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.