The importance of sand monitoring in separator tanks - part two

Vibrating fork level detectors

An effective solution is provided by the latest vibrating fork level detectors. These devices operate using the concept of a tuning fork and have traditionally been employed to monitor liquid point levels. The device has two prongs and when in air they are oscillated by an internal piezo-electric crystal at a frequency of approximately 1400 Hz. When the prongs are immersed into the medium within the tank, the frequency changes. The greater the density of the medium, the lower the frequency – thus when liquid covers the prongs, the frequency drops significantly. The frequency is continuously monitored, with changes immediately detected by the device’s electronics, and the output state switching to operate an alarm, pump or valve.

This principle also enables the device to continuously monitor liquid-to-sand interface via special ‘sand switch’ functionality. This makes it suitable for separator applications. Data can be transmitted from the device to the control room, enabling the build-up of sand deposits to be monitored remotely, and avoiding the need for workers to visit the well pad.

Configuring these devices is straightforward, with different sensitivity settings to ensure they can successfully detect least, medium, high or most compacted sand. In addition to detecting sand build-up, they can also be used within a control system to automate a chamber’s clean-out cycle. This eliminates the need to perform this process manually and therefore keeps personnel away from potentially hazardous areas.

Vibrating forks are compact, lightweight and easy to install. The shape of the prongs makes it less likely that sticky or viscous material will attach itself to the device, and it therefore drains away quickly. In addition, there are no moving parts that can freeze or get stuck, which increases reliability and makes the device virtually maintenance-free.

The use of HART^® communications enables these devices to deliver the benefits of advanced smart diagnostics. This provides insight into device condition and helps to identify potential problems before they become serious, supporting predictive maintenance practices. Potential issues that can be detected early include damage to the prongs or the sensor, corrosion and over-temperature. By monitoring fork frequency, it is also possible to detect build-up on the prongs. Vibrating fork technology has good resistance to light-to-moderate build-up, but heavier deposits can cause an incorrect wet signal if left unchecked, especially if the prongs become bridged. The ability to monitor build-up can be especially useful in oil and gas production, due to the presence of coating materials such as paraffin wax. Diagnostic information can be accessed from the control room, thereby removing the need for field trips and increasing worker safety.

Reliable detection at an oil and gas plant

A good example of how the latest vibrating fork level detectors are successfully being used to provide effective detection of sand build-up can be seen at an oil and gas processing plant in Sichuan, China. As part of the shale gas extraction process, the plant uses a separator tank for de-sanding the mixture of oil, gas and water. The level of sand in the tank must be carefully monitored to prevent crude with a high sand content from reaching the pumps, which could lead to pipeline corrosion and pump abrasion. The plant operator therefore needed to install a detection device that would provide an alarm when the level of sand build-up in the separator reached a certain level. The media also contains hydrogen sulfide, so a corrosion-resistant measurement technology was needed to fulfil the requirements of the NACE® MR0175 materials standard.

Figure 3. The Rosemount^TM 2140 Vibrating Fork Level Detector from Emerson

The operator solved this challenge by installing a Rosemount™ 2140 Vibrating Fork Level Detector from Emerson, which is providing reliable detection of sediment build-up. The diagnostics in the device include a ‘wet sand’ setting that enables it to detect sediment build-up in the tank at a critical point, transmit data via HART communications, and issue an alarm. Clean-out can then be proactively scheduled, negating the risk that the deposits will be drawn into the pumps and cause damage through corrosion or abrasion. In addition, the Rosemount 2140 fulfils the requirements of NACE MR1075, enabling the operator to achieve reliable measurement of the media in a corrosive environment.

This is part two of a two-part article. Part one is available to read here.

Author: Marianne Williams, Emerson

Read the article online at: https://www.oilfieldtechnology.com/special-reports/22102019/the-importance-of-sand-monitoring-in-separator-tanks--part-two/