Pump Station Operation

This section presents an overview of the key aspects of the pump station control. The following basic control operations are involved in controlling a pump station p roperly:

Control of pumping units including their driver, ·

Control of station valves and possibly line valves, ·

Alarm annunciation to correct an operation for limit violation, ·

Emergency shutdown to prevent possible damage to facilities. ·

The first two control operations are usually interlocked for starting and stopping opera- tions of stations and/or pipeline system.

Nowadays, pipeline system control is automated to provide the capabilities of operat- ing pipeline systems reliably, efficiently and thus economically. Automated system control enables monitoring and control of pipelines, pump stations, metering stations, and other facilities through a SCADA system. In addition, it can be extended to other applications such as storage management, energy optimization, volume accounting, leak detection, etc.

It is now generally an accepted practice that stations are automated and operated under remote control from a central SCADA control center. This is possible because a centralized system provides the capability to monitor the complete pipeline system and control the entire pipeline system in a safe and efficient manner. Only under abnormal conditions or during some maintenance tasks will the station be under local control. Some stations may be completely unmanned whereas others will have maintenance staff on site but who will not normally be in control of the station equipment. Figure 4-48 shows a typical implementation of a pipeline system and station control.

The key criteria of deciding if the station should be unmanned are that the sta- tion can be operated reliably and robustly and the control system has a high level of availability. In addition, transfer of control from “remote” to “local” must be easily supported in the event of an abnormal situation.

The following factors are usually taken into account in deciding if the station is to be unmanned:

Safety can be enhanced by minimizing potential human errors; ·

Reliable and quick control response can be achieved; ·

Stations can be more efficiently operated remotely to reduce the operation cost. ·

Overall, the costs of installing the automation system and of maintaining the unmanned station must be compared against the costs of manning the station.

A typical station control system consists of several components; station control, unit control, driver control, and other auxiliary unit control. A station can be operated locally as well as interfaced to a SCADA system to enable remote control from a cen- tral control center. References [14, 15] discuss the control in more detail.

A properly designed remote control system will provide the ability to:

Monitor all equipment associated with the station including station auxiliary ·

systems;

Provide two-way communication between the station and the host; ·

Monitor the starting and stopping sequence of the drivers and pump units; ·

Control and monitor sequencing of station valves; ·

Initiate an emergency shutdown of the station or unit. ·

These extra system control capabilities can meet the following objectives for station operation:

Operate the station safely and reliably, while maintaining cost efficiency; ·

Allow constant monitoring of critical components of the station; ·

Shorten response time to potential problems; ·

Eliminate mundane tasks for the station operators. ·

Pumps and Pump stations n 205 Unplanned outages can cost a pipeline company tens of thousands of dollars. Therefore, the station control system must be reliable, robust and have a high level of availability in order to minimize business interruptions and maintain a safe environment for personnel and equipment. It must also be able to transfer control from “remote” to local in the event of an emergency or an abnormal situation. From the perspective of the overall operation of a pipeline, a pump station can be viewed as a “black box” that maintains product flow by offsetting pressure losses in the pipeline. The pipeline operator may only be interested in set- ting pressures at the various stations and not be concerned with the control of the individual units. In this situation, the station control system would receive station set points rather than individual unit set points from the SCADA system. It would then determine how many units should be operating and the set points for each unit.

An alternate control scheme is to include the station control system within the SCADA system. The system operator would then be initiating start/stop commands and relaying them to individual units as well as sending them the required set points. Through the SCADA system, the following data required for monitoring and control- ling pump units are displayed:

Pump unit status ·

Flow rate ·

Product name and density or API gravity ·

Suction pressure and its set point ·

Discharge pressure and its set point ·

Throttle pressure or difference between the pump casing pressure and the sta- ·

tion discharge pressure Holding pressure ·

Station electrical load ·

Communication status ·

Alarms ·

In practice, the local station control system is implemented with a programmable logic con- troller (PLC). PLCs are now the heart of station control for station equipment (pumps, driv- ers, lube oil systems). PLCs consist of a programmable microprocessor unit, communication modules, and input/output modules for connection to field devices. The PLC has overall control capability for the station. This includes all equipment not under the direct control of a unit control system. It ensures that the station operates within the parameters for the station and mainline piping (above minimum inlet pressure, below maximum allowable operating pressure, etc.). In addition, the PLC determines the required set points for the operating units based on the required station set points received from the pipeline operator via the SCADA system. The individual set points sent to each unit will be determined based on a load shar- ing strategy. This will vary depending on the type of units installed and the overall pipeline operating strategy. They may include such strategies as:

Base loading: one or more units may be operated at a constant load while other ·

more efficient units are used to compensate for small changes.

Optimum load sharing: set points for each unit are determined based on know- ·

ing the individual unit operating curves and allocating load to minimize overall energy consumption.

With the increase in computing capability, it is now more common for pipeline com- panies wanting to optimize their pipeline operations to consider having a system opti- mizer that would optimize pumping usage on the entire pipeline. This is discussed in more detail in Reference [14].