of needs, including measurement of interface between two immiscible liquids; and suitable for use at high pressures and temperatures, where other technologies may fail – particularly important for hydrocarbon applications

Displacer technology

The two types of displacer transmitter in common use today are 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 consider the effect of the upper fluid which, even if a vapour, will influence the buoyancy force, particularly if the vapour space is at a high pressure. The difference between the two types of displacer transmitters involves 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 pneumatic, signal proportional to the liquid level change.

Torque tube technology

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 (Figure 1). The torque tube is hollow and 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. However, it is bulky and 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.

Spring operated instrument, Delta-Mobrey

This newer, more elegant design of displacement transmitter overcomes many of the problems associated with torque tube devices. Just as reliable, 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 element is hung (Figure 2).

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, detected by the instrument electronics and converted into a signal proportional to the liquid level.

Key benefits of MLT100

• Smaller, lighter and more robust
• Smaller mounting envelope
• Lighter and easier to install
• No critical welds under stress
• Lower maintenance cost

Spring operated displace transmitter

The spring is a heavy-duty coil made from typically 3mm or 4mm gauge specially selected alloy wire. The coil operates at approximately 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.

Some of the best instruments on the market are those with coils made from Nimonic©, a nickel alloy that gives the spring a perfectly linear expansion over the full operating temperature range of the instrument, proving highly accurate level measurement. However, the operating range is not quite as wide, although specifications vary between manufacturers.

Specification and calibration

Whichever transmitter is chosen for the application, the size and weight of the displacer is critical, 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 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 very easily trim, re-calibrate, or re-range the instrument on site. As mechanical devices, displacer transmitters have traditionally needed regular maintenance, cleaning and checking of calibration.

When considering investing in this sort of instrumentation it is worth exploring 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.

Conclusion

Displacement transmitter technology is a tried and trusted method of level measurement in high temperature and high-pressure environments. The evolution from torque tube to spring operated instruments and the additional of high accuracy LVDTs, sophisticated electronics, and digital communication options have made significant gains in functionality and operability in the field.

Along with the wide portfolio of products, Delta Mobrey has a reputation of developing solutions in collaboration with customers to suit their specific needs. Please get in touch.

About Delta Mobrey

For more than 100 years, Delta Mobrey Limited has been a world-leading specialist in the design and manufacture of quality process instrumentation for the measurement and control of fluids and gases in all industrial plant and equipment.

Trusted quality – proven value: quality and reliability have always formed the cornerstone of our success, recognised by industry with international approvals covering every aspect of our manufacturing and product portfolio, together with certifications spanning all areas of hazardous and regulated environments.

Global team – local support: as a global organisation, Delta Mobrey is totally committed to delivering the best possible customer service and technical support, ensuring a lifetime value of ownership, together with a flexible and responsive approach to meet our customers’ individual demands