How should the design pressure for positive displacement pump systems(rotary pumps)be determined?

In process industries such as chemical,petroleum,and pharmaceutical,positive displacement pumps(e.g.,rotary pumps,gear pumps,screw pumps)are widely used due to their stable flow rate and suitability for high-viscosity media.However,determining the design pressure for these pump systems does not simply rely on"shut-off pressure"like general centrifugal pumps do.Instead,it requires a comprehensive analysis from the perspective of overall system safety and economic efficiency.The design pressure serves as the core basis for the strength design of equipment and piping.

A key characteristic of positive displacement pumps is that their output pressure is primarily limited by the pump casing strength and driver torque.When the outlet flow is restricted,the pressure continuously rises until it reaches the so-called"stoppage pressure"–the pressure difference required to cause the driver to stop operating.This pressure value is typically much higher than the pump's normal operating pressure.If the"stoppage pressure"were directly used as the design pressure for equipment within the system,it would lead to excessive equipment wall thickness,material waste,and unnecessary investment increases.Therefore,the design pressure for equipment on the discharge side of a positive displacement pump should not be simplistically equated to the pump's stoppage pressure.It should be based on the maximum working pressure proposed by the chemical process,supplemented by various additional conditions that may arise during system operation.

So,how is this design pressure determined scientifically?

First,the pump must be analyzed dynamically within the context of the entire process system,rather than viewing individual equipment in isolation.Factors such as system pressure fluctuations,the location of safety valves or rupture discs,the operation method of pump discharge valves,and changes in pipeline resistance all affect the actual pressure bearing at the top of vessels or pipelines.For instance,when a safety valve is installed on a vessel at the pump discharge,the design pressure should not be less than the safety valve set pressure,and the fluid pressure drop from the vessel to the safety valve must be considered.If the system lacks pressure relief devices,the pump's shut-off pressure needs consideration,supplemented by external pressure checks to prevent equipment instability due to vacuum or external pressure conditions.

Furthermore,medium characteristics are equally critical.For positive displacement pump systems handling low-boiling point fluids like liquefied gases or refrigerants,the design pressure must reference the saturated vapor pressure of the medium at its potential maximum temperature.If a valve is mistakenly closed or fluid stagnates,vaporization of the medium could cause a rapid pressure increase,necessitating that the maximum saturated vapor pressure serves as the design basis.Simultaneously,for flammable,toxic,or high-value media,the selection of design pressure requires greater caution,often involving an appropriate increase in the safety margin beyond standard specifications.

In practical engineering design,the design pressure for piping in positive displacement pump systems must also follow the"higher rather than lower"principle:The design pressure of any pipeline connected to equipment should not be lower than the design pressure of the connected equipment;for sections that might become blocked or isolated from overpressure relief devices,design must proceed based on the maximum working pressure that could occur.It is important to note that the pressure value used in the pipeline wall thickness calculation is the design pressure for that pipeline section,and the corresponding working temperature is its design temperature.

So,is it clear now?

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