Level products are used widely at the wellhead for a broad range of measurement and control applications, ensuring plant and operator safety, and contributing to process efficiency. On new installations, the ability to deploy level instrumentation easily and quickly is important to help operators get to first oil as quickly as possible. On existing installations, replacing older technology with the latest level measurement devices reduces maintenance and enables accurate measurement. Where no cabling exists wireless presents an easy and cost effective way to add visibility at the wellhead.
Typical applications include level measurement for the storage tanks used for well production, and tanks used to deliver water and injection chemicals. The need to accurately monitor storage tanks to optimise levels and prevent overspills is never greater than when handling potentially polluting and hazardous liquids. Accurate and reliable instrumentation helps avoid the risk of oil spills that impact the environment and require expensive labor and equipment to clean up. As well as the associated risks and dangers to the work force, accidental spills are time consuming and can seriously damage a company’s reputation.
Other level monitoring applications at the wellhead include slug-catchers, heater treaters, Vapor Recovery Units (VRU’s) and separators. Within slug catcher applications, level transmitters are used to monitor liquid level in the chamber and activate a pump to drain the vessel ready for the next slug. Accurate level measurement is critical in heater treater vessels to enable the oil and water to be reliably separated. This improves its quality and reduces oil contamination to the downstream water treatment plant. In Vapor Recovery Units, level sensors are used to ensure there is sufficient space between the liquid and the fixed roof of the storage tank for vapors to be collected.
In separator vessels, level sensors are used to maintain the oil/gas interface at the correct level. In addition, high and low level sensors are required to protect the system from ‘carry over’ of liquid in the gas or ‘gas blow-by’ which occurs when a low level causes gas to exit via the oil output causing high pressure downstream.
Level transmitters based on GWR technology are ideal for these wellhead applications where accurate and reliable measurements are required to determine the level in liquid in a tank or vessel. In these installations, the GWR transmitter is mounted on the top of the tank or chamber, with the probe extending to the full depth of the vessel.
GWR is based on a low energy pulse of microwaves being sent down the probe. When the pulse reaches the media surface, a reflection is sent back to the transmitter. The transmitter measures the time taken for the pulse to reach the media surface and be reflected back. An on-board microprocessor accurately calculates the distance to the media surface using ‘time-of-flight’ principles.
GWR is not density dependent and is unaffected by high turbulence or vibrations. Because there are no moving parts to stick or wear, maintenance costs are reduced and the problems of false readings, which can result in hazardous situations, are avoided. These characteristics make GWR level transmitters an excellent choice for these applications.
In addition to tank level instrumentation, most storage tank applications incorporate high and low level devices to trigger alarms. High level overflow protection on tanks, storage vessels and separators, involves shutting down the supply source either by switching off a pump or closing a valve. Here, level measurement helps prevent overspills and subsequent environmental and/or safety incidents. Shutdowns when levels are low provide protection to pumps in buffer tanks, column bottoms and storage tank applications. In this type of application it is important to monitor levels to prevent pumps from running dry. This protects pump bearings and prevents mechanical damage.
Vibrating fork level switches are ideal for these applications. The switches operate on the principle of a tuning fork, whose frequency varies depending on whether the fork is immersed or dry. They can be used in applications with liquids and slurries, including coating and aerated liquids. Sensing is virtually unaffected by flow, turbulence, bubbles, foam, vibration, solid particles, build-up, or fluid properties. Unlike many other level switch technologies, the vibrating fork technology does not have parts that can get stuck and therefore is less prone to failure.
Many switches based on vibrating fork technology now provide basic diagnostics, which allow the status of the device to be accessed locally. Some of the latest devices offer enhanced diagnostics to detect conditions such as a damaged fork, damaged sensor, media build up on the fork and excessive corrosion on the fork.
Wireless Technology speeds installation
WirelessHART devices are typically installed and operating in just a few hours – reliably transmitting data, via a wireless gateway, to a control system or data historian. A wireless network will usually comprise one or more wireless gateways connected to up to 100 wireless transmitters. Wireless networks are far ranging, with up to 200 meters device separation. More distant well pads can be connected to the central network using IEEE802.11 Wi-Fi access points.
WirelessHART self-organizing mesh networks allow individual devices to serve as alternative communication paths. This allows signals to easily find their way around fixed or moving obstacles - typically providing greater than 99 per cent communications reliability. Once a wireless network has been established, any WirelessHART enabled device can join the network. This makes it easy to make late changes or move devices around to meet specific requests.
Software applications provide a comprehensive overview of the wireless network making it easy to organise wireless diagnostics across multiple wireless gateways. Update rates and communication routes can be optimised to extend battery life which is in the order of years. Diagnostics allow battery voltage to be monitored from the control room, enabling replacements to be scheduled - reducing maintenance requirements and eliminating unnecessary field trips.
Wireless level transmitters greatly improve network solidarity and measurement integrity by eliminating the vulnerabilities of wire, simplifying installation and configuration procedures, and improving diagnostic abilities.
By increasing well automation capabilities, operators gain better visibility into what is going on at the wellhead and be better equipped to make the right decisions, without having to go back into the field to find out more. The result is improved production and lower costs. The use of wireless at the wellhead has been embraced by operators. Helping to overcome the issue of understaffing, limited experience, expanding fields and poor automation, wireless is enabling operators to reach first oil quicker and ensuring production is efficient and safe. DP pressure transmitters for reliable, accurate and fast flow measurements provide good visibility into wellhead performance and temperature transmitters to help achieve optimal efficiency and compensate in flow calculations. Complimenting these devices are now wireless GWR and vibrating fork transmitters ensuring important level measurements are collected efficiently and reliably.
Part One of this article can be accessed here.
Edited by Katie Woodward
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/29052014/emerson_wireless_level_technology_part_two/