Pump Unit Control

Pump units are controlled in one of two ways: discharge pressure control by throttling a control valve and pump speed control. Throttling is a common way of controlling the discharge going through a centrifugal pump and the control valve is installed on the dis- charge side of the pump. Another way to control the discharge flow is to control the speed of the pump, which in turn is controlled by the speed of the pump driver. Unlike valve closing, reducing the flow by reducing the pump speed does not waste energy. This con- trolling action results in changes in flow through the pump and pressure in the pipeline. In operating a pump unit, several problems occur: the violation of the required min- imum flow and the presence of vapour in the pump. The minimum flow requirements of the pump must be carefully taken into account during the design and operation of pumping units. If the flow is slow, energy is converted to heat due to low pumping efficiency and the heat cannot be carried away quickly. The liquid in the pump will heat and eventually vapor- ize. Typically, the pump manufacturer will place a minimum flow requirement of about 40% of design flow for pumps associated with the pipeline industry. For most of the time, this does not limit operations but care must be taken during the start-up of the line.

Pumps and Pump stations n 209 Typically, at the inlet station, some method of recirculation is provided so that the inlet pumps can be brought on-line safely. When the pipeline is running at a large base flow, this recirculation valve may be manual, but for lines where the product is stopped and started, it may be controlled automatically by the unit control logic so that it is open until the minimum flow requirement down the line has been established. The goal is to have the pump operate at or near the most efficient point-labeled Best Efficiency Point (BEP) in Figure 4-50.

The pump may be allowed to continue to run for a short time (at most a few minutes) after the valve is closed and thus the discharge flow is zero. If the valve is closed or even slightly opened, but the pump keeps running, energy is wasted, resulting in an overheated and highly pressurized pump and subsequently shortening the life of the pump.

Another problem is the presence of vapor in a pump. If the pressure drops below the vapor pressure of a liquid at the pump suction, the liquid vaporizes and the vaporized liquid forms bubbles. These bubbles move with the flow into the pump impeller and volute where the pressure of the liquid increases sharply. Then, the bubbles collapse in the high pressure area and the collapsing bubbles can generate localized high-pressure, causing damage in the form of surface pitting. The problem is normally avoided by increasing the pressure of the flow on the suction side of the pump — making the avail- able NPSH higher than the required NPSH.

However, when a pump is shut down, vapor can fill in the pump unit and station piping. If the pump is started under such a condition, the pump impeller will be spinning without liquid flowing through the pump, and thus the liquid cannot be drawn into the pump fully and the flow is slow. As a result, the pump can be overheated if such an operation lasts a long time. To prevent this from happening, the pump must be primed with liquid before starting. After the priming is done, the flow is allowed to increase until it reaches the desired flow level. If a control valve is installed for a fixed speed pump, the control valve should be opened gradually after the flow starts flowing through the pump.

4.10.6 Throttling vs. Speed Controls

This section will consider pump station operation using both constant speed and variable speed electrically-driven pumps. Fixed-speed electric motors provide a cost-effective

solution for base load applications where electrical power is available and reliable. They have the advantage of low maintenance costs and are simple to operate. Variable speed drive (VSD) motors are becoming the standard for pump stations that have varying flow or prod- uct density requirements such as on batched product pipelines. Despite their control systems being more complex than for a constant speed motor, variable speed motors are much more energy efficient. This is because pump capacity can be controlled without the disadvantage of pressure loss incurred by the throttling through a discharge control valve.

Variable speed pumps control the flow and pressures by varying the speed of the driv- ers with maximum power override. For a pump station that contains both fixed-speed and variable speed motors, the control strategy is to run the fixed speed units at a base load with minimal throttling and use the variable speed units to adjust for the required station set point. Figure 4-58 exhibits the performance curves of a variable speed pump.

In applications that require flow or pressure control, the most energy efficient op- tion is an electronic VSD, referred to as a Variable Frequency Drive (VFD). The most common form of VFD is the voltage-source, pulse-width modulated frequency con- verter. The converter develops a voltage directly proportional to the frequency which produces a constant magnetic flux in the motor. This type of speed control can be related to set points of discharge pressure or flow.

4.10.6.1 Throttling for Fixed Speed Pumps

As discussed in Section 4.7.1, there is only one operating point for a fixed speed pump. As shown in the figure below, a throttling action is required to match the system head curve to the pumping head curve of a fixed speed pump at a particular flow rate other than the design flow. Fundamentally, throttling changes the system curve. A control valve is used to throttle the fluid flow, and is installed downstream of the pump.

Figure 4-51 shows that the pump operates at H1 for the design flow rate Q0. If a

throttle valve is partially closed in the pump discharge line, the throughput drops from Q0 to QT, and additional friction pressure drop occurs through the partially closed

valve. As a result, the pump will operate at a new operating point, H2.

Pumps and Pump stations n 211 The throttle pressure is the difference between the case pressure and the dis- charge pressure. The casing pressure of a pump is the available pressure generated by a pump, and the discharge pressure is the pipeline pressure on the discharge side of the pump station. The discharge is the pressure required to transport the liquid to the next pump station or terminal. The throttle pressure is unused pressure developed by the pump and thus results in wasted power. Figure 4-52 illustrates the energy losses caused by throttling; the greater the throttle pressure, the greater the energy loss. 4.10.6.2 Speed Control for Variable Speed Pumps

Pipeline systems operate at flow rates different from the design conditions, because supply or demand changes, liquid properties also can vary as in the case of batch opera- tion, or other operating conditions. Such varying conditions demand flow control. De- pending on the varying conditions, there are several ways of controlling flow rates:

Install a control valve at each pump station to throttle the flow rate; ·

Install multiple pumping units to provide sufficient discharge head that can be ·

matched to the flow requirement; Install variable speed pumps. ·

Variable speed pumps control the flow and pressures by varying the speed of the driv- ers with maximum power override. For a pump station that contains both fixed-speed and variable speed motors, the control strategy is to run the fixed speed units at a base load with minimal throttling and use the variable speed units to adjust for the required station set point.

Even though variable speed pumps are more expensive, it is advantageous to in- stall and operate variable speed pumps because energy cost can be saved and it they easily applicable to a wide range of flow changes.

Compared to fixed speed pumps, variable speed pumps can produce significant energy or power savings as illustrated in Figure 4-52 [16]:

Figure 4.53 shows a pump operating at the operating point B, where the flow rate and pressure are QH and PB, respectively. Here, for discussion purposes, it can be as-

sumed that the operating point is the best efficiency point (BEP), where the efficiency

of the pump is highest. The power required by the pump for the high flow condition is PWH = QH ´ PB/hBEP, where hBEP = 84%, the pump efficiency at the BEP, and is

represented by the area of rectangle, PB0QHB, in the figure.

To achieve the lower flow rate QL, the control valve is partially closed for the fixed

speed pump or the pump speed is lowered for the variable speed pump, as illustrated in the figure; C is the operating point of the fixed speed pump and A that of the variable speed pump for the lower flow rate. At C, the flow rate is reduced to QL but the pump

pressure is increased to PC for the fixed speed pump, and thus the power required by

the fixed speed pump is PWC = QL ´ PC/hC, where the pump efficiency, hC is lower

than the pump efficiency at BEP. On the other hand, the operating point of the vari- able speed pump is A for the same flow rate. There, the power is PWA = QL * PA/hA,

where the pump efficiency, hA may be lower than the pump efficiency at BEP but will

be higher than hC. As shown by rectangles in the figure, PA0QLA, the power required

by the variable speed pump is lower than the power required by the fixed speed pump, because the pressure requirement is lower and the pump efficiency is higher.

In summary, the fixed speed pumps waste energy by throttling the flow to achieve a lower flow rate, because:

The pump operates at a reduced efficiency, ·

The pump is required to produce an increased pressure. ·

Energy savings can result from using variable speed pumps, and thus it is advantageous to use them from the viewpoint of reducing the energy cost. In addition, variable speed pumps offer the following advantages:

Pressure surge can be small, particularly during pump start-up and shut-down ·

operations, because changes in flow and pressure occur gradually. They provide flexibility of controlling flow over a wide range. ·

Pumps and Pump stations n 213

4.11 STATION ELECTRICAL CONTROL

A pump station using electric motor drivers requires a reliable source of electricity. This may be supplied from a commercial source or generated at the station. Economic and reliability considerations usually determine the choice of power source.

The electrical supply usually will have high voltage feeders, voltage reduction equipment, and be a multi-bus operation with its associated transfer equipment. All the bus and equipment protection required to support such a system is normally provided with the electrical equipment. Controls for this equipment may be incorporated into stand-alone control equipment or they may be part of the station control system.

The electrical protection is always contained in stand-alone, specialized equip- ment package that will protect against:

Over and under voltage ·

Over and under frequency ·

Over current and short circuits · Ground fault · Voltage imbalance · Phase reversal ·

Transformer gas and high temperature ·

The electrical supply control system monitors the electrical system and sends the fol- lowing information back to the station control system:

Voltage and current values ·

Real power, power factor ·

Electrical energy consumption ·

Circuit breaker and disconnect position ·

Frequency ·