Speed manages streams

InTech, Jul 2004 by Gibson, Ian, Irvine, Geoff

The case for variable frequency drives as a final control element spreads.

The concept of using variable frequency drives P to control electric-driven centrifugal pumps is distinguished. But they seldom work in the petroleum industry. Why?

The reasons given range from misinformed economic data and an unease of using unfamiliar electrical technology to an ignorance of how these devices react as control devices. One still sees the majority of flow, pressure, and temperature control happening through constant-speed centrifugal machines that a control valve throttles to provide the desired control of fluids.

The use of variable frequency (VF) drives-adjustable frequency drives or variable speed (VS) drives-on electric motors driving centrifugal pumps offers many advantages over using control valves for controlling the fluid flow. Some of the advantages are energy savings, efficiency, power factor, installation, specification, ability to control, maintenance, fugitive emissions, and reduced wear on the bearings and seals of the pumps.

Although this method of control has gained wide acceptance in general industry, there is still reluctance to use it in the petroleum industry. The reasons for this vary, but the most common reasons are:

* a lack of understanding about how these devices work in comparison to the more commonly used control valve

* a common misconception that these devices are expensive compared to control valves

* a reluctance to use these devices on explosionproof motors found in petroleum facilities

* a fear of the reliability of these fundamentally electronic devices

* a lack of knowledge about the failure modes of these devices for the hazard and operability analysis

There is no doubt that in the early days of development of the variable frequency drive some of the issues were true. However, over time the cost, reliability, functionality, and knowledge of these devices has improved to a point where we believe they should be viable final control elements for petroleum facilities.

Flow by discharge throttling

Discharge throttling is widely used in the petroleum industry. It involves putting a control valve on the discharge side.

With this method of control the control valve varies the pressure drop across the valve, and hence the flow. It has the effect of increasing the slope of the system curve, and therefore takes the operating point to a different point on the pump curve.

The insertion loss of a wide-open control valve is about 10% of the other dynamic losses. This requires the pump H-Q curve to be higher at the design point to compensate for this loss.

Most control valve design guides call for the valve to be no more than 70% to 80% open at the design point.

However, when speed control operates to control the pump flow the effect on the pump curve is that it "moves" up and down the system curve.

The speed control method of flow control ensures that just enough energy goes to the pump to get the desired flow rate. Compared to controlling via discharge throttling, this method of control is much more energy efficient.

There is an analogy to driving a car. Discharge throttling is similar to having the car engine going at constant revolutions per minute (rpm) (a brick on the accelerator) and using the brake to control the speed. Speed control of the pump is similar to using the accelerator-the accelerator is depressed enough to supply the desired speed.

Also, some points that are germane to this type of control are:

1. The effectiveness of this type of control is very much dependent on the shape of the pump curve and the system curve. The diagrams shown here are "idealistic" for the purpose of demonstrating a point. One needs to be aware of the effects of curves shaped differently than the ones shown here, and how to best implement a W drive for these cases.

2. There are pump designs (axial flow, mixed flow) that do not approximate a parabolic relationship, and some of these cannot use throttling-recycle is imperative. These are common in deep-well turbine pump designs. Speed control of such pumps is often ineffectual.

3. There can be a loss of efficiency in the pump operation that is similar to the loss of efficiency when using discharge throttling on the pump.

Energy saving VF drives

Thus, the main incentive for using variable speed drives is the energy savings from their implementation. This is due to the pump affinity laws governing a centrifugal pump (or fan) operation.

The operation of a variable frequency drive uses a number of simple units.

The first part of the VF drive is a rectifier to convert the alternating current (AC) voltage to a pulsating direct current (DC) voltage. Then the intermediate DC circuit filters the pulsating DC voltage to a DC current or voltage. The inverter uses the DC current or voltage from the intermediate DC circuit to produce an AC current or voltage having the desired frequency. The control unit oversees the operation of the components of the VF drive.

The most common method of controlling the frequency is pulse width modulation. With this method the intermediate circuit produces a constant DC voltage, and the inverter then produces a synthesized sine wave of the required voltage and frequency to control the speed and direction of the AC motor. The synthesized sine wave consists of switching pulses of varying width, producing an equivalent root-means-square sine wave the motor recognizes. The reason one needs to control frequency and voltage at the same time requires some explanation.