Servo level gauges are prone to measurement inaccuracy, have a poor reliability record and require regular maintenance and recalibration. Despite this, many aging tank gauging systems based on this level measurement technology remain in use on bulk liquid storage tanks, supporting both overfill prevention and inventory management.
Many terminal operators would like to replace their servo devices with modern radar level gauges, which provide extremely accurate and reliable measurements, have minimal maintenance needs, and include advanced diagnostics to automatically monitor device health during operation. However, as I explained in a Tank Storage article titled ‘A Cost-efficient Tank Gauging Solution’, communication compatibility issues can prevent them from doing so. The article states:
…Historically, most manufacturers of tank gauging equipment have provided proprietary fieldbus options for the communications between their devices and the control room. Because the electrical interface and protocol software is specific to each manufacturer, this prevents devices developed by other manufacturers from communicating within the implemented tank gauging system.
So, if an organisation wanted to partially upgrade their system or replace individual devices, the only option would be to buy equipment from the original system supplier. Installing equipment from another supplier would typically require separate cabling installed for another fieldbus network, a second operator display in the control room and a second interface added to the existing distributed control system – which would be costly and time-consuming.
Rather than upgrading individual devices, organizations could replace their entire tank gauging system in one go. This would eliminate the issue of proprietary communications but can also prove costly, complex and time-consuming. The article continues:
…A solution to this challenge is provided by gauge emulation. Gauge emulation makes it possible to perform easy and cost-efficient tank gauging system upgrades incrementally. Emulation refers to the ability of the latest gauges to ‘speak the language’ of servo devices, enabling these older gauges to be seamlessly replaced by modern radar gauges from alternative vendors. Emulation enables the legacy technology to be replaced when the budget is available – one gauge at a time if necessary – with minimal disruption.
The Rosemount™ 5900 Series from Emerson is a modern radar level gauge that can emulate a broad range of legacy devices and different proprietary fieldbus protocols, when used in conjunction with a Rosemount 2410 Tank Hub for communicating with the host system. Because it can be installed within an existing tank opening, it can quickly and easily replace servo gauges. The Rosemount 5900 can also communicate with open standard protocols, such as Foundation Fieldbus or Modbus, enabling connection to these communication networks.
As well as emulating servo gauges, the Rosemount 5900 can also emulate older and/or less reliable radar devices, as was the case at a large gas separation plant in Thailand. In this application, radar level gauges were providing critical level and volume measurements in spherical liquefied petroleum gas (LPG) tanks, but were proving unreliable. The plant owners wanted to upgrade to more reliable radar level gauges, but to communicate with the control room system, the field device communication protocol needed to be compatible with the existing communication protocol being used. The article explains:
…The entire tank gauging system could have been replaced in one go but this was considered cost-prohibitive. Instead, on each of its 19 tanks, the company replaced the unreliable devices with Rosemount 5900S Radar Level Gauges with an LPG antenna, specially designed for spherical tanks. Using emulation technology, the upgrade was performed step-by-step. Consequently, the plant now has a safe and reliable tank gauging system, with the upgrade performed at a pace and budget that suited the company. The plant owners estimate that this upgrade with emulating gauges saved them around $45,000 in downtime, operational expenditure and man-hours.
In addition to gauge emulation, Emerson also provides host system emulation, enabling old tank management system equipment to be upgraded but still communicate with existing legacy gauges. Using the Rosemount 2460 System Hub, data can be collected from servo or radar gauges from a broad range of vendors using a variety of proprietary communication protocols, and forwarded to the new tank management system.
The article describes an example of host system emulation, at a tank terminal owned by a sugar cane ethanol manufacturer in Brazil:
…The site was equipped with legacy tank gauging equipment, including servo gauges and communication interface units (CIU). Data communication between the tank gauging field devices and the host system was based on a proprietary protocol. Technical support for the CIU was lacking and spare parts were expensive or unavailable, making the terminal highly vulnerable to device faults. In addition, the CIU didn’t offer the possibility of integration with modern automation equipment and host systems.
When the terminal’s owners decided to replace the existing CIU, Emerson provided its Rosemount 2460 System Hub with emulation. The Rosemount 2460 could emulate the protocol communication with both the existing servo gauges and the host system. No changes to the servo gauges or in the host system were required. The system hub also enables future communication updates, redundancy capability, plus inventory management calculations.
The manufacturer now benefits from a modern, future-proof communications device, fully supported by Emerson. There is no longer any need to risk operational disturbances from obsolete data communication equipment.