Pumping systems are composed of various components including motors, pumps, drives, control valves, piping and ancillary equipment. At each of these components, there are inefficiencies. Because every system component impacts total system efficiency in different ways, focusing exclusively on individual components can overlook potential cost savings. To maximize the overall cost-effectiveness of their operations, end users must turn to optimization of the entire system.
The graphic above depicts a simple closed-loop heat exchanger system. Below and on the following pages are explanations of how each of the system’s parts work together and for optimization tips.
- Suction Source
- Suction Piping
- Block Valve
- Suction Gauge
- Primary Base Plate
- Mechanical Seal
- Discharge Gauge
- Flow Meter
- Discharge Piping
- Check Valve
- Bypass Valve
- Bypass Line
- Control Valve
- Input Power/Power Cable
- Process (heat exchanger)
1. Suction Source
Suction fluid level must provide adequate net positive suction head (NPSH) to ensure the pump can meet design conditions without cavitating. The NPSH available (NPSHA) is a function of the following attributes of the pump and piping system:
- elevation difference between the liquid level and the eye of the impeller
- pressure on the liquid surface
- head loss in the pump’s suction pipelines
- pump suction nozzle flow velocity
- vapor pressure and density (as function of temperature) of the liquid being pumped
- barometric pressure at the pump site
2. Suction Piping
The majority of hydraulic problems encountered in pumping systems originate in the suction piping. It is important to provide the best possible suction piping layout. Piping must be large enough to carry the volume. Piping configuration must be such that the liquid is properly led to the pump. Ideal piping configuration should provide a minimum of 10 diameters of straight pipe between the suction source and the pump suction. The head loss component of NPSHA is based on the losses in the pump suction piping. These losses can be significant and increase with the square of the increased ratio of flow rate. Sometimes pump performance is limited by NPSHA. It may be possible to reduce the piping head losses by increasing the suction piping diameter.
3. Block Valve
Block valves are used to isolate the pump for repair and service without having to evacuate the system. Block valves should be installed at the suction and discharge side of the pump, and they should never be used for process control.
4. Suction Gauge
The suction gauge should be liquid-filled and sized according to the suction pressure seen by the pump. If the pump experiences a vacuum condition, a gauge that reads vacuum as well as pressure should be used.
The success of a reliable pumping system begins with the sub-base. The sub-base is the foundation on which the primary base plate is installed. The sub-base must be of appropriate size and mass to support the primary base plate as well as the components installed on the base plate.
6. Primary Base Plate
Torsional stiffness, rigidity and flatness (0.002 inches TIR at machined surfaces) are the most important considerations with respect to the primary base plate. By design, the base plate (when grouted) should be approximately five times the mass of the components it supports.
Selecting a pump for an application is based on the system head versus flow requirements, the pump performance characteristics, the pumping application, the footprint required for the pump and driver, application specifications, codes, regulations, reliability and maintainability considerations, and energy cost considerations.
The specifying engineer may need to work closely with the pump manufacturer to select the best pump, size, speed, power requirements, type of drive, mechanical seal, and any ancillary components for the application. Proper pump selection is a multistep, multidiscipline process that requires a clear picture of the process system and piping, a thorough understanding of system operation and energy requirements, and knowledge of the economics over the life of the system.
8. Mechanical Seal
Both the mechanical and support system must be selected to meet the system requirements.
- Metal parts must be corrosion-resistant.
- Mating faces must also resist corrosion and wear (stationary and rotating).
- The proper type of seal is based on the pressure on the seal and on the seal size.
- Temperature can determine the use of the sealing members as materials must be selected to handle liquid temperature.
9. Discharge Gauge
As with the suction gauge, the discharge gauge should be liquid-filled and sized appropriately to meet the maximum discharge pressure of the system with a slight margin.
10. Flow Meter
Various types of flow meters are available for pumping systems, including intrusive and non-intrusive design. Intrusive designs consist of orifices, venturies, nozzles, rotameters, pitot tubes, turbine, vortex and calorimetric. Non-intrusive designs include ultrasonic doppler; this design is installed externally on the piping and reads flow through motion of sound. Flow meters should be selected based on type of pumpage, temperature and accuracy of flow readings.
11. Discharge Piping
A piping system is composed of individual pipelines connecting the other system components. Because a pipeline is the basic building block of a pump system and pipelines are one of the major friction losses in a pump system, it is important to examine the losses associated with piping to understand the total frictional head loss in the system.
12. Check Valve
An essential element in the design of a pumping system is the proper selection of the check valve, whose purpose is to automatically open to allow forward flow and return to the closed position to prevent reverse flow when the pump is shut down. Several types of check valves are available, including lift check valves, swing check valves and pump control valves.
13. Bypass Valve
When evaluating what types of valves are suitable for installation in bypass piping, end users must consider the role of the bypass line. What was the purpose of the installation of the bypass line? Bypass piping can roughly be divided into two categories:
- Bypass piping that acts as a backup line to allow operation to continue while damaged equipment such as pressure-reducing valves and steam traps are isolated and shut down during replacement or repair
- Bypass piping designed to supplement the performance of pressure-reducing valves and traps
These distinct objectives require valves with different features, so the optimal valve models will be different.
14. Bypass Line
The bypass line should be sized to accommodate no less than 50 percent of the design flow of the pump. Proper sizing will minimize the risk of premature pump failure should the system requirements force the pump to operate back on the curve.
15. Control Valve
Control valves are used to control conditions such as flow, pressure, temperature and liquid level by fully or partially opening or closing in response to signals received from controllers. As with other pump system components, the control valve should be selected based on type of pumpage, temperature, pressure and volumetric flow rate. Control valves increase the friction head loss in the system, resulting in wasted energy and potentially off-design operating conditions.
The basic function of the coupling is to transmit power, accommodate misalignment and compensate for axial movement (end movement of shafts). Factors in coupling selection include torque, chemical compatibility, temperature, speed, starts and stops, physical dimension, starting method, and system alignment.
When selecting an alternating-current (AC) motor and associated equipment for an application, the following points should be considered:
- Environment: Conditions such as ambient temperature, air supply, and the presence of gas, moisture or dust should all be considered when choosing a motor.
- Speed Range: The minimum and maximum speeds for the application will determine the motor base speed.
- Speed Variation: The allowable amount of speed variation should be considered. Does it require constant speed at all torque values, or will variations be tolerated?
- Torque Requirements: The starting torque and running torque should be considered when selecting a motor. Starting torque requirements vary from a small percentage of the full load to a value several times full-load torque. The starting torque varies because of a change in load conditions or the mechanical nature of the machine. The motor torque supplied to the driven machine must be more than that required from start to full speed. The greater the excess torque, the more rapid the acceleration.
- Acceleration: The necessary acceleration time should be considered. Acceleration time is directly proportional to the total inertia and inversely proportional to the torque (motor vendor).
- Duty Cycle (RMS Calculation): Selecting the proper motor depends on whether the load is steady, varies, follows a repetitive cycle of variation or has pulsating torques. The duty cycle, defined as a fixed repetitive load pattern over a given period of time, is expressed as the ratio of on-time to the cycle period. When the operating cycle is such that the motor operates at idle or a reduced load for more than 25 percent of the time, the duty cycle becomes a factor in selecting the proper motor.
- Heating: The temperature of an AC motor is a function of ventilation and losses in the motor. Losses such as operating self-ventilated motors at reduced speeds may cause above-normal temperature rises. De-rating or forced ventilation may be necessary to achieve the rated torque output at reduced speeds.
18. Input Power/Power Cable
Input power cable selection is dependent on whether or not a variable frequency drive (VFD) is used. Using the proper (shielded) cable when connecting the VFD to the motor can impact system reliability. Refer to the motor and drive manuals to make sure you are using the correct cable and implementing the grounding according to the VFD and motor manufacturers’ instructions. Every VFD has a maximum motor cable length. Before installation, you should know the maximum distance from the motor to the drive. The reason for maximum cable distance is the firing/gating of the insulated gate bipolar transistors (IGBTs) can be adversely affected by the capacitance of the cable. Conductors have a very small capacitance, but the longer the wire, the more capacitance is introduced to the system.
Selection criteria for applying a VFD to a centrifugal pump system:
- What is the voltage available on-site?
- Does the VFD rated output voltage equal motor voltage?
- Is it single-phase input?
- Motor full-load amps: Is the VFD output rating sufficient?
- Motor service factor (SF) amps: Will the motor run into the SF?
- Load type: Variable torque (110 percent) or constant torque (150 percent)?
- Temperature: Is enclosure cooling or heating needed?
- Ingress protection: Dust, moisture, corrosive gas?
- Elevation: More than 3,300 feet above sea level?
- Hazardous site: Explosion-proof?
–Motor cable length
- VFD output filters to protect motors or to avoid nuisance tripping issues
- Output sine wave filters
- dV/dt filters
- Common mode filters
- Look at the big picture of the motor, pump and drive as a complete system.
–Harmonic level: IEEE 519
- Line reactor
- Direct-current (DC) choke
- Active/passive filter
- 12, 18 or 24 pulse rectifier VFD
- Active front end VFD
20. Process (heat exchanger)
The graph is a closed-loop system with one pump feeding a heat exchanger. In order to size the pump to the system, we must establish a system curve. The system curve represents the head required to move fluid through the system at various flow rates. It has two components: friction head and static head. The system will operate where the pump and system curve intersect. The process (heat exchanger) creates resistance to flow (friction) and is a critical factor when determining the required head and flow during the pump selection process.