Twin screw positive displacement pumps provide performance solutions for many chemical batch applications. Normal, double Screw pump They are "the last resort", because they can solve many pumping challenges, but this skill is not widely known. Compared with centrifugal pump, the unit price of twin-screw pump may be more expensive. However, when comparing the total cost of ownership (TCO) of the system optimized by twin-screw pump, twin-screw pump soon becomes an attractive choice.
In any application, if the process may touch air pockets, some gas or steam content, flow, temperature, pressure, liquid characteristics or viscosity characteristics are in danger of changing, then the use of twin-screw pump should be considered. Twin screw performance has advantages in chemical processes and transportation applications undergoing one or more changes, as well as in any operating conditions that may be difficult to reach or exceed other more common pump limits.
The twin-screw planning in chemical processing application can flow at a flow rate of up to 3000 m3/h (m3/h), the pressure difference exceeds 100 bar, and the product temperature reaches 400 ℃ (752 Fahrenheit), Three screw pump The viscosity of the product ranges from less than 1 centipoise (cP) to 1000000 centipoise (cP), making it a durable machine. They can deal with various process conditions existing in chemical production and processing industry, such as dry running, self-priming, large capacity, flushing, cleaning and flow control.
Variable frequency drive (VFD) is a powerful supporting skill for twin-screw pump. Now, the use of VFD allows users to take full advantage of the flexibility of the twin screw pump, because the power of the pump depends on the speed, and does not affect the pressure difference. With VFD, users can control all performance scales from 10% to 100% by controlling the speed.
When evaluating TCO during startup, operation, protection and shutdown, twin-screw pump may be the most economical solution.
Basic principles of twin-screw skills
Twin screw pump can handle a variety of liquids or products with expanded viscosity scale in batch process. In addition to liquids containing entrapped gases, they can also deal with various liquid properties, such as smooth, non smooth, corrosive and abrasive liquids.
The impeller of the centrifugal pump will form a low-pressure area at the impeller hole. When the liquid containing trapped gas is pumped, the gas will be separated and collected at this moment. This phenomenon is called steam determination. When steam is determined to be generated, IMO pump The air ring at the hole will restrict the flow of liquid through the impeller.
The only way to correct the steam blockage is to remove the gas. This includes stopping the pump, removing air from the system, and restarting the pump. The twin-screw pump is a positive displacement pump. The trapped gas entering the pump cavity is discharged with the liquid discharged from the pump. Since the pump discharge outlet is blocked from the pump suction inlet, there is no steam determination.
The second advantage of twin-screw pump is energy efficiency. The volume power of twin-screw and other rotary positive displacement pumps is the same as that of viscosity. This is because the internal clearance becomes more effective when sealing the pump chamber.
Due to its planning, the capacity and power of the centrifugal pump will decrease with the addition of viscosity. The loss of internal friction of centrifugal pump will be added along with the addition of viscosity, resulting in the decrease of capacity, head and power. The capacity of centrifugal pump will also change with the change of system pressure, so additional control equipment is required, such as control valve and variable speed drive equipment.
The requirements for dynamic pressure drop at pump inlet (NPSHr) of twin-screw pump are low. This allows users to plan to pump different liquids with one pump. The NPSH of twin-screw pump is low, because it reduces the resistance of liquid entering the pump chamber; The generation of cavitation is to increase the pressure drop generated with the increase of flow resistance, and then increase the possibility of cavitation.
In the process of planning the system, the following features should be included in how to reduce the liquid flow resistance of the twin-screw pump:
• Diverter channel, diverting the incoming liquid to the output
• Lower axial speed in the pump chamber as a function of pump speed and selected pitch
• Avoid liquid acceleration
A typical intermittent process may change the characteristics of the products and components used to pump data as the intermittent process progresses. Each type of product requires a certain pressure and temperature in the batch process to start the appropriate chemical response. These reactions generally require a pump, which can circulate the product from the reactor vessel through the heat exchanger, and then return to the vessel. Therefore, it is necessary for the pump to be able to handle all product viscosities from the characteristics of the reverberant to the final product, and to empty the reverberator after each batch of reverberation.
Low NPSHr and dry running capacity become very important at this stage, because any product in the reactor is unavailable
For further application. The power and stability of the plant can be improved by pumping out the reverberator with a low liquid level and emptying it with an air plug, plus the dry running capacity of the pump. The mechanical seal support of the twin screw pump and the internal clearance adhered by the rigid timing gear enable it to meet this process requirement. This allows the twin-screw pump to pump the low viscosity liquid touched in the solvent or caustic cleaning cycle, as well as the high viscosity final product produced in the reverberator.
Other planning considerations
The twin-screw pump planning provides an optional sheath for the pump body and the housing of the fixed case, and adds rated operational flexibility under starting, closing, cleaning cycle and abnormal conditions. The screw hole and pitch can be customized to meet specific operating requirements and supply specific power. The fixed box can also be customized to provide a variety of industry standard sealing solutions. Many models include a rotor plan that provides a rear pull-out function to help with viewing and maintenance. This eliminates the need to disturb pipes or drives.
Special hard coatings, such as ceramic, chromium, nickel, and cobalt based materials, can also be used to add wear resistance to critical pumping surfaces, such as valve body bores and screws. The pump casing can be made of any machinable alloy, depending on the corrosion protection requirements used. Pump manufacturing materials include cast iron, ductile iron, white cast iron, nickel resistance, steel, low temperature steel, stainless steel, duplex stainless steel and Hastelloy.
Flow control and energy recovery
The twin screw pump delivers the fluid to the closed chamber formed by the screw. Therefore, the capacity of the pump directly and primarily depends on the speed of the pump. Pump speed control will allow users to optimize process control. Flow control is a function of pump speed. As long as the outlet pressure of the pump is higher than the inlet pressure, it can work like a pump, adding energy to the fluid and consuming energy.
However, when the inlet pressure is higher than the outlet pressure, the pump will act as a hydraulic motor and perform heat regeneration
