Screw pumps are widely used in the transportation of water, wet sludge and flocculant liquids in sewage treatment plants due to their characteristics of variable delivery, strong self-priming ability, reversibility, and the ability to transport liquids containing solid particles. . Screw pump selection should follow the principles of economy, reasonableness and reliability. If the design is poorly selected, it will cause troubles for future use, management, and maintenance. Therefore, using a practical and reliable screw pump in accordance with actual production needs can not only ensure smooth production, but also reduce repair costs.

    First, the screw pump speed selection

    The flow rate of the screw pump has a linear relationship with the rotation speed. Compared with the low-speed screw pump, the high-speed screw pump can increase the flow rate and the lift, but the power is obviously increased. The high rotation speed accelerates the abrasion between the rotor and the stator, and must ensure that The screw pump fails prematurely, and the fixed rotor length of the high-speed screw pump is very short, which makes it easy to wear, thus shortening the service life of the screw pump.

    By reducing the speed through a speed-reducing mechanism or a stepless speed-regulating mechanism, the speed is maintained within a reasonable range below each minute of three hundred revolutions, and the service life can be extended several times as compared with a high-speed screw pump.

    Second, the quality of the screw pump

    There are many kinds of screw pumps on the market. Relatively speaking, the imported screw pumps are designed with reasonable materials and high quality. However, the prices are relatively high, some services are not in place, the accessories are expensive, and the long order period may affect the normal production. run. Most of the domestic production of imitation of imported products, product quality varies greatly, in the selection of domestic production of products, when considering its cost-effective, choose low-speed, long lead, transmission components, material quality, long life rating of the product.

    Third, ensure that debris does not enter the pump body

    Solid debris mixed in the wet sludge will damage the rubber stator of the screw pump, so it is very important to ensure that debris does not enter the pump cavity. Many sewage plants add a pulverizer in front of the pump, and some are installed. Grid device or filter to block debris from entering the screw pump. The grid should be cleaned in time to avoid plugging.

    Fourth, avoid breaking materials

    The screw pump must not be allowed to operate in the event of a broken material. Once it has occurred, the rubber stator at the outlet of the screw pump is dry due to dry friction. It instantly generates high temperatures and burns out. Therefore, the crusher is in good condition and the grill is unblocked. This is necessary for the normal operation of the screw pump. One of the conditions, for this reason, some screw pumps are also equipped with a shut-off stop device on the pump body. When the cut-off occurs, due to the self-priming function of the screw pump, a vacuum will be generated in the chamber and the vacuum device will stop the screw pump. Operation.

    Fifth, maintain a constant outlet pressure

    The screw pump is a positive displacement rotary pump. When the outlet end is blocked, the pressure will gradually increase to exceed the predetermined pressure value. At this point, the motor load increases dramatically. The load of the related parts of the transmission machinery will also exceed the design value. In severe cases, the motor will be burned and the transmission parts will be broken. In order to avoid damage to the screw pump, a bypass relief valve is generally installed at the outlet of the screw pump to stabilize the outlet pressure and maintain the normal operation of the pump.

 

Single-screw pump working principle, characteristics and selection method

   ●How the pump works

    A single-screw pump is a volumetric pump with an internally engaged eccentric rotation. The main components of the pump are: a single-screw rotor and a stator with a double screw, usually made of elastic material, when the rotor is wound in the stator cavity When the axis of the planet rotates, the closed wrist formed between the rotating stators is displaced along the rotor spiral; therefore, the medium is continuously, evenly, and constantly delivered from the suction inlet to the extrusion end. Characteristics, single-screw pump is especially suitable for work under the following conditions.

    Delivery of high-viscosity media: According to the size of the pump can be transferred from 37000-200000 centipoise media.

    Media containing particles or fibers: The particle diameter can be 30mm (no more than rotor eccentricity). The fiber length can be 350mm (equivalent to the pitch of a 0.4-position rotor). Its content is generally up to 40% of the medium pit. If the solid content in the medium is a fine powder, the highest content can reach 60% or more can also be transported. When the delivery pressure is required to be stable and the inherent structure of the medium does not like to be destroyed, the single screw pump is the ideal choice for transportation.

    ● The characteristics of the pump

    Single-screw pump has many advantages compared with piston pump centrifugal pump, vane pump and gear pump because of its structure and working characteristics.

    Can transport high solid content media;

    Even flow pressure is stable, and it is more obvious at low speed;

    The flow is proportional to the speed of the pump and therefore has good variable regulation;

    A pump can be used to transport different viscosity media;

    The installation position of the pump can be arbitrarily tilted;

    Suitable for conveying sensitive goods and articles that are susceptible to damage such as centrifugal force;

    Small size, light weight, low noise, simple structure, easy maintenance.

    ● Purpose of the pump

    The single-screw pump can be widely used in the following industrial sectors to transport various media.

    Chemical industry: Acid, alkali, salt solution, outlet pressure of screw pump Various viscous, paste-like milky chemical slurry.

    Exploration and Mining: All kinds of drilling mud, mining water, floating slurry and liquid.

    Shipbuilding industry: Bottom sewage, oil, various fresh fuel oil.

    Ceramic industry: clay, clay, glaze.

    Energy industry: all kinds of fuel oil, coal slurry, coal slurry, slime and nuclear waste.

    Sewage treatment: sewage, oil, sludge.

    Paper industry: all kinds of cellulose pulp, paint, black liquor.

    Medicine, food, makeup industry: all kinds of cereal berry pulp, starch paste, ointment, mother liquor, mashed potatoes, wine and distiller's grains.

    Other agricultural, forestry, animal husbandry, and sub-fisheries: It is also widely used.

    ● Selection of pumps

    The pump specifications are determined based on the nature of the liquid being transported, the flow rate, and the pressure. The speed of the pump is selected by the viscosity and the corrosive nature of the delivered liquid as the main parameters to ensure the reliable operation of the pump. Recommended for this (Figure 2) for reference.

    The design speed of the pump is to allow the maximum rotation speed when transporting similar non-corrosive liquids such as fresh water or clean water. In actual use, the medium properties and life requirements are different. When used, the pump speed should generally be lower than the design speed, and the high viscosity and medium containing particles Generally, the design speed is 1/2-1/3 or lower. The low speed is beneficial to the wear of the pump, but after the use, the flow rate due to the wear of the stator is reduced. In this case, the speed can be increased to compensate for the decrease in flow rate. Regular policy. The temperature of the conveying medium should be between -10°C and 80°C. Special conditions can reach up to 120°C.

    The normal pressure of each stage of the pump (one lead of the stator) is 0.6 Mpa, and up to 0.8 Mpa is allowed within a short time (not more than 30 minutes), and the volumetric efficiency at the same output pressure is gradually reduced when worn.

    The maximum suction range when pumping fresh water is 8 meters. The actual usage recommendation is no more than 6.5 meters. When pumping high viscosity media, it should be sucked with positive pressure.

 

Selection of single screw pump

1. The pressure of the single-screw pump is determined: The maximum output pressure of the single-screw pump is determined according to the number of bushings, that is, the number of lead of the bushing: Class 1: The maximum working pressure is 0.6 MPa; Class 2: The maximum working pressure is 1.2 MPa. Grade 4: The maximum working pressure is 2.4 MPa. Due to the different conditions of the transmission medium, 3qgb screw pump is for medium with severe wear, please refer to Table 1 to select the number of bushings. Table 1: Selecting the Number of Bushing Stages Based on the Abrasion of the Medium Abrasion Level 1

 

 

No 0.6MPa 1.2MPa

General 0.4MPa 0.8MPa

Serious 0.2MPa 0.4MPa

2. Selection of single screw pump speed: Due to its structural characteristics, the single screw pump is mostly used for conveying higher viscosity liquids and liquids containing particles, so the choice of the number of revolutions is very critical. Based on experience, Table 2 and Table 3 are listed. You can consider them according to your specific situation. Table 2: Select the pump speed according to the wear of the medium Wearability Media name Speed ​​(rpm)

No fresh water, coagulant, oil, juice, minced meat, paint, soapy water, blood, glycerin 400-1000

General sludge, suspension, industrial wastewater, paint pigments, waste silk (sugar), mortar, fish, wheat bran, rapeseed oil filtered sediment 200-400

Serious Mortar, clay, plaster, clay 50-200

*Special examples of specific media and their wear characteristics are given in the table. Please note that the characteristics of the media change with its concentration and temperature. * When the size of the pump is large, the speed should be selected lower. Table 3: Select pump speed according to medium viscosity Medium viscosity (cst) 1-1000 1000-10000 10000-100000 100000- 1000000

Speed ​​(rpm) 400-1000 200-400 <200 <100

* When selecting the speed, it is also based on experience, because some other factors also affect the choice of speed, in the final determination of the above values ​​and the best consultation with the manufacturer. 3. Selection of single-screw pump bushing rubber material: The single-screw pump bushing is a rubber product, and it is also a consumable part of a single-screw pump. Its selection is good or bad, and directly affects the service life of the bushing. Normally, the bushing is normal. The service life is 3-6 months. If it is not properly selected, the bushing may come off the steel pipe or the rubber may drop out. Therefore, we are required to understand the basic properties of rubber and the adaptability of rubber to various media. Tables 4 and 5 are listed based on our experience.