Pipeline Controls
Transport pipelines are often used to transport gas or liquid fuel. The most common fuels transported are natural gas and petroleum products. In the U.S. alone, there is nearly 500,000 miles of pipeline. Nearly half of this total is dedicated to petroleum products. These pipelines consist of long pipes with large diameters that move the product over long distances. Along the line there are injection, pumping, and delivery stations. The control of the entire pipeline is usually done though a main control room. Here, field data is collected and the operations system can monitor the hydraulic conditions of the line and send commands to the various stations. Some of these commands include the starting/stopping of pumps and the open/close of valves.
The data for the production system is typically processed through a SCADA system. This system receives information from remote terminal units (RTU) at each station. The RTUs collect and transmit real-time data including pressure, flow, valve position, temperature, etc. This data is used to perform leak detection, leak location, batch tracking, composition tracking and more. Much of the pressure measurement is done with pressure switches.
There are several features that are particularly desirable for pressure switches that are employed in pipeline applications:
SENSITIVITY - Sensitivity relates to how accurately the device can be calibrated to the aim setting. If a device is not sensitive, it is very difficult to “fine tune” the device. For pipelines, small changes in pressure can lead to large changes in product throughput. Many switches are used to directly control the operation of pumps and valves in the system. If the switches are not adjusted accurately, then the result can be lost production of the line.
LOW DRIFT - Drift is amount that the switch setting will change over time and operation. It is desirable for the drift to be as small as possible. The reasons for this are the same as those mentioned under sensitivity. Accurate settings lead to optimized production.
LOW DEADBANDS - Deadband is the difference between the set point and the reset point for a pressure switch. This is also called switch hysteresis or differential. All pressure switches have some deadband. Many of the pressure switches used in pipeline applications are for high or low limit alarms. These alarms may interrupt or reduce the flow of product. It is often desirable for the deadbands to be as small as possible so alarms can be cleared quickly in order to allow production to restart.
RUGGEDNESS - Pipelines often pass through severe environments. Controls must be able to withstand cycling, shock, vibration, earthquakes, and all types of weather.
OVERPRESSURES and VACUUM - Many switches are used at relatively low settings but will often be exposed to high operating system pressures. Conversely, some of the switches may be exposed to vacuum during operation of maintenance. It is not acceptable for the setting to shift when exposed to these over/underpressures.
EXPLOSION PROOF - The product flowing through pipelines is often flammable or explosive. Many of the locations are classified as hazardous requiring explosion proof equipment. This includes all control equipment in these locations.
EASE OF USE/MAINTENANCE - The Department of Transportation (DOT) regulates pipelines in the U.S. The DOT requires that all controls must be checked periodically. In addition, desired switch setting may change based on adjustments to production or changes to the line. It is important that controls allow personnel to easily perform this maintenance.
The Solon pressure switch design is ideally suited to pipeline applications. Most pressure switches employ an “inline” mechanism design. This design has inherent friction that changes over time. The Solon mechanism employs a unique leaf hinge with no guide bushings. When this is coupled with the amplified movement of the sensing element, the result is a device that is very sensitive and less likely to drift. Since deadband is a function of contact gap and friction, the minimal friction in the Solon device results that deadbands that may be 1/4 to 1/8 of comparable range units.
Many of Solon’s devices have rugged cast enclosures and are available with weather tight and explosion proof ratings. In addition, all Solon switches are designed to withstand full vacuum and overpressure without shifting set points beyond their tolerances. Furthermore, many models are available with a number of useful features such as manual resets and external adjustments.
Pipeline applications are demanding. That is why many pipeline companies have their own assessment programs that must be past before a switch is used. After extensive testing including drift, overpressure, and seismic criteria, Solon was selected as the primary supplier of pressure switches for the Trans-Alaskan Pipeline. Most recently, Solon passed a 3 year appraisal program with Colonial Pipeline Company. Consider Solon for your pipeline pressure switch applications.
