Minimum Flow Control System Design Factors

Pump Size Capacity, power, specific speed, and suction specific speed are all factors that must be examined when designing a bypass system. These factors have a direct impact on the cost of building and operating the bypass system. Use of a continuous bypass system will require an even larger pump and driver to supply both the process and bypass flow requirements simultaneously.

Discharge Pressure High discharge pressures result in high head loss in bypass valves, components, and lines. Liquids that can flash and cavitate demand special precautions to minimize damage in valving, orifices, and piping.

Available Heat Sink Bypass flow must be reintroduced into the system far enough upstream to prevent progressive temperature buildup or flow disturbance in the pump suction. This may mean a simple discharge back to an uninsulated inlet line or discharge to a receiving tank or cooler with enough area and enough inflow of cool liquid to handle the thermal load. Bypass flow can discharge into a deaerator storage tank, a condenser, a flash tank, or a cooling pond. Elevation, distance, and pressure inside the receiving tank are also factors, as is the fact that the interior must be available for inspection and for repair of spargers, spray or distribution pipes, orifices, and backpressure regulators.

Pump Design A pump's design and materials of construction often affect the minimum allowable percentage of flow. With thermal effects, pumps vary in the length of time that they will tolerate shut off or low flow. This is important in designing the bypass system valves, instrumentation, and controls.

Hydraulic effects at low flow are most apparent in high-energy pumps. The pump manufacturer should state the continuous minimum hydraulically acceptable flow for a given pump—and how it was determined. With axial-flow pumps, the shape of the pump head curve may be a factor in selecting the required bypass flow percentage. If feasible, a witness shop test of the pump should be specified to demonstrate and verify minimum flow recommendations.

Liquid Pumped Liquids that flash and cavitate generally required a high bypass flow percentage. Examples are liquids near the boiling point or at high pressure. Abrasives in the liquid may require more bypass than would be needed for thermal reasons alone.

Energy Costs A high energy cost to operate the pump requires careful consideration of bypass system design. The evaluation should compare equipment installation costs, maintenance, and energy costs for various bypass configurations. Figure 1 shows the annual pumping costs for a continuous bypass type system based on bypass flow, pressure and energy rates. The example shows a pump with a discharge head of 500 ft (152

FIGURE 1 Annual pumping cost estimate for continuous bypass systems (metric conversions: ft X 0.3048 = m, gpm X 0.277 = m3/h, hp X 0.746 = W)

m) and a bypass flow requirement of 400 gpm (91 m3/h). Based on an energy cost of 5.0 cents/KWH, the annual cost for continuous bypass is $24,000. An automatic bypass system will only open the bypass when process flow demand is low. The total design life energy costs of a continuous bypass system can easily exceed the hardware costs of an automatic bypass system.

Noise Considerations Bypass systems can easily exceed OSHA requirements for occupied spaces if not designed properly. High pressure drops and high fluid velocity increase noise. Multi-stage pressure reduction, heavy wall pipe, insulation, and silencers will all combine to reduce noise to acceptable levels. See Section 8.4 for a further discussion of this topic.

Process System Design and Operational Expectations Bypass system design will depend on the plant design life and the expected process operational requirements. For example, a swing-loaded electric power plant will have far different pump operating requirements than a low-pressure emergency fire water system. The bypass system designer must evaluate installation, maintenance, and operating costs for the life of the process system with the expected utilization. If the actual operating conditions differ significantly from the design, the configuration of the bypass system should be reevaluated. For example, a process designed for normal operation with one pump at 75% of maximum capacity may put severe demands on the bypass system if the pump is operated continuously at 20% of maximum capacity.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

Get My Free Ebook


Post a comment