1. Flow
The amount of fluid delivered by the pump in unit time is called flow.It can be expressed by volume flow qv, and the common unit is m3/s,m3/h or L/s；It can also be expressed by mass flow qm, and the common unit is kg/s or kg/h.
The relationship between mass flow and volume flow is:
qm=pqv
Where, p — density of liquid at delivery temperature, kg/m ³.
According to the needs of chemical production process and the requirements of the manufacturer, the flow of chemical pumps can be expressed as follows: ① The normal operating flow is the flow required to reach its scale output under the normal operating conditions of chemical production. ② Maximum required flow and minimum required flow When chemical production conditions change, the maximum and minimum required pump flow.
③ The rated flow of the pump shall be determined and guaranteed by the pump manufacturer. This flow shall be equal to or greater than the normal operating flow, and shall be determined with full consideration of the maximum and minimum flow. In general, the rated flow of the pump is greater than the normal operating flow, or even equal to the maximum required flow.
④ Maximum allowable flow The maximum value of the pump flow determined by the manufacturer according to the pump performance within the allowable range of structural strength and driver power. This flow value should generally be greater than the maximum required flow.
⑤ Minimum allowable flow The minimum value of the pump flow determined by the manufacturer according to the pump performance to ensure that the pump can discharge liquid continuously and stably, and that the pump temperature, vibration and noise are within the allowable range. This flow value should generally be less than the minimum required flow.
2. Discharge pressure
Discharge pressure refers to the total pressure energy (in MPa) of the delivered liquid after passing through the pump. It is an important sign of whether the pump can complete the task of conveying liquid. For chemical pumps, the discharge pressure may affect the normal progress of chemical production. Therefore, the discharge pressure of chemical pump is determined according to the needs of chemical process.
According to the needs of chemical production process and the requirements for the manufacturer, the discharge pressure mainly has the following expression methods.
① Normal operating pressure， The pump discharge pressure required for chemical production under normal operating conditions.
② Maximum discharge pressure， When chemical production conditions change, the pump discharge pressure required by the possible working conditions.
③Rated discharge pressure, the discharge pressure specified and guaranteed by the manufacturer. The rated discharge pressure shall be equal to or greater than the normal operating pressure. For vane pump, the discharge pressure shall be the maximum flow.
④ Maximum allowable discharge pressure The manufacturer determines the maximum allowable discharge pressure of the pump according to the pump performance, structural strength, prime mover power, etc. The maximum allowable discharge pressure shall be greater than or equal to the maximum required discharge pressure, but shall be lower than the maximum allowable working pressure of the pump pressure parts.
3. Energy head
The energy head (head or energy head) of the pump is the increment of the energy of the unit mass liquid from the pump inlet (pump inlet flange) to the pump outlet (pump outlet flange), that is, the effective energy obtained after the unit mass liquid passes through the pump λ Is expressed in J/kg.
In the past, in the engineering unit system, the head was used to represent the effective energy obtained by the unit mass liquid after passing through the pump, which was represented by the symbol H, and the unit was kgf · m/kgf or m liquid column.
The relationship between energy head h and head H is:
h=Hg
Where, g – gravity acceleration, the value is 9.81m/s ²。
Head is the key performance parameter of vane pump. Because the head directly affects the discharge pressure of the vane pump, this feature is very important for chemical pumps. According to the chemical process needs and the requirements of the manufacturer, the following requirements are proposed for the pump lift.
①The pump head determined by the discharge pressure and suction pressure of the pump under normal working conditions of chemical production.
② The maximum required head is the pump head when the chemical production conditions change and the maximum discharge pressure (suction pressure remains unchanged) may be required.
The lift of chemical vane pump shall be the lift under the maximum flow required in chemical production.
③ Rated lift refers to the lift of vane pump under rated impeller diameter, rated speed, rated suction and discharge pressure, which is determined and guaranteed by the pump manufacturer, and the lift value shall be equal to or greater than the normal operating lift. Generally, its value is equal to the maximum required lift.
④ Shut down the head of the vane pump when the flow is zero. It refers to the maximum limit lift of vane pump. Generally, the discharge pressure under this lift determines the maximum allowable working pressure of pressure bearing parts such as pump body.
The energy head (head) of the pump is the key characteristic parameter of the pump. The pump manufacturer shall provide the flow energy head (head) curve with the pump flow as the independent variable.
4. Suction pressure
It refers to the pressure of the delivered liquid entering the pump, which is determined by the chemical production conditions in chemical production. The suction pressure of the pump must be greater than the saturated vapor pressure of the liquid to be pumped at the pumping temperature. If it is lower than the saturated vapor pressure, the pump will produce cavitation.
For vane pump, because its energy head (head) depends on the impeller diameter and speed of the pump, when the suction pressure changes, the discharge pressure of the vane pump will change accordingly. Therefore, the suction pressure of the vane pump shall not exceed its maximum allowable suction pressure value to avoid the pump overpressure damage caused by the pump discharge pressure exceeding the maximum allowable discharge pressure.
For the positive displacement pump, because its discharge pressure depends on the pressure of the pump discharge end system, when the pump suction pressure changes, the pressure difference of the positive displacement pump will change, and the required power will also change. Therefore, the suction pressure of the positive displacement pump cannot be too low to avoid overloading due to excessive pump pressure difference.
The rated suction pressure of the pump is marked on the nameplate of the pump to control the suction pressure of the pump.
5. Power and efficiency
The pump power usually refers to the input power, that is, the shaft power transferred from the prime mover to the rotating shaft, expressed in symbols, and the unit is W or KW.
The output power of the pump, that is, the energy obtained by the liquid in unit time, is called the effective power P. P=qmh=pgqvH
Where, P — effective power, W;
Qm — mass flow, kg/s; Qv — volume flow, m ³/ s。
Due to various losses of the pump during operation, it is impossible to convert all the power input by the driver into liquid efficiency. The difference between the shaft power and the effective power is the lost power of the pump, which is measured by the efficiency force of the pump, and its value is equal to the effective P
Ratio of ratio and shaft power, namely: (1-4)
Corpse P.
The efficiency of the pump also indicates the extent to which the shaft power input by the pump is used by the liquid.
6. Speed
The number of revolutions per minute of the pump shaft is called the speed, which is expressed by the symbol n, and the unit is r/min. In the international standard system of units (the unit of speed in St is s-1, that is, Hz. The rated speed of the pump is the speed at which the pump reaches the rated flow and rated head under the rated size (such as impeller diameter of vane pump, plunger diameter of reciprocating pump, etc.).
When a fixed speed prime mover (such as a motor) is used to directly drive the vane pump, the rated speed of the pump is the same as the rated speed of the prime mover.
When driven by a prime mover with adjustable speed, it must be ensured that the pump reaches the rated flow and rated head at the rated speed, and can operate continuously for a long time at 105% of the rated speed. This speed is called the maximum continuous speed. The adjustable speed prime mover shall have an overspeed automatic shutdown mechanism. The automatic shutdown speed is 120% of the rated speed of the pump. Therefore, the pump is required to be able to operate normally at 120% of its rated speed for a short time.
In chemical production, the variable speed prime mover is used to drive the vane pump, which is convenient to change the working condition of the pump by changing the pump speed, so as to adapt to the change of chemical production conditions. However, the operating performance of the pump must meet the above requirements.
The rotating speed of positive displacement pump is low (the rotating speed of reciprocating pump is generally less than 200r/min; the rotating speed of rotor pump is less than 1500r/min), so the prime mover with fixed rotating speed is generally used. After being decelerated by the reducer, the working speed of the pump can be reached, and the speed of the pump can also be changed by means of speed governor (such as hydraulic torque converter) or frequency conversion speed regulation to meet the needs of chemical production conditions.
7. NPSH
In order to prevent cavitation of the pump, the additional energy (pressure) value added on the basis of the energy (pressure) value of the liquid it inhales is called cavitation allowance.
In chemical production units, the elevation of the liquid at the suction end of the pump is often increased, that is, the static pressure of the liquid column is used as the additional energy (pressure), and the unit is meter liquid column. In practical application, there are two kinds of NPSH: required NPSH and effective NPSHa.
(1) NPSH required,
Essentially, it is the pressure drop of the delivered fluid after passing through the pump inlet, and its value is determined by the pump itself. The smaller the value is, the smaller the resistance loss of the pump inlet is. Therefore, NPSH is the minimum value of NPSH. When selecting chemical pumps, the NPSH of the pump must meet the requirements of the characteristics of the liquid to be delivered and the pump installation conditions. NPSH is also an important purchase condition when ordering chemical pumps.
(2) Effective NPSH.
It indicates the actual NPSH after the pump is installed. This value is determined by the installation conditions of the pump and has nothing to do with the pump itself
NPSH. The value must be greater than NPSH -. Generally NPSH. ≥ (NPSH＋0.5m)
8. Medium temperature
The medium temperature refers to the temperature of the conveyed liquid. The temperature of liquid materials in chemical production can reach – 200 ℃ at low temperature and 500 ℃ at high temperature. Therefore, the influence of medium temperature on chemical pumps is more prominent than that of general pumps, and it is one of the important parameters of chemical pumps. The conversion of mass flow and volume flow of chemical pumps, the conversion of differential pressure and head, the conversion of pump performance when the pump manufacturer conducts performance tests with clean water at room temperature and transports actual materials, and the calculation of NPSH must involve the physical parameters such as the density, viscosity, saturated vapor pressure of the medium. These parameters change with temperature. Only by calculating with accurate values at temperature can correct results be obtained. For pressure bearing parts such as pump body of chemical pump, the pressure value of its material and pressure test shall be determined according to the pressure and temperature. The corrosivity of the delivered liquid is also related to the temperature, and the pump material must be determined according to the corrosivity of the pump at the operating temperature. The structure and installation method of pumps vary with temperature. For pumps used at high and low temperatures, the influence of temperature stress and temperature change (pump operation and shutdown) on installation accuracy should be reduced and eliminated from the structure, installation method and other aspects. The structure and material selection of the pump shaft seal and whether the auxiliary device of the shaft seal is required shall also be determined by considering the pump temperature.
Post time: Dec-27-2022