Speed Control of Motor driven Cross Country Pipeline Petroleum Pumps

Last Updated 13 years ago by Kenya Engineer

: -A Comparison of Fluid couplings with Variable speed drives

Article by Eng.Tony Washika
Kenya Pipeline Company

Transportation of Petroleum Products by Pipeline is efficient, reliable, and safe. Cross country Pipelines are responsible for transporting both crude and refined Petroleum Products worldwide across vast territories and distances.

Typical Pipeline Lengths are 500-1000 km, the longest Pipeline in the world being the 5327 km long Druzhba  oil Pipeline, crossing through Russia, Belarus, Ukraine, Poland, Germany, Slovakia, Czech republic and Hungary .

In contrast the Kenya Pipeline Company network is about 1225 Km long. A Pipeline network features an Origin pump station and booster pump stations located along the network. It is necessary to control the pressure along the length of the Pipeline at the suction and/or discharge of each pump in the pipeline, and for this, it is required to vary the speed of the Pumps, and various methods are employed to do this.

Traditionally, most cross country pipeline pumps in the MW range have employed Fluid couplings to vary the pump Speed. However, all over the world, some companies have changed to variable speed drives, an example being AEC Pipelines of Canada which employed a 3500 HP VFD (Variable frequency drive) in 1987 on a crude oil pipeline, the first application of it’s kind in Canada.

The application of a 3500 Hp variable frequency drive for Pipeline Pump Control.IEEE transactions on Industry applications 25. In Pipeline Operators, discussions have been ongoing about whether to change to Variable speed drives from Fluid couplings. This paper explores the various arguments.

Fluid coupling Vs Variable Speed drive
Fluid Coupling-This method is also called hydro-Kinetic transmission, meaning that power is transmitted by Kinetic Energy. The input shaft of the Fluid coupling is connected to the drive motor, and the output shaft is connected to the driven load, which in this case is the pump. A typical petroleum pipeline pump with Fluid coupling speed control is shown in Fig 1 below.

The fluid coupling is filled with mineral oil, which is the medium for transmission of kinetic energy from the impeller to the runner. However, there still are systems that use synthetic oils, and water as a medium of power transmission.

The level of oil in the working circuit determines the output speed of the Fluid coupling. This level is controlled by a tube called a scoop tube, whose position is controlled by a control motor whose position input is a 4-20 MA signal. For pumps in the range of MW, the fluid coupling method is very efficient up to 96% and has been the preferred method.

This method has been in use for more than 80 years. Kenya Pipeline Company has got a total of 17 No. Mainline pump motors along the Mombasa- Nairobi Pipeline, rated 1600 Kw, 3.3 Kv AC and 1760 Kw, 6.6 Kv AC. Along the Nairobi-Eldoret  Parallel Pipelines, there are a total of 12 No. 1MW and 1.5 MW 3.3 KV  AC mainline pump motors. All the mainline pumpsets in KPC employ fluid couplings for pump speed control.

Variable frequency Drives-These are also called variable speed drives, or inverter drives. These operate by varying the frequency and voltage applied to the AC motor, resulting in a variation in Motor speed and Torque. The motors most commonly used in Industry are AC squirrel Cage Induction Motors, accounting for 50% of all electricity usage in industrialized countries.

It is worth noting that there are differences between low power VFDs and the multi-MW VFDs used on petroleum pipelines. These differences are mostly brought about by limits in power ratings of the components used in low power VFDs.

Traditionally, due to power limitations on IGBTs, most multi MW VFDs employed SCRs, but with IGBTs in the market rated over 3000A and 6600 V, Multi MW VFDs are now incorporating IGBTs for commutation (switching). This allows use of pulse width Modulation techniques, increasing efficiencies up to 96%.

About the author
Eng. Tony Washika undertook an MSc in Sustainable Energy Engineering at the Royal Institute of Technology (KTH) in Sweden part time( 2006-2012) . He has worked in Mumias Sugar Company, Agro Chemicals and Food Company Muhoroni and is currently a Senior Engineer (Instrumentation and Control) at Kenya Pipeline Company as Electrical Engineer. Eng Washika is a Registered Engineer, member of the Institution of Engineers of Kenya, and a member of the Institute of Electrical and Electronics Engineers (IEEE).      
Email contact: tonywashika@ieee.org, or tonywashika@yahoo.com.