When selecting the twin screw pump basic technical parameters are needed. Include flow, inlet, and outlet pressure difference, and viscosity of the medium. In addition, it is also necessary to provide the working temperature of the medium, whether it is corrosive or abrasive, etc. With these parameters, you can start the selection. Please follow the process below for selection:
1 Determine the speed of the twin screw pump according to the viscosity
1) Working temperatures
First of all, the speed of the pump should be determined according to the viscosity of the medium being conveyed. At different working temperatures, the viscosity of the medium is different, and it is necessary to determine the maximum medium viscosity that may occur at the current working temperature. If the actual viscosity is higher than the estimated viscosity, there may be insufficient power.
According to the viscosity of the medium at the working temperature, the number of revolutions n of the pump is determined: generally, when V≤1000mm2/s, n=1450r/min is allowed, and when V≤3000mm2/s, n=750~950r/min is allowed. When V>3000mm2/s, n=600~750r/min is allowed.
2) NPSH value of the pump
Please carefully consider the NPSH value of the twin screw pump and the NPSH value of the pipeline; the higher the viscosity, the more attention should be paid. Generally speaking, when the viscosity exceeds 1000mm2/s, the inlet resistance of the pipeline should be calculated before selection. If the inlet resistance does not meet the requirements, it will cause damage to the pump. Therefore, remind the user to design the calculation or submit it to the technical department of our company for calculation.
3) NPSH value of the pipeline
When high-viscosity materials are transported in the pipeline, it should be noted that if the pipeline resistance is too large, the load will increase sharply, and the conventional shaft power calculation will cause the problem of insufficient power. When handing over to our company for calculation, the user should provide the corresponding inlet pipeline information, including the pipeline length, height difference, filter elbow tee, and other accessories, and our company’s technical department will provide the selection.
2. Determine the model of the pump according to the parameters provided by the user
The model of the twin screw pump can be determined according to the flow rate, the pressure difference between the inlet and the outlet, the viscosity of the medium, and the speed of the pump.
Our company’s pump model consists of 8 parts. The fifth “specification” has been described above, but how to determine the model of the pump? This is further explained here:
2.1 According to the flow and pressure determine whether to choose a double-suction product or a single-suction product.
Note that as long as the conditions are met, try to use single-suction products, because single-suction products have a simple structure and are easy to load and unload.
Generally speaking, when the flow rate is not large and the pressure is not high, use single-suction products (ie BY2G or BY2GX); on the contrary, when the flow is large or the pressure is high, use double-suction products (ie BY2GS…series). The single-suction product has a simple structure and is easy to maintain. It is preferred to use it as far as possible.
2.2 Determination of the sealing form of the twin screw pump
General users will put forward some requirements, and what kind of sealing form is adopted depends on the needs of the working conditions or the user’s preference in many cases. However, it should be noted that the use of packing seals will produce more or less trace leakage. Leakage that meets the requirements of industry standards and national standards is allowed, but when transporting toxic and harmful liquids, it is forbidden to use packing seals. such substances, double mechanical seals should be used to achieve long-term zero leakage. For the transportation of high-viscosity chemical materials, if there is no suitable sealing liquid, the packing seal should be used to realize it. Otherwise, the mechanical seal may be scrapped in a very short period of time due to the inability to obtain timely cooling, or be affected by high-viscosity materials. effect and fail.
2.3 The heat preservation method and installation method
The heat preservation method and installation method are the relevant structures of the pump selected by the user according to the conveying needs of the medium. When the user does not make a request, our company will produce according to the content of the default item.
2.4 The bypass valve
The bypass valve plays an overload protection role in the pump unit. It is configured according to the user’s needs. Our company will not configure the bypass valve if the user does not ask for it. Our company recommends using other methods to protect the pump unit from overload, such as automatic shutdown of the motor when the motor is overloaded, pressure relays in the pump discharge pipeline system, and installation of solenoid valves, etc., which are simpler, more reliable and more sensitive.
3. Determine the drive model of the twin screw pump
The motor is the most commonly used driver for the pump unit. After selecting the pump specification, you can find out the shaft power N corresponding to the flow rate of the twin screw pump of this specification under the working pressure P from the table under the corresponding viscosity V. With the shaft power, you can determine the motor power. The optional motor power Nm should be slightly larger than the found shaft power N. When the power is relatively small and the pressure is relatively low, the reserve should be larger. The applied power reserve is Nm≥K·N, and the K value is shown in the table below. With the motor power, the model and specifications of the motor (or geared motor) are determined according to the explosion-proof requirements and other conditions required by the user.
After the user selects the pump, please ask the company for the size of the pump and motor unit required for the foundation construction design.